Distribution of immunoreactive alkaline phosphatase in the adult rat ileum by immunoperoxidase staining at the light microscopic level.

Our prior immunocytochemical studies using monospecific antibody to alkaline phosphatase, Bouin's fixation, and paraffin sections demonstrated a decreasing gradient of villus brush border staining from the proximal to the distal rat small intestine. In addition, we noted an unusual pattern of staining in the terminal centimeter of the adult rat ileum: the villus brush border staining was less intense than crypt brush border staining. To determine whether this pattern of staining was present throughout the entire ileum, we examined alkaline phosphatase staining in two separate jejunal sites and the entire lowest third of the intestine of adult Wistar rats. With Bouin's fixation and paraffin embedding, both conventional and germ-free rats showed the same unusual staining pattern throughout the entire ileum. This pattern suggested that bacterial proteases were not responsible for the diminished ileal brush border alkaline phosphatase. However, when acetone fixation and cryostat sections were used with the avidin-biotin-peroxidase complex system, the previously noted reversed gradient of staining between the ileal villus and crypt areas was no longer present. Rather, ileal crypt brush border staining was less than ileal villus brush border staining. With either methodology, jejunal villus brush border staining was significantly more intense than ileal brush border staining, whereas the deep crypt brush border staining was not significantly different between the two regions. The present study reinforces the need for a combination of methodologies in order to best and most accurately localize certain antigens with immunocytochemistry. It also confirms a decreasing proximal to distal gradient for villus brush border alkaline phosphatase despite similar deep crypt brush border staining throughout the small intestine.