Ultrastructural and Immunohistochemical Study on the Lamina Propria Cells Beneath Paneth Cells in the Rat Ileum

Diurnal changes in bacterial settlement on the Peyer's patch and surrounding mucosa in the rat ileum and its effect against the intestinal immune system

Our previous study revealed the diurnal change in the indigenous bacteria settling on the terminal region of the rat ileum. In the present study, we investigated the diurnal change in indigenous bacteria on the most distal ileal Peyer's patch (PP) and surrounding ileal mucosa and explored how stimulation from indigenous bacteria for a day affects the intestinal immune system at the beginning of the light phase. Histological measurement revealed that bacteria adjacent to the follicle-associated epithelium of PP and to the villous epithelium of the surrounding ileal mucosa are more abundant at zeitgeber time (ZT)0 and ZT18 than at ZT12. On the other hand, tissue-section 16S rRNA amplicon sequencing revealed no significant difference between ZT0 and ZT12 in the bacterial composition on the ileal tissue including the PP. One-day treatment with an antibiotic (Abx) successfully impaired the settlement of bacteria around the ileal PP. In transcriptome analysis, 1-day Abx treatment led to the downregulation of several chemokines in both PP and ordinary ileal mucosa at ZT0. Histological analysis of the 1-day Abx group revealed decreases in both CD68⁺ macrophages in PP and naphthol AS-D chloroacetate esterase stain-positive mast cells in the ileal villi. Together, these findings suggest that the colonies of indigenous bacteria on the distal ileal PP and surrounding mucosa expand during the dark phase, which might lead to the expression of genes to regulate the intestinal immune system and contribute to the homeostasis of at least macrophages in PP and mast cells in the ileal mucosa.

Region specificity of fibroblast-like cells in the mucosa of the rat large intestine

Our previous studies using immunohistochemistry and serial block-face scanning electron microscopy (SBF-SEM) clarified that fibroblast-like cells (FBLCs) in the rat ileal mucosa are classifiable into several subtypes, but their characteristics throughout the large intestine remain unknown. In this study, we investigated the region-specific characteristics of FBLCs in the rat large intestine using histological analysis including SBF-SEM. Immunohistochemistry revealed that CD34⁺CD31^- FBLCs were localized in the lamina propria beneath the crypt bases throughout the large intestine and were more abundant in the descending colon than in the other regions. In addition, platelet-derived growth factor receptor α (PDGFRα)⁺ FBLCs were ubiquitously present just below the epithelium throughout the large intestine, and those at the crypt base were slightly more abundant in the descending colon than in the other regions. SBF-SEM analysis revealed that there were two types of FBLCs around the crypt base in both the cecum and the descending colon: sub-epithelial FBLCs localizing just beneath the epithelium in the manner of PDGFRα⁺ FBLCs, and lamina propria FBLCs localizing farther away from the epithelium than sub-epithelial FBLCs in the manner of CD34⁺CD31^- FBLCs. The lamina propria FBLCs were closely apposed to various immune cells in the lamina propria, and their endoplasmic reticulum in the descending colon exhibited various dilatation levels, unlike that in the cecum. These findings indicate that FBLCs, especially around the crypt base, differed in each region of the large intestine with respect to localization, abundance, and ultrastructure, which could lead to the region-specific microenvironment around the crypt base.

Histological study of diurnal changes in bacterial settlement in the rat alimentary tract

The composition of fecal bacteria is reported to change throughout the day, whereas the circadian rhythmicity of indigenous bacteria that settle on the epithelium is mostly unknown. The present study aimed to clarify the diurnal changes in the settlement of indigenous bacteria in the rat alimentary tract using histological analysis. The settlement of indigenous bacteria on the mucosal epithelium throughout the day and the diurnal changes in settlement levels were observed in the esophagus, the nonglandular area of the stomach, and the ileum. The peak of zeitgeber time (ZT) in the settlement level differed by segment: ZT 12 in the esophagus, ZT 6 in the nonglandular area of the stomach, and ZT 0 in the ileum. Moreover, 16S rRNA amplicon sequencing using tissue sections revealed that the compositions of the indigenous bacteria in the ileum differed among ZT. In the intervillous spaces of the ileum, the formation level of the mucus layer, one of the most fundamental host defenses against bacteria, was lowest at ZT 0. Bacteria were preferentially adjacent to the villous epithelium in the area without coverage by the mucus layer at ZT 0. These findings collectively suggest that the settlement level and possibly the composition of the indigenous bacteria changed diurnally in various segments of the alimentary tract, and the formation of the mucus layer might be the most likely to lead to such diurnal changes in indigenous bacteria, at least in the ileum.

