Reproduction 12 Flashcards
Describe female germ cells
Primordial germ cells- capable of infinite mitosis, first visible in week 3, expanded by mitosis. In the absense of SRY, female gonads devleop
Ovarian function is to porduce oocytes and hormones
Oogonia- mitosis
Primary oocytes- 1st meiotic division Arrest before birth
Secondary oocytes- 2nd meiotic division Primary oocytes arrest in prophase 1 and enter a prolonged restoring state Meiosis leads to the formation of polar bodies (by products) that are discarded
1st polar body has twice as much DNA then the 2nd. Presence of polar body signifies sperm entry and completion of 2nd meiotic division
Describe folliculogenesis
2 main types of follicular cells: Granulosa cells (produce oestrogens and progestagens) and Theca cells (produce angrogens and progestagens)
A primordial follicle is a primary oocyte surrounded by a single layer of flattened granulosa cells
The theca amd zona pellucida become visible
Secondary follicle- granulosa cells proliferate and become 3-6 cells deep and secrete follicular fluid, theca forms two distinct layers- 10-15 secondary follicles rescued per cycle by FSH
Graafian follicle: one dominant follicle per cycle, egg surrounded by granulosa cells and attached by the cumulus oophorus
A few primordial germ cells matured each day, hormone independent
Primordial follicle -> Primary follicle -> secondary follicle -> early antral follicle -> graafian follicle
Describe atresia
The degeneration and reabsorption of follicles before they reach maturity
Earliest signs of apoptosis in Graafian follicles are condensation of chromosomes, wrinkling of nuclear envelope and oocyte free-floating in follicular fluid
Describe the formation of the zona pellucida (ZP)
Glycoprotein layer
Islands of material are produced by the oocyte between granulosa cells and fuse together, granulosa cell processes transverse ZP and provide the egg with nutrition; lactate and pyruvate
Following ovulation the egg continues to be surrounded by the ZP and cumulus cells (corona radiata)
Describe extra-ovarian hormone actions
Hypothalamus- pulsatile release of GnRH Anterior pituitary- FSH- acts on ovary and stimulates development of follicles LH- acts on ovary and stimulates follicle maturatuamd development of the corpus luteum Both stimulate secretion of oestradiol and ovulation
Describe ovarian hormones
Oestrogens- (17beta oestradiol)- secondary sexual characteristics, follicle maturation, preparation of the endometrium for pregnancy and thinning of cervical mucous
Progesterone- completes the preparation of the endometrium for pregnancy and stimulates the development of mamary glands (breasts)
Describe the menstrual cycles
Follicular phase- follicles mature, endometrium proliferates, oocyte released
Luteal phase- corpus luteum, endometrium prepared for blastocyst implantation
Menses- if no pregnancy. Start of cycle, endometrium is lost, the basal layer is left behind following mestruation. Endometrium grows again during cycle
Describe the two-cell hypothesis of oestrogen production
LH stimulates theca cells to produce androgens,
FSH stimulates granulosa cells to convert the androgens to oestrogens via aromatase
Oestrogens suppresses FSH and LH production by the anterior pituitary by negative feedbacks
Follicular phase
Hypothalamus gives pultatile GnRH release
Causes anterior pituitary secreting FSH and LH
Up to 15 follicles are rescued by FSH and start to mature (oestrogen is produced by the developing follicle)
Describe stimulation of ovulation
Dominant follicle has the highest FSH receptor density,
granulosa cells in the dominant follicle express LH receptors,
high oestrogen at mid cycle stimulate the hypothalamus to release GnRH via positive feedback which causes the LH surge and FSH spike.
LH surge is critical for oocyte to be released
Describe the process of ovulation
Increase in number granulosa cells and accumulation of follicular fluid,
cumulus oophorus loosens,
follicle wall weakens,
protease produced,
increased osmotic pressure
stigma forms and then ovulation (FSH and LH levels fall)
Describe the luteal phase
Formation of the corpus luteum is stimulated by the LH surge
Fibrin clot forms in ruptured follicle Granulosa forms large lutein cells and the theca cells form the small lutein cells
LH maintains the corpus luteum,
LH also stimulates progesterone and oestrogen that maintains the endometrium and limits new follicular growth
If there is no hCG secreted by an implanting blastocyst the corpus luteum degenerates forming the corpus albicans so progesterone and oestrogens levels fall and the cycle recommences
If pregnancy occurs hCG acts like LH to maintain the corpus luteum to produce progesterone to support pregnancy until the placenta takes over
