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ODESSA STATE MEDICAL UNIVERSITY
Department of histology, cytology and embryology
BASICS OF GENERAL AND COMPARATIVE EMBRYOLOGY
Odessa - 2009
BASICS OF GENERAL EMBRYOLOGY
Embryology is the science that studies the natural laws of human and animal organism’s development since
the fertilization and up to the formation of the structural patterns of an adult organism in the embryo and the
development of the ability to independent nutrition. The knowledge of basic human embryology, the
physiological pattern of development and its special features at the various stages of the embryonic period
enables the doctor to understand and to adjust – if possible – the development pathology. In some cases
successful prophylaxis is possible.
The general stages of development, typical for spinal animals are preserved during the embryonic
development of human organism. However, some special features make human embryonic development
different from the development of other spinal animals.
Before describing the embryogenesis let’s mention the native scientists, that contributed to the development
of embryology as a science. At the beginning of XIX century the proceedings of St Petersbourg Medical
Academy academicians Caspar Frederick Wolf and Carl Maximovich Bar were published. The development
of birds – using the chicken model – was described thoroughly by C.F. Wolf, while K.M. Bar was the first to
describe the structure of human oocyte. By the middle of XIX century the investigations of the lancelet’s
embryogenesis, conducted by Illya Illyich Mechnikov and Alexander Onufrievich Kovalevsky had proven the
common nature of animal’s development pattern. That was the basis of the biogenetic law of general
development. Therefore, I.I. Mechnikov and A.O. Kovalevsky are considered to be the founders of
comparative embryology.
The human embryonic development consists of the three periods: initial (first week), embryonic (2-8 weeks)
and fetal (from 9-th weeks to birth).
Human embryogenesis is a part of onthogenesis (individual development of the organism). It includes the
following basic stages:
I Fertilization and zygote formation
II Cleavage and blastule formation (blastocytes)
III Gastrulation (formation of germinal layers)
IV Histogenesis and organogenesis of embryonic and extra-embryonic tissues
V Genesis of systems
Comparative embryology is a subdivision of embryology. It compares the development of embryo’s in
animals of various evolutional ranking. These data help to figure out the basic stages of embryogenesis,
similar for all animals and humans and to determine the exact position of humans in the evolution succession.
This is especially important, because the impossibility of experiments with humans limits the material supply
in the field of human embryogenesis. Moreover, some early stages of human embryogenesis are rapid and not
fully manifested. All these are the premises for comparing of the embryogenetic patterns of more primitive
animals in order to understand the human embryogenesis better.
PROGENESIS
Embryogenesis is closely related to progenesis (development and maturation of sex cells) and to early post-
embryonic period.
A sperm cell is the final cell of the spermatogenesis process. It is the unique cell. It has an elongated shape.
All its body is highly specialized. The head of the sperm cell contains a nucleus. About 2/3 of it is covered by
the acrosome. Nucleus with a haploid set of condensed chromosomes (23 chromosomes with an allosome) is
the main component of the sperm cell’s head. The density of chromosomal condensation is high enough, so
that the volume of nucleus is 30-fold smaller than the volume of nucleus in the haploid cells right after
meiosis. Such a rate of condensation is promoted by special basic proteins that are contained in the nucleus.
These proteins differ from typical histones and are rich with arginine and cysteine. The condensed condition
protects the genetic material from damage that might occur during the penetration of the oocyte membrane by
the sperm cell.
The frontal part of the nucleus is covered by the big acrosome that fits it as a double cap. Acrosome is the
flattened vesicle that is formed by Golgi’s complex; it’s structure resembles lysosome. It contains the
enzymes required for the penetration of spermatozoon into the oocyte (collagenaze, hyaluronidaze, acrosine,
acid phosphatase, etc). The head and the tail of the spermatozoon are covered by the plasmatic membrane.
The head area of this membrane contains special proteins. Some of them have the negative charge and
promote the direct movement course of the sperm cell towards the oocyte (at close distances). Other proteins
participate in the process of binding to the oocyte. The post-acrosomal zone of the spermatozoon’s head is the
area, where its cytoplasmic membrane merges with the membrane of the oocyte after the fertilization occurs.
The tail of the spermatozoon is its longest part. It consists of the neck, the intermediate, main and terminal
parts. Its short neck contains two centrioles. The proximal centriole lies close to the nucleus, while the distal
one is the beginning of the axis line, or the axoneme, that runs throught the cell’s tail. The structure of
axoneme is typical for all villi or filaments. It is formed by microtubules – the scheme of their location is
(9Ч2)+2. The microtubules interact with each other with a help of dineine “handles”. This enables the
neighboring duplets to slide and the sperm cell’s tail wriggles.
In the tail’s intermediate part the axoneme is surrounded by 9 pairs of peripheral microtubules. They act as
passive elastic structures. The mitochondrial sheath is located there too. It is formed by mitochonders that are
located in a spiral pattern. The coils of this spiral are pressed close to each other.
In the tail’s main part the axoneme is surrounded by 9 pairs of external fibrillae and a fibrous sheath (sheath
of thin fibrillae).
Finally, at the terminal part of the tail the axoneme is covered by the plasmatic membrane (however, it is
present in all other parts of the tail).It contains a few contractile filaments.
The oocyte’s shape is spheroid. The volume of its cytoplasm is much bigger, than the one of spermatozoon. It
can’t move by itself.
The classification of oocyte cells is based on the presence, quantity and distribution of the nutrients that are
used only after the fertilization. The main nutrients are yolk, lipids and glycogen. Yolk consists of crystalline
protein-like substances, usually of phosphoproteins. These crystals look like granules or like platelets of
various shapes, surrounded by membrane. The amount of yolk deposit in the oocyte is greatly variable and
depends on the species. It is strictly determined by the genetic set of the species and would not depend on the
nutrition rate of female organism. The yolk contains A, B1, B2, D and E vitamins. Most lipids are represented
by neutral fats; the rest are phosphatides, phospholipids and cholesterol.
