The mucosa-associated lymphoid tissues (MALT) are dispersed aggregates of nonencapsulated organized lymphoid tissue within the mucosa, which are associated with local immune responses at mucosal surfaces. The three major regions of MALT are the gut-associated lymphoid tissue (GALT), bronchus-associated lymphoid tissue (BALT) and nasal-associated lymphoid tissue (NALT). When oral studies are performed it is advisable to evaluate the GALT and when inhalation studies are performed the BALT and NALT should be evaluated. The organization of the MALT is similar to that of lymph nodes with variable numbers of follicles (B-cell area), interfollicular areas (T-cell area), and efferent lymphatics although afferent lymphatics are lacking. The overlying follicle associated epithelium (FAE) is typically cuboidal with variable numbers of goblet cells and epithelial cells with either microvilli or numerous surface microfolds (M cells). In addition to the organized lymphoid structures, single lymphocytes can be observed within the epithelium, mucosa and lamina propria. All MALTs are morphologically similar although there are location and species differences in the percentage of T and B cells (Haley, 2003
). The article by Cesta may be referred to for a more comprehensive review of the normal structure and function of MALT (Cesta, 2006
There are special features of stimulated and non-stimulated MALT that should be noted. In the rat, the basal lamina of the NALT and GALT epithelium is often interrupted where there is B and T lymphocyte and macrophage infiltration and should not be confused with ulceration (Kuper et al., 1990
). This feature is not typically present in BALT but is reported to occur after intratracheal antigen administration with a concomitant increase in the number of nonciliated cells (Van der Brugge-Gamelkoorn et al., 1986
According to the STP position paper: Best Practice Guideline for the Routine Pathology Evaluation of the Immune System (Haley et al., 2005
), the separate compartments in each lymphoid organ should be evaluated separately and descriptive rather than interpretive terminology should be used to characterize changes within those compartments. The evaluation of MALT for enhanced histopathology would include a careful comparison with appropriate controls and an indication of changes in the number and size of follicles and germinal centers and changes in the size and density of the interfollicular area. Other changes to evaluate are hypertrophy of the high endothelial venules (HEV), and the presence, severity and location of apoptotic cells and tingible body macrophages.
Although not strictly a component of enhanced histopathology, other items that can be noted during the evaluation are the location and severity of necrosis, plasma cells, granulocytes, pigmented macrophages, granulomas, etc. An example of a checklist for the changes to be noted in MALT for enhanced histopathology is given in . This table is intended to be an example of a guideline that the pathologist can use during histological evaluation rather than a format for reporting lesions. The diagnoses listed in this table are descriptive rather than interpretive, consistent with the STP position paper (Haley et al., 2005
MALT: specify type (e.g., GALT, BALT, NALT).