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Cardiac skeleton

The cardiac skeleton, also known as the fibrous skeleton of the heart, is a rigid framework composed of dense collagenous connective tissue located at the base of the ventricular mass, where it separates the atria from the ventricles and anchors the four cardiac valves.^[1] This structure primarily consists of four fibrous rings, or anuli fibrosi, that encircle the orifices of the mitral, tricuspid, aortic, and pulmonary valves, along with interconnecting components including the right and left fibrous trigones, the central fibrous body, and membranous portions of the interatrial and interventricular septa.^[2] In humans, the skeleton is predominantly fibrous, though cartilaginous elements termed cartilago cordis—often hyaline cartilage—may occur within the trigones or near the atrioventricular node in some individuals or species.^[3] The cardiac skeleton serves multiple critical functions essential to heart mechanics and electrophysiology. Structurally, it provides attachment sites for the valve leaflets, chordae tendineae, and papillary muscles, while resisting deformation to maintain open valve orifices during cardiac cycles and preventing excessive dilation of the annuli.^[1] Electrically, the dense fibrous tissue acts as an insulator, preventing direct propagation of impulses from the atria to the ventricles except through the specialized atrioventricular conduction system—the bundle of His—which penetrates the central fibrous body to ensure coordinated contraction.^[2] The right fibrous trigone, the thickest component at the junction of the aortic, mitral, and tricuspid annuli, exemplifies this dual role by offering robust mechanical support and contributing to the membranous septum's integrity.^[1] Developmentally, the cardiac skeleton arises from epicardially derived fibroblasts during embryogenesis, with the fibrous rings and trigones coalescing to support valve formation from endocardial cushions.^[1] Pathologically, degeneration such as annular calcification—common in aging or conditions like aortic stenosis—can impair valve function, cause conduction blocks by encroaching on the His bundle, or lead to complications like paravalvular leaks following surgical interventions, often evaluated via CT or MR imaging for precise assessment.^[1] These aspects underscore the cardiac skeleton's essential role in maintaining hemodynamic efficiency and rhythmic synchronization across vertebrate hearts.^[3]

Anatomy

Gross Structure

The cardiac skeleton, also known as the fibrous skeleton of the heart, is a dense connective tissue framework located at the base of the heart that separates the atria from the ventricles and encircles the annuli of the four cardiac valves.^[4] This structure provides a rigid scaffold for the attachment of valvular leaflets and the origins of the myocardium, ensuring the geometric integrity of the heart chambers during contraction and relaxation.^[4] In humans, it is composed primarily of fibrous tissue without bony elements in the typical adult heart.^[5] The key components of the cardiac skeleton include four fibrous rings, known as the annuli fibrosi, which surround the orifices of the mitral, tricuspid, aortic, and pulmonary valves.^[4] These rings are interconnected by the left and right fibrous trigones, which are triangular regions of dense connective tissue that form the strongest portions of the skeleton; the right trigone lies between the aortic and tricuspid annuli, while the left trigone connects the aortic and mitral annuli. The rings are further interconnected by the central fibrous body, which lies between the mitral and tricuspid annuli and is penetrated by the atrioventricular bundle.^[4] Additionally, the membranous interventricular septum, a thin fibrous sheet, forms part of the skeleton and separates the right and left ventricles at their base.^[4] The leaflets of the atrioventricular valves (mitral and tricuspid) attach to the fibrous rings of the skeleton and are prevented from prolapsing by chordae tendineae that extend from the leaflets to papillary muscles in the ventricles.^[4] Spanning the central region at the heart's base, the cardiac skeleton occupies a pivotal position posterior to the sternum and encircles the origins of the great vessels (aorta and pulmonary trunk), thereby anchoring the valves and facilitating their coordinated function.^[4] The fibrous rings and trigones collectively form a nearly continuous annular structure in a figure-eight configuration around the atrioventricular orifices, interrupted only at the site where the atrioventricular node penetrates to connect the atrial and ventricular conduction systems.^[4] This layout can be visualized in anatomical diagrams as a compact, Y-shaped or basal ring system viewed from the apex, with the aortic annulus centrally positioned and the trigones bridging to the adjacent valve rings.^[4] In humans, the cardiac skeleton typically lacks an os cordis (heart bone), a cartilaginous or ossified structure present in some mammals for added rigidity.^[5] However, rare cases of mineralization or ossification within the fibrous trigones have been reported, often associated with aging or cardiovascular pathology, as in the first documented instance of an os cordis in a human heart discovered incidentally during autopsy.^[5]

