The effects of alterations in villus shape on the intestinal mucosal surface of the albino rat; the relationship between mucosal surface area and the crypts.

The villi and crypts of the small intestine of the albino rat (Rattus norvigicus) have been enumerated and measured at different ages from 2 weeks to 8 months. The number of villi increased up to the third month of age and the number of crypts up to the eighth month. In the proximal intestine, the mean length of the villus base increased up to the fifth month as ridge-shaped villi were formed. Villus height was greater proximally than distally and this, and the crypt depth, remained constant from the end of the first month of age. The total villus circumference per unit area of intestine, and the villus circumference per crypt was the same proximally and distally, and was relatively constant after the first month of age. The total circumference of the crypt mouths per square millimeter of intestine was the same proximally and distally and, at all ages, was greater than the total villus circumference. The villus surface area per square millimeter of intestine, or per crypt, remained relatively constant after the first month and was greater proximally than distally, due only to the taller villi proximally. The change from finger-shaped to ridge-shaped villi did not affect the villus mucosal surface area. The changes in villus shape are probably not determined by differences in the rate of crypt cell production.