Oogenesis is a process by which ova are formed from the primary germ cells of the ovary. It is a type of gametogenesis.
Phases of Oogenesis
It is divided into three phases
- Multiplication phase
- Growth Phase
- Maturation Phase
Let us discuss them one by one.
1. Multiplication Phase
The first phase is similar to that of spermatogenesis. The cells of the germinal epithelium divide repeatedly by mitosis to produce many oogonia or egg mother cells.
Each oogonium is diploid. It ceases to divide before entering into the next phase.
2. Growth Phase
In this phase, the oogonial cells increase in size due to the accumulation of yolk and other nutritive substances. The nucleus increases many times in size.
The chromosomes inside the nucleus increase greatly in length and develop side loops to facilitate the rapid synthesis of yolk.
These chromosomes with loops are called lampbrush chromosomes and are found in diverse groups of animals including human beings.
After these changes, the cell is called a primary oocyte. A vitelline membrane is formed around the oocyte.
The growth phase is prolonged and may vary from few days to two or more years.
3. Maturation Phase
The primary oocyte divides twice in this phase. The first maturation division is by meiosis and two unequal cells are produced.
The larger cell is called the secondary oocyte while the smaller cell is called the first polar body. The number of chromosomes is reduced to half (n) in both the secondary oocyte and first polar body.
The secondary oocyte divides mitotically during second maturation division to form a larger cell called ovum and a small cell called the second polar body.
The first polar body usually divides into two polar bodies. These polar bodies do not take part in reproduction and soon degenerate, Polar bodies or polocytes contain little amount of cytoplasm and a haploid set of chromosomes.
These are formed so as to get rid of an excess number of chromosomes. In the vertebrates, the first polar body is formed usually after the release of the primary oocyte from the ovary and its entrance into the oviduct.
The second polar body is formed only after the entry of sperm head into the secondary oocyte during fertilization.
Structure of Ovum or egg:
The mature ovum or egg is a spherical or oval cell. It is nonmotile and much larger than the sperm.
The ovum of the hen is about 30 mm in diameter. A frog’s ovum is only 2 mm in diameter, whereas the human ovum is still smaller being only 0.15 mm.
An ovum contains a variable amount of yolk in its cytoplasm for the nourishment of the future embryo. In addition to the yolk, other nutritive substances such as glycogen, lipid, and oil globules are stored in the egg.
There is a prominent nucleus at the center of the cytoplasm. The cytoplasm of the egg is called the ooplasm.
It also contains the endoplasmic reticulum, mitochondria, and Golgi body.
The ovum is surrounded by a thin vitelline membrane formed by the hardening of the outer peripheral layer of the cytoplasm.
More commonly outside the primary vitelline are present secondary and tertiary membranes. The egg is polarised with one pole being different from the other. These are known as animal poles and vegetal poles respectively.
The nucleus lies in the animal pole and the nutritive materials at the vegetal pole. During maturation of the ovum, the polar bodies are given out at the animal pole. In a frog egg, the yolk is denser than the cytoplasm towards the vegetal pole.
The line passing through these two poles is called the primary axis or animal vegetal axis.
The human egg, like other placental mammals, is almost without yolk (micro-lecithal and isolecithal). The cytoplasm is bulky with a centrally located nucleus.
There is a delicate vitelline membrane surrounded by zona pellucida which is a clear homogeneous layer.
Outside of this layer is an invertment of radially arranged centers cellular investment cling to the ovum for some time after it is called corona radiata.
The cells of the corona radiata cling to the ovum for some time after it is released.
Type of eggs
The eggs are classified according to the amount and distribution of yolk.
a. Egg types based on the amount of yolk :
- Microlecithal or Oligolecithal: These are small-sized eggs that contain very little amount of yolk. They are found in Hydra, sea urchin, Amphioxus, marsupials, and placental mammals. Assuming that there is no yolk in the eggs of placental mammals, some authors describe these as alecithal.
- Mesolecithal eggs: These eggs contain a moderate amount of yolk and are found in annelids, mollusks, Petromyzon, Dipnoi (lungfishes), and amphibians.
- Macrolecithal or Megalecithal or Polylecithal eggs: These eggs contain an enormous amount of yolk and are found in insects, Myxine, elasmobranch fishes, reptiles, birds, and egg-laying mammals.
b. Egg types based on the distribution of yolk :
- Isolecithal or homolecithal : In microlecithal eggs, the quantity of yolk is so little that it is found scattered uniformly throughout the egg cytoplasm. Such eggs with evenly distributed yolk in ooplasm are known as isolecithal or homolecithal.
- Telolecithal eggs: These types of eggs have a polarized distribution of yolk and are found in mesolecithal and macrolecithal eggs. Because of the uneven distribution of yolk, such an egg is said to have a vegetal pole where the concentration of yolk is the greatest and an animal pole where the concentration of yolk is the smallest. The telolecithal eggs may be either moderately telolecithal (e.g. Amphibia. Petromyzon and Dipnoi) or highly telolecithal (e.g. cartilaginous and bony fishes, reptiles, and birds.
- Centrolecithal eggs: In insects, the yolk is concentrated in the center of the egg and the cytoplasm forms a thin peripheral layer around the yolk. Such eggs are called centrolecithal eggs.
c. According to covering (Shell)
- Cleidoic eggs: Fully laden with yolk and surrounded by albumen and a water proof COCa3 shell. For example: in Reptiles and Birds
- Non-cleidoic eggs: Shell absent. For example in fishes, amphibia and eutherian mammals.
d. According to the types of development
- Determinate or Mosaic Eggs: Definite fate of every blastomere of early blastula. For example: Platyhelminthes, Annelida, Arthropoda and Mollusca etc.
- Intermediate or Regulative Eggs: No predetermination of the fate of various egg parts. For example, in Echinoderms and Chordates.
Comparison of egg and sperm
Both the gametes are morphologically complete cells. Each has a nucleus and cytoplasm containing representatives of centrosomes, mitochondria, and Golgi bodies. Each has a plasma membrane.
Yet neither is capable of independent continued existence nor physiologically balanced. The egg is large and inert containing a vast store of metaplasm.
It is protected by the egg envelope and has lost the power of continued division.
The sperm is small, highly motile, and contains little cytoplasm and no metaplasm. It is devoid of protective membranes and in itself has lost the power of continued division.
Differences Between Spermatogenesis and Oogenesis
|1. It takes place in the testes.||1. It takes place in the ovaries.|
|2. This process results in the formation of four sperms from each primary spermatocytes and all the sperms are functional.||2. This process results in the formation of only one large ovum and three polar bodies from each primary oocytes. The ovum is functional but polar bodies are non-functional.|
|3. Sperms are very small in size, motile and active adapted to swimming by the lashing movement of the tail.||3. The ovum is large, on-motile and contain large amount of yolk. Due to yolk it is much passive and receptive.|
|4. Numerous sperms are produced by this process.||4. Only few ova are produced in this process.|
|5. This process is completed within the testes.||5. The formation of ovum is completed outside the ovary.|
|6. No polar body is formed.||6. Polar bodies are formed in this process.|