Ultrastructure of cells constituting lymphoid tubules and circulating hemocytes in Penaeus monodon

The two main groups of cells in the lymphoid tubule wall of Penaeus monodon are fixed cells and migrating hemocytes. Fixed cells include endothelial, stromal, and capsular cells. Together, they form the scaffold that defines the structure of the lymphoid tubule and provide physical support as well as a niche for transmigrating hemocytes. The luminal surface of lymphoid tubule was lined by elongated, spindle-shaped endothelial cells with a centrally located nucleus and rather thick plasma membrane. Stromal cells were the smallest type of fixed cell. They are stellate cells located between the inner endothelial and outer capsular cells. These cells formed a cyto-reticular network for migrating hemocytes. Capsular cells have a flattened and irregular shape with a ruffled border with long filamentous microvilli. The nucleus is centrally located within a small mass of cytoplasm. Together they form the outermost layer of the lymphoid tubular wall. Transmigrating hemocytes within the lymphoid tubules, as opposed to circulating hemocytes, were classified into hyaline (HH), small granular (SGH) and large granular (LGH) hemocytes. The HH have very few granules and a few cytoplasmic organelles, reflecting low synthetic activity. The granular hemocytes (SGH and LGH), despite being different in size, have similar ultrastructural characteristics. They contain high amounts of rough endoplasmic reticulum, ribosomes, mitochondria, and three types of granules. These characteristics implicate their higher synthetic as well as immunologic activities. Based on these characteristics we believe that all the hemocytes belong to a single line of cell differentiation.