Electron microscopic study of the open circulatory system of the shrimp,Caridina japonica. I. Gill capillaries

Absence of endothelium in invertebrate blood vessels: significance of endothelium and sympathetic nerve/medial smooth muscle in the vertebrate vascular system

In the course of evolution, two remarkable changes seem to have occurred in vertebrate circulation: the appearance and development of the "endothelium or endothelial tubular system" and "sympathetic nerve/medial smooth muscle system". In the present article, some relevant literature is reviewed and discussed. Absence of endothelium in the vascular wall of most invertebrates had been known and was confirmed by recent electron microscopic studies. The medial smooth muscle is rather proper to vertebrate vessels. It seems to have appeared after emergence of and in association with the endothelial tubular system. Phylogenetically, the parasympathetic nervous system is thought to be older than the sympathetic system. The former is distributed to viscera and the latter developed in close relation with the vascular system. It is assumed that during evolution, a circulatory system composed of the heart and endothelial tubular system first formed in vertebrates, medial smooth muscle then appeared for regulation of the system, and innervation of the muscle tissue took place. This sequence of development assumed for phylogenesis is actually realized in the ontogenetic processes. We thus propose a hypothesis that the "sympathetic nerve/medial smooth muscle system" may be regarded as a new neuroeffector mechanism that developed for systemic regulation of the endothelium-lined closed vascular system in vertebrates. A few implications of the hypothesis are presented.

Ultracytochemical location of Na(+)/K(+)-atpase activity and effect of high salinity acclimation in gill and renal epithelia of the freshwater shrimp Macrobrachium olfersii (Crustacea, Decapoda)

Accumulation sites of lead phosphate reaction product consequent to Na(+)/K(+)-ATPase activity in gill and renal epithelia of the freshwater shrimp Macrobrachium olfersii were located ultracytochemically by para-nitrophenyl-phosphate hydrolysis and lead precipitation, and quantified per unit membrane area and cytoplasmic volume. In shrimps in freshwater (<0.5 per thousand S, 20 mOsm/kg H(2)O, 0.7 mEq Na(+)/liter), numerous sites of electron-dense, Na(+)/K(+)-ATPase reaction product accumulation were demonstrated in the membrane invaginations of the mitochondria-rich, intralamellar septal cells (12.5 +/- 1.7 sites/microm(2) membrane, 179 +/- 22 sites/microm(3) cytoplasm, mean+/- SEM, N </= 7) and in the basal region of the medial renal tubules (19.8 +/- 1.8 sites/microm(2) membrane, 437 +/- 53 sites/microm(3) cytoplasm), but not in the pillar cells whose apical flanges form the primary interface with the external medium. A putative, ouabain-insensitive Na(+)- or H(+)-ATPase was found in the apical microvilli of the medial renal tubules (17.4 +/- 1.7 sites/microm(2) membrane, 629 +/- 101 sites/microm(3) cytoplasm). This restricted location of Na(+)/K(+)-ATPase activity within the gill epithelium suggests that during uptake, Na(+) moves across the apical pillar cell membrane, passes through specialized, basolateral coupling junctions into the septal cell cytoplasm and is pumped into the hemolymph via the Na(+)/K(+)-ATPase in the invagination membranes. In shrimps acclimated to a high-salinity medium (21 per thousand S, 630 mOsm/kg H(2)O, 280 mEq Na(+)/liter) for 2 and 5 days, the mean number of sites of para-nitrophenylphosphatase activity/microm(2) membrane and /microm(3) cytoplasm for both epithelia increases markedly by 83 and 163%, respectively. However, after 10 days acclimation, the number of sites declines dramatically, attaining values far below those for shrimps in freshwater. These acclimation-induced alterations in numerical density/microm(3) cytoplasm cannot be accounted for by corresponding changes in membrane surface density (microm(2) membrane/microm(3) cytoplasm) and reflect a real alteration in the number of Na(+)/K(+)-ATPase reaction product sites/unit membrane area. These data suggest that neither the gill nor the renal Na(+)/K(+)-ATPase systems function at maximal activity in shrimps in freshwater, possibly due to the low Na(+) concentration, and are initially stimulated by the increase in external ionic concentration. However, these powerful Na(+) transport systems respond to salt loading by a notable reduction in the number of hydrolysis sites, possibly through the incorporation of the Na(+)/K(+)-ATPase into isolated membrane vesicles in the basal invaginations of the medial renal tubules, together with ultrastructural alterations like the spatial isolation of the mitochondria by multiple membrane stacks in the intralamellar septal cells. J. Exp. Zool. 284:617-628, 1999.

