WEEK 1: IMPLANTATION AND EARLY EMBRYO DEVELOPMENT

Question 1

When does mitotic cell division starts post-fertilization?

Answer 1

Mitotic cell division starts12 hours post-fertilization

Question 2

Describe the mitotic division of a fertilized egg to form a blastocyst. Include the time frame each one happens.

Answer 2

1. Zygote formed
2. 12hours: 2 cells
3. 24-36 hours: 4 cells
4. 36-72 hours: 8 cells
5. Day 4: 16 cells (Morula), Morula enters the endometrial cavity
6. Day 5: Morula will evolve to a fluid filled cavity referred as Blastocyst.

Question 3

Describe the two cell layers of the blastocyst.

Answer 3

The blastocyst consists of two distinct cell layers:
*The outer trophoblast
*The inner cell masses.

Question 4

How do the cell layers of the Blastocyst differentiate.

Answer 4

*The trophoblast differentiates into two layers: the inner layer of cytotrophoblast and the outer layer of syncytiotrophoblast.

*The inner cell mass gives rise to the embryonic disc, which will later develop into the embryo.

Question 5

State the functions of the syncytiotrophoblasts and cytotrophoblasts.

Answer 5

*It is the outer layer of thetrophoblastsand activelyinvadestheuterine wall, duringimplantation, rupturing maternalcapillariesand thus establishing an interface between maternal blood and embryonicextracellular fluid, facilitating passive exchange of material between the mother and the embryo.

*Cells from the cytotrophoblast invade maternal arteries to make them larger, remodeling them and allowing good blood supply to fetus

Question 6

Describe the process of implantation.

Answer 6

1. Starts by “embryo hatching” from zona pellucida
2. Apposition and Adhesion:

Cell Adhesion Molecules (CAMs):
CAMs on the surface of trophoblast cells interact with CAMs on the surface of endometrial cells, facilitating adhesion.

Integrins:
Integrins play a role in the adhesion process by mediating cell-cell interactions.

3. Penetration and Invasion:

Matrix Metalloproteinases (MMPs):
Trophoblast cells release MMPs, enzymes that break down components of the extracellular matrix in the endometrial tissue.

MMPs facilitate the penetration of trophoblast cells into the endometrial lining.

4. Syncytiotrophoblast Formation:

*hCG (Human Chorionic Gonadotropin):
hCG is produced by the syncytiotrophoblast.

It supports the corpus luteum, preventing its degeneration and maintaining the production of progesterone during early pregnancy.

5. Chorionic Villi Formation:

Cytotrophoblast and Syncytiotrophoblast:
These trophoblast layers differentiate to form chorionic villi, the finger-like projections that extend into the endometrial tissue.

6. Decidual Reaction:

Progesterone:
Progesterone, produced by the corpus luteum and later by the placenta, plays a role in the decidualization of endometrial cells.

Insulin-Like Growth Factor (IGF):
IGF is involved in the regulation of decidualization.

7. Maternal Recognition of Pregnancy:

Human Chorionic Gonadotropin (hCG):
hCG is released by the syncytiotrophoblast.
It signals the presence of the developing embryo to the maternal body and prevents the regression of the corpus luteum.

8
Insulin-Like Growth Factor (IGF):
IGF plays a role in trophoblast invasion and placental development.

Question 7

Describe Early embryonal development at week 1.

Answer 7

Week 1:
Morula (consisting of blastomers)
Blastocyst(reaches endometrial cavity on day 5)
Nidation (contact with endometrium)

Question 8

Describe Early embryonal development at week 2.

Answer 8

Week 2
Trophoblast infiltrates endometrium & spiral arteries
Develops uteroplacental circulation
Amniotic cavity develops
Formation of ectoderm and endoderm
Chorionic vili forms by day 13

Question 9

Describe Early embryonal development at week 3.

Answer 9

Week 3
Mesoderm develops to form trilaminar germ layer
Embryonal folding will occur
Amniotic cavity will be formed
Fetal vascular system develops
Placenta will be formed

Question 10

What is differentiation?

Answer 10

The transformation of unspecialized embryonic cells to a specialized cells, tissues and organs.

Question 11

An embryo has three germ layers.

State them.

Answer 11

Ectoderm
Mesoderm
Endoderm

Question 12

Outline the structures that the ectoderm differentiates into.

Answer 12

*Nervous system including brain, spinal cord and nerves
*Lining of the mouth, nostrils, and anus

*Epidermis of skin, sweat glands, hair, nails

Question 13

Outline the structures that the mesoderm differentiates into.

Answer 13

Bones and muscles
Blood cells and blood vessels
Reproductive and excretory systems
Inner layer (dermis) of skin

Question 14

Outline the structures that the endoderm differentiates into.

Answer 14

Lining of digestive tract
Lining of trachea, bronchi, and lungs
Liver, pancreas
Thyroid, parathyroid & thymus glands
urinary bladder

Question 15

State the 6 components of the developmental history of human.

Answer 15

1. Gametogenesis
   2.Fertilization
2. Embryogenesis
3. Gastrulation
   5.Organogenesis
4. Maturation