GAMETOGENSIS - OOGENESIS

STD-12      UNIT-6     CHA-3

HUMAN REPRODUCTION

GAMETOGENSIS - OOGENESIS

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 
• Maturation . 

Multiplication phase . 

• In the foetal development , certain cells in the germinal epithelium of the ovary of the foetus are larger than others . 
• These cells divide by mitosis , producing a couple of million egg mother cells or oogonia in each ovary of the foetus . 
• No more oogonia are formed or added after birth . 
• The oogonia multiply by mitotic divisions forming the primary oocytes .

Growth phase . 

• This phase of the primary oocyte is very long . 
• It may extend over many years . 
• The oogonium grows into a large primary oocytes .
• Each primary oocyte then gets surrounded by a layer of granulosa cells to form primary follicle . 
• A large 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 results of maturation divisions in oogenesis 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 a 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 maturation of 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 II whereas the secondary oocyte proceeds as far as the metaphase stage of meiosis II . 
• How ever , it then stops advancing any further ; it awaits the arrival of sperm for completion of meiosis II . 
• Entry of the sperm restarts the cell cycle breaking down MPF ( M - phase promoting factor ) and turning on APC ( Anaphase promoting complex ) . 
• Completion of meiosis II 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 the growth of Graafian follicles and also the development of egg oocyte within the follicle to complete the meiosis I to form secondary oocyte . 
• FSH also stimulates the formation of oestrogens . 
• LH induces the rupture of the mature Graafian 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 Graafian follicle . The remaining part of the Graafian 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 

• One oogonium produces one ovum and three polar bodies . 
• 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 chromosomes in the ovum . 
• During meiosis first crossing over takes place which brings about variation . 
• Oogenesis occurs in various organisms . Therefore , it supports the evidence of basic relationship among the organisms .

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