There are various types of oocyte cells - polylecytal, or cells containing big amount of yolk (they are found in
most arthropoda, fishes and birds), mesolecytal or cells containing moderate amount of yolk (amphibians,
some sturgeon fishes), oligolecytal or cells containing small amount of yolk (invertebrates, mammals) and
alecytal – or cells containing no yolk at all (tapeworms). The oligolecytal oocyte cells are subdivided into
primary (lancelet) and secondary (placentar mammals, humans) types. Usually, the yolk content of the
oligolecytal cells is distributed evenly. These cells are called isolecytal. In placental mammals and in humans
the nutrition of embryo is provided by maternal organism, therefore, there is no need to store the nutrients in
the oocyte. However, to differentiate the mammal oocyte from the oocyte of the lower chordata species, that
has a similar structure, it is called secondary isolecytal.
More nutrients are required for the development of highly organized animals, except for mammals. The
polylecytal cells are subdivided according to yolk distribution: there are centrolecytal cells (arthropoda) and
thelolecytal cells (fishes, except for sturgeon, amphibia). The polarity of oocyte is determined by the type of
yolk distribution. The nutrients are stored at one pole (vegetal pole), while the other one is occupied by the
germination material (animal pole). Such oocyte cells are called the thelolecytal ones. Some animal species
have thelolecytal oocyte cells with mild polarity rate (amphibia), while the others have cells with a significant
grade of polarity (fishes, birds).
The size of human oocyte is about 130 mkm. Its cytolemma is surrounded by a transparent or shining zone
(zona pellucida). It consists of glycoproteins and glycosaminoglycans. The shiny zone is surrounded by the
layer of follicular cells that manage the trophic and protective functions. The cell’s cytoplasm contains the
well-developed protein synthesizing organells and Golgi’s complex which are located in the peripheral
cytoplasm. These organells produce the cortical granules. The cortical granules contain glycosaminoglycans
and proteolythic enzymes that take part in the cortical reaction.
FERTILIZATION
Fertilization is the beginning of the embryo’s development. Fertilization is a process of male and female sex
cells fusion that is followed by the formation of zygote. Zygote has a basic difference from sex cells – it
contains a diploid chromosomal set.
Fertilization is merging of the spermatozoon and the secondary oocyte that is followed by the fusion of their
nuclei into the single nucleus of the fertilized oocyte, which is called zygote. Spermatozoon has two functions
– the first is the activation of the oocyte and stimulation of its development, The oocyte development without
the sperm cell’s contribution is called parthenogenesis. The other function of spermatozoon is the introduction
of new genetic material into the oocyte.
The interaction of sex cells is divided into three phases:
1 Distant interaction and rapprochement of gametes
2 Contact interaction and activation of the oocyte
3 Penetration of the spermatozoon into the oocyte and the following fusion– or the syngamy.
Distant interaction of gametes. After the ejaculation spermatozoons lose the glycoproteins and the sperm
plasma proteins of the acrosomal area. This process is mediated by the secretion of the female genital
pathway. It is called capacitation. As a result of this process the sperm cells gain fertilizing abilities. The main
biological task of gametes is to promote the meeting of the sex cells and, however, to prevent the penetration
of additional spermatozoons into the oocyte. In some species the chemotaxis process occurs at this stage – in
this case the movement of spermatozoon is mediated by the concentration gradient of certain compounds,
which are produced by the oocyte (bony fishes). In most cases the distant influence of oocyte isn’t directed.
This kind of influence is mediated by two classes of compounds, which are called hynogamones I and II.
Hynogamones I are the non-protein compounds of low molecular weight. They are produced by the sheaths of
the oocyte. They activate the movement of spermatozoon. Hynogamones II (fertilizines or isoagglutinines)
are glycoproteins that induce the adhesion of sperm cells through the process of immune reaction with
androhamone II (anti-fertilizine). The molecules of this substance are embedded in the membrane of the
spermatozoon. The sperm cells produce androhamone I – the non-protein substance which antagonizes
hynogamone I and suppresses the mobility of sperm cells.
Contact interaction of gametes. It is the second phase of fertilization process. During this stage the sperm
cells approach the oocyte and interact with its membrane. The movement of the sperm cells tails promotes the
spinning of the oocyte. The acrosomal reaction at the sperm cells occurs during the contact of male and
female gametes. It is characterized by the increase of the spermatozoon’s membrane permeability to Ca2+ ions
and further depolarization. It promotes the fusion of the plasmolemma with the anterior membrane of the
acrosome, which is followed by the release of the acrosomal content (spermolyzines). This process results in
the rapid de-condensation of the oocyte sheaths in the area of contact with a sperm cell. The enzymes digest
the zona pellucida, then the sperm cell penetrates it ad enters the peri-vitelline space, which is located
between the zona pellucida and the oocyte’s plasmolemma. In a few seconds the properties of the oocyte’s
plasmolemma change and the cortical reaction starts. The cortical reaction is the fusion of oocyte’s
plasmolemma with the membrane of cortical granules. It is followed by the release of the cortical granules
content. It enters the peri-vitelline space and interacts with the glycoprotein molecules of the zona pellucida.
At the same time the protrusions appear on the acrosomal membrane. It results in the formation of acrosomal
tubules. They elongate and contact with the oocyte’s plasmatic membrane. The cleavage of cell membrane
occurs and the spermatozoon’s nucleus and centrioles enter the oocyte along the acrosomal tubules.
Introduction of the spermatozoon into the oocyte. The head and the intermediate part of the
spermatozoon’s tail penetrate into the ooplasma. The cortical reaction is terminated when the factor,
promoting the consolidation of zona pellucida manifests. After that zona pellucid forms the fertilization layer
that prevents polyspermia (penetration of other sperm cells). The spermatozoon, which has entered the
oocyte, turns with its neck forwards instantly. The chromatine of its nucleus undergoes de-spiralization. Since
this time the nucleus of the spermatozoon is called the male pro-nucleus. The similar processes occur in the
nucleus of the oocyte. This nucleus is called female pro-nucleus. Inside the fertilized oocyte (zygote) the
spermatozoon’s centrioles become the center of motion. The fusion of two pronuclei is called the synkarion.
During the embryogenesis the fertilized oocyte develops into the multi-cellular organism with various organs
and tissues. Therefore, the embryonic development is based on the following processes:
1 Reproduction of cells
2 Cells specification, tissue specification in various directions (differentiation)
3 Growth
4 Movement of single cells and cellular clusters – formation of primordial organs
CLEAVAGE
The newly formed zygote gradually turns into the mutlicellular organism by mitotic division. However, the
newly formed cells, also called blastomeres, have no time to reach the size of the progenitor cells. Therefore,
the size of the newly formed blastomeres decreases gradually. That’s why this phase of embryogenesis is
called cleavage. The cells, which are formed during the several first division sessions usually are not specific.