A morphological study on gills of a crab acclimated to fresh water

The gills of the fully euryhaline Chinese crab Eriocheir sinensis were studied by light and electron microscopy. In these Phyllobranchiates, the gills consist of a double row of lamellae extending laterally from a central shaft. Haemolymph flow pattern inside the gill is described and the existence of a complex secondary vascularization inside the platelets is reported. It is shown that important differences exist between the ultrastructure of the three anterior and the three posterior pairs of large gills. The epithelium of the posterior gills is much thicker and possesses an extensive elaboration of the plasma membranes in the form of infoldings, crypts and interdigitations, along which are packed numerous mitochondria. The presence of such a complex membrane system opening to the extracellular space and closely associated with mitochondria is common to all salt-transporting tissues. This study corroborates the idea that the posterior pairs of gills of Eriocheir sinensis are the only ones implicated in active Na+ uptake when the crab lives in dilute aquatic environment. The epithelium of anterior gills is much thinner and the cells poor in intracellular organelles. It seems to be involved essentially in respiration. Thus this work clearly corroborates the existence already suggested by physiological approach of a functional difference between the different pairs of E. sinensis branchiae with respect to their participation in the respiration and in the regulation of the blood ions content. Common to both types of gills is the presence of a lamellar septum separating the haemolymph space into two compartments. The part played by that structure in determining the pattern of haemolymph flow, together with periodic bridges forming pillars across the haemolymph space, is emphasized.

A proposed solution to a fine-structural puzzle: the organization of gill cuticle in a crayfish (Panulirus)

Crayfish gill cuticle is approximately 2 micron thick and comprises an epicuticle and an endocuticle, which is subdivided into outer and inner layers. Sections demonstrate indistinct lamellae in the outer endocuticle and vertically striated lamellae in the inner endocuticle. Microfibrils cannot be seen in sections. Difficulties in interpretation of the fibrous architecture of the cuticle from thin sections have been overcome by examining tilted series of micrographs of sections and also by making freeze-fracture replicas of the cuticle, which reveal the microfibrils clearly. A model for the endocuticle based on a helicoidal configuration of microfibrillar laminae is proposed and the vertically striated structures seen in sections of the outer layer are accounted for by including regular rows of particles oriented perpendicular to microfibrils. The model is compared with cuticles and coverings reported from other invertebrates.

A morphological study on gills of the brown shrimp, Penaeus aztecus

The gills of Penaeus aztecus were examined by light and electron microscopy. They are dendrobranchiate, consisting of a central axis with biserially arranged branches that subdivide into bifurcating filaments. A septum divides the lumina of these structures into afferent and efferent channels. Hemolymph from the sternal sinus flows through the afferent channels into the filaments where it is directed into the efferent channels and finally to the pericardial cavity. In addition to these channels, numerous blood vessels permeate the gill. The cuticle covering the gill overlies a thin epithelium which is separated from hemolymph by a basal lamina. The epithelium, which is active in cuticle secretion, has a series of pillar processes that form subcuticular lacunae. The apical membranes of epithelial cells become folded in shrimp exposed to hypo- and hyperosmotic salinities. Granular cells that contain elaborate Golgi apparati and several types of granules are present throughout the gill. Nephrocytes resembling glomerular podocytes line the efferent channels. A large nerve traverses the septum in the axis.