Gametogenesis PDF

Life Sciences I Cell biology GAMETOGENESIS  Gametogenesis is the process by which gametes, or germ cells, are produced in an organism. The formation of egg cells, or ova, is technically...

Life Sciences I Cell biology GAMETOGENESIS  Gametogenesis is the process by which gametes, or germ cells, are produced in an organism. The formation of egg cells, or ova, is technically called oogenesis, and the formation of sperm cells, or spermatozoa, is called spermatogenesis. 2 https://www.yourarticlelibrary.com/  Gametogenesis is the production of gametes from haploid precursor cells. In animals and higher plants, two morphologically distinct types of gametes are produced (male and female) via distinct differentiation programs. Animals produce a tissue that is dedicated to forming gametes, called the germ line. Individual germline cells are called germ cells. During the process of gametogenesis, a germ cell undergoes meiosis to produce haploid cells that directly develop into gametes. Hence, in animals, meiosis is an integral part of gametogenesis. 3 E.M. Maine, Gametogenesis, Editor(s): Stanley Maloy, Kelly Hughes, Brenner's Encyclopedia of Genetics (Second Edition), Academic Press, 2013, Pages 154-156, ISBN 9780080961569, https://doi.org/10.1016/B978-0-12-374984-0.00575-1.  Meiosis E.M. Maine, Gametogenesis, Editor(s): Stanley Maloy, Kelly Hughes, Brenner's Encyclopedia of Genetics (Second Edition), Academic Press, 2013, Pages 154-156, ISBN 9780080961569, https://doi.org/10.1016/B978-0-12-374984-0.00575-1.  Sperms and ova are formed respectively in the male and female gonads (testes and ovaries). Both spermatogenesis and oogenesis comprise similar phases: (i) multiplication phase, (ii) growth phase, (iii) maturation phase. The gametes differ from all other cells (= somatic cells) of the body in that their nuclei contain only half the number of chromosomes found in the nuclei of somatic cells. 5 https://www.yourarticlelibrary.com/  Spermatogenesis: The process of formation of sperms is called spermatogenesis. It occurs in the seminiferous tubules of the testes. The seminiferous tubules are lined by germinal epithelium. The germinal epithelium consists largely of cuboidal primary or primordial germ cells (PGCs) and contains certain tall somatic cells called Sertoli cells (= nurse cells). Spermatogenesis includes formation of spermatids and formation of spermatozoa. 6 https://www.yourarticlelibrary.com/ Section of the human testis. Cross section shows the diversity of cellular anatomy within the testis. 7  Spermatogonia, the primitive germ cells next to the basal lamina of the seminiferous tubules, mature into primary spermatocytes. This process begins during adolescence. The primary spermatocytes undergo meiotic division, reducing the number of chromosomes. In this two-stage process, they divide into secondary spermatocytes and then into spermatids, which contain the haploid number of 23 chromosomes. The spermatids mature into spermatozoa (sperm). As a single spermatogonium divides and matures, its descendants remain tied together by cytoplasmic bridges until the late spermatid stage. This arrangement helps ensure synchrony of the differentiation of each clone of germ cells. The estimated number of spermatids formed from a single spermatogonium is 512. The formation of a mature sperm from a primitive germ cell by spermatogenesis in humans spans approximately 74 days. 8 https://www.yourarticlelibrary.com/  Seminiferous epithelium. Note that maturing germ cells remain connected by cytoplasmic bridges through the early spermatid stage and that these cells are closely invested by Sertoli cell cytoplasm as they move from the basal lamina to the lumen. (Reproduced with permission from Junqueira LC, Carneiro J: Basic Histology: Text & Atlas, 10th ed. New York, NY: McGraw-Hill; 2003.)  Formation of Spermatids (It includes the following phases): (a) Multiplication Phase: At sexual maturity, the undifferentiated primordial germ cells divide several times by mitosis to produce a large number of spermatogonia (Gr. sperma = seeds, gonos- generation). Spermatogonia (2N) are of two types: type A spermatogonia and type В spermatogonia. Type A spermatogonia serve as the stem cells which divide to form additional spermatogonia. Type В spermatogonia are the precursors of sperms. 