What is Oogenesis and Ovulation ?

OOGENESIS

The process of formation of a mature female gamete (ovum) is called Oogenesis. It occurs in the ovaries (female gonads). it consists of three phases: multiplication, growth and maturation

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(a) Mul­ti­pli­ca­tion Phase : 

In the fetal devel­op­ment Cer­tain Cells in ger­mi­nal epithe­li­um of the ovary of the foe­tus a larg­er than others.

The cells divide by mito­sis pro­duc­ing a cou­ple of mil­lion in moth­er cells or gonia in each ovary of the foetus.No more are formed or added after birth.

The Oogo­nia mul­ti­ply by mitot­ic divi­sions form­ing the pri­ma­ry Oocytes.

(b) Growth Phase :

This phase of pri­ma­ry oocyte is very long. it may extend over many years.

The oogo­ni­um grows into a large pri­ma­ry oocyte. Each pri­ma­ry oocyte then gets sur­round­ed by a lay­er of gran­u­losa cells to form pri­ma­ry follicle. 

A large num­ber num­ber of these fol­li­cles degen­er­ate dur­ing the peri­od from birth to puberty .

So at puber­ty only 60,000 — 80,000 pri­ma­ry fol­li­cles are left in each ovary . 

The flu­id filled cav­i­ty of the fol­li­cle is called antrum.

© Mat­u­ra­tion Phase :

Like a pri­ma­ry sper­ma­to­cyte, each pri­ma­ry oocyte under­goes two mat­u­ra­tion divi­sions. First mei­ot­ic and the sec­ond mei­ot­ic . The result of mat­u­ra­tion divi­sion in oogen­sis are how­ev­er , very dif­fer­ent from those in spermatogenesis.

In the first, mei­ot­ic divi­sion , the pri­ma­ry oocyte divides into two very unequal hap­loid daugh­ter cells — a large sec­ondary oocyte and a very small first polar body or polocyte.

In the sec­ond mat­u­ra­tion divi­sion , the first polar body May divide to form two sec­ond polar bodies.

The sec­ondary oocyte again divides into unequal daugh­ter cells , a large ootid and a very small sec­ond polar body . 

The ootid grows into func­tion­al hap­loid ovum. Thus from one oogo­ni­um one ovum and three polar bod­ies are formed. The Ovum , is the actu­al female gamete . the polar bod­ies take no part in repro­duc­tion and hence , soon degenerate.

In human beings , ovum is released from the ovary in the sec­ondary oocyte stage. the mat­u­ra­tionof sec­ondary oocyte is com­plet­ed in the moth­er’s oviduct (fal­lop­i­an tube) usu­al­ly after the sperm has entered the sec­ondary oocyte for fertilization.

In humans (and most ver­te­brates), the first polar body does not under­go meio­sis ll . How­ev­er, it then stop advanc­ing any fur­ther , it awaits the arrie arrival of sperm for com­ple­tion of meio­sis ll. 

Entry of the sperm restarts the cells cycle break­ing down MPF (M phase pro­mot­ing fac­tor )and tur­ing on APC (Anphase pro­mot­ing com­plex) . Com­ple­tion of meio­sis ll con­verts the sec­ondary oocyte into a fer­til­ized ovum( egg) or zygote (and also a sec­ond polar body )

Hor­mon­al Con­trol Of Oogenesis:

GnRH secret­ed by the hypo­thal­a­mus stim­u­lates the ante­ri­or lobe of pitu­itary gland to secrete LH and FSH. 

FSH stim­u­lates teh growth of Graf­fi­an fol­li­cles and also the devel­op­ment of egg/ oocyte with­in the fol­li­cle to com­plete the meio­sis l to form sec­ondary Oocyte .

FSH also stim­u­lates the for­ma­tion of oestrogen.LH induces the rup­ture of mature Graf­fi­an fol­li­cle and there­by the release of sec­ondary oocyte . Thus LH caus­es ovulation .

In brief Ovu­la­tion in human beings May be defined as the release of the sec­ondary oocyte from the Graf­fi­an fol­li­cle . The remain­ing part of the Graf­fi­an fol­li­cle is stim­u­lat­ed by LH to devel­op into cor­pus Luteum (“yel­low body”).

The ris­ing lev­el of prog­es­terone inhibits the release of GnRH., which in turn , inhibits pro­duc­tion of FSH , LH and progesterone.

Sig­nif­i­cance of Oogenesis:

(i) One Oogo­ni­um pro­duces one ovum and three polar bodies

(ii) polar bod­ies have small amount of cyto­plasm . it helps to retain suf­fi­cient amount of cyto­plasm in the ovum which is essen­tial for the devel­op­ment of ear­ly embryo. For­ma­tion of Polar bod­ies main­tains half num­ber of Chro­mo­some in the ovum.

(iii) Dur­ing meio­sis first cross­ing over takes place which brings about variation.

(iv). Ooge­n­e­sis occurs in var­i­ous organ­isms . There­fore, it sup­ports the evi­dence basic rela­tion­ship among the organisms.

OVULATION

In humans , ovum is released from the ovary in the sec­ondary oocyte stage . thus in human being, ovu­la­tion is the release of the sec­ondary oocyte from the ovary.

The wall of the ovary gets rup­tured to release the oocyte. In humans Ovu­la­tion occurs about 14 days before the onset of the next menstruation. 

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Ovu­la­tion is induced by LH. The mat­u­ra­tion of the ovum is com­plet­ed in the moth­er’s Fal­lop­i­an tube usu­al­ly after sperm gas entered the sec­ondary oocyte dur­ing fertilization .