Heart valves

Earlier it was thought that all heart valves are simple structures whose contribution to unidirectional blood flow is simply a passive movement in response to the acting pressure gradient. This understanding of "passive structures" led to the creation of "passive" mechanical and biological valve substitutes.

It now becomes apparent that heart valves have a more complex structure and function. Therefore, the creation of an "active" heart valve substitute suggests a significant similarity in its structure and function with a natural heart valve, which in the long term is quite feasible due to the development of tissue engineering.

Heart valves develop from the embryonic buds of the mesenchymal tissue during the insertion of the endocardium. In the process of morphogenesis, the atrioventricular canal (tricuspid and mitral cardiac valves) and ventricular outflow tract (aortic and pulmonary heart valves) are formed.

[1], [2], [3], [4], [5]

How are heart valves arranged?

The beginning of the study of the blood supply to the valves was laid by Luschka (1852), using an injection of heart vessels with contrasting mass. He found numerous blood vessels in the valves of the atrioventricular and semilunar valves of the aorta and pulmonary artery. However, in a number of guidelines on pathological anatomy and histology, there were indications that unchanged human heart valves do not contain blood vessels, and the latter appear in the valves only in various pathological processes - atherosclerosis and endocarditis of various etiologies. Information on the absence of blood vessels was based mainly on histological studies. It was assumed that in the absence of blood vessels in the free part of the valves, their nutrition occurs by filtering the fluid from the blood plasma that sweeps the valves. The penetration of a few vessels along with fibers of the striated muscle tissue into the base of the valves and tendon chords was noted.

However, with the injection of cardiac vessels with various dyes (carcasses in gelatin, bismuth in gelatin, aqueous suspension of black carcass, carmine or trypan blue solutions), it was found that the vessels penetrate into the atrioventricular heart valves, aortic and pulmonary artery valves along with cardiac muscle tissue , slightly not reaching the free edge of the leaf.

In the friable fibrous connective tissue of the valves of the atrioventricular valves, separate main vessels were found, anastomosing with the vessels a number of located areas of cardiac transversal muscle tissue.

The largest number of blood vessels was located at the base and comparatively less - in the free part of these valves.

According to KI Kulchitsky et al. (1990), a larger diameter of the arterial and venous vessels is found in the mitral valve. At the base of the valves of this valve, there are mainly main vessels with a narrow-looped network of capillaries penetrating into the basal part of the valve and occupying 10% of its area. In the tricuspid valve, arterial vessels have a smaller diameter than in the mitral valve. In the valves of this valve there are mainly scattered type vessels and relatively wide loops of blood capillaries. In the mitral valve, the front leaf is more intensively blood flowing, in the tricuspid valve, the anterior and posterior valves, which carry the main closure function. The ratio of the diameters of arterial and venous vessels in the atrioventricular valves of the heart of mature people is 1: 1.5. The capillary loops are polygonal and located perpendicular to the base of the valve flaps. Vessels form a planar network located under the endothelium from the side of the atria. Blood vessels are also found in the tendon chords, where they penetrate from the papillary muscles of the right and left ventricles to a distance of up to 30% of the length of the tendon chords. Numerous blood vessels form arched loops at the base of the tendon chords. Heart valves of the aorta and pulmonary trunk for blood supply are significantly different from atrioventricular. The main vessels of relatively smaller diameter fit the base of the semilunar valves of the aortic and pulmonary valves. The short branches of these vessels terminate in the capillary loops of an irregular oval and polygonal shape. They are located, mainly, near the base of the semilunar wings. The venous vessels in the base of the valves of the aorta and pulmonary artery also have a smaller diameter than at the base of the atrioventricular valves. The ratio of the diameters of arterial and venous vessels in the valves of the aorta and pulmonary artery of the heart of mature people is 1: 1.4. From the larger vessels, short lateral branches branch out, ending with capillaries of the wrong oval and polygonal shape.

With age, cohesion of connective tissue fibers, both collagen and elastic, is observed, as well as a decrease in the amount of loose fibrous unformed connective tissue, sclerosis of the valves of the valves of the atrioventricular valves and half-moon valves of the aorta and pulmonary artery valves. The length in the valves of the fibers of the cardiac transversal muscle tissue decreases, and, consequently, its quantity and number of blood vessels penetrating into the heart valves decrease. In connection with these changes, the heart valves lose their elastic and elastic properties, which affects the mechanism of closing the valves and hemodynamics.