10 https://www.yourarticlelibrary.com/  Formation of Spermatids: (b) Growth Phase: Each type В spermatogonium actively grows to a larger primary spermatocyte by obtaining nourishment from the nursing cells. 11 https://www.yourarticlelibrary.com/  Formation of Spermatids: (c) Maturation Phase: Each primary spermatocyte undergoes two successive divisions, called maturation divisions. The first maturation division is reductional or meiotic. Hence, the primary spermatocyte divides into two haploid daughter cells called secondary spermatocytes. Both secondary spermatocytes now undergo second maturation division which is an ordinary mitotic division to form, four haploid spermatids, by each primary spermatocyte. 12 https://www.yourarticlelibrary.com/  Formation of Spermatozoa from Spermatids (Spermatogenesis): The transformation of spermatids into spermatozoa is called spermiogenesis or spermateliosis. The spermatozoa are later on known as sperms. Thus four sperms are formed from one spermatogonium. After spermiogenesis sperm heads become embedded in the Sertoli cells and are finally released from the seminiferous tubules by the process called spermiation. 13 https://www.yourarticlelibrary.com/  Significance of Spermatogenesis: 1. During spermatogenesis, one spermatogonium produces four sperms. 2. Sperms have half the number of chromosomes. After fertilization, the diploid chromosome number is restored in the zygote. It maintains the chromosome number of the species. 3. During meiosis I crossing over takes place which brings about variation. 4. Spermatogenesis occurs in various organisms. Thus it supports the evidence of the basic relationship of the organisms. 14 https://www.yourarticlelibrary.com/  15 Basic Histology: Text & Atlas, 11th ed. New York, NY: McGraw-Hill; 2005.)  Human spermatozoon Each sperm is an intricate motile cell, rich in DNA, with a head that is made up mostly of chromosomal material. Covering the head like a cap is the acrosome, a lysosome-like organelle rich in enzymes involved in sperm penetration of the ovum and other events associated with fertilization. The motile tail of the sperm is wrapped in its proximal portion by a sheath holding numerous mitochondria. 16 Function of the male reproductive system. Barrett K.E., & Barman S.M., & Brooks H.L., & Yuan J.J.(Eds.), (2019). Ganong's Review of Medical Physiology, 26e. McGraw- Hill. https://accessmedicine.mhmedical.com/content.aspx?bookid=2525&sectionid=204296105  Human spermatozoon, profile view. Note the acrosome, an organelle that covers half the sperm head inside the plasma membrane of the sperm. (Reproduced with permission from Junqueira LC, Carneiro J: Basic Histology: Text & Atlas, 11th ed. New York, NY: McGraw-Hill; 2005.) 17  Mammalian sperm 1. Head: It contains anterior small acrosome and posterior large nucleus. Acrosome is formed from Golgi body of the spermatid. Acrosome contains hyaluronidase proteolytic enzymes which are popularly known as spermlysins that are used to contact and penetrate the egg (ovum) at the time of fertilization. 2. Neck: It is very short and is present between the head and middle piece. It contains the proximal centriole towards the nucleus which plays a role in the first cleavage of the zygote and the distal centriole which gives rise to the axial filament of the sperm. 18 https://www.yourarticlelibrary.com/  Mammalian sperm 3. Middle piece: The middle piece of human sperm contains the mitochondria coiled round the axial filament called mitochondrial spiral. They provide energy for the movement of the sperm. So it is the “power house of the sperm”. At the end of the middle piece there is a ring centriole (annulus) with unknown function. Posterior half of nucleus, neck and middle piece of sperm are covered by a sheath called manchette. 4. Tail: The tail is several times longer than the head. In its most part called main piece, the axial filament is surrounded by a thin layer of cytoplasm. The part behind the main piece is called end piece which consists of naked filament alone. The sperm swims about by its tail in a fluid medium. 19 https://www.yourarticlelibrary.com/  20 https://www.yourarticlelibrary.com/  Summary keynotes: Spermatogenesis: 1. It occurs in the testes. The walls of the seminiferous tubules of vertebrates contain primordial germ cells. 2. The primordial germ cells begin to enlarge with increased metabolism and undergo mitotic division, and produce spermatogonia. 3. The spermatogonium divides meiotically (first meiosis) resulting in two spermato- cytes. 4. Due to second meiotic division (mitosis) of spermatocyte four haploid-spermatids are formed. 5. Spermatids metamorphose to spermatozoa. This is spermiogenesis. (a) In this process spermatid increases in size; centriole divides into two; distal centriole forms the main axis of the tail the axial filament is surrounded by a fibre coat; mitochondria form a spiral sheath; golgi complex forms acroblast and give rise to acrosome. 6. The mature spermatozoon has three distinct zones head, trunk and tail. It can move actively in fluid. 21 https://www.yourarticlelibrary.com/  Oogenesis The process of formation of a mature female gamete (ovum) is called oogenesis. It occurs in the ovaries (female gonads). It consists of three phases: multiplication, growth and maturation. 22 https://www.yourarticlelibrary.com/  Oogenesis (a) Multiplication phase: In the foetal development, certain cells in the germinal epithelium of the ovary of the foetus are larger than others. These cells divide by mitosis, producing a couple of million egg mother cells or oogonia in each ovary of the foetus. No more oogonia are formed or added after birth. The oogonia multiply by mitotic divisions forming the primary oocytes. 23 https://www.yourarticlelibrary.com/  Oogenesis (b) Growth phase: This phase of the primary oocyte is very long. It may extend over many years. The oogonium grows into a large primary oocytes. Each primary oocyte then gets surrounded by a layer of granulosa cells to form primary follicle. A large number of these follicles degenerate during the period from birth to puberty. So at puberty only 60,000- 80,000 primary follicles are left in each ovary. The fluid filled cavity of the follicle is called antrum. 24 https://www.yourarticlelibrary.com/  (c) Maturation phase: Oogenesis Like a primary spermatocyte, each primary oocyte undergoes two maturation divisions, first meiotic and the second meiotic. The results of maturation divisions in oogenesis are, however, very different from those in spermatogenesis. In the first, meiotic division, the primary oocyte divides into two very unequal haploid daughter cells— a large secondary oocyte and a very small first polar body or polocyte. In the second maturation division, the first polar body may divide to form two second polar bodies. The secondary oocyte again divides into unequal daughter cells, a large ootid and a very small second polar body. The ootid grows into a functional haploid ovum. Thus from one oogonium, one ovum and three polar bodies are formed. The ovum, is the actual female gamete. The polar bodies take no part in reproduction and, hence, soon degenerate. In human beings, ovum is released from the ovary in the secondary oocyte stage. The maturation of secondary oocyte is completed in the mother’s oviduct (Fallopian tube) usually after the sperm has entered the secondary oocyte for fertilization. https://www.yourarticlelibrary.com/  26 https://www.yourarticlelibrary.com/  In humans (and most vertebrates), the first polar body does not undergo meiosis II, whereas the secondary oocyte proceeds as far as the metaphase stage of meiosis II. However, it then stops advancing any further; it awaits the arrival of sperm for completion of meiosis II. Entry of the sperm restarts the cell cycle breaking down MPF (M- phase promoting factor) and turning on APC (Anaphase promoting complex). Completion of meiosis II converts the secondary oocyte into a fertilized ovum (egg) or zygote (and also a second polar body). 27 https://www.yourarticlelibrary.com/  Significance of Oogenesis: 1. One oogonium produces one ovum and three polar bodies. 2. Polar bodies have small amount of cytoplasm. It helps to retain sufficient amount of cytoplasm in the ovum which is essential for the development of early embryo. Formation of polar bodies maintains half number of chromosomes in the ovum. 3. During meiosis first crossing over takes place which brings about variation. 4. Oogenesis occurs in various organisms. Therefore, it supports the evidence of basic relationship of the organisms. 28 https://www.yourarticlelibrary.com/  Summary keynotes: Oogenesis: 1. It occurs in the ovary. The phases of division in multiplication are similar to those in spermatogenesis. 2. The primary germ cells of the ovary undergo several mitotic divisions to produce oogonia. 3. The oogonium gives rise to primary oocytes mitotically. 4. A primary oocyte undergoes first meiotic division and produces one large secondary oocyte and one small first polar body or polocyte. 5. The oocyte and the polocyte divide mitotically (second meiotic division). The oocyte produces one large ootid and one small second polar body and the polocyte two polar bodies. 6. In oogenesis one haploid ootid and three polar bodies are formed. On maturity, 29 the ootid is transformed into ovum. https://www.yourarticlelibrary.com/  30 https://www.yourarticlelibrary.com/  Ovum or Egg: The mature ovum or female gamete is spherical in shape. Its cytoplasm is called ooplasm containing large nucleus, termed the germinal vesicle. The nucleus contains a prominent nucleolus. The cytoplasm is enveloped by a cell membrane (plasma membrane). The membrane forming the surface layer of an ovum is called the vitelline membrane (it is the primary egg membrane, e.g., in insects, molluscs, amphibians and birds). In tunicates and fishes the vitelline membrane is known as the chorion. The mammalian eggs contain similar membrane and in them is known as the zona pellucida. 31 https://www.yourarticlelibrary.com/  Ovum or Egg: A narrow perivitelline space is present between the zona pellucida and plasma membrane. The secondary egg membranes are secreted by the ovarian tissues around the primary egg membranes. They are composed of chitinous shells in insects, ascidians and cyclostomes or jelly coats in amphibians. The tertiary egg membranes are secreted by the oviduct or uterus e.g., jelly that protects the egg of bony fishes and amphibians and albumen (egg white), shell membranes and the outer most calcareous porous shell of the egg of reptiles and birds. The side of the ovum which extrudes polar bodies is termed the animal pole. The opposite side is called the vegetal pole. 32 https://www.yourarticlelibrary.com/  33 https://i.pinimg.com/originals/6b/f0/a3/6bf0a3775ef124a83a5d2d708155b803.jpg  Egg Envelops All membranes (covering) which are present outside to plasma membrane of oocyte or ovum is known as egg membranes. Egg envelope on the basis of origin are of three types: (1) Primary egg envelops - These are secreted by egg. e.g. Vitelline membrane-(Zona pellucida & Zona Reticulate) (2) Secondary egg envelops - These are secreted by ovary. e.g. - Chorion on insects. (3) Tertiary egg envelops - These are secreted by oviduct. e.g.- Jelly coat of frog, Albumin, shell membrane and shell of Hen. 34  Types of Eggs: Based on the distribution of yolk in the cytoplasm eggs are of the following types: 1. Isolecithal eggs: In isolecithal eggs, the yolk is uniformly distributed throughout the ooplasm (eg. Urchin). 2. Telolecithal eggs: In eggs containing moderate or large quantity of yolk, the distribution of yolk is not uniform. lt is concentrated more towards the vegetal pole. 3. Mesolecithal eggs: In amphibian, Dipnoi and Petromyzon the amount of yolk present is moderate and is not high. 4. Centrolecithal eggs: The yolk is localized at the centre e.g., eggs of insects. 5. Alecithal eggs: When the egg contains no yolk, it is called alecithal egg. Eg.: the eggs of eutherian mammals. 35 https://www.yourarticlelibrary.com/  36 https://i1.wp.com/manabu-biology.com/wp-content/uploads/2017/01/7f2cbc77-1.png?fit=173%2C214  37 https://qph.fs.quoracdn.net/main-qimg-934e19b9fcc6a2034eae51bb6afeb801  Literature 1. Function of the male reproductive system. Barrett K.E., & Barman S.M., & Brooks H.L., & Yuan J.J.(Eds.), (2019). Ganong's Review of Medical Physiology, 26e. McGraw- Hill. https://accessmedicine.mhmedical.com/content.aspx?bookid=2525&sec tionid=204296105. 2. Raghavan, V. (1989). Gametogenesis and fertilization. In Developmental Biology of Fern Gametophytes (Developmental and Cell Biology Series, pp. 238-258). Cambridge: Cambridge University Press. doi:10.1017/CBO9780511529757.013 3. https://www.yourarticlelibrary.com/ 38 www.lsmu.lt