Embryology
Embryology – the study of the development of an organism
from the time an egg is fertilized until the time when all major organ
systems are formed
A. Historical Aspect
1. Preformation – this was a concept held during the 17th and
early 18th century which stated that sex cells (gametes)
contained a completely formed, but greatly miniaturized
human inside them. This miniaturized person was termed a
homunculus
2. By the mid-18th century, another view began to become
popular known as epigenesis. This concept stated that the egg
contained material from which the embryo was built and
directed the formation of the individual by some unknown
“creative principal”.
3. Roux’s experiment (using amphibian eggs) 1888
4. Driesch’s experiment (using echinoderm eggs) 1892
V. Steps in Embryonic Development
A. Definitions:
1. Gamete – sex cell
2. Sperm – male gamete
a) motile, flagellated
b) usually much smaller than the female
gamete
c) haploid (contains only one set of
chromosomes for that species) 1n
3. Egg or ovum – female gamete
a) non-motile
b) up to 1,000 times larger than a sperm cell
c) haploid {1n also}
d) within the egg are most of the enzymes, proteins, and
other information needed for development for the new
embryo
4. Fertilization – union of sperm and egg (fertilization is also called
syngamy)
a) the result of fertilization is a diploid zygote
b) diploid – having two complete sets of chromosomes
resulting from syngamy
c) zygote – the cell produced from the union of egg and
sperm that will divide mitotically, forming a new
organism
B. First Step: Fertilization (syngamy)
1. Sperm must penetrate the gel coat surrounding
the egg
2. Enzymes called lysins, located in an
organelle in the head of the sperm called the
acrosome, dissolves the gel coat membrane
around the egg
C. Second Step : Egg Activation
1. Egg activation begins when the acrosomal membrane of
one sperm fuses with the egg membrane
2. Early changes to the egg ensures that only one sperm will
fertilize that egg:
a) the first sperm that unites with the egg membrane is
surrounded by the microvilli located on the membrane,
which wraps around the penetrating sperm
b) the nucleus of this one sperm is drawn into the center
of the egg (contains the DNA)
c) within milliseconds of the initial penetration of this
sperm, ionic changes occur in the egg membrane
making the membrane unresponsive to any other sperm
d) a protective barrier called the fertilization membrane
prevents any other sperm from entering
e. In certain species, especially amphibian eggs, a structure called
the gray crescent forms on the side opposite of the site where the
sperm initially penetrated
D. Third Step : Metabolic and Nuclear Events
1. Very little mRNA synthesis occurs within the early zygote.
Almost all of the mRNA directing early development and
division was stored within the egg itself. This original
mRNA is called maternal mRNA since it was formed by the
mother’s cells. (This is the main reason an egg cell is so
much larger than a sperm cell)
2. The maternal mRNA directs protein synthesis early on in
embryonic development
3. The DNA only has to be concerned about replicating, not
transcribing, and rapid replication occurs (mitotic divisions)
4. Egg regions:
a) animal pole – area of the egg that contains less yolk,
more mitochondria, more ribosomes and is more
metabolically active
b. vegetal pole – contains more yolk (food) and is less
metabolically active
E. Fourth Step : Cleavage
and Egg Types
1. Definitions:
a) cleavage – cell
divisions (the
result of
cytokinesis)
b) blastomeres – the
cells resulting
from cell divisions
c) Yolk – lipids, glycogen, proteins, carbohydrates and other
organic compounds used for food
d) Synchrony – term used for simultaneous division of cells
2. Cleavage patterns
a) Holoblastic cleavage – division of a cell that divides it into
two equal daughter cells, uniformly across the whole ball of
cells
b) Meroblastic cleavage – egg cells with a lot of yolk divide
only at one end (not equal)
i) examples include bird eggs and reptilian eggs
c) spiral cleavage – cleavage patterns occur at obtuse angles to
one another
i) this type is typical to lower invertebrates (determinant
development)
d) radial cleavage – cleavage patterns oriented directly over
other cells at right angles to each other
i) typical of highest invertebrates and chordates
e) Determinant development – zygotes, even by the four cell
stages, have become specialized (lower invertebrates)
f) Indeterminant development – cells become specialized
much later in the gastrula stage
i) unspecialized cells are called totipotential cells or
undifferentiated cells
ii) undifferentiated cells are also called stem cells
3. Embryo development as a result of cleavages
a) The one cell (zygote) divides by mitosis to become
two cells, two cells divide to become four, four to
eight, and so forth
b) Soon a hollow fluid filled ball of cells forms called the
blastula (the cavity within is called the blastocoel)
c) Some cells on the blastula begin to grow inward
(invaginate) which forms a double walled structure
called the gastrula (process is called gastrulation)
d) The opening created by this invagination of cells is
called the blastopore and the new cavity is called the
archenteron
e) The outer layer of cells will become the ectoderm and
the inner layer of cells will become the endoderm
VI. Primary Germ Layers – beginnings of differentiation formed
during gastrulation
A. There are three primary germ layers:
1.Ectoderm (outer tissue)- origins of the
following tissues:
a) nervous tissue including brain
b) epidermis, including skin, nails, hair, fur, teeth,
etc.
c) sensory organs associated with the skin
2. Endoderm (inner tissue) – origins of the following
tissues:
a) gut tract lining
b) respiratory tract lining
c) digestive glands including pancreas and liver
3. Mesoderm (middle tissue) -origins of the following
tissues:
a) connective tissues (bones, cartilage, blood,
tendons, ligaments)
b) muscle tissue (smooth, striated, and cardiac)
c) notochord in chordates
d) dermis (inner layer of skin)
e) outer covering of internal organs
f) excretory organs
g) reproductive organs
Patterns of Development
• Protostomes • Deuterostomes
– Blastopore =Mouth – Blastopore =Anus
– Spiral-Determinate – Radial-
Cleavage Indeterminate
– Ectodermal Cleavage
Skeleton – Mesodermal
Skeleton
Tissue Organization
• Acellular
– Single-celled, No tissues
• Diploblastic
– Body parts organized into layers that are derived
embryologically from two tissue layers, Ectoderm and
endoderm
• Triploblastic
– Body parts organized into layers that are derived
embryologically from three tissue layers, Ectoderm,
mesoderm, endoderm
VII. Vertebrate Embryology
A. Chordate characteristics – during some stage of the
development or life of a chordate, it has:
1. a hollow, dorsal nerve cord
2. a stiff, but flexible, rod-like structure used as
support in their back called a notochord
3. post-anal tail
4. pharyngeal gills slits
B. Formation of a hollow tube (the neural tube) in the back
of vertebrates occurs after gastrulation…this process is called
neurulation. It occurs by the invagination of the ectoderm
C. Avian and Reptilian Embrylogy
1. Both of these groups undergo meroblastic development
2. The egg in these groups is the “true egg” plus several
protective membranes:
a) vitelline membrane – surrounds and protects the
true egg
b) albumin – “egg white” – nutritional and consumed
along with yolk during development
c) shell – composed of calcium carbonate and is porous for
the exchange of gases
d) yolk – food reserve
e) amniotic sac – encloses developing embryo in a fluid for
protection
f) chorion sac – found near the shell and aids in gas
exchange
g) allantois – “waste sac”, helps to accumulate uric acid
wastes and break those toxic compounds into non-harmful
products