Question

List the early embryonic stages starting with the zygote. Report the number of days after fertilization each stage occurs Explain the changes that occur in the embryo and the process of implantation. Explain the source and role of human chorionic gonadotropin and the source of the hormones replacing it.

Answer 1

After fertilisation ,Sperm and ovum combine to form Zygote .

Zygote undergo cleavage

1. 2 cell stage on Day 2 f/b. 4 cell stage on Day 3-4 of fertilisation

3. 8 cell stage known as early Morula on Day 4 f/b Late Morula 16 cell stage ( Mulberry morula ) same day .Late morula have inner cell mass which form embryo proper and outer cell which form trophoblast cells .

4 .On Day 5 Blastocyst is formed where trophoblast appears .Late blastocyst on Day 6-7 in which cavity is present .Implantation occur on Day 8-9 in Late blastocyst stage after Zona pellucida is shed off .

5. Implantation starts and blastocyst attaches to endometrium .It occurs in posterior wall of body of uterus close to fundus.It correspond to day 20 of regular menstrual cycle .Implantation is process by which embryo attaches the uterine wall then penetrates the epithelium further penetrate the circulatory system of Mother tobform Placenta .

Endometrium is prepared for implantation by various growth factors lipid modulated by sex hormone Progesterone --- pregnancy hormone .Endometrium is receptive for implantation for only few days .

Implantation begins with adhesion and apposition of blastocyst to endometeium mediated by cytokines and adhesion molecules integrin that interact with extracellular components especially lamnin and fibronectin.

Trophoblast invasion follows adhesion of blastocyst and mediated by proteinase degradaion of extarcellular matrix .

6.Day 8 - Bikaminar germ disc is formed .Trophoblast differentiate into cytotrophoblast ,hypoblast and epiblast

7) Day 9 -- yolk sac

8) Day 12 - Extraembryonic mesoderm

9) Day 13;14 -- Primiary chorionic villus ,Primitive streak ,prochordal plate ,secondary yolk sac,chorionic frondosum

10 ) Day 15 - Neural tube

11) Day 16 - Gastrulation fromation of 3 germ layers

12) Day 18 - Trilminar germinal disc

13) Day 21 ; Fetal circulation started

HCG ( Human chorionic gonadotrophic hormone ) glycoprotein hormone ,chemically functionally similar to Pituitary LH ,FSH and TSH

In early pregnancy less than 5 weeks ,it is synthesised in both Syncytiotrophoblasts and Cytotrophoblast cells of blastocyst but later eventually by Syncytiotroohoblast of Placenta.

Action - HCG stimulates production of progesterone by Corpus luteum of pregnanay LH like action ( Luteinising hormone ) .Maintains corpus luteum till 7 ;wks also have role in prevention of rejection of Fetus on foreign agent .

Further it's action is done by Progesterone --- pregnanch hormone .