Ultrastructural and phenotypical diversity of macrophages in the rat ileal mucosa

Several types of macrophages have been reported in the intestinal mucosa, but their histological localization remains ambiguous. Here, we obtained detailed information about ultrastructural and phenotypical diversity of macrophage-like cells (MLCs) in the rat ileal mucosa using immunofluorescent analysis and serial block-face scanning electron microscopy (SBF-SEM). The results revealed that the cells immunopositive for CD68, the pan-macrophage marker, included CD163^-CD4⁺, CD163⁺CD4⁺, and CD163^-CD4^- cells in the lamina propria (LP) of the intestinal villus and around the crypt. CD68⁺CD4⁺CD163^- cells seemed to be preferentially localized in the intestinal villus, whereas CD68⁺CD163⁺CD4⁺ cells were frequently localized around the crypt. SBF-SEM analysis identified three types of MLCs in the ileal mucosa, which were tentatively named types I-III MLC based on aspects of the 3D-ultrastructure, such as the localization, quantity of lysosomes, endoplasmic reticulum, and exoplasm. Type I and II MLCs were localized in the villous LP, while type III MLCs were localized around the crypt, although type II MLCs were a minor population. All three MLC types extended their cellular processes into the epithelium, with type I MLCs showing the greatest abundance of extended processes. Type I MLCs in the upper portion of the intestinal villus showed a higher level of attachment to intraepithelial lymphocytes (IELs) compared to type III MLCs around the crypt. These findings suggest that macrophages of the rat ileal mucosa differed by region along the longitudinal axis of the villous tip-crypt from the perspective of ultrastructure, cellular composition, localization, and interactions with IELs.

Three-dimensional analysis of neural connectivity with cells in rat ileal mucosa by serial block-face scanning electron microscopy

The comprehensive targets of innervation in the intestinal mucosa are unknown, partly because of the diversity of cell types and the complexity of the neural circuits. Herein, we investigated the comprehensive targets of neural connectivity and analyzed the precise characteristics of their contact structures in the mucosa of rat ileum. We examined target cells of neural connections and the characteristics of their contact structures by serial block-face scanning electron microscopy at four portions of the rat ileal mucosa: the apical and basal portions in the villi, and the lateral and basal portions around/in the crypts. Nerve fibers were in contact with several types of fibroblast-like cells (FBLCs), macrophage-like cells, eosinophils, lymphocyte-like cells, and other types of cells. The nerve fibers almost always ran more inside of lamina propria than subepithelial FBLC, and thus contacts with epithelial cells were very scarce. The contact structures of the nerve fibers were usually contained synaptic vesicle-like structures, and we classified them into patterns based on the number of nerve fiber contacting the target cells at one site, the maximum diameter of the contact structures, and the relationship between nerve fibers and nerve bundles. The contact structures for each type of cells occasionally dug into the cellular bodies of the target cells. We revealed the comprehensive targets of neural connectivity based on the characteristics of contact structures, and identified FBLCs, immunocompetent cells, and eosinophils as the candidate targets for innervation in the rat ileal mucosa.

Intrinsic Control of Surface Immune and Epithelial Homeostasis by Tissue-Resident Gut Stromal Cells

The epithelial layer creates a chemical and physical barrier at the forefront of intestinal mucosa, and immune cells beneath the surface epithelium are poised to react to extrinsic factors, to maintain tissue homeostasis. Importantly, the nexus of epithelial-immune responses at mucosal surfaces is dexterously modulated by intrinsic stromal-mesenchymal cells. First, organogenesis of lymphoid tissues, including Peyer's patches, requires dynamic interplay between lymphoid cells and stromal cells, which have become known as "lymphoid organizers." Second, correct spatiotemporal interaction between these cell populations is essential to generate the infrastructure for gut immune responses. Moreover, immune cells at the intestinal barrier are functionally modulated by stromal cells; one such example is the stromal cell-mediated differentiation of innate immune cells, including innate lymphoid cells and mast cells. Ultimately, mucosal stromal cells orchestrate the destinations of epithelial and immune cells to maintain intestinal immune homeostasis.

Three-dimensional analysis of fibroblast-like cells in the lamina propria of the rat ileum using serial block-face scanning electron microscopy

Serial block-face scanning electron microscopy (SBF-SEM) is useful for three-dimensional observation of tissues or cells at high-resolution. In this study, SBF-SEM was used to three-dimensionally analyze the characteristics of fibroblast-like cells (FBLCs) in the rat ileal lamina propria (LP). The results revealed that FBLCs in LP could be divided into four types, tentatively named FBLC type I-IV, based on the external cellular appearance, abundance or shape of each organelle, detailed distribution in the LP and relationship with surrounding cells. FBLC-I and -II localized around the intestinal crypt (InC), FBLC-III localized from the lateral portion of InC to the apical portion of the intestinal villus (InV), and FBLC-IV localized in the InV. FBLC-I, -II and -III, but not FBLC-IV, localized beneath the epithelium. FBLC-II possessed thin lamellar-shaped endoplasmic reticula, whereas FBLC-I possessed expanded endoplasmic reticula that occasionally showed a spherical shape. FBLC-III and -IV possessed a cytoplasmic region with high-electron density and no organelles immediately beneath the cellular membrane; this region was found at the epithelial sides in FBLC-III and scattered in FBLC-IV. FBLC-IV were in constant close proximity to villous myocytes throughout the InV and also in contact with FBLC-III especially in the apical portion of the InV. FBLC-I, -II and -IV, but not -III, were constantly found to be in contact with various immunocompetent cells in the LP. Three-dimensional analysis using SBF-SEM indicates that four types of FBLC localized in the rat ileal LP.