The ultrastructure of the amebocyte-producing organ in Biomphalaria glabrata.

The amebocyte-producing organ (APO) in normal and echinostome-sensitized Biomphalaria glabrata was studied at the ultrastructural level. The APO in unexposed snails consists of small clusters of primary ameboblasts resting on the epithelial cells lining the pericardium. The ameboblasts are held in a loose reticulum formed by extensions from smooth muscle and few fibroblastic cells. Secondary ameboblasts and amebocytes constitute further stages of this cell line. Amebocytes, resembling cells in the snail's circulation, appear in the blood sinus coursing through the interior of the APO. Exposure of snails to echinostome miracidia results in significant morphological changes in the organ. Large clusters of primary and and secondary ameboblasts appear, many of these cells undergoing mitosis. Fully activated APOs consist of masses of cells loosely arranged in zones of progressive maturation. Blood cells in activated APOs were significantly larger than those seen in normal APOs.