Today we will discuss about the first topic of Human Reproductive Physiology and that is Gametogenesis.
Now let us dig out the meaning of Gametogeneis
In the word Gamet/o/genesis.
a) O is the combining vowel, which is joining two word roots gamete and genesis
b) Gamete means a mature haploid male or female germ cells, which when join it forms zygote
c) Genesis means production
So, Production of gametes is known as Gametogenesis
It is the process of formation and differentiation of haploid gametes sperms and ova from the diploid primary germ cells gametogonia that are spermatogonia and oogonia present in primary sex organs called gonads, which are testes and ovary
Gametogenis is of two types spermatogenesis and oogenesis
It is the formation of haploid, microscopic and motile male gametes called spermatozoa from the diploid spermatogonia in the testes of male sex organ
It is a continuous process and it is completed in about 74 days
It has got two parts A. Formation of spermatids and B. Spermiogenesis
- Formation of Spermatids: It is divided into three phases
Multiplicative phase: It is a rapid mitotic division where spermatogonia (2n) are formed from germ cells (2n)
Growth phase: Here the diploid spermatogonia increases in size about twice to form primary spermatocytes
Maturartion phase: It is characterized by meiosis. Diploid primary spermatocyte undergoes meiosis-I division to form two haploid secondary spermatocytes each containing 23 chromosomes. It is immediately followed by meiosis-II in each secondary spermatocyte to form two haploid spermatids. So each diploid spermatogonium produces four haploid spermatids
- Spermiogenensis: The transformation of a non-motile, rounded and haploid spermatid into a functional and motile spermatozoan is called spermiogenesis or spermioteliosis.
It involves the formation of haploid female gametes ova from the diploid egg mother cells oogonia, of ovary of female organism
Period of oogenesis is different in different animals. In human female, there are about 1700 primary germ cell in the undifferentiated female gonad at one month of foetal development. These proliferate to form about 600000 oogonia at two months of gestation period and by fifth month, the ovaries contain over 7 million oogonia under the influence of mother’s oestrogen hormone. Many oogonia about 50 % undergo atresia or degeneration and at the time of puberty, ovaries contain about 300000 primary follicles. Oogenesis is completed only after the onset of puberty and only one out of 500 is stimulated by FSH to mature. So oogenesis is a discontinuous process. When oogenesis is completed, no additional primary oocytes are created, in contrast to the male process of spermatogenesis where spermatocytes are continuously created. The primary oocytes reach their maximum development at 20 weeks of gestation. The succeeding phase of ootidogenesis or oogenesis occurs when the primary oocyte develops into an ootid. This is achieved by the process of meiosis. However, this process begins at prenatal age, it stops at prophase I. In late foetal life, all oocytes, still primary oocytes, have halted at this stage of development, called the dictyate (prolonged diplotene). After menarche these cells then continue to develop, although only a few do so every menstrual cycle. The Meiosis I of oogenesis begins during embryonic development of the foetus, but gets stopped in the diplotene stage of prophase I until puberty.
Like the spermatogenesis, oogenesis is formed of three phases
In this certain primary germ cell of germinal epithelium of ovary undergo rapid mitotic divisions to form groups of diploid egg mother cells, oogonia.
Growth phase of oogenesis is of very long duration than that of spermatogenesis. The oocyte increases in size about 200 times, 2000 times in frog. Vitellogenesis occurs in this phase, which is the accumulation of egg yolk. Nucleus becomes bloated with nucleoplasm and is called germinal vesicle. A thin vitelline membrane is secreted around the oocyte. Increase in number of mitochondria, endoplasmic reticulum and Golgi body.
Formation of lampbrush chromosomes occurs in amphibians.
It is characterized by meiosis. In this phase the diploid and fully grown primary oocyte undergoes meiosis-I (reductional division) to form two unequal haploid cells. The smaller cell is called first polar body and has very small amount of cytoplasm. The larger cell is called secondary oocyte. Both of these are haploid and each has 23 chromosomes.
Secondary oocyte undergoes meiosis-II (equational division) to form two haploid cells. The smaller cells are called second polar bodies and have very little of cytoplasm, while the larger cell is called ootid. This ootid differentiates in to Ovum.
So in oogenesis a diploid oogonium forms one haploid ovum and three polar bodies while in spermatogenensis a diploid spermatogonium forms four haploid sperms.
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