The purpose of the present investigation was to describe ultrastructural characteristics of hepatic bile ducts and the gall bladder in larvae of the sea lamprey, Petromyzon marinus, using freeze--fracture replicas as well as ultrathin sections. Comparison of these structures with those of other vertebrates was necessary to provide a basis in future studies for characterization of biliary degeneration during metamorphosis. The bile ducts were composed of a simple cuboidal to columnar epithelium with the cells separated by wide lateral intercellular spaces and containing a prominent brush border. Vacuole-like intracytoplasmic cisternae formed a peripheral network within the cells and were confluent with intercellular spaces at the site of numerous pores in the lateral plasma membranes. The cells were joined apically by well developed zonulae occludentes surmounting zonulae adhaerentes. The zonulae occludentes, as observed in freeze--fracture replicas, appeared as a honeycomb-like meshwork. Frequent gaps in P-face ridges suggested a 'leaky' epithelium. The cytoplasm of bile duct cells contained few organelles except for large numbers of mitochondria; many microfilaments were present. The ultrastructural features of those cells reflected an epithelium specialized for absorption and transport and they were similar to cells of the bile ducts in other vertebrates. The general organization of epithelium in the gall bladder resembled that of bile ducts, but intercellular spaces were narrower, peripheral pores and cisternae were absent laterally, and the brush border was less extensively developed at the apical surface. The cytoplasm also contained large pools of glycogen and numerous microfilaments were situated in the apical ectoplasm. The overall appearance of the gall bladder of lampreys suggested that the epithelium was less specialized compared to the water transporting organs of other vertebrate species.
The renal corpuscle of the garter snake was examined using scanning and transmission electron microscopy, freeze-fracture and tracer experiments. The renal corpuscle was small and glomerular architecture simple. The visceral epithelium was composed of irregular, highly branched podocytes with broad, interdigitating foot processes. Numerous cell junctions, including fasciae occludentes and adhaerentes, occurred between podocytes. The podocytes bore numerous microvilli which projected into the urinary space and contained large bundles of microfilaments. The podocytes lay on a basal lamina which appeared to act as a primary filtration barrier.
Freeze–fracture revealed unusual structures of unknown origin in plasma membranes of proximal tubular cells in the nephron of the garter snake. These areas are closely associated with gap junctions and are characterized primarily by the relative paucity of intramembranous particles.
Thin sections and freeze fracture replicas were used to study the structure of the hepatocytes of the parasitic adult lamprey (Petromyzon marinus L.). Despite the absence of bile ducts and bile canaliculi, the hepatocytes have some features which resemble those of cells in the livers of other vertebrates. Hepatocytes are characterised by large gap junctions, many cytoplasmic inclusions, and large deposits of iron. The latter is present throughout the cytoplasmic matrix and within large inclusion bodies which may arise through sequestration of parts of the cytoplasm by membrane isolation. There is no evidence for the involvement of hepatocytes in glucose metabolism but their fine structure reflects the production of bile products and the processing of lipoproteins. The accumulation of bile products within cytoplasmic inclusions resembles the situation resulting from biliary atresia or other cholestatic conditions in higher organisms. There is little folding of the plasma membrane facing the perivascular space (of Dissé), perhaps indicating limited involvement of this surface in the transport of bile products. Nerve endings in close apposition to hepatocytes suggest possible nervous control or metabolic function or the presence of sensory receptors in lamprey liver.
The interlamellar areas of the gills of the anadromous sea lamprey, Petromyzon marinus, undergo structural modification during the process of metamorphosis of larvae to young adults. The initial changes involve a degeneration of presumed ion-absorptive superficial cells of the larvae and this is followed by a differentiation of adult-type chloride cells presumably from intermediate cells. Adult-type chloride cells are characterized primarily by the presence of an extensive tubular reticulum, which is continuous with lateral and basal plasma membranes. The tubules first appear in peripheral regions of intermediate cells and subsequent development involves a proliferation of the tubules toward the interior of the cell. Autoradiography with [ 3 H]thymidine indicates that extensive DNA synthesis occurs synchronously in the interlamellar basal cells throughout the gill filaments. This DNA synthesis and division of both basal and intermediate cells is most common immediately preceding the first appearance of immature chloride cells. This suggests that cells are produced in apparent anticipation of a requirement for adult-type chloride cells and that they subsequently differentiate. The timing of differentiation of chloride cells is well correlated with developmental changes in other organs during metamorphosis.
