- Egg cell or ovum, also known as female gamete is formed by the process of oogenesis in female organisms.
- They are produced by follicle cells present in the ovaries.
- Egg cells are released in the secondary oocyte stage by the process called ovulation.
- They are haploid in nature and are obtained by meiosis.
- Ovum is the largest cell in human beings. They are larger than sperms.
Structure:
- Ovum is round (nearly spherical) and is non-motile as it is devoid of a tail.
- It consists of a central nucleus, cytoplasm and the cell membrane.
- Nucleus consists of genetic material which once fertilized by sperm, helps to determine the characteristics of the off spring.
- The cytoplasm surrounding the nucleus is called yolk or ooplasm, which consists of nutrients required for the developing embryo.
- The peripheral layer of ooplasm is known as cortex which consists of many microvilli and cortical granules.
- An unfertilized egg in birds and reptiles also consists of outer transparent albumen surrounding the yolk.
- In telolecithal and mesolecithal eggs, the upper black pigmented half the cell containing a nucleus is called animal pole and the lower whiter half containing yolk is called a vegetal pole.
- The egg cell is surrounded by a thin double layered membrane.
- The inner layer is called vitelline membrane and the outer layer is called zona pellucida.
- The space between the two layers is called a pre-vitelline membrane.
- Eggs of birds and reptiles are also covered externally by a hard calcareous shell.
- On the basis of amount and distribution of yolk, eggs are of various types.
1. On the basis of amount of yolk present, eggs are of following types:
- Alecithal egg:
- The ovum consists of almost no yolk in it.
- Animals like human beings, marsupials and eutherian animals produce such eggs.
- Microlecithal egg:
- Such eggs consist of small amount of yolk in them.
- They are also called oligolecithal eggs.
- e.g. eggs produced by sea urchin, Branchiostoma
- Mesolecithal egg:
- There is moderate amount of yolk present in it.
- e.g. lungfishes, frogs, toads etc.
- Megalechital or Macrolecithal or Polylecithal egg:
- They contain huge amount of yolk in them.
- e.g. sharks, bony fishes, reptiles, birds, insects and prototherians
2. On the basis of distribution of yolk, eggs are of following types:
- Homolecithal or Isolecithal egg:
- Yolk is evenly or uniformly distributed inside an egg.
- Such distribution is found in alecithal and microlecithal eggs.
- Telolecithal egg:
- In such eggs, most of the yolk lies near the vegetal pole.
- The cytoplasm and nucleus near the animal pole.
- Such distribution is observed in mesolecithal and macrolecithal eggs.
- In chicken, the huge amount of yolk occupies the entire ovum but the cytoplasm and nucleus lie in the animal pole. This condition may be referred to as highly telolecithal.
- Centrolecithal egg:
- In such eggs, yolk is centrally placed and surrounded by thin cytoplasm.
- Some insects have centrolecithal eggs.