Fertilization Process
Following ovulation, egg is capable of fertilization for only 12 to 24 hours.
Sperm (acrosome) in between corona radiata cells, acrosome cap attach with zona pellucida, acrosomal cap release hyaluronidase.
Acrosome reacted sperm penetrating the zona pellucida.
Acrosome reacted sperm in perivitelline space.
Incorporated sperm with vesiculating head into oolema.
Soon after sperm infusion, penetration of other sperm is prevented by zona pellucida reaction (hardening) or oolema block due to release of cortical granules by exocytosis from oocyte.
Formation of male and female pronuclei with completion of second polar body.
Male and female pronuclei unite at centre with restoration of diploid number of chromosome.
Zygote formed (contain both maternal and paternal genetic material).
Antigen present on cortex (called fertilizin) and it’s coat of ovum react with antibody called antifertilizin release at plasma membrane of sperm head.
Antifertilizin present in plasma membrane of sperm.
Thus union of two gametes immunological reaction (chemotaxis).
Sex of child determined by sex chromosome supplied by spermatozoon.
If spermatozoon contain X chromosome; female embryo is formed (46,XX).
If spermatozoon contain Y chromosome, male embryo is formed (46, XY).
Embryonic development
After zygote formation, mitotic devision of zygote producing two blastomere.
16 cell stage called morula (solid mass of blastomere).
Morula
Two cell stage (called morula) reached approximately 30 hours after fertilization.
Each blastomere; equal cytoplasmic and chromosome number.
Morula after spending about 3 days in uterine tube enters uterine cavity through narrow uterine ostium on 4th day in the 16-64 cell stage.
Central cell of morula known inner cell mass (form embryo).
Peripheral cell called outer cell mass (form protective and nutritive membranes of embryo).
Blastocyst
On 4th and 5th day morula in uterine cavity covered by film of mucus.
Fluid pass through canaliculi of zona pellucida; separate cells of morula called blastocyst.
Due to blastocyst enlargement zona pellucida become stretched, thinned and gradually disappears.
Lysis of zona pellucida and escape of embryo called zona hatching.
Outer side of morula become trophectoderm (adhesion of blastocyst to uterine epithelialium.
Inner side of morula become inner cell mass.
Changes in Blastocyst
Trophoblast cell adjacent to inner cell mass primarily involved in adhesion to endometrial cell.
Responsible for blastocyst attachment are;
Selectins, Integrins, trophinin, tasin, EGF (epidermal growth factor), proteoglycan, Heparin sulphate.
Implementation
Implementation of blastocyst into endometrium occur 6-7 days after fertilization.
Implementation occurs in endometrium of anterior or posterior wall of body endometrium near fundus on 6th day after fertilization (20th day of regular menstrual cycle).
Endometrium in secretory phase crossponding to 20-21 days of cycle.
Microvilli on surface of trophectoderm integrate with decidual cells of endometrium.
Blastocyst differentiated into-
1. Trophoblast
a. Chorion (outer layer of embryo cell)
b. Placenta
2. Inner mass cell
a. Amnion (inner layer of embryo cell)
b. Foetus
c. Umbilical cord
3. Blastocele
Implementation
Implementation stages are;
Stage 1
Apposition
Stage 2
Adhesion
Stage 3
Penetration and invasion
1. Apposition
Pinopods; long finger like projection (microvilli) form endometrial cell surface.
Endometrial fluid secreted by endometrial gland cells, pinopods absorb endometrial fluid.
Endometrial fluid rich in glycogen and mucin provide nutrition to blastocyst.
2. Adhesion (Attachment)
Adhesion of blastocyst to endometrium occurs through adhesion molecules like integrin, selectin and cadherin (glycoprotein).
3. Penetration and invasion
Trophoblast cells of blastocyst attach to endometrial epithelium. Invasive trophoblast cells cross endometrial epithelial basement membrane and invade.
Uterine tissue breakdown and embryo grow.
Deep penetration of human blastocyst called interstitial implantation.
Decidua formation (blastocyst covered on all sides by endometrium (decidua).
Trophoblast
Trophoblast is cell of blastocyst.
