In order to clarify the fundamental structure of the host defence
mechanism in chicken caeca, a detailed
analysis of the distribution of lymphoid nodules (LNs) was carried out
on
longitudinal sections of both the
mesenteric (side of the ileocaecal ligament) and the antimesenteric mucosa.
An overwhelming majority of
solitary or aggregated LNs were located in the mesenteric mucosa, although
a few were also found in the
antimesenteric mucosa. Of the total LNs, 45.7% were detected at
the proximal 7.8% section in the caecal
tonsil. LNs (21.4%) were also concentrated in the distal
22.0% section corresponding to the apex. A
moderate concentration of LNs (13.1%) was found at the
transitional 20.0% region between the base and
body. Approximately 80.2% of total LNs were found at the above
3 regions in the mesenteric mucosa. In
many cases, the frequency of LNs in the caecal tonsils was opposite to
that at the apices. Aggregated LNs
were mainly found in the caecal tonsils, transitional region and apex.
Almost all aggregated LNs consisted
of fundamental nodular units possessing M cells in their follicle
associated epithelia. The aggregated LNs in
the above 3 regions therefore could provide immunological surveillance
against caecal luminal contents. In
particular, the cooperative function between LNs of the caecal tonsil
and apex might be highly important in
maintaining the caecal microenvironment.