A profound transformation of classical cytology began when it became possible to identify cells not only by their morphology, but also by their physiological characteristics. Through the new techniques, a cell type is defined by the appearance of its colonies, its mode of locomotion as recorded by cinematography, its effect on the medium, its rate of growth, the nature of the substances which inhibit its multiplication, the nature and concentration of the substances required for proliferation, etc. The advantages of this conception of cytology are obvious. If all the manifested and hidden potentialities of every cell type were known, the behaviour of a tissue under given conditions could be predicted. The events in which the tissues take part are determined by the response made to the physicochemical conditions of the medium by each cell according to its innate qualities. On the mode of response of a cell type to other cells and to the humours depend the character istics of the community.
Physiological Factors of Growth.—The effect of blood plasma on tissue cells remained unknown until it was tested on pure cultures of fibroblasts and epithelium. These experiments showed that, first, blood serum is not a nutrient medium for epi thelial cells and fibroblasts; it inhibits their proliferation when added to a nutrient medium. Secondly, blood serum is an excellent nutrient medium for monocytes, tissue macrophages, and sarcom atous macrophages. Thirdly, the proteins and amino acids of the blood do not promote the proliferation of connective tissue and epithelial cells. Serum proteins or lipoids possess the property of restraining cell proliferation. But blood serum taken from an animal a few weeks old neither stimulates nor inhibits the growth of colonies of fibroblasts. When the age of the animal increases, its serum becomes more and more growth-inhibiting. The pro gressive increase in the growth-inhibiting properties of blood serum takes place rapidly at the beginning of life and very slowly in old age. The curve expressing this phenomenon resembles that of the decrease in function of the age of the patient in the rate of healing of a wound, as expressed in du Noiiy's equation. The age of an animal can be ascertained approximately by the growth index of its serum. The inhibiting power of serum is due chiefly to lipoids. After extraction of the lipoids, the remaining proteins are far less inhibiting than the whole serum. On the contrary, the
isolated lipoids are toxic. In old age, the inhibiting effect of serum is augmented because both proteins and lipoids are more concentrated.
The nature of the substances responsible for cell multiplication remained unknown until fibroblasts and epithelium were found to multiply with great velocity in the presence of embryo juice.
This juice is a complete food. For 16 years, a strain of fibroblasts has been fed on it, and has built up many thousands of colonies from the substances that it contains. The substance responsible for the unlimited growth of the cells is a protein, which is mixed with a lipoid having some inhibiting power. The primary split products of certain proteins produce as extensive a multiplication of fibroblasts as embryo juice does. The incomplete digests of ox fibrin, casein, egg albumin, liver, testi, thyroid, thymus, pituitary gland, etc., increase the rate of growth of fibroblasts, epithelial cells and macrophages. The split products of crystalline egg al bumin, on the contrary, possess little growth-promoting power. When the digestion is carried further and the digests are com posed chiefly of amino acids, their effect on the growth of tissue cells disappears almost completely. It is probable that the proteins of embryo juice are not absorbed as such by the cells after they have been hydrolyzed. The power of tissue juice to cause growth may possibly be attributed to the ease with which its proteins can be transformed into polypeptides by the tissue ferments. Mono cytes and macrophages do not require embryo juice or protein split products in their medium in order to proliferate and manifest other forms of activity. They are voracious cells and feed upon red blood corpuscles, protein precipitates and muscle fragments, and also on plasma, embryo juice and protein split products at a low concentration.
Through the method of tissue culture, the fundamental char acteristics of the main cell types and the mechanisms that make them an organized whole are being progressively elucidated. Al though the techniques will be still further improved, they have reached such a degree of development as to permit the construc tion of a new science of cytology based not only on the mor phology, but chiefly on the physiological properties of the cells.