Abstract The ultrastructure of hepatocytes, bile canaliculi, and hepatic sinusoids of the larval lamprey, Petromyzon marinus , was examined using thinsectioned and freeze‐fractured tissues. The liver is a “tubular gland” with hepatocytes arranged in a tubular fashion around large bile canaliculi. Hepatocytes are roughly conical in shape, with their tapered apices facing a bile canalicular lumen. They possess extensive rough and smooth endoplasmic reticulum, a well‐developed Golgi complex, abundant mitochondria, and varying numbers of large secondary lysosomes. Both secondary lysosomes and the Golgi complex are concentrated in the apical or peribiliary cytoplasm, indicating a possible role in bile secretion. The apical surfaces of the hepatocytes bear numerous elongate microvilli and occasional cilia, which project into the bile canaliculi. The hepatocytes are joined, apically, by junctional complexes composed of zonulae occludentes and adhaerentes. In freeze‐fracture, the zonulae occludentes are of variable apicobasal depth and consist of honeycomb‐like meshworks of fibrils. Spaces of variable width frequently appear in the P‐face grooves, indicating that the zonulae occludentes are “leaky.” Numerous communicating (gap) junctions join the hepatocytes laterally. Varying numbers of lateral microvilli project into the intercellular spaces and, basally, the plasma membrane is deeply infolded, resulting in the formation of apparently interdigitating basal processes resting upon a thin basal lamina. Sinusoids are composed of both a heavily‐fenestrated, continuous endothelium, and phagocytic reticulo‐endothelial (Kupffer) cells. Despite the difference in arrangement of their hepatocytes, the mammalian and lamprey livers show similar ultrastructural features.
The chloride cells in the interlamellar areas of the gills of young adult, anadromous sea lampreys, Petromyzon marinus L., captured in fresh water undergo structural modification during the adaptation of these animals to sea water. In fresh water the chloride cells are partially overlapped by mucus-secreting superficial cells and contain an extensive reticulum of cytoplasmic tubules, which are confluent with both lateral and basal plasma membranes, numerous mitochondria, a Golgi complex of moderate size, and numerous apical vesicles. Adaptation to sea water results in a retraction of the superficial cells, exposing the entire apical surface of the chloride cells, and a proliferation of both cytoplasmic tubules and mitochondria. Extensive enlargement of the Golgi complex in the chloride cells of these animals suggests the involvement of this organelle in the proliferation of cytoplasmic tubules. The extracellular tracer, ruthenium red, enters the tubules from the lateral or basal intercellular spaces in both freshwater- and seawater-adapted animals but never enters either tubules or vesicles from the apical surfaces, indicating that these are not confluent. The presence of dividing basal cells and newly-forming chloride cells, combined with evidence of degeneration of chloride cells, suggests that there is a turnover of this cell type. Both superficial and basal cells are phagocytic and involved in heterophagy of degenerating chloride cells. This phenomenon occurs in both fresh water and sea water indicating that the chloride cells may be functional in both environments.
Abstract Ultrastructural features of the tubular nephron of the garter snake, with special reference to modifications for conservation of water, were studied using transmission electron microscopy, freeze‐fracture and tracer experiments. Although a nephric loop (loop of Henle) is lacking, the tubules appear to be structurally well adapted for efficient ion and water reabsorption. The most prominent features are well developed microvilli in the proximal tubule and elaborate lateral folds, particularly in the distal tubule and collecting ducts. The latter structures are highly interdigitated, creating complex intercellular channels, perhaps facilitating transepithelial fluid transport. Only the proximal tubule actively absorbs and degrades protein tracers from the lumen. The cells of the collecting duct secrete mucus which may precipitate and bind urate salts in the lumen. This may be significant in the excretion of these salts, a process which combines maximal removal of salts and nitrogenous wastes with minimal loss of water.
