Gametogenesis

• Gametogenesis involves the production of sex cells capable of fusing at fertilization and producing a new being.
• Gametogenesis is divided into four phases: the origin of the primordial germ cells and their migration to the gonads, the multiplication of the germ cells in the gonads through mitosis, reduction of the chromosome number by half during meiosis, and the final stages of maturation and differentiation of the gametes into spermatozoa and ova.
• Primordial germ cells (PGCs) that will give rise to the gametes are recognizable at an early stage of development of vertebrates.
• In anuran amphibians, germ plasm/PGCs can be recognized in the vegetal region of a fertilized egg or during cleavage then into the entodermal floor of the primitive gut.
• PGCs arise in the epiblast of the early embryo and take temporary residence in extra-embryonic tissues then later on return to the body of the embryo proper.
• PGCs are recognizable in the germinal crescent, located beyond the future head region of the embryo.
• PGCs then migrate through the vitelline veins until they settle in the genital ridge, which is the future gonad of the organism.
• In mammals, PGCs are found in the posterior wall of the yolk sac near the origin of the allantois.
• These cells reach the future gonads by migration around the wall of the posterior gut and then through the dorsal mesentery.
• Once they are in prophase I, they are now called oocytes.
• Oogonia proliferate in numbers by mitosis, depending on the species, with variations that may occur.
• Once the Polar Cell Group (PCG) reside in the ovaries, they are now called oogonia.
• Gametogenesis, or the formation of gametes, occurs in the ovaries of several animal creatures, with varying features and events.
• Further growth of the egg occurs.
• Variations may exist in the organization of an egg in different species of animals.
• Some oogonia are committed to undergo meiosis.
• Many substances are either transported to the eggs or the eggs themselves produce important substances for development such as yolk, shells, hormones, etc.
• Associated with every developing egg are supporting cells with varied names among different species.
• The functional ovum produced by oogenesis is highly organized, with large amounts of materials stored inside the egg that are essential in the development of a new being so well-arranged.
• In oogenesis, only one functional egg is produced from each Polar Cell Group/oogonium.
• Prophase I in meiosis involves the alignment of homologous chromosomes or pairs, known as tetrads, in the equatorial plate.
• Telophase I in meiosis results in each daughter cell containing the haploid (1n) number of chromosomes and each chromosome still consisting of two chromatids (2c) connected by a centromere.
• Anaphase I in meiosis involves the movement of one chromosome of the homologous pair towards one pole of the spindle, and the other chromosomes towards the opposite pole.
• Teratomas develop when PGCs migrating along the gut are misdirected to the gonads and go elsewhere, resulting in bizarre growths of a mixture of highly differentiated tissues such as skin, hair, cartilage, teeth, etc in one site.
• Teratomas can be found in the sarcoccygeal region, oropharyngeal region, and mediastinum.
• In bird eggs, the Vegetal pole is the rest of the egg where yolk is abundant, while the Animal pole is the germinal disk.
• Cortical granules are abundant vesicles located near the surface of the egg and have roles in fertilization.
• The germinal vesicle is an egg nucleus and is somewhat large.
• The yolk material, a great source of nutrients, is assembled in organelles called yolk platelets that are unevenly distributed.
• Pigment granules are present in amphibian eggs and come in various hues/shades that may be localized.
• The reproductive pattern of amphibians is quite different from most amniotes such as birds and animals.
• In frog and zebrafish eggs, the Vegetal pole is the bottom part, while the Animal pole is the top part.
• The maturation of amphibian egg cells has three phases: Previtellogenic phase before the deposition of yolk, Vitellogenic phase major period of cell division, and Maturation phase oocyte is released from its meiotic block through the action of progesterone.
• Fertilization of amphibian eggs is external and a large number of eggs and their release at one time during the year is their reproductive pattern.
• Alecithal/microlecithal amount of yolk is isolecithal distribution.
• The cortex is seen under the electron microscope and is the cytoplasmic periphery of the egg that is structurally different from the inner cytoplasm.
• Eggs can be categorized based on the amount and distribution of yolk.
• The albumen is not part of the egg cell, it is an extracellular covering.
• In an egg with uneven distribution of yolk, the Vegetal pole is the yolk-rich side or region, while the Animal pole is the less yolky region.
• Growth of oocytes in the frog during the first three years of its life occurs inside the ovaries, with three batches of eggs formed.