Lecture 3 - Embryology

Question 1

Embryo

Answer 1

Beginning of 4th week to end of 8th week (2nd month)

Question 2

Fetus

Answer 2

3rd month to birth

Question 3

Ovulation

Answer 3

A secondary oocyte is released from the ovary and swept into the oviduct

Question 4

Fertilisation

Answer 4

A single sperm penetrates the secondary oocyte. The sperm and egg nuclei will fuse, resulting in a fertilised egg/zygote.

Question 5

Cleavage

Answer 5

Zygote undergoes rapid mitotic division as it moves along the oviduct towards to the uterus. (16 cells)

Question 6

Morula

Answer 6

By day 4, successive division produces a cluster of 16 cells called Morula and enters cavity of uterus.

Question 7

Blastocysts (overview)

Answer 7

Morula is filled with fluid and forms blastocyst. Zona pellucida degenerates and blastocyst hatches into uterine walls

Question 8

Implantation

Answer 8

Blastocyst attached to uterine lining (endometrium) and begins to digest it’s way inwards.

Question 9

What are the two types of cells in a blastocyst?

Answer 9

Blastocyst has two types of cells: outer epithelial layer (trophoblast) and inner cell mass

Question 10

What does the trophoblast form?

Answer 10

It forms some extra embryonic structures (ex: parts of placenta)

Question 11

What happens to the inner cell mass in the late blastocyst?

Answer 11

The inner cell mass start to differentiate into a bilaminar embryonic disc composed of two cell layers; hypoblast (primitive endoderm) and epiblast

Question 12

Gastrula

Answer 12

After bilaminar disc has formed, cavity appears called the amniotic cavity (from epiblast). Cells of hypoblast migrates, forming the yolk sac.

Question 13

Egg/Zygote

Answer 13

Fertilisation to end of 3rd week

Question 14

How long does it take to form 1 somite pair?

Answer 14

40 minutes

Question 15

Gastrulaton

Answer 15

The bilaminar embryonic disc differentiates to form 3 germ layers.
- a line thickens along the midline in the epiblast (at caudal end) of the disc (primitive streak -invagination)

Question 16

Invagination

Answer 16

Cells of epiblast migrates inwards toward streak, detaches from epiblast and slips beneath it into interior of embryo.
- cells in interior become mesoderm, cells remaining from epiblast become ectoderm

Question 17

Neurulation

Answer 17

Neural plate forms at cranial end (thickens) of embryo and grows from the cranial to caudal
- formation of notochord (derived from mesoderm)

Question 18

Notochord

Answer 18

Somites form with the notochord

Question 19

Neural tube

Answer 19

Neural plate forms a grove into the mesoderm to form a tube.

• formation of tube begins in middle and extend out in both cranial and caudal ends
• some cells from the neural fold detach to form new population of cells call neural crest

Question 20

What does the neural crest become?

Answer 20

It forms the peripheral nervous system

Question 21

Which planes does the trilaminar disc fold?

Answer 21

The horizontal and median plane

Question 22

The folding in a horizontal plane for what type of body folds?

Answer 22

It forms 2 lateral body folds

Question 23

How does the rotation of the cilia determine the left and right sides of the embryo?

Answer 23

The rotation of the cilia causes the flow of fluids to migrate to the left more than the right. The side with the larger amount of fluid becomes the left side of the embryo.

Question 24

What does the folding in the median plane create?

Answer 24

The plane forms at the cranial and caudal ends. The folding causes the endoderm to move inwards to form the gut tube.