Nem’s Notes… Phase 1
HUMAN LIFE CYCLE 2 (page 1 of 2)
Fertilisation & Implantation
Fertilisation This usually occurs in the Ampulla of the Uterine Tube within a hours of ovulation. It
involves the fusion of gametes producing a zygote. Sex is determined by the sperm. A
sperm with an X chromosome will produce a female and a Y a male. Meiosis of the
zygote allows further recombination and crossing over of genetic material.
Week 1 At about 30 hours after fertilisation (after meiosis II), the zygote cleaves by mitosis into
smaller and smaller cells called blastomeres. First it divides into two blastomeres,
then four, then eight and so on. After the nine cell stage compaction occurs whereby
the cells form a tight ball which allows greater cell-cell communication.
At about 3 days after fertilisation, when there are 12-15 blastomeres, the developing
human is called a morula which has an inner cell mass and an outer cell layer. The
morula enters the uterus at this stage.
At about 4 days, when the morula enters the uterus, a fluid-filled space called the
blastocyst cavity appears in the morula which divides the inner cell mass from the
outer cells which are now called the trophoblast. The conceptus is now called a
blastocyst.
At about 6 days the blastocyst attaches to the endometrial endothelium usually
adjacent to the inner cell mass. Finger-like processes called the syntrophoblast
extend into the endometrium and connective tissue below releasing invasive
proteolytic enzymes that allow the blastocyst to burrow into the endometrium.
Superficial implantation has occurred.
Week 2 From day 6 to day 14 the syntrophoblast continues implantation and starts to produce
human chorionic gonadotrophin (hCG) which maintains the action of the ovarian
corpus luteum and is the basis of the pregnancy test (which may be performed after
the second week).
From day 8, a cavity appears in the inner cell mass which will become the amniotic
cavity. At the same time a bilaminar embryonic disc forms which divides the
amniotic cavity from the yolk sac. Surrounding the two cavities is the
extraembryonic mesoderm.
At day 9, Lacunae (holes) appear in the syntrophoblast which fill with maternal blood
and provide nutrition by diffusion of nutrients.
At day 10 the conceptus is completely implanted in the endometrium and lacunar
networks form which will give rise to the placenta.
At day 12 small spaces within the extraembryonic mesoderm fuse to make one large
cavity surrounding the yolk sac and amniotic cavity. This is called the
extraembryonic coelom. The yolk sac decreases in size because of this.
At day 13 chorionic villi start to appear which arise from the trophoblastic cells.
At day 14 some hypoblastic cells on the underside of the embryonic disc take on a
columnar shape and become the prechordal plate which will organise head
development and shows the position of the mouth.
more online at http://homepage.virgin.net/nemonique.sam/noteindx.htm page 1 of 2
Nem’s Notes… Phase 1
HUMAN LIFE CYCLE 2 (page 2 of 2)
Fertilisation & Implantation
Week 3 At the beginning of the third week the primitive streak starts to develop on the
epiblast of the embryonic disc at the caudal end (away from the prechordal plate). It is
a result of the creation of intraembryonic mesoderm between the epiblast and
hypoblast. As the primitive streak moves further up it forms the primitive groove. The
disc is now in three layers (ie trilaminar) consisting of ectoderm, mesoderm and
endoderm.
Between the ectoderm and endoderm a notochord starts to develop moving toward
the cranial end from the primitive pit (part of the primitive streak). It stops at the
prechordal plate.
Above the notochord on the ectoderm, a neural plate develops which gives rise to the
CNS. At day 18 the plate obtains a groove called the neural groove which has neural
folds either side of it. As the groove deepens the folds fuse together over the top
giving rise to the neural tube which will become the spinal cord. The neural crests
develop into the spinal ganglion on either side of the neural tube.
Toward the end of the third week the mesoderm either side of the notochord
proliferates into somites. They will eventually give rise to the axial skeleton.
The lateral mesoderm develops small coelomic cavities which coalesce into a single
horse-shoe shaped intraembryonic coelom. This will eventually give rise to the
major body cavities such as the pericardial, pleural and peritoneal cavities.
Week 4 The intraembryonic coelom is placed more ventrally in the embryo by means of the
folding which occurs due rapid growth at the edges of the embryonic disc (including
the head). The developing heart is moved into the ventral surface along with the future
pericardial cavity. Lateral folding of the disc gives rise to the peritoneal cavity as the
coelom is pushed onto the ventral surface of the embryo. The future parietal surface is
derived from the somatic mesoderm and the visceral from the splanchnic mesoderm.
The visceral surface will be in contact with the organs while the parietal will be in
contact with the thoracic cavity.
The Germ Each of the germ layers gives rise to specific organs and tissues.
Layers The ectoderm gives rise to: (a) Epidermis
(b) Central & Peripheral Nervous Systems
(c) Retina of the Eye
The mesoderm gives rise to: (a) Smooth muscle coats
(b) Connective tissue
(c) Cardiovascular System
(d) Bone marrow and blood cells
(e) Skeleton
(f) Reproductive system
The endoderm gives rise to: (a) Epithelial linings of respiratory & GI tracts
(b) glands (within the liver, pancreas & GI tract)
Spina Bifida Spina bifida is a failure of the spine to fuse properly. There are two forms:
(a) Occulta – occurs in 10% of normal people in L5-S1 verterbrae
usually no clinical symptoms.
(b) Cystica – occurs with cyst on the back & can be of three further subtypes
(i) with meningocele – cyst containing meninges & CSF
(ii) with meningomyelocele – contains spinal cord/roots
(iii) with myeloschisis – open back due to lack of fusion
Anencephaly is the absence of brain and may accompany spina bifida cystica (ie it is
a neuralation defect, whereas spina bifida is a canilisation defect).
These defects can be tested for using amniocentisis or ultrasonography.
more online at http://homepage.virgin.net/nemonique.sam/noteindx.htm page 2 of 2