Gametogenesis Lecture Notes PDF

(001) GAMETOGENESIS DR.OANDASAN | 09/27/2020 OUTLINE I. WHAT IS GAMETOGENESIS II. THE CHROMOSOME THEORY OF INHERITANCE...

(001) GAMETOGENESIS DR.OANDASAN | 09/27/2020 OUTLINE I. WHAT IS GAMETOGENESIS II. THE CHROMOSOME THEORY OF INHERITANCE III. MITOSIS AND MEIOSIS IV. OOGENESIS V. PRIMORDIAL GERM CELL VI. GAMETES VII. OOGENESIS CONT. VIII. MATURATION OF HAIR FOLLICLE IX. SPERMIOGENESIS Additional facts:  FOUR PHASES OF GAMETOGENESIS: 1. The extraembryonic origin of the germ cells and their I. WHAT IS GAMETOGENESIS migration into the gonads 2. An increase in the number of germ cells by mitosis  Process by which the diploid germ cells undergo a number 3. A reduction in chromosomal number by meiosis of chromosomal and morphological changes to form mature 4. Structural and functional maturation of the eggs and haploid gamets. spermatozoa  Process involved in the maturation of the two highly specialized cells, spermatozoon in males and ovum in females before they unite to form zygote. II. THE CHROMOSOME THEORY OF INHERITANCE Spermatozoon + Ovum = Zygote  23,000 genes on 46 chromosome  Genes on the same chromosome tend to be inherited together and so are known as linked genes.  In somatic cells, chromosomes appear as 23 homologous pairs to form the diploid number of 46.  22 pairs of matching chromosomes- DIPLOID  22 – autosomes and 1 pair of sex chromosome  XX – the individual is genetically female.  XY- the individual is genetically male. III. MITOSIS AND MEIOSIS Mitosis  Process whereby one cell divides, giving rise to two daughter cells, that are genetically identical to the parent cell. o Prophase o Prometaphase o Metaphase o Anaphase o Telophase  (001) GAMETOGENESIS DR.OANDASAN | 09/27/2020 Meiosis  The primordia of the gonads give a chemostatic attractant for the germ cells. o Cell division that takes place in the GERM CELLS to generate male and female gametes, sperm and egg cells. o Meiosis requires two cell divisions, Meiosis I and Meiosis II, to reduce the number of chromosomes to the haploid number of 23. o As in mitosis, male and female germ cells (spermatocytes and primary oocytes) at the beginning of meiosis I replicate DNA so that each of the 46 chromosomes is duplicated into sister chromatids. o Crossovers, critical events in the meiosis I, are the o Interchange of chromatid segments between paired homologous chromosomes.  Germ cells directed toward the gonads by the mature cellular o Segments of chromatids break and are and non-cellular microenvironment that surround them Or by exchanged as homologous chromosomes tissue rearrangement that occur in the early embryos separate.  If they fail to reach the ridges, the gonads will not develop. o As separation occurs, points of interchange  So, it has the inductive influence on the development of are temporarily united and form an X-like gonads into ovary and testes structure, a CHIASMA.  Determine sex of the embryo. o Genetic variability is enhanced through Crossover, which redistributes genetic material. o Random distribution of homologous chromosomes to the daughter cells. VI. GAMETES  Gametes are derived from primordial germ cells (PGCs) that are formed in the epiblast during the second week, move through the primitive streak during gastrulation, and migrate to the wall of the yolk sac  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. Mitotic divisions increase their number during their migration and also when they arrive in the gonad. In preparation for fertilization, germ cells undergo gametogenesis, which includes meiosis, to reduce the number of chromosomes and cytodifferentiation to complete their maturation. IV. OOGENESIS The process involved in the development of a mature ovu From yolk sac – Primitive germ cells at 3rd week V. PRIMORDIAL GERM CELL  Appears in the wall of the endodermal layer of the yolk sac due to their large size and high content of alkaline phosphatase, and migratby amoeboid movement toward the hind gut epithelium and then through dorsal mesentery reach to the primordia of the gonads (primitive sex glands).  Once PGCs have arrived in the gonad of a genetic female,  Become recognizable at 24 days post-fertilization. they differentiate into oogonia  Invading the genital ridges in the 6th week of development.  (001) GAMETOGENESIS DR.OANDASAN | 09/27/2020  These cells undergo a number of mitotic divisions, and by the end of the third month, they are arranged in clusters surrounded by a layer of flat epithelial cell.  The majority of oogonia continue to divide by mitosis, but some of them arrest their cell division in prophase of meiosis I and form primary oocytes. VII. OOGENESIS CONT.  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 7th month 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  As primordial follicles begin to grow, surrounding follicular epithelial cells. follicular cells change from flat to cuboidal and  A primary oocyte, together with its surrounding flat proliferate to produce a stratified epithelium of epithelial cells, is known as a primordial follicle. granulosa cells, and the unit is called a primary  Near the time of birth, all primary oocytes have follicle. started prophase of meiosis I, but instead of  Granulosa cells rest on a basement membrane proceeding into metaphase, they enter the separating them from surrounding ovarian diplotene stage, a resting stage during prophase connective tissue (stromal cells) that form the that is characterized by a lacy network of theca folliculi. chromatin.  