Gametogenesis PDF

WE MAKE DOCTORS GAMETOGENESIS DR. GUADALUPE RODRIGUEZ EMBRYOLOGY OBJECTIVES: Understand the process involving the Primordial Germ Cells. Formation Migration Proliferation (mitosis) Understand the process of Gametogenesis (gender-specific). Identify all the cells and molecules involved....

WE MAKE DOCTORS GAMETOGENESIS DR. GUADALUPE RODRIGUEZ EMBRYOLOGY OBJECTIVES: Understand the process involving the Primordial Germ Cells. Formation Migration Proliferation (mitosis) Understand the process of Gametogenesis (gender-specific). Identify all the cells and molecules involved. Oogenesis Spermatogenesis PRIMORDIAL GERM CELLS Development begins with fertilization, the process by which the male gamete, the sperm, and the female gamete, the oocyte, unite to give rise to a zygote. Gametes are derived from primordial germ cells (PGCs) that are formed in the epiblast during the second week. During the fourth week, these cells begin to migrate from the yolk sac toward the developing gonads, where they arrive by the end of the fifth week. In preparation for fertilization, germ cells undergo GAMETOGENESIS, which includes meiosis, to reduce the number of chromosomes and cytodiflerentiation to complete their maturation. Primordial Germ Cells and Teratomas Oropharyngeal teratoma These tumors may arise from PGCs or from epiblast cells, both of which are pluripotent. Tissues within the tumors include derivatives of all three germ layers and may include gut, bone, skin, teeth, and so forth. MITOSIS MEIOSIS is the cell division that takes place in the germ cells to generate male and female gametes, sperm and egg cells. Meiosis requires two cell divisions, meiosis I and meiosis II, to reduce the number of chromosomes to the haploid number of 23.  Homologous pairs then separate into two daughter cells, thereby reducing the chromosome number from diploid to haploid. Shortly thereafter, meiosis II separates sister chromatids. Each gamete then contains 23 chromosomes. Crossover Segments of chromatids break and are exchanged as homologous chromosomes separate. Approximately 30 to 40 crossovers (one or two per chromosome) with each meiotic I division are most frequent between genes that are far apart on a chromosome. As a result of meiotic divisions: Genetic variability is enhanced through Crossover, which redistributes genetic material Random distribution of homologous chromosomes to the daughter cells Each germ cell contains a haploid number of chromosomes so that at fertilization, the diploid number of 46 is restored. Polar Bodies During meiosis, one primary oocyte gives rise to four daughter cells, each with 22 plus 1 X chromosomes. Only one of these develops into a mature gamete, however, the oocyte; the other three, the POLAR BODIES, receive little cytoplasm and degenerate during subsequent development. Similarly, one primary spermatocyte gives rise to four daughter cells, two with 22 plus 1 X chromosomes and two with 22 plus 1 Y chromosomes  Oogenesis Process whereby oogonia differentiate into mature oocytes.  BEFORE BIRTH Once PGCs have arrived in the gonad of a genetic female, they differentiate into OOGONIA. These cells undergo a number of mitotic divisions, but some of them arrest their cell division in prophase of meiosis I and form PRIMARY OOCYTES. During the next few months, oogonia increase rapidly in number, and by the fifth month of prenatal development, the total number of germ cells in the ovary reaches its maximum, estimated at 7 million. At this time, cell death begins, and many oogonia as well as primary oocytes degenerate and become atretic. By the seventh month, the majority of oogonia have degenerated except for a few near the surface All surviving primary oocytes have entered prophase of meiosis I, and most of them are individually surrounded by a layer of flat follicular epithelial cells A primary oocyte, together with its surrounding flat epithelial cells, is known as a PRIMORDIAL FOLLICLE. A. Primordial follicle consisting of a primary oocyte surrounded by a layer of flattened epithelial cells. B. Early primary follicle recruited from the pool of primordial follicles. C. Mature primary [preantral] follicle with follicular cells and the presence of a well defined zona pellucida. Near the time of birth, all primary oocytes have started prophase of meiosis I, but instead of proceeding into metaphase, they enter the diplotene stage, a resting stage during prophase. Primary oocytes remain arrested in prophase and do not finish their first meiotic division before puberty is reached. This arrested state is produced by oocyte maturation inhibitor (OMI) The total number of primary oocytes at birth is estimated to vary from 600,000 to 800,000. Approximately 40,000 are presented by the beginning of puberty Fewer than 500 will be ovulated  PUBERTY Each month, 15 to 20 follicles selected from this pool begin to mature. As primordial follicles begin to grow, surrounding follicular cells change from flat to cuboidal and proliferate to produce a stratified epithelium of granulosa cells, and the unit is called a PRIMARY FOLLICLE. Granulosa cells and the oocyte secrete a layer of glycoproteins on the surface of the oocyte, forming the ZONA PELLUCIDA. As follicles continue to grow, cells of the theca folliculi organize into: Inner layer of secretory cells: theca interna Outer fibrous capsule: theca externa As development continues, fluid-filled spaces appear between granulosa cells. Coalescence of these spaces forms the ANTRUM, and the follicle is termed a vesicular or an antral follicle. Initially, the antrum is crescent-shaped, but with time, it enlarges. At maturity, the mature vesicular (graafian) follicle may be 25 mm or more in diameter. It is surrounded by the theca interna, which is composed of cells having characteristics of: Steroid secretion Rich in blood vessels The theca externa With each ovarian cycle, a number of follicles begin to develop, but only one reaches full maturity. The others degenerate and become atretic. When the secondary follicle is mature, a surge in luteinizing hormone (LH) induces the preovulatory growth phase. Meiosis I is completed, resulting in formation of two daughter cells of unequal size, each with 23 double-structured chromosomes. The secondary oocyte, receives most of the cytoplasm; the other, the first polar body, receives practically none. The first polar body lies between the zona pellucida and the cell membrane of the secondary oocyte, in the perivitelline space. The cell then enters meiosis II but arrests in metaphase approximately 3 hours before ovulation. Meiosis II is completed only if the oocyte is fertilized; otherwise, the cell degenerates approximately 24 hours after ovulation. Spermatogenesis Includes all of the events by which spermatogonia are transformed into spermatozoa At BIRTH, germ cells in the male infant can be recognized in the sex cords of the testis as large, pale cells urrounded by supporting cells. Supporting cells, which are derived from the surface epithelium of the testis in the same manner as follicular cells, become sustentacular cells or Sertoli cells. Before puberty, the sex cords acquire a lumen and become the seminiferous tubules. At the same time, PGCs give rise to spermatogonial stem cells. At regular intervals, cells emerge from this stem cell population to form type A spermatogonia, and their production marks the initiation of SPERMATOGENESIS. Type A cells undergo a limited number of mitotic divisions to form clones of cells The last cell division produces type B spermatogonia, which then divide to form PRIMARY SPERMATOCYTES Primary spermatocytes then enter a prolonged prophase (22 days) followed by rapid completion of meiosis I and formation of SECONDARY SPERMATOCYTES. During the second meiotic division, these cells immediately begin to form haploid SPERMATIDS. The products of meiosis during spermatogenesis in humans. Spermatogenesis is regulated by LH production by the pituitary gland LH binds to receptors on Leydig cells and stimulates testosterone production, which in turn binds to Sertoli cells to promote spermatogenesis Follicle-stimulating hormone (FSH) is essential because its binding to Sertoli cells stimulates testicular fluid production and synthesis of intracellular androgen receptor proteins  Spermiogenesis The series of changes resulting in the transformation of spermatids into spermatozoon These changes include: Formation of the acrosome: covers half of the nuclear surface and contains enzymes to assist in penetration of the egg and its surrounding layers during fertilization Condensation of the nucleus Formation of neck, middle piece, and tail Shedding of most of the cytoplasm as residual bodies that are phagocytized by Sertoli cells Approximately 300 million sperm cells are produced daily. When fully formed, spermatozoa enter the lumen of seminiferous tubules. From there, they are pushed toward the epididymis by contractile elements in the wall of the seminiferous tubules. They obtain full motility in the epididymis. ABNORMAL GAMETS Abnormal Spermatozoa: Abnormal spermatozoa are seen frequently (up to 10% have observable defects). The head or the tail may be abnormal, spermatozoa may be giants or dwarfs, and sometimes, they are joined. Sperm with morphologic abnormalities lack normal motility and probably do not fertilize oocytes. References: Chapter 2. Gametogenesis: Conversion of Germ Cells into Male and Female Gamets Bevis, R. (1992). Langman’s Medical Embryology, 6th edition T W Sadler Langman’s Medical Embryology, 6th Edition Williams & Wilkins 412pp £20.50 0-683- 07473-8. Nursing Children and Young People, 4(3), 14. https://doi.org/10.7748/paed.4.3.14.s14 Thank You!