Heart valves have lymphatic capillary networks and a small number of lymphatic vessels equipped with valves. Lymphatic capillaries of the valves have a characteristic appearance: their lumen is very irregular, the same capillary in different areas has a different diameter. In the junction of several capillaries, extensions are formed-lacunae of various shapes. The network loops are often irregular polygonal, less often oval or round shape. Often, the lymphatic network loops are not closed, and the lymphatic capillaries end blindly. The lymphatic capillary loops are oriented more often in the direction from the free edge of the valve to its base. In a number of cases, a two-layered network of lymphatic capillaries was found in the valves of the atrioventricular valve.

Nerve plexuses of the endocardium are located in its various layers, mainly under the endothelium. At the free edge of valve flaps, nerve fibers are located, mainly radially, connecting with those of the tendon chords. Closer to the base of the valves is a large-plexus plexus that connects to the plexus around the fibrous rings. On the semilunular valves, the endocardial neural network is more rare. At the place of attachment of the valves, it becomes thick and multilayered.

Cellular structure of the heart valves

The valve interstitial cells responsible for maintaining the valve structure have an elongated shape with a large number of thin processes that extend through the entire valve matrix. There are two populations of valve interstitial cells, differing in morphology and structure; some have contractile properties and are characterized by the presence of contractile fibrils, others have secretory properties and have a well-developed endoplasmic reticulum and Golgi apparatus. The contractile function resists hemodynamic pressure and is additionally supported by the production of both cardiac and skeletal contractile proteins, which include the heavy chains of alpha and beta-myosin and various isoforms of troponin. The contraction of the valve of the heart valve was demonstrated in response to a number of vasoactive agents suggesting the coordinating action of the biological stimulus for the successful functioning of the valve.

Interstitial cells are also necessary components of the reductive system of structures such as heart valves. The constant movement of the valves and the deformation of the connective tissue associated with it, produces damage to which valve interstitial cells react to maintain the integrity of the valve. The recovery process is vital for the normal functioning of the valve, and the absence of these cells in modern models of artificial valves is probably a factor contributing to structural damage to bioprostheses.

An important direction in the study of interstitial cells is the study of the interaction between them and the surrounding matrix, mediated by the focal adhesion of molecules. Focal adhesions are specialized cell-matrix interactions that bind the cytoskeleton of a cell to matrix proteins through integrins. They also act as signaling sites for transduction, transmitting mechanical information from the extracellular matrix, which can elicit responses, including, but not limited to, cell adhesion, migration, growth and differentiation. Understanding the cellular biology of valvular interstitial cells is vital to establishing the mechanisms through which these cells interact with each other and the environment, so that this function can be reproduced in artificial valves.

In connection with the development of a promising direction of tissue engineering of cardiac valves, interstitial cell studies are conducted using a wide range of techniques. The presence of the cytoskeleton of the cells is confirmed by staining for vimentin, desmin, troponin, alpha-actin and smooth muscle myosin, heavy chains of alpha and beta-myosin, light chains-2 cardiac myosin, alpha and beta tubulin. Cell contractility is confirmed by a positive response to epinephrine, angiotensin II, bradykinin, carbachol, potassium chloride, endothelium I. Cellular interrelation is defined by functional gap interactions and is checked by microinjection of carboxyfluorescein. Matrix secretion is established by staining for prolyl-4-hydroxylase / collagen type II, fibronectin, chondroitin sulfate, laminin. The innervation is established by the proximity of the motor nerve endings, which is reflected by the activity of the neuropeptide Y tyrosine hydroxylase, acetylcholinesterase, vasoactive intestinal polypeptide, substance-P, capcytonin of the gene-bound peptide. Mitogenic factors are estimated by platelet-derived growth factor, the main fibroblast growth factor, serotonin (5-HT). The investigated fibroblasts of interstitial cells are characterized by an incomplete basal membrane, long, thin cytoplasmic processes, close connection with the matrix, a well developed uneven endoplasmic reticulum and the Golgi apparatus, a wealth of microfilaments, and the formation of adhesive bonds.

Valvular endocardial cells form a functional atrombogenous envelope around each heart valve, similar to the vascular endothelium. The widely used method of valve replacement eliminates the protective function of the endocardium, which can lead to the deposition of platelets and fibrin on artificial valves, the development of bacterial infection and tissue calcification. Another likely function of these cells is the regulation of the underlying valvular interstitial cells, similar to the regulation of smooth muscle cells by the endothelium. Complex interaction exists between the endothelium and neighboring cells, partially mediated by soluble factors secreted by endothelial cells. These cells form a huge surface, covered with micro-growths on the luminal side, thus increasing exposure and possible interaction with the metabolic substances of the circulating blood.

