Gametogenesis, Fertilisation and the Blastocyst

Question 1

What is gametogenesis?

Answer 1

the conversion of germ cells into the male and female gametes

Question 2

What is the primary follicle?

Answer 2

primary oocyte (diplotene) surrounded by a single layer of follicular cells

Question 3

What happens to the primary follicle?

Answer 3

FSH, 5-12 primary follicles develop

Follicular cells multiply and form multiple layers. This is separated from the ovum by the septum pellucidum.

Question 4

What happens to the 5-12 primary follicles?

Answer 4

All but 1 degenerate, these form the corpus atreticum/scar.
The remaining cells of the growing follicle secrete a fluid to form a fluid-filled antrum
Ovarian non-gamete cells form the thecal layer

Question 5

What is the thecal layer?

What are the two types?

Answer 5

Cells surrounding the follicle
Vascular theca interna - produces oestrogen
Theca externa (inactive)

Question 6

When does the follicle become a mature/graafian follicle?

Answer 6

once the antrum, stratum granulosum and cumulus oophorus form
also is a secondary oocyte

Question 7

What is the stratum granulosum?

Answer 7

follicular cells which produce progesterone

Question 8

What is the cumulus oophorus?

Answer 8

cells which surround the oocyte

Question 9

What happens to the mature follicle?

Answer 9

follicle rupture! ovum is released (metaphase II).

Question 10

Structure of released ovum

Answer 10

Covering of follicular cells - corona radiata

Septum pellucidum expands to form the zona pellucidum

Question 11

What happens to the rest of the follicular cells?

Answer 11

Cells of theca interna and stratum granulosum enlarge and turn yellowish. This is the corpus luteum and secretes large amounts of progesterone

Question 12

What happens to the corpus luteum if fertilisation does not occur?

Answer 12

Degenerates after around 12 days, forming the corpus albicans. Cessation of hormonal output leads to menstruation

Question 13

What happens to the corpus luteum if pregnancy does occur?

Answer 13

Sustained by HCG produced by conceptus (early embryo), forming CL of pregnancy, then the large corpus albicans

Question 14

Describe the process of spermatogenesis

Answer 14

stem cells divide by mitosis several times to form:
spermatogonia which grow into:
primary spermatocytes, which undergo 1st meiotic division to form:
secondary spermatocytes, which undergo 2nd meiotic division to form:
spermatids (no tail), which develop into:
spermatozoa

Question 15

Differences in gametogenesis between males and females

Answer 15

1. f: discontinuous, no stem cells retained
   m: continuous from puberty, stem cells retained
2. f: all primary oocytes present from birth, suspended
   partially through meiosis
   m: takes around 9 weeks

Question 16

Differences in male and female gametes

Answer 16

1. f: non-motile
   m: motile
2. f: very high cytoplasmic to nuclear ratio
   m: low (almost none in sperm head)

Question 17

Steps of fertilisation

Answer 17

1. sperm reaches secondary oocyte

2. sperm and oocyte fuse

Question 18

What processes occur for the sperm to reach the oocyte?

Answer 18

Sperm reaches isthmus, where they undergo capacitation. They travel to the ampulla, following chemoattractants from cumulus cells.
Ovum released from abdominal cavity and is caught by the fallopian tube. (fertilisation occurs in ampulla)

Question 19

What is capacitation?

Answer 19

conditioning of the sperm to become more accustomed to the environment (acrosomal head loses glycoprotein coat and becomes more mobile again)

Question 20

What occurs during the meeting of the sperm and oocyte?

Answer 20

penetration of corona radiata and zona pellucida prompts completion of 2nd meiotic division of oocyte
zona reaction occurs and ZP hardens, preventing further penetration
male nucleus injected, as plasma membranes fuse

Question 21

What helps the sperm to penetrate the CR and ZP?

Answer 21

acrosomal enzymes

Question 22

What happens after fertilisation?

Answer 22

cleavage of the cell to form a blastomere, then further cleavage until a morula is formed. Compaction occurs and the ZP splits to form a blastocyst

Question 23

What does a blastomere look like?

Answer 23

four cleaved cells

Question 24

What occurs in compaction?

Answer 24

outer cells of morula tighten, forming gap junctions and desmosomes

Question 25

Structure of blastocyst

Answer 25

inner cell mass - primitive endo and ectoderm

outer (trophoblast) - synctio and cyto extraembryonic mesoderm