Intracellular sites of activity in the histogenesis of tunicate vanadocytes

The stages in the formation of haemovanadin in fibroblasts of Ascidia pygmaea Michaelson have been investigated with an electron-microscope. The histochemical and vital studies of Endean (1960) on the blood-cells of Phallusia mammillata and the chemical analyses of haemovanadin by Bayer, Bielig, Califano, and Wirth (1954) have been used as a guide to the interpretation of the structures observed. Pinocytosis of the pharyngeal and intestinal epithelia occurs, during which vanadium, probably bound to acid mucus, is absorbed into the blood-spaces. Amoebocytes pick up vesicles of mucus which are then dissolved in vacuoles. Vanadium is seen to accu- mulate on the vacuolar membranes as the contents of the vacuoles are concentrated and chelated with sulphate and protein. The complex formed is considered to be a precursor of the haemovanadin molecule and can be used as a marker in later stages of histogenesis. When first formed on the vacuolar membrane the vanadium-complex particles are about 3 mμ in diameter. When they are precipitated into the large vacuole formed later by fusion of small ones, the granules are compound and measure from 60 to 100 mμ. Collection and concentration appear to cease when the amoebocyte turns into a ‘signet-ring’ cell, with a single large storage vacuole compressing the cytoplasm and nucleus into a polar cap. At this stage the synthesis of haemovanadin begins in the cytoplasm as oval grains of protein, 150 by 300 mμ, contained in vacuoles formed at the vesicular ends of Golgi bodies and surrounded by ribosomes. The grains of protein are then surrounded by vanadium-complex material which has been removed from the large vacuole by a process similar to the ‘ropheocytosis’ of ferritin in erythroblasts. The new vanadium-protein compound is distributed in vacuoles around the periphery of the cell, and finally the contents become acid and active haemovanadin proper is formed. The cell is now a ‘compartment cell’. The vacuoles increase in size, owing probably to their absorption of sugars in the blood near the gut, and they become turXgid with fluid so that the cell resembles a morula. In the early vanadocyte extremely electron-dense particles are visible on the vacuolar membrane, about 20 mμ in diameter. It is probable that they are particles of haemovanadin. In the older vanadocyte the turgid vacuoles become filled with grains of cellulose-like polysaccharide, 30 to 50 mμ in diameter. The test of the tunicate shows amoeboid vanadocytes with long pseudopodia, from which polysaccharide fragments are deposited in the matrix. It is concluded that the formation of haemovanadin in several separate stages is the result of the activity of vacuoles whose membranes change their specialized enzymic and permeability properties, while the vacuoles retain their identity.