Johannes A.G. Rhodin

Ultrastructure of mammalian venous capillaries, venules, and small collecting veins

A combination of light microscopical and electron microscopical techniques were used in this investigation of the venous drainage system of the subdermal microvessels of rabbit skin. The vessels were sectioned longitudinally to facilitate analysis of different segments of the venules. By using as criteria the luminal diameter, the perivascular elements, and the relationship of the vessel to the local vascular pattern, the venous part of the microcirculatory bed is divided into the following segments. (a) Venous capillary: capillaries with a diameter of up to about 8 μ which can be identified as being continuous with the larger postcapillary venules. The endothelium is usually thin, and occasionally displays fenestrations. Occasional pericytes occur. The ratio of luminal diameter to vascular wall thickness is about 20:1. (b) Postcapillary venule: microvessels with a diameter of about 8–30 μ , continuous with venous capillaries. With increasing luminal diameter, these vessels show a gradual increase in the number of pericytes and veil cells (fibroblasts). The ratio of luminal diameter to vascular wall thickness averages 10:1. (c) Collecting venule: 30–50 μ . Vessels with one complete layer of pericytes and a complete layer of veil cells. Occasional primitive smooth muscle cells occur. The ratio of luminal diameter to vascular wall thickness is 30:1. (d) Muscular venule: 50–100 μ . Nearly all periendothelial cells are smooth muscle cells that overlap and sometimes form two layers. Myoendothelial junctions occur, but are rare. The ratio of luminal diameter to vascular wall thickness is 50:1. (e) Small collecting veins: 100–300 μ . Vessels with a prominent media consisting of continuous layers of smooth muscle cells. The ratio of luminal diameter to vascular wall thickness averages about 100:1.

The ultrastructure of mammalian arterioles and precapillary sphincters

A combination of techniques was utilized whereby the monolayer of microcirculatory vessels of the fascia of the rabbit medial thigh muscles was studied in vivo and the same site subsequently was fixed in situ for both light and electron microscopy. The microvessels were sectioned longitudinally and the identical segment was analyzed by phase contrast microscopy as well as by low and high magnification electron microscopy. Dilated arterioles with an inner diameter of less than 100 μ, terminal arterioles, and precapillary sphincters were studied with special reference to the organization of the endothelium, the muscular and connective tissue components, and the nervous supply. A system of myoendothelial junctions was discovered which establishes close membranous contacts between endothelium and the media of the terminal arterioles and the precapillary sphincters. The functional implications of such a system is discussed, and it is proposed that it may be part of a receptor mechanism for humoral transmitter substances.

The ultrastructure of the adrenal cortex of the rat under normal and experimental conditions

The adrenal glands of rats were fixed by intra-arterial perfusion of glutaraldehyde and osmium tetroxide under normal conditions and after short-term administrations of Dexamethasone (a synthetic glucocorticoid) and adrenocorticotropic hormone (ACTH). Sections of the entire cortex and part of the medulla were analyzed in the electron microscope at magnifications ranging between 200 × and 60 000 ×. This allowed for great accuracy in identifying the zones of the cortex. In all cases, blood vessels and sinusoids remained open and the fenestrated endothelium was well preserved throughout the cortex and the medulla. The parenchymal cells were also well preserved, including those of the zona reticularis, and signs of cell death were no more common in one or the other zone. The mechanism of accumulation and depletion of lipid droplets was studied, particularly in the zona fasciculata. Under the influence of Dexamethasone, lipid droplets accumulated in large numbers 25 hours after administration. The ACTH effect was evidenced 10 minutes after injection by a disappearance of lipid droplets and by a reduced amount of smooth endoplasmic reticulum. The lipid droplets appeared to arise either from the smooth endoplasmic reticulum or from Golgi elements. As the droplets enlarged, they became surrounded by an increasing number of profiles of smooth endoplasmic reticulum. The profiles merged to form a continuous membranous casing near the boundary membrane of the droplet. As a final step, the peripheral part of the membranous casing fused with the surface membrane of the cell. The content of the lipid droplet was discharged into the subendothelial space together with the boundary membrane and the central part of the casing of the smooth endoplasmic reticulum. A membranous ghost remained temporarily. It is proposed that this represents the elaboration and release of corticosteroid hormones by a modified apocrine secretion which we would term endoplasmocrine secretion for the purposes of satisfactorily distinguishing it from other modes of endocrine and exocrine secretion.

The ultrastructure of mouse lymph node venules and the passage of lymphocytes across their walls

Postcapillary venules of mouse lymph nodes are vessels situated between capillaries in the cortex and paracortex and efferent veins in the hilar region. They are characterized by a high endothelium. Their vessel diameters and periendothelial ultrastructure are similar to these same elements in the postcapillary venules of other tissues. The high endothelial cells are variable in shape. Their well-developed organelles are indicative of intense synthetic activity. Distinctive particles, multivesicular bodies with dense matrices, are associated with a prominent Golgi complex. An endothelial cell coat is present at the luminal surface. Many lymphocytes, singly or in groups, are within the venule walls. The majority have traversed the endothelium and lie adjacent to the pericytes, which constitute the major obstacle to lymphocyte emigration from the venules. Most lymphocytes that have not yet passed the endothelial layer lie between two or more endothelial cells. Analysis of serial sections supports the contention that lymphocytes pass between, rather than within endothelial cells.

