Ultrastructure of Taste Buds in the Spotted Dogfish Scyliorhinus caniculus (Selachii)

Abstract

The taste buds (TBs) of animals of different systematic groups differ considerably in structure [1], but our knowledge of these sensory organs across the vertebrate lineage is currently too limited to derive a TB phylogenetic tree. Since only a single report exists on gustatory receptors of selachians [2], the present study of the structure of TBs of the spotted dogfish, Scyliorhinus caniculus, was initiated to learn more of the interrelationships among TBs of vertebrates. Spotted dogfish were immersion-fixed in a glutaraldehyde-paraformaldehyde-picric acid solution. Pieces of skin removed from different parts of the head and oral cavity were processed conventionally for transmission electron microscopy. In the spotted dogfish, TBs were rare and occurred exclusively at the tip of epidermal papillae within the oral mucosa. In longitudinal section, the TBs were pear-shaped, were approximately 50 μm high and 30 μm wide, were located in the upper third of the squamous non-keratinized epithelium, and were surrounded by marginal cells, the normal epidermal cells. The base of each TB rested on the marginal cells and not on the underlying dermal (corium) papilla, which projected into the basal portion of the epithelium. Each TB was innervated by a nerve that ran through the corium papilla, penetrated the basal lamina of the epithelium, and ascended between the epithelial cells to the TB. Blood capillaries were present in the dermal papilla, but did not reach the TB. The sensory epithelium within the TB consisted of a total of about 25 slender light and dark cells bearing microvilli and situated parallel to the TB’s longitudinal axis. Light cells, which stained less electron dense than dark cells, had a cytoplasm rich in mitochondria, smooth endoplasmic reticulum (sER), and microtubules, and their apical ends terminated in three to five large and divided microvilli. Intermediate (tono)-filaments were rarely observed. Dark cells occurred in two forms. In form one, the cell ended apically in long, slender, and divided microvilli. This dark cell was rich in mitochondria, sER, and dark-staining secretory vesicles of about 200 nm diameter, and contained numerous intermedate filaments. Form two of the dark cell showed long, thin, and undivided microvilli. These form two dark cells contained numerous mitochondria, numerous profiles of the sER, and solitary intermediate filaments. Also, small filament bundles occurred in the basal portions of the cell. Some of the basal marginal cells, which were also rich in organelles, appeared to be basal stem cells, and were different from those of teleosts [1,3,4]. In the spotted dogfish, basal cells comparable to the ones in teleosts did not form the base of the TB, but lay directly on top of the corium papilla. The TB was innervated by unmyelinated nerve fibers, and presynaptic 130-nm dense core and 40-nm clear vesicles occurred mainly at the bases of both the light and dark (form 1) cells. Efferent synapses were not observed. Thus, the TBs of the spotted dogfish differed in the following ways from those of other vertebrates: (1) the TBs were not located directly on top of the corium papillae; (2) the TBs contained at least three types of elongated cells which reached the epithelial surface; and (3) the TBs did not contain basal cells analogous to those of teleosts and amphibians [1,3,4].