Studies On The Structure And Changes Of Prereproduction And Post Reproduction Of Caudal Neurosecretory System Of Silurus Meridionalis Chen

From February 2006 to April 2007, the total of 81 specimens of Silurus meridionalis Chen which contain some 3-month old parrs, 6-month old parrs, 1~+old parrs,2~+old parrs and adults were collected in Hechuan-Beibei segment of Jialing River and reticulate cage breeding rooms of Tongliang etc..The development of caudal neurosecretory system (CNSS) in parr and the structure of CNSS of adult in different gonadal stage in Silurus meridionalis have been studied in detail by means of anatomy, histology, histochemisty, enzyme cytochemistry and transmission electron microscope (TEM) technology. The results were as follows:The CNSS in Silurus meridionalis is mainly composed of three parts: caudal neurosecretory cell (Dahlgren cell), axon and neurohemal area (urophysis). The Dahlgren cell is mainly distributed in the last three caudal spinal cord, its soma is large and has many processes, and the nuclear is large and obvious, nearly round or oval, nucleolus is distinct. The Dahlgren cell can be divided into two types: typeⅠand typeⅡaccording to the size, shape, staining ability, distribution locations and so on. The axon of Dahlgren cell is usually amyelinated nerve fibers, its contents including neuroprotofibril, mitochondria, neurosecretory granules and some other electronic transparent vesicles. Axon terminals can be divided into three types according to the electron density and the size of inclusions: ending type I mainly contain electron transparent vesicles; typeⅡcontain electron-dense granules and some electronic transparent vesicles; typeⅢcontain only a few large vacuoles, and has no other distinct contents. The three endings may represent different types of neurons. The urophysis of Silurus meridionalis belongs to the style of single and continuousness, it has two different forms which are obviously different from other fishes: one form is similar to long oval from ventral sight, the anterior extremity is round and expand near the spinal cord and the subsequent gradually narrowing from side-glance; the other form can be divided into two continuous parts, the enlargement of anterior part is obvious and large, similar to round, the posterior part is plat in dorsum-ventral side and has depression in intermediate longitudinal direction. The volume of urophysis in Silurus meridionalis increases with the increase of age from 3-month old parr to 6~+old adult, it is only 1008.31μm~2×270.13μm~2×312.33μm~2 in 3-month old parr, at the age of 6~+, it reaches 5134.00μm~2×1843.00μm~2×2038.00μm~2, an increase of nearly five times. The volume of urophysis in Silurus meridionalis is significantly larger which compared with other species, it may be related to the large individuals and fast growth.Comparing the neurosecretory cell of prereproductive and post reproductive in Silurus meridionalis, we found that the microstructure and ultrastructure are both significantly different. Under light microscope, Dahlgren cell soma is large, H.E, Mallory staining are deeper, and full of contents in prereproduction; but in post reproduction, soma minish obviously and staining become feint. Under TEM, the processes and endings of Dahlgren cell are full of two types of secretory granules which are electron dense-density and low-density. The ribosomes, mitochondria, rough endoplasmic reticulum (RER), lysosomes, Golgi apparatus are all abundant in soma, and lysosomes contain some rudiments such as secretory granules, transparent vesicles, RER and so on, while the nuclear invaginate significantly, and at the the entrance of invagination, there has a large number of aggregate secondary lysosomes; in post reprodution, Dahlgren cell form a bubble-like, and secretory granules are rare, while ribosome declines sharply, mitochondrial vacuolizate, RER lumen enlarge, the number and size of lysosomes are both reduced but staining is deepened.The development of CNSS in parr of Silurus meridionalis is earlier than other species'. In 3-month old parr, the main structure of CNSS has been formed, and the final section of the spinal cord has been slightly upwards, Dahlgren cells, axon and neurohemal area can be seen under light microscope. From 3-month old to 6-month old parr, and then to 1~+old parr, Dahlgren cells increase gradually and the soma become large, their axons and endings are also increased correspondingly, while capillary vessel become slender from thick and the number are increased gradually. Ultrastructure indicates that the mitochondria in soma of Dahlgren cell turn the curvy long-club to small and oval, while the number are increased; the cavity of RER are increasing; the number of lysosome is decreased, but its cross-sectional area become smaller slightly, it can be seen the lysosome melt with electronic transparent vesicles.In summary, the characteristics of morphologic structure and development of CNSS in Silurus meridionalis are as follows: First, the distribution of caudal neurosecretory cells is limited relatively, while the urophysis has two types of configuration and is large in size; Second, the morphologic structure of caudal neurosecretory cells changes significantly in prereproduction and post reproduction, which indicates the tightly relation with gonadal development; Third, the development of CNSS in parr occurs early, it is mainly the number increasing and soma growing in Dahlgren cells from 3-month old to 1~+old parr.