They produce DNA and proteins that are required for the process of cellular division, not for specific
biochemical activity. In various species the cleavage process is organized in a different way. One of the basic
factors these differences are based on is the amount of yolk in the oocyte.
These special characteristics of oocyte’s structure determine the type of cleavage. Actually, the cleavage of
the oligolecytal oocyte is similar to simple mitosis. The whole zygote participates in the cleavage. The newly
formed cells have the same size. They are smaller than the progenitor cell. This type of cleavage is called the
full or holoblastic. Isolecytal and thelolecytal cells with mild polarity grade are dividing in this way. The
cleavage is called equal (lancelet) if the newly formed blastomeres gave the same size. If the blastomeres
have different sizes, the cleavage is called non-equal (amphibian, mammals). The polylecytal oocyte cells
promote the partial (meroblastic or discoid) cleavage of zygote. In this case the cleavage occurs in the animal
pole material only. After the fusion the zygote’s nucleus is divided into multiple nuclei that move to the outer
layer of cytoplasm, free from yolk, along the cytoplasmic bridges. There these nuclei form the cellular layer,
or blastoderm. In polylecytal oocyte cells of both centrolecytal and thelolecytal types, the cytoplasm forms a
thin layer. The huge mass of yolk (as compared to the volume of pure cytoplasm) which forms the vegetal
pole material, would not undergo any division. This kind of cleavage is called partial, incomplete or
meroblastic. In some polylecytal cells the discoid cleavage occurs (birds, fishes). In this case the cleavage
with blastomeres formation occurs in the thin cytoplasmic disc only that is located at the animal pole. I
various zygotes the cleavage process of blastomeres runs at different speed. Therefore, there are synchronic
and asynchronic types of cleavage. Synchronic or proper cleavage occurs at the same speed, therefore, the
complete geometric progression of the cell’s number is found (2, 4, 8, 16 etc). In case of asynchronic cleavage
the cleavage speed at various areas of the embryo is not uniform. Therefore, the geometric progression of the
number of cells is disrupted.
After the few cycles of cleavage, with each cycle resulting in the doubling of the embryo’s cells, the emryo
looks like a small mulberry (morula – in Latin).
BLASTULATION; TYPES OF BLASTULES
In most eggs even at the early stages of cleavage the inner walls of blastomeres start moving aside and
therefore form a small cavity of cleavage (blastocele) in the compact mass of cells that grows larger
gradually. The shape of blastomeres is no longer spheroid, the areas of interacting contacts increase.
Therefore, the walls of embryo undergo epithelization and now look like a layer of epithelium. Epithelization
is an important premise of cellular interaction and their coordinated movement during he further stages of
development. At this very stage the embryo is called blastula. The process of blastula formation is called
blastulation. Blastula consists of the wall (called blastoderm) and the cavity (called blastocele). The cavity is
filled with liquid, which is produced by the blastomeres. Blastoderm is formed by the theca (animal pole
material), fundus (vegetal pole material) and the marginal zone which is located between them.
The structure of blastula depends on the type of egg’s cleavage. In oligolecytal oocyte cells with a full equal
cleavage (lancelet) the blastula is formed by the thin one-layer cell mass and a large centrally positioned
blastocele. This type of blastula is called celoblastula.
In case of full non-equal cleavage (lampreys, frogs) the amphiblastula is formed. It consists of multi-layered
blastoderma and the eccentrically positioned blastocele. The theca of this blastula is formed by small
blastomeres, and the fundus consists of large blastomeres with high yolk content.
The polylecytal eggs of birds and fishes have a discoid type of cleavage. During it the disc, formed by several
layers of cells appears. It is located on the single body of yolk. The disc curves over the surface of yolk and
therefore forms a cavity (blastocele). This type of blastula is called discoblastula.
The full asynchronic cleavage that is typical for mammals produces the embryonic vesicle or the blastocyst. It
is formed by the flat external layer of cells – trophoblast and the nodular blastomeric formation on its internal
side, called the embryoblast. In mammals the conversion of morula into blastula occurs very fast. The central
part of the embryo contains a large cavity filled with fluid, or the blastocele. The fluid is produced by
blastomeres. The trophoblastic cells flatten and therefore the permeability rate in this layer decreases, and the
free passage of fluid is hampered. During the early stages the blastocysts of mammals are covered by zona
pellucida.
GASTRULATION
In multicellular organisms the process of gastrulation occurs. It is the metamorphosis of the blastula, which is
formed by the single layer of cells (or the blastoderm) into the double-layered structure. In spinal animals the
three-laminar embryonic disc or gastrula is formed. It consists of the external germinal lamina, or ectoderm,
and the internal germinal lamina, or endoderm. In spinal animals the gastrulation process is followed by the
formation of the third germinal lamina, or mesoderm. The formation of germinal layers is the first stage of
embryonic cellular material differentiation. Later these germinal layers form the axial progenitors of organs.
The formation of certain axial germinal organs occurs along with the gastrulation process (example - chord).
Gastrulation is mediated by the inducing factors that influence the different areas of embryo and promote their
differentiated development. Proteins, nucleoproteins, steroids and other chemical compounds may act as the
inducing factors.
There are various types of gastrulation. Gastrulation depends on the type of blastula, which, in its turn, is
based on the type of oocyte.
1 Invagination of the blastoderma vegetal pole towards the animal pole. The embryo looks like the
cup with the two-layer wall. The inner layer forms the primary endoderm, while the outer gives
start to the primary ectoderm. The gastrula cavity, which is formed by the endodermal invagination
is called he gastrocele. The opening that joins the gastrocele with the external environment is
called the primary oral opening, or the blastopore. This kind of gastrulation is typical for
Echinoderma and the lower chordate (lancelet).
2 Epiboly is the growth of the animal pole of blastula over the vegetal pole. Therefore, the
vegetative part occupies internal position. The material, which resides on the germinal surface after
the gastrulation is ectoderm, while the invaginated material becomes endoderm. These two types
of gastrulation may be parallel. As a result of epiboly the cells that would undergo invagination
approach to the marginal part of blastopore and overlap over its labia and then take their places in
the internal structure of the embryo. In some animals the gastrulation may be combined by two or
more types – for examples, in Amphibian the gastrulation occurs as a combination of invagination
and epiboly.
