Structure of the Toad's Urinary Bladder as Related to Its Physiology

Abstract

The structure of the urinary bladder of the toad Bufo marinus was studied by light and electron microscopy. The epithelium covering the mucosal surface of the bladder is 3 to l0 microns thick and consists of squamous epithelial cells, goblet cells, and a third class of cells containing many mitochondria and possibly representing goblet cells in early stages of their secretory cycle. This epithelium is supported on a lamina propria 30 to several hundred microns thick and containing collagen fibrils, bundles of smooth muscle fibers, and blood vessels. The serosal surface of the bladder is covered by an incomplete mesothelium. The cytoplasm of the squamous epithelial cells, which greatly outnumber the other types of cells, is organized in a way characteristic of epithelial secretory cells. Mitochondria, smooth and rough surfaced endoplasmic reticulum, a Golgi apparatus, "multivesicular bodies," and isolated particles and vesicles are present. Secretion granules are found immediately under the plasma membranes of the free surfaces of the epithelial cells and are seen to fuse with these membranes and release their contents to contribute to a fibrous surface coating found only on the fi'ee mucosal surfaces of the cells. Beneath the plasma membranes on these surfaces is an additional, finely granular component. Lateral and basal plasma membranes are heavily plicated and appear ordinary in fine structure. The cells of the epithelium are tightly held together by a terminal bar apparatus and sealed together, with an intervening space of only 0.02 mu near the bladder lumen, in such a way as to prevent water leakage between the cells. It is demonstrated in in vitro experiments that water traversing the bladder wall passes through the cytoplasm of the epithelial cells and that a vesicle transport mechanism is not involved. In vitro experiments also show that the basal (serosal) surfaces of the epithelial cells are freely permeable to water, while the free (mucosal) surfaces are normally relatively impermeable but become permeable when the serosal surface of the bladder is treated with neurohypophyseal hormones. The permeability barrier found at the mucosal surface may be represented, structurally, either by the filamentous layer lying external to the plasma membrane, by the intracellular, granular component found just under the plasma membrane, or by both of these components of the mucosal surface complex. The polarity of the epithelial sheet is emphasized and related to the physiological role of the urinary bladder in amphibian water balance mechanisms.

The Surface Charge of Isolated Toad Bladder Epithelial Cells

“…The epithelial cells of the toad bladder fall into this latter category. In the intact toad bladder, they are tightly apposed near their luminal surfaces by a junctional complex (21,22), which appears to conform to the tripartite structure recently described by Farquhar and Palade (23). Below the junctional complex, the lateral and basal surfaces of adjoining cells are thrown into convoluted folds; desmosomes also contribute to cell attachment in this region.…”

“…In addition to the differences between the surfaces of any given cell, there are well recognized differences in the cell types which are present in the epithelial layer of the toad bladder (21,22). The dominant cell type is the granular cell, which has been estimated to comprise about 75% of the major cell types of the epithelial layer (22).…”

Removing the rubbish: frogs eliminate foreign objects from the body cavity through the bladder

“…In all cases, the beads were first surrounded by a thin, transparent tissue layer that progressively grew from the bladder to surround the bead (figure 1c). After the bead was surrounded by this thin tissue, a thicker and obviously vascularized tissue, presumably composed mainly of granular cells similar to those in the bladder epithelium [4,5], grew around the bead until it was fully enveloped (figure 1d -f ). Once in the bladders, the beads apparently floated freely in the urine, and were expelled if they happened to be near the duct connecting the bladder to the cloaca when the bladder was voided.…”

The Kinetics of Sodium Transport in the Toad Bladder

“…In addition, the hormone increases the passive permeability of these tissues to water (Koefoed-Johnsen and Ussing, 1953;Hays and Leaf, 1962). In the latter case, it has been shown clearly that the site of action of the hormone is on the mucosal side of the cells (MacRobbie and Ussing, 1961;Peachey and Rasmussen, 1961), although it must be added to the serosal medium to have any effect. Studies of the site of action of the hormone on sodium transport, on the other hand, have not been so definitive.…”