The nephron of the euryhaline freshwater salmonids is composed of the renal corpuscle and the renal tubules. Throughout much of the renal corpuscle, only the lamina densa separates the fenestrated processes of the endothelial cells from the foot processes of the visceral epithelium. The renal tubule consists of five distinct segments. The neck segment is short and intermittently ciliated; it lacks the mucous cells which appear in the neck segment of some teleosts. The proximal segment bears a dense bruch border and is both structurally and functionally divisible into a first and a second segment. The first portion is typified by the presence of short apical tubules, variously sized apical vacuoles, and numerous lysosomes. The second proximal segment is distinguished by the abundance and distribution of mitochondria throughout the cytoplasm. Infoldings of the basilar plasmalemma are especially prominent in this region. A cilated intermidate segment intervenes briefly between the proximal and distal portions of the tubule. The distal segment consists of cuboidal cells which bear scattered, short microvilli, small vesicles and multivesicular bodies. Renal tissue from several species of trout was examined in order to establish the basis for a common pattern of histological and ultrastructural characteristics within the family Salmonidae. In all species examined, renal structure was very similar and could readily be compared with that previously described in other freshwater and marine species.
In previous studies, a marked diminution in left coronary flow during systole has been demonstrated. This phenomenon has been said to result from capillary compressin by the contraction of myocardium and perhaps also from active changes in the state of the smooth muscle in the wall of the coronary vessels. In order to determine the morphological factors involved in this occurrence, and to gain a three-dimensional concept of the myocardial microvasculature, casts were prepared by perfusion of the coronary arteries with methyl methacrylate. Because of the conditions of the perfusion, these casts were considered to demonstrate a state of muscular contraction. Following maceration, portions of the casts were prepared for scanning electron microscopy (SEM). Arterioles of larger diameter were characterized by the helical configuration of the vascular replicas. Smaller vessels were categorized according to their directional pattern and their diameter. The arterioles gave rise to numerous large-diameter capillaries and also to short vessels that were guarded by sphincters. True capillaries, in some cases guarded by precapillary sphincters, were packed in densely stacked parallel sheets with numerous short anastomotic connections between them. Postcapillary venules were found to enter at regular intervals into larger venules arranged perpendicular to the capillary sheets. Most of these characteristics were found to be consistent throughout the microvasculature. These structural features were considered to be factors in the active regulation of blood flow through the myocardial microvasculature.
The use of methyl methacrylate corrosion casts prepared for portions of the vascular system has made it possible to examine numerous and extensive areas of microscopic structures on a 3-dimensional scale with the scanning electron microscope. By this means we have examined the arterial microvasculature of intracranial vessels among three domestic animal species. In addition, these vessels have been compared with the terminal branches of abdominal arteries in the dog. The results of this study suggest that the sphincteric control mechanisms of the vessels in the two regions may be structurally different from one another. In the case of the intracranial vessels, the terminal portion of the arteriole is continued by a precapillary arteriole composed of a chain-like series of muscular constrictions. This is most suitably described as a precapillary sphincter area, which terminates at the capillary. In the abdominal vessels, the precapillary arteriole is generally followed by a single precapillary sphincter at the origin of the capillary. These morphological characteristics may account, in part, for the difference in response of vessels in these two regions in hypovolemic shock.
The use of methyl methacrylate corrosion-casts has made it possible to examine the intracranial microvasculature on a three-dimensional scale with the scanning electron microscope. By this means we have compared regions of four cerebral and cerebellar arteries among three domestic animal species. The results of this study suggest that there are from one to three different levels of interarteriolar anastomosis between branches of the same or adjacent vessels. In the horse and ox anastomoses were demonstrated (1) at the level of the precapillary arterioles, (2) along the arterioles, and (3) between small pial arteries. In the dog only the first-named anastomoses were evident in this study. These morphological characteristics may explain in part, the shunting mechanism by which hypoxia may be reduced among intracranial capillary networks.
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