The Peri-Renal Tissue of Protopterus A Contribution to the History of the Adrenal
The animal studied was the lungfish Protopterus aethiopicus; the investigation was histological and cytochemical, the material being freshly taken from East African lakes. The peri-renal tissue is a substantial organ which, because it surrounds the kidney, extends through the greater part of the body cavity. Those who have investigated its structure in recent years have considered it to be a haematopoietic organ. A wide variety of histological, histochemical, and haematological techniques was used. The organ is shown to contain five characteristic kinds of tissue; these are named: lipid, endothelial, pigmented, round-cell, and steroid tissue respectively. The lipid tissue is characterized by cytoplasmic liposomes--droplets containing phospholipid. In one type of cell, the ‘small lipid cell’, which may be related to the eosinophil granulocytes of the blood, the droplets have a relatively small diameter. In a second type, the ‘large lipid cell’, the droplets grow to a much larger diameter, and show evidence of cycles of activity involving increase and disappearance of the phospholipid. Histochemical interpretations are made difficult by the finding that this phospholipid does not colour with Sudan black. The endothelial tissue is composed of phagocytic (macrophage) cells some of which are elongated and line vascular sinusoids while others are spherical and contain larger inclusions. These inclusions have some lipid characteristics and may be a manifestation of the translocation of lipid within the organ. The pigmented tissue is composed of melanophores and of a second, round, type of cell which shows evidence of relationship with the macrophage system. Both kinds of cell contain ‘melanin’; the second also has inclusions with different histochemical properties. The round-cell tissue, found in nodules, has a general resemblance to lymphoid tissue. It is doubtful whether its cells are lymphocytes, but it has not been found possible to characterize them in any other way. The steroid tissue is composed of cells which react positively to the histochemical tests comprised in the ‘steroid repertoire’, as used in the study of the mammalian adrenal. Attention is drawn to technical difficulties which may have confused the study of the peri-renal tissue as a haematopoietic organ. The connexion between the lipid tissue and circulating granulocytes has features which distinguish it from the processes of granulocyte formation in the spleen. There are strong indications of an evolutionary connexion between peri-renal tissue and the cortex of the adrenal gland of higher animals. The lungfish may offer the best starting-point for the study of the comparative histology of the adrenal of tetrapods. The association between lipid, steroid, haematopoietic, macrophage, and pigment tissue has parallels in function in the complex interrelations of the mammalian adrenal.