Microscopic studies of the paraphysis of the turtle Trachemys (scripta) dorbigni (Dumeril & Bibron, 1835)

A microscopic investigation was conducted on the paraphysis of the turtle Trachemys dorbigni. The paraphysis is a highly vascular neuroepithelial structure lined by a simple cuboidal epithelium. The prominent ultrastructural features observed were many dense bodies, mitochondria and lipid droplets. The cells of the paraphysis exhibit extensive microvillar borders, large intercellular spaces, many mitochondria, dense bodies and lipid droplets. Few PAS (periodic acid of Schiff) positive granules were found in the cytoplasm of the epithelial cells. Large intercellular spaces were seen when samples were fixed by perfusion. Scanning electron microscopy revealed that surface epithelial cells have microvilli and cilia distributed in tufts in the center of the cell. Macrophagic cells were frequently seen on the surface of the epithelial cells. The connective tissue of the paraphysis presented many sinusoid vessels, mast cells, fibroblasts, and collagen fibers. Intra-arterial administration of Evans blue showed the absence of the blood-brain barrier. This is probably related to the presence of a fenestrated endothelium, which characterizes this structure as a circumventricular organ (CVO). The presence of vesicles in the cytoplasm of epithelial cells, fenestrations, and macropinocytosis vesicles in the vascular endothelium suggest absorption and secretion functions.