Fine structural and immunohistochemical detection of collar enamel in the teeth of Polypterus senegalus, an actinopterygian fish.

This is the first detailed report about the collar enamel of the teeth of Polypterus senegalus. We have examined the fine structure of the collar enamel and enamel organ of Polypterus during amelogenesis by light and transmission electron microscopy. An immunohistochemical analysis with an antibody against bovine amelogenin, an antiserum against porcine amelogenin and region-specific antibodies or antiserum against the C-terminus, middle region and N-terminus of porcine amelogenin has also been performed to examine the collar enamel matrix present in these teeth. Their ameloblasts contain fully developed Golgi apparatus, rough endoplasmic reticulum and secretory granules. During collar enamel formation, an amorphous fine enamel matrix containing no collagen fibrils is found between the dentin and ameloblast layers. In non-demineralized sections, the collar enamel (500 nm to 1 µm thick) is distinguishable from dentin, because of its higher density and differences in the arrangement of its crystals. The fine structural features of collar enamel in Polypterus are similar to those of tooth enamel in Lepisosteus (gars), coelacanths, lungfish and amphibians. The enamel matrix shows intense immunoreactivity to the antibody and antiserum against mammalian amelogenins and to the middleregion- and C-terminal-specific anti-amelogenin antibodies. These findings suggest that the proteins in the enamel of Polypterus contain domains that closely resemble those of bovine and porcine amelogenins. The enamel matrix, which exhibits positive immunoreactivity to mammalian amelogenins, extends to the cap enameloid surface, implying that amelogenin-like proteins are secreted by ameloblasts as a thin matrix layer that covers the cap enameloid after enameloid maturation.

Villiform processes in the pharynx of the soft-shelled turtle, Trionyx sinensis japonicus, functioning as a respiratory and presumably salt uptaking organ in the water.

Some species of soft-shelled turtle have been known to use a conspicuous mass of villiform processes of the pharyngeal mucosa as an aquatic respiratory organ when staying underwater for prolonged periods, such as hibernation. Using hibernating turtles, Trionyx sinensis japonicus, the present study employed scanning electron microscopy to demonstrate for the first time the detailed morphology and distribution of these villiform processes. Two types of processes, complex and simple, could be identified. Light microscope observation of the transverse sections of the villi demonstrated a rich vascularization in the connective tissue of the villi, comprising arterioles and venules running in the core and capillaries in the periphery. Most of the capillaries were invaginated into the multilayered cuboidal epithelium. Near the tip of the villi they became swollen, forming sinusoidal capillaries. Transmission electron microscopy clarified the fine structure of the blood-water barrier, which consisted of a non-fenestrated endothelium and an attenuated epithelium that sandwiched a connective tissue with a discontinuous subendothelial and a continuous subepithelial basement lamina. The epithelium consisted of secretory cells, mitochondria-rich cells, and basal cells. The mitochondria-rich cells contained a cytoplasmic area filled with tubulovesicular elements. Based on their ultrastructural resemblance with the chloride cells in the fish and tadpole, these cells are suggested to be involved in the uptake of Na+ and Cl from fresh water for keeping ionic balance in the blood.

The existence of Na+/K+-ATPase-immunoreactive cells in the pharyngeal villiform-papilla epithelium of the soft-shelled turtle, Trionyx sinensis japonicus.

The pharyngeal villiform processes of the hibernating soft-shelled turtle, Trionyx sinensis japonicus, were studied by immunohistochemistry for Na+/K+-ATPase in combination with a mitochondrion staining. Mitochondria-rich cells were recognized in the epithelium constituting the distal part of most processes, and exclusively showed the Na+/K+-ATPase immunoreactivity. These cells tended to attract each other to form clusters. When considering the physiological and histological data previously obtained in corresponding cells in the fish gill epithelium, the mitochondria-rich cells in the hibernating turtle were suggested to be involved in the electrolyte (Na+) uptake from the aquatic habitat.

The existence of Merkel cells in the lingual connective tissue of the Surinam caiman, Caiman crocodilus crocodilus (order Crocodilia).

The tongue of the Surinam caiman (a reptilian species) was studied by light microscopy including immunohistochemistry for protein gene product 9.5 (PGP 9.5), and transmission electron microscopy. The connective tissue immediately under taste buds housed a cluster of cells immunoreactive for PGP 9.5. These cells synapsed on nerves, and their cytoplasm contained characteristic granules of 90 nm in the mean diameter, glycogen particles, and bundles of intermediate filaments. In light of these ultrastructural features, they were identified as Merkel cells. The Merkel cells were also surrounded by Schwann cells. These findings indicate that the present Merkel cell-neurite-Schwann cell complex is comparable to the avian Merkel corpuscle. On the basis of the granule localization in the cytoplasm, the caiman Merkel cell was presumed to be involved in not only mechanoreception but also endocrine or paracrine functions.

The vascular supply of the villiform processes in the pharynx of the soft-shelled turtle, Trionyx sinensis japonicus. A scanning electron microscopic study of corrosion casts.

Following our observations of the fine structure of the pharyngeal villiform processes of the hibernating soft-shelled turtle, Trionyx sinensis japonicus (Yokosuka et al., 2000), this paper deals with a scanning electron microscope study of the resin casts of blood vessels supplying those processes. Each villiform process contained arterioles and venules which ran in the axial portion of the process; capillaries formed a network at the periphery of the connective tissue core of the villus. In the distal portions of the villus, the capillaries increased markedly in their caliber to form sinusoidal capillaries. Such a vascular architecture supports the view that the villiform processes serve in the aquatic respiration of the soft-shelled turtle. The casts indicated an occurrence of sphincters in the vascular bed of the villi.