Functional morphology of the salivary gland of the snail, Helix pomatia: a histochemical and immunocytochemical study

Histological Studies of Snails at Different Periods of Their Life Activity

Large species of terrestrial mollusks are used for human consumption and are bred for this purpose. They have become a part of national cuisine in many countries. Histology is used in many fields of humane and veterinary medicine. Its main essence is that this method can diagnose the presence of formations and inclusions. It is used to determine this process's structure and stage of development. Samples of different tissues can be used as material for histological examination. The specialist analyzes and describes the histological preparations obtained after processing the material. Thus, we proposed a histological method of studying snails. This method can provide the data of snails’ state as a whole. Snails of the species Helix aspersa maxima, Helix aspersa muller, and Helix pomatia were the material for the study. Ten snails of each species were selected for the experiment at different periods: immediately after hibernation, five days after wakening, 14 days after wakening, and at the time of their active life – in July. First, we conducted histological studies of snails during their active life, i.e., in the summer months. Therefore, for this purpose, snails for the experiment were taken in July. It was determined that if the snail eats and has an everyday life, its digestive gland generally consists of digestive tubules separated by intertubular connective tissue containing hemolymphatic sinuses and hemocytes. Thus, in our research, we studied the structure of the digestive gland. A circular muscle layer surrounds each tubule. There are three different cell types in the epithelium lining the tubules of the digestive glands, which differentiate into digestive cells, calcium cells, and excretory cells. Digestive cells are the most numerous cellular component of the tubular epithelium of the digestive gland. Basally located nuclei of digestive cells have a round or oval shape. During the rest period, the digestive gland of snails is characterized by a complete loss of digestive function and the transformation of digestive cells into excretory ones. Metabolic products are not excreted into the environment. When they accumulate, they fill all cavities in the body of snails, namely the lumens of glandular tubules, excretory ducts, stomach, and intestines. Food intake in snails increases the level of metabolism. It stimulates the differentiation of digestive cells, the functional activity of excretory cells, and the volume of excretory products. It leads to the complete liberation of the digestive tract from accumulated excretion products during the rest period. The structure of the digestive gland is completely restored.

PACAP has anti-apoptotic effect in the salivary gland of an invertebrate species, Helix pomatia

Pituitary adenylate cyclase activating polypeptide (PACAP) shows a remarkable sequence similarity among species and several studies provide evidence that the functions of PACAP have also been conserved among vertebrate species. Relatively little is known about its presence and functions in invertebrates. The aim of the present study was to investigate whether the well-known anti-apoptotic effect of PACAP can also be demonstrated in invertebrates. This effect was studied in the salivary gland of a molluscan species, Helix pomatia. In this work, we first showed the presence of PACAP-like immunoreactivity in the Helix salivary gland by means of immunohistochemistry. Radioimmunoassay measurements showed that PACAP38-like immunoreactivity dominated in the salivary gland of both active and inactive snails and its concentration was higher in active than in inactive animals in contrast to PACAP27-like immunoreactivity, which did not show activity-dependent changes. PACAP induced a significant elevation of cAMP level in salivary gland extracts. Application of apoptosis-inducing agents, dopamine and colchicine, led to a marked increase in the number of terminal uridine deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive apoptotic cells in the salivary gland, which was significantly attenuated by PACAP treatment. In a similar manner, the number of caspase-positive cells was reduced after co-application of dopamine and PACAP. Taken together, the data indicate that PACAP activates cAMP in a molluscan species and we show, for the first time, that PACAP is anti-apoptotic in the invertebrate Helix pomatia.

Electrical properties and cell-to-cell communication of the salivary gland cells of the snail, Helix pomatia

The aim of the present study was to assess the cellular mechanism of secretion in the salivary gland of the snail, Helix pomatia, using electrophysiological, electron microscopic and immunohistochemical techniques. A homogeneously distributed membrane potential (-56.6 +/- 9.8 mV) was determined mainly by a K+ -electrochemical gradient and partly by the contribution of the electrogenic Na+ -pump and Cl- conductance. Low resistance electrical coupling sites were identified physiologically. Transmission electron microscopy and innexin 2 antibody revealed the presence of gap-junction-like membrane structures between gland cells. It is suggested that gap-junctions are sites of electrotonic intercellular communication, which integrate the gland cells into a synchronized functional unit in the acinus. Stimulation of the salivary nerve elicited secretory potentials (depolarization) which could be mimicked by local application of acetylcholine, dopamine or serotonin. In voltage-clamp experiments four major conductances were identified: a delayed rectifier (IK), a transient (IA) and a Ca2+ -activated outward K+ current (IK(Ca)) and Ca2+ -inward currents (ICa). It is suggested that one or more of these conductances may give rise to a stimulus activated secretory potential leading to excitation-secretion coupling and subsequent the release of the mucus from the gland cells.