Human beings, like most other animals, are multicellular organisms that perpetuate our species through the type of reproduction that we know as sexual. This type of reproduction, which results in the emergence of individuals with genetic characteristics from two individuals, something that gives species much greater variability than that offered by asexual reproduction.
For sexual reproduction to produce a new being, it will be necessary for a certain type of cells to fuse: sex cells or gametes It is about these that we are going to talk about in this article.
They receive the name gametes or sexual cells from a specific type of cell which Its main function is to generate a new being perpetuating the species and the genes of the parents.
Sexual cells have different shapes, specifically there are two types whose union will generate the zygote from which a new individual will eventually develop. The specific name of these cells depends on the type of living being we are talking about, with one male and one female.
This type of cells It has half the number of chromosomes that the species in question has , something that when the new being appears through the union or fusion of two cells from two different individuals allows the daughter organism to end up having the same number of chromosomes as its parents, although with genetic information different from that of any of the previous ones. After their union, a genetic recombination of the genetic information from both cells occurs, generating a unique genetic code through said recombination.
In the case of humans, we have a total of 46 chromosomes divided into 23 pairs. Of them, 22 pairs correspond to somatic chromosomes and are the same regardless of sex. However par 23 differs between men and women , these being the sex chromosomes that mark our genetic sex. Specifically, men have one X and one Y chromosome, while women have two X chromosomes.
When we talk about sex or sexual cells, the first thing we think about is the type of reproduction and cells that we human beings have and that other animal species also have: sperm and eggs.
The sperm
The sexual cells of the male gender are called sperm, and which have half of the genetic information necessary to form a new living being. It is a type of cell that is very small in size, smaller than that of female gametes, and is formed in large quantities within the testicles of the males of each species.
For fertilization to occur, it is necessary for the sperm to travel to the egg, of which only one (generally, although there are exceptions) will be able to enter the egg and combine its genetic material with it. That is why the sperm has morphological adaptations that allow such movement.
Its basic morphology is the following:
First of all we can observe the existence of a large head (the largest part of the sperm) within which we can find the core , in which the genetic information in question can be found, and the acrosome or layer formed by various enzymes that allow the sperm to enter the female gametes. In addition to this, we can find different substances that nourish and allow the movement of the sperm.
The other main part is the tail or flagellum, thanks to which the sperm can move through the interior of the female body until they reach the egg. Inside it we can first find a small neck through which it is attached to the head, then an intermediate piece in which we can find different mitochondria which allow sufficient energy to be produced (through the substances present both in the sperm itself and in the rest of the semen) and finally the flagellum or final part, which moves to allow movement.
Eggs are the female sex cells, which carry half of the genetic information necessary for the genesis of a new being. It is a type of large cell, which is sphere-shaped and They are produced by the ovaries of the females of different species
Eggs have the characteristic that they are not always available for fertilization, there being an entire cycle through which an egg is produced, matures, remains available for possible reproduction and is released if not fertilized, this being the menstrual cycle. About one is generated per month (actually, it’s usually 28 days).
Likewise, unlike sperm, which are present in large quantities throughout life, there is only a certain number of them in each female. During reproduction itself, the egg remains motionless, until the sperm reach it and the egg finally manages to penetrate it (if it can).
The structure of this cell is as follows, from inside to outside:
First of all, and inside, the nucleus stands out, in which the genetic information is found that would allow the formation of a new being by joining a sperm. We can also find vitellium inside , a series of substances as an energy reservoir that would allow the survival of the zygote until the formation of a placenta. All of this would be surrounded by a plasma membrane that limits the cell and through which chemical elements can enter and leave that allow its interior to be chemically balanced.
around the membrane We can find a protective gelatinous layer, called the layer pellucida , which acts as protection while allowing the entry of the first sperm and ends up hardening to prevent more than one from entering. A final layer, the outermost, is the corona radiata. This will have special relevance in regulating sexual hormones and generating the placenta if there is fertilization.
Sperm and eggs are not the only types of sexual cells that exist, being only those of animals. Plants and other vegetables also have sexual reproduction in many cases its sexual cells being the oosphere and pollen.
The oosphere
The type of female sexual cell in plants that have the ability to reproduce sexually is called the oosphere. This type of cell can be found inside the so-called seminal rudiments located in the embryonic sacs of plants, located in the flowers.
Like animal eggs, it has half as many chromosomes as the rest of the cells of the parent individuals. The pollen or male gamete at the plant level comes into contact with it through the stigma of the flowers.
The pollen
Pollen would be the plant equivalent of sperm: the male sexual cell of plants. These are small grain-shaped particles that form in the stamens of plants. It joins the oosphere in the process known as pollination (for which they need the wind or the help of animals.
These grains, whose content is half of the genetic information necessary to produce a new being, enter the stigma and join the oosphere. To do this, once on the stigma, the pollen generates a small extension called a pollen tube in order to transport its genetic material to the oosphere.
