Light and electron microscopic studies of the morphological features of immature and mature rodlet cells in Catostomus commersoni are presented emphasizing the cells' association with epithelial tissues. The peripheral fibrillar layer is lacking from the apex and from the base of the cell. A cytoplasmic extension from the base may be a feeding mechanism whereby the rodlet cell obtains nutrient at the expense of adjacent cells leaving intercellular spaces often containing myelin figures. RNAase digestion studies demonstrate the presence of RNA in the electron dense rodlet core.The structure and histochemistry of the rodlets which do not appear to disintegrate upon expulsion from the cell are compared to the cytoplasmic inclusions of both normal fish cells and protozoan parasites. The possible association of the rodlet cell with various pathological conditions is briefly reviewed and the authors conclude that it is premature to disregard the possibility that this cell could be a parasite or infective agent.

Peripheral blood and haemopoietic tissues of spleen and kidney of the icefish, Chaenocephalus aceratus were examined using LM and EM techniques. The peripheral blood contained cellular elements from all the recognized cell lines usually seen in other teleost groups. Erythrocytes were very rare; when found, they were mature or senile and fragile. Thrombocytes of two morphologies, several cell types considered to be part of the lymphoid series and monocytes/macrophages were present. Two distinctive types of granulocytes also were found; their morphologies and granulation were so different from teleost granulocytes hitherto described that their identification was impossible.

This study was undertaken to identify an ubiquitous granular leukocyte found in Catostomus commersonni Lacépède. The cell contains large, numerous, strongly PAS-positive cytoplasmic granules, an eccentric nucleus and prominent, persistent juxtanuclear space. It develops in the hemopoietic tissue of the kidney, and mature cells are found not only in kidney and peripheral blood but also in areas of connective tissue where mast cells are usually located. Electron microscopy confirms the presence of a large Golgi apparatus, unlamellated cytoplasmic granules and extensive rough-surfaced endoplasmic reticulum. Histochemical studies show that the cytoplasmic granules are alcianophobic, non-metachromatic and unstained by acridine orange. Histamine is detectable spectrophotometrically in kidney tissue, but the PAS-positive granular leukocyte does not consistently degranulate after treatment with histamine liberator 48/80. The authors suggest that while the PAS-positive granular leukocyte is not identical with classical basophils/mast cells, which are absent in C. commersonnii, it may represent an evolutionary precursor of these cells.

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Developmental stages of the two circulating blood granulocytes, the heterophil and the periodic acid-Schiff-positive granular leukocyte (PAS-GL) are described. There are no metachromatic basophils in this fish and eosinophils, when present, are confined to extravascular tissues. Fish were killed up to 24 h after injection with histamine liberator 48/80. Morphological (L/M and E/M) and histochemical studies of blood, haemopoietic tissue and gills from experimental fish were compared with those of untreated control fish.Blood cells other than PAS-GL and heterophils were generally unaffected by 48/80. Most PAS-GL and heterophils showed alterations in granules and other cytoplasmic features. Stretching and splitting of the PAS-GL nucleus occurred. Very little, if any, histamine was detectable in PAS-GL and heterophils.The authors support the postulate that during evolution PAS-GL become basophilsl mast cells through two major stages, (1) sulphation of the granule polysaccharide to give heparin resulting in metachromasia of the granules and (2) storage of large quantities of histamine in the granules.