The Functional Differentiation of the Small Intestine

T H E epithelium of the duodenal mucosa of vertebrates is strikingly uniform. In all but the cyclostomes and a few fishes, the epithelium is made up of tall columnar absorbing cells, with mucus-producing goblet cells interspersed. The columnar cells have a high cytoplasmic/ nuclear ratio, with the cytoplasm arranged into an apical zone containing a prominent Golgi apparatus, and a basal zone. The apical surface is featured by a sheet of microvilli that are extremely rich in alkaline phosphatase. The attainment of these characteristics has been shown to take place in a brief critical period, just before hatching in the chick embryo (Moog, 1950) and just before birth in the mouse (Moog, 1951), although in the latter animal a second period of phosphatase accumulation precedes weaning. The guinea-pig has now been added to this comparative series, because the protracted gestation period (about 67 days) might permit some further insight into the problems of duodenal differentiation. The newborn guinea-pig is quite mature and is at once able to eat solid food. According to the extensive work of Flexner and his colleagues (cf. Flexner, 1955), the cerebral cortex of the foetal guinea-pig reaches a high degree of maturity a full month before birth. The lungs also attain an advanced state of differentiation during prenatal life (Sorokin, Padykula, & Herman, 1959). In the duodenal epithelium slight phosphatase activity is detectable histochemically by the end of the 7th week (Verne & Hebert, 1949). The striated border is present by that time, but other aspects of chemical and cytological differentiation are said to be delayed until the 2nd or 3rd day after birth (Peyrot, 1955). This delay in differentiation appears to be related to the fact that the epithelial cells, during most of the foetal period, are choked with glycogen (Peyrot, 1957). The present study