In the present study, the wall of large intestine was comprised of four layers typical to the histo-architecture of hollow organs
viz. tunica mucosa, tunica sub-mucosa, tunica musculosa and tunica serosa which was in accordance with
Banks (1986). The solitary lymphatic nodules (SLN) and lymphoglandular complexes (LGCs) were observed in the tunica submucosa of colon and rectum. The SLN was a well demarcated encapsulated lymphocytic aggregation but LGC consisted of partially encapsulated lymphocytic aggregate (Fig 1). Variable numbers of lymphatic nodules with germinal centres were observed in the tunica submusoca. The surface epithelial lining of LGC of large intestine showed invagination into the follicular complex and the inter nodular lymphoid tissue were penetrated by the mucosal gland from the tunica mucosa (Fig 2) which was similar to the findings of
Morfitt and Pohlenz (1989) in porcine.
In the colon and rectum of crossbred pigs the LGCs typically consisted of a pore like depression on surface epithelium from which extensions of mucosal glands passed through the circular gap into lamina muscularis mucosae and penetrated submucosal lymphoid tissue (Fig 2). This might be an access route of gut antigens to the lymphog landular complex. The present findings was in consonance with the findings of Kapoor and Singh (2016) in buffalo calves. However, in some LGCs no pore like depression and disrupted muscularis mucosae could be observed. The muscularis mucosae layer distinctly separated the SLN and LGC from the luminal side (Fig 1). This might be due to faviform type of LGC. The present findings were not in agreement with the findings of
Zhaxi et al., (2014) in Bactrian Camels,
Gautam et al., (2013) in goat and
Morfitt and Pohlenz (1989) in porcine.
The lymphatic aggregation of LGC in the lamina propria and tunica submucosa showed three distinct zone
viz. follicle, interfollicular area and dome area (Fig 3) and distinct FAE could be appreciated in the absorptive epithelium and crypts in the submucosa. Occasionally the rectal LGC was observed as one distinct lymphoid follicle that was surrounded by a glandular epithelium (Fig 4).
In the present study, the mesenteric lymph node associated with the large intestine was covered by connective tissue capsule with distinct cortex and medulla. Distinct secondary lymphoid follicle with inner germinal centre and outer corona could be seen in cortico-medullary area (Fig 5). However,
Kalita et al., (2014) reported that the lymph nodes of Mizo pigs consisted of cortex like tissue and medulla like tissue.
The connective tissue covering the SLN and LGC was well appreciated and scanty collagen fibres (Fig 6) were observed in the inter-follicular glands, which was not in agreement with the findings of
Kapoor and Singh (2016) in buffalo calf. The reticular fibres were present on the outer cortical area of the follicles and in the inter follicular areas. Similar findings were also reported in pig by Dev
Choudhury et al., (2017). In the present study, nerve fibre innervations were seen around the crypt area, lamina propria and tunica sub mucosa of large intestine. A fine network of nerve fibres was also observed around the lymphoid follicle and observed penetrating the LGC (Fig 7).
In the present investigation FAE and sub epithelial dome (SED) were appreciated in semi thin sections stained with toluidine blue. In rectal LGC distinct FAE could be seen. However,
Gautam (2015) in piglet reported that distinct FAE was absent in LGC. Presence of germinal centres in LGC were observed in the tunica submucosa. Disrupted lamina muscularis mucosae could also be observed (Fig 2) which was in consonance with the findings of
Bayraktaroglu et al., (2016) in Angora goat.
The epithelium lining the SLN showed PAS positive reaction. The mucous substance showed strong reaction in the gland in LGC (Fig 8). Again, the PAS stained sections (Fig 9) revealed that in the FAE epithelium, the number of goblet was less.
Frappier (2007) also opined that in the FAE there were less number of goblet cell and more number of M cells. This might be due to the transport of antigen through M cell to the underlying dome area from the FAE.
Columnar cells predominated the FAE and the crypts. Abundant goblet cells were present in the crypt epithelium. The M cells showed presence of inter epithelial lymphocyte and basally placed heterochromatic nucleus. However,
Owen (1999) opined that M cells of humans, rodents and chickens contained basolateral pockets where migrating lymphocytes and macrophages were present. The enterocytes were distinct with round to oval euchromatic nucleus (Fig 10 and 11).
Beyaz et al., (2010) in Angora rabbit reported that M cells were observed with intraepithelial lymphocyte and basally placed euchromatic nucleus similar to the present findings.
