Gametes are sex cells or reproductive cells produced by gametogenesis from the germinal cell.
Gametes contain only one copy (set) of dissimilar chromosomes or half the genetic material necessary to form a complete organism. It is a haploid cell.
During the cell division (meiosis), gametes are produced. A diploid parent cell undergoes two fission or two separate cycles of nuclear division, resulting in the formation of four haploid cells, which are developed into male gamete sperm or female gamete ova.
Zygote (diploid cells containing paired chromosomes) are formed during fertilization, where male and female gametes fuse.
In humans, 23 chromosomes are present in each gamete, and after fusion, zygotes are formed as diploid zygotes containing 46 chromosomes.
In animals, gametes (i.e, sex cells) are produced in respective male and female reproductive organs called gonads.
Types of Gamete
A gamete is classified as a male gamete and a female gamete which are described below:-
In animals, sperm is the male gamete produced from the male parents for sexual reproduction.
In-plant male gamete present in the pollen of a plant, plant male gamete formed in the anther of angiosperm, in the archegonial head of liverworts and moss, and the pollen cones of gymnosperm.
- In fertilization, sperm fuse with an ovum of the female to produce a new offspring.
- Mature sperm looks like a small tad-pole motile cell and consists of two distinguishable parts, an head and a tail.
- The shape and size of the sperm head are not fixed. It varies for each animal species.
- The human sperm’s head is flattened and almond shape, and size about 4 to 5 mm long and 2 to 3 mm wide.
- In the head portion, the cell nucleus comprises chromosomes (genetic material).
- The sex of the future child depends on the X or Y chromosome present in the sperm cell.
- Sperm have a long tail-like projection called a flagellum that connects the head and middle portion.
- The size of the tail is about 50mm long, and the tail helps in sperm cell movement.
Female gametes are also called ova or eggs. It is found in the ovary of animals and the ovary of plants.
- Ovum, when fused with a sperm cell., produces a new individual.
- It is produced by sexually reproducing animals, plants, and fungi. In humans and animals, they are formed by follicle cells, and they are produced by ovules found inside the ovary in the plant.
- The shape of a mature ovum is usually spherical, non-motile with yolky cytoplasm, and enclosed in one or more egg envelopes.
- Depending upon the amount of yolk present in the ovum, the size of an ovum varies in different animals from 10 microns to a few centimeters.
- The central nucleus contains genetic material which is present in the ovum.
- The cell plasma or yolk surrounds the nucleus for developing the egg cells as it contains a nutritional element.
Gametes are also categorized into two types based on morphological type (shape and size).
- Isogamy: male and female sex cells or gametes are almost identical. In some species of algae, protozoans, and fungi, male and female gametes are similar, and usually, both are motile.
- Anisogamy (Heterogamy): Gametes may be dissimilar morphological types gametes of many green algae of Chlamydomonas are heterogamy.
- An advanced form of heterogamy is found in all higher plants, animals, and some algae and fungi, known as oogamy. In oogamy, one of the gametes is larger and non-motile, which is an egg, and another is small and motile.
Gametogenesis (Gamete Formation)
The process of gamete formation is called gametogenesis.
- Gametogenesis is the process of gamete formation from precursor cells, it takes place in the gonad.
- Two morphological dissimilar types of gametes (male and female) are produced in animals and higher plants via the differentiation method.
- The formation of male gamete (sperm cell or spermatozoa) is known as spermatogenesis, and the formation of female gamete (ova or egg cell) is called oogenesis.
Spermatogenesis is the process by which haploid spermatozoa develop from germ cells in the seminiferous tubules of the testis and finally produce sperm.
Sperm is produced from the cell of germinal epithelium invertebrates, but certain animals such as mammals and Mollusca have somatic cells lying between germinal cells. These somatic cells are also called Sertoli cells.
The sperm formation process can be studied in 2 stages first is the formation of spermatids, and second is spermiogenesis.
1. Formation of spermatids
- During embryonic development, the primordial cell (germ cells) containing chromatin riched nuclei migrate to the testis, where they undergo multiple mitotic divisions to become spermatogonia.
- At puberty, the spermatogonia (immature germ cell) begin to pass through mitotic division and continually proliferates and differentiates through definite stages of development to form sperm.
- The spermatogonia proliferate rapidly by mitosis and produce two diploid primary spermatocytes this process is called Spermatocytogenisis.
- These primary spermatocyte divides meiotically meiosis I reducing the chromosome number to form two haploid secondary spermatocytes.
- Each secondary spermatocyte is divided into two equal haploids and produces four spermatids by meiosis II or heterotypic division.
- These spermatids are immature, so they cannot act as gametes, so they pass through the next stage.
- The process of transformation of spermatids into sperm refers to spermiogenesis.
- Spermatozoa and sperm are motile; to provide motility to the sperm, the superfluous material of the developing sperm is discarded.
- Except for the change in nuclei, centriole, acrosome, mitochondria, and axial filaments, another cytoplasmic portion is discarded for the reduction of the weight of the sperm.
- The separation takes place between the fully developed, haploid male sex cells and mature spermatozoa.
- In sperm production results, four individual sperm cell is produced from each precursor germ cell.
Oogenesis is the process by which mature haploid ovum develops from germ cell, it takes place in the ovaries. This process is completed in 3 Stages.
1. Multiplication phase
- During early embryonic development, primordial germ cells from the dorsal endoderm of the yolk sac migrate along the mesentery of the hindgut to the outer surface of an ovary. cells are divided repeatedly during migration.
- Once these primordial germ cells reach the germinal epithelium, they migrate into the substance of the ovarian cortex and become oogonia or primordial ova.
- Oocytogenesis process by which oocyte develops; during Oocytogenesis, the oogonia undergo mitosis to form the diploid primary oocyte, which passes through the growth phase.
- The oogonia completes mitotic replication and the first stage of meiosis by the fifth month of fetal development.
2. Growth phase
- The size of the primary oocyte increases enormously in this stage and becomes large, which contains a large amount of fat and protein in the yolk, RNA, DNA, ATP, enzyme, mitochondria, Golgi bodies ribosome, etc.
- A primary oocyte is ready for the maturation when nucleus and cytoplasm are completely developed.
3. Maturation phase
- The primary oocyte undergoes a meiosis splitting process where the genetic material is divided into two daughter cells. However, the primary oocyte arrested in prophase I, the oocyte, remains in this stage until it is triggered to continue during the female fertile period.
- The oocyte continues through the end of meiosis I, forming a large ovum secondary oocyte and small first polar body (non-viable cell produced at the end of meiosis in females, containing genetic material).
- Oocytes undergo first meiotic division only after puberty.
- The secondary oocyte is arrested in metaphase II of meiosis until fertilization occurs.
- The first polar body may or may not pass through a meiosis II and then disintegrates.
- Upon fertilization, the cell completes meiosis II and produces an ootid (fertilized egg cell or mature oocyte) and another polar body.
- Guyton AC and Hall JE (2003), Medical Physiology, 10th 2. Standring S (2005), Gray’s Anatomy: The Anatomical Basis of Clinical Practice, 39 Edition, Indian Print: Saunders
- Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (1 ed.). S . Chand and company Ltd.
- Singh, R. (2013). Brenner’s Encyclopedia of Genetics || Gametes, Mammalian. 149–150.