To study the discrete stages of gametogenesis in mammalian testis and ovary.
The primary sex organs, the testis in the males and the ovaries in the females produce gametes i.e., sperms and ovum respectively by the process called gametogenesis. In the biological process of gametogenesis, diploid or haploid precursor cells divide and differentiate into mature haploid gametes. Gametogenesis can take place through mitotic and meiotic division of diploid gametocytes into numerous gametes.
The mammalian testis is a crucial organ in the male reproductive system responsible to produce sperm cells (spermatogenesis) and the synthesis of male sex hormones, primarily testosterone. It plays a vital role in the process of sexual reproduction. Here is a detailed explanation of the mammalian testis:
The testis is a pair of oval-shaped, glandular organs located in the scrotum, which is a sac-like structure situated outside the male body cavity. This position outside the body allows the testis to maintain a lower temperature than the core body temperature, which is essential for proper sperm production.
Each testis is composed of several distinct parts, including:
Seminiferous Tubule: These are highly coiled, microscopic tubes within the testis where spermatogenesis takes place. Sperm cells are produced within the seminiferous tubules through a complex process involving mitosis and meiosis.
Leydig Cells: Also known as interstitial cells, Leydig cells are found in the spaces between the seminiferous tubules. They are responsible for producing and secreting testosterone, the primary male sex hormone. Testosterone plays a crucial role in the development of male secondary sexual characteristics, such as facial hair, deepening of the voice, and muscle growth.
Sertoli Cells: These cells are found within the seminiferous tubules and provide physical support and nourishment to developing sperm cells. They also secrete hormones necessary for sperm production and regulate the microenvironment within the tubules.
The hypothalamus-pituitary-gonadal axis regulates testicular function. The hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary gland to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH and FSH act on the testis to regulate testosterone production and sperm maturation. FSH is primarily responsible for regulating spermatogenesis in the testis.
Testosterone is the primary male sex hormone produced by Leydig cells in the testis. It is crucial for the development of male secondary sexual characteristics, the maintenance of muscle mass and bone density, and the regulation of libido and mood.
This is the process by which spermatogonia stem cells within the seminiferous tubules differentiate and undergo multiple cell divisions to produce mature sperm cells. It has 23 haploid chromosomes. It involves both mitosis (to increase the number of cells) and meiosis (to produce haploid sperm cells with half the normal chromosome number). The entire process takes about 64-72 days in humans.
The formation of sperms in testis occurs through cell division and is shown below:
Spermatogonia → Primary Spermatocytes → Secondary Spermatocytes → Spermatids → Spermatozoa (sperms)
Spermatogenesis is the process of sperm cell production in the testis. Here are the key steps in spermatogenesis, presented in a concise manner:
Spermatogonia (diploid stem cells) undergo mitosis to produce identical diploid daughter cells.
Primary spermatocytes (resulting from mitosis) undergo meiosis I to produce two haploid secondary spermatocytes.
Secondary spermatocytes undergo meiosis II to produce four haploid spermatids.
Spermatids undergo structural changes and condense into spermatozoa (sperm cells).
After spermiogenesis, sperm heads become embedded in the Sertoli cells, and are finally released into the lumen of the seminiferous tubules and transported to the epididymis for further maturation and storage. This is called spermiation.
The mammalian ovary is a crucial organ in the female reproductive system responsible for producing and releasing eggs (ova) and secreting female sex hormones, primarily oestrogen and progesterone. Here is a detailed explanation of the mammalian ovary:
In mammals, including humans, females typically have two ovaries. These paired organs are situated in the pelvic cavity, one on each side of the uterus.
Ovarian Cortex: The outer layer of the ovary is called the cortex. It contains structures known as ovarian follicles, which house immature eggs (oocytes) at various stages of development. Each follicle consists of an oocyte surrounded by granulosa cells.
Ovarian Medulla: The inner part of the ovary is called the medulla. It contains blood vessels, nerves, and connective tissue.
Oestrogen: The ovaries are a primary source of oestrogen production in females. Oestrogen plays a significant role in the development of secondary sexual characteristics (e.g., breast development and the menstrual cycle), as well as in regulating the menstrual cycle and supporting pregnancy.
Progesterone: After ovulation, the empty follicle transforms into a structure called the corpus luteum. The corpus luteum secretes progesterone, which helps prepare the uterine lining for potential implantation of a fertilized egg. If fertilization does not occur, the corpus luteum degenerates, leading to a drop in progesterone and the onset of menstruation.
By mitosis, these cells divide and give rise to egg mother cells or oogonia. After birth, the formation of oogonia ends. By mitotic division, oogonia multiplies to create primary oocytes, reach prophase-I of the meiotic division and get temporarily arrested at that stage. Granulosa cells covered the primary oocyte to create the primary follicle. The antrum is the fluid-filled cavity of the follicle.
It is a very long phase of the primary oocyte. Inside a large primary oocyte, the oogonium grows by taking nutrition from the surrounding follicle cells after puberty.
Two maturation divisions take place in each primary oocyte, i.e., first mitotic and second meiotic. The two unequal haploid daughter cells are formed by the division of the primary oocyte. A large haploid daughter cell is a secondary oocyte, and a small haploid daughter cell is the first polar body. The first polar body divides into the second polar body. The secondary oocyte splits and forms two unequal daughter cells. A large daughter cell is an ootid and a small daughter cell is the second polar body. The ootid develops into the haploid ovum. Hence, from oogenesis, one ovum and three polar bodies are formed. The ovum is the main female gamete.
During the menstrual cycle, which typically occurs monthly in humans, one or more ovarian follicles mature in response to hormonal signals. When an egg is fully developed within a mature follicle, the follicle ruptures and releases the egg into the fallopian tube. This process is known as ovulation. The LH surge (luteinizing hormone) triggers the release of the secondary oocyte from the ovary during ovulation.
The phases of follicle development are shown below:
1°follicle → 2°follicle → 3°follicle → Graafian follicle → Corpus luteum
After ovulation the ovarian follicle transforms into a corpus luteum. The corpus luteum and mature follicles are found in the cortex. Follicle formation begins at around 16–18 weeks of fetal life. Meiosis is a process that only occurs in germ cells and leads to the development of a single haploid ovum.
Students should be able to: