gametogenesis Flashcards

Question

explain the first stage of oogenesis in the embryo how does this align with follicle development?

Answer 1

In the embryo - primordial germ cells differentiate into oogonia (equivalent of spermatogonia). These oogonia undergo mitotic divisions to make more of them (still in the embryo) By the end of the third month of foetal development, these oogonia differentiate further and enter meiosis I. now they are primary oocytes, arrested in PROPHASE I. (born with 1-2 million primary oocytes, by puberty this has declined to around 400,000, and over the lifetime roughly 400 of these will fully mature and be released) FOLLICLE - Oogonium undergoing mitotic divisions -IN the primordial follicle. This is before birth. From puberty onwards, a primordial follicle will begin the develop into a ‘preantral follicle’

Answer 2

meiosis resumes and the oocyte fully develops… Each month a few follicles (each containing one oocyte) are recruited and begin to develop, one will become doinant and complete meiosis I. in this first asymmetric division, the genetic material halves as normal, but the majority of resources and cytoplasm go with just one of these halves, producing one secondary oocyte and then the ‘first polar body’ which is mostly genetic material and gets lost/degraded. The second asymmetric division (MII) - genetic material in the secondary oocyte is split again, but like division I, produces a second polar body with half this genetic material, and a fully mature ovum with the majority of cytoplasm etc… I THINK this is triggered by the LH surge so is in the ANTRAL follicle, the last stage we've got written for follicle development

Answer 3

perm binding is required to trigger completion of 2nd meiotic division (so you only get the 2nd polar body if fertilisation occurs. otherwise , while MII is initiated in an egg being released, it stops at metaphase II (until fertilisation)

Answer 4

the primordial follicle, in embryo before birth, it is at this point that the oogonia are undergoing mitotic divisions. and MI? Primordial follicles are continuously and gradually recruited to begin growth this happens independently of menstrual cycles and is ongoing. These develop through the following stages: Primary follicle Secondary (preantral) follicle Early antral follicle from puberty onwards, a few primordial follicles will begin to develop into the antral follicle... one becoms dominant, Graffian

Answer 5

growth of the follicle (20 um in diameter to ~200-400 um) The primary oocyte finishing its own growth (60-120 um, still arrested in prophase I) Large amounts of mRNA and rRNA produced to build organelles and generate protein stores Oocyte secretes glycoproteins which condense to form the zona pellucida - protective, needed once oocyte leaves follicle, holds the oocyte and early embryo intact, important in fertilisation Granulosa cells proliferate to form thick layer around oocyte; contact between granulosa cells and oocyte is maintained through cytoplasmic processes (just like sertoli cells in spermatogenesis) Ovarian stromal cells condense to form thecal layer; separated from granulosa layer by membrana propria

Answer 6

Thecal layer expands to form two layers (not important) theca interna vs externa Granulosa cells secrete fluid, creating fluid filled cavities - antrum. This increases follicle size The granulosa layer changes so that the oocytes is suspended in follicular fluid by a stalk of (cumulus) granulosa cells, connecting it the the (mural) granulosa cells Oocyte continues to synthesize RNA and make proteins

Answer 7

granulosa cells, like sertoli cells, are connected to the oocyte through cytoplasmic processes Gap junctions form between granulosa cells and at the oocyte surface - this is all required for transfer of amino acids and nucleotides to the growing oocyte which does not have a blood supply

Answer 8

Very early development of the PRIMORDIAL follicle - is stimulated by local growth factors and cytokines, and then a few follicles recommence growth every day… However - if the follicle is to continue developing, it required both FSH and LH from the pituitary (FSH-knockout mice arrest follicular development at the preantral stage; LH-knockout mice stop at the antral phase) if not you get atresia (cell death of the follicle) NOTE - important to consider for next bit, but during the follicular phase, thecal cells bind LH and granulosa to FSH, but the expression of receptors changes (granulosa cells later express LH receptors)

Answer 9

Thecal cells - stimulated by LH to produce androgens - testosterone and androstenedione,3 and progestogens Granulosa cells - stimulated by FSH to produce oestrogens (oestradiol 17β and oestrone), by converting some of the androgens produced by the thecal cells, via enzyme aromatase, ,whose expression is triggered by these androgens^^^ LATER PRODUCE PROGESTOGENS, SWITCH TO LH

Answer 10

we know they exert negative at the hypothalamus and pituitary, but actually in a way they increase their own expression because these steroid hormones being produced by the antral follicles stimulate proliferation - so more cells grow, produce more hormones etc…

Answer 11

Primordial follicles begin developing months before ovulation, progressing through preantral and antral stages. In each cycle, antral follicles are recruited by FSH, and one becomes dominant. Just before ovulation, the oocyte in the Graafian follicle completes meiosis I, enters MII and is ovulated as a secondary oocyte arrested in metaphase II

Answer 12

GnRH is rising, secreted by the hypothalamus Acts at ant. pituitary to stimulate LH and FSH release Rise in FSH stimulates follicle maturation, and the preantral follicle produces oestrogen (from granulosa cells) At LOW concentrations, this oestrogen exhibits negative feedback on hypothalamus and pituitary (so LH and FSH are prevented from rising higher as the follicles mature). Remember - the steroid hormones being produced will also be causing proliferation of these somatic cells, so oestrogen continues to rise…

Answer 13

Oestrogen levels reach a critical value, at which it exhibits positive feedback instead, causing a surge in LH (and little peak for FSH), which triggers ovulation of one egg from the most mature follicle: Nuclear membrane breaks down and meiosis resumes up to metaphase II. Half of the chromosomes but majority of cytoplasm go to one cell (the secondary oocyte); rest is the first polar body (dies) Cytoplasmic maturation occurs (synthesis of specific set of proteins, reorganisation of microtubules) (all stuff said previously in oogenesis description) Follicle ruptures and oocyte is carried out in follicular fluid

Answer 14

follicle is now a the highly vascularised corpus luteum. Large lutein cells (which were the granulosa cells) produce progestogens; small lutein cells (which were the thecal cells) produce progesterone and androgens Progesterone - most important. Granulosa cells now produce progesterone under LH stimulation (as said, LH surge coincides with granulosa cells starting to express LH receptors) . Forms positive feedback loop where granulosa cells are then stimulated by their own progesterone resulting in exponential increase in progesterone release Progesterone (and inhibin) exhibit negative feedback on the pituitary AND hypothalamus (AND ENHANCE negative feedback of estradiol, also blocking its positive feedback) to reduce FSH and LH - don't want more follicles to be recruited to maturation as the body is preparing for a pregnancy

Answer 15

inhibin and oxytocin: important in maintaining the corpus luteum, and for luteolysis (its degeneration) when the time comes Prolactin, oestrogen and progesterone note - corpus luteum undergoes luteolysis after 12-14 days

Answer 16

Its primary function is to increase endometrial growth and prepare the uterus for implantation. However if the corpus luteum degenerates and no implantation has occurred, its hormones drop, progesterone (and oestrogen) no longer playing an inhibitory role or maintaining lining, period, rise in GnRH = new cycle

gametogenesis Flashcards

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