Gametogenesis, Fertilisation and the Blastocyst Flashcards
What is gametogenesis?
the conversion of germ cells into the male and female gametes
What is the primary follicle?
primary oocyte (diplotene) surrounded by a single layer of follicular cells
What happens to the primary follicle?
FSH, 5-12 primary follicles develop
Follicular cells multiply and form multiple layers. This is separated from the ovum by the septum pellucidum.
What happens to the 5-12 primary follicles?
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
What is the thecal layer?
What are the two types?
Cells surrounding the follicle Vascular theca interna - produces oestrogen Theca externa (inactive)
When does the follicle become a mature/graafian follicle?
once the antrum, stratum granulosum and cumulus oophorus form
also is a secondary oocyte
What is the stratum granulosum?
follicular cells which produce progesterone
What is the cumulus oophorus?
cells which surround the oocyte
What happens to the mature follicle?
follicle rupture! ovum is released (metaphase II).
Structure of released ovum
Covering of follicular cells - corona radiata
Septum pellucidum expands to form the zona pellucidum
What happens to the rest of the follicular cells?
Cells of theca interna and stratum granulosum enlarge and turn yellowish. This is the corpus luteum and secretes large amounts of progesterone
What happens to the corpus luteum if fertilisation does not occur?
Degenerates after around 12 days, forming the corpus albicans. Cessation of hormonal output leads to menstruation
What happens to the corpus luteum if pregnancy does occur?
Sustained by HCG produced by conceptus (early embryo), forming CL of pregnancy, then the large corpus albicans
Describe the process of spermatogenesis
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
Differences in gametogenesis between males and females
- f: discontinuous, no stem cells retained
m: continuous from puberty, stem cells retained - f: all primary oocytes present from birth, suspended
partially through meiosis
m: takes around 9 weeks
Differences in male and female gametes
- f: non-motile
m: motile - f: very high cytoplasmic to nuclear ratio
m: low (almost none in sperm head)
Steps of fertilisation
- sperm reaches secondary oocyte
2. sperm and oocyte fuse
What processes occur for the sperm to reach the oocyte?
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)
What is capacitation?
conditioning of the sperm to become more accustomed to the environment (acrosomal head loses glycoprotein coat and becomes more mobile again)
What occurs during the meeting of the sperm and oocyte?
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
What helps the sperm to penetrate the CR and ZP?
acrosomal enzymes
What happens after fertilisation?
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
What does a blastomere look like?
four cleaved cells
What occurs in compaction?
outer cells of morula tighten, forming gap junctions and desmosomes
Structure of blastocyst
inner cell mass - primitive endo and ectoderm
outer (trophoblast) - synctio and cyto extraembryonic mesoderm
