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EURASIAN JOURNAL OF MEDICAL AND NATURAL SCIENCES
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HISTOLOGY OF THE BLOOD VASCULAR SYSTEM:
I
f STRUCTURE, FUNCTION, AND CLINICAL IMPLICATIONS
Ergasheva Husnora Fakhriddinovna
Assistant at the Alfraganus University Email: [email protected] Orcid Id: 0009-0003-6262-5145 https://doi.org/10.5281/zenodo.14545936
ARTICLE INFO
Received: 16th December 2024 Accepted: 22th December 2024 Online: 23th December 2024
KEYWORDS Tunica intima, tunica media, tunica externa, atherosclerosis, vasculitides, diabetic
neuropathy, capillaries,
endothelial cells, vascular pathophysiology, thrombotic microangiopathy, Takayasu arteritis, vascular diagnostics.
ABSTRACT
The vascular system is a vital component of human physiology, serving as a network for the transportation of oxygen, nutrients, hormones, and metabolic waste products. Its intricate structure and functions are closely tied to its role in maintaining homeostasis and adapting to the diverse needs of tissues and organs. The vascular system is composed of three primary layers—tunica intima, tunica media, and tunica externa—each of which varies in structure and function based on the vessel type, including arteries, veins, and capillaries. Arteries deliver oxygen-rich blood under high pressure, veins return deoxygenated blood while serving as a reservoir for 70% of blood volume, and capillaries facilitate the exchange of gases, nutrients, and waste at the tissue level. Advances in histological techniques, including hematoxylin and eosin (H&E) staining, Verhoeff staining, and electron microscopy, have enhanced our understanding of vascular morphology and its functional adaptations. Pathophysiological conditions, such as atherosclerosis, vasculitides, and diabetic neuropathy, highlight the vascular system's susceptibility to various diseases. Atherosclerosis, characterized by fatty plaque formation, represents a significant health burden due to its implications for blood flow and thrombotic risks. Additionally, conditions like thrombotic microangiopathy, Takayasu arteritis, and familial hemorrhagic telangiectasia emphasize the diverse clinical manifestations of vascular dysfunction. Emerging research, including the use of immunofluorescent markers like CD31 and advanced imaging methods, continues to provide insight into vascular pathologies and potential therapeutic interventions.
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Understanding the vascular system's intricate structure, function, and disease associations is critical for advancing diagnostics, treatment strategies, and regenerative medicine approaches.
Introduction
Blood vessels are vital components of the cardiovascular system, playing a key role in the dynamic transportation of essential substances and blood products to every cell in the body. The vascular network originates at the heart's outlets, extends throughout the body, and returns at the heart's major venous inlets. This intricate vascular system facilitates the delivery of oxygen, nutrients, blood cells, and pharmacological agents to tissues. Simultaneously, it ensures the removal of cellular byproducts, carbon dioxide, and toxic substances from tissues, maintaining systemic balance.
Histologically, the vascular system is categorized into two components: macrovasculature and microvasculature. Macrovasculature includes vessels that are visible to the naked eye, while microvasculature comprises vessels smaller than 100 microns, requiring microscopic examination for observation.
Issues of Concern
Pericytes are critical cells that support the health, development, and function of blood vessels. Initially discovered by Rouget in the 19th century and later named "pericytes" by Zimmermann, these cells have distinct roles depending on their location. In the brain and retina, they are essential for forming the blood-brain barrier and blood-retinal barrier, respectively. In the kidneys, they play a pivotal role in the filtration process of the glomeruli, while in the liver, they are crucial for extracellular matrix remodeling.
Recent studies highlight their additional roles in immune response and phagocytic activity within blood vessels. Notably, the structure and function of blood vessels vary based on their anatomical location and the specific demands of the surrounding tissue, making their study highly context-dependent. These differences emphasize the necessity of tailored approaches when investigating the histology and function of blood vessels in different organ systems.
Structure
The structure of the vascular system's vessels is highly specialized to meet their individual functional and anatomical requirements. This specialization is evident in the three shared layers of the vessel wall. The outermost layer, the tunica externa or adventitia, is responsible for maintaining vessel integrity and resisting mechanical strain. This layer contains the vasa vasorum and nervi vasorum, which are vessels and nerves that supply the cells of the vessel wall itself, earning them the nicknames "vessels of the vessel" and "nerves of the vessel." Collagen in the tunica externa is essential for anchoring the vessel to adjacent tissues and structures.
The middle layer, the tunica media, is generally thicker in arteries than in veins and is composed of transversely arranged smooth muscle cells. These cells regulate the vessel's lumen diameter, allowing for changes in blood flow and pressure. The thickness of the tunica media varies significantly between vessel types, being well-developed in arteries and almost negligible in some veins. It also contains elastic lamellae and is surrounded by the external elastic membrane, which separates it from the tunica externa.
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The innermost layer, the tunica intima, directly interacts with the contents of the lumen. It consists of a single layer of simple squamous endothelial cells supported by a basal lamina. In some vessels, the tunica intima includes a subendothelial layer composed of smooth muscle, collagen, and elastic fibers, varying in thickness. This layer is enclosed by the internal elastic membrane, which separates it from the tunica media. The tunica intima is thickest in arteries, particularly muscular arteries, and thinner in veins.
Although these three layers are common to all blood vessels, they undergo modifications—known as segmental differentiation—to adapt to their specific roles within the vascular system. This differentiation begins with the arterial system at the heart's outlets, where blood flows from large arteries to progressively smaller vessels.
Large arteries, or elastic arteries, such as the aorta and pulmonary arteries, have a thick tunica intima with prominent smooth muscle in the subendothelial layer. The tunica media in these arteries is primarily composed of smooth muscle, responsible for secreting ground substance and regulating lumen diameter. The adventitia in large arteries is relatively thin, accounting for less than half of the vessel wall's total thickness, and contains the vasa vasorum and nervi vasorum, which transport nutrients, oxygen, and waste to and from the vessel wall.
Muscular arteries, or medium arteries, are characterized by a thick tunica media made up of spiral or transverse smooth muscle. Their subendothelial layer is so thin that it is often difficult to visualize in histological images, giving the appearance of the media abutting the endothelium.
Small arteries and arterioles have one or two layers of smooth muscle cells, which play a critical role in increasing peripheral resistance. Unlike larger arteries, arterioles lack an external elastic membrane and lead into the capillary network.
Capillaries, the smallest vessels in diameter (4-10 microns), have the simplest structure, consisting of an endothelial layer surrounded by connective tissue. Pericytes, located within the basement membrane, are essential for neurovascular signaling and regulating blood pressure. Capillaries are highly specialized based on their function and anatomical location and can be classified into three types: continuous, fenestrated, and discontinuous. Continuous capillaries, found in muscle, connective tissue, skin, lungs, and the central nervous system, have uninterrupted endothelium and a continuous basal lamina, facilitating selective transcytosis. Fenestrated capillaries, located in endocrine organs, kidneys, and the intestinal tract, have circular fenestrations that enhance exchange. Discontinuous capillaries, found in the liver, spleen, and bone marrow, have large gaps in both the endothelium and basal lamina, allowing the passage of large molecules and cells.
Blood begins its return to the heart through the venous system, starting with venules. Venules, which have diameters up to 0.1 mm, are directly connected to capillary beds and contain sparse subendothelial elastic fibers. Pericytes within the basement membrane contribute to signaling. As venules develop a more defined tunica media further upstream, they are classified as muscular venules.
Small veins, with diameters between 0.1 mm and 1 mm, are the first class of veins to exhibit all three tunics. By this stage, pericytes are no longer present. Medium veins, which can reach diameters of up to 1 cm, include many named veins and are defined by the presence
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of valves that prevent retrograde blood flow. These valves are especially common in the lower extremities. Medium veins have a thin tunica media compared to arteries and a thick tunica externa composed of collagen and elastin fibers.
Large veins, such as the portal vein, inferior vena cava, and superior vena cava, have diameters exceeding 1 cm. These veins lack a clear distinction between the tunica intima and tunica media and do not have an internal elastic membrane. Their tunica externa is thick and contains longitudinal smooth muscle, providing structural support. Unlike arteries, the distinct layers in veins are less pronounced and can vary significantly among individuals.
Function
The function of a blood vessel is closely tied to its structure and anatomical location. Despite their differences, arteries and veins share general roles in circulation. Arteries are primarily responsible for delivering oxygenated blood and nutrients to tissues throughout the body, while veins return deoxygenated blood and metabolic waste to the heart and excretory organs. Veins, equipped with one-way valves, can effectively move blood against gravity, despite their limited contractile ability. Their high compliance, due to the absence of significant smooth muscle, allows veins to accommodate approximately 70% of the total blood volume.
Arterioles, often referred to as "resistance vessels," play a critical role in generating peripheral resistance. By contracting the smooth muscle layers in their tunica media, arterioles create significant resistance across a broad parallel network. This function is essential for protecting capillaries and venules from high-pressure blood flow, ensuring a controlled and efficient microcirculation.
Capillaries, the smallest and most abundant vessels in the vascular system, are integral to material exchange between blood and tissues. Their expansive parallel network supports efficient diffusion and filtration. Fenestrated capillaries, found in endocrine organs, the gastrointestinal tract, and kidneys, facilitate selective filtration of luminal contents through small openings in their endothelial cells. Pinocytotic vesicles within endothelial cells further enhance the uptake of specific substances. These fenestrations and vesicles, visible only under an electron microscope, distinguish fenestrated capillaries from discontinuous capillaries.
Discontinuous capillaries, characterized by large and irregular gaps, are specialized for the transport of blood and large proteins. Found in the liver, spleen, and bone marrow, they enable the movement of molecules that other capillaries cannot accommodate.
Pericytes, also known as Rouget cells, are critical components of the microvascular wall, found within the basement membrane of capillaries and venules. These circumferentially arranged cells communicate with endothelial cells through direct contact and paracrine signaling, playing a vital role in endothelial cell maturation, maintaining the integrity of the blood-brain barrier, and regulating microvascular blood flow. Pericytes also contribute to clearing cellular debris and supporting neurovascular health.
The blood-brain barrier, a unique feature of the vascular system, provides protection for the brain by insulating its parenchyma from excess edema and limiting the exchange of chemicals. This barrier is maintained by tight junctions between endothelial cells and a denser basement membrane, ensuring a controlled environment essential for proper brain
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function. The strict regulation of chemical traffic and fluid dynamics highlights the vascular system's adaptability to the specific demands of different tissues.
Tissue Preparation
The preparation of blood vessels for histological visualization commonly involves routine hematoxylin and eosin (H&E) staining. Initially, the vessel specimen is fixed in a formalin solution for 24 to 48 hours to preserve the tissue structure. Following fixation, the specimen is embedded in paraffin wax to facilitate sectioning. This process begins with dehydration using graded alcohol solutions, followed by clearing the alcohol with xylene. The dehydrated specimen is then placed in a paraffin mold with sufficient wax to secure the section. Once embedded, a cassette top is added to the mold and filled with paraffin, which rapidly cools and solidifies the block.
The embedded vessel specimen is then sectioned using a rotary microtome, producing extremely thin slices to allow light to pass through for microscopic examination. After sectioning, the specimen is de-waxed through progressive alcohol rinses and hydrated with water to prepare the tissue for staining. Hematoxylin, combined with a mordant aluminum salt, is applied to stain the nuclei of the tunics a red hue. The specimen is rinsed with water, a process known as "bluing," which involves exposure to a weakly alkaline solution, converting the hematoxylin to a dark blue color. The tissue is then inspected to ensure proper nuclear staining and contrast.
Subsequent steps involve de-staining with weak acid alcohol, followed by another water rinse to repeat the bluing process. The tissue is counterstained with aqueous or alcoholic eosin Y, which imparts a pink hue to non-nuclear elements. The specimen is dehydrated again through a series of alcohol changes, with the remaining alcohol removed via xylene rinses. The final result is a transparent slide ready for light microscopy.
An alternative staining method, Verhoeff stain, is used to specifically visualize elastic tissue within the vessel wall. While the preparation process is similar, this method employs a combination of hematoxylin, iron (III) chloride, and iodine for staining. The specimen is then counterstained with picric acid and fuchsin to highlight collagen, providing contrast against the hematoxylin-stained elastin.
For larger vessel sections, the en face preparation method offers an alternative approach, allowing for the examination of extensive areas of blood vessel surfaces, such as the aorta. This method is particularly useful for analyzing endothelial cell organization along the lumen and identifying specific lesion locations. It is often combined with immunofluorescent staining for enhanced visualization and detail.
Histochemistry and Cytochemistry
CD31, a platelet endothelial adhesion molecule, is widely utilized in immunofluorescence to identify endothelial cells under microscopy. This marker is crucial in assessing tumor angiogenesis, providing insight into the rate of tumor growth. Additionally, CD31 is instrumental in identifying vascular tumors such as angiomas and angiosarcomas, both of which retain this adhesion molecule.
Light Microscopy
The structural organization of blood vessel walls is clearly visible under light microscopy, particularly in large vessels. The tunica intima appears as a single layer of simple
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squamous cells. Surrounding the intima, the internal elastic membrane is evident as a dark basophilic, undulating line adjacent to the lumen. The tunica media is prominent in some vessels, with the cytoplasm of smooth muscle cells staining pink and the nuclei appearing blue, elongated, and spheroid-like. As smooth muscle lacks the organized actin and myosin filaments found in striated muscle, no striations are observed.
The tunica externa is also discernible outside the external elastic membrane, which stains deeply basophilic. This layer is particularly pronounced in large vessels, such as the inferior vena cava, where it constitutes the thickest portion of the vessel wall. Wavy bundles of eosinophilic collagen fibers, interspersed with smooth muscle cells and nuclei, are visible within the externa.
Electron Microscopy
Electron microscopy reveals ultrastructural details of blood vessels that are not observable with other techniques. Endothelial cells, often challenging to distinguish at lower magnifications, become readily visible, as do their morphological responses to mechanical forces like shear stress. When subjected to increased shear stress, endothelial cells elongate and narrow, reflecting their adaptability to mechanical strain.
Electron microscopy also provides detailed views of the structural differences among capillary subtypes—continuous, fenestrated, and discontinuous—that are otherwise indistinguishable. At this level of magnification, pericytes, which are small cells located in capillaries and venules, are clearly visible. These cells play critical roles in regulating endothelial cell maturation and neuroendocrine signaling within the microvascular environment.
Pathophysiology
The extensive network of blood vessels plays a direct or indirect role in numerous pathological conditions, with atherosclerotic disease being one of the most prevalent. This condition begins when low-density lipoprotein (LDL) is absorbed by endothelial cells lining the vessels. As LDL accumulates in the tunica intima, macrophages work to digest the oxidized lipid. When LDL levels in the intima become excessive, macrophages are overwhelmed and transform into foam cells, leading to the formation of a fatty streak within the subendothelial space of the intima. Inflammatory cytokines trigger the proliferation of smooth muscle cells in the tunica media, which secrete a fibrous cap that encapsulates the fatty streak. These changes to the vascular wall increase the risk of thrombotic and embolic events, restrict blood flow, and can eventually result in complete occlusion. In severe cases, fatty streaks may become so prominent that they are visible to the naked eye in gross specimens.
In response to stress or inflammation, cytokines can cause separation of the endothelial lining, allowing fluid to leak into the surrounding tissues. This process involves endothelial cell retraction, which permits otherwise impermeable large molecules to escape into the extravascular space. This hyperpermeability, combined with angiogenic growth and endothelial proliferation, is a key feature in the pathophysiology of vascular tumors such as angiomas and angiosarcomas.
Diabetic neuropathy is believed to result from sclerotic changes and increased resistance in the small blood vessels that supply peripheral nerves, known as the nervi
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vasorum. These vascular changes impair blood flow to nerves, contributing to neuropathy and associated symptoms.
On histological examination, myocardial fibers may sometimes extend into the walls of the pulmonary veins and vena cava. These extensions, known as myocardial sleeves, represent continuations of myocardial tissue into the vessel wall and can serve as potential foci for the development of atrial fibrillation. These structural and functional changes highlight the vascular system's involvement in both localized and systemic disease processes.
Clinical Significance
The vascular system, due to its critical role and extensive presence throughout the body, is associated with a wide range of diseases and pathological conditions.
• Atherosclerosis: This condition involves the deposition of fatty plaques in the walls of blood vessels, leading to restricted blood flow and increased risk of thrombotic events.
• Arteriolosclerosis: Characterized by the hardening and loss of elasticity in small arteries and arterioles, this condition is commonly associated with hypertension and diabetes mellitus.
• Vasculitides: These are immune-mediated conditions that result in inflammation and damage to blood vessels, affecting their structural integrity and function.
Connective Tissue Disorders: Conditions such as scleroderma can impact blood vessels, leading to fibrosis and functional impairments.
Emerging research suggests that mobilized peripheral blood stem cells may offer a potential therapeutic option for treating lymphedema, though further studies are required to validate this approach.
During fetal development, disruptions in vascular endothelial growth factor (VEGF) or Angiopoietin/Tie2 signaling by pericytes within placental vessels can lead to significant placental diseases, such as pre-eclampsia, posing risks to both the mother and fetus.
Thrombotic Microangiopathy is a pathological condition involving alterations in the vascular wall that result in the formation of multiple small thrombi within the microcirculation. This can lead to organ dysfunction and microangiopathic hemolytic anemia. It is categorized into primary forms, such as thrombotic thrombocytopenic purpura and hemolytic-uremic syndrome, and secondary forms related to conditions like malignant hypertension or scleroderma. Though rare, with an incidence of about six cases per million annually, it remains a critical diagnosis due to its high untreated mortality rate of 90%. Prompt treatment, including plasmapheresis, can significantly reduce mortality, though early death may still occur within 24 hours of onset, particularly in women.
Takayasu Arteritis is an inflammatory disease affecting the aorta, its branches, and pulmonary arteries, primarily in young women. The cause is unknown, but the inflammation can lead to arterial stenosis, occlusions, aneurysms, or dilations. Symptoms include asymmetric pulses, differing blood pressure readings between limbs, claudication, reduced brain perfusion, and hypertension. Diagnosis is achieved through aortic arteriography or MRI angiography. Treatment involves corticosteroids, immunosuppressants, and surgical interventions, such as bypass procedures, in severe cases of ischemia.
Familial Hemorrhagic Telangiectasia (Osler-Weber-Rendu Syndrome) is a hereditary condition characterized by direct arteriovenous malformations, bypassing
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capillaries. The most common manifestation is spontaneous and recurrent nosebleeds, typically starting around age 12. About 25% of patients experience gastrointestinal bleeding, often beginning at age 40. Severe bleeding can also occur in the brain or lungs, leading to significant complications.
Soft Tissue Sarcomas are rare malignant tumors that originate in the body's soft tissues, including muscles, connective tissues, blood or lymphatic vessels, nerves, ligaments, and adipose tissue. Over 50 subtypes are recognized, and these tumors can develop in nearly any part of the body. A definitive diagnosis requires a biopsy, which identifies the specific type of sarcoma and its tissue of origin.
These conditions highlight the clinical importance of understanding the vascular system's role in maintaining health and its involvement in a diverse array of diseases.
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