Using scanning electron microscopy, the structure of the apical surface of epidermal cells in the head skin and epithelium in the oral cavity of the carp Cyprinus carpio specimens, intact and with chronic olfactory and visual deafferentiation, has been studied. It is shown that sensory-deprived fish differ from intact fish by significant changes in the morphology of the apical surface of epidermal cells. The regular ring-shaped pattern on the apical surface of cells is disturbed but complex patterns or lamellar protrusions of ring-shaped ridges appear. The location and orientation character, as well as the density of distribution and the pattern on the apical surface of even neighboring cells can be completely different. A high concentration of cells with the modified apical surface was found in sensory zones of taste and tactile reception. The changes are similar in epidermal cells of the head skin and epithelial cells of the oral cavity. The individual specificity of structural changes in epidermal cells was not found. The possible relationship between the changes in the apical surface of epidermal cells and activation of the skin reception system and other sensory systems is discussed.

Tactile reception in cyprinid fish (silver crucian carp Carassius auratus gibelio and common carp Cyprinus carpio) was investigatedby noninvasive recording of action potentials from the head skinin response to rhythmic tactile stimulation. Responses shaped aselectric potentials following synchronously with changes in thepressure force on the skin were recorded using chloro-silver electrodes.It was shown that fish, like all terrestrial vertebrates, have phasicand tonic tactile receptors. Three main types of tactile responseswere distinguished: (1) tonic responses as potential fluctuationsrepeating the dynamics of the tactile stimulus, (2) phasic responsesas on-, off-, and on-off potentials, and (3) slow deflections fromthe resting potential. Compound responses comprising simultaneouslydifferent types of responses were also recorded. The response latency averaging6.1 ms for tonic and 4.4 ms for phasic responses was clearly independentof the stimulus direction (switch-on, switch-off). The thresholdlevel of tactile sensitivity in the perioral and gular head areaswas 0.05–0.2 g/mm2. Tactile receptorson the silver crucian carp and common carp heads exhibited zonaldistribution patterns, with sensory areas differing in their tactilesensitivity. Most sensitive areas were on the upper and lower lips,in the perilabial skin and the gular area on the ventral side ofthe head. Less sensitive areas were situated rostrally and ventrallyto the eyeball. The areas situated dorsally and caudally to theeyeball proved to be insensitive to the applied tactile stimulation.In all sensory areas on the fish head, most intense responses wereevoked by sliding tactile stimuli. The distribution specificityof tactile sensory areas meets their functional significance forfish feeding behavior.

— A comparative study into morphological features of the intraoral gustatory apparatus in juveniles of the common carp Cyprinus carpio and White Sea cod Gadus morhua marisalbi of the same age has been performed using scanning electron microscopy. Juveniles of these two fish species have a similar system of distribution of taste receptors in the oral cavity into three interrelated parts. The interaction between these parts provides a complex multistep system for taste analysis of food items. Features of morphology of the taste receptor apparatus in the same sensory zones of the oral cavity in juvenile common carp and cod show the similarity of their adaptation to the same type of feeding both on planktonic and benthic organisms. Species-specific features have been revealed in the morphology of the sensory field of taste buds. Some structural differences in morphology of the same sensory zones in the oral cavity of juvenile common carp and cod have an adaptive orientation due to species specificity of taste testing processes.

The morphology and distribution of taste buds in the outer integument of the body and in the oral cavity of two forms (blind cave and sighted terrestrial ones) of the astyanax Astyanax fasciatus and in intact and blinded individuals of the Buenos Aires tetra Hyphessobrycon anisitsi have been studied using electronic scanning and light microscopy. In sighted individuals of both species, the morphometric parameters of the taste apparatus and the distribution of taste receptors are similar; the taste apparatus in the oral cavity is more developed than in the outer covers. Morphologically different taste zones were found in the oral cavity of characins. In blind fish, the taste apparatus of the maxillary zones is distinguished by smaller taste buds and a greater density of their distribution. The sensory field of taste buds in blind and sighted individuals of astyanax and tetra has a similar ultrastructure; it is formed by taste cells of three types. In blind astyanaxes and blinded individuals of tetra, numerous modified epidermal cells were found for the first time in the epithelium of the taste zones and in contact with taste buds, which are regarded as tactile receptors and a constituent element of polysensory taste-tactile complexes localized in blind fish in mainly ventral sensory zones.

The first data on the responses of tactile receptors of the tonic, phasic, and mixed types to mechanic stimulation of the surface of head skin of fish obtained by means of noninvasive recording of potentials are presented. The sensitivity of skin tactile receptors is the highest in the circumoral and gular zones, which reflects their functional role in foraging behavior.

