meiosis ii does not take place in the oocyte until ovulation.
meiosis ii does not take place in the oocyte until ovulation.
meiosis ii does not take place in the oocyte until?
ovulation
after migration primordial germ cells will become oogonia in the forming gonad (ovary). the oogonia proliferate extensively by mitotic divisions, up to 5-7 million cells in humans. but then many of these oogonia die and about 50,000 remain. these cells differentiate into primary oocytes. in week 11-12 "post coitus" the first meiotic division begins (before birth for most mammals) and remains arrested in prophase i from a few days to many years depending on the species. it is in this period or in some cases at the beginning of sexual maturity that the primary oocytes secrete proteins to form a coat called zona pellucida and they also produce cortical granules containing enzymes and proteins needed for fertilization. meiosis stands by because of the follicular granulosa cells that send inhibitory signals through gap junctions and the zona pellucida. sexual maturation is the beginning of periodic ovulation. ovulation is the regular release of one oocyte from the ovary into the reproductive tract and is preceded by follicular growth. a few follicle cells are stimulated to grow but only one oocyte is ovulated. a primordial follicle consists of an epithelial layer of follicular granulosa cells enclosing an oocyte. the pituitary gland secrete follicle-stimulating hormones (fshs) that stimulate follicular growth and oocyte maturation. the thecal cells around each follicle secrete estrogen. this hormone stimulates the production of fsh receptors on the follicular granulosa cells and has at the same time a negative feedback on fsh secretion. this results in a competition between the follicles and only the follicle with the most fsh receptors survives and is ovulated. meiotic division i goes on in the ovulated oocyte stimulated by luteinizing hormones (lhs) produced by the pituitary gland. fsh and lh block the gap junctions between follicle cells and the oocyte therefore inhibiting communication between them. most follicular granulosa cells stay around the oocyte and so form the cumulus layer. large non-mammalian oocytes accumulate egg yolk, glycogen, lipids, ribosomes, and the mrna needed for protein synthesis during early embryonic growth. these intensive rna biosynthese are mirrored in the structure of the chromosomes, which decondense and form lateral loops giving them a lampbrush appearance (see lampbrush chromosome). oocyte maturation is the following phase of oocyte development. it occurs at sexual maturity when hormones stimulate the oocyte to complete meiotic division i. the meiotic division i produces 2 cells differing in size: a small polar body and a large secondary oocyte. the secondary oocyte undergoes meiotic division ii and that results in the formation of a second small polar body and a large mature egg, both being haploid cells. the polar bodies degenerate. oocyte maturation stands by at metaphase ii in most vertebrates. during ovulation, the arrested secondary oocyte leaves the ovary and matures rapidly into an egg ready for fertilization. fertilization will cause the egg to complete meiosis ii. in human females there is proliferation of the oogonia in the fetus, meiosis starts then before birth and stands by at meiotic division i up to 50 years, ovulation begins at puberty.
germ cell
interphase is followed by meiosis i and then meiosis ii. meiosis i separates replicated homologous chromosomes, each still made up of two sister chromatids, into two daughter cells, thus reducing the chromosome number by half. during meiosis ii, sister chromatids decouple and the resultant daughter chromosomes are segregated into four daughter cells. for diploid organisms, the daughter cells resulting from meiosis are haploid and contain only one copy of each chromosome. in some species, cells enter a resting phase known as interkinesis between meiosis i and meiosis ii.
meiosis