3 Delamination - the cleavage of the single layer into external (ectoderm) and internal (endoderm)
germinal layers. This type of endoderm differentiation is typical for some invertebrates
(arthropoda) and higher vertebrates (birds, placentar mammals, humans).
4 Immigration – some cells of the blastoderm migrate to the blastocele and form a second germinal
lamina, or endoderm. The cells which stay in the former position turn into ectoderm. This type of
gastrulation has been discovered by I.I. Mechnikov at 1886. It is considered to be the most ancient
evolution type of gastrulation. It is found in lower organisms. However, the migration of cells is
found in some higher vertebrates (birds), placental mammals and in humans. In these cases it is a
component of the second stage of gastrulation and results in the formation of the intermediate
germinal layer, or mesoderm.
The development of lower vertebrates occurs in water. All these animals lay eggs in water, where the
embryonic and larval stages of development take place.
The higher vertebrates live on dry land and lay their eggs there. It influences all embryonic processes. The
differentiation and superior organization of animals requires the increase of yolk volume that is provided
during the formation of the oocyte in the ovaries. This fact also impacts the processes of cleavage and
gastrulation. The development in the air environment requires special shielding formations that would protect
the embryo from parching, mechanical damage, etc (egg and embryonic shells). The formation of hard and
dense shells on the egg surface in reptiles and birds makes the external fertilization impossible. Therefore, the
sperm cells have to penetrate the oocyte when these shells are absent yet. Internal insemination and
fertilization is the only option.
BASICS OF COMPARATIVE EMBRYOLOGY
DEVELOPMENT OF LANCELET
Oocyte type – primary isolecytal. Zygote’s cleavage type – holoblastic, equal, synchronic.
Celoblastule is formed. Its external part consists of theca (upper part), fundus (lower part) and marginal zone
(which lie between the theca and the fundus).
Gastrulation type – invagination and formation of a two-layered goblet-like gastrula. Its gastrocele is
connected to the environment with the wide blastopore, which forms the caudal side of the embryo in future.
The invaginated cells form the internal germinal layer or endoderm. The external wall of gastrula is formed
by theca cells or the external germinal lamina, the ectoderm. The formation of ectoderm and endoderm occurs
at early stages of gastrulation. The third germinal lamina or the mesoderm is formed by the invagination of
the epidermis forming two “pockets” that protrude between the two other germinal layers. Later these three
germinal layers form the axial organs – the chord, the neural and the intestinal tubes.
Later the mesodermal bladders (or wings) are divided into the dorsal (myotome) and ventral (splanchnotome)
parts. Along the whole body the dorsal areas of mesoderm are segmented into somites (10-11 pairs. Humans
have 44 pairs of somites). Caudal part is the exception. Splanchnotome is not segmented. It consists of two
layers that separate the secondary body cavity or celome. The parietal layer lies close to ectoderm, while the
visceral layer lies close to endoderm. The margin between the dorsal and ventral parts is occupied by cell
formation called the nephrotome. It is segmented as well as the myotome – the segmentation is found in its
cranial and body parts (segmental pediculi), while it caudal part is not segmented (nephrogenic strands).
The germinal layers and the axial organs are connected by the mobile branched cells that migrate from
various areas of mesoderm (mostly from somites and splanchnotome). These cells form the mesenchyma.
After mesenchyma is formed, it spreads between the germinal organs and forms the internal media that
provides metabolic processes between various organs.
All organs of the lancelet body are formed by germinal laminae (ectoderm, endoderm and mesoderm), the
axial organs (neural and intestinal tubes) and mesenchyma.
DEVELOPMENT OF AMPHIBIAN
Oocyte type – thelolecytal with poor polarity rate. Cleavage type – holoblastic, unequal, non-synchronic.
Morula consists of large blastomeres. Blastula consists of multi-layered blastoderm. The blastocele is located
close to animal pole, where smaller blastomeres are located (amphiblastula). Gastrulation starts with
invagination which results in the formation of the elongated structure – the falciform groove – at the margin
between the animal and vegetal poles. Later gastrulation is continued by the growth of blastomeres on the
animal pole of the vegetative part. This kind of gastrulation is called epiboly. Finally, the vegetative part
occupies the internal area and fills half of the internal space of the spheroid gastrula. The gastrula walls
contain all three germinal layers. The gastrulation in amphibian is characterized by the simultaneous
formation and rapid differentiation of the germinal layers.
DEVELOPMENT OF FISHES
Oocyte type – thelolecytal, with significant polarity rate. Cleavage type – meroblaastic, unequal, non-
synchronic. Cleavage results in the formation of the flat germinal disc, not the spheroid morula. Later the
germinal disc is converted into the discoblastula. First phase of gastrulation – delamination or invagination
(depending on the species), second phase of gastrulation is immigration.
The development of fishes is different from the development patterns found in lancelet and Amphibian – in
fishes the extra-embryonic organ is formed. It is called the yolk sack. Its formation is started at the early
gastrula stage, when the inner layer consists of the germinal (intestinal) endoderm and the extra-germinal
(yolk) endoderm, that is located at the periphery of the germinal disc. The development of the yolk sack is
provided by the gradual growth of the germinal disc margin over the yolk (the extra-germinal parts of
endoderm and the visceral mesoderm layer grow over the yolk and therefore form the yolk sack). In the
peripheral areas ectoderm and the parietal layer of mesoderm join these layers. The nutrition of embryo is
provided by the blood vessels that are formed in the walls of the yolk sack. They run through the yolk stalk,
which connects the embryo to the yolk sack. In fishes the yolk sack has trophic and hemopoetic functions. In
higher vertebrates its functions shift because of the environmental changes.
DEVELOPMENT OF BIRDS
Oocyte type – thelolecytal, with significant polarity rate. It consists of several shells (yolk, white, two sub-
shell layers with an air chamber located at one of the poles between them, and the eggshell itself). The
formation of these shells is explained by the development of birds in atmospheric environment, not in water.
