Cell reproduction

The cyclic continuity of living beings finds in the phenomena of reproduction the connecting links between successive generations.

Reproduction is carried out at different levels of the evolutionary scale, in the different branches of the vegetable and animal kingdoms, in the different living species, with such a variety of mechanisms that alone justifies an entire treatise.

A first classification of the phenomena of reproduction must distinguish the unicellular organisms from the multicellular ones, since only in the first the cell division coincides with the reproduction.

In multicellular reproduction can be agamic or sexual (or gamica).

Agamic reproduction, relatively less frequent, is based on the mechanism of mitosis, so that the variability of the species is entrusted rather to the repeated occurrence of mutations.

There are also different mechanisms, such as strobilation, sporulation, etc., while in plants we find well-known regenerative forms in agriculture (cuttings, layering, etc.).

The most widespread reproductive mechanism in the higher forms is, however, the sexual one, corresponding to the appearance of meiosis, the formation and gametes and their fusion in the zygote (fertilization).

In primitive species the gametes are not morphologically differentiated: in this case we speak of isogametia. There are, however, always two series of gametes, identified with the symbols (+) and (-) and fertilization can take place only from the encounter between gametes of opposite sign: there is therefore a biological difference, not yet expressed morphologically.

As the evolutionary scale progresses, the morphological and functional differentiation appears, in which we distinguish a type of female gametes generally endowed with abundant reserve material (the deutoplasm or calf, which will ensure the development of the embryo until it can be metabolically independent ) and a type of male gametes endowed with motility to reach female ones. The gametes are always haploid and are the result of meiosis. Their fusion gives rise to the diploid zygote.

Between meíosi and zygote can occur a series of haploid cellular generations, as between zygote and meiosis can occur a series of diploid cellular generations, with the different varieties of generative cycles expressed by the alternation of generations.

A diplonte species (with a diploid organism) is characterized by gametic meiosis: meiosis produces directly the gametes, which melt immediately reconstitute the diploid state. This is the prevailing case of the Metazoans, including man.

Gametogenesis

Taking into account the reproduction with gametic meiosis, as occurs in man, let's clarify how meiosis is inserted in gametogenesis (gamete formation).

In embryonic development, of male and female gametogenesis (called spermatogenesis and oogenesis), there is an early differentiation between cells destined to form the body (somatic line) and those destined to produce gametes (germ line). The initial cells of the germ line are called protogons. With the differentiation of the gonad in the male or female sense, germ cell differentiation occurs respectively in spermatogonia and ovogonas.

Looking at spermatogenesis, we see that there is a series of cellular generations in spermatogonia, a series that continues throughout life. Only a part of the spermatogonia so continuously produced differs from the normal mitotic cycle and instead begins the meiotic cycle.

The germ cell in which meiosis will begin (reduplication and then first division) is called first order spermatocyte; its division originates two second order spermatocytes, which with the second division give rise to a total of four spermatids.

We can graduate the reduction of the chromosomal outfit from 4n of first-order spermatocytes (after reduplication there are four chromatals for each pair of homologues) to the second of the second-order spermatocytes and to the n of the spermatids, as already seen studying meiosis, which so it ends. Spermatids are therefore already haploid, but are not yet mature gametes. From the structure of a haploid cell the functional maturation (called sperm mythogenesis) transforms the spermatids into spermatozoa, that is mature male gametes.

In female gametogenesis (or ovogenesis), different differences are noted. First of all, the number of gametes to be prepared is much smaller. It is estimated that about 5 X 105 ovogons are prepared in the gonads of a female of the human species; of these, only about 400 are interested in follicle maturation and subsequent dejection, in a cycle that usually involves only one follicle a month for a fertile period of about 35 years.

The different number of gametes prepared in the two sexes corresponds to the difference in function and behavior already mentioned: the spermatozoa are small, mobile and numerous in relation to the need to search for the egg and to the low probability of finding it; the eggs are large, inert and few, in relation to the function of guaranteeing to the embryo the reserve material and to the protection provided to them by internal fertilization (naturally, in species with external fertilization even the eggs must be more numerous).

The need to supply gametes with reserve material corresponds to the presence, in oogenesis, of a phase of arrest of meiosis, during which the chromosomes partly despiralize. We then observe the so-called «feathery chromosomes», in which a series of extroflexions identifies the traits in which the genes responsible for the synthesis of deutoplasm are derepressed.

The smaller number required for female gametes also corresponds to the fact that, of the four haploid cells produced by meiosis, only one receives the entire reserve material and becomes a gamete, while the other three (the polocytes or polar bodies), which contain only chromosomal material, cannot give rise to zygotes and embryos and are destined to regress.

Fertilization

In the animal kingdom we can observe the passage from external fertilization (gametes exposed to every environmental risk and therefore necessarily very numerous in the two sexes) to internal fertilization, to which parental care is further connected up to the metabolic relationship of the Mammals between mother and fetus.

Fertilization, once the encounter between gametes of the opposite sex has taken place, must take place by guaranteeing two conditions: specificity and uniqueness. In other words, it must be ensured that the spermatozoon is of the same species as the egg and that, once it has entered the first, no others enter it.

The specificity is ensured by the biochemical characteristics of the acrosome and the surface of the egg. In fact, there is talk of reactions between «fertilisine» and «antifertilisine», with specificity comparable to that of the encounter between enzymes and substrate.

The uniqueness of the fertilization is guaranteed by a modification of the surface structure of the ovule ("cortical reaction") which begins precisely at the time of the first specific fertilisin / antifertilisin reaction; after this reaction the membrane of the egg is altered, so that the other possible spermatozoa that reach it are no longer able to start the specific fertilization reaction.

Following fertilization, the tail of the spermotozoan remains external to the egg, while the chromosomal material penetrates. This, called "male pronucleus", joins the "female pronucleus" of the egg thus forming the diploid nucleus of the zygote.

Edited by: Lorenzo Boscariol