Lecture: Anatomy 3: Principles of embryology

Question 1

What happens during day 1 of fertilisation?

Answer 1

1. Sperm and ovum meet in uterine Tube
2. Sperm penetrates corona radiate and zona pellucida
3. Genetic material fuses and undergoes 2nd meiotic division; zygote with 46 chromosomes

Question 2

What does the ovum do to ensure only 1 sperm interacts with egg?

Answer 2

Acrosome reaction makes ovum impermeable to other sperm

Question 3

What happens during days 2-3 of fertilisation?

Answer 3

1. Cleavage is the rapid process of mitotic divisions

2. By day 3, 16 cell embryo; cells are referred to a blastomere.

Question 4

What is the solid sphere of embryo cells (blastomeres) referred to as?

Answer 4

Morula

Question 5

What happens during days 4-5 of fertilisation?

Answer 5

1. Morula develops a cavity (blastocoele) and becomes known as a blastocyst.
2. Outer layer thins out and becomes the trophoblast which helps form the placenta; helps to form embryonic pole.
3. Blastocyst has now reached the uterine lumen and is ready for implantation.

Question 6

What happens during days 6-7 of fertilisation?

Answer 6

1. Inner cell mass differentiates into two layers- epiblast and hypoblast
2. Amniotic cavity develops within the epiblast mass

Question 7

What do hypoblasts form?

Answer 7

Hypoblast forms extraembryonic membranes

Question 8

What do epiblasts form?

Answer 8

Epiblast forms embryo

Question 9

What happens during days 6-8 of fertilisation?

Answer 9

1. Exocoelomic membrane formed from the hypoblast.

2. Primary Yolk Sac forms

Question 10

What is the role of the Primary Yolk Sac?

Answer 10

Contains nutrients that supply the embryo before the placenta functions.

Question 11

What happens during week 2 of fertilisation?

Answer 11

1. Blastocyst has reached the uterine cavity
2. Corpus luteum produces progesterone to maintain endometrium
3. Trophoblast differentiate into cytotrophoblasts and syncytiotrophoblast

Question 12

What are the 4 stages of implantation?

Answer 12

1. Shredding/Hatching of the zona pellucida
2. Apposition to ensure embryonic pole is in contact
3. Adhesion via molecular communication
4. Invasion

Question 13

What happens during week 3 of fertilisation?

Answer 13

1. Embryo develops 3 distinct layers which is initiated by the primitive streak.
2. Epiblasts become ectoderm. Hypoblasts degenerate to become epiblasts and form endoderm. Epiblasts also give rise to mesoderm.

Question 14

What are the 3 distinct layers of the embryo?

Answer 14

Ectoderm, mesoderm, and endoderm.

Question 15

What is the role of the 3 distinct layers of the embryo?

Answer 15

The 3 layers divide amniotic sac and yolk sac but also allow communication between the 2 regions.

Question 16

What does the ectoderm form?

Answer 16

Forms nervous system, skin, eyes, and ears.

Question 17

What does the mesoderm form?

Answer 17

Forms musculoskeletal system, muscular system, and renal system, as well as cardiovascular system

Question 18

What does the endoderm form?

Answer 18

Forms cardiovascular system and gastrointestinal system.

Question 19

What happens during week 4 of fertilisation?

Answer 19

Flat disc folds in 2 direction (longitudinal and lateral) to envelop and engulf yolk sac.

Question 20

What does lateral (transverse) folding do to the developing embryo?

Answer 20

Brings the amniotic cavity down, creating the future gut tube inside the peritoneal cavity; day 18

Question 21

What does longitudinal folding do to the developing embryo?

Answer 21

Brings head and tail are brought closer; day 21.