Figure 9.

Neuronal like structures identified within mesothelial cells connecting to the splenic capsule.

Panel A-B; Osmium stained electron micrographs of mesothelial cells on the inferior edge of a Sprague Dawley rat splenic capsule located close to the branch point of a connection. Mesothelial cells were identified by their location above the collagen layer of the splenic capsule and their numerous micro villi. Structures within the mesothelial cell cytoplasm resemble similar structures fond in neuronal tissue. In these images electron dense structures similar to that of synaptic junctions (S) can be observed between two circular structures. In panel A, one of these low density circular structures contains a single mitochondrion. These structures are similar to that observed in dendrites of neuronal cells (d). More densely filled structures run adjacent to the dendritic structure separated by (S), which resemble axons (a) in their arrangement with the dendritic structures and greater electron density. No clear vesicular structures were observed within (a).

Panel C and E; Many of the ‘axonal looking’ structures (a) were found to contain a dense elongated core, similar to synaptic ribbons (r) observed in the axons of rapidly firing neural cells.

Panel D and F; Shown in panel d and f are urethane stained sections which provide less ultrastructural contrast but allow antigen specific staining. Immuno-gold labeling of antibodies against ribeye, a core component of synaptic ribbons, demonstrated gold particle deposition on the dark banded structures within mesothelial cells that resembled synaptic ribbons (r).

Panel G; In this low magnification osmium stained section, a tissue connection with the splenic capsule can be observed. Mesothelial cells, identified by their microvilli can be seen lining the entire length connection before making contact with other mesothelial cells on the splenic surface. At higher magnification more than 15 object pairs, similar to that observed in panels c and d containing all of axon (a), ribbon (r) and dendritic (d) like structures can be seen in this image.

Panels H and I; Paraffin embedded sections of rat spleen stained with anti-ribeye, a core component of synaptic ribbons. As shown in panel H, mesothelial cells located on structures that connect to the splenic capsule as well as mesothelial cells immediately adjacent to these junctions, stain positive for ribeye. As demonstrated in panel I, mesothelial in areas where these junctions are not present are negative for ribeye while the splenic parenchyma is positive.