|
Fertilisation
- sperm binds to receptor molecule on zona pellucida
- release degradative enzymes (acrosome reaction)
- on penetrating oocyte membrane cortical granules released preventing
further sperm penetration
- induces oocyte to pass through second meiotic division
- male and female pronuclei fuse
- forms zygote
Cleavage
- zygote increases in cell number but not cell mass
- cells formed are blastomeres
- 3 days: form 6 to 12 cells
- 4 days: 10 to 32 (morula)
Blastocyst
- early stage of embryonic development
- consists of hollow ball of cells with a localised thickening (inner cell
mass)
- blastocoele is cavity will become yolk sac
- inner cell mass differentiates into two layers - epiblast and hypoblast
- hypoblast backs onto blastocoele
- amniotic cavity forms within inner cell mass (epiblast)
- outer cell layer known as trophoblast
- fluid filled cavity known as blastocoele
- exists from about 5 to 7-12 days
Hatching
- blastocyst enzymatically bores hole in zona pellucida
- squeezed out into uterus
Implantation
- occurs at about day 6
- blastocyt attaches to uterine wall
- endometrial cells differentiate into decidual cells
- endometrium supported by progesterone from corpus luteum
- corpus luteum supported by hCG released by
blastocyst, else degerates after ~13 days
Gastrulation
- epiblast cells migrate through primitive streak
- form layer between epiblast and hypoblast (some hypoblast cells may be
included)
- these form trilaminar germ disc of
- ectoderm (epidermis and appendages, surface epithelial derivatives,
nervous system)
- mesoderm (dermis, musculature, vascular system)
- endoderm (epithelial lining og gut + derivatives)
13.4.2 Fertilization, Cleavage and Implantation [Embryol]
See 13.3.3.
13.4.3 General Principles of Embryology
| Concepts and examples of : multipotentiality,
epigenesis, induction, teratogenesis, genetic and environmental
interactions in causality of development, |
|
| Embryonic, organogenetic, foetal periods of
development. Trimesters. Determination of embryonic and foetal age. |
|
| Pre-somite and somite periods, crown-rump
length, crown-heel length, skeletal maturity. |
|
| Ultrasound |
Amniocentesis, chorionic villus sampling |
13.4.4 Formation of the Three-layered Embryo
| Cleavage, trophoblast and inner cell mass
formation, blastula formation and formation of the two-layered embryo (epiblast,
hypoblast). |
|
| Primitive streak and node; gastrulation;
formation of ectoderm, mesoderm and endoderm. |
|
|