Gametogenesis

It is the process of formation of gametes or sex cells, Gametes are haploid. The cells producing gametes are called germ or germinal cells. They are generally diploid. Therefore, gametogenesis iS generally accompanied by meiosis. It occurs inside gonads.

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Context 

1. Spermatogenesis 
2. Harmonal Control Of Spermatogenesis
3. Oogenesis
4. Harmonal Control Of Oogenesis

However, primordial germ cells (PGCS) are actually extragonadal in origin being formed from extraembryonic mesoderm and entering the gonads of developing embryo through yolk sac endoderm. Gametogenesis is accomplished in three phases 

(i) Multiplication Phase. Here multiplication of the germ cells occurs through mitosis so as to increase their number.

 (ii) Growth Phase. The germ cells increase in size. 

(iii) Maturation Phase.where Germ cells undergo meiosis that produces haploid gametes. The latter are of two types, male or sperm and female or ova.

Accordingly, gametogenesis is of two types, spermatogenesis and oogenesis.

1. Spermatogenesis

It is the prcocess pf formation of hapliod spermatozoa (= sperms) from diploid spermatogonia inside the testes of the male. Spermatogenesis begins n human males during puberty at the age 13-14 rears and continues uninterrupted throughout the life of adult though it declines in later life. Inside the testes, the sites of spermatogenesis are seminiferous tubules. The epithelium lining the seminiferous tubules is called germinal epithelium.

Germinal epithelium has two types of cells, primary germ cells and indifferent or supporting cells. The latter gives rise to large sized elongated pyramidal or coloumnar polygonal cells called Sertoli cells.

Sertoli cells develop processes for supporting and nourishing products of spermatogenesis. For their functioning they require pituitary hormone, FSH.

Primary germ cells undergo spermatogenesis.

Spermatogenesis has four phases-multiplication, GRowth, maturation and differentiation.

(i)  Multiplication Phase. Primary germ cells are cuboidal in outline. They divide mitotically to form mall spermatogonial cells of about 12 um in diameter. At the time of sexual maturity, the spermogonial ells divide many times by mitosis to form a large number of spermatogonia (Gk, sperma-seed, ganos eneration). Spermatogonia are of two types, type A and type B. Type A spermatogonia function as stem ells or mother spermatogonia which give rise to second type of spermatogonia whenever required. Type-B spermatogonia are progenitor cells which function as precursors of spermatozoa.

(ii)  Growth Phase. It is also called spermatocytogenesis. Spermatocytogenesis is the phenomenon of formation of primary spermatocytes from B-type spermatogonia. The latter grow in size, accumulate nutrients and meiotic factors to form nearly double-sized cells of primary spermatocytes.

(iii) Maturation Phase or Formation of Spermatids. Primary spermatocytes are large diploid cells aving 44 + XY chromosomes. They undergo meiosis I to produce small sized haploid secondary spermatocytes of two types, 22 + X and 22 + Y. Secondary spermatocytes, however, possess replicated chromosomes having undergone crossing over in their chromatids. Secondary spermatocytes divide by meiosis II to produce haploid spermatids (with 1 N chromosomes and 1 N DNA). As meiosis is a double vision, one primary spermatocyte or type B-spermatogonium gives rise to four spermatids.

(iv) Differentiation Phase or Spermiogenesis. It is the conversion of spermatid into spermatozoa. For s the spermatid must remain in contact with sertoli cell which under the influence of FSH of anterior pituitary produces the required spermiogenic factors. The different changes which occur during spermiogenesis (a) Formation of acrosome by Golgi apparatus. The latter then degenerates. (b) Elongation and condensation of nucleus. (c) Separation of centrioles. (d) Formation of axial filament from distal centriole.

Development of mitochondrial spiral around upper part of axial filament. (f) Differentiation of a limiting mbrane and formation of flagellum. This produces a spermatozoa. The unused components of thespermatid degenerate. The whole complex of changes which convert a spermatogonium into mature spermatozoa is known as spermatogenesis (=spermioteleosis). It requires about 64 days.  After their maturation, spermatozoa detach from sertoli cells.

The process is called spermiation. The released sperms are stored in epididymis and first portion of vasa deferentia for upto one month. Here they gain motility. Nutrition is provided by epithelium of epididymis.

