Ultrastructural observations on haemocytopoiesis in adult horseshoe crab (Tachypleus tridentatus)

Haemocytopoiesis was studied in adult Tachypleus tridentatus by using transmission electron microscopy. Haemocytopoiesis occurred in connective tissues adjacent to the site of gametogenesis with basement membrane as a boundary between them. The haemocytopoiesis could be divided into three stages: prohaemocyte, plasmatocyte and granulocyte. In the male, haemocytes were developed in a reticular structure formed by boundary basement membrane which separated them from seminiferous tubules. The nucleus of early prohaemocyte was euchromatic, while heterochromatin arranged in nuclear periphery of late prohaemocyte, in which rich vesicles were distributed in the cytoplasm. The nuclei of plasmatocytes, which were larger than prohaemacytes, varied in shape as the cells did and contained rough, web-like heterochromatin. Cytoplasm of plasmatocytes looked empty and was low in electron density. There were a few protein-deposited vesicles in various sizes in the cytoplasm of late Granulocytes were rich in large granules about 1.5 mu m in diameter and small granules about 0.5 mu m in diameter. The number of granules increased along with the cell development. The mature haemocytes were 10-15 mu m in size.