THE APICAL CECAL DIVERTICULUM OF THE CHICKEN IDENTIFIED AS A LYMPHOIDORGAN

A peculiar structure, observed as a dome-like protrusion at the apex o f the caecum, was investigated macroscopically and histologically in h ealthy White Leghorn chickens. It was hemispheric or spherical in shap e and as it consisted of a lumen with a wall occupied by lymphoid tiss ue, this structure was designated the apical caecal diverticulum (ACD) . ACD were detected in 25.2% of examined chickens and had a mean diame ter and height of 1.9 mm and 1.2 mm respectively. Histologically, both the lamina propria mucosae and the submucosa of ACD consisted of well developed aggregated lymphoid nodules. Each nodule was covered by fol licle-associated epithelium which contained cells resembling M eels. S ome secondary nodules extended into the subserosa. The muscularis muco sae and the stratum circulae of the tunica muscularis disappeared near the entrance to ACD. The stratum longitudinale also gradually decreas ed in thickness around the entrance, becoming an extremely thin layer in the diverticulum wall. At the caecal apex, each stratum of the tuni ca muscularis was thinner than in the caecal body and separated into s everal muscle bundles. These bundles were occasionally displaced by de veloped lymphoid nodules, causing them to protrude into the subserosa. The high frequency of ACD suggests that caecal apex may be sites for immunological surveillance in the chicken caecum. In addition to the i ntense and frequent antiperistalsis at the apex suggested by Yasukawa (1959), possible causes for the formation of ACD included (1) the frag ility of the tunica muscularis at the ACD, and (2) the local removal o f the physical supporting structures by the development of lymphoid no dules.