In this condition, which affects the cells of tissues, we have a change in the chemical composition of the cell-contents; the albuminous constituents split up and yield fat.
Fat may be formed in the animal body either from the carbo-hydrates or albuminous substances of the food. In the case of fatty degeneration the fat is derived from the albuminous constituents of the tissues. This implies that these constituents break up, yielding their nitrogen in some lower form of combination which is usually carried off, leaving the fat in the tissue. In accordance with this we find that, where general and extensive fatty degeneration occurs, there is simultaneously an excess of urea or other extractives in the urine. The splitting up of the albuminous constituents of cells implies a most serious alteration in their chemical constitution. In all such cases, therefore, it is to be inferred that the vitality of the cells is greatly reduced, and in some cases the condition approaches to, or is associated with, necrosis.
That fat may be formed from carbo-hydrates is shown by the fact that bees produce wax when fed on honey alone, this being a solution of sugar (Gundlach). The formation of fat from nitrogenous substances is proved by various facts. Thus the . tissues of the body, including the nitrogenous constituents, are sometimes changed after death into Adipocere. a peculiar waxy substance, composed of fatty acids combined with ammonia and lime instead of glycerine, and therefore more strictly a soap than a fat. This substance is occasionally found in graves, and it has been produced by leaving the dead body in running water for a time. Again, it has been shown that, in lactating animals fed with animal food carefully deprived of fat, the milk is even more abundant and rich in fat than in animals fed on diet containing much fat. Again, fresh milk becomes richer in fat and poorer in caseine (which is nitrogenous) during the first day after its withdrawal from the mamma; (Hoppe-Seyler).
The most conclusive proof that albuminous tissues yield fat is afforded by the effects of poisoning by phosphorus. When a dog has been deprived of food till all its spare fat has been exhausted and the nitrogen in the urine has .reached a constant minimum of 8 grammes in the twenty-four hours, the administration of small doses of phosphorus causes a marked increase of the nitrogen, which may reach nearly 24 grammes. This is coincident with a very large increase of fat in the tissues of many internal organs.
The causes of fatty degeneration may be divided into those which, depending on some morbid condition of the blood, act on many organs, and those which have simply a local influence. So we may speak of a general and local fatty degeneration.
General fatty degeneration is produced by certain poisons, preeminently by phosphorus, but also by arsenic, antimony, iodoform, chloroform, and others. It occurs also in some general diseases in which the blood is greatly altered, in acute yellow atrophy of the liver, in some fevers, in pernicious anaemia, and in some other forms of anaemia. It has been produced artificially by confining animals in an over-heated space for thirty-six hours (Cohnheim). In these cases the altered blood has acted on the cells of the tissues and caused them to alter their chemical constitution. The change occurs mostly in the parenchyma of organs, as in the hepatic cells, the renal epithelium, and striated muscular tissue, especially that of the heart, but it is also seen in some cases in other structures, such as the intima of arteries.
Local fatty degeneration is frequently the result of deprivation of blood, as where an artery is occluded. In the brain, occlusion of arteries is followed by softening of the cerebral tissue, a species of necrosis, but this is associated with the appearance of cells filled with finely-divided fat (the so-called compound granular corpuscles of Gluge). It is true that the fat here may be partly derived from the myelin of the nerve-fibres, but these fat-filled cells are not confined to the white substance, which alone contains myeline, but are present also in the grey substance where there is no myeline, and fat is also visible in the walls of the blood-vessels (as in Fig. 42). Inflammation is a frequent cause of local fatty degeneration, especially in certain parenchymatous organs, where the cloudy swelling often goes on to fatty degeneration. (See Fig. 41.) In quickly growing tumours, and even in slowly advancing cancers, the cells frequently undergo fatty degeneration. Lastly, nerve-fibres which have been divided show not only atrophy, but also a fatty degeneration.
Fig. 42. - Fatty degeneration in the cerebral vessels in softening of the brain. (Pagkt).
Some authors have endeavoured to account for fatty degeneration on the supposition that it is due to a deficiency of oxygen. It is said that in general fatty degeneration the blood is deficient in oxygen, and in the local form the tissues are deprived of oxygen. In the former case, however, there is usually an obvious alteration of the blood apart from simple anaemia, while in many local fatty degenerations there is no deprivation of oxygen, as in inflammations, in tumours, and after section of nerves.
The degeneration occurs mainly in the cells of the tissues. The fat, arising as it does by the chemical decomposition of the protoplasm of the cell, appears in the form of fine drops or granules, which are strongly refracting. (See Figs. 42, 43, 44.) These granules are separated from each other by the remains of the cell contents and are therefore isolated. It may happen in this way that, as in Fig. 43, a fatty degeneration occurring in a structure may render its constituent cells unusually distinct, their form being brought prominently out by the fat in them. As time goes on, the fat granules increase till the whole cell is filled with fine refracting oil drops, which remain isolated (see Fig. 44), being each surrounded by an albuminous envelope. The process is, in fact, very much like what occurs in the cells of the mammary gland in the secretion of milk, the colostrum cells being like the fully degenerated cells, which latter are often described as compound granular corpuscles.
Fig. 43. - Fatty degeneration in an atheromatous aorta. The shapes of the cells brought out by the fat; a, from internal coat; b, muscle cells from middle coat, x 350.
Fig. 44. - Fatty degeneration of cells in a cancer of the mamma; a, slighty affected; b, more so; c, completely fatty - the compound granular corpuscle, x 350.
When finely-divided fat suspended in fluid is present in the living tissues, it is very readily absorbed. We know how readily the emulsionized fat in the alimentary canal is taken up by the epithelium, and passed on into the lacteals. When milk is injected into the abdominal cavity of a living animal, or even laid on the surface of the diaphragm after death, it very quickly passes into the lymphatics. In the case of fatty degeneration of cells, if fluid be present, the cells disintegrate, an emulsion is formed, and absorption occurs just as in the case of milk.
But sometimes the fatty degeneration occurs in connection with structures not adapted to absorption, as in a hydrocele, where the tendency is rather to transudation, or as in an ovarian tumour. In that case, the fat undergoes further changes, resulting usually in the production of crystals of cholestearine (Fig. 45) or margarine.
Fig. 45. - Crystals of cholestcarine; a, large ones from an old hydrocele; b, from stagnant, bile in gall bladder. x350.
Cholestearine. which is an alcohol, occurs as a normal constituent of the central nervous tissue and bile, in which latter it is dissolved. Its crystals are rhombic tables whose angles measure 79° 30' and 100° 70'. On adding strong sulphuric acid to a crystal of cholestearine, letting it gradually run in and act on it, the crystal appears to melt from the edge inwards, and take on a fatty appearance, and by and by it gathers into a brown drop. On adding iodine and sulphuric acid to a crystal, there is at first a beautiful display of colours. Margarine occurs in the form of radiating needles such as one sees frequently inside the fat cells in adipose tissue.
Local fatty degeneration is not infrequently followed by Calcareous-infiltration, where, from deficiency of fluid or otherwise, the fat is not absorbed.
We have already seen that in Caseation fatty degeneration is associated with necrosis.