Oogenesis and the menstrual cycle

what does oogenesis refer to?

development of an immature oogonium into a secondary oocyte, which is ready to be fertilized

when does oogenesis begin?

begins during the embryonic development of the female

what does diploid mean?

two copies of each chromosome in each cell

which human cells are NOT 2n?

gametes

what are the x-shaped chromosomes comprised of?

two sister chromatids

using n and c, what is the amount of DNA in a cell before replication?

2n2c

using n and c, what is the amount of DNA in a cell after replication?

2n4c

using n and c, what is the amount of DNA in a cell after cell division?

2n2c

what happens during meiosis I?

one 2n2c mother cell replicates its DNA to become 2n4c and during telophase it divides, forming two 1n2c daughter cells

what happens during meiosis II?

the two 1n2c cells remain 1n2c until telophase, then they form four 1n1c daughter cells

what happens to the four daughter cells in oogenesis?

* one develop into a mature gamete → the one that gets most of the cytoplasm
* the remaining receive barely any cytoplasm and are called polar bodies → they will degenerate

when and where does the primordial germ cells migrate to?

* during fetal period of the female
* migrate to the primordial gonads

what happens to the primordial germ cells in the primordial gonads?

they differentiate into oogonia

what happens to the oogonia between week 4 to month 7 of gestation?

they replicate by mitosis

what are the oogonia that start the process of meiosis I called?

primary oocytes

in which phase are the primary oocytes arrested in?

prophase I

what has happened to the oogonia after month 7 of gestation?

either they have matured into primary occytes or died

how long are the primary oocytes arrested in prophase I?

until the time when ovulation nears

what causes the primary oocyte to finish meiosis I? what does it become when finishing meiosis I?

LH surge ca. one day before ovulation

becomes a secondary oocyte

in which phase is the secondary oocyte arrested in? when is it arrested?

in metaphase, ca 3 hours before ovulation

what happens if the secondary oocyte is fertilized?

then the secondary oocyte will complete meiosis II, and it will become a mature ovum

what happens if the secondary oocyte isn’t fertilized?

it will degenerate, never finish meiosis II

describe the primordial follicle

flat epithelial cells surround each oogonium

where does the follicular cells originate from?

from the surface epithelium of the ovary

which cells arrest the primary oocyte in prophase of meiosis I?

follicular cells

describe the primary follicle

* flat follicular cells → cuboidal granulosa cells
* basement membrane separates granulosa cells from ovarian stroma
* cells from ovarian stroma form the thecal layer outside of the BM
* zona pellucida

what forms the zona pellucida?

granulosa cell and the oocyte secrete glycoproteins onto the surface of the oocyte

when does primary follicles mature into primordial follicles?

during puberty

what is the antral follicle characterised by?

by formation of an antrum

what does the granulosa cells which surround the oocyte form?

cumulus oophorus

how many primary follicles begin to mature every menstrual cycle?

15-20

describe the graafian follicle

* mature vesicular follicle
* 25mm in diameter

when does the antral follicle mature into the graafian follicle?

ca 37 hours before ovulation

what is the 1st menstrual cycle called?

menarche

how long does one menstrual cycle last ca?

28 days (21-35 days)

what marks the start of the menstrual cycle?

menstrual bleeding

which two cycles are the menstrual cycle comprised of?

the ovarian cycle and the endometrial (or uterine) cycle

which parts of the brain regulate the secretion of the hormones that regulate the menstrual cycle?

the hypothalamus and the anterior lobe of the pituitary gland

what is the function of the hypothalamus in regards to the menstrual cycle hormones?

it instructs the ant. pituitary which hormones to produce and when

what is the function of the anterior pituitary in regards to menstrual cycle hormones?

synthesises and release the hormones into the blood

what do we call the cooperation between the hypothalamus, the ant. pituitary and the ovaries?

the hypothalamic-pituitary-gonadal axis

what is the concept the hypothalamic-pituitary-gonadal axis is based on?

the hypothalamus produces a hormone called gonadotropin-releasing hormone, which stimulate the ant. pituitary to release gonadotropins

what are the names of the gonadotropins?

follicle-stimulating hormone and luteinzing hormone

what does FSH do?

stimulate the maturation of the ovarian follicles

describe the mechanism by which the maturing follicles produce estrogen

* the theca interna cells of the follicle produce androgens like testosterone and andreostenedione
* these male sex hormones will then travel to the granulosa cells which converts them into female sex hormones like oesterone and oestradiol by the help of the enzyme aromatase

what is the effect of the LH surge?

* causes oocyte to complete meiosis I and initiate meiosis II
* stimulate production of progesterone by follicular stromal cells
* stimulates collagenase
* increase local prostaglandin levels in the ovary, causing local muscle contractions in the ovarian wall → extrudes oocyte of graafian follicle → ovulation

what is the function of collagenase?

digest collagen fibers surrounding the follicle

descibe the follicular phase of the ovarian cycle

* takes place in the first half of the menstrual cycle
* FSH stimulate development of several follicles of the ovaries
* ca. 15-20 primary follicles will mature into antral follicles, one will mature into a graafian follicle
* other follicles degenerate and become atretic

what happens to the atretic follicles?

they will degenerate and be replaced by CT, formin a corpus atreticum

describe the menstrual (or desquamation) phase of the endometrial cycle

* corpus luteum degenerates as no pregnancy occured
* progresterone level decrease → induce vasospasms in the uterine spiral arteries, which causes the compact and spongy layers of the endometrium to die due to being deprived of blood
* the dead layers of the endometrium leave the body through the vagina along with blood, causing the menstrual bleeding
* only basal layer of endometrium remains

describe the proliferation phase of the endometrial cycle

the estrogens produced by the ovarian follicles stimulates proliferation of the basal layer of the endometrium

the proliferation eventually forms new spongy and compact layers

what does the granulosa cells comprizing the cumulus oophorus form? what is the function of the formed structure?

corona radiata

* functions as a barrier which spermatozoa must penetrate

describe the luteal phase of the ovarian cycle

* after ovulation the remains of the graafian follicle develops into the corpus luteum under the influence of LH
* corpus luteum produce progresterone, which aids pregnancy
* if pregnancy occurs: syncytiotrophoblast of embryo will produce hCG → keeps corpus luteum alive → progesterone is produced → corpeus lutein will grow into a corpus luteum of pregnancy
* no pregnancy: corpus luteum will degenerate due to lack of stimulation from hCG, degenerates into a mass of fibrotic scar tissue called corpus albicans

describe the secretory phase of the endometrial cycle

* progresterone produced by the corpus luteum causes the new functional layer of the endometrium to change in preparation of implantation by fertilized egg
* the uterine spiral arteries grow deeper
* endometrium start to produce glycogen-rich secretions