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Developmental Biology/Embryology Gametogenesis: 1 Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college GE-2T: Developmental Biology/Embryology Gametogenesis: 1. What is spermatogenesis ? Spermatogenesis is the process by which haploid spermatozoa develop from germ cells in the seminiferous tubules of the testis. This process starts with the mitotic division of the stem cells located close to the basement membrane of the tubules. These cells are called spermatogonial stem cells. 2. Describe the process of spermatogenesis. In the process of spermatogenesis, spermatogonial stem cells develop into mature sperms. It occurs in the male gonads testis. Testes are made up of many seminiferous tubules lined by germinal epithelium. Cells of this layer divide to form spermatozoa in the following four steps: (1) Multiplication Phase, (2) Growth Phase, (3) Maturation Phase, and (3) Spermiogenesis. 1) Multiplication Phase: At maturity, the primordial germ cells divide by mitosis to produce a large number of spermatogonia. Type A spermatogonia is the stem cells which divide to form spermatogonia. Type B spermatogonia are the precursors of sperms. Spermatogonia have spherical or oval nuclei, and rest on the basement membrane. 2) Growth Phase: Type B spermatogonium actively grows to a primary spermatocyte. It obtains nourishment from the nursing cells. Cells in prophase of the first meiotic division are primary spermatocytes. They are characterized by highly condensed chromosomes giving the nucleus a coarse chromatin pattern and an intermediate position in the seminiferous epithelium. This is a long stage, so many primary spermatocytes can be seen. 3) Maturation Phase: Each primary spermatocyte undergoes two maturation divisions. The first maturation division is reductional and forms two haploid daughter cells called secondary spermatocytes. Both secondary spermatocytes then undergo second maturation division to form four haploid spermatids. Spermatids are spherical cells with interphase nuclei, positioned high in the epithelium. All of these progeny cells remain attached to each other by cytoplasmic bridges. The bridges remain until sperm are fully differentiated. Page | 1 GE-2T: Developmental Biology/Embryology Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college 4) Spermiogenesis: This is the metamorphosis of spherical spermatids into elongated spermatozoa. The spermatozoa are then known as sperms. No further mitosis or meiosis occurs. During spermiogenesis, the acrosome forms, the flagellar apparatus forms, and most excess cytoplasm (the residual body) is separated and left in the Sertoli cell. Spermatozoa are released into the lumen of the seminiferous tubule. A small amount of excess cytoplasm (the cytoplasmic droplet) is shed later in the epididymis. Figure: Process of spermatogenesis 3. What is oogenesis ? Oogenesis is the process of the formation of a mature ovum (female gamete) from the oogonia in females. It takes place in the ovaries. Oogenesis is initiated in the embryonic stage. 4. Describe the process of oogenesis. Oogenesis is process by which female gametes (egg cells) are produced in the ovaries of the foetus before birth. It begins during fetal development when oogonia are formed from primordial germ cells by mitosis. It occurs in three phases i.e., 1) Multiplication phase, 2) Growth phase and 3) Maturation phase Page | 2 GE-2T: Developmental Biology/Embryology Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college 1) Multiplication phase: The germinal epithelial cells of ovary divide repeatedly by mitotic division until many diploid oogonia are formed. The oogonia grow to form primary oocytes. Primary oocytes begin first meiotic division but cell division ceases in Prophase I until on set of puberty. Primary oocytes surrounded by a layer of follicle cells. 2) Growth phase: In the growth phase, the increase in the size of the primary oocyte is very considerable. It is a slow process and may take a long period. In human female, the primary oocytes are already formed at the time of birth and continue growth only after attaining sexual maturity. It contains a diploid number of chromosomes. 3) Maturation phase: At puberty some follicles develop each month in response to FSH produced by pituitary gland. Primary oocyte completes first meiotic division to forms two cells of different sizes due to unequal distribution of cytoplasm. The one with less cytoplasm become the first polar body (n) which eventually degenerates. The larger cell becomes secondary oocyte (n). The follicle cells surrounding the primary follicle develop into the secondary follicle. The follicle layer of the secondary oocyte thickens and folds to form the Graafian follicle. When the Graafian follicle become matures, it will move towards the surface of the ovary wall and rupture to release the secondary oocyte (n). The secondary oocytes proceeds to meiosis II & stops at prophase II. Meiosis II is completed if cell is