Reticular meshwork of the spleen in rats studied by electron microscopy

Abstract

The reticular meshwork of the rat spleen, which consists of both fibrous and cellular reticula, was investigated by transmission electron microscopy. The fibrous reticulum of the splenic pulp is composed of reticular fibers and basement membranes of the sinuses. These reticular fibers and basement membranes are continuous with each other. The reticular fibers are enfolded by reticular cells and are composed of two basic elements: 1) peripheral basal laminae of the reticular cells, and 2) central connective tissue spaces in which microfibrils, collagenous fibrils, elastic fibers, and unmyelinated adrenergic nerve fibers are present. The basement membranes of the sinuses are sandwiched between reticular cells and sinus endothelial cells and are composed of lamina-densalike material, microfibrils, collagenous fibrils, and elastic fibers. The presence of these connective tissue fibrous components indicates that there are connective tissue spaces in these basement membranes. The basement membrane is divided into three parts: the basal lamina of the reticular cell, the connective tissue space, and the basal lamina of the sinus endothelial cell. When the connective tissue space is very small or absent, the two basal laminae may fuse to form a single, thick basement membrane of the splenic sinus wall. The fibrous reticulum having these structures is responsible for support (collagenous fibrils) and rebounding (elastic fibers). The cells of the cellular reticulum--reticular cells and their cytoplasmic processes, which possess abundant contractile microfilaments, dense bodies, hemidesmosomes, basal laminae, and a well-developed, rough-surfaced endoplasmic reticulum, and Golgi complexes, which are characteristic of both fibroblasts and smooth muscle cells--are considered to be myofibroblasts. They may play roles in splenic contraction and in fibrogenesis of the fibrous reticulum. The contractile ability may be influenced by the unmyelinated adrenergic nerve fibers that pass through the reticular fibers. The three-dimensional reticular meshwork of the spleen consists of sustentacular fibrous reticulum and contractile myofibroblastic cellular reticulum. This meshwork not only supports the organ but also contributes to a contractile mechanism in circulation regulation, in collaboration with major contractile elements in the capsulo-trabecular system.

Neural Signaling in the Spleen Controls B-Cell Responses to Blood-Borne Antigen

“…ASCs that have migrated successfully to the RP often are associated with reticular fibers in the RP that are known to guide their migration toward vessels where they secrete antibodies (9). We found that these conduits are innervated frequently which permits direct exposure of migrating cells to neurotransmitters.…”

“…These activated B cells differentiate into antibody-secreting cells, a process that involves the reexpression of syndecan-1 (CD138) and the downregulation of adhesion molecules (5,6). Finally, the antibody-secreting cells migrate along reticular fibers to the red pulp (RP) (7), where they release antibodies into the blood (8,9). MZ and B1 B cells have a preactivated phenotype and express membrane immunoglobulins mainly of the immunoglobulin M (IgM) isotype (10); thus, they can initiate a rapid antibody response to bacterial polysaccharide which is detectable in the serum as early as 48 h after infection (8).…”

Cytochemical Studies on Carbohydrates in the Reticular Cells of the Rat Splenic Cord.

“…Thus, it seems that the abundance of cytoplasmic glycogen in the reticular cells of the rat splenic cord can also be well connected with the contractility of the cells, since the reticular cells in the splenic cord contain a large number of contractile microfilaments which regulate the intrasplenic blood volume [6,7].…”

“…In the latter cells, a role of the acidic glycoconjugates remains to be determined. However, reticular cells and reticular fibers are known to form a meshwork of the splenic cord in the spaces containing a variety of blood cells [6,7]. Therefore, the acidic glycoconjugates on the reticular surface of the rat splenic cord exposed to the retaining blood seem to reflect functional activities to store large numbers of red blood cells in the splenic cord in the mammalian spleen [3,10].…”

Extramedullary hematopoiesis: Elucidating the function of the hematopoietic stem cell niche (Review)

“…For this reason, c-kit-positive/CD34-negative cells in the EMH-positive spleen may potentially be a type of hematopoietic precursor cell. Bone marrow-derived HSCs in the arterial blood should enter into an open blood system comprised of the red pulp of the splenic cords, so that maturing cells may infiltrate into the venous sinuses (26,(44)(45)(46)(47)(48). Previous studies have reported the presence of bone marrow-derived HSCs within the sinuses and adjacent to endothelial cells of the spleen (5,49).…”