Intestinal absorptive surface in mammals of different sizes

The present treatise is primarily concerned with the structural and morphometric parameters of the cecum and large intestine of mammals. Over the past century, numerous accounts have visually presented the variation and diversity of the large intestine of mammals. This includes above all comprehensive works on the macroscopic anatomy. More recently, the microscopic anatomy of various animals at the light, electron, and scanning electron microscopy levels has been covered, especially for rodents and primates. In the past two decades, progress has been made by adding a new dimension to the previous structural studies, namely, the morphometrical analysis of the intestine of various animals and subsequent employment of this data in an analysis based on principles of scaling. The present account follows essentially this outline, but presented in a slightly different order. First, in the introductory section, the scientific aims and general prologue to this field of study are presented. Included in this section is a short, concise literature survey that deals with the major literature available on the subject of the large intestine at the macroscopical and microscopical level, as well as the most recent morphometric analyses of the intestines. The main focus of the present work is on the methodological; a new method is described to measure the intestines of animals ranging in size from the harvest mouse to the horse. This technique may also be applicable to other hollow or tube-like organs. Heretofore, previous techniques have been based on obtaining the area of the surface mucosa by measuring lengths and widths and calculating the area by multiplying the two measurements. Alternatively, some methods have taken probes and made measurements at the light microscopy level and then extrapolated these results to determine the entire area. The former method is inadequate, while the latter possesses the inherent disadvantages of all sampling techniques. The present technique has two levels: (1) obtaining the basal surface area of the entire intestine and (2) accounting for any increase in the mucosal surface area that is due to microscopically visible folds, villi, or other such structures. The former is accomplished by flattening appropriately sized pieces of intestine between two glass plates and tracing the contours onto transparent paper. The entire intestine is processed in this manner, resulting in a basal surface area, the contours having been submitted to analysis on a semi-automatic image analyzer to determine the area in square millimeters. The second-level measurements determine a factor of surface enlargement by calculating a ratio of the distance along the surface contour of the mucosa on a histological section cut perpendicular to the width of the intestine to a second distance (reference line) drawn straight beneath the mucosa but not tracing the enlargements. The measurements obtained at both levels are multiplied to give a final surface area. In addition to a detailed description of the technique complete with a flow sheet and pictorial diagram, the various aspects of proper fixation, tissue embedding, shrinkage, and determination of sampling sites (for the second level of measurement) are discussed. A pilot experiment to determine the surface enlargement due to microvilli is presented from material taken from the giant pouched rat. This was performed by measuring video sequences of microvilli taken from electron microscopy images. Cecal microvilli increase the surface area 15-fold, while in the colon the increase is approximately 19- to 20-fold. In the discussion, the choice of using three animals per group is discussed, based on simple statistical tests. Section 3 is entitled "Morphology of the Mammalian Cecum." Chronologically, it marks the onset of the entire investigation. Before having developed the method described in Sect. 2, these morphological investigations at the light, electron and scanning electron microscopy