Cleavage type – meroblastic, unequal, non-synchronic. The cleavage results in the formation of the germinal
disc, which is later converted into discoblastula. Delamination occurs at the first stage of gastrulation. At this
stage the single layer of cells which forms the discoblastula is divided in two layers. The upper one is called
epiblast, and the lower one – hypoblast. The hypoblast is formed by the flat cells. It covers the yolk and
doesn’t form any embryonic tissues. The epiblast consists of cylinder-shaped cells. Its central part (the
germinal shield) forms all three germinal layers – ectoderm, mesoderm and endoderm. The immigration of
the germinal epiblast cells and the formation of mesoderm and axial organs occur in the same way as it goes
in mammals and humans.
In birds the formation of yolk sack is followed by the formation of other extra-embryonic organs – amnion,
serosa and allantois.
Amnion (or water membrane) and serosa are formed by the protrusion of ectoderm and parietal layer of
mesoderm (splanchnotome). The amniotic folds are formed in this way. These folds spread over the embryo
(on top and from behind) and join with each other thus forming the amniotic membrane. Later it is filled with
liquid. This liquid is produced by the amniotic membrane ectodrmal cells. It contains proteins and
carbohydrates. At the same time the external parts of the amniotic fold grow over the whole embryo and the
yolk sack. Therefore, the same germinal layers grow over the internal surface of eggshell. They form another
extra-embryonic organ – the serosa. It participates in the process of gas exchange and provides the embryo
with oxygen (it is the provisory respiratory organ). The functions of amnion are trophic and protective. It
surrounds the embryo with liquid and protects it from any harmful impact.
Allantois is developed as the protrusion from the ventral side of the posterior intestine which is formed by
endoderm and the visceral layer of mesoderm. The shape of allantois is elongated. It grows and enters the
fissure between the amnion and the serosa. Then it grows in the direction of the egg’s air chamber. In birds
the allantois participates in the process of gas exchange. It is the reservoir for waste products, which are
formed during the embryogenesis. The blood vessels of allantoic mesoderm provide oxygen into the allantois
and remove the metabolic products from the embryo.
DIFFERENTIATION OF GERMINAL LAYERS
Differentiation is the part of gastrulation. During the differentiation the cells become more specific in
biochemical and morphologic respect and therefore become different one from another. Therefore the
possibility range of further development decreases. At the same time, differentiation occurs at all stages of
embryogenesis (starting from the stage of zygote) and continues in the adult organism.
ECTODERM ENDODERM
NEURO- PLACODES SKIN PRECHORDIAL INTESTINAL
ECTODERM ECTODERM PLATE ENDODERM
Neural Neural Epithelium of Epidermis, Epithelium Epithelium of Epithelium of
tube crest internal ear sweat and of vagina esophagus, stomach,
sebaceous and anus respiratory intestine,
glands, hairs, system, liver,
epithelium of thymus, pancreas
cornea and thyroid
oral cavity, and para-
teeth enamel thyroid
and cuticle glands
Spinal cord, Ganglionic plates
brain, retina, (peripheral neural
olifactory ganglion).
Organ APUD cells,
Medulla of
suprarenal gland
MESODERM
NOTOCHORD 44 SOMITES NEPHROGONOTOMES PARAMESO- SPLANCHNOTOMES
NEPHRIC
Myotomes Dermatomes Sclerotomes CHANNEL Visceral Parietal
Sceletal Mesenchyma layer layer
muscles
epithelium
of urinary
s ystem,
Intervertebral gonads epithelium Mesothelium
discs nuclei and deferens (primary):
Skin derma, duct vagina,
bones and uterus,
cartilages, uterine tubes
connective Myocardium,
tissue, blood epicardium,
vesels, suprarenal
blood cells, glands cortex
myocytes,
microglia
EMRYONIC DEVELOPMENT OF PLACENTAR MAMMALS AND HUMANS
The embryogenesis of mammals and humans differs significantly from the embryo genetic patterns of other vertebrates.
That is explained by the development of embryo inside the maternal organism. The initial stages of embryonic
development in mammals and humans have the same pattern.
Oocyte type: secondary isolecytal. Fertilization type: internal; it occurs at the proximal part of the uterine tube. The cleavage
of zygote begins there too. Cleavage type: holoblastic, unequal, non-synchronic. The first cleavage session results in the
formation of two blastomeres with different sizes. One of them is light and small, another is dark and large. During the first
2-3 days the cleavage speed is slow. The embryo moves along the uterine tube due to the flow of fluid that is produced by
the contractions of uterine tubes muscles and the movements of its epithelium cilia. Since the 3-rd day the cleavage speed
increases. At the 4-th day the embryo consists of 7-12 blastomeres. During the following cleavage sessions the number of
light blastomeres increases rapidly. They surround the dark blastomeres with a solid layer, so that they become covered by
the dense formation. The layer of light blastomeres that surrounds the dark ones is called trophoblast. It has a trophic
function and absorbs the serous fluid from the lumen of the uterine tube. Later it absorbs the fluid from the uterine cavity.
Trophoblast provides the dark blastomeric formation, which is called embryoblast, with nutrients. At this stage the embryo
is called stereoblastula or morula. At the 3 - 4-th day the formation of the blastocyst begins. Blastocyst is the hollow vesicle
filled with fluid. Due to the accumulation of liquid under the trophoblastic layer, the emryoblast moves away and takes
position close to the pole. It forms the germinal knot, fixed to the trophoblast from within. The implantation process starts.
The germinal knot flattens and turns into the germinal plate, which prepares to the first stage of gastrulation.
The development of mammals and humans has the same pattern up to the stage of blastocyst. Certain differences become
evident later.
Implantation is the introduction of the embryo into the mucosa of the uterine cavity. There are two stages of implantation:
1 Adhesion or sticking (the embryo sticks to the endometrium)
2 Invasion or settling down (the embryo invades the uterus mucosa)
After the adhesion the trophoblastic cells start a rapid proliferation (division).
At the 5 – 6-th day the blastocyst stays at the uterine cavity and is not fixed to it walls. It is called the free blastocyst,
surrounded by the fertilization membrane.
The cavity of human blastocyst is filled by serous fluid. The extra-embryonic mesoderm cells (mesenchyma) are separated
from the embryoblast. They become branched and cover the inner surface of the trophoblast surrounding the embryoblast.
