INCREASED C EBP IN FETAL-RAT SMALL-INTESTINE PRECEDES INITIATION OF DIFFERENTIATION MARKER MESSENGER-RNA SYNTHESIS/

Morphogenesis, initiation of differentiation marker gene expression, a nd their correlation with CCAT/enhancer binding protein (C/EBP) expres sion were analyzed in the developing fetal rat small intestine. Expres sions of mRNAs for lactase-phlorizin hydrolase (LPH), intestinal alkal ine phosphatase (IALP), carbamoyl-phosphate synthetase (CPS), and thre e isoforms of C/EBP were simultaneously determined by Northern blot an alysis from 15 to 19 days of gestation. At 17 days of gestation, prior to villus formation as demonstrated by light and electron microscopy, only CPS and C/EBP alpha, -beta, and -delta expression could clearly be detected. Both LPH and IALP mRNA were definitely detectable in prox imal and middle intestine on day 18, as soon as the stratified epithel ium of the early intestine had been transformed into a single layer of columnar epithelium lining villi. This distribution was confirmed by in situ hybridization for LPH mRNA. During the period of transformatio n when the columnar epithelium and villi were forming, no LPH or IALP mRNA was detectable in the immature distal one-third of the fetal inte stine. Preceding villus morphogenesis, immunostaining demonstrated nuc lear localization of C/EBP alpha protein in intestinal epithelial cell s, with continued expression in all enterocytes through 19 days of ges tation. Enhanced expression of C/EBP alpha mRNA and protein began 24 h prior to the initiation of the differentiation markers, suggesting th at it map play a role in regulation of fetal intestinal differentiatio n.