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, lung, and kidney) in the form of solitary lymph nodules, especially prominent in pathological conditions, and in the oropharynx and intestine as patches of aggregate lymph nodules. These lymphoid aggregates are called gut-associated lymphoid tissue (GALT) and bronchio- lar-associated lymphoid tissue (BALT). Cecal patches (cecal tonsils; Figure 37-20/10 and p. 798) are particu­larly evident.

The thymus consists of several separate lobes that accompany the jugular veins (Figure 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 has been described (p. 798; Figure 37-22/9). Like the thymus, the bursa is a lymphoepithe­lial organ; it displays a thin wall made uneven by the lobules it encloses, surrounding an irregular lumen. In the second week of embryonic development (in the chicken), lymphoid precursor cells migrate into the developing organ (Figure 37-23, B), and longitudinal plicae form and protrude into the lumen. Nodular epi­thelial formations, originating from the plicae, now begin to penetrate the lamina propria; when lymphopoi­esis is initiated, lymphoid cells invade these buds from the lamina propria. By active proliferation of lymphoid cells, these buds have considerably increased by the 18th day. 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 (Figure 37-23, C). The main function of the bursa as a primary lymphatic organ is thought to be the antigen­independent differentiation of B lymphocytes. 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. 799; Figure 37-24/5). Its structure resem­bles that of the mammal, although the distinction between the red and white pulp is less marked.