DEVBIOLEC L2: Str of Gametes and Gametogenesis

• Parts of mature sperm are head, neck/midpiece, and tail.
• The acrosome is the cap-like structure at the tip of the sperm cell that contains enzymes needed to penetrate the egg's protective coat.
• Axoneme is the motor portion of the flagella structure of sperm.
• Mammalian sperm have different shape and size depending on the species.
• Mature egg stores all material for beginning growth and development.
• Ovum can be alecithal, meiolecithal, mesolecithal, or mega/polylecithal depending on the amount of yolk present by species.
• Alecithal have no yolk, eg mammalian.
• Meiolecithal have small amount of yolk, eg amphioxous.
• Mesolecithal have moderate amount of yolk, eg frog, toad.
• Mega or poly lecithal have large amount of yolk, eg birds, chicken, ostrich.
• Ovum synthesises and stores proteins (like yolk) as reservoirs for developing embryo.
• Part of the egg cell include the cytoplasm, haploid nucleus, cell membrane, vitelline envelope, zona pellucida (mammals), cumulus (mammals), cortex, cortical granules, and the egg jelly (some species).
• The ovum cytoplasm has many components.
• The cell membrane of the ovum fuse with the sperm plasma membrane after fertilization and becomes the fertilization membrane.
• The ova vitelline envelope is outside the plasma membrane and contains glycoproteins essential for species specificity and sperm binding.
• Zona pellucida (mammals) is the extra coating in ovum made of extracellular matrix.
• Cortex is beneath the cell membrane of the ova and is a gel-like cytoplasm that may help the sperm enter into the cell.
• Cortical granules are inside the cortex that are membrane bound vesicles that help prevent polyspermy.
• Egg jelly in ovum of some species help attract or activate sperm.
• Eggs on land have particularly elaborate envelope: vitelline envelope, fibrous layer, outer layer of albumin, and the shell made of calcium carbonate.
• The egg cytoplasm consists of proteins, mRNA, ribosomes, tRNA, morphogenetic factors, and protective chemicals.
• mRNA in egg cytoplasm provides early instructions for development.
• Ribosomes and tRNA in egg cytoplasm aid in protein synthesis early in development.
• Morphogenetic factors in egg cytoplasm are molecules that affect differentiation of various cell types (can be localized to specific areas of the cell).
• Protective chemicals in egg cytoplasm are UV filters, DNA repair enzymes, antibodies (birds)
• Four phases of gametogenesis include

  1. Extra-embryonic origin of the germ cells and their migration
  2. Increase in number of cells by mitosis
  3. Reduction of chromosome number by meiosis
  4. Structural and functional maturation of the reproductive cells
• Sperm are produced at the testis and stored in epididymis.
• Sperm production begins between ages 11 to 15 and continues til death.
• Site of sperm formation

  • Spermatic cord
  • Testicular veins
  • Testicular artery
  • Vas deferens
  • Epididymis
  • Seminiferous tubules
• Spermatogenesis

  1. Spermatogonium
  2. Primary spermatocyte
  3. Secondary spermatocyte
  4. Spermatids
  5. Spermatozoa
• In spermatocytogenesis the primitive sex cells appear earliest in the fourth week of intrauterine life in the wall of yolk sac as primordial germ cells. They migrate to the developing testes and lie dormant among the cells lining the seminiferous tubules. At puberty the germ cells awaken and start the actual process of spermatogenesis. These cells increase in number by simple mitosis to form cells known as spermatogonia; type a and type b. Type b spermatogonia replicate dna to have a 46 double structured chromosomes to begin meiosis-1 and are called primary spermatocytes.
• In spermiogenesis, spermatids are rounded cells. They modify to assume specific shape of the sperm. They elongate and reorganize internal structure and acquire the particular shape.
• There are four steps in spermiogenesis.

  Nuclear condensation

  Acrosome formation

  Flagellum formation

  Cytoplasmic reduction
• The parts of spermatozoa include neck, midpiece, principal piece, and tail.
• In nuclear condensation the nucleus becomes smaller denser and takes on a characteristic flattened form. There is also thickening and reduction of the nuclear size. Also, condensation of the nuclear contents into the smallest space.
• In acrosome formation there is an enlargement at one end of the cell which contains digestive enzymes that will be used during fertilization. This is called the acrosomal vesicle or cap.
• In flagellum formation, the generation of the sperm cell tail occurs. it consists of the neck, midpiece, principle piece, & tail.
• In cytoplasmic reduction the elimination of all unnecessary cytoplasm occurs. The cytoplasm of the spermatides that is no longer needed is phagocytized by Sertoli cells or is disposed of in the lumen of the tubules. A clamp of cytoplasm can remain on the net and midpiece of the sperm for a little while.
• The changes in spermiogenesis are the following

  1. Golgi apparatus, forms acrosomal cap with proteolytic enzymes.
  2. Nucleus, condensed.
  3. Centrioles, make collar and neck.
  4. Microtubules, form flagellum.
  5. Mitochondria, arranged as spiral and neck.
  6. Excess cytoplasm, cast off as residual body.
  7. Cytoplasmic bridges, break and sperms release from Sertoli cells to lie free in lumen of seminiferous tubules.
• About 64 days are required to go from spermatogonium to a sperm.