Ultrastructural study of the kidney in the coelacanth Latimeria chalumnae (Rhipidistia: Coelacanthini).

The morphology of the nephrons of the coelacanth Latimeria chalumnae was investigated by electron microscopy. Each nephron is composed of a large renal corpuscle with well vascularized glomerulus, ciliated neck segment, proximal tubule divided into first and second proximal segments, ciliated intermediate segment, distal tubule, collecting tubule, and duct. The podocytes of visceral epithelium contain large bi-lobed nuclei and their surface membranes pinch off vesicles into the cytoplasm. The processes of the podocytes give rise to pedicels that enclose narrow filtration slits. The endothelium of glomerular capillaries is attenuated and fenestrated. The short cytoplasmic processes of mesangial cells do not penetrate deeply into the sub-endothelial lamina. The glomerular basement membrane is about 286 nm in thickness. The pedicels also arise from podocyte cell bodies, and are connected by diaphragms and enclose slits, which open into narrow urinary spaces between podocytes. The cuboidal cells of the short neck segment display cilia with a characteristic pattern of 9+2 microtubules. The first proximal tubule segment differs from the second proximal segment in having densely packed microvilli, prominent endocytotic-lysosomal apparatus, and numerous basal membrane infoldings associated with mitochondria. The lateral cell membranes like those of other segments are straight and joined by desmosomes and apical adhering and tight junctions. The distal tubules display few short luminal microvilli and numerous basal mitochondria. The distal tubule, collecting tubule and duct are devoid of intercalated cells. The ultrastructure of the L chalumnae nephrons correlates well with their osmoregulatory function and resembles that of freshwater rainbow trout.

Electron microscopic study of the anterior midgut in Cenocorixa bifida Hung. (Hemiptera: Corixidae) with reference to its secretory function.

The epithelium of anterior midgut of adult Cenocorixa bifida was examined with light and electron microscopy. The folded epithelium is composed of tall columnar cells extending to the lumen, differentiating dark and light cells with interdigitating apices and regenerative basal cells in the nidi surrounded by villiform ridges that penetrate deeply into the epithelium. The columnar cells display microvilli at their luminal surface. Microvilli lined intercellular spaces and basal plasma membrane infoldings are associated with mitochondria. These ultrastructural features suggest their role in absorption of electrolytes and nutrients from the midgut lumen. The columnar cells contain large oval nuclei with prominent nucleoli. Their cytoplasm is rich in rough endoplasmic reticulum, Golgi complexes and electron-dense secretory granules indicating that they are also engaged in synthesis of digestive enzymes. The presence of secretory granules in close proximity of the apical plasma membrane suggests the release of secretion is by exocytosis. The presence of degenerating cells containing secretory granules at the luminal surface and the occurance of empty vesicles and cell fragments in the lumen are consistent with the holocrine secretion of digestive enzymes. Apical extrusions of columnar cells filled with fine granular material are most likely formed in response to the lack of food in the midgut. The presence of laminated concretions in the cytoplasm is indicative of storageexcretion of surplus minerals. The peritrophic membrane is absent from the midgut of C. bifida.

Design of the labial cuticle in Cenocorixa bifida Hung. (Hemiptera: Corixidae) with reference to ionic transport.

The surface topography and ultrastructure of the labial cuticle of Cenocorixa bifida were examined by scanning and transmission electron microscopy. The dorsal wall of the labium consists of seven sclerotized transverse bars each displaying two rows of semicircular grooves and pores. The cuticle is about 20 microm thick and is composed of epicuticle and lamellate exocuticle and endocuticle, the latter separated from the underlying epidermis by subcuticle containing amorphous material. The epicuticle is subdivided into an electron-dense very thin outer epicuticle and a homogenous thick inner epicuticle, which is penetrated by grooves. The exocuticle is filled with electron-dense blocks of material, which may provide mechanical support to the labial wall. The elongate epidermal cells display extensive infoldings of the apical plasma membrane (facing the cuticle) and contain abundant mitochondria in the cytoplasm. The presence of deep epicuticular grooves and pores in the thin labial cuticle and extensive apical membrane infolding and abundant mitochondria in the epidermal cells suggest that the labium in C. bifida is the site of osmoregulatory ionic uptake.