THE pathological new-formation of tissue occurs by processes analogous to those concerned in the physiological formation of tissue in the process of growth.
Karyomitosis ( = nucleus, = a thread or fibre). - The newformation of tissue, whether physiological or pathological, implies cell-division. According to the views of Remak this process consisted in a direct division of the nucleus and cell. The observations of Flemming and Strassburger show that in the growth of both animal and vegetable tissues the process is not so direct, but involves certain changes in the nucleus of a striking and peculiar nature. To this process the names Indirect division, Karyokinesis, Karyomitosis have been applied.
This process occupies a comparatively short time, seconds or minutes, and is not readily seen in the products of post-mortem examinations. In order to observe it the tissues should be obtained from the living body and immediately subjected to the proper fixing and hardening processes, preparatory to microscopic examination.
The general outlines of the process may be followed in Figure 67.
The Nucleus in a state of rest is not a homogeneous body. It has a limiting membrane, inside which the contents are composed of two substances. One of these is deeply stained by certain reagents and is hence called Chromatic substance, while the other is less stained and is called Achromatic. The chromatic substance forms a finely fibrous stroma (see a, Fig. 67) between which the achromatic substance and the nucleoli lie. It is the fibrous stroma which is mainly concerned in the processes which precede division. The membrane and nucleoli disappear and the fibres become thicker and stain more deeply than before (see b, Fig. 67), being converted into a convoluted fibre, or system of fibres. These fibres seem to have a power of movement according to which they alter their arrangement as the process proceeds (hence the name karyokinesis, from = nucleus, and = movement).
Fig. 67. - Division of cells. Explanation in text. (After Flemming).
From this convoluted form develops the stellate form of the fibres (c), the fibres by longitudinal division becoming finer (d). The fibres now arrange themselves in the equator of the nucleus (at d and e), and here they divide into two. The two halves now diverge from one another towards the poles of the cell, forming there the fibres of the new nuclei, and having at first a stellate arrangement (/). This by degrees gives place to a convoluted arrangement, and the nucleus subsides into the state of rest, the fibres to a great extent disappearing, and the cell membrane being formed. After the polar separation of the daughter nuclei the cell itself shows signs of division, and so the process is completed.
The process of karyomitosis is to be observed in all kinds of pathological new-formation. It is abundantly evident in growing tumours, in tubercles (according to Baumgarten), in inflammatory new-formations, and it is probably the chief, if not the only method, of new-formation of permanent cells. In fact, the activity of new-formation may in general be estimated by the number of cells in which karyomitosis is visible.
According to Arnold the process is not so uniform as that described above, but follows two types which he designates segmentation and fragmentation, but he suggests that the two may be simply modifications of the same type. There is probably, in addition to this, a direct division of cells, which occurs chiefly in leucocytes.
An important question in regard to new-formations is, as to their origin and their relation to the existing tissues. It will be found that the new-formed tissue always conforms in the details of its structure to one or other of the normal tissues. No new-formation is therefore foreign to the organism, there is no heterologous or heteroplastic new formation in this sense. For the most part, also, the new-formed tissue obviously springs from and is in close relation with tissue of its own kind. It sometimes happens, however, in the case of tumours that their tissue extends to structures of a different nature from their own, and in this sense the term heterologous is sometimes used.
In its earlier periods, after the cleavage of the germinal vesicle, the embryo consists of a mass of round cells which, from the fact that they present no visible differentiation of structure, may be called Indifferent cells. Whilst themselves devoid of visible special structure yet these early embryonic cells contain in them the powers requisite for the development of the whole being of whose cells they are the parents. These cells form the three germinal layers, the epiblast, hypoblast, and mesoblast. When once this differentiation has occurred these layers remain distinct, and each produces its own special tissues. The same distinction is presumably carried out in pathological new-formations It was, indeed, suggested by Virchow that the connective tissue which exists in every part of the body, penetrating amongst other tissues, may be regarded as the remains of the undifferentiated embryonic tissue. In this sense he regarded connective tissue as the essential agent in new-formation of tissue, and as capable of producing epithelial tissues as well as those of its own kind. In more recent times there was a tendency to ascribe to the leucocytes which emigrate from the bloodvessels a somewhat similar position, and to ascribe at; least some new-formations to the development of these cells. The general belief now is that the differentiation of the germinal layers holds for pathological as well as for physiological new-formation, and that tissues do not originate except from tissues of their own nature. The greatest difficulty in this respect is in regard to cancers, under which heading the subject will recur.
A certain interchangeability is manifested among the different tissues belonging to the connective tissue series. These really form a single tissue having certain modifications, so that fibrous tissue, bone, cartilage, mucous tissue, adipose tissue, may not only be developed to a certain extent from each other, but may even be converted, when mature, into each other. Here we have a true metaplasia. Thus, adipose tissue, by absorption of the fat, becomes loose connective tissue, connective tissue by attraction of lime-salts becomes osseous tissue, cartilage also develops into bone, or, by a different change in its matrix, forms mucous tissue.
The new-formation of tissue occurs under three different circumstances. It may be virtually a continuation of normal growth, the new tissue being produced to subserve the normal functions of the body. Secondly, it may occur in consequence of the application of an irritant which directly stimulates the tissues. Inflammatory new-formation is an example of this, and we have a further example in the specific or infective new-formations. Lastly, there is a group in which no cause is apparent for the new-formation; the tissue simply grows, without any apparent stimulus and without any purpose in the economy. This comprises the group of tumours proper, in regard to some of which, however, the existence of parasitic organisms as the stimulating agents has been at least suspected.
In the present section the first of these groups will be considered.
Karyomitosis - Flemming, Virch. Arch., vol. lxxvi., and Zellsubstanz, Kern- und Zelltheilung, 1882; Strassburger, Zellbildung und Zelltheilung, 1876, Kerntheilung, 1884; Eberth, Virch. Arch., vol. lxvii.; Arnold, Virch. Arch., xciii., xcv., xcvii.; Baumgarten, Tuberkel und Tuberkulose, 1885; Martin, Virch. Arch., lxxxvi.; Klein, Quarterly Jour, of Micr. Science, xvii. and xix.; Bizzozero, Virch. Arch., ex. (here a very good method of staining which the author has found very efficient).