Gametes are haploid reproductive cells in sexually reproducing organisms that fuse with one another during fertilization. Fertilization produces a diploid cell that undergoes repeated rounds of cell division to produce a new individual. Gametes are the physical carriers of genetic information from one generation to the next. They carry recombinant chromosomes produced at the end of meiosis.
Often, species that reproduce sexually have two morphologically distinct types of individuals that produce different gametes. The larger gamete produced by the female is usually called the egg or ovum. The smaller one is the sperm. Similar distinctions also exist in the plant world, with the female gamete being called the ovule and the male gamete going by the name of pollen.
Types of Gametes
In many species, there are two types of gametes whose form and function are distinct from one another. In humans and other mammals, for instance, the ovum is much larger than the sperm. The sperm also has a distinctive tadpole-like appearance with special adaptations for its primary function of traveling through the female reproductive tract and fertilizing the egg. In a similar manner, the ovum has a number of structural adaptations that aid the process of accurate fertilization and subsequent implantation. Species that have obvious differences in the appearance of gametes are said to display anisogamy.
In addition, most species are also heterogametic – containing a different set of chromosomes in each type of gamete. In mammals, the female gamete contains a single X chromosome in addition to 22 somatic chromosomes. On the other hand, the male gamete, the sperm, could carry either an X or a Y chromosome as the 23rd chromosome. Depending on the chromosome present in the sperm, the resultant diploid zygote could either be a female (XX) or a male (XY). In birds, this form of heterogamy is reversed. Females produce gametes that could contain either the W or the Z chromosome and males produce a single type of gamete.
Examples of Gametes
The two most common gametes are sperm and ova. These two haploid cells can undergo internal or external fertilization and can differ from each other in size, form, and function. Some species produce both sperm and ova within the same organism. They are called hermaphrodites. However, the majority of sexually reproducing organisms have distinct sexes with each producing a single type of gamete.
Structure and Function of Sperm
Human sperms are highly specialized cells that have undergone an extensive period of differentiation. sperms contain four morphological regions – the head, neck, midpiece, and tail. These generic terms are in fact referring to different subcellular organelles that have been adapted to aid the sperm in its function.
The ‘head’, for instance, contains the genetic material. The DNA in a mature sperm is highly compacted, has nearly non-existent transcriptional activity and all the chromosomes are tightly condensed. They even have special proteins called protamines to pack the DNA more tightly than histones. The head is also surrounded by a cap-like structure containing hydrolytic enzymes called the acrosome. Acrosomal enzymes act on the outer membranes of the egg, allowing the DNA in the sperm access to the plasma membrane of the ovum.
The neck of the sperm is made of a pair of centrioles. The proximal centriole enters the oocyte during fertilization and even duplicates within the zygote. The distal centriole gives rise to filamentous structures that form the lashing tail of the sperm.
The tail is made of flagella that allow this cell to travel along the female reproductive tract – from the cervix, through the uterus towards the fallopian tubes where fertilization can occur. This motility is even necessary for species that undergo external fertilization. Sperm flagella contain a central cytoskeletal axonemal filament that is surrounded by 2 fibrous sheaths. The axoneme has a pair of extended microtubules that mediate movement through motor proteins called dynein.
The energy for flagellar movement is provided by spirally arranged mitochondria in the tubular midpiece. Some energy is also derived from glycolysis that occurs in the fibrous sheaths of the flagellum. The carbohydrate needed for glycolysis, aerobic respiration and oxidative phosphorylation is transported into sperm either from the semen or the mucus membranes of the female genital tract.
The sperm does not have many organelles that are commonly seen in most cells. For example, sperm do not have an endoplasmic reticulum or ribosomes since most protein and lipid synthesis is completed during spermatogenesis. Even after an extensive period of differentiation, however, sperm need to undergo another process called capacitation after ejaculation, before they become fully functional. This usually involves changes to the membrane, activation (and deactivation) of some enzymes and protein modifications.
A major difference between male and female gametes, especially in humans, is their manner of being produced in the body. Spermatogenesis begins after puberty in the testes and can continue for the rest of the lifespan of the individual, in the absence of any disease or disorder. Sperm ‘mother cells’ also known as spermatogonia, can divide continuously through mitosis and generate cells that differentiate into mature sperms after meiosis. Every diploid spermatocyte can result in 2 haploid cells carrying an X chromosome and 2 haploid cells containing a Y chromosome. All these 4 nuclei remain connected to each other through cytoplasmic bridges so that even spermatids that have a Y chromosome can benefit from the proteins produced from X-chromosome gene expression.
The egg cell (ovum, plural: ova) is the female gamete. This is usually a non-motile cell. In birds, reptiles, amphibians and invertebrates, the egg is either fertilized externally or the egg is laid before a new organism emerges. In mammals, both fertilization and embryonic development happen inside the female.
The ovum is produced from oogonia or ovum ‘mother cells’ through a process called oogenesis in the ovary. The ovum is not only among the largest cells of the body, it is also specialized to ensure accurate fertilization by exactly one sperm cell. The egg also contains nutrients that sustain a growing zygote initially. In many organisms, these nutrients are seen as a fatty yolk and a protein-rich albumin. In mammals, however, the egg is implanted in the uterus and directly derives nutrients from the mother’s body after the first few rounds of mitotic replication.
Protective Membranes of the Ovum
The egg in humans contains two major protective layers – the corona radiata containing follicular cells, and the zona pellucida. The corona radiate can be made of 2 or 3 layers of cells while the zona pellucida is a clear thick membrane made of glycoproteins. The corona radiata needs to be enzymatically overcome by sperm before reaching the zona pellucida. Binding of the sperm to this inner glycoprotein membrane induces the release of hydrolytic enzymes from the acrosome. This mediates the fusion of the sperm membrane with the plasma membrane of the egg, facilitating the fertilization of the two haploid nuclei. The release of digestive enzymes and the subsequent steps are called the acrosomal reaction and it elicits a response from the egg membranes as well. The ovum forms a vitelline membrane that prevents the further entry of any other sperm. Egg membranes are also believed to play a role in maintaining species specificity during fertilization, preventing the egg membranes from being accessed by sperm of a different species.
Sex Determination in Birds
In birds (as well as some fish), the female produces two different types of eggs, since they are the heterogametic sex. This means that a diploid somatic cell in adult female birds has two different types of sex chromosomes. These two chromosomes are called the Z and W chromosomes to distinguish them from the XY sex-determination system. Males have two Z chromosomes and therefore produce sperm, all of which contain only a Z chromosome. In essence, it is the genetic composition of the egg that determines the sex of the offspring, in direct contrast with the genetics of humans and many other animals.
Each haploid gamete should have exactly half the number of chromosomes of a somatic, diploid cell. However, errors during meiosis can result in gametes that have either fewer or greater numbers of chromosomes. When such gametes participate in fertilization, the resulting zygote is aneuploid. Many aneuploid zygotes are non-viable. That is, they do not complete embryonic development and result in spontaneous abortions. However, sometimes aneuploidy can result in disorders that become apparent only after birth. The most common among these is trisomy 21, also known as Down’s Syndrome. It arises when one haploid gamete carries 2 copies of chromosome 21 – either the entire DNA molecule or large stretches of it.
When sex chromosome aneuploidy occurs, it can result in the individual having more than 2 sex chromosomes. Sometimes, it could also result in a person having only a single X chromosome in all their cells. These individuals are usually sterile, and their external sexual characteristics are often at variance with their internal genetic composition.