2) Origins of the Gametes Flashcards
Describe the process by which germ cells form gametes:
Proliferate by mitosis, genetic reshuffling and reduction to haploid in meiosis
What are the functions of meiosis?
Reduce chromosome number to 23
Ensure every gamete is unique
By what processes does genetic variation arise?
Crossing over
Random segregation
Independent assortment
Which part of the gonad do male germ cells colonise?
Seminiferous cords in medulla
Describe the route sperm take in the testis:
Seminiferous tubules -> rete testis -> ductili efferentes -> epididymis
Where do spermatozoa develop?
Seminiferous tubules in association with Sertoli cells
What does the blood testis barrier separate?
And what is its other function?
Surrounding interstitial tissues and germ cells from seminiferous tubules
Stops sperm being recognised as foreign
What cells secrete testosterone and where are they found?
Leydig cells in interstitial tissue
What is the name of the cells that are the ‘raw material’ for spermatogenesis?
Spermatogonia
What two cells do spermatogonium divide into?
Ad spermatogonium (maintain stock) Ap spermatogonium (primary spermatocytes)
Describe the formation of sperm from primary spermatocytes:
Primary spermatocytes become secondary spermatocytes and then spermatids by meiosis. Spermatids differentiate into sperm
What is the spermatogenic cycle?
Time taken for re-appearance of same stage within a given segment of tubule (16 days)
What is a spermatogenic wave?
Distance between same stage in a tubule
What is spermiogenesis?
Remodelling of spermatids and gaining of motility as they pass to epididymis
What allows spermatids to travel to the epididymis?
Sertoli cell secretions and peristaltic contractions
What secretions make up semen?
Seminal vesicle
Prostate gland
Sperm (via vas deferens)
Bulbourethral gland
What are the seminal vesicle secretions for?
AA, citrate, fructose and prostaglandins
Energy source
What are the bulbourethral gland secretions for?
Mucoproteins help lubricate and neutralise acidic urine in distal urethra
What is the final maturation step for sperm?
Sperm capacitation
What occurs in sperm capacitation?
Conditions in female genital tract stimulate:
Removal of glycoproteins and cholesterol from sperm membrane
Activation of sperm signalling pathways
What does sperm capacitation allow?
Allows sperm to bind to zona pellucida of oocyte and initiate acrosome reaction
Describe the colonisation by female germ cells:
Colonise gonadal cortex and differentiate into oogonia
What stage are primary oocytes in?
Prophase I
How are primary oocytes arranged?
Surrounded by flat epithelia cells called follicular cells forming primordial follicle
What could be a reason that there is increasing risk of fetal chromosomal abnormalities in older pregnant women?
Primary oocytes remain in resting stage for many years, increasing chance of cell damage
Why are there so few oocytes when puberty is reached?
Undergo atresia in late gestation and childhood
In what stages do oocytes mature?
Pre-antral, antral and pre-ovulatory
What changes occur to follicular cells in the pre-antral stage?
Change from flat to cuboidal before proliferating to produce stratified epithelium of granulosa cells
What layers form around the oocyte in the pre-antral stage?
Zona pellucida, granulosa cells, inner and outer theca folliculi
What are the granulosa cells surrounding the oocyte called?
Cumulus oophorus
What occurs in the antral stage?
What does this development depend upon?
Fluid filled spaces appear in between granulosa cells to from an antrum
Development depends upon FSH binding to granulosa cells and LH binding to theca cells
What occurs in the pre-ovulatory stage?
Oocyte completes meiosis I forming secondary oocyte (most of cytoplasm) and polar body. Cells enters meiosis II but arrests at metaphase
When is meiosis II completed?
If oocyte is fertilised
What causes oocyte release?
LH surge increasing collagenase activity and prostaglandins amplifying response and causing local contractions in ovarian wall
Describe the formation of the corpus luteum:
Remaining granulosa and theca interna cells become vascularised, develop yellow pigment and change to lutein cells
What does the corpus luteum secrete and what effect does this have?
Progesterone and oestrogen under influence of LH
Stimulates uterine mucosa to enter secretory stage in preparation for embryo implantation
What causes the oocyte to move into the fallopian tube?
Fimbriae, uterine tube contracting and cilia
What happens if there is no fertilisation?
After 14 days, corpus luteum degenerate forming corpus albicans
Decreased progesterone = menstrual bleeding
What happens to corpus luteum if fertilisation occurs?
Degeneration prevented by human chorionic gonadotropin secreted by embryo
Forms corpus luteum graviditatis
What happens to progesterone secretion if fertilisation occurs?
Cells of CL secrete progesterone until 4th month where placental secretion becomes adequate
What is the effect of FSH in women?
Stimulate follicle growth and maturation
What is the effect of LH in women?
Stimulate follicle maturation, ovulation due to surge and promotes corpus luteum development
When is the follicular stage of the ovarian cycle?
0-14 days
What is the luteal stage of the ovarian cycle?
14-28 days
