Birds (and other animals with a cloaca) have a bursa, or outpouching of the cloaca, in which b-cells mitose, mature, then migrate out into the general body tissues. The bursa is a blind sac that extends from the dorsal side of the cloaca, the common portal of the reproductive, urinary and digestive systems. Within the bursas of young animals there are extensive leaf-like folds composed of simple, columnar epidermis and a loose connective tissue that is permeated with blood vessels and lymph nodules. With maturation of the animal, b-cells (think of them as bursa-derived cells) will migrate to the blood and lymph vessels, then into the rest of the body. Atrophy of a recognizable bursa with significant numbers of defensive cells generally occurs around time of sexual maturation.

In this slide, the surface epithelium of the bursal folds is - not surprisingly - highly folded. Blood located within the bursal lumen is artifactual. Note that lymphocytes are not crossing the epithelium. Their route of transit is via blood and lymph vessels, and their function is not directly related to the cloaca or the bursa.

Return to modules

On to villi