Early Embrology

Question 1

Describe the the development of primordial germ cells

Answer 1

Primordial germ cells develop from the wall of the yolk sac during the 2nd week of development, migrate into the embryo to the gonadal ridges, where they undero rapid mitotic divisions.

Question 2

What is the difference of proliferation of primordial germs cells between the sexes

Answer 2

Oogina multiply rapidly until the 5th month reaching 7 million, then many undergo atresia

Spermatogenia continue to slowly proliferate from puberty throughout life

Question 3

In the production of gametes how many meiotic cell divisions occur?

Answer 3

2 Diploid -> Haploid

Question 4

What happens to the primodiary follicle before & after birth until puberty

Answer 4

In fetal life differentiate into primary oocyte, at birth enter prophase of first meiotic division & remain suspneded until puberty (suspended by oocyte maturation inhibitor OMI)

Question 5

After puberty the primodiary follicle changes to primary follicle - how does he physiology change

Answer 5

primodiary follicle becomes surrounded in granulosa cells & ovarian connective tissue (theca folliculi)
The theca folliculi then divides into theca interna (vascualr/secretory) and theca externa (outer fibrous layer

Question 6

When does fertilisation occur

Answer 6

12-24hours post ovulation

Question 7

5 stages of fertilisation

Answer 7

1) Sperm activation & penetration of the corona radiata
2) Attachement & penetration of zona pellucida
3) Fusion of oocyte & sperm cell membrne
4) Completetion of meiosis in the oocyte & formation pronucli
5) Formation of the zygote

Question 8

What is the name of the process sperm undergo to be capable of penetrating the ovvum? What happens?

Answer 8

Capacitation
Removal of glycoprotein & cholesterol of the acromsomal head frmo the secretions of he cervix/uterus
When come into contact with the corona radiata, they release the enzyme hyaluronidase
zona pellicuda -> reaction to prevent further sperm penetrating

Question 9

What happens to the oocte on entry of the spermatzoa?

Answer 9

Second meitoic division, forming the mature ooytes and a second polar body.

The chromosomes of oocyte/sperm condense to form pronuclei -> then mix to form zygote

Question 10

What is morula?

Answer 10

After mitotic division of the zygote, a 16 cell embro forms, each cell is called a blastomere, as each cell divisides the cells become smaller, they are all pluripotent.

Question 11

Explain the anatomy of a blastocyst

Answer 11

Outer layer of cells: trophoblast
Inner cell mass
Blastocele inner cavity

Question 12

How long after fertilisation is & where in the anatomy

1) 2 cell embryo

Answer 12

1) 30 hours middle of the tube

Question 13

How long after fertilisation is & where in the anatomy

morula

Answer 13

3 days

junction of uterine tube & uterus

Question 14

How long after fertilisation is & where in the anatomy

blastocyst

Answer 14

4-5 days

uterine lumen

Question 15

During implantation

what is the name of the invading trophblast into maternal endometirum?

Answer 15

Syncytiotophoblast

Question 16

During implantation the inner cell mass divides into 2 layers - what are these called
which layer a splits to form amniotic cavity

Answer 16

epiblast & hypoblast

epiblast splits to form amniotioc cavity

Question 17

What is he primary yolk sac? What does it form from?

Answer 17

Fluid filled sac that is a source of nutrition for the embryp before the placenta is fully formed.

Forms from hypoblast -> bilaminar embryonic dis

Question 18

At day 13 what does the cytotrophoblast form?

Answer 18

Primary chorionic villi which are finger like protrusions into the lacunae.

Question 19

At day 13 describe the form of the embryo proper

Answer 19

The epiblast and hypoblast applied to each other (the bilaminar embryonic disc), the amniotic cavity above and yolk sac below the hypoblast.

Question 20

At the end of the second week what does the syncytiothrophoblast produce? What is its role?

Answer 20

hCG, maintains the corpus lutem in the ovary which sustains the thickness of the endometrium

Question 21

What is granulation?

Answer 21

Formation of 3 germ layers
Epiblast → ectoderm → mesoderm
Hypoblast → Endoderm

Question 22

In general what tissues do each other layers form?

• Ectoderm
• Mesoderm
• Endoderm

Answer 22

Ectoderm: Epidermis and nervous system
Mesoderm:
Endoderm: Linning of GI and resp system
Mesoderm: Skeletal, connective tissue, muscle

Question 23

How is the mesoderm layer produced?

Answer 23

End of the second week, a groove in the bilaminar embryonic disc called the primitive streak forms. Cells from the ectoderm migrate toward the streak and then detach from it and spread laterally beneath it forming the mesoderm. New layer lies between ectoderm and endoderm

Question 24

The ectoderm and endoderm remain attached in 2 areas, the cloacal membrane and buccophargeal membrane.

What does the buccophargeal membrane form?

Answer 24

The oral cavity

Question 25

What is the notochord? How does it develop and what is its significance?

Answer 25

Cells from the primitive streak migrate cranially towards the buccopharygeal membrane, creation the notochordal plate which forms folds to form a solid cylinder the notochord. This underlie the future neural tube

forms a longtiduninal axis for the embryo, and the centre nuclei polposi of intervertebral disc.

Question 26

What is the importance of neural crest cells?

Answer 26

At the edges of the neural tube, where the neuroectroderm is continous wth the ectoderm, neural crest cells form - these detach from the neural tube and migrate from discrete structures

These contribute to the formation of autonomic ganglia, connective tissue, adrenal medulla, enteric ganglia, schwann cells, meninges, teeth.

Question 27

What are the 3 segments of the mesoderm?

Answer 27

Paraxial mesoderm
Intermediate mesoderm
Lateral plate mesoderm

Question 28

What does the paraxial mesoderm from?

Answer 28

Skeletal muscle of trunk and limbs, skeleton (except skull) dermis and connective tissue

Question 29

What are somites?

Answer 29

The paraxial mesoderm form paired blocks of tissue in the craniocaudal direction called somites, ranging up to a number of 42-44 pairs depending on the age of the embryo.

Question 30

What 2 layers to the somites divide into?

Answer 30

Outer dermomyotome (connective tissue and skeletal muscle)
Inner sclerotome (bone and cartilage) 

Cells from the sclerotome surround the notochord and give rise to the vertebral column

Question 31

The myoterms split into?

Answer 31

Dorsal epimeres and ventral hypomere.

Epimere → epaxial muscles/spine mucks
Hypomere → muscle of body wall

ventrolateral give buds off that give future limbs.

Question 32

What does the lateral plate mesoderm form?

Answer 32

Serous membrane of the body, pleural/pericardium/peritoneum. Smooth muscle connection tissue

That in contact future body → parietal layer and that in contact with endodermal gut → visceral layer

Question 33

What does the intermediate mesoderm form?

Answer 33

The urogenital tract

Question 34

How is the yolk sac lost?

Answer 34

During the fourth week, lateral fold around the embryo the amniotic cavity pinches the the communication between the yolk sac and the gut → forms a narrowed communication the vitelline duct which is later lost.

4th week is all about the folding of the embryo.