By the 11-th day the extra-embryonic mesoderm fills the cavity of blastocyst. The implantation process starts at the 7-th day
of the embryonic development (invasion of the embryo into the uterine mucosa occurs). The trophoblast and the
embryoblast change. These changes are related to the preparation to implantation. Trophoblast is the major component of
this process. In the trophoblastic cells the increase of lyzosomes number occurs. Their enzymes are required to digest the
uterine tissues, so that the embryo could reach the lower layers of uterine mucosa. As the embryo settles down into the
endometrium, it forms the implantation pit and sinks into the lower layers of mucosa. By that time the trophoblast becomes
two-layered due to the division of cells. Its inner layer consists of cuboid cells and is called cytotrophoblast, while the
external layer loses its cellular structure and becomes the single layer of cytoplasm with the dividing nuclei within. It is
called the chorionic syncytium or the symplasto-trophoblast. It produces the proteolytic enzymes. It forms the cilia (that
would become chorionic cilia later) which destroy the fertilization membrane gradually. After the complete invagination of
the embryo the endometrial pit is filled by the coagulated blood. Later it is filled by the connective tissue. By the 11-13-th
day it is covered by epithelium. Lacunae, filled with maternal blood are forming around the embryo. The connective tissue
cells develop the decidual reaction: the blood vessels grow, edema occurs, the cell’s volume increases and they accumulate
glycogens and lipids.
During the first 2 weeks the trophoblast consumes the products of maternal tissues destruction. It is the hystiotrophic type of
nutrition. Later the nutrition of the embryo is provided by the maternal blood. This is called the hematotrophic type of
nutrition. The embryo’s metabolism waste is excreted into the maternal blood. Later the formation of the hemochoroidal
type of placenta occurs. It provides the improved level of embryo’s metabolism.
Along with the implantation and chorion formation, the material of the germinal plate undergoes the process of gastrulation.
In humans the gastrulation consists of two phases.
The first phase of gastrulation occurs at the 7-th day. It is the delamination which goes along with implantation. The group
of embryoblastic cells flattens (resembling the germinal disc of birds) and the embryoblast divides into the hypoblast (the
lower, or the internal layer) and the epiblast (the upper, or the external layer). The hypoblast is formed by small light cubical
cells, while the epiblast consists of the cylinder-shaped cells that look like epithelium. The nuclei of these cells are located at
different levels (pseudo-multinuclear structure).
The hypoblast participates in formation of the extra-embryonic organ – the yolk sack. It grows on the internal surface of the
trophoblast and forms the extra-embryonic endoderm.
In higher animals and in humans the small cavities are formed between the epiblastic cells. These cavities join and form the
single amniotic cavity. After the movement of the cells that form the cover of this cavity the epiblast undergoes one more
division and forms two layers – the embryonic epiblast that forms the fundus of the amniotic cavity and the amniotic (extra-
embryonic) ectoderm, which forms the cover of amniotic cavity. The amniotic ectoderm takes part in the formation of
amnion (another extra-embryonic organ). In other animals the formation of amnion occurs in a different way.
Embryonic epiblast is the major progenitor of the embryonic tissues formation. At the second phase of gastrulation it
becomes the source of all three germinal laminae.
At the period between the gastrulation phases the extra-embryonic organs are formed. They are required to provide the
successful development of the embryo.
Therefore, the first stage of gastrulation results in the separation of the germinal layers source (the embryonic epiblast). It is
the beginning of the extra-embryonic organs formation process.
The second phase of gastrulation is the immigration of cells. It begins at the 14-th day and continues up to the 17-th day. It
results in the formation of mesoderm and the axial organs complex.
The migration of cells in the epiblastic region results in the formation of the following structures:
1 Primitive streak – the thickened epiblast with a groove in the medial part
2 Primitive knot (Henzen’s tuberculum) – the protrusion located in the anterior part of the primitive streak with a
pit (primitive pit) in its center.
These are the places of active cellular migration that goes from the external surface of the embryo to its internal part. One
more thickened formation appears in the anterior part of epiblast. It is called the prechordal plate. Later the epiblast cells
start to migrate inside through the primitive streak. Some cells penetrate it, move the hypoblast cells aside and form the
germinal endoderm. Other cells spread as the friable layer between the epiblast and the hypoblast (actually, the endoderm).
They give start to the chord and the germinal mesoderm by forming of the mesodermal wings along the sides of chord. The
cells that stay on the surface of epiblast form the germinal ectoderm. The hypoblast resides and forms the extra-germinal
endoderm, which participates in the development process of the yolk sack.
By the 15-17-th day the structure of the embryo is as follows: the trophoblast and the extra-germinal mesoderm form the
chorion. The germinal area holds yolk vesicle and amniotic vesicle. This part of embryo is fixed to the chorion by the
amniotic stalk, which is formed by the dense strand of the extra-germinal mesoderm cells. The amniotic stalk is the
progenitor of the umbilical chord.
Since the 20-th day the mesoderm from each side differentiates and forms 3 parts: dorsal mesoderm, ventral mesoderm
(splanchnotome) and intermediate mesoderm (nephrotome), Later the migration of cells occurs. They move mostly from
somites. These cells become star-shaped and fill the space between the germinal layers. They form the extra-germinal
mesoderm or mesenchyme. Some authors call it the fourth germinal layer.
The embryo’s body starts the individual formation since the 20-th day. The germinal plate rises over the yolk sack and turns
inward, separating itself from the yolk sack by the trunk fold. During this process the germinal endoderm forms an intestinal
tube by fusion of its edges. However, the middle part of the intestinal tube is still connected to the yolk sack. Except for the
lateral flexion the longitudinal flexion occurs so that the cranial and caudal parts of the embryo bend towards the yolk sack.
As a result the prechordal plate moves from the dorsal surface to the ventral one. At this very place the oral opening of the
embryo is formed after the plate breaks.
Finally, the embryo is connected to the yolk sack by the stem which is the part of the amniotic stalk, and later – of the
umbilical chord. Since this time the amniotic membrane surrounds the embryo (except for the place where the amniotic
stalk is fixed).
EXTRA-EMBRYONIC ORGANS IN PLACENTAL MAMMALS AND HUMANS
The extra-embryonic organs which develop during the embryogenesis process outside the embryo’s body have multiple
functions which provide growth and development of the embryo. The extra-embryonic organs are amnion, yolk sack,
allantois, chorion and placenta.
