| The loach(Paramisgurnus dabryanus)belongs to the family Cobitidae of the order Cypriniformes,mainly distributed in middle and lower streams of Yangtze River.Its meat is delicious.P.dabryanus is a small-scale economical fish living in benthos.In recently years,the demand of P.dabryanus has raised year by year.The artificial culture scale enlarge constantly,and demand of fry of P.dabryanus increased gradually.But high mortality rate in the seed breed period has been a big bottleneck.Therefore,inorder to understand formation and development process,carry on a preliminary discussion about gut air-breathing organization structure characteristic,we study in histological,histochemical and digestive enzyme fluctuations of lavae and juvenile digestive systems.We also hope to provide fundmental knowledge of early life history,and theory suppot of seed rearing.The main results are shown as follows:Histological and histochemical methods were used to observe the post-embryonic development of digestive system in P.dabryanus The results showed that newly hatched larval buccopharynxgeal cavity was closed and digestive tract was a straight tube.At 3 DAH,The anterior part of intestine inflated,size of larval yolk sac decreased significantly.At 4 DAH,the mouth opened,taste buds and goblet cells could be found,the digestive tract was developing quickly,the esophagus was divided into two different parts;larval yolk sac was depleted,Liver was reticular structure and hepatic sinusoid appeared at 5 DAH.At 7 DAH,larval taste buds and mucous cells had a substantial increase in buccopharynxgeal cavity and lipid vacuole appeared in liver.After 15 DAH,the structure of anterior and posterior esophagus had significant differences,posterior esophagus was ?I? shape.The esophagus had abundant mucous cells at 20 DAH.At the same time,the development of intestines,liver and pancreas gradually completed.At 50 DAH,the anterior intestine had abundant mucous folds and mucous cells,the number of blood capillaries turned more while the wall turned thinner from middle intestine to posterior intestine,formed obvious digest and air-breathing region.The results suggested that,it was a critical period of P.dabryanus development of digestive system from 3 DAH to 20 DAH.The digestive system of P.dabryanu turned to be completed at 20 DAH;Air-breathing function of the digestive tract formed was from 10 to 20 DAH,it formed may be due to hydrostatic hypoxia living environment.Development and distribution of mucous cells in digestive tract of larval and juvenile P.dabryanus was observed and analyzed by the method of histological sections and AB-PAS histochemical staining techniques.The results indicated that there were four types of mucous cells(I,II,III,IV)in digestive tract of P.dabryanus.Mucous cells initially appeared in buccopharyngeal cavity and esophagus at 4 DAH.There was a rapid increase of the number of mucous cells in buccopharyngeal cavity and esophagus between 10 and 15 DAH.The number of all types mucous cells in intestine increased significantly between 15 and 20 DAH.Mucous cells were widespread in digestive tract after 20 DAH.As the growth of larvae,mucous cells in digestive tract were mainly III and IV typesAccording to our researches,the sensitive period of the development of larval and juvenile P.dabryanus were 5 ~ 10 DAH and 15 ~ 20 DAH,respectively.Digestive function gradually developed consummately after 20 DAH.Specific growth rate,soluble protein content and the activities of digestive enzymes were examined during P.dabryanus larval and juvenile period.The results showed that the activities of trypsin,lipase,amylase and alkaline phosphatase could be detected at 1 DAH.Soluble protein content of larvae was firstly decreased,then gradually increased.The minimum value was 0.158±0.01 mg/mL observed on 3 DAH.The fluctuation patterns were different between the total and specific activities of the digestive enzymes.Total activities of these four enzymes showed a gradual increase through the fish growth.Trypsin's specific activities reached to a peak(78.97±8.09 U/mg protein)on 7 DAH,then significant decreased(P<0.05)until a significant(P<0.05)increased after 15 DAH,and reached a stable level after 25 DAH.Lipase's specific activities had two obviously peaks on 5 DAH(0.322±0.02 U/mg protein)and 20 DAH(0.835±0.13 U/mg protein)respectively.Amylase's specific activities reached to a peak(2.65±0.09 U/mg protein)on 5 DAH,and decreased slowly towards to be stable after 20 DAH.Alkaline phosphatase's specific activities significant increased from 2 to 5 DAH,then decreased gradually.It had a certain increase from 15 to 20 DAH,tended to be stable after 25 DAH.Observe morphology and ultrastructure of intestinal tract of P.dabryanus with light and electron microscopies.Intestinal tract was divided into three parts.Morphologically complex folds were formed on surfaces of anterior and middle intestines where many secretory cells were present.Highly developed junction complex were seen in anterior intestine.Cytoplasm contained abundant mitochondria and pinocytotic vesicles.Epithelium of posterior intestine was thin,translucent and squamous with dense blood capillary network underground,while there were no folds on mucous membrane surface.Distance between blood capillary network and intestinal cavity was(1.95±0.34)?m.Respiration epithelial cells of posterior intestine are a kind of modified squamous epithelial cells which show the ultrastructural characteristics of both types I and II pneumocyte of lung alveoli in mammals.The blood is composed of three layers: the thin layer of the extended cytoplasm of respiratory epithelial cells(1.47±0.23)?m,the thin sheet of cytoplasm of endothelial cells(0.21±0.03)?m and the basement membrance(0.27±0.05)?m between them is similar to that of the respiratory organs in other vertebrates.Intestinal tract has digestion and respiration functions.Anterior intestine performs digestion/absorption.Posterior intestine performs gas exchange/accessory respiration.In middle intestine,digestion and respiration overlap.Results indicate that P.dabryanus larval and juvenile opened mouth at 4 DAH.Not until 20 DAH is sensitive period of development.Gut air-breathing function was completely formed.After 20 DAH digestive function gradually developed consummately.Posterior intestine is the place where performs gas exchange/accessory respiration.P.dabryanus is able to digest carbohydrate,protein,lipid,and has a high utilization rate of carbohydrate at early larval stage.With the completion of larvae's digestive system of P.dabryanus,the utilization rate of protein and lipid increased gradually.In the seed breeding process,diet prescription adjustment according to the digestive enzyme fluctuation in different development period could help promote larvae and juveniles growth.In order to improvesurvival rate,it was necessary to strengthen larvae and juveniles daily feeding and management from 5 to 20 DAH. |
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