Blood Cell Ultrastructure of the Ascidian Botryllus schlosseri.

In the blood of B. schlosseri these cell types are constantly present: hemoblasts, granulocytes, macrophages, morula cells, nephrocytes and pigmented cells. All blood cells seem to have a common origin from the hemoblast, which is an undifferentiated cell with large nucleus, prominent nucleolus and cytoplasm filled with ribosomes. At least two kinds of granulocyte (microgranular and macro‐granular amoebocyte) are recognizable on the basis of granule size. Furthermore, the microgranular amoebocyte contains small vesicles, glycogen and peripheral bundles of microfilaments. The macrogranular amoebocyte displays a well developed RER formed by elongated cisterns that contain a finely granular material and swell to form large sac‐like structures. Specialized amoebocytes (macrophages) can phagocytize cell fragments and entire cells, thus forming large necrotic masses. The morula cell is characterized by large vacuoles in which are strongly electron dense masses that contain iron, as revealed by X‐ray microanalysis. Both young and mature morula cells are described and the probable formation of the masses from the Golgi complex is suggested. The nephrocyte (purinic cell) shows very few giant vacuoles containing numerous granules, mostly electron lucent and often of geometric form, that are presumably the same as the refringent granules visible in vivo.