Ontogenic changes in secretory component expression by villous and crypt cells of rat small intestine

In order to determine whether the rat small intestine exhibits quantitative changes in the synthesis of the secretory component (SC) during growth, epithelial villus and crypt cells were isolated from jejunal segments at intervals after birth up to adulthood. SC concentration was measured in each cell fraction by immunoradiometric assay and compared to sucrase activity, an enzyme marker of the differentiated villus enterocyte. The following results were observed. (i) Adult rats showed a characteristic decreasing concentration gradient of SC from the crypts (mean concentration in crypt cells: 636 +/- 173 ng/mg protein) to the villus tip (mean concentration in villus cells: 152 +/- 17 ng/mg protein). This gradient was the reverse of that found for sucrase activity. (ii) In young sucklings (10 days old), SC was virtually absent in both villus and crypt cells, but its concentration progressively increased in weanling rats and reached adult levels by day 40 postpartum. (iii) The crypt to villus cell gradient of SC, absent in sucklings up to day 20, developed during the fourth postnatal week. (iv) Treatment of 10-day-old suckling pups with pharmacological doses of either corticosterone or L-thyroxine for 3 consecutive days failed to induce the precocious synthesis of SC by jejunal enterocytes, but produced significant (P less than 0.01) decreases in concentration. Under the same conditions, sucrase activity was markedly enhanced. In conclusion, major changes in the ability of the immature crypt cell to produce the specific receptor for transepithelial transport of polymeric immunoglobulins occur during the fourth week of rat life. The initiation of this ontogenic process is not triggered by the dietary and hormonal changes known to control the maturation of other functions linked to the differenciated villus cell, such as sucrase activity.