3- Folliculogenesis Flashcards
describe the formation of primordial follicles during foetal development
primordial germ cells (PGCs) from the yolk sac multiply by mitosis, invade the genital ridges. they differentiate into male/female gametes depending on SRY expression and the sexual differentiation of the gonads
in females, PGCs enter ovaries and become oogonia – diploid precursors to oocytes.
oogonia multiply by mitosis, then enter meiosis as primary oocytes. meiosis begins and arrests at a certain stage.
primary oocytes remain on the outer layer of the ovary in meiotic arrest for many years until puberty – become surrounded by protective layers.
primitive sex cords invade the genital ridges and surround the follicles as a single cell layer, differentiate into granulosa cells. granulosa cells secrete the basal lamina – a basement membrane layer that keeps everything intact.
this entire structure – oocyte surrounded by a single granulosa cell layer and secreted basal lamina = primordial follicle.
what secretes the basal lamina?
granulosa cells
what is the purpose of the basal lamina?
protective, acellular basement membrane layer
what is mitosis?
cell division resulting in two identical daughter cells – same number of chromosomes as parent cell
what are the 6 stages of mitosis?
interphase, prophase, metaphase, anaphase, telophase, cytokinesis
what is meiosis?
cell division that reduces chromosome number by half - occurs in two stages called meiosis I and II
in the first division the chromosome number is reduced. in the second division the replicated chromatids of each chromosome are separated. we go from one diploid cell to four haploid cells.
describe oogenesis during its stages of mitosis and meiosis
oogonia undergo mitosis – produce many identical oogonia daughter cells. once these oogonia care committed to meiosis = primary oocytes.
unequal division of cytoplasm during meiotic cytokinesis.
primary oocyte undergoes meiosis I during foetal development – divides and produces two non-identical daughter cells.
daughter cell with the majority of the cytoplasm = secondary oocyte. contains the majority of the nutrients and organelles needed for survival.
daughter cell with the least cytoplasm – 1st polar body – is non-functional and will degenerate.
secondary oocyte arrested in meiosis II until fertilisation – can complete meiosis II once fertilised.
divides into two non-identical daughter cells. daughter cell with the majority of the cytoplasm = mature ovum – has the majority of cytoplasm and organelles needed for embryonic development.
other daughter cell is the 2nd polar body.
atresia
degenerative process for ovarian follicles
what are the two stages of follicular development?
pre-antral and antral
describe the development of pre-antral follicles during puberty
females are born with a pool of primordial follicles – contain oocytes in meiotic (I) arrest, follicles aren’t growing
after puberty a cohort of primordial follicles from the resting pool initiate growth.
granulosa cells multiply, oocyte grows despite being in meiotic arrest, zona pellucida is secreted by the oocyte around it as a protective layer and marker for growth.
with the growth of the follicle, a second cell layer condenses around the follicles and differentiates into theca cells, developing a blood supply. these are pre-antral follicles.
describe the follicular development of antral follicles
follicle continues growing, granulosa cells continue multiplying – gaps form in the granulosa cell layers which are filled with follicular fluid.
follicular fluid is the exudate from blood filtration from the theca cells, which travels through follicle layers and picks up other substances before filling in the gaps between granulosa cells.
the gaps as fluid-filled spaces form together to form an antrum – follicles with antrum are called antral/ secondary follicles.
granulosa cells are pushed to the side as the antrum enlarges – eventually the egg sits on the edge of the follicle by a tiny stalk of cells.
describe the follicular development of a pre-ovulatory follicle
granulosa cells change into cumulus cells, and mural granulosa cells form.
theca cells divide into theca interna and theca externa (facing adjacent and away respectively from the basal lamina).
follicular fluid continues building up from theca cell layer blood supply.
antral follicles become Graafian/ pre-ovulatory follicles.
what factors drive the initiation of folliculogenesis?
FSH drives most of folliculogenesis but early growth is independent of FSH – seems to be drive by local factors from within the ovary and adjacent follicles.
primordial follicle pool is in a constant state of inhibition – inhibition needs to be turned off by activating factors for follicular growth.
describe the three stages of folliculogenesis
follicle initiation – the cohort of follicles leave the resting pool of follicles, grow continuously. undergo pre-antral and antral stage. dependent on FSH.
follicle recruitment – follicles at the right stage of development at the right time, correspond with an FSH rise and continue to grow.
dominant follicle selection – from the group of recruited antral follicles, one is selected for ovulation. the others die.
why is theca vasculature important?
circulating hormones such as FSH and LH can reach the follicle
steroids produced by the follicle can enter the bloodstream
