Evagination of vascular smooth muscle cells during the early stages of Crotalaria pulmonary hypertension

Chronic pulmonary hypertension--the monocrotaline model and involvement of the hemostatic system

Monocrotaline (MCT) is a toxic pyrrolizidine alkaloid of plant origin. Administration of small doses of MCT or its active metabolite, monocrotaline pyrrole (MCTP), to rats causes delayed and progressive lung injury characterized by pulmonary vascular remodeling, pulmonary hypertension, and compensatory right heart hypertrophy. The lesions induced by MCT(P) administration in rats are similar to those observed in certain chronic pulmonary vascular diseases of people. This review begins with a synopsis of the hemostatic system, emphasizing the role of endothelium since endothelial cell dysfunction likely underlies the pathogenesis of MCT(P)-induced pneumotoxicity. MCT toxicology is discussed, focusing on morphologic, pulmonary mechanical, hemodynamic, and biochemical and molecular alterations that occur after toxicant exposure. Fibrin and platelet thrombosis of the pulmonary microvasculature occurs after administration of MCT(P) to rats, and several investigators have hypothesized that thrombi contribute to the lung injury and pulmonary hypertension. The evidence for involvement of the various components of the hemostatic system in MCT(P)-induced vascular injury and remodeling is reviewed. Current evidence is consistent with involvement of platelets and an altered fibrinolytic system, yet much remains to be learned about specific events and signals in the vascular pathogenesis.

Ultrastructure of rat pulmonary arterioles after neonatal exposure to hypoxia and subsequent relief and treatment with monocrotaline

A group of rats was born in and spent the first 4 weeks of life at a simulated altitude of 3550 m. Two animals were killed immediately afterwards and the remaining 16 were allowed to recover for various times up to a maximum of 12 weeks at sea-level atmospheric pressure. On ultrastructural examination, the pulmonary arterioles of hypoxic rats showed muscularization, the new layer of mature smooth muscle cells containing abundant organelles and myofilaments. These cells were bounded by prominent elastic laminae. During the recovery period, the medial layer became progressively thinned, but the cells still retained some characteristics of smooth muscle by 12 weeks' recovery. When a similar group of ten hypoxic rats was allowed to recover for 12 weeks before being given monocrotaline, there was early enlargement of the residual smooth muscle cells in the media of pulmonary arterioles and within 5 weeks there was again a thick layer of medial smooth muscle. This was in contrast to the sparse, weakly muscularized arterioles seen in eight similarly treated rats born under normoxic conditions. The relevance is discussed of these findings to the rare occurrence of primary pulmonary hypertension in people who were born at high altitude but returned to sea-level during childhood.

Ultrastructure of the lung in chronic hypoxia

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Exploration of the pulmonary circulation. Festschrift to Professor Donald Heath

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The ultrastructure of pulmonary arteries and arterioles in emphysema

The lungs from three cases of pulmonary emphysema obtained at heart-lung transplantation were examined by electron microscopy to determine the origin of intimal longitudinal muscle and the formation of muscular tubes in small pulmonary arteries and arterioles. The earliest change consisted of migration of mature smooth muscle cells from the media of small pulmonary arteries, through gaps in the internal elastic lamina, into the subendothelial space. Most of these cells then adopted a longitudinal orientation, maintained a muscular phenotype, and became enmeshed in a web of elastic fibres. A small minority, immediately subjacent to the endothelium, were orientated circularly and, in some vessels, were enclosed by rudimentary internal and external elastic laminae to form early muscular tubes. Pulmonary arterioles, which are normally devoid of a media, contained several layers of circularly orientated smooth muscle cells, some of which also formed muscular tubes. It is postulated that the limited migration of mature smooth muscle cells seen in states of chronic hypoxia is mediated by a different stimulus from that causing the florid invasion of the intima by immature smooth muscle, with subsequent transformation into myofibroblasts, which characterize plexogenic pulmonary arteriopathy.

The ultrastructure of plexogenic pulmonary arteriopathy

The lungs from 16 cases of plexogenic pulmonary arteriopathy obtained at heart-lung transplantation, half of which had primary pulmonary hypertension, were examined by electron microscopy. From these the probable pathogenesis of pulmonary arterial intimal fibrosis in plexogenic pulmonary arteriopathy was deduced. The earliest detectable change was migration of smooth muscle cells from the media, through the internal elastic lamina into the intima. These cells collected beneath the endothelium and lost many of their myofilaments to become myofibroblasts. They were associated with ground substance but scanty collagen fibrils. As the quantity of interstitial collagen increased, the myofibroblasts reverted to a muscular structure, became elongated, and assumed a regular, circumferential orientation. This later stage coincided with the development of plexiform lesions. At both early and later stages, the muscular pulmonary arteries were contracted but not markedly so, and muscular evaginations were not seen. On the other hand, the cellular intimal proliferations developed early and were occlusive. This suggests that occlusion of small pulmonary arterial vessels by myofibroblasts may be at least as important as vasoconstriction in the early elevation of the pulmonary vascular resistance in primary pulmonary hypertension.

