Background:   Changes in food constituent during suckling, weaning and post-weaning periods documented histological and functional adaptation in rat small intestine.
Objectives: identify histological changing events, which represent such adaptation in the rat duodenal mucosa.
 Material and methods: Twenty four male rat pups are divided into equal 4 groups including day one, 7 &15 (pre-weaning) and day 21 (weaning). In addition, two other groups including 12 male adult rats equally divided, aged 45 days (post-weaning) and 90 days, were used. The animals are euthanized at the fore mentioned days and the duodenum is processed for light and scanning electron microscopic examination. Paraffin sections are stained with haematoxylin & eosin and combined alcian blue/ PAS. Computer image analysis and statistical study are done for the height of the villi, depth of the crypts and villous / crypt ratio.
 Results: The villi are broadening with the advance of age. The height of the villi   and the depth of the crypts show a statistical non-significant increase in the pre-weaning groups. At weaning, there is a significant decrease in the height of the villi with a decrease of the villous/crypt ratio. The villi are covered by brushed columnar enterocytes and some vacuolated enterocytes are seen only in pre-weaning period. Acidic and neutral mucin positive stained goblet cells are distributed on villi and within the crypts   of pre-weaning pups. At weaning, goblet cells are positive for acidic mucin only. Crypts are lined with pyramidal enterocytes and paneth cells are dominating in day 21. Both villi and crypts at postnatal day 21(weaning) acquire structural appearance similar to that of adult.