Gametogenesis Mona A. Elsafadi Gametogenesis a process of formation and development of specialized generative cells (gametes)
Involves chromosomes and cytoplasm of the gametes. Prepares sex cells for fertilization. Chromosomes number is reduced by half. The shape of the cells is altered. Mona A. Elsafadi Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half and results in the formation of gametes. Mona A. Elsafadi Meiosis Involves two meiotic divisions Takes place in germ cells only.
Produces haploid gametes ( sperms and oocytes). Occurs during gametogenesis Mona A. Elsafadi Meiosis Homologous chromosomes: pairs of chromosomes, one inherited from each parent and are similar in shape and size and have identical gene loci. X and Y chromosomes are not homologues but they have homologous segments at the tips of their short arms. They pair at these regions only. Mona A. Elsafadi Meiosis Mona A. Elsafadi Meiosis I has three unique features:
Synapsis: a process of forming complexes of homologous chromosomes following chromosome replication. Homologous recombination (crossing over): the genetic exchange process that occurs between paired homologous chromosomes, so the sister chromatids in meiosis II are not always identical. Reduction division: at the end of meiosis each cell contains only half the original complement of chromosomes. Mona A. Elsafadi Second Meiotic Division
Has no DNA replication. The haploid number of chromosomes is retained. Each daughter cells has the reduced haploid number of chromosomes with single chromatid chromosomes. Similar to an ordinary mitosis except that the chromosome number of the cell entering the second meiotic division is haploid. Mona A. Elsafadi Importance of meiosis:
Provides constancy of chromosome number by reducing it from diploid to haploid. Allows random assortment of maternal and paternal chromosomes between the gametes. Relocates segments of maternal and paternal chromosomes by crossing over of chromosome segments. Mona A. Elsafadi Spermatogenesis The sequence of events by which spermatogonia are transformed into spermatozoa (mature sperms) Mona A. Elsafadi Spermatogenesis Purpose: to produce mature sperms which are able to fertilize the oocytes to produce the zygotes. Location: Seminiferous tubules of the testes Epididymis Duration: about 64 days in human Timing: begins at puberty and continues until death. Mona A. Elsafadi Mona A. Elsafadi Stages of spermatogenesis
Spermatocytogenesis: at puberty spermatogonia proliferate by mitosis to yield primary spermatocytes. Meiosis: Meiosis I: formation of secondary spermatocytes (1N) Meiosis II: formation of spermatids (IN) Spermiogenesis: maturation process by which spermatids are transformed into mature sperms and supported by Sertoli cells. Mona A. Elsafadi Spermiogenesis Mona A. Elsafadi Sertoli Cells Lining the seminiferous tubules.
Support and nurture the germ cells. Involved in the regulation of spermatogenesis. Mona A. Elsafadi Epididymis Elongated coiled duct along the posterior border of the testis Continuous with the vas deferens which transports the sperms to the urethra. Where sperms are stored and become functionally mature after transporting passively form seminiferous tubules. Mona A. Elsafadi Mature Sperm Is a free-swimming, actively motile cell consisting of a head, a neck and a tail Mona A. Elsafadi Sperms Head Forms most of the bulk of the sperm.
Contains haploid nucleus. Contains Acrosome: Covers the anterior two-third of the nucleus Caplike saccular organelle containing several enzymes, which facilitate sperm penetration of the corona radiata and zona pellucida during fertilization. Mona A. Elsafadi Sperms Tail Provides the motility of the sperm.
Consists of three segments: Middle piece Principal piece End piece The middle piece contains mitochondria, which provides ATP for activity. Mona A. Elsafadi Genes and molecular Factors
Bcl-2 family proteins involved in: germ cells maturation germ cells survival at different stages. Y chromosome: Essential for normal spermatogenesis Microdeletions result in defective spermatogenesis and infertility Mona A. Elsafadi Oogenesis Is the sequence of events by which oogonia are transformed into mature oocytes. Mona A. Elsafadi Oogenesis Timing: No. of oocytes: begins before birth
Is completed after puberty Continues to menopause No. of oocytes: 2 millions primary oocytes at birth 40,000 remain at adolescence 400 become secondary oocytes and ovulated Mona A. Elsafadi Oogenesis Mona A. Elsafadi Prenatal Maturation During early fetal life, oogonia proliferate by mitosis. Before birth, oogonia enlarge to form primary oocytes. Primordial follicle: primary oocyte surrounded by a single layer of flattened epithelial cells Primary follicle: enlarged primary oocyte surrounded by zona pellucida and a single layer of columnar follicular epithelial cells Secondary follicle: primary follicle with more than one layer of follicular cells Mona A. Elsafadi Oocyte Maturation Mona A. Elsafadi Prenatal Maturation First meiotic division
Begins before birth Completed after adolescence (11-19 yrs) Oocyte maturation inhibitor: Secreted by primary oocytes maintains the meiotic arrest Mona A. Elsafadi Postnatal Maturation The primary oocytes remain dormant until puberty.
Shortly before ovulation, primary oocyte increases in size and complete first meiotic division to form secondary oocyte with first polar body. At ovulation, secondary oocyte begins second meiotic division but arrested at metaphase. If fertilization occurs, second meiotic division is resumed and produces second polar body. Mona A. Elsafadi Postnatal Maturation Begins during puberty
One follicle matures and ovulates each month Long duration of first meiotic arrest may cause: High frequency of meiotic errors (non-disjunction) Primary oocytes to be vulnerable to environmental agents (radiation) Mona A. Elsafadi Comparison of Gametes No primary oocytes form after birth
Division of cytoplasm is unequal Massive immotile cells Surrounded by zona pellucida and a layer of follicular cells. Only one kind of secondary oocyte: 23,X Continuous production of primary spermatocytes Division of cytoplasm is equal Microscopic motile cells Sparse cytoplasm and have heads covered with acrosomes and tails. Two kinds of normal sperm: 23,X and 23,Y Mona A. Elsafadi Thanks Mona A. Elsafadi