Essay on Gametogenesis in Male and Female Gonads !
Gametogenesis, carried out in gonads, is the formation of gametes for sexual reproduction. It is controlled by gonadotrophic hormones (FSH, LH, ICSH etc.) secreted by pituitary gland. Meiosis forms the most significant part of the process of gametogenesis.
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1. Spermatogenesis:
It is the formation of sperms by the testes. It takes place in seminiferous tubules at the time of puberty and continues throughout life. The process includes the formation of spermatogonia from germinal epithelium (primordial germ cell) through mitosis. Finally they stop undergoing mitosis, grow and become primary spermatocytes.
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Each spermatocyte undergoes meiosis. First maturation division is reductional, and produces two secondary spermatocytes.
The latter divides by equational division (second maturation division) to form four haploid spermatids. Spermatids receive nourishment from the Sertoli cells to form sperms. This step is called spermiogenesis.
Significance of spermatogenesis:
1. During this process one spermatogonium produces four sperms.
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2. Sperms have half number of chromosomes.
3. After fertilization the diploid chromosomes number is restored in zygote. It maintains the chromosomes number of species.
Sperm:
A mammalian sperm is a motile cell consisting of three parts, namely the head, the middle piece and the tail.
The head consists of acrosome at the anterior end followed by the nucleus. The acrosome is derived from the golgi complex and contains hydrolysing enzymes for the sperm penetration. The nucleus is the seat of genetic information and contains haploid set of chromosomes.
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Middle piece contains spiral row of mitochondria. Mitochondria provide energy (ATP) and strength to the sperm for locomotion.
Between the nucleus and the middle piece lies neck which contains a proximal and a distal centriole. From the distal centriole is produced axial filament which passes through the middle piece and extends into the tail.
Tail is fine, vibrating posterior portion of sperm which helps in swimming. This ability to swim, called motility is essential for male fertility as the sperm has to swim up the vaginal canal, cervix and uterine canal to reach to the ovum.
Sperms are constantly being produced in the seminiferous tubules which always contain sperm in various stages of development. The final maturation of a sperm takes place in the epididymis lying on the surface of each testis.
2. Oogenesis:
Stages of Oogenesis:
The formation of eggs by the ovaries (called oogenesis) starts only after the female has attained puberty (around 13 yrs. of age). The process is induced by FSH from the anterior pituitary. It leads to the growth of a single graafian follicle in one of the two ovaries every month. As the developing ovary is colonised by primordial germ cells prior to birth which differentiate into oogonia.
These enlarge within the follicle under the influence of mitotic division to form primary oocyte containing diploid number of chromosomes. These undergo reductional division (1st meiotic division) to form a secondary oocyte and first polar body. The ‘egg’ is released at secondary oocyte stage under the effect of LH.
The empty follicle develops into corpus luteum. Second maturation division occurs just before fertilization when the secondary oocyte releases second polar body and itself gets transformed into egg.
The first polar body may also divide to form two polar bodies of equal sizes which do not take part in reproduction and ultimately degenerates. Significance of oogenesis
(i) One oogonium produces one ovum and three polar bodies
(ii) Polar bodies containing- small amount of cytoplasm helps to retain sufficient amount of cytoplasm in the ovum which is essential for the development of early embryo.
(iii) Formation of polar bodies maintains the half number of chromosomes in the ovum.
Egg/Ovum:
Human egg is a small, spherical structure surrounded by a non-cellular membrane zona pellucida outside which is an envelope of radiating follicle cells forming corona radiata.
There is a haploid nucleus called germinal vesicle in the centre of the cytoplasm. Cytoplasm of the ovum is called ooplasm. The membrane forming the surface layer of cytoplasm is called vitelline membrane.
A narrow previtelline space, present in between the zona pellucida and plasma membrane (vitelline membrane, inner one), consists of 1-3 polar bodies. The side of the ovum which extrudes polar bodies is termed as animal pole and the opposite end is vegetal pole. Human ovum loses its ability to be fertilized about 24 hours after ovulation. Yolk is absent, and thus human eggs are alecithal.