Game to Genesis 1

8/6/2019 Game to Genesis 1

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Gametogenesis


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GametogenesisDevelopment begins with the fertilization, the process by which maleand female gamate unite to form zygote.The sperm and oocyte, the male and female gametes, are highly specialized sex cells . Each of these cells contains half the number of chromosomes (haploid number)Gametes are derived from primordial germ cells (PGCs) that areformed in the epiblast during the second week and that move to thewall of the yolk sac.During the fourth week, these cells begin to migrate from the yolksac toward the developing gonads, where they arrive by the end of the fifth week.In preparation for fertilization, germ cells undergo gametogenesisGametogenesis is the process of conversion of germ cells intospecialized cells called gametes (in other words, gametogenesis isthe process of formation and development of specialized generativecells, gametes )Gamete maturation in males is called spermatogenesis and infemales is called oogenesis


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S permatogenesis

Germ cells after reaching the developing gonads remaindormant until puberty.Just before puberty, the germ cells which have beenremain dormant since fetal period develop into

spermatogoniaThe process of transformation of spermatogonia intomature sperm is called the spermatogenesis .In humans, the time required for a spermatogonium todevelop into a mature sperm is approximately 64 days


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S permiogensisThe process of transformation of a circular spermatid to a sperm is known asspermiogenesis and the changes include Condensation of nucleus into head. Golgi complex develop into acrosomal cap. Formation of neck Formation of middle piece, principle piece and end piece of tail. S hedding of most cytoplasm.


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S tructure of mature sperm

Consists of head, neck and tail.Head Contains haploid nucleus. Nucleus covered with

acrosome.Neck Constriction between head

and tail.Tail consists of three segment Middle piece (covered with

mitochondria) Principle piece Tail piece.


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O ogenesis

O ogenesis is the process of transformation of oogonia into mature oocyte.This maturation process begins before birth andis completed after puberty.O ogonia are derived from germ cells.All the oogonia to be used throughout life are

produced before birth ( N o oogonia form after birth in females in contrast to the continuousproduction of spermatogonium in males.)


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23 X

46 XX

Germ cell O ogonium

46 XX

46 XX

23 X

Primary oocyte

23X

S econdary oocyte

Enlarge/maturation

1 st meiotic div.

23 X

O vum

2nd meiotic div.

1 st polar body

2nd polar body

Before birth

During puberty

C ontd..


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Prenatal growth of oocyte

O nce the germ cells have arrived in the gonad of a genetic female, they differentiate into oogonia

A B


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O ogonia enlarge to form primary oocytes before birthAs a primary oocyte forms, the connective tissue cells around it becomeflattened and surround the oocyte.A primary oocyte, together with its surrounding flat epithelial cells, is knownas a primordial follicleNear the time of birth, primary oocytes have entered prophase of meiosis Ibut instead of proceeding into metaphase remain arrested in prophaseand do not finish their first meiotic division before puberty is reachedThis arrested state is produced by oocyte maturation inhibition (OMI)secreted by the follicular cells surrounding the primary oocyte

DC


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Postnatal growth of oocyteAt puberty, 15-20 primordial follicles begin to mature passing throughdifferent successive stages but only one reaches full maturity while the

others degenarate.As the primary oocyte begin to grow, surrounding flat follicular cells changesfrom flat to cuboidal and then proliferate to produce a stratified epitheliumand the unit is called a primary follicle. The follicular cells may now becalled granulosa cellsgranulosa cells and the oocyte secrete a layer of glycoproteins on thesurface of the oocyte, called zona pellucida

A B

Growing follicle

Primary follicle


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DC

S econdary follicle

As development continues, fluid-filled spaces appear among thegranulosa cells. Coalescence of these spaces forms the antrumThe wall of the granulosa cells become thin, the oocyte lieeccentrically and the follicle is termed a secondary

/vesicular/graafian/ovarian follicle .The eccentric oocyte is surrounded by some granulosa cells andthat are given the name cumulus oophorusThe connective tissue cells around the follicle condensed to formcapsule called theca granulosa/folliculi which soon differentiatesinto two layers, an internal vascular and glandular layer, the thecainterna , and a fibrous capsule-like layer, the theca externa


