R. Calvert

Establishment of regional differences in brush border enzymatic activities during the development of fetal mouse small intestine

SummaryIn order to study the establishment of regional differences in brush border enzymic activities during the development of fetal mouse small intestine we have followed (1) the differentiation of microvilli by morphometry, and (2) the developmental pattern of three brush border enzymes (lactase, glucoamylase and alkaline phosphatase). From day 16 to day 19 of gestation, the height of duodenal microvilli increases 2.4 times on the absorptive cells located near the tip of the villi. During the same period in the upper half of the duodenal villi, the number of microvilli per square μm rises by a factor of 2.4 and the microvillous surface area increases by a factor of 5.2. The differentiation of ileal microvilli follows a similar pattern but they are always shorter and less numerous than those of the duodenum. Lactase activity appears at 18 days of gestation; the other two brush border enzymes are first detected at 16 days of gestation. Afterwards all three enzyme activities increase rapidly and a decreasing gradient of activity is established from the proximal to the distal segment of the small intestine. Hence, the structural development of the microvilli and the appearance of brush border enzyme activities occur simultaneously and a proximo-distal gradient is already established at 16 days of gestation.

The occluding junctions of mouse duodenal enterocytes during development

SummaryThe architecture of occluding junctions during the differentiation of the mouse duodenum was studied in freeze-fractured material. Irregular zonulae occludentes (ZO) (Type I) are numerous during fetal life, and are characterized by their irregular width, and by the presence of basal open-ended extensions fused with the discontinuous basal strand of the ZO. Regular ZOs (Type II), typical of the adult villous epithelium, appear after Type I junctions by day 16 of gestation. Two patterns are distinguishable: in the first, parallel strands of ridges and furrows are found without crossing branches; in the second pattern, the junction zone is organized like a network of short branches forming various types of polygons. In fetal and adult mice fasciae occludentes (FO) (Type III) are present on the lateral cell membranes; in unfixed specimens particles are found in the furrows of the E-face and pits on the ridges of the P-face. In fixed tissues, the particles are aligned on the ridges of the P-face. These results indicate that fixation with glutaraldehyde modifies considerably the affinity of junctional particles toward the P-face during the fracture process. Moreover, the presence of numerous large FOs on the lateral cell membranes of enterocytes during late fetal life and in the adult, is possibly related to cell movement along the intestinal villi.