Embryology

Question 1

27.1 Describe the germinal phase with reference to: zygotes, morulae, blastocysts and formation of a 3-layer embryonic disk and the fates of the endoderm, mesoderm and ectoderm in the developing embryo.

Answer 1

Germinal phase

= Last 12/14 days

= Encompasses: Fertilisation, cleavage, differentiation, implantation, separation of blastocyst into layers

= It starts at conception, when the sperm and egg combine, forming a zygote.

1. Fertilisation (Occurs within uterine tube 12-24 hours after ovulation).
2. Cleavage (first cleavage completed about 30 hours after fertilisation).
3. Morula (3-4 days after fertilization)
4. Blastocyst (4 ½ - 5 days after fertilization)
5. Implantation (occurs about 6 days after fertilization).

Embryologic stage:

Separation of the blastocyst into layers

disc into the 3 layers is that part of the same part.

1. Blastocyte becomes attached to the endometrium.
2. Superficial cells of the trophoblast (outer layer of the blastocyst) fuse together to form the syncytiotrophoblast (another layer as a digger), which grows into the endometrium (of the mother) and derives nutrients into it.
3. The endometrium responds by growing over and eventually enveloping the blastocyst.
4. The syncytiotrophoblast secretes the hormone HCG to instruct the corpus luteum to continue progesterone production (which maintains pregnancy).
5. Progesterone (has many functions) also stimulates the growth of nutrient-rich decidual ceels to feed the early embryo.
6. Around the middle of week 2 the embryoblast starts to form a two-layered embryonic disc = Hypoblast + Epiblast, at the mid-plane of blastocyst, with a cavity on either side. This delaminar disc sets the dorsal/ventral axis of embryo, with the epiblast being the dorsal side.
7. By the end of second week, the 2 layers grow around the cavity on their respective side and form the yolk sax and amnion.
8. The yolk sac provides nutrients absorbed from the trophoblast, as well as gas exchange for the embryo during weeks 2 and 3, before the placenta takes over these functions at week 4. Also being the first site of embryonic blood cell production, before the liver.

3 mins in

https://www.youtube.com/watch?v=g7FExgKr-x4

Question 2

27.2 Discuss the process of implantation and how an ectopic pregnancy creates potential problems for future fertility.

Question 3

27.3 Outline the process of placentation and briefly describe the structure and major functions of the umbilical cord and placenta.

Question 4

27.4 Indicate the major events occurring during the phases of embryonic and fetal growth.

Question 5

27.5 Discuss the effects of teratogen on fetal development.

Question 6

** Human gestation (40 weeks) it is divided into 3 trimesters: **

All major systems are developed by the start of the 2nd trimester (12 weeks).

Question 7

What happens in the germinal phase, and for how long?

Answer 7

Germinal phase (14 days):

Mitosis of cells to form zygote, morula and blastocyst.

Day 1 - Fusion of haploid gametes to form a diploid zygote.

Day 4 - Post fertilisation, blastocyst implants into the uterine wall.

Question 8

What happens in the
Embryogenic phase and for how long?

Answer 8

Embryogenic phase (2-10 weeks):

Cells start to develop and mature into the systems of the body:

• Gastrulation, Nervous system, Cardiovascular system, organs and limbs.

Question 9

What happens in the foetal phase, and for how long?

Answer 9

Foetal phase (last 10 weeks - 9 months)

Characterised by growth and development of the organs and the foetus.

Question 10

How many weeks is human gestation (pregnancy)?

Answer 10

40 weeks