Early Embryology Simple Summary

Topics: Embryology, Developmental biology, Mesoderm Pages: 11 (2273 words) Published: May 23, 2011
Week 1
Ovarian cycle:
GnRH from hypothalamus FSH and LH from pituitary cyclic changes in the ovary. Follicular phase:
* 15-20 preantral follicles start to grow due to FSH
* 1 reaches full maturity released
* Others undrgo atresia
* Estrogens are released which;
1. Proliferate endometrium
2. Thin the cervical mucus
3. Stimulate pituitary to release LH
* The LH;
1. Elevate concentrations of maturation promoting factor (oocytes complete meiosis one and begin meiosis II, stopped at metaphase) 2. Stimulate progesterone production by follicular stromal cells (luteinisation) 3. Cause follicular rupture


* High concentration of LH makes the collagenase active breaks down collagen by bulging follicle rupture. * Oocyte + cumulus oophorus breaks free ovulation.

Luteal Phase:
* After ovulation, under influence of LH, theca and granulose cells become lutein cells. * Corpus luteum secretes progesterone.
* Eventually (~10 days after ovulation), corpus albicans is formed. (if no fertilization!) * If fertilised, syncitiotrophoblasts release hCG

* Swept in by fimbria of the oviduct and cilia.
* Once in tube, cumulus cells withdraw their cytplasmic processes from the zona pellucida and lose contact with the oocyte. Fertilisation
* Sperm must undergo (a) capacitation (glycoprotein coat and seminal plasma proteins removed) * (b) Acrosome reaction- after bind to zona pellucid, acrosome releases proteins needed to bind. * Fusion of gametes oocyte membrane becomes impenetrable to other sperm, zona pellucid alter structure to prevent further binding. * Meiosis II is completed when sperm enters.

Blastocyst formation
* 2 cell stage 4 cell 8 cell 16 cell (morula)
* Morula has inner cell mass (make embryo) and outer cell mass (trophoblast i.e. placenta) * ~ when morula enters uterus, fluid penetrates zona pellucida blastocele.

Uterus is in secretory phase, zona pellucida has degenerated.

Week 2 Bilaminar Germ Disc
Day 8
* Blastocyst is partially embedded
* Trophoblast = cytotrophoblast (inner) + syncitiotrophoblast (outer) * Inner cell mass (embryoblast) = hypoblast(small cuboidal) + epiblast (high columnar)

Day 9
* Hypoblast + exocoelomic membrane become primitive yolk sac * Vacuoles appear in syncitium fuse form lacunae utero-placental circulation

Day 12
* Completely embedded
* Sinusoids formed
* Extra embryonic mesoderm grows.

Day 13/14
* Chorionic cavity forms
* Definitive yolk sac formed

Week 3
* Cells migrate through primitive streak to form endoderm, ectoderm and mesoderm. * Ectoderm skin epithelium and nervous system
* Mesoderm CT , muscle, skeleton
* Endoderm GIT and respiratory epithelium (also gives glands) * Notochord forms in the mesoderm which gives patterning and orientation. * Cloacal and Buccopharyngeal membranes are formed.

Week 3-8 The Embryonic Period:
Disc Folding
Endoderm folds to form yolk sac. Ectoderm folds backward to form the amniotic sac. All threelayers then fold ventrally, around the notochord and toward the yolk sac (pinching off a portion). The middle of the tube becomes the GIT.

Above the notochord is the buccopharyngeal membrane with mesoderm above that. Below the notochord is the primitive streak with cloacal membrane below that. Dorsally, the neural tube and endoderm lie. Ventrally the mesoderm and then endoderm.

* Neuralation:
* Ectoderm overlying the notochord thickens to become the neural plate. This expands toward primitive streak. At the end of the third week, neural folds and groove are seen. * Neural folds fuse

* Neural tube communicates with amoniotic cavity via neuropores. Day 25,...
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