Ultrastructure of the nephron in the soft-shelled turtle, Pelodiscus sinensis (Reptilia, Chelonia, Trionychidae).

The structure of the nephron in adult soft-shelled turtles (Pelodiscus sinensis) was studied by light microscopy, transmission and scanning electron microscopy. The kidney contained 5-6 renal lobes. Nephrons of P. sinensis are composed of a renal corpuscle (RC) and of a renal tubule that appears divided morphologically into five distinct segments: neck segment (NS) (This segment is only present in approximately 10% of the nephrons), proximal tubule (PT), intermediate segment (IS), distal tubule (DT), and collecting duct (CD). The RCs and most of the convoluted DTs lie in the central zone, while the PTs and the CDs lie in the peripheral zone of the renal lobe. The renal corpuscle is relatively large with especial processes in podocytes and a thick basement membrane. The podocyte processes covering a large capillary area can be observed by TEM, and the major podocyte processes formed a very specific pattern in SEM. The podocyte processes expand to form a flattened network over the whole capillary loops surface, and only may observe little filtration slits in glomerular area. The neck segment when presentis short and has a relatively narrow lumen, consisting of cuboidal or squamous cells. There is a well-developed endocytic-lysosomal apparatus in the apical cytoplasm of the PT. The proximal tubule and intermediate segment cells show some differences between male and female. It showed that proximal tubule cells of male soft-shelt turtle contain lateral intercellular spaces, into which extensions of the cell membrane protrude, and the basal cell membrane forms a conspicuous labyrinth. Whereas, the basal and lateral cell membranes of the female are smooth, and no later-basal intercellular spaces. The differences between male and female in the middle segment cells is similar to proximal tubule cells. Not previously reported in vertebrate kidneys. The IS is the narrowest nephron segment, formed by multiciliated as well as nonciliated cells. In DT cells, basolateral interdigitations and infoldings are particularly well-developed. The CD contains clear cells with numerous secretory granules and dark cells with dense mitochondria and an elaborate Golgi complex. This study was undertaken in order to disclose specific kidney features in P. sinensis that could be related to function. In addition, the ultrastructure of the nephrons in P. sinensis are discussed in relation to other turtles and vertebrates.