Amnion is a temporary organ which provides the water media for the embryo’s development. It evolutional development
was promoted by the migration of vertebrates to the dry land. The inner surface of the amniotic vesicle is covered by the
extra-embryonic (amniotic) ectoderm. Its external surface is covered by the extra-embryonic mesenchyma. They form the
connective tissue component of amnion. By the end of the 7-th day of embryo’s development the amnion’s connective
tissue contacts with the connective tissue of chorion. The epithelium of amnion grows over the amniotic stalk that is turns
into the umbilical chord.
At the early stages the amnion is covered by the simple squamous epithelium with an active rate of cleavage. It turns into
the prismatic epithelium gradually. The microvilla are formed on the surface of the epithelial cells. The cytoplasm of these
cells contains the small drops of lipids, glycogen granules and glycosaminoglycans.
The main function of amnion is the production of amniotic fluid that provides the medium suitable for embryo’s
development and protects t from mechanical damage. The epithelium of amnion reabsorbs the amniotic fluid.
Yolk sack has appeared as an organ of nutrients storage (yolk). These nutrients were required to provide the development of
embryo. Om humans the yolk sack is formed by the extra-embryonic endoderm and the extra-embryonic mesoderm
(mesenchyme). In birds the yolk sack is filled with yolk. On the contrary, in humans and other mammals it contains serous
fluid and, in fact, has a very small amount of yolk. It is formed at the 2-nd week and wouldn’t participate in the process of
embryo’s nutrition, because at the 3-rd week the contact of the embryo with the maternal organism establishes and the
hematotrophic nutrition begins. Blood insulae and primary blood vessels appear in the yolk sack’s wall. They provide the
conduction of oxygen and nutrients to the embryo. The yolk sack hemopoetic function is maintained up to 7-8 th week.
Then the yolk sack undergoes regression and occupies the umbilical chord as the thin tube, which serves as the conductor of
the blood vessels to the placenta.
Allantois is a small finger-like protrusion located in the caudal part of the intestinal endoderm and the visceral layer of
mesoderm. It is the derivate of the yolk sack and consists of extra-embryonic endoderm and visceral mesoderm. The
embryo is located between the amniotic and yolk vesicles. It is connected to chorion by the amniotic stalk, which allantois is
growing into. The proximal part of allantois is located along the yolk stalk, while its distal part grows into the space between
amnion and chorion. In humans the allantois is not significantly developed. However, it provides the nutrition and
respiration of the embryo, because the blood vessels are growing towards the chorion along it. It is the organ of gas
exchange and excretion. At the 20nd month of embryogenesis the allantois undergoes reduction and becomes a part of the
umbilical chord together the reduced yolk sack.
Umbilical chord connects the embryo and the placenta. It is the elastic stalk covered by the amniotic membrane. The basis
of the umbilical chord consists of the mucous connective tissue with fibers – it is also called the Warton’s jelly. It is formed
by the extra-embryonic mesoderm of the amniotic stem with two umbilical arteries and one umbilical vein along with the
rudiments of yolk sack and allantois.
The mucous connective tissue provides the elasticity of the umbilical chord and protects the umbilical blood vessels from
compression. Therefore, it provides the continuous flow of nutrients and oxygen to the embryo. It also protects the embryo
by blocking the invasion of extra-vascular harmful agents from placenta into embryo.
New provisory organs provide the connection of embryo and the maternal organism in mammals and humans. They are
chorion and placenta. Chorion is formed by trophoblast and the extra-embryonic mesoderm. By the middle of the 2-nd
week the embryo is staying deep in the endometrium and its trophoblast forms the primary villi – the branched protrusions
of symplasto-trophoblast and cyto-trophoblast. After the embryo implants into the uterus wall, they provide the connection
with the maternal organism. Since this time trophoblast is called chorion. At the beginning of the 3-rd week the extra-
embryonic mesenchyma enters the villi of the chorion. It grows over all surfaces, including the inner surface of trophoblast,
or chorion. Therefore, the connective tissue stroma of villi is formed, and they become the secondary epithelio-
mesenchymal villi of chorion.
By the end of the 3-rd week the mesenchymal cells located in the chorionic villi form the blood vessels. Their structure is
similar to the one of blood capillaries. They contact with the blood vessels of chorion’s mesodermal plate. The villi
containing the well-developed blood vessels are celled tertiary. They form the structural and functional unit of placenta, that
is called cotyledone. The chorion’s villi are sunk into the lacunae in the endometrium. They are surrounded by the blood
coming from the damaged blood vessels of the endometrium. Therefore, the utero-placentar circulation is formed and the
hematotrophic type of embryo’s nutrition is set. The chorion’s villi grow bigger and become branched. The chorion’s
surface turned to the endometrium (maternal blood is coming from that side constantly) is covered by the branched villi that
grow intensively and later form the so-called branched chorion. The surface of the chorion facing the uterus cavity is
covered by villi that undergo gradual reduction and form the smooth chorion. The branched chorion and the neighbouring
areas of endoometrium form the new extra-embryonic organ – the placenta, which provides nutrients and oxygen to the
developing embryo. The terminals of the villi form the anchors that are fixed to the endometrium. Therefore, these areas of
villi are not covered by the symplasto-trophoblast. They are covered by the cytotrophoblast only, that provides tight
fixation.
The different kinds of animals have various types of placenta. The types of placenta structures are as follows:
1 Epithelio-chorionic placenta – the chorionic villi grow into the uterus glands and contacts to their epithelium
(horses, pigs, dolphins, whales). That contact provides the chorion with proteins from maternal tissues and the
fission of these proteins up to the level of amino-acids.
2 Desmo-chorionic placenta – its chorion partially destroys the uterus’ glands epithelium and grows into the
underlying connective tissue (cows, sheep). These types of placenta provide the development of the embryo up
to the level, when independent feeding and mobility become possible right after birth.
3 Endothelio-chorionic placenta – the chorionic villi destroy the epithelium and the connective tissue of uterus.
The contact the vascular endothelium (predators: feline, canine, walruses and seals).
4 Hemo-chorinonic placenta – the wall of uterus’s blood vessels is destroyed, and the chorionic villi contact the
maternal blood directly (humans, primates). These two types of placentas provide the embryo with maternal
proteins, which are required for the development of tissues.
