The Lymphatic Structures

Only the goose and duck (among domestic poultry) have lymphoid tissue encapsulated as true lymph nodes—a pair of cervicothoracic nodes in the thoracic inlet and a pair of lumbar nodes close to the kidneys.

Lymphatics are less numerous than in mammals. They accompany (and wind around) the blood vessels, are valved, and present microscopic lymph nodules scattered at intervals in their walls. They conduct the lymph to the thoracic inlet, where it is discharged into the cranial venae cavae.

Although true lymph nodes are absent, much lymphatic tissue occurs in various organs (liver, pancreas, lungs, and kidneys) in the form of solitary lymph nodules, especially prominent in pathologic conditions, and in the oropharynx and intestine as patches of aggregate lymph nodules. These lymphoid aggregates are called gut-associated lymphoid tissue (GALT) and bronchiolar- associated lymphoid tissue (BALT). Cecal patches (cecal tonsils; Fig. 37.20/10) are particularly evident.

The thymus consists of several separate lobes that accompany the jugular veins (Fig. 37.15/9). The lobes are divided into lobules, each of which consists of a dark cortex and a pale medulla. The thymus, best developed in the young, regresses with the onset of sexual maturity.

The cloacal bursa, which has been described earlier (Fig. 37.22/9), is a lymphoepithelial organ and has a thin wall made uneven by the enclosed lobules that surround an irregular lumen. In the second week of embryonic development (in the chicken), lymphoid precursor cells migrate into the developing organ (Fig. 37.23B), and longitudinal plicae form and protrude into the lumen. Nodular epithelial formations, originating from the plicae, now begin to penetrate the lamina propria. With the initiation of lymphopoiesis, lymphoid cells invade these buds from the lamina propria. The buds increase considerably by day 18 through active proliferation of lymphoid cells. The bursa reaches its greatest size approximately 6 weeks after hatching, when the plicae are completely filled by large epithelial accumulations (or bursa follicles), which results in the histology of the organ showing many similarities with that of the thymus (Fig. 37.23C). The bursa is the site of antigen­independent differentiation of B lymphocytes and antibody production. The bursa gradually regresses from the age of 2 to 3 months, but a small nodule remains in the adult. In young birds the bursa is an important organ for investigating and diagnosing several viral infections (e.g., circovirus infections).

The location and shape of the spleen have been described (p. 785; Fig. 37.24/5). Its structure resembles that of the mammal, although the distinction between the red and white pulp is less marked.