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Gametogenesis:Spermato Genesis and Oogenesis

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Gametogenesis:Spermato Genesis and Oogenesis Gametogenesis:Spermato g e n e s i s a n d o o g e n e s i s Dr. Pooja Agrahari Assistant professor Department of zoology c. M. science college, darbhanga Gametogenesis • Gametogenesis is a highly specialized nuclear and cytoplasmic event in which haploid gametes are forms from diploid germ cell. The formation of these gametes is controlled by reduction division. The formation of male gametes and female gametes are called spermatogenesis and oogenesis, respectively. Gametogenesis is divided into four phase- • Extra gonadal Origin of primordial germ cells • Proliferation of germ cell by mitosis • Meiosis • Structural and functional maturation of the spermatozoa and ova. Spermatogenesis • Spermatogenesis occurs in seminiferous tubules of testis. The process of spermatogenesis can be studied in under following- • Phase of multiplication • Phases of growth • Phases of maturation • Phase of multiplication- During multiplication phase primordial germ cell divide by repeated mitotic cell division and produce spermatogonial cell. The spermatogonial cells are diploid cell found next to the basal membrane of seminiferous tubule. • Phase of growth- After multiplication phase spermatogonial cell undergo growth phase. During this phase the size of nucleus and spermatogonial cell increases. This enlargement in the size is due to absorption of nutritive material from sertoli cells. This enlarge cells are called primary spermatodes having diploid number of chromosomes. • Phase of maturation- During this phase first maturation division (meiosis I) occurs resulting in the formation of haploid secondary spermatocyte. Now the secondary spermatocyte undergoes second maturation division (meiosis II) to form spermatids. Thus from each primordial spermatocyte 4 haploid spermatids are produced. Primordial germ cell Multiplication Growth First maturation division Second maturation division Structure of human sperm • The body of mature sperm can be divided into the following parts- • Head- It is anterior most part of sperm and has acrosome derived from “Golgi complex” and nucleus. In human beings, it is spoon shaped. • Head is divided into two parts- • Acrosome- It is anterior most part of the sperm head is smaller than the nuclear part. It help the sperm to penetrate through egg membrane. • Nucleus- It constitutes the major portion of head and consists of a solid and homogenous mass of basochromatid. The sperm nucleus does not contain any nucleolus, RNAs and fluid content. • Neck- It is present between the middle piece and head. It contains proximal centriole of the spermatids, which help in initiation of cleavage in the fertilized egg. • Middle piece- It consists of mitochondria and axial filament. The number of mitochondria depends upon distance travelled by sperm to fertilized egg. • Tail- It is the largest and posterior part of sperm. It is in the form of elongated filament envelope by cytoplasmic sheath and whole structure of tail is surrounded by plasma membrane. Spermiogenesis or spermioteleosis • The spermatids formed from the spermatocytes by maturation division. Spermatids are non-motile spherical bodies and from these spermatids, motile elongated and functional gametes are formed by process known as spermatogenesis or spermiogenesis. The process of spermiogenesis can be studied under the following- • Formation of head- Nuclear morphogenesis- During the process of spermiogenesis the nucleus of spermatid losses its intact fluid content, its RNAs, nucleolus while DNA content remains unchanged. The general shape of sperm nucleus also changes i.e., the spherical form of nucleus become elongated. • Acrosome formation- Acrosome of the sperm is derived from the Golgi apparatus. during early stage the Golgi complex is around in concentric manners near the nucleosome and its vascular start enlarging at at the the time of acrosome vacuoles is small Pro acrosome granule arise. The proacrosomal granules form the core of acrosome. The vacuole now losses its liquid content and its wall spread over the proacrosomal granule and front half of the nucleus. The remaining part of the Golgi complex is discarded gradually • Formation of middle piece- It is derived from the mitochondria of spermatids. In the middle piece the mitochondria remain in the form of mitochondrial helix, which provide energy for the movement of sperm. Mitochondria form a spiral ring behind the neck around the distal centriole and proximal part of axoneme. This is called as nebenkern. • Formation of tail- Tail of sperm is derived from distal centriole. the proximal centriole of spermatid after second meiotic division moves just behind the sperm nucleus. here construction develop around the proximal centriole forming neck. the distal centriole after elongation give rise to the axial filament of the sperm tail. Here a thin film of cytoplasm is present around the axial filament known as cytoplasmic sheath. Oogenesis • Oogenesis is a process in which eggs are produced from the primordial germ cell. The egg and three polar bodies are formed in a programmed series of division. The process of oogenesis involved two phase- • A) pre meiotic Phase- Multiplication • B) meiotic face- Growth/ maturation • Premeiotic phase- It involves phase of multiplication, in which primordial germ cell multiplied by mitosis to form a population of oogonial cell. This process occurs in all vertebrates, but its timing differ. • Meiotic phase-During this phase oogonial cells formed during pre meiotic phase undergo growth phase and maturation phase. Here prophase I is very prolonged as during this stage oogonia undergoes increase in size, due to accumulation of nutrient material. After these changes each oogonial cell is called primary oocyte. During growth phase following changes takes place- • Changes in nucleus- • Changes in cytoplasm- • Vitellogenesis- Synthesis of yolk development is called vitellogenesis. Yolk is a nutritive material which accounts for the rapid increase in the size of primary oocyte. This yolk is utilized during the development of embryo. Chemically yolk is made up of protein, lipid especially phospholipid, a little amount of carbohydrate and water. In some cases several growing oocyte fuse into one to produce a large oocyte, and all the nuclei except one degenerate. In most cases vitellogenesis involve the surrounding cell, namely follicle and nurse cell. The possible organ and their role in vitellogenesis are as follows- • Role of accessory cells- Involved Follicle cell and Nurse Cells • Role of micropenocytosis • Role of mitochondria • Role of endoplasmic reticulum and Golgi complex • Maturation Phase- It involves the completion of meiotic division arrested in growth phase. The completion of meiotic division results in the formation of a single egg and three polar bodies. This involves equal nuclear division but an equal cytoplasmic division. In first meiotic division the diploid nucleus of the primary oocyte divides to form two haploid nucleus. One of which migrate out from the vitellium membrane. This migrated nucleus along with very little amount of cytoplasm forms first polar body. The remaining part of cytoplasm with other haploid nucleus is called secondary oocyte. Thie secondary oocyte undergoes IInd mitotic division to form a second polar body and an egg. The first polar body may or may not divide. If divide it forms two polar bodies. These three polar bodies and one egg are formed from the primary oocyte. Egg membrane • In most of the animals oocyte maturation does not complete until the egg membranes are developed outside the oolema of egg. The egg membranes vary in different animal group and reflect the adaptation made by the animal to ensure development of the young in a particular environment. Depending upon the origin egg membranes can be classified into following group- • Primary egg membrane • Secondary egg membrane.
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