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What is the Importance of Gametogenesis in Human Reproduction?
The process of gamete formation in sexually reproducing animals is called gametogenesis. Gametogenesis is the process by which gamete is formed inside the testes and ovary. In the case of human beings production of sperm and ovum occured in the male and female bodies respectively.
Types of Gametogenesis
The gametogenesis is of two types, e.g., spermatogenesis (formation of male gamete) and oogenesis (formation of female gamete).
Spermatogenesis is the process in which spermatozoa are produced from male primordial germ cells by mitosis and meiosis. The initial cells in this pathway are called spermatogonia, which yield primary spermatocytes by mitosis. The primary spermatocytes are further divided by meiotic division to form secondary spermatocytes. Each secondary spermatocyte divides mitotically to give rise to two spermatids. These spermatids do not divide further but undergo morphological changes to form spermatozoa.
- Location: Spermatogenesis occurs in the seminiferous tubules of the testes.
- Time: Sperm production starts in adolescence (13-14 years) periods and declines after the age of 60-65 years.
- Spermatogenesis occurs from spermatogonia cells through successive stages as follows.
Stages of Spermatogenesis:
(i) Formation of Spermatids: Spermatogonia are considered primitive or primordial germ cells that serve as a pool of undifferentiated stem cells from which spermatozoa develop.
Two types of spermatogonia can be found-
- Type A Spermatogonia, serves as the stem cells.
- Type B Spermatogonia is considered a precursor of sperm.
Phase I: Each type B spermatogonium divides mitotically 5 times to give rise 32 type B spermatogonia.
Phase II: In this phase, 64 primary spermatocytes are formed from 32 type-B spermatogonia by mitotic division.
Phase III: Now each primary spermatocyte undergoes 1st meiotic division to give rise to 128 primary spermatocytes. These primary spermatocytes further undergo 2nd meiotic division to form 256 secondary spermatocytes.
Phase IV: Now each secondary spermatocyte divides mitotically to give rise to two spermatids. Thus from 256 secondary spermatocytes, 512 spermatids are formed.
(ii) Formation of Spermatozoa from Spermatids or Spermiogenesis: The transformation of spermatids into spermatozoa is called spermiogenesis. But it is to be noted that spermatids only undergo morphological changes to form sperms. The spermatids mature into sperms or spermatozoa in the deep folds of the cytoplasm of the Sertoli cell. The spermatozoa are later known as sperm. Various parts of spermatids take part in the formation of spermatozoan, e.g., nucleus, Golgi apparatus, mitochondria, cell membrane, etc. After spermiogenesis sperm heads become embedded in the Sertoli cells and are finally released from the seminiferous tubules by the process called spermiation. Thus mature spermatozoa become free in the lumen of the seminiferous tubules.
Spermatogenesis is highly dependent upon optimal conditions for the process to occur correctly and is essential for sexual reproduction. DNA methylation and histone modification have been implicated in the regulation of the process. It starts at puberty and usually continues uninterrupted until death, although a slight decrease can be discerned in the quantity of produced sperm with an increase in age. The whole process of spermatogenesis is controlled by the hypothalamic-hypophyseal-testicular axis, through the feedback mechanism process. Here LH and FSH provide a very important role in controlling spermatogenesis.
Differences between Spermatogenesis and Spermiogenesis:
|1. It is the total process of the formation of spermatozoa or sperm from the diploid sperm mother cells (spermatogonia).||1. It is the process of transformation of spermatids into spermatozoa.|
|2. It involves the multiplication phase, growth phase, maturation phase, and differentiation phase.||2. It is a part of spermatogenesis where only transformation takes place.|
|3. One spermatogonium produces four spermatozoa.||3. One spermatid develops into one spermatozoa.|
Hormonal Control of Spermatogenesis:
Hormonal control of spermatogenesis varies among the species. In humans, the mechanism is not completely understood. However, it is known that initiation of spermatogenesis occurs at puberty due to the interaction of the hypothalamus, pituitary gland, and Leydig cells. Spermatogenesis is initiated due to the increase of gonadotrophin-releasing hormone (GnRH) by the hypothalamus. GnRH acts on the anterior pituitary to secrete LH (luteinizing hormone) and FSH (follicle-stimulating hormone). If the pituitary gland is removed, spermatogenesis can still be initiated by the FSH and testosterone. The testosterone secretes by the action of LH, acts on the leydig’s cell of the testis. FSH stimulates both the production of androgen binding protein (ABP) by Sertoli cells and the formation of the blood-testis barrier. ABP is essential to concentrating testosterone in levels high enough to initiate and maintain the process of spermatogenesis.
