GAMETOGENESIS

GAMETOGENESIS (formation of gametes)

Spermatogenesis:

·         Formation of sperm from the germ cell in the testes is spermatogenesis.

·         The process begins at puberty.

·         Spermatogonia present in the lining of seminiferous tubules undergo mitotic division to increase their number.

          

·         Each spermatogonium is diploid (2n) which contain 46 chromosomes.

·         Innermost layer of spermatogonial becomes larger called primary spermatocyte.

·         Primary spermatocyte undergoes meiosis-I to form two equal haploid (n) secondary spermatocytes (n).

·         Each secondary spermatocyte undergoes meiosis-II to form two equal, haploid spermatids.

·         Each primary spermatocyte produces four spermatids.

·         Spermatids transformed into spermatozoa (sperms) by the process called spermiogenesis.

·         The sperm head embedded in the Sertoli cell.

·         Release of sperm from the seminiferous tubule is called spermiation.

          

Hormonal control of spermatogenesis:

·         This process is initiated at puberty due to secretion of gonadotrophins releasing hormone (GnRH)

·         GnRH secreted form hypothalamus and stimulate anterior pituitary to secrete two gonadotrophins.

o    Luteinizing hormone (LH) and

o    Follicle stimulating Hormone (FSH)

·         LH acts on Leydig cells and stimulates synthesis of androgens.

·         Androgen stimulates spermatogenesis.

·         FSH acts on Sertoli cells and stimulates spermatogenesis in other ways.

Structure of sperm:

·         Ultrastructure of sperm consists of a headneck, a middle piece and a tail.

·         Whole body of sperm surrounded by plasma membrane.

·         The sperm head contain an elongated haploid nucleus.

·         Above the nucleus a cap like structure present called acrosome.

·         The acrosome contains enzymes which help in fertilization of ovum.

·         The middle piece contains mitochondria, which provide energy for movement of tail that facilitate sperm motility.

·         Human male ejaculates 200-300 million sperms during coitus.

·         60 percent must have normal shape and size and 40 percent of them must show vigorous motility.

·         Sperm released from seminiferous tubules enters into accessory ducts.

·         On their way fluids from seminal vesicle and prostate gland added which collectively called as Semen.

·         The function of male accessory ducts and glands are maintained by testicular hormone androgen.

 

Oogenesis:

·         Formation of a mature female gamete or ovum is called oogenesis.

·         Oogenesis starts during embryonic stage, 25th week of the foetal age.

·         Germinal epithelium of ovary divided mitotically to produce millions of gamete mother cell or oogonia.

·         No oogonia formed or added after birth.

·         Oogonia enters into meiosis-I and proceeds up to diakinesis of Prophase-I and get suspended, at this stage called primary Oocytes.

·         Each primary oocyte surrounded by layers of granulose cells and then called primary follicle.

·         At puberty only 60,000 to 80,000 primary oocytes are left in each ovary.

·         After puberty primary follicles get surrounded by more layers of granulosa cells and a new theca to form secondary follicles.

·         The secondary follicle transformed into tertiary follicle, characterized by a fluid filled cavity called antrum.

·         The theca layers organized into an inner theca interna and outer theca externa.

·         During the growth of primary follicle into tertiary follicle during puberty, the primary oocyte restarts its first meiotic division and completes it within tertiary follicle resulting two unequal haploid cells.

o    Large haploid cell is called secondary oocyte.

o    A tiny cell called first polar body.

·         The secondary oocyte retains bulk of the nutrient rich cytoplasm of primary oocyte.

·         The tertiary follicle having secondary oocyte further changes into Graafian follicle.

·         The secondary oocyte surrounded by a new membrane, zona pellucida.

·         The secondary oocyte undergoes second meiotic division continued upto metaphase-II and get suspended until entry of sperm.

·         At this stage Graafian follicle releases secondary oocyte from the ovary by the process called ovulation.

·         On entry of a sperm into the secondary oocytes stimulates it to complete meiosis-II and there is formation of a haploid ovum and a second polar body (n).

Menstrual cycle:

·         Reproductive cycle of female primates is called menstrual cycle.

·         The first menstruation begins at puberty is called Menarche.

·         Menstrual cycle repeated at an average interval of 28/29 days.

·         One ovum is released in the middle of each menstrual cycle.

 

Menstrual cycle has following phases:

Menstrual phase:

·         1st phase of menstrual cycle.

·         Menstrual flow occurs.

·         Lasts for 3-5 days.

·         Breakdown of endometrial lining and blood vessel.

·         Mucus and blood comes out through vagina.

·         It occurs only when ovum released but no fertilization.

·         Lack of menstruation is the indication of pregnancy.

Follicular phase:

·         Menstrual phase followed by follicular phase.

·         Primary follicle becomes Graafian follicle.

·         Regeneration and proliferation of uterine endometrium.

·         LH and FSH level increases gradually in follicular phase.

·         Level of estrogen increases as it is secreted from growing follicle.

·         It lasts for 5-13 days.

Ovulatory phase:

·         FSH and LH attain peak level in this period (14th day).

·         This is called LH surge, which induces rupture of Graafian follicle and release of ovum from the ovary called ovulation.

Luteal phase:

·         Remaining part of Graafian follicle transformed into corpus luteum.

·         Corpus luteum produces large amount of progesterone.

·         Progesterone maintains the uterine endometrium, and prepares it for implantation.

·         Thickness of uterine endometrium increase in many folds, due to proliferation.

·         If there is fertilization, corpus luteum grows further and pregnancy continued, menstrual cycle stopped.

·         In the absence of fertilization corpus luteum degenerates.

·         Disintegration of endometrium leading to menstruation.

·         Menstrual cycle ceases around 50 years of age, called menopause.

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