Gametogenesis: Conversion of Germ Cells Into Male and Female Gametes PDF

# Chapter 1: Gametogenesis: Conversion of Germ Cells Into Male and Female Gametes ## Sperm Each gamete contains a haploid number of 23 chromosomes, and the union of the gametes at fertilization restores the diploid number of 46. ## Mitosis - Mitosis is the process whereby one cell divides, givi...

# Chapter 1: Gametogenesis: Conversion of Germ Cells Into Male and Female Gametes ## Sperm Each gamete contains a haploid number of 23 chromosomes, and the union of the gametes at fertilization restores the diploid number of 46. ## Mitosis - Mitosis is the process whereby one cell divides, giving rise to two daughter cells that are genetically identical to the parent cell. - Each daughter cell receives the complete complement of 46 chromosomes. - Before a cell enters mitosis, each chromosome replicates its deoxyribonucleic acid (DNA). - During this replication phase the chromosomes are extremely long, they are spread diffusely through the nucleus, and they cannot be recognized with the light microscope. - With the onset of mitosis the chromosomes begin to coil, contract, and condense. These events mark the beginning of prophase. - Each chromosome now consists of two parallel subunits, chromatids, that are joined at a narrow region common to both called the centromere. - Throughout prophase the chromosomes continue to condense, shorten, and thicken. - At prometaphase the chromatids become distinguishable. - During metaphase the chromosomes line up in the equatorial plane. ## Figure 1.2 **Various stages of mitosis** * **Prophase:** Chromosomes are visible as slender threads. * **Metaphase:** Doubled chromatids become clearly visible as individual units. * **Anaphase:** Members of a chromosome pair do not unite. ## Part One: General Embryology ### Figure 1.1 **An embryo at the end of the third week, showing the position of primordial germ cells in the wall of the yolk sac, close to the attachment of the future umbilical cord.** * From this location, these cells migrate to the developing gonad. * Some evidence suggests that PGCs that have strayed from their normal migratory paths could be responsible for some of these tumors. * Another source is epiblast cells migrating through the primitive streak during gastrulation. ## The Chromosome Theory of Inheritance * Traits of a new individual are determined by specific genes on chromosomes inherited from the father and the mother. * Humans have approximately 35,000 genes on 46 chromosomes. * 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. * There are 22 pairs of matching chromosomes, the autosomes, and one pair of sex chromosomes. * If the sex pair is **XX**, the individual is genetically female; if the pair is **XY**, the individual is genetically male. * One chromosome of each pair is derived from the maternal gamete, the oocyte, and one from the paternal gamete, the sperm. ## Chapter 1: Gametogenesis: Conversion of Germ Cells Into Male and Female Gametes ### 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 and that move 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. ### Clinical Correlate **Primordial Germ Cells (PGCs) and Teratomas:** * Teratomas are tumors of disputed origin that often contain a variety of tissues, such as bone, hair, muscle, gut epithelia, and others. * It is thought that these tumors arise from a pluripotent stem cell that can differentiate into any of the three germ layers or their derivatives.