fertilized by a sperm (n). The final products of meiosis II are the haploid ootid or ovum and the second polar body. Page | 3 GE-2T: Developmental Biology/Embryology Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college Page | 4 GE-2T: Developmental Biology/Embryology Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college 5. What are the differences between spermatogenesis and oogenesis ? Aspect Spermatogenesis Oogenesis Site of process Entirely in the testes Mostly in the ovaries Cells produced Sperm Ova or egg Size of cells Small Big Cell structure Consist of the head, middle Round pieces and tail Number of Four funcional cells One funcional cells and three gamates non-funcional polar bodies produced. Meiosis Equal division of cells. Unequal division of cytoplasm. Occurs continuously Not continues. Stops at meiosis I. Meiosis II occurs only if sperm penetrates the secondary oocyte. Density of Many mitochondria, less Fewer mitochondria, more organelles cytoplasm. cytoplasm. Parent cells Infinite number of cells can Limited number of cells can become sperm. become ova. 6. What are the similarities of spermatogenesis and oogenesis ? a) Both occur in the reproductive organs. b) Both produces haploid(n) gamates that are involved in fertilization. c) Both involve meiosis. 7. What is Spermiogenesis ? Spermiogenesis is a process of metamorphosis from a round cell with typical organelles to a highly specialized, elongated cell well adapted for traversing the male and female reproductive tracts and achieving fertilization of an egg. Page | 5 GE-2T: Developmental Biology/Embryology Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college Figure: Transition of spermatid to spermatozoan 8. Describe the ultra structure of sperm. A spermatozoan is a haploid male gamete whose primary function is to fuse with ovum to restore diploid and to transmit paternal characters to the offspring’s. A mammalian sperm is minute, microscopic, flagellated and motile gamete with no nutritive material, protective envelopes and most of cell- organelles like ribosome, endoplasmic reticulum, etc. The whole body of sperm is enveloped by plasma membrane only. It is basically formed of four parts, each performing a specific function: 1. Head: Shape of head varies in different mammals. It is generally oval and flat (in man, bull, rabbit). Basically the head is formed of two parts: Acrosome: It is small cap-like pointed structure present at the tip of nucleus. It is formed from a part of Golgi body of spermatid. During the sperm entry, the acrosome secretes a lytic enzyme, called hyaluronidase, which helps in the penetration of ovum. Nucleus: It is generally long, narrow and pointed but is flat and oval in human sperm. It is formed by condensation of nuclear chromatin of spermatid and loss of RNA, nucleolus and acidic proteins. Chemically, the nucleus is formed of deoxyribonucleoprotein (DNA+basic proteins). It is the carrier of genetic information. Acrosome and anterior half of nucleus are covered by a fibrillar sheath galea. Page | 6 GE-2T: Developmental Biology/Embryology Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college 2. Neck: It is the smallest part of spermatozoan and may be indistinct. It is formed of two centrioles perpendicular to each other and is formed from the centrosome of spermatid. Each centriole is a micro tubular triplet structure having 9+0 arrangement. Proximal centriole lies in a depression in the posterior surface of the nucleus and is perpendicular to main axis of the sperm. Distal centriole is along longitudinal axis of the sperm. Centrioles form spindle for the first cleavage of zygote. Distal centriole acts as basal body and gives rise to axoneme of the sperm-tail. 3. Middle piece: It lies behind the neck and is cylindrical in the human sperm. It is formed of a mitochondrial spiral, nebenkem, around the proximal part of axoneme. The mitochondria are the carriers of the oxidative enzymes and the enzymes which are responsible for oxidative phosphorylation. So the middle piece is the powerhouse of a sperm. Posterior half of nucleus, neck and middle piece of sperm are covered by a sheath, manchette. 4. Tail (flagellum): It is the longest part of sperm. It is slender and tapering part. It is formed of two parts: Central, contractile and micro tubular part called axoneme or axial filament, and outer protoplasmic sheath. Axoneme is formed of 11 proteinous microtubules arranged in 9+2 manner. Sometimes, a ring centriole may be present at the junction of middle piece and flagellum. Tail shows lashing movements which provide forward push to the sperm. Sometimes, the distal part of axoneme is uncovered and is called end piece. Viability: It is the period up to which the sperm is able to fertilize an ovum. Viability of human sperm is about 24 hours. Page | 7 GE-2T: Developmental Biology/Embryology Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college 9. Describe the ultra structure of ovum.
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