A male gamete (sperm) fuses with a female gamete (ovum or egg) to produce a zygote that develops into an individual. But humans are diploid while gametes are haploid.
So the cells undergo meiosis to produce haploid gametes. This process is called gametogenesis.
In men, it occurs as spermatogenesis, and in women, it occurs as oogenesis.
Spermatogenesis and oogenesis Steps
Spermatogenesis occurs in testes of man.
The process of development of male germ cells into spermatozoa is called spermatogenesis. The duration of spermatogenesis is 65-74 days. The primitive or immature germ cells called spermatogonia are present in the testis. The testes also contain supporting cells called Sertoli cells, and testosterone-producing cells called Leydig (interstitial) cells. Spermatogonia are located attached to the basement membrane of the seminiferous tubule.
Spermatogenesis starts at puberty when under the influence of rising levels of gonadotropin and testosterone, the inactive germ cells are activated and spermatogenesis is initiated. Thereafter spermatogenesis continues throughout life. Spermatogenesis can be divided into three distinct phases-
The primitive germ cells present in the semeniferous tubules undergo repeated mitotic divisions to form primary spermatocytes. This process is called spermatocytogenesis. These mitotic divisions produce two types of spermatogonia A and spermatogonia B.
- Spermatogonia A- they resemble the original spermatogonia and are the source for the subsequent spermatogonia in the testis. They serve as a reserve and undergo further division to produce more spermatogonia.
- Spermatogonia B- they grow and enter into the add nominal component where they develop into primary spermatocytes.
The primary spermatocytes are diploid and have to undergo two meiotic divisions to produce haploid cells. The first meiotic division, produces two secondary spermatocytes are haploid. The second meiotic division of these secondary spermatocytes produces haploid spermatids. These spermatids contain 23 chromosomes i.e., 22 autosomes and 1 sex chromosome. Thus Each primary spermatocyte forms four spermatids two of which contain an X chromosome and two of which containing the Y chromosome. Each spermatogonium divides into 512 spermatids after these two stages.
Spermiogenesis is the process of development of the haploid spermatids into mature spermatozoa. Spermatids are small and round and have to undergo many structural changes to form spermatozoa. The changes it undergoes are-
- A large reduction in the cytoplasm- cytoplasmic fragments are discarded in the form of residual bodies.
- Nucleus elongates to become head of spermatozoa
- An acrosomal cap is formed, which contains enzymes needed to penetrate the ovum forms over the head.
- The middle piece and tailpiece form so that the spermatozoa can move swiftly within the uterus.
These changes occurring in mature sperm help them survive in the acidic environment of the vagina and help them recognize, move towards, and fertilize the ovum.
Spermatozoa, after being formed stick to the Sertoli cells in the lumen of seminiferous tubules. The detachment of the head of spermatozoa and its release into the luminal fluid is called spermiation.
This occurs in the female genital tract. Motility increases and an acrosomal reaction occurs.
The process of development of female germ cells into spermatozoa is called oogenesis. While spermatogenesis starts at puberty and continues throughout life, the process of oogenesis starts at fetal life and ceases at menopause. The process of spermatogenesis is short while the development of each oocyte begins in intrauterine life and is completed at ovulation, which occurs during the menstrual cycle. Many sperms can be produced within a few days while a single ovum is produced during each cycle.
In embryonic life, the primordial germ cells migrate from yolk sac to the genital ridge in the 6th week of gestation. These oogonia undergo repeated mitotic divisions until the number reaches 7 million. After mitosis ceases, the oogonia are called oocytes.
The development of oocyte occurs in three stages- oogonium becoming primary oocyte, primary oocyte converted to the secondary oocyte, and finally secondary oocyte developing to mature ovum.
Oongoium becoming primary oocyte
Oogonium converts to oocytes in embryonic life. These diploid oocytes are called primary oocytes. They undergo two meiotic divisions.
The first meiotic division occurs in primary oocytes in intrauterine life in the 8th week of gestation. The cells get arrested in prophase state, but the oocytes grow in size.
Oocyte degeneration starts from intrauterine life such that only about 1 million primary oocytes remain at the time of birth. By the time of puberty, only 200,000 remain in the ovary. In a woman’s life, only about 400 oocytes are ovulated and others degenerate. Thus the process of oogenesis is limited as new oogonia cannot be manufactured in the ovary.
Primary oocyte converted to secondary oocyte
the primary oocyte that is destined for ovulation completes its first meiotic division just before ovulation; this division results in the production of two structures the larger daughter cell in the secondary oocyte containing 23 chromosomes and the other first polar body which is smaller. thus the cytoplasmic division is grossly unequal and the polar body is completely non-functional.
Secondary oocyte forming ovum
Secondary oocyte undergoes second meiotic division after ovulation, but the process is arrested in metaphase. The division is completed only when the sperm penetrates the egg. Thus the egg or ovum contains 23 chromosomes and the second polar body forms during the second meiotic division. Each primary oocyte produces only one ovum.
The oocyte grows throughout its life in the ovarian follicle till ovulation when the ovum is released from the follicle. Along with the development of oocyte in ovarian follicles, follicles also grow in different phases.
Each primary oocyte gets surrounded by a layer of granulosa cells to form the primary follicle. The primary follicles get surrounded by more layers of granulosa cells and new theca to form secondary follicles.
The tertiary follicle forms from the secondary follicle and has a characteristic fluid-filled antrum. The theca layer is distinguished into inner theca interna and outer theca externa. In this stage, the primary oocyte completes its first meiotic division.
The tertiary follicle matures into the final Graafian follicle. A membrane called zona pellucida is formed around the secondary oocyte. The Graafian follicle ruptures to release the ovum during ovulation.
Similarities between oogenesis and spermatogenesis
- Both take place in the gonads or reproductive organs of the body.
- Both produce haploid gamete cells from the original diploid cell.
- Both undergo 2 meiotic divisions to divide chromosome number by half.
- In both cases, nuclear and cytoplasmic changes occur in the respective cells.
- In both cases, the first mitosis occurs to cause the multiplication of cells.