Discrete localization of distinct alkaline phosphatase isozymes in the cell surface of silkworm midgut epithelium

Alkaline phosphatase (ALP) isozymes were localized in the larval midgut epithelium of the silkworm, Bombyx mori (Insecta: Bombycidae) by immunohistochemistry with the peroxidase‐conjugated antibody technique. The monospecific antibodies were produced against purified membrane‐bound ALP (m‐ALP) and soluble ALP (s‐ALP), respectively. The anti‐(m‐ALP)‐antibody did not cross‐react with the purified s‐ALP and vice versa. The m‐ALP was localized at the brush border of columnar cells in the middle and posterior midgut epithelium, but very weakly at that in the anterior midgut. In contrast, the s‐ALP was localized at the apical surface of goblet cells throughout the length of midgut. The extent of positive staining gradually increased toward the posterior midgut, where strong staining was also detected in the goblet cavity. From these results, it is concluded that two ALP isozymes distribute in the different cell types of Bombyx midgut epithelium and that the regional differentiation along the length of midgut exists on columnar and goblet cells.