Giant spherical cell, spherical or ovoid, 0.1 mm diameter in humans
Contains all proteins required for the development of the embryo into an individual
Nutritional reserve – yolk is rich in lipids, proteins and polysaccharides present within yolk granules. In some species, each yolk granule is membrane enclosed
In oviparous egg, 95% volume of the cell is yolk. In mammals, there is little yolk. Embryos are mainly nourished by their mothers
Egg coat is a specialized form of extracellular matrix, rich in glycoproteins, some of which is secreted by the egg itself and the others deposited on it by surrounding cells
Major coat that just surrounds the egg is called the vitelline membrane in non-mammalian cells and zona pellucida in mammalian cells. Protects from mechanical damage and acts as a barrier allowing sperms from only the same species to fertilize it
Secretory vesicles called cortical granules are present under plasma membrane in the cortex of the egg cytoplasm. When the egg is fertilized by a sperm, these cortical granules release their contents by exocytosis which alter the egg coat in order to prevent more than one sperm from fusing the egg
Development of Egg
Primordial germ cells migrate to the genital ridge to form oogonia which undergo mitosis, thereby increasing the number of oogonia. These oogonia eventually develop into primary oocyte
Primary oocyte undergoes meiosis and remains arrested in prophase I for few days to many years. This is when it synthesizes egg coat and cortical granules. Non-mammalian oocytes accumulate ribosomes, yolk, glycogen, lipid and mRNA that encode developmental proteins required for embryogenesis
Primary oocytes then undergo oocyte maturation which usually does not occur until sexual maturity, when they are stimulated by hormones. They resume meiosis resulting in cells with half the original number of chromosomes. This is followed by cytokinesis giving rise to a polar body and secondary oocyte which is larger in size than the polar body
Secondary oocyte contains 2 sister chromatids of a chromosome. It undergoes meiosis II giving a second small polar body and mature large ovum with haploid set of chromosomes
Because of asymmetric division, the oocytes maintain their large size. The 2 small polar bodies degenerate
In most vertebrates, the oocyte maturation is held in metaphase II pre-birth. At ovulation, the secondary oocyte undergoes completion of meiosis II and is released from the ovary ready for fertilization
Oogenesis
Growth of Ovum size
Ovum possess extra gene copies at the prophase I stage when it is still a primary oocyte. It contains diploid chromosomes in duplicate, hence, it has twice as much DNA available for RNA synthesis as does an average somatic cell. Oocytes of some species prolong this phase even further to make more extra copies of specific genes. Thus, the eggs require a larger number of ribosomes to support the synthesis of enormous amounts of proteins during early embryogenesis. Thus, in the oocytes, the rRNA genes are specifically amplified. Example, some amphibian eggs have 1-2 million copies of genes
The oocytes also depend on the surrounding cells for their growth. Yolk is usually synthesized outside the ovary and is imported into the oocyte. In birds, amphibians and insects, yolk proteins are made by liver cells and secreted into blood which are taken up by the oocytes by receptor-mediated endocytosis from extracellular fluid. Nutritive help comes from 2 cell types –
Nurse cells – A specialized form of oogonia. Connected to the oocyte by cytoplasmic bridges that transport ribosomes, mRNA, proteins etc. to the oocyte. This happens in invertebrates. The vertebrates manufacture these themselves
Follicle cells – Found in both vertebrates and invertebrates. Arranged as an epithelial layer around the oocyte to which they are connected by gap junctions. They exchange only small molecules, hence, transport only precursors of macromolecules. They also secrete macromolecules that contribute to egg coat or are taken up by receptor-mediated endocytosis by the oocyte or act on egg cell surface receptors to control the spatial patterning and axial symmetries of the ovum