Histology

The cardiac skeleton is primarily composed of dense irregular connective tissue, characterized by a predominance of type I collagen fibers that provide tensile strength and structural integrity.^[6] Interwoven with these collagen fibers are lesser amounts of elastin, which contributes to flexibility, and proteoglycans embedded in the ground substance, aiding in tissue hydration and rigidity.^[7]^[8] Cellular components are sparse, consisting mainly of fibroblasts responsible for matrix maintenance and remodeling, with minimal vascularization throughout the structure and acellular matrix regions in the core areas that enhance its insulating properties.^[9]^[4] Zonal variations exist within the cardiac skeleton, featuring denser collagen alignment near the valve annuli for reinforced anchorage, while the trigones exhibit a looser arrangement; the ground substance, rich in proteoglycans, plays a key role in modulating overall rigidity across these zones.^[6] Under histological staining, the cardiac skeleton appears eosinophilic with hematoxylin and eosin (H&E), highlighting the collagen-rich matrix, and exhibits birefringence under polarized light, revealing the oriented collagen fibers.^[6] Age-related histological changes include progressive fibrosis, with increased collagen deposition, and calcification, particularly in the central fibrous body and annuli, contributing to stiffening without altering the fundamental tissue architecture.^[10]^[4]

Function

Structural Support

The cardiac skeleton serves as a critical mechanical framework that anchors the atrioventricular valves (mitral and tricuspid) and semilunar valves (aortic and pulmonary) to the surrounding myocardium, preventing valve prolapse and annular dilation during the high-pressure phases of the cardiac cycle. This anchoring is achieved through the dense fibrous tissue of the annulus fibrosa and central fibrous body, which distribute contractile forces evenly across the valve bases, ensuring stable leaflet positioning and competent closure under systolic pressures exceeding 100 mmHg. By resisting excessive deformation, the skeleton maintains valve integrity over billions of cycles, averting regurgitation due to mechanical failure.^[11] In addition to valve support, the cardiac skeleton contributes to the structural integrity of the atrioventricular and interventricular septa by integrating fibrous elements that reinforce these partitions against the shearing forces generated by myocardial contraction. The fibrous trigones and membranous septum components of the skeleton act as load-bearing connectors, channeling tensile and compressive stresses from ventricular walls to prevent septal buckling or rupture during diastole and systole. This force distribution preserves the geometric alignment of cardiac chambers, allowing efficient pressure gradients without compromising barrier function.^[12] The skeleton interacts with the chordae tendineae and papillary muscles to facilitate proper valve coaptation, particularly in the atrioventricular valves, where chordae attach to leaflet edges and transmit papillary muscle tension to the fibrous annuli. This tethered system counteracts ventricular shortening, keeping leaflets aligned for tight closure and minimizing leakage, with the skeleton providing a fixed proximal anchor point for these dynamic elements.^[13] Biomechanically, the cardiac skeleton exhibits high tensile strength derived from its collagen-rich composition, where type I and III fibers are predominantly oriented circumferentially and longitudinally to optimize load-bearing capacity. This orientation enables resistance to both tensile stresses—characterized by minimal elongation under pulling forces, akin to a stiff spring in the linear stress-strain regime—and shear stresses that arise from twisting motions between atrial and ventricular contractions. Such properties ensure long-term durability under repeated cardiac cycles. The fibrous rings and trigones, as key anchors, exemplify this collagen-based rigidity. Overall, these features of the cardiac skeleton help define and preserve heart geometry by limiting excessive chamber expansion, particularly at the base, where it constrains annular dilation and maintains proportional atrioventricular alignment during volume changes. This stabilizing role supports efficient ejection fractions and prevents maladaptive remodeling under normal hemodynamic loads.^[14]

Electrical Insulation

The cardiac skeleton serves as a critical electrical insulator, separating the atrial and ventricular myocardium to prevent the direct propagation of action potentials from the atria to the ventricles. This isolation is achieved through its composition as an avascular, dense matrix primarily consisting of collagen fibers, which lacks the excitable cardiomyocytes and gap junctions necessary for electrical conduction. The electrically inert nature of