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S hortly before ovulation when the secondary follicle ismatured, primary oocyte completes its first meioticdivision to give rise to two daughter cells of unequal size,a secondary oocyte and the first polar body, eachwith 23 chromosomes .Unlike the corresponding stage of spermatogenesis,however, the division of cytoplasm is unequal. Thesecondary oocyte receives almost all the cytoplasmand the first polar body receives very little.The polar body is a small, nonfunctional cell that soondegenerates, lies between the zona pellucida and thecell membrane of secondary oocyte called perivitellinespace


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At ovulation, the nucleus of the secondaryoocyte begins the second meiotic division butarrest in metaphaseMeiosis II is completed only if the oocyte isfertilized; and most cytoplasm is again retainedby one cell, the fertilized oocyte .The other cell, the second polar body , also asmall nonfunctional cell, soon degenerates.If the fertilization is failed to occur then the cell

degenerates approximately 24 hours after ovulation.


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Comparison of gametsThe oocyte is a massive cell compared with the spermand is immotile whereas the microscopic sperm is highlymotileO ne primary spermatocyte gives rise to four spematozoa

where as one primary oocyte gives only one ovumWith respect to sex chromosome constitution, there aretwo kinds of normal sperm : 23, X and 23, Y whereasthere is only one kind of normal secondary oocyte : 23, XWhen primary spermatocyte divides, its cytoplam isequally distributed between the two secondaryspermatocytes whereas when primary oocyte divides,almost all of its cytoplasm goes to the secondary oocyteand very little to the 1 st polar body


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O vulationThe shedding of the ovum from the ovary is calledovulationAround mid cycle, the ovarian follicle, under theinfluence of F S H and LH, enlarges producing a swellingor bulge on the surface of the ovary.A small avascular spot, the stigma , soon appears at theapex of this swellingProstaglandin levels increase in response to the LH andcause contractions of smooth muscle in the thecaexterna and in the ovarian wall.O wing to those contractions, the stigma soon ruptures,

expelling the secondary oocyte with the follicular fluid .The expelled secondary oocyte is surrounded by thezona pellucida and one or more layers of cumulusoophorus cells, which are radially arranged as thecorona radiata


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S tructure of ovum at the time of ovulationThe ovum that is shed from the ovary is not fully mature but is

exactly the secondary oocyte which is likely undergoing 2nd

meiosisMeiosis completes only when fertilization takes place.


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Transport of oocyteThe secondary oocyte is expelled at ovulation from the ovarian follicle withthe escaping follicular fluidDuring ovulation, the fimbriated end of the uterine tube becomes closelyapplied to the ovary. The fingerlike processes of the tube, fimbriae , moveback and forth over the ovaryThe sweeping action of the fimbriae, by motion of cilia on the epithelial liningand fluid currents produced by the fimbriae "sweep" the secondary oocyteinto the funnel-shaped infundibulum of the uterine tube.The oocyte passes into the ampulla of the tube, mainly as the result of

peristalsis -movements of the wall of the tube characterized by alternatecontraction and relaxation-that pass toward the uterus. The movement of cilia also helps to propel the oocyte forward


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Corpus luteumAfter ovulation, granulosa cells remaining

in the wall of the ruptured follicle, together with cells from the theca interna, collapseand are thrown into folds.They become vascularized bysurrounding vessels and develop intoglandular structure called corpus luteumCorpus luteum secrete progesterone

Progesterone, together with estrogenichormones, causes the uterine mucosa toenter the secretory stage in preparationfor implantation of the embryoUnder the influence of LH, these cellsdevelop a yellowish pigment in their cytoplasm called lutein and and now arecalled luteal cellsThis yellow pigment gives the abovementioned structure yellow colour. That iswhy it is called corpus luteum (= yellowbody)


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If the oocyte is not fertilized, The corpus luteum persists for about 14 days. During this periodit secretes progesterone and is called corpus luteum of

menstruation There after it degenerates and is subsequently transformed into

white scar tissue in the ovary called corpus albicans (=whitebody)

With degenaration of corpus luteum, progesterone productiondecreases which causes uterine mucosa to shed off (menstrualphase)

If the oocyte is fertilized, the corpus luteum enlarges to form a corpus luteum of

pregnancy and increases its hormone production for three tofour months.

During these early 3 to 4 month, degeneration of the corpusluteum is prevented by human chorionic gonadotropin, ahormone secreted by the syncytiotrophoblast of the blastocyst

After 4 month, progesterone is secreted by placenta, so there isno need of corpus luteum.

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Game to Genesis 1