ULTRASTRUCTURAL AND AUTORADIOGRAPHIC STUDIES OF ERYTHROPOIETIN-INDUCED RED CELL PRODUCTION*

Many aspects of hematopoiesis remain unclear, such as the chemical and structural nature of events that initiate the process of blood cell maturation. A better understanding of these early events would provide a new basis for explaining how formed elements of the blood emerge to replace aging cells and might clarify one widely held theory, i.e. the derivation of mature elements from “totipotent” stem cells. To this end, several problems of blood cell origin are dealt with in this study. The most important considerations constitute an effort to characterize the initial features of erythropoietin-induced red cell maturation. There is little definitive information on the mode of erythropoietin action. Some investigations point to the late stages of erythroid maturation as those responsive to erythropoietin (Borsook et al., 1962; Gallagher et al., 1963); others to a “primitive” precursor cell as the target element (Alpen & Cranmore, 1959; Erslev, 1964). Similarly, there is considerable disagreement concerning the characteristics of the molecular changes that occur in cells during their response to this factor. Literature can be cited to support the idea that erythropoietin enhances the rate of hemoglobin synthesis (Stohlman et al., 1963), that it leads to a shorter maturation period (Brecher & Stohlman, 1962), or that it initiates the process of erythroid cell differentiation (Kurtides et a!., 1963; Krantz & Goldwasser, 1965). In adult mice splenic red cell production can be altered by changing the environmental or humoral conditions, e.g. lowered oxygen tensions being countered by increased erythropoiesis (Brecher & Stohlman, 1959). Conversely, when the hypoxic stimulus is removed. polycythemic mice show a markedly inhibited erythropoietic activity. In this state virtually no definitive nucleated erythroid elements are seen in the spleens. At this time such spleens are sensitive to exogenous erythropoietin and respond to a single injection by producing a cohort of erythroid elements that undergo a synchronized process of maturation. Histological preparations of splenic tissue reveal the presence of proerythroblasts 24 hr after activation, and erythroblasts at 48 hours. Peak peripheral reticulocytosis occurs between 72-96 hr, after which the erythroid wave subsides (Filmanowicz & Gurney, 196 1 ) . The present study, undertaken to identify the morphological and chemical bases of erythropoietin-induced activity, extends our earlier autoradiographic findings (Orlic et al., 1965a), which for the first time, indicated that the stem cell response to erythropoietin involves an almost immediate or simultaneous incorporation of 3H-thymidine. In addition, this work provides information on the molecular and ultrastructural changes that occur during the early hours of response, as well as later stages when erythropoietin-activated transitional cells are replaced by more mature red cell elements. Identification of erythropoietin-sensitive stem cells is

The ultrastructure of calf heart conducting fibers with special reference to nexuses and their distribution.

The fibers of the conducting system of the calf heart have been examined by electron microscopy. The individual cells comprising the fibers and the nature of their cell contacts were studied with particular attention given to the incidence and frequency of nexuses. Nexuses were found to occupy 3.3% of the surface of the 27 cell borders which were measured. The frequency of nexuses did not vary whether the cell border was from the periphery or the center of the fiber. Other junctional specializations found were desmosomes, fasciae adherentes, and junctions exhibiting degrees of structural differentiation which were between these two. These various junctional complexes comprised approximately 45% of the cell borders and occurred with equal frequency and in no particular sequential order. The fibers were surrounded by dense connective tissue, including occasional elastic fibers. Individual cells contained varying numbers of myofibrils, both peripherally and centrally, as well as large numbers of glycogen particles and scattered filaments. Among the mitochondria was found a type which had one central tubular crista. Occasional lipofuscin granules were seen. The role of the nexus and other factors in the conduction of impulses in the conducting system is discussed.

Fine structure of elasmobranch arteries, capillaries and veins in the spiny dogfish, Squalus acanthias☆

Abstract The ultrastructure of the intestinal vasculature of the spiny dogfish is described and compared with blood vessels of similar caliber in mammals. Arteries and arterioles have a great resemblance to their mammalian counterparts, including the so-called myoendothelial junctions of the arterioles. The capillaries, venules and veins differ somewhat. One example is the lack of a basal lamina in the connective tissue capillaries and venules of the spiny dogfish. Functional similarities and differences are discussed in the light of the ultrastructural architecture of the blood vessels described.

Fixation of microvasculature for electron microscopy.

Abstract Small blood vessels in mammals can be fixed chemically for electron microscopy either by topical application of the fixative or by intra-arterial perfusion. These two methods of chemical fixation are described in detail. Advantages and disadvantages are discussed.