Growth

GROWTH. - It is the biological problem of the increase in volume of organisms during development. As regards mammals and man, it takes place through characteristic phases, the study of which, for the human species, is of great value since it is connected with the constitutional habit.

With the activation of the egg there is a first phase in which development without growth is found, in that the egg cell divides into cells (blastomeres) that are progressively smaller (segmentation phase) up to the morula stage. Then the increase in volume of the germe begins, both through the formation of an internal cavity and above all in relation to the fact that the cells deriving from the division of a cell grow until they reach the dimensions of the mother cell; moreover, the constitution of embryonic tissues with a considerable quantity of intercellular substance (mesenchyme) is another cause of this increase. From the mesenchyme the connective tissues will originate, in which the interstitial part, between the cells, partly produced by the cells but not alive, acquires a notable predominance over the cellular part.

Cell reproduction does not maintain a uniform rhythm in all the tissues of the germ: fundamental in this regard are the studies of Bizzozero and his classification of tissues into tissues with perennial elements (which cease reproduction early, such as nervous tissue, and no longer have the faculty of reproducing), into tissues with stable elements (which cease their reproductive activity with the end of bodily growth, such as for example the liver and other glands), into tissues with labile elements (which continue to reproduce even after the phase of bodily growth throughout life, to make up for their continuous functional wear, such as for example the epidermis and the blood).

Growth is studied by relating weight or volume to age. Every organism has its own rhythm of growth, and in general well-determined periods of slowing down and acceleration are noted in relation to the activity of particular glands of internal secretion (hypophysis, thyroid, thymus, gonads). According to Quetelet, in man there is a maximum of growth from the 6th to the 8th month of intrauterine life and postnatal maxima around the fifth and a half year and around the 15th-16th year, the age of puberty.

If one represents the growth curve of the most varied organisms, or of organs or parts of them, one observes that it has the form of an S. This curve has been compared to those of autocatalytic monomolecular reactions (Robertson, Ostwald). D'Ancona emphasizes the similarity of the curves of all those phenomena that are self-limiting through the state they themselves provoke; just as in autocatalytic reactions there is self-limitation through the production of the substance itself, in organisms the limitation is given by the differentiation of the tissues.

In the growth of organisms every phase of differentiation is characterized by a slowing down: sexual maturity provokes the most conspicuous slowing down or the arrest of growth. In some cases, as in the lampreys (Cotronei), there can even be a bodily reduction. On growth, besides hormonal factors and other internal factors, external and environmental factors act: the quantity of food, temperature, the quantity of oxygen, and space are those most directly connected.

A. Stefanelli