Gastrointestinal System

The rat is anatomically similar to the mouse. As in mice, salivary gland sexual dimorphism also occurs in rats. Intestinal Paneth cells have smaller granules than in

Pathology of Laboratory Rodents and Rabbits, Fourth Edition.

FIG. 2.1. Splenic white pulp of a rat, depicting the prominent marginal zone in normal rats (arrows).

FIG. 2.3. Uterus of a female rat during proestrus, in which the

uterine horns are filled with clear fluid.

mice. Unlike mice, the rat liver is consistently lobated into 4 major lobes and lacks a gall bladder. Hepatocytes in the rat are more uniform in size than in the mouse. However, polyploidy is a common morphological fea­ture in adult animals, and the number of binucleate cells increases with age. Bile is not concentrated. Foci of “ altered hepatocytes” are well documented in this spe­cies. They are generally of more concern to pathologists working in the pharmaceutical industry than to pathol­ogists in the diagnostic laboratory. Focal hepatitis with mononuclear cell infiltration in the absence of an iden­tifiable infectious agent occasionally occurs as an inci­dental finding in laboratory rats. The liver may also reflect general disease states. Atrophy of hepatic cords is a common manifestation of reduced (or absence of) food intake (Fig. 2.2). Atrophy of hepatic cords has been

FIG. 2.2. Liver from an adult rat with atrophy of hepatic cords. The reduction in hepatocellular cytoplasmic volume is consistent with reduced food intake and is often seen in disease states.

observed in adult male and female rats following fasting periods as short as 18 hours. The rat pancreas is diffuse.