Gametogenesis Flashcards
What happens to germ cells after colonising the gonad?
- proliferate by mitosis
- reshuffle genetically + reduce to haploid by meoisis
- cytodifferentiate into mature gametes
What are the two main functions of meiosis?
- reduce the chromosome number in gamete to 23
- genetic variation in each gamete
What allows for genetic variation in meiosis?
- crossing over
- independent assortment
- random segregation
What is crossing over?
When does this occur?
Exchange of regions of DNA between 2 homologous chromosomes at chiasma
Prophase 1
What is independent assortment?
Random organisation of bivalents along metaphase plate
What is random segregation?
Random distribution of alleles among the 4 gametes
Outline spermatogenesis
- spermatogonia divide by mitosis > Ad spermatogonium + Ap spermatogonium
- Ap spermatogonium produce type B spermatogonia > primary spermatocytes
- primary spermatocytes divide by meiosis > secondary spermatocytes > 4 spermatids
- spermatids differentiate into spermatozoa (spermiogenesis)
What do spermatogonia divide into and how?
- mitosis
- Ad spermatogonium: resting
- Ap spermatogonium: active > maintain stock + continue along spermatogenesis pathway
What do primary spermatocytes divide into and how?
- meiosis
- secondary spermatocytes > 4 spermatids
What is spermiogenesis?
- spermatids differentiate into spermatozoa
- when spermatids are released into lumen of seminiferous tubules + ends when they reach epididymis
Where does spermatogenesis occur?
Seminiferous tubules (Sertoli cells)
What is the spermatogenic cycle?
Time taken for reappearance of the same stage spermatids within a given segment of seminiferous tubule
What is the spermatogenic wave?
Distance between spermatids groups at the same level of maturation
Describe the structure of sperm
- head: contains nucleus (haploid genetic information to fuse with oocyte) | acrosome layer allows his to happen at contact with ovum
- tail: motility for sperm
Why is zinc an important component of semen?
Where is it released from?
Needed for motility of sperm
Prostate
What is sperm capacitation?
- occurs in female reproductive tract
- final maturation step > fertile
- removal of glycoproteins + cholesterol from sperm membrane
- activation of sperm signalling pathways
- allow sperm to bind to zona pellucida of oocyte + initiate acrosome reaction
Where does sperm capacitation occur?
Female reproductive tract
What step is needed to make sperm fertile in IVF?
Incubated in capacitation media
Outline maturation of oocyte before birth
- germ cells in ovary differentiate into oogonia
- oogonia proliferate by mitosis + enter meiosis but stop at prophase 1 > primary oocyte
- primary oocyte are surrounded by follicular cells > primordial follicle
- cells degenerate (atresia)
What are the maturation stages of oocyte during puberty?
Preantral
Antral
Preovulatory
Outline the preantral stage of oocyte maturation
- primordial follicle from before birth
- follicular cells change from flat > cuboidal > stratified cuboidal epithelium of granulosa cells
- granolas cells secrete glycoproteins > forms zona pellucida
- primary follicle formed
Outline the antral stage of oocyte maturation
- fluid filled spaces appear between granulosa cells > forms the antrum
- follicle is called the secondary follicle
- outer fibrous layer develops into theca externa + interna
Outline the preovulatory stage in oocyte maturation
- induced by surge in LH
- follicle completes meiosis I > 2 haploid cells
- secondary oocyte receives most of cytoplasm + polar body receives little
- cell enters meiosis II > only completed if fertilisation occurs
Outline what occurs during ovulation
- FSH + LH stimulate rapid growth of follicles > graafian follicle to 2.5cm
- LH surge increases collagenase activity
- prostaglandins increase response to LH > contractions in ovarian wall
- oocyte breaks from ovary
What forms the corpus luteum?
- Remaining granulosa + theca interna become vascularised
- develop yellow pigment + change to lutein cells
What does the corpus luteum do?
- secrete oestrogens + progesterone
- this stimulates uterine mucosa to enter secretory stage in prep for embryo implantation
- dies after 14 days if no fertilisation occurs > corpus albicans
What happens to corpus luteum if fertilisation doesn’t occur?
- CL degenerates
- forms fibrotic scar tissue - corpus albicans
- decrease in progesterone > menstrual bleeding
What happens to corpus luteum if fertilisation occurs?
- degeneration of CL prevented by human chorionic gonadotropin
- CL grows > corpus luteum graviditatis
- progesterone secreted until 4th month (then placenta takes over)
What prevents the degernation of corpus luteum if fertilisation occurs?
Human chorionic gonadotropin hCG
Secreted by embryo
Outline oocyte transport
- frimbriae ‘catch’ oocyte using sweeping movements
- uterine tubes contract rhymtically
- oocyte moved by peristaltic muscular contractions + cillia
Compare spermatogenesis + oogenesis:
- amount produced
- when does it start?
- when does it end?
- motile gametes?
- where do stages occur?
Spermatogenesis:
- 200 million sperm a day
- 4 spermatids formed: no polar bodies + even cytoplasm division
- starts in puberty
- continues during adult life
-motile gametes
- all stages in testes
Oogenesis:
- 1 ovum per 28 day
- 1 ovum formed: 3 polar bodies + uneven division of cytoplasm
- starts in fetus
- ends at menopause
- non motile gametes
- last stage of meoisis II in oviduct
What is spermiation?
Release of spermatid into lumen of seminiferous tubules
Where do sperm become mobile?
How are they transported before this point?
- epididymis
- Sertoli cell secretions + peristalsis
How many sperm are produced each day?
200 million
When does oogenesis start, continue + complete in women?
- Starts: in utero
- Continues: at ovulation (puberty)
- Completes: at fertilisation
What replenishes the stock of spermatogonia avaliable for the production of sperm?
Type A spermatogonia
Are germ cells diploid or haploid?
Diploid
Karyotype of androgen insensitivity syndrome
46 XY
Karyotype of klinefelters syndrome
47 XXY
Karyotype of Turner’s syndrome
45, XO
Function for rete testis
Helps move sperm from testicle to epididymis