Методом электронной сканирующей микроскопии исследованы изменения в морфологии и ультраструктуре вкусового рецепторного аппарата в трёх отделах орофарингиальной полости у рыб (радужная форель Parasalmo mykiss) в условиях хронического выключения зрительной и обонятельной рецепции. Показано, что выключение как обонятельной, так и зрительной рецепции вызывает достоверные изменения в морфологии интраорального вкусового аппарата, сходные по интенсивности и динамике их проявления. В условиях хронической сенсорной депривации происходит активация роста вкусовых сосочков в слизистой выстилке сенсорных зон, иннерви-руемых ветвями лицевого и тройничного нервов. Одновременно во вкусовых почках челюстного и центрального отделов ротовой полости наблюдалась активация процессов морфогенеза вкусовых рецепторных элементов. В фарингиальном отделе, получающем иннервацию от языкоглоточного и блуждающего нервов, подобных изменений не обнаружено. Изменения в морфологии вкусового рецепторного аппарата ротовой полости, вызванные сенсорной депривацией, сохранялись на одном уровне в течение длительного времени (9 мес). Полученные данные указывают на существование у рыб хорошо развитых обонятельно-вкусовых и зрительно-вкусовых взаимодействий на уровне интраоральной вкусовой субсистемы.

На молоди радужной форели в электрофизиологических экспериментах исследовали вкусовую чувствительность четырёх сенсорных зон ротовой полости (дентальной, максиллярной, твёрдого нёба и языка) к аминокислотам и классическим вкусовым стимулам. В тех же зонах исследованы особенности ультраструктурной организации поверхности сенсорных полей вкусовых почек. В каждой зоне обнаружена высокая вариабельность количественного соотношения субтипов клеток, формирующих вкусовые почки. Ответные реакции на химические стимулы в разных сенсорных зонах имели сходный характер и проявляли доза-зависимый эффект. Вместе с тем вкусовые рецепторы разных зон различались на 1-3 порядка по чувствительности к одинаковым стимулам. Сенсорные зоны ротовой полости форели характеризовались сходной ранжированностью чувствительности к применяемым аминокислотам и классическим вкусовым раздражителям. Максимальная чувствительность во всех исследованных зонах зарегистрирована к L-цистеину. Рецепторы всех сенсорных зон были более чувствительны к аминокислотам, чем к классическим вкусовым стимулам. Анализ распределения ответных реакций на классические вкусовые стимулы в сенсорных зонах ротовой полости указывает на отсутствие у рыб топической локализации рецепторов кислого, солёного, горького и сладкого. Выявленные черты сходства и специфичности в структурных и функциональных свойствах сенсорных зон ротовой полости радужной форели можно рассматривать как проявление системной организации вкусового аппарата у данного вида рыб.

Study of the structural organization of gustatory apparatus in rainbow trout Parasalmo mykiss performed using electron scanning microscopy demonstrated that external taste buds are absent in this species in skin covers of the head and in the circumoral region. In the oropharyngeal cavity (oral and gill cavities and pharynx) of the rainbow trout, a well-developed gustatory receptor apparatus was found. In correspondence with specific features of morphology and anatomy of the skull, taste buds form seven gustatory zones. Morphometric analysis demonstrated differences between gustatory zones in the pattern and density of distribution of taste buds, as well as in average sizes of their sensory field. Zones of similar innervation have many common features in morphology. Morphologically similar zones form three regions in the oropharyngeal cavity: rostral, central, and caudal. A tendency for a decrease in the concentration of taste buds in the rostrocaudal direction common for all sensory zones was revealed. The highest concentration of taste buds was recorded at papillae of rostral regions near big teeth. A typical feature of taste buds in rainbow trout is irregular shape of the taste pore. Analysis of ultrastructural specific features of apical processes of taste cells allows us to distinguish five cell shapes in the composition of taste buds. The numeric ratio of cell shapes varies in buds of different localization. The quantitative distribution of taste buds over sensory zones, specific features of morphology and sizes of their sensory field are discussed in relation to the feeding pattern of the species.

Investigation of the structure of the gustatory apparatus and morphometric features of the brain in two age groups (1.5 and 6 months) of Siberian sturgeon Acipenser baerii shows a well-developed gustatory system in all studied exemplars. In the oral cavity of the sturgeon, 14 sensory zones are separated, and eight zones are supplied by gustatory receptors. During the growth of the juveniles, morphometric parameters of the zones can change. The zones of the dorsal and ventral linings of the oral cavity are similar by morphology and size, and they are complementary. The largest zone of the oral cavity of the sturgeon with a complex structure is the palate organ. The taste buds of the sturgeon are characterized by a large variation in shapes of apical protrusions of receptor cells. Weight indices of the brain parts and estimated sensory coefficients show an increasing degree of development of the gustatory and olfactory systems during growth of the juveniles. Two age groups of the juveniles are characterized by similar morphometric parameters of the gustatory receptor apparatus with external localization, but the morphology of the gustatory apparatus of the oral cavity is different. The features of the gustatory system morphology are discussed in connection with biology of the species.