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
(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.
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 .