Quantitative analysis of factors contributing to expansion of microvillous surface area in the coprodaeum of hens transferred to a low NaCl diet.

A stereological study of the lower intestine (coprodaeum) of the domestic hen was undertaken using combined light and electron microscopy. Numbers of columnar absorptive epithelial cells and the dimensions and numbers of microvilli were estimated. The aim was to identify the main factors contributing to an increase in microvillous surface area following transfer of hens from a high to a low NaCl diet on which they were kept for at least 3 wk. The principal contributor to observed changes between organs was cell number. Birds adapted to the low NaCl diet had 57% more cells than high NaCl controls. The average cell had a larger microvillous surface (55%) and this could be explained by the presence of longer (34%) and more densely packed (38%) microvilli. The total number of microvilli per coprodaeum doubled (from 35 x 10(9) on the high NaCl to 71 x 10(9) on the low NaCl diet). The increase in cell number accounted for 67% of the change in surface area and 78% of the change in number of microvilli per organ. These findings emphasise that, when assessing the form and function of a whole organ, it is important to monitor cell populations as well as single cells. This is especially true when studying renewing and expanding (rather than static) populations.