Differentiation of the cardiac and pulmonary toxicity of monocrotaline, a pyrrolizidine alkaloid

Monocrotaline, given to rats as a 20 mg/l solution in drinking water for 3 weeks, doubled the mass of the right heart and lung. The rise in lung mass preceded that of the heart. These increases were accompanied by increases in the absolute protein content of the two organs, together with increases in the rates of both protein and RNA syntheses. The increase in lung mass was not accompanied by a change in total collagen content, as measured by two independent methods: 4-hydroxyproline content and detergent fractionation. In contrast, the right ventricle showed more than a 4-fold increase in total collagen content. Total pulmonary lipids increased by 86%, but the lipid: protein ratio was unchanged. Right ventricular lipids were unchanged in amount but the lipid: protein ratio fell by 29%. Lung DNA:RNA ratio decreased 49% and right ventricle DNA:RNA ratio decreased 69%, indicating that both of these organs were responding to monocrotaline with hypertrophy. These results suggest that the processes of hypertrophy differ in the two organs: in the lung, there was no fibrosis despite a marked increase in dry weight, while right ventricular hypertrophy was characterized by increased collagen deposition. There was no alteration in the left ventricle in any of the parameters investigated.

Electron microscopy of the plexiform lesion

Pulmonary arteries from a case of plexogenic pulmonary arteriopathy were studied with the electron microscope. Many muscular pulmonary arteries showed intimal fibrosis of concentric or "onion-skin" type. The cells embedded within this fibrosis resembled smooth muscle but since they also possessed some features of fibroblasts we refer to them as myofibroblasts. Myofibroblasts also occurred in plexiform lesions together with fibroblasts and "fibrillary cells". These fibrillary cells contained numerous, prominent filaments with a random orientation. They lined the vascular channels of the plexiform lesions as well as being present within the stroma. They appeared to phagocytose fibrin and then to organise the plexiform lesion into a fibrous scar. Fibrillary cells closely resemble vasoformative reserve cells and the cells of the cardiac myxoma and so-called "papillary tumour" of heart valves. They may, therefore, be primitive multipotential cells found throughout the entire cardiovascular system.

The ultrastructural effects of acute decompression on the lung of rats: the influence of frusemide

Rats were placed in a decompression chamber and the chamber and the pressure reduced to 265 mm Hg over a period of 1 hr. Other rats were subjected to the same treatment after having been given an injection of frusemide. An ultrastructural study of the lungs of these rats showed that acute decompression of this magnitude can cause swelling and disintegration of type I alveolar epithelial cells. Although capillary endothelial cells showed only minor cytoplasmic damage many capillaries were ruptured with diapedesis of red cells. This is believed to be caused by gross capillary dilatation associated with pulmonary congestion. Pretreatment with frusemide prevented capillary rupture and reduced the degree of epithelial degeneration. However, frusemide induced swelling of capillary endothelial cells which was unassociated with acute decompression.

The electron microscopy of "fibrinoid necrosis" in pulmonary arteries

Fibrinoid necrosis was induced in the pulmonary arteries of five male Wistar albino rats by feeding them on a diet adulterated by the addition of 0.07% ground Crotalaria spectabilis seeds by weight. Electron microscopy of the arteries affected by the process showed fibrin in the thrombus occluding their lumens and in the arterial intima, held up from further penetration of the media by the inner elastic lamina. Naturally occurring gaps in this lamina were found, and it is postulated that they determine the characteristic histological configuration of fibrinous vasculosis. The smooth muscle cells of the media of the pulmonary veins showed clear evaginations, devoid of myofilaments and organells, indicative of sustained constriction compatible with viability of the cells. In contrast, the smooth muscle cells of the media of the pulmonary arteries showed loss of myofilaments leading to frank necrosis. Other cells seen in the media include fibrocytes and "vasoformative reserve cells." The authors consider that the latter have considerable and varied potential. Once liberated during the process of fibrinoid necrosis in the arterial media they may play an important part in pulmonary vascular pathology as, for example, in the formation of the plexiform lesion.