Comparative Studies On The Gross Anatomy And Histological Structure Of The Central Nervous System Between Triplophysa Bleekeri And Triplophysa Rosa

Fishes are the largest vertebrate group on earth with more than 30,000 species. Their great diversity is attributed to their tremendous ability to adapt to the changing environment. Fishes from different inhabiting environments may demonstrate different morphological features not only on their external morphology but also on their internal anatomy, for instance the central nervous system, In the present study, we compared the gross anatomy and histological structure of the central nervous system (CNS) between two balitorid fish species from the same genus (Triplophysa bleekeri and Triplophysa rosa), which inhabiting different water environments; Triplophysa bleekeri lives in open river system or shallow streams with rock beaches while Triplophysa rosa lives in underground river system or caves. The aims of this study are (1) trying to see if there is any morphological/histological difference of the CNS between the two species with different inhabiting environments; (2) to understand the structural basis of functional specialization and the effects of the habitat shift on morphological structure; and (3) to gain the insights on the adaptive evolution of the fish CNS based on comparative studies. The results show:(1) There is no significant difference between male and female samples of T. bleekeri and T. rosa in the relative brain weight, the relative size of each part of the brain, and the external morphology. Thus there is no sexual dimorphism of the brain structure for the two species.(2) The brains of the two species are both spindle shaped - the middle part slightly wider, tapering at both ends, and are divided into five subdivisions:telencephalon, diencephalon, mesencephalon, cerebellum, and myelencephalon, which structure are similar to other teleost fishes. (3) The olfactory lobes of the two species belong to "carp type", which differentiate into two parts:bulbus olfactorius and tractus olfactorius. Bulbus olfactorius is close to the olfactory capsule, and the parts are connected by short olfactory nerve. The tractus olfactorius of T. rosa is 3.37■0.19 mm in length, longer than that of T. bleekeri. The two species have the same stratification of the olfactory bulb. The surface is epithelium. The margin is rich in nerve fibers, from outside to inside which is divided into three layers:outer neural fiber layer, glomerular layer, and inner neural fiber layer. The central of the olfactory bulb is the small cell layer. There is no clear boundary between the cells of different layers.(4) The cerebrums of the two species are both in long-cylindrical shape. The cross-section of T. rosa cerebrum is much shorter and wider than that of T. bleekeri. The cerebrum is composed of the dorsal pallium, the ventral corpus striayum, and the common ventricle. The corpus striayum is the major component of the cerebrum. The distribution of nervenucleus in cerebrum of the two species is similar. The cells (3-5) of the dorsal olfactory nucleus and lateral olfactory nucleus in T. rosa are closely arranged in rows, making the distribution of the cells layered. There is no report described this feature in other fishes so far.(5) The mesencephalon is made up of the tectum opticum and tegmentum. The two species have the same stratification of the tectum opticum, from inside to outside which is divided into six layers. The nerve fibers and cell layers are arranged alternately. The tectum opticum in T. rosa is extremely degradation, thus the valvula cerebelli is completely exposed. However, in T. bleekeri most of the valvula cerebelli is covered by the tectum opticum, and only a small part the valvula cerebelli is exposed. T. rosa does not have the forearm structure, but the posterior commissure is more developed than that of T. bleekeri. However the torus semicircularis in T. bleekeri is more developed than that of T. rosa.(6) The structure of cerebellum in the two species is similar, composed of the valvula cerebelli, corpus cerebelli, and auricles. From outside to inside it is divided into three layers:the molecular layer, purkinje cell layer, and granular layer.(7) The myelencephalon is composed of the lobus facialis, lobus vagus, ontology structure, and the fourth ventricle. The lobus facialis in T. bleekeri is more developed than that of T. rosa, while the lobus vagus in T. rosa is more developed than that of T. bleekeri. The histological structures of the two species are similar. In the front, the lobus facialis is slightly apart, backward to blended, so the author support Evans’s point of view:there is not one, but two lobus facialis in teleosts. Observed one lobus facialis is due to the fusion of the two.(8) The spinal cord is extended to the last caudal vertebra, and tapering from front to back. There is no arm or lumbar enlargement or lumbar enlargement. Each side of the spinal cord gray matter, the dorsal horn, and ventral horn are notablely developed, but lateral horn is undeveloped. The largest cell in the gray matter is the motor cell, which is the multipolar neuron. The cells are mainly distributed in the lateral and ventral horn, and distributed more in the front of the spinal cord, the more backward the fewer the number.(9) The relative size of each brain part or of the whole brain in T. bleekeri is larger than those in T. rosa. The brain weight, bulbus olfactorius, and lobus inferior are in significant differences. There are also significant differences in the mesencephalon, tegmentum, dorsal myelencephalon, and lobus facialis between the two species. There is no significant difference of cerebrum and corbus between the two species. Regarding the cell size, the cell diameters of ventral precommissure nucleus and lateral olfactory nucleus are in significant differences, the cells in T. bleekeri are smaller than those in T. rosa. And the cell diameters of ganglian layer are in significant differences, the cells in T. rosa are smaller than those in T. bleekeri.(10) The gliocytes of the central nervous system in T. bleekeri are mainly distributed in the fiber-concentrated regions and in the deep layer of the middle ventricle. This kind of distribution is related to their functions, such as support, nutrition, and isolation of neuron.(11) The nucleus is big and slightly stained under the observation through transmission electron microscope. The nissl substance has no obvious boundary, composed of many developed parallel arrangement rough endoplasmic reticulum and free ribosomes which always gather into flower shapes. The nissl bodies are distributed in perikaryon and neurodendrite, but not in axon hillock and axon. The golgi complex is close to the nissl bodies. Most of the mitochondria are round, oval or rod-shaped, randomly distributed in the perikaryon, but rare near the nissl bodies. In the smaller dendrites and axons, the mitochondria are mostly thin rod. In the perikaryon, neurofilaments and microtubules occupy all the space which is not filled by the other organelles.(12) The nucleuses of oligodendrocytes are oval or irregular in shape. The chromatin is sparsely distributed in karyoplasm or under the karyolemma. There are some microtubules in the cytoplasm. The microglia is smaller than oligodendrocyte. The nucleuses of microglias are mostly oval shaped, in triangular, and strongly basophilic. The chromatin is densely distributed in karyoplasm or under the karyolemma. There is no microtubule in the cytoplasm, but the cytoplasm is rich of rough endoplasmic reticulum.In summary, the central nervous systems of T. bleekeri and T. rosa are generally similar. These differences largely depend on their living environments and habitats.