When rachitic chicks are given 1,25-dihydroxyvitamin D3 in amounts as low as 50 ng/bird, the appearance of the duodenal mucosa is altered within 2 h of the administration of the hormone. The changes are most readily apparent on scanning electron microscopy and include: a more plump appearance of villi with loss of furrows and pits on their surfaces, elongation of villi and a smoother, more uniform microvillus surface. These changes occur within 2 h of the administration of the hormone and persist for as long as 24 h. The morphological change precedes the increase in calcium absorption induced by 1,25-dihydroxyvitamin D3 in the intestine. These observations suggest that 1,25-dihydroxyvitamin D3 may play an important part in maintenance of the structure of the duodenal mucosa.
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