HISTOLOGICAL AND HISTOCHEMICAL-CHANGES IN THE DIGESTIVE-TRACT OF WHITE STURGEON LARVAE DURING ONTOGENY

Ontogenetic changes in digestive tract histology and digestive enzyme histochemistry were investigated II to 36 days post-hatch in white stu rgeon Acipenser transmontanus larvae. From initiation of exogenous fee ding (12 days post-hatch), larvae were fed a commercial salmonid diet for the ensuing 24 days. The digestive system of white sturgeon displa yed a high degree of morphologic organization and functionality at the onset of exogenous feeding. An enhancement of digestive capacities oc curred with transition to active feeding. On day 2 of feeding, there w as a clear increase of alkaline phosphatase, aminopeptidase M, dipepti dyl peptidase IV, and gamma-glutamyl transpeptidase activity in the br ush border of the spiral intestine. This strong activity is an apparen t confirmation of the importance of this segment of the intestine for protein digestion and nutrient absorption. The functional development of the pyloric intestine occurred on day 4 and was concomitant with an increase in the activity of brush border and cytoplasmic enzymes such as acetylcholinesterase, dipeptidyl peptidase II, beta- and beta-gala ctosidases. The absence of acetylcholinesterase, lactase, nonspecific esterase, and weak activity of exopeptidases and alkaline phosphatase in the anterior intestine suggests that this segment of the intestine may be less important in nutrient absorption than the pyloric and spir al intestines. The observed quantitative and qualitative differences i n enzyme activity along the intestine indicate a high degree of specia lization of each segment for specific digestive and absorptive process es.