FSH may initiate the sequestering of testosterone in the testes, but once developed only testosterone is required to maintain spermatogenesis. The hormone inhibin acts to decrease the levels of FSH. Recent studies suggest that both LH and FSH support the process of spermatogenesis by suppressing the pro-apoptotic signals and therefore promote spermatogenic cell survival. The Sertoli cells themselves mediate parts of spermatogenesis through hormone production. They are capable of producing the hormones estradiol and inhibin. The Leydig cells are also capable of producing estradiol in addition to their main product testosterone. FSH acts on spermatogonia to stimulate sperm production.
Significance of Spermatogenesis:
- One spermatogonium produces four sperm.
- Each sperm has half the number of chromosomes. After fertilization, a diploid zygote is formed.
- During meiosis crossing over takes place which brings about variation.
Structure of Spermatozoa:
A mature spermatozoon (about 0.05 mm in length) is made up of four parts: Head, Neck, Body, and Tail.
(a) Head: It is flat oval and made mainly of a nucleus. It is 4-5 mm in length and 2.5-3.5 mm in diameter. Its front part is covered by a cap-like structure called acrosome which is formed from the Golgi body of the spermatid. The acrosome is filled with enzymes (sperm lysins). These enzymes help in the process of fertilization of the ovum.
(b) Neck: This part is short and weak. It contains proximal and distal centrioles. From the neck region, an axial fibre made up of 11 fibrils extends through the body upto the tail.
(c) Body: It is columnar which measures 5-7 mm in length and 1 mm in diameter. Around the axial fibre extended through the body, a double-layered sheath formed from mitochondria remains spirally arranged. The mitochondria provide energy in the form of ATP for the locomotion of the sperm. The head, neck, and body of sperm remain covered by a membrane. This is called the sperm membrane. At the end of the body, there is a ring centriole (annulus) with an unknown function.
(d) Tail: The motile portion of sperm is called the tail or flagellum. The tail portion is divided into 3 parts: The middle piece, Principle piece, End piece. It is 45 mm in length and 0.5 mm in diameter. The axial fibre of the tail remains covered by a double-layered fibrous cytoplasmic sheath. The mitochondria synthesize ATP which supplies energy to the motile tail. The end part of the tail is very fine and uncovered. It looks like a cilium. It is about 5 mm long.
It is the process by which a mature ovum is formed in the ovary.
- Location: Oogenesis occurs in the ovary of a female.
- Time: Ovum production starts during adolescence (8-13 years) period and declines after the age of 40-50 years. But ovum production temporarily ceased during pregnancy and the lactation period of a woman.
Stages of Oogenesis
Phase I: The primitive germ cells of the ovary divide mitotically producing oogonia. These oogonia are stem cells that proliferate by mitosis to form primary oocytes.
Phase II: Primary oocytes from the oogonia enter a prolonged prophase (diplotene stage) of the first meiotic division and remain in this stage until ovulation occurs after puberty. These primary oocytes are now covered by single granulosa cell layers thus forming primordial follicles.
- Phase I: During this phase, the primary oocytes which are in the prophase of the first meiotic division since fetal life completes the 1st meiotic division thus forming secondary oocytes and 1st polar body formed.
- Phase II: Now secondary oocytes immediately begin the second meiotic division but this division also stops at metaphase and is completed only if fertilization occurs. During this time the second polar body is formed along with a mature ovum. This second polar body undergoes atresia, and the mature ovum is involved in fertilization.
Hormonal Control of Oogenesis:
The process of oogenesis (egg production) is controlled by the two centres in the brain, the hypothalamus and the anterior lobe of the pituitary gland. In the center of this hormonal control is the hypothalamic hypophysial (pituitary gland) system with the two hypophysial gonadotropins FSH and LH. The pulsatile liberation of GnRH by the hypothalamus is the fundamental precondition for normal control of the cyclic ovarian function. This cyclic activity releases FSH and LH, from the pituitary gland, both of which stimulate the maturation of the follicles in the ovary and trigger ovulation. During the ovarian cycle, oestrogen is produced by the theca interna and follicular cells and progesterone by the corpus luteum.
