Histological development and histochemical localization of enzymes in rumen and reticulum in bovine fetuses.

Thirty Holstein fetuses from 100 to 251 d after conception were utilized to study prenatal anatomical development of the epithelium of the rumen and reticulum. Four calves at birth were included in the study for comparison. Tissue sections were frozen and stained to locate specific enzymes. At 100 d, the epithelial layer of the rumen was differentiated into a thin basal zone and a thickened superficial zone of undifferentiated cells. The basement membrane was straight, and in both zones cells were perpendicular to it. At 120 to 141 d, low, primary undulations were detected in the basal zone, basement membrane and underlying lamina propria. At 150 to 166 d, secondary undulations and incipient papillae began to resemble the papillae of mature mucosa. In rumen papillae of 192-d to 215-d fetuses, shallow furrows began to separate papillae apexes from the mass of epithelium. In fetuses 244 to 251 d, the papillae began to be a separate entity. At birth, the basal position of the papillae still remained fused. An incipient separation between the papillae was seen. Several dehydrogenase enzymes, including those associated with the Krebs cycle and reductase associated with energy transformation, were observed in both ruminal and reticular tissue. Alkaline phosphatase activity was localized in the stratum corneum and in blood vessels. Development of the honeycomb configuration of reticular epithelium was evident in the 100-d fetus and progressed rapidly with age.