embryology 2

• Gametogenesis


  Biological process by which diploid precursor cells undergo cell division and differentiation to form mature haploid gametes
• Gametogenesis


  • Includes spermatogenesis (production of sperm) and oogenesis (production of eggs)
  • Takes place through the process of meiosis
• Spermatocyte

  Male gametocyte, from which a spermatozoon develops
• Oocyte


  Cell that develops into an egg or ovum; a female gametocyte
• Polar body


  One of the small cells that are by-products of the meiosis that forms an egg
• Mitosis


  The division of a cell nucleus in which the genome is copied and separated into two identical halves. It is normally followed by cell division
• Meiosis


  Cell division of a diploid cell into four haploid cells, which develop to produce gametes
• Formation of gametes

  1. Primordial germ cells (PGCs) formed in the epiblast during the second week

  2. PGCs move through the primitive streak during gastrulation

  3. PGCs migrate to the wall of the yolk sac

  4. PGCs begin to migrate from the yolk sac toward the developing gonads by the end of the fifth week

  5. Mitotic divisions increase PGC number during migration and when they arrive in the gonads
• Germ cells undergo meiosis in preparation for fertilization to reduce the number of chromosomes and undergo cytodifferentiation to complete their maturation
• Teratomas

  • Tumors that often contain a variety of tissues, such as bone, hair, muscle, gut epithelia, and others
  • Arise from pluripotent stem cells that can differentiate into any of the three germ layers or their derivatives
  • Evidence suggests that PGCs could be responsible
  • Another source may be epiblast cells
• Chromosome theory of inheritance

  • Chromosomes appear as 23 homologous pairs to form the diploid number of 46
  • Genes on the same chromosome tend to be inherited together (linked genes)
  • 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 and one from the paternal gamete
  • Each gamete contains a haploid number of 23 chromosomes, and the union of the gametes at fertilization restores the diploid number of 46
• Mitosis

  1. Cell division in which replicated chromosomes are separated into two new nuclei

  2. Cell division gives rise to genetically identical cells in which the total number of chromosomes is maintained

  3. Before a cell enters mitosis, chromosome replicates its DNA

  4. Prophase: chromosomes begin to coil, contract, and condense

  5. Prometaphase: chromatids become distinguishable

  6. Metaphase: chromosomes line up in the equatorial plane, and their doubled structure is clearly visible

  7. Anaphase: the centromere of each chromosome divides, migration of chromatids to opposite poles of the spindle

  8. Telophase: chromosomes uncoil and lengthen, the nuclear envelope reforms, and the cytoplasm divides
• Meiosis

  1. Cell division that takes place in the germ cells to generate male and female gametes (sperm and egg cells)

  2. Requires two cell divisions

  3. After DNA replication, homologous chromosomes align in pairs (synapsis)

  4. Homologous pairs then separate into two daughter cells, thereby reducing the chromosome number from diploid to haploid

  5. Meiosis II separates sister chromatids, each gamete then contains 23 chromosomes

  6. DNA synthesis does not occur between the two stages
• Crossovers in meiosis I

  • Interchange of chromatid segments between paired homologous chromosomes
  • Segments of chromatids break and are exchanged as homologous chromosomes separate
  • Chiasma (an X-like structure) is formed
  • 30 to 40 crossovers (one or two per chromosome) with each meiotic I, frequent between genes that are far apart on a chromosome
• Meiosis

  • Enhances genetic variability
  • Produces haploid cells, so fertilization can restore the diploid number of chromosomes
• Oogenesis


  1. Diploid oogonium go through mitosis until one develops into a primary oocyte, which will begin the first meiotic division, but then arrest

  2. The secondary oocyte begins the second meiotic division and then arrests again; it will not finish this division unless it is fertilized by a sperm
• Spermatogenesis

  1. Diploid spermatogonia go through mitosis until they begin to develop into gametes

  2. One develops into a primary spermatocyte that will go through the first meiotic division to form two haploid secondary spermatocytes

  3. The secondary spermatocytes will go through a second meiotic division to each produce two spermatids

  4. The spermatids will eventually develop flagella and become mature sperm