Principles of Embroyology

Question 1

Day 1 - Describe the process of Fertilisation (12-24 hours) after Ovulation (4).

Answer 1

1. Sperm and Ovum meet in the Uterine Tube (ampulla).
2. The Sperm penetrates the corona Radiata and Zona Pellucida
3. Acrosome Reaction: This fuses sperm for 2nd meiotic division and makes the ovum impeneratrable to other Sperm.
4. Zygote produced (diploid)

Question 2

Describe Days 2-3 following fertilisation (4)

Answer 2

1. CLEAVAGE: Rapid process of mitotic divisions. (first around 30 hours) after fertilisation.
2. By 3rd day 16 cell embryos. Each cell is called a BLASTOMERE.
3. Cells form solid sphere called MORULA.

Question 3

Describe Days 4-5 (4)

Answer 3

1. Morula forms a cavity (blastocoele), changing into BLASTOCYST.
2. Outer layer of Blastocyst thins out, becoming TROPHOBLAST. This helps form Placenta.
3. The rest of cells are pushed up to form a pole, an inner cell mass called EMBRYOBLAST.
4. Blastocyst reaches uterine lumen and is ready to be implanted.

Question 4

Describe Day 6-7 (3)

Answer 4

1. The blastocyst is implanted into the uterine lumen. It forms two layers called BILAMINAR DISC
2. The inner layer (around the embryonic pole) forms EPIBLAST (this develops into embryo) and HYPOBLAST (this develops into extraembryonic membranes).
3. AMNIOTIC cavity develops within the epiblast layer.

Question 5

Describe Day 6-8 (2)

Answer 5

1. EXOCOELOMIC membrane forms below from Hypoblast layer.

2. This creates a cavity called Yolk Sac. This contains nutrients which supply embryo before Placenta arrives.

Question 6

Describe week 2 (3)

Answer 6

1. Blastocyst reaches Uterine Cavity (endometrium). This needs to oxygen, nutrients and waste removal.
2. CORPUS LEUTUM produces Progesterone to maintain endometrium.
3. The blastocyst is implanted, causing a decidual reaction - the maternal cells contribute to placenta.

Question 7

Describe the 4 stages of Implantation of blastocyst in uterine cavity (endometrium). (4)

Answer 7

1. The Zone pellucida Sheds.
2. Apposition (arranging) of blastocyst to ensure embryonic pole stays in contact with uterine cavity.
3. Adhesion of blastocyst via molecular communication.
4. The trophoblast differentiates into cytotrophoblast and syncytiotrophoblasts. These invade and implant into the Uterine cavity.

Question 8

What is the area invaded by syncytiotrophoblasts called and what hormone is produced? (2)

Answer 8

1. Area is called Syncytium.

2. HCG (human chorionic gonadotropin) horomone is produced (checked during pregnancy test).

Question 9

> week 3. What does the bilaminar disc form? What is the process called? (3)

Answer 9

1. The bilaminar disc differentiates into 3 germ layers (trilaminar disc), initiated by the PRIMITIVE STREAK.
2. The epiblast becomes the ECTODERM. Epiblast cells invades the hypoblast layer, creating MESODERM in middle and then ENDODERM as last layer.
3. This process is called Gastrulation.

Question 10

Describe > 4th week (4)

Answer 10

1. The flat disc (trilaminar disk) folds in 2 directions.
2. Longitudinally (CEPHALOCAUDAL) (day 21) begins so head and tail are brought close together.
3. Laterally (transverse) (day 18) The amniotic cavity increases down, creating future GUT TUBE inside peritoneal cavity.

Question 11

What parts of the body are formed by Ectoderm, Mesoderm and Endoderm? (4)

Answer 11

1. Ectoderm - Nervous System, skin, ears, eyes.
2. Mesoderm - Skeletal system, muscular system, Renal System.
3. Mesoderm + Endoderm - Cardiovascular system
4. Endoderm - Gastrointestinal system.