Functions of placents: trophic, respiratory, excretory, endocrine, protective, etc. Amino-acids, glucose, lipids, electrolytes,
vitamins, hormones, oxygen, drugs and viruses cross the placenta and enter the embryo’s circulation. . The embryo excretes
carbon dioxide and metabolic waste into maternal circulation.
As a result of complicated development process all provisory organs get developed by the fifth week. They provide all the
conditions required for the development of organism. The processes of organo- and histogenesis start.
SPECIAL HISTOLOGY
NERVOUS SYSTEM AND SENSORY ORGANS EMBRYOGENESIS
Nervous tissue is being developed from ectoderm. The inducing activity of the notochorde, which is growing
forward, promotes the formation of the thickened area in the medium part of th ectoderm. It grows forward
from the primary node and is called the neural plate. At the 18-th day the neural plate starts to bend, so that
the neural groove and neural cylinders are formed. Later, the 4-th week the sides of the neural groove move
closer to each other gradually and join together, thus separating from the skin ectoderm. However, the
junction of sides along the groove is not simultaneous – first it occurs at the neck area, then – at the caudal
area and after all, the merging at the cranial part occurs. After all, the unpaired neural tube is formed. The
formation of the neural tube is accompanied by the formation of neural folds, or the thickened areas that are
located between the neural groove and the skin ectoderm. As the neural tube separates from the skin
ectoderm, the cell clusters of the neural folds get isolated and form a solid layer between the ectoderm and the
neural tube. It is called the neural plate or the neural crests. Some cells of the ganglious plate migrate into
various directions thus forming the neural gangles. The material of the neural plate itself gets segmented and
forms the spinal nodes.
Therefore, the subdivision of nervous system into three parts occurs at the early stages of embryogenesis.
These parts are head, medial and caudal regions. The fastest development rate is found at the medial region,
where the spine is formed. Later, the head region develops into brain. The development of the caudal region
into the neural tube occurs later, then the development of the upper areas. Its development is hampered early –
actually it is the source of the caudal areas of the spine, which are characterized by low grae of differentiation.
DEVELOPMENT OF SPINAL CORD
The predecessor of the spinal cord is the tube which consists of cylinder cell. These cells undergo intensive
mitotic cleavage. As a result, the walls of the neural tube thicken and become multilayered. At the transversal
cut of the neural tube 4 layers can be found: dorsal (or the cover plate), ventral (or the bottom plate) and two
lateral plates. The thick lateral sides of the neural tube are divided by the longitudinal stripe into the dorsal
plate and ventral (or basal) plate.
Later the cells of the neural tube enter two lines of differentiation – some of them become spongioblasts,
which are the source of neuroglia; the other become neuroblasts – later they develop into neurons.
At this stage the neural tube consists of three layers:
1 The inner, or ependymal layer, that covers the neural tube channel from inside
2 The medium mantle layer which contains neuroblasts and the neurons, that are developed from neuroblasts.
It also contains the primitive neuroglial matrix, which is the progenitor area for neuroglial cells (astrocytes
and oligodendrocytes)
3 External layer, or the marginal mantle. It contains no neuroblasts but is formed by the branches of the of the
cells located in the ependymic and mantle layers. These branches are later developed into the neural pathways
of the spinal cord, or the white substance. Later the cells of the ependymic layer turn into the cylinder-shaped
ependymic cells, or glial cells that cover the channel of the spinal cord from inside. The mantle layer develops
into the grey substance with a groups of fast-growing cells. These are the progenitors of the motoric nuclei.
Their neuritis gro from the spinal cord to the periphery thus forming the radices of the spinal nerves.
At the same time the walls of the neural tube grow irregularly. The cover plate and the bottom plate grow
slower. Their width is insignificant until the very end of the development. As the spinal cord develops, the
regions of dorsal and ventral plates bend, while its right and left regions move closer to each other. Therefore,
the glial and connective tissue septa is formed in its posterior part, while the narrow fissure develops in the
anterior area.
The neurites of the sensory cells, located in the spinal ganglia, enter the posterior horns of the spinal cord.
These neurites form the posterior radices of the spinal cord.
DEVELOPMENT OF SPINAL AND VEGETATIVE GANGLIA
The ganglious plate is the source of development for spinal and vegetative ganglia.
At the first stage the ganglious plate consists of the solid layer of cells, located between ectoderm and neural
tube. Later the segmentation of the neural tube occurs along with the segmentation of the embryo. Each
segment of the ganglious plate is divided in two equal parts. These halves of each segment are later developed
into spinal cord ganglia. Due to the intensive development of the neural tube, the progenitors of the spinal
ganglia move from the dorsal area of the neural tube to its lateral parts, therefore, they occupy the sideway
position. The cells of theganglia progenitors are differentiated in two directions: some of them become
neuroblasts, while the other turn into spongioblasts. Later the spongioblasts are developed into the ganglionar
oligodendroglia (mantle cells). The neuroblasts are differentiated into the bipolar neurons.
The shape bipolar neurons of the superior chordate animal and of humans changes during the development
process. Their branches move closer to each other, and their basal parts join each other. Therefore, the body
of such neuron produces a T – shaped branch, which is divided into dendrite and neurite. This is how the
pseudo-unipolar cells are differentiated.
Later the cells of oligodendroglia form a capsule around the neurons and the sheaths of the neural fibers.
The source of the vegetative neural ganglia are the cellular elements of the ganglious plate, which move to the
periphery, where the future neural ganglia would be located. They differentiate in two directions – some of
them become neuroblasts, other turn into spongioblasts. Later the neuroblasts form the multi-polar neurons,
and the spongioblasts turn into glial cells.
Except for the ganglia cells, chromaffinoblasts migrate to the periphery from the ganglious plate. Later these
cells form the tissue of the adrenal glands.
FORMATION OF THE PERIPHERAL NERVES
The peripheral branches of the pseudo-unipolar cells grow close to the anterior radices of the spinal cord and
form peripheral nerves together with them. The neurites of the certain multipolar neurites that belong to the
vegetative ganglia join the peripheral nerves that go to various organs.
DEVELOPMENT OF BRAIN
The width of the neural plate in the capital part of the embryo exceeds the width of its medial and
caudal parts.