2. Harmonal Control Of Spermatogenesis 

Spermatogenesis is under control of endocrine hormones. It begins at the time of puberty due to large scale secretion of gonadotropin releasing hormone or GnRH by hypothalamus. It acts on anterior pituitary to proudce two gonadotropins, luteinising hormone (LH) and follicle stimulating hormone (FSH). LH, ICSH produce or interstitial cell stimulating hormone acts on interstitial or Leydig cells which androgens like testosterone. Testosterone is essential for formation of sperms, atleast spermatogenesis part by Sertoli cells. Growth hormone is also required for spermatogenesis (Guyton and Hall, 2000).

Under influence of FSH, Sertoli cells develop androgen binding protein (ABP). The latter helps in concentrating testosterone in the seminiferous tubules for spermiogenesis. However, excess of testosterone inhibits LH/ICSH by anterior pituitary and G RH production by hypothalamus. Sertoli cells also produce a glycoprotein called inhibin. Inhibin suppresses FSH synthesis by anterior pituitary and GnRH synthesis by hypothalamus. Therefore the normal release of testosterones are under negative feedback control.

3. Oogenesis

   It is the process of formation of functional haploid ova from the diploid germinal cells in the ovary or female gonad. Early stages of oogenesis occur by the time female foetus is only 25 weeks old. At this time all the oogonia have been formed and entered into primary oocyte stage.

Multiplication Phase. Certain cells of the germinal epithelium of ovary which are larger than others function as germ cells. They undergo repeated mitotic divisions to form diploid oogonia. Some two million oogonia or gamete mother cells are formed in each foetal ovary. The oogonia form egg tubes of Pfluger which grow into cortex and give rise to a multicellular mass called egg nest.

Growth Phase. One oogonium of the egg nest grows in size and functions as primary oocyte. The other cells of egg nest form an epithelial covering over it. The same is called follicular sheath or epithelium. The ensheathed primary oocyte is called primary follicle. A number of these follicles degenerate between birth and puberty. At puberty, an ovary contains 60,000-80,000 primary follicles.

Maturation Phase. It begins in the foetal stage of females but is interrupted quite early. The primary oocyte begins meiosis I but the division is arrested in diakinesis stage of prophase I. It is in this phase that ovarian follicles containing the oocytes occur in the foetus and remain so till the female attains puberty.

After attainment of puberty, follicles and their contained oocytes resume development, usually one at a time and once a month. Follicular sheath differentiated into layers of granulosa cells. Another layer called theca develops from cortex. With the formation of theca, primary follicle is transformed into secondary follicle. Theca differentiates into two layers, theca externa and theca interna. Granulosa cells secrete fluid that cause the development of a cavity or antrum around the primary oocyte. The stage is called tertiary follicle. Primary oocyte grows further and completes meiosis I. It produces a very large number of secondary oocyte and a small polar body. Both are haploid. The polar body has a very small amount of cytoplasm but the whole chromosome set. In human females, the polar body doesn't divide further. The bulk of nutrient rich cytoplasm is retained in the secondary oocyte. The follicle grows is of maximum size and is known as Graafian follicle. Zona pellucida develops around secondary oocyte. The secondary oocyte proceeds with meiosis II but the division gets arrested in metaphase II stage due to accumulation of metaphase promoting factor (MPF). It is at this stage of oocyte that the ovum is shed during ovulation. It passes into oviduct, where in the ampulla part, cell cycle will resume only after the entry of sperm. It triggers the breakdown of MPF and promotes synthesis of anaphase promoting complex (APC). Meiosis II is completed. A second polar body is extruded. The oocyte is now changed into ovum or ootid which soon gets changed into zygote by the fusion of male and female pronuclei.

4. Hormonal Control Of Oogenesis

In response to production of GnRH or gonadotropin releasing hormone, anterior pituitary secretes two hormones, FSH (follicle stimulating hormone) and LH (luteinizing hormone). FSH stimulates follicular growth and maturation of oocyte. Granulosa cells of developing ovarian follicle produce estrogen. In the presence of high titre of both estrogen and LH, ovulation occurs. High concentration of estrogen inhibits secretion of both FSH and GnRH. This is negative feedback control. LH helps in converting ruptured Graafian follicle into corpus luteum. The latter secretes progesterone which prepares the uterus to receive fertilised ovum. High concentration of progesterone inhibits further release of LH from anterior pituitary and GnRH from hypothalamus. This is another example of negative feedback loop.

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