The control circuit of the hormonal cycle has two essential control elements:
- The pulsatile liberation of GnRH, as well as FSH and LH.
- The long-loop feedback effect of estrogen and progesterone on the hypothalamic hypophyseal system (these two hormones are synthesized in the [ready to rupture] follicle and so originate in the ovary, thus the name “long loop”).
Significance of Oogenesis:
The significances of oogenesis are as follows:
- One ovum and three polar bodies are formed from a single oogonium.
- The formation of polar bodies maintains half the number of chromosomes of the ovum.
- During meiosis crossing over takes place which brings about variation.
Differences between Spermatogenesis and Oogenesis:
|1. It occurs in the testes.||1. It occurs in the ovaries.|
|2. Spermatogonia change to primary spermatocyte.||2. Oogonia change to the primary oocyte.|
|3. A primary spermatocyte divides to form two secondary spermatocytes.||3. A primary oocyte divides to form one secondary oocyte and one polar body.|
|4. A secondary spermatocyte divides to form two spermatids.||4. A secondary oocyte divides to form one ootid and one polar body.|
|5. No polar body is formed.||5. Polar bodies are formed.|
|6. Four spermatozoa are formed from one spermatogonium.||6. One ovum is formed from an oogonium.|
|7. Sperm is motile and yolkless.||7. Egg or ovum is non-motile and with the yolk.|
Similarities between Spermatogenesis and Oogenesis:
- They are meant for producing male and female gametes from cells of gonads.
- Both processes involve three phases – The multiplication phase, the Growth phase, and the Maturation phase.
- Mitotic divisions occur in the multiplication phase. It increases the number of spermatogonia and oogonia.
- There is the growth of spermatocyte as well as oocyte prior to undergoing meiosis.
Differences between Spermatocyte and Oocyte:
|1. Primary spermatocytes are formed from spermatogonia in the seminiferous tubules of the testes.||1. Primary oocytes are formed from the oogonia in the fetal ovary.|
|2. After meiosis 1 each primary spermatocytes produce two secondary spermatocytes (n).||2. After meiosis 1 each primary oocytes produce secondary oocyte (n) and 1st polar body (n).|
|3. Each secondary spermatocyte then further undergoes meiosis II and forms four spermatids (n).||3. Each secondary spermatocyte then further undergoes meiosis II and forms one ootid and 2nd polar body.|
|4. Each primary spermatocyte forms four (n) spermatids.||4. Each primary oocyte forms one ootid and three polar bodies.|
|5. Spermatids then form sperms.||5. Ootid then form ovum.|
The process by which the mature Graafian follicle ruptures and liberates the ovum on the 14th day of the menstrual cycle is known as ovulation. Ovulation is controlled by LH and Oestrogen hormones.
Structure of Ovum:
The ovum is nearly spherical in shape. It has an outer covering called a vitelline membrane. External to that there is another covering called zona pellucida. The space between the vitelline membrane and zona pellucida is called perivitelline space. The ovum discharged from the Graafian follicle of the ovary has a layer of granular cells. This is called corona radiata.
The cytoplasm of the ovum is called ooplasm. The cytoplasm is covered by a plasma membrane. Very small vesicles termed cortical granules are present under the plasma membrane. The cytoplasm contains a large nucleus, called a germinal vesicle. There are no centrioles in the ovum. The human ovum loses its ability to be fertilized about 24 hours after ovulation.
Differences between Sperm and Ovum:
|1. Sperms are produced in the testes.||1. Ova are produced in the ovaries.|
|2. Four sperms are formed from one spermatogonium.||2. One oogonium produces one ovum.|
|3. Sperms are motile, sperms eject hyaluronidase upon the wall of the ovum for penetration purposes.||3. Ovum are non-motile. It engulfs sperm by forming a reception cone or cone of attraction.|
|4. A mature sperm is made of four parts head, neck, body, and tail.||4. Such types of parts are absent.|
|5. Sperm possess a very small quantity of cytoplasm.||5. Ovum has a large quantity of cytoplasm, termed as ooplasm.|
|6. Mitochondria form a spiral in the body.||6. Mitochondria are scattered.|