Spermatogenesis | Sperm | Oogenesis | Menstrual cycle |

  Gametogenesis  

It is the process of producing haploid gametes from diploid germ cells in the gonads, i.e. sperms and ova are produced in the testis and ovary, respectively. It is of the following two types (7) Spermatogenesis (17) Oogenesis 

💥Spermatogenesis: -

It is the production of sperms in males. 

(i), In tastes the immature male germ cells (spermatogonia) produce sperms by spermatogenesis that begins at puberty. 

(ii) Spermatogonia (sing. spermatogonium) present on the inside wall of seminiferous tubules multiply by mitotic division and increase in numbers.

(iii) Each spermatogonium is diploid and contains 46 chromosomes. Some of the spermatogonia called primary spermatocytes periodically undergo meiosis. 

(iv) The primary spermatocytes undergo first meiotic division leading to two equal, haploid cells called secondary spermatocytes, which contains only 23 chromosomes each.

(v)The secondary spermatocytes undergo the second meiotic division to produce four equal, haploid spermatids which contain only 23 chromosomes each.

(vi) The spermatids are transformed into spermatozoa (sperms) by the process called spermiogenesis. Thus, each sperm too contains only 23 chromosomes.

(vii) Sperm heads are embedded in the Sertoli cells and are finally released from the seminiferous tubules by the process called spermiation.

Role of Hormones in Spermatogenesis: -

The following hormones play an essential role in spermatogenesis.

(i) Spermatogenesis starts during puberty with the significant increase in hypothalamic hormone called Gonadotropin Releasing Hormone (GnRH). 

(ii) The increased level of GnRH acts on the anterior pituitary gland and stimulates the secretion of two gonadotropins, i.e., LH (Luteinizing Hormone) and FSH (Follicle stimulating Hormone) 

(iii) LH acts on the Leydig cells and stimulates the synthesis and secretion of androgens, which in turn stimulate the process of spermatogenes1s. 

(iv) FSH acts on Sertoli cells and stimulates the secretion of some factors, which help in the process of spermiogenesis.

💥Structure of a Sperm: -

(i) A sperm is composed of a head, neck, a middle piece and a tail. 

(ii) A plasma membrane encloses the whole body of sperm. 

(ii) Head contains an elongated haploid nucleus, the anterior portion of which is covered by a cap-like structure called acrosome. 

(iv) The acrosome is filled with enzymes that help in fertilization of the ovum. 

(v) Neck contains two centrioles, a proximal centriole, which is necessary for first cleavage division of zygote and a distal centriole, that gives rise to axial filament of tail. 

(vi) Middle piece possesses many mitochondria to produce energy for the movement of tail to facilitate sperm motility. 

(vii) Tail of the sperm consists of an axial filament. It helps in the movement of sperm inside the female reproductive tract towards the ovum for fertilization.

0ogenesis: -

It is the process of formation of a female gamete or ova in the ovary. 

(i) It starts during embryonic stage in a female. 

(ii) About a million oogonia are formed in the ovary of the female fetus which is 25 weeks old. No new oogonia are formed after birth.

(iii) The oogonial cells start meiotic division, enter into prophase I, get temporarily arrested at that stage and are called as primary oocytes.

(iv) Each primary oocyte then gets surrounded by a layer of granulosa cells called primary follicle.

(v) A large number of primary follicles degenerate during the phase from birth to puberty. As a result, about 60,000-80,000 primary follicles are left in each ovary at puberty.

(vi) The primary follicles get surrounded by more layers of granulosa cells and a new theca and are called secondary follicles.

 

(vii) The thecal layer in secondary follicles becomes organized into an outer theca externa and an inner theca interna. This stage is called tertiary follicle. 

(viii) Tertiary follicle is characterized by a fluid-filled cavity called antrum. 

(ix) At this stage, the primary oocyte within the tertiary follicle grows in size and completes its first meiotic division. 

(x) The first meiotic division, which is unequal results in the formation of haploid secondary oocyte and tiny first polar body. 

(xi) Secondary oocyte retains bulk of the nutrient rich cytoplasm of the primary oocyte. 

(xii) The tertiary follicle develops into a mature follicle or Graafian follicle. 

(xiii) The secondary oocyte forms a new membrane called zona pellucida surrounding it.

(xiv) The Graafian follicle ruptures to release the secondary oocyte (ovum from the ovary by the process called ovulation.

Ovum: -

Human egg or ovum is non-cleidoic (i.e. without shell), alecithal (i.e. yolk is absent), microscopic with about 0.1 -0.13 mm or 100-130 um diameter. The ovum possesses three coverings, i.e. inner plasma membrane, middle glycoprotein, zona pellucida and outer cellular corona radiata with radially elongated scattered cells held in mucopolysaccharide (hyaluronic acid).

✒ Menstrual Cycle: -

It is a rhythmic change in the reproductive organs of the female primates (monkey, apes and humans). 

(i) The first menstruation begins at puberty and is called menarche. 

(ii) Average interval of menstruation in human female is about 28-29 days. 

(iii) The cyclic events starting from one menstruation till the next one constitutes one menstrual cycle. 

(iv) The four phases of menstrual cycle are: -

✅Menstrual Phase: -

(a) Cycle starts with this phase and the menstrual flow occurs for 3-5 days.

 (b) It occurs due to breakdown of endometrial with lining of the uterus and blood vessels which along unfertilized ovum comes out through the vagina. 

(c) Menstruation occurs only if fertilization does not take place.

(d) Lack of menstruation generally indicates pregnancy, but may also be due to stress, poor health, diseases, etc. 

✅Follicular or Proliferative Phase: - 

(a) Menstrual phase is followed by the follicular phase.

(b) The primary follicle in the ovary grows to become a fully mature Graafian follicle and simultaneously the endometrium of uterus regenerates through proliferation.

(c) Pituitary and ovarian hormones induce the changes in ovary and uterus. 

(d) LH and FSH levels increase gradually during this phase and stimulates follicular development. 

(e) The growing follicles secrete oestrogens. 

(f) Both LH and FSH attain highest level during mid-cycle (about 14th day)

✅Ovulatory Phase: -

(a) Rapid secretion of LH leading 1o its maximum level during the mid-cycle is called LH surge. 

(b) The LH surge induces rupture of Graafian follicle, releasing the ovum and this process is called ovulation.

✅Luteal Phase/Secretory Phase: -

(a) Ovulatory phase is followed by the luteal phase. 

(b) The remaining parts of the ruptured Graafian follicle change into corpus luteum. 

(c) Corpus luteum secretes large amount of progesterone, which is required for the maintenance of endometrium. 

(d) The thickened endometrium is necessary for the implantation of fertilized ovum and maintenance of pregnancy. 

(e) During pregnancy, all the events of menstrual cycle stop, and menstruation does not occur. 

(f) In case of no fertilization, the corpus luteum degenerates and now known as corpus albicans, which does not secrete progesterone. This causes rupture of endometrium leading to menstruation - initiation of a new cycle. 

(v) In human females, menstrual cycle stops around the age of 50 years and this phase is called as menopause. 

(vi) Menstrual cycle is absent temporarily during pregnancy and lactation periods and permanently after menopause. 

(vii) Menstruation is also called 'Weeping of uterus for the lost ovum' or "Funeral of unfertilized eggs'.