Also, granulosa cells and the oocyte secrete a  Primary oocytes remain arrested in prophase and layer of glycoproteins on the surface of the 00-cyte, do not finish their first meiotic division before forming the zona pellucida puberty is reached. VIII. MATURATION OF HAIR FOLLICLE  This arrested state is produced by oocyte maturation inhibitor (OMI), a small peptide secreted by follicular cells.  At puberty, a pool of growing follicles is established and continuously maintained from the supply of primordial follicles.  Each month, 15 to 20 follicles selected from this pool begin to mature.  Some of these dies, whereas others begin to accumulate fluid in a space called the antrum, thereby entering the antral or vesicular stage.  Fluid continues to accumulate such that, immediately prior to ovulation, follicles are quite swollen and are called mature vesicular follicles or graafian follicles.  The antral stage is the longest, whereas the mature vesicular stage encompasses approximately 37 hours prior to ovulation.  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 Granulosa cells surrounding the oocyte remain intact and form the cumulus oophorus.  (001) GAMETOGENESIS DR.OANDASAN | 09/27/2020  At about 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.  With each ovarian cycle, a number of follicles begin to develop, but usually, only one reaches full maturity.  When the secondary follicle is mature, a surge in luteinizing hormone (LH) induces the preovulatory growth phase.  Meiosis I is completed.  One cell, 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.  Primary spermatocytes then enter a prolonged prophase (22  The cell then enters meiosis II but arrests in metaphase days) followed by rapid completion of meiosis I and formation approximately 3 hours before ovulation. of secondary spermatocyte.  During the second meiotic division, these cells immediately  Meiosis II is completed only if the oocyte is fertilized; begin to form haploid spermatids. otherwise, the cell degenerates approximately 24 hours after  Spermatogenesis is regulated by LH production by the ovulation. pituitary gland  The first polar body may undergo a second division.  LH binds to receptors on Leydig cells and stimulates  At birth, germ cells in the male infant can be recognized in testosterone production, which in turn binds to Sertoli cells the sex cords of the testis as large, pale cells surrounded by to promote spermatogenesis. supporting cells.  Follicle-stimulating hormone (FSH) is also essential  Supporting cells, which are derived from the surface because its binding to Sertoli cells stimulates testicular fluid epithelium of the testis in the same manner as follicular cells, production and synthesis of intracellular androgen receptor become sustentacular cells or Sertoli cells. proteins. IX. SPERMIOGENESIS  The series of changes resulting in the transformation of spermatids into spermatozoa A. Formation of the acrosome, which covers half of the nuclear surface and contains enzymes to assist in penetration of the egg and its surrounding layers during fertilization. B. Condensation of the nucleus. C. Formation of neck, middle piece, and tail. D. Shedding of most of the cytoplasm as residual bodies that are phagocytized by Sertoli cells.  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.  (001) GAMETOGENESIS DR.OANDASAN | 09/27/2020  Although initially only slightly motile, spermatozoa obtain full e. Secondary spermatocyte motility in the epididymis. 4. In a routine chest x-ray examination, the radiologist sees what appear to be teeth in a mediastinal mass. What is the likely diagnosis, and what is a probable embryological explanation for its appearance? 5. When does meiosis begin in the female and in the male? 6. At what stages of oogenesis is meiosis arrested in the female? 7. What is the underlying cause of most spontaneous abortions during the early weeks of pregnancy? 8. What is the difference between spermatogenesis and spermiogenesis? 9. The actions of what hormones are responsible for the changes in the endometrium during the menstrual cycle? 10. Sertoli cells in the testis are stimulated by what two major reproductive hormones? REFERENCE OF ADDITIONAL FACTS AND REVIEW QUESTIONS: Book by Bruce M. Carlson, Human Embryology and Developmental Biology (updated edition) Clinical Correlates: Numerical Abnormalities  Trisomy 21 (Down Syndrome)  Trisomy 18  Trisomy 13  Klinefelter Syndrome  Turner syndrome Structural Abnormalities  Cri-du-chat syndrome  Angelman syndrome  Prader-willi syndrome  Miller-dieker syndrome  22q11 syndrome  Fragile X syndrome REVIEW QUESTIONS: 1. During spermatogenesis, histone is replaced by what to allow better packing of the condensed chromatin in the head of the spermatozoon? a. Inhibin b. Prostaglandin E c. Testosterone d. Protamine e. Androgen-binding protein 2. Which cell type is located outside the blood-testis barrier? a. Spermatozoon b. Secondary spermatocyte c. Spermatid d. Primary spermatocyte e. Spermatogonium 3. Which of the following cells normally participates in mitotic divisions? a. Primary oocyte b. Oogonium c. Primary spermatocyte d. Spermatid