Embryology

Question 1

What are the key processes by which one cell becomes a multicellular body?

Answer 1

Growth, morphogenesis (development of form and structure), and differentiation.

Question 2

What are the three periods of embryonic development?

Answer 2

Weeks 1-2 are the pre-embryonic period, weeks 3-8 are the embryonic period, and week 9+ (~38) are the fetal period.

Question 3

What happens during fertilisation in the pre-embryonic period?

Answer 3

The oocyte is released from the ovary and fertilised in the ampulla of the Fallopian tube, becoming a zygote.

Question 4

What occurs after fertilisation?

Answer 4

Cleavage (the first mitotic division) occurs about 30 hours after fertilisation, resulting in two blastomeres.

Question 5

What is the zona pellucida?

Answer 5

A glycoprotein ‘shell’ that forms to prevent access by any further sperm.

Question 6

What is a morula?

Answer 6

A ball of about 16 cells, all of which are totipotent, and the same size as the original zygote.

Question 7

What is compaction?

Answer 7

The process when the first cavity forms, and the zona pellucida still surrounds the cells.

Question 8

What is the blastocyst cavity?

Answer 8

A large fluid-filled space created by fluid released from the cells.

Question 9

What are the two masses of cells in the blastocyst?

Answer 9

The inner mass called the embryoblast (which will form the being) and the outer mass called the tropoblast (which forms supporting tissues).

Question 10

What happens after compaction?

Answer 10

The cells become pluripotent.

Question 11

When does the blastocyst hatch from the zona pellucida?

Answer 11

About day 5, as the risk of being fertilised by another sperm is low.

Question 12

What is required for the blastocyst around day 6-7?

Answer 12

The blastocyst needs to implant itself in the uterine lining.

Question 13

What does the tropoblast do during implantation?

Answer 13

It communicates with the endometrial cells.

Question 14

What are the two cell layers generated by the tropoblast in week 2?

Answer 14

The syncytiotropoblast and cytotropoblast.

Question 15

What does the embryoblast become by the end of week 2?

Answer 15

A two-layered disk called the bilaminar disk, comprised of the epiblast and hypoblast.

Question 16

What cavities are present in the conceptus by the end of week 2?

Answer 16

The amniotic cavity and yolk sac, suspended by a connecting stalk within the chorionic cavity.

Question 17

What occurs during implantation?

Answer 17

The blastocyst buries itself into the endometrium and establishes maternal blood flow within the placenta to access nutrients and remove waste.

Question 18

What is ectopic pregnancy?

Answer 18

Implantation at another site, most commonly the Fallopian tube.

It can also occur in the ovaries and may become a life-threatening emergency.

Question 19

What can occur when the embryo implants in the lower uterine segment?

Answer 19

Placenta praevia can occur, which may block a normal delivery and require a c-section.

Question 20

What happens by day 9 of implantation?

Answer 20

The syncytiotrophoblast undergoes rapid development at the embryonic pole.

Question 21

What forms at the abembryonic pole by day 9?

Answer 21

The primitive yolk forms, which is in contact with the cytotrophoblast.

Question 22

What pushes the primitive yolk sac away from the cytotrophoblast?

Answer 22

An acellular extraembryonic reticulum at about day 11.

Question 23

What occurs at day 12 of implantation?

Answer 23

Maternal sinusoids are invaded by the syncytiotrophoblast for blood supply, forming continuous lacunae.

Question 24

What begins at day 12?

Answer 24

The uteroplacental circulation begins.

Question 25

What forms at day 13?

Answer 25

A secondary yolk sac forms by pinching off the primitive yolk sac.

Question 26

What is suspended by the mesoderm at day 14?

Answer 26

The embryo and its cavities.

Question 27

What is the embryonic period?

Answer 27

Weeks 3 - 8, during which the greatest change occurs and the highest risk of major congenital malformation exists.

Question 28

What happens in the 3rd week of the embryonic period?

Answer 28

A primitive streak with its pit and node appears, leading to the formation of the three germ layers.

Question 29

What are the three germ layers formed during gastrulation?

Answer 29

Ectoderm, mesoderm, and endoderm.

Question 30

What does the ectoderm develop into?

Answer 30

Organs and structures in contact with the outside world.

Question 31

What does the mesoderm develop into?

Answer 31

Supporting tissues.

Question 32

What does the endoderm develop into?

Answer 32

Internal structures.

Question 33

What is the notochord?

Answer 33

A solid rod of cells formed during gastrulation with important signalling roles.

Question 34

What role does the notochord play?

Answer 34

It directs the overlying ectoderm into the neuroectoderm.

Question 35

Somite derivatives

Answer 35

dermatome, myotome, sclerotome

Question 36

Cephalocaudal folding

Answer 36

Ectoderm folds over endoderm, this pushes the cardiogenic field down