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Home , Gamete, Gametogenesis, Oocyte

CHAPTER-1: Gameogenesis Prepared by: Dr. Md. Ariful Alam, Associate Professor

GAMETOGENESIS

Gametogenesis is the processes of formation in the sexually reproducing . The sexually reproducing animals contain two types of cells in their body such as and the germinal cells. The somatic cells form various organs of the body and provide a phase for the maturation, development and formation of the germinal cells. The somatic cells always multiply by mitotic division. The germinal cells form the (testis and ) in the body. These cells produce the gamete cells by successive mitotic and meiotic divisions. The male gamete is known as spermatozoa or and the gamete is known as ovum or . The process of sperm production is known as and the process of ovum production is known as .

Spermatogenesis: The process of sperm production is known as spermatogenesis which occurs in the male gonads or testis. The testis of the vertebrate are composed of many seminiferous tubules which are lined by the cells of germinal epithelium. The cells of the germinal epithelium form by the process of spermatogenesis. The spermatogenesis is a continuous process and it has four stages:

1. Multiplication/ stage 2. Growth stage 3. Maturation or stage 4. Differentiation stage

Multiplication phase: The undifferentiated germ cells or primordial cells contain large-sized and chromatin rich nuclei. These cells multiply by repeated mitotic divisions and produce the cells which are known as the spermatogonia with 2n each. All the spermatogonia produced do not participate in spermatogenesis which ensures that stem cells never run out of supply. Growth The two produced daughter cells initially have chromosomes with just one . These cells enter into a period of growth, known as , which allows the cells to replenish their chromosomal material. The end result is two cells with double-chromatid chromosomes. These cells are known as primary . Maturation or Meiosis The primary spermatocytes enter into meiosis, which is a double division. The first division, meiosis I, is called the reductional division. It produces two daughter cells each with n

1 CHAPTER-1: Gameogenesis Prepared by: Dr. Md. Ariful Alam, Associate Professor chromosomes (half the starting number, 2n). These daughter cells are called secondary spermatocytes. It is worth noting that the chromosomes of these cells have two .

The second division, meiosis II, is called the equational division, and is nearly identical to mitosis. However, it is very important to note that there is no period of "intermission," or interphase, between the meiosis I and meiosis II. The previously obtained daughter cells each produce two new daughter cells, which bring the total of daughter cells to four. The newly formed daughter cells have n chromosomes but with one chromatid. They are called . All of these progeny cells remain attached to each other by cytoplasmic bridges. The bridges remain until sperm are fully differentiated.

Differentiation or Spermiogenesis Differentiation is essentially the process of transforming spermatids into bona fide, mature sperm cells, with all the features necessary for the task they’re designed to do—most important of all, motility. Once all the previous divisions are done, and spermatids are formed, differentiation starts. Differentiation is also known as spermiogenesis, not to be confused with spermatogenesis. During differentiation, the is gradually molded into an elongated shape, and large portions of the are shed off. Most notably, the spermatid develops an acrosome, produced by the Golgi apparatus, and a that looks roughly like a long tail and is responsible for motility. The flagellum is produced by the present in the spermatid. The nucleus is squeezed into an elongated shape and forms with the acrosome the head of the sperm, the acrosome occupying the topmost part of the head. The mitochondria which act as power for cells are arranged in the middle piece of the sperm along with the . Once differentiation is complete, the sperm cell separated and migrates into the lumen of the seminiferous tubules.

2 CHAPTER-1: Gameogenesis Prepared by: Dr. Md. Ariful Alam, Associate Professor

Fig: Flow diagram of spermatogenesis process

Oogenesis The process of formation of ova which occur in the cells of the germinal epithelium of the , such cells are known as primordial germinal cells. The oogenesis completed in the following three successive stages: 1. Multiplication/ mitosis stage 2. Growth stage 3. Maturation or Meiosis stage

1. Multiplication stage: For oogenesis some cells of germinal epithelium become large sized and multiply mitotically to form a population of egg mother cells or oogonia. When mitosis stops the oogonia are

3 CHAPTER-1: Gameogenesis Prepared by: Dr. Md. Ariful Alam, Associate Professor called . The enters the of meiosis I and now is called primary oocytes. Before the completion of first meiosis the primary oocytes pass through a phase of growth and differentiation.

2. Growth stage: In the growth phase, the size of the primary oocyte increases enormously. In the primary oocyte large amount of fats and become accumulated in the form of and due to is heavy weight it is usually concentrated towards the lower portion of the egg forming the vegetal pole. The cytoplasm of the oocyte becomes rich in RNA, DNA, ATP and enzymes. Moreover the mitochondria, golgi complex, , etc. become concentrated in the cytoplasm of the oocyte. During the growth phase, tremendous change also occur int nucleus of the primary oocyte. The nucleus becomes large due to the increased amount of the nucleoplasm. The chromosomes change their shape and become giant lampbrush . When the growth of the cytoplasm and nucleus of the primary oocyte is completed it becomes ready for the maturation phase.

3. Maturation Phase: The maturation phase is accompanied by the maturation or meiotic division. The maturation division of the primary oocyte differs greatly from the maturation division of the . Here after the meiotic division of the nucleus, the cytoplasm of the oocyte from a singe large sized haploid egg and three polar bodies or polocytes at the end. Therefore these divisions allow one cell out of the four daughter cells to contain most of the cytoplasm and reserve food material which is sufficient for the developing . For the shake of convenience this maturation division can be grouped in the following two stages:

(i) First maturation division: During frirst maturation division or first meiosis the homologous chromosomes of the primary oocyte nucleus pass through the pairing or , duplication, chiasma formation and crossing over. Soon after the nuclear membrane breaks and the bivalent chromosomes move towards the opposite poles due to concentration of chromosomal fibres. A new nuclear envelope is developed around the daughter chromososme by the endoplasmic reticulum.

After karyokinesis the unequal cytokinesis occurs and a small haploid and a large haploid secondary oocyte are formed.

(ii) Second meiotic division: The haploid secondary oocyte and first polar body pass through the second meiotic division. Due to a second meiotic division the secondary oocyte forms a

4 CHAPTER-1: Gameogenesis Prepared by: Dr. Md. Ariful Alam, Associate Professor mature egg and a second polocyte. By the second meiotic division the first polar body also divides into two secondary polocyte. These polocytes degenerate while the haploid becomes ready for the fertilization.

Fig: Flow diagram of oogenesis process

GENETIC SIGNIFICANCE OF MEIOSIS

The meiosis has following genetic significances: 1. The basic cytogenetical significance of meiosis is the formation of four monoploid (haploid) nuclei from a single diploid one, in two successive divisions, thus balancing the doubling of chromosome number that results from syngamy (fertilization). 2. The crossing over which occurs in its prophase I, provides new combinations of genetical substance and hence new combinations of characters in offsprings. 3. The two members of a homologous pair of chromosomes pass on two different daughter cells. This process is called segregation of chromosomes and it results in different combination of chromosomes and consequently different combination of characters in both daughter cells.

5 CHAPTER-1: Gameogenesis Prepared by: Dr. Md. Ariful Alam, Associate Professor

Fertilization in The pathway leading to the fusion between a single sperm and a previously quiescent egg, and the subsequent union of male and female pronuclei, consists of a sequence of events.

The chorion and the micropyle Mature ovulated eggs of most teleost fishes are enclosed by a complex, acellular and intricate membrane called chorion. The male gamete gains access to the egg surface by passing through a specialized opening in the chorion known as the micropyle. The chorion of teleosts egg typically possesses a single micropyle. It consists of a peripheral depression or vestibule which is continuous with an inner micropylar canal. The micropylar canal is typically funnel shaped. The micropyle is formed with the differentiation and growth of the chorion during oocyte development.

Organization of egg prior to fertilization A. The cortical cytoplasmic layer (Cortex) Fully grown physiologically immature eggs of most teleosts appear to be compartmentalized into a central mass of yolk and a peripheral layer of ooplasm known as the cortical cytoplasm or cortex.

B. The cortical granules (alveoli) The cortical cytoplasm of the eggs contains membrane-limited cortical granules. The endoplasmic reticulum and the Golgi complex of the developing oocyte are involved in the formation of cortical granules and their contents (lipid droplets). This consists of glycoprotein.

C. The sperm entry site The site at which the fertilization sperm enters the egg is topographically restricted by the micropylar canal of the chorion.

Interactions and fusion of Fertilization in freshwater fishes appears to be very rapid and efficient event. A. Egg substances and sperm behaviour The swarming of many sperm in or near the micropyle during teleost fertilization suggests that the egg proper and/or the chorion may release a substance that attracts homologous spermatozoa. The egg envelope produces initiating factors that chemotactically attract sperm toward the egg at fertilization.

6 CHAPTER-1: Gameogenesis Prepared by: Dr. Md. Ariful Alam, Associate Professor

B. Recognition and binding Sperm bind to the walls of the micropylar canal during fertilization suggests that the first level of recognition between sperm and egg is at the chorion.

C. Gamete fusion/fertilization The process of fertilization involves the direct fusion of the plasma membrane of the egg with the plasma membrane of the sperm.

Polyspermy preventing mechanisms Millions of sperm are produced by the testis for each egg cell. Fertilization involves a single sperm nucleus fusion with the egg nucleus. If more than one sperm fuses with the egg nucleus the results are disastrous and usually lethal for the developing . Polyspermic zygote typically encounters difficulties during and dies shortly after fertilization. The presence of the chorion by itself reduces the collision of many sperm with the egg surface. The block to polyspermy appears to be primarily mechanical and guaranteed through the structural design of the micropylar apparatus. In some fishes the inner opening of the micropyle is so narrow that only one sperm can enter into the egg (example Zebrafish, medaka). Blocks to polyspermy in teleost fishes may also be mediated through modification of the egg structure at the site of sperm entry and the secretion of substances that influence sperm behaviour.

The condition of monospermy is maintained by a constellation of strategies In the carp for example, restriction of access to the cell surface by the presence of a single micropyle in the chorion, the formation of a fertilizer cone which pushes supernumerary sperm out of the micropylar canal, the agglutination of excess sperm by discharged cortical granule exudate, and reduction in the effective diameter of the inner micropylar aperture by the cluster of sperm entry site microvilli are factors that cooperate to block polyspermy. Fertilization cone A localized movement of the cytoplasm leads to the formation of a protuberance called the fertilization cone or ooplasmic protrusion at the site of sperm entry.

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