Endothelium often reflects the morphological and functional differences caused by the shear stresses on the vessel wall arising from the movement of the blood, the same applies to valvular endocardial cells that accept both elongated and polygonal shape. Changes in the structure of the cell can occur due to the action of local hemodynamics on the components of the cytoskeleton of the cell or the secondary effect caused by changes in the underlying extracellular matrix. At the level of the ultrastructure valvular endocardial cells have intercellular connections, plasma vesicles, uneven endoplasmic reticulum and the Golgi apparatus. Despite the fact that they produce the vWF, both in the living organism and in the artificial environment, they lack the Weibel-Palada bodies (specific granules containing the von Willebrand factor), which are organelles that are characteristic of the vascular endothelium. Valvular endocardial cells are characterized by strong joints, functional gap interactions and overlapped by marginal folds.

Endocardial cells retain their metabolic activity even in vitro: they produce von Willebrand factor, prostacyclin, nitric oxide synthase, demonstrate the activity of angiotensin converting enzyme, intensively secrete the adhesion molecules ICAM-1 and ELAM-1, which are important for the binding of mononuclear cells in the development of the immune response. All these markers should be taken into account when growing the ideal cell culture to create an artificial valve by the method of tissue engineering, but the immunostimulating potential of valvular endocardial cells themselves may limit their use.

The extracellular metric of the heart valves consists of fibrous collagen and elastin macromolecules, proteoglycans and glycoproteins. Collagen is - 60% of the dry weight of the valve, elastin - 10% and proteoglycans - 20%. The collagen component provides the basic mechanical stability of the valve and is represented by collagens I (74%). II (24%) and V (2%) types. Bunches of collagen filaments are surrounded by an elastin sheath that interacts between them. The glycosaminoglycan side chains of proteoglycan molecules tend to form a gel-like substance in which other matrix molecules interact to form permanent relationships and other components are deposited. Glycosaminoglycans of the human heart valve consist mainly of hyaluronic acid, to a lesser extent - from dermatan sulfate, chondroitin-4-sulfate and chondroitin-6-sulfate, with a minimum amount of heparan sulfate. Remodeling and renewal of matrix tissue are regulated by matrix metalloproteinases (MMPs) and their tissue inhibitors (TIs). These molecules also participate in a wider range of physiological and pathological processes. Some metalloproteinases, including interstitial collagenases (MMP-1, MMP-13) and gelatinases (MMP-2, MMP-9) and their tissue inhibitors (TI-1, TI- 2, TI-3), are found in all valves of the heart. The overabundance of metalloproteinase production is typical for pathological conditions of the heart valve.

[6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]

Heart valves and their morphological structure

Heart valves consist of three morphologically different and functionally significant layers of the matrix of the valve - fibrous, spongy and ventricular.

The fibrous layer forms a load-proof frame of the valve flap, consisting of layers of collagen fibers. These fibers are arranged radially in the form of folds for the possibility of stretching the arterial valves upon closure. The fibrous layer lies near the outlet outer surface of these valves. The fibrous layers of the atrioventricular valves serve as a continuation of the collagen beams of the tendon chords. It is located between the spongy (entrance) and ventricular (exit) layers.

Between the fibrous and ventricular there is a spongy layer (spongiosa). The spongy layer consists of a poorly organized connective tissue in a viscous medium. The dominant matrix components of this layer are proteoglycans with arbitrarily oriented collagen and thin layers of elastin. The side chains of the molecules of proteoglycans carry a strong negative charge, which affects their high ability to bind water and form the porous gel of the matrix. The spongy matrix layer reduces the mechanical stresses in the valves of the heart valves and maintains their flexibility.

The ventricular layer is much thinner than others, and is replete with elastic fibers that allow tissues to withstand constant deformation. Elastin has a spongy structure that surrounds and connects collagen fibers, and ensures their maintenance in a neutral folded state. The inlet layer of the valve (ventricular for arterial valves and spongy for atrioventricular valves) contains a greater amount of elastin than the outlet, which provides a softening of the hydraulic impact when closing the valves. This relationship between collagen and elastin allows the expansion of the valves to 40% without permanent deformation. Under the influence of a small load, the collagen structures of this layer are oriented in the direction of loading, and its resistance to further growth of the load increases.

Thus, the idea of heart valves as an idle endocardium duplication is not only simplistic, but also, in fact, incorrect. Heart valves are an organ with a complex structure, including striated muscle fibers, blood and lymphatic vessels, and nerve elements. Both in their structure and functioning, the valves form a single whole with all the structures of the heart. The analysis of the normal function of the valve must take into account its cellular organization, as well as the interaction of cells between themselves and the matrix. The knowledge gained from such studies is leading in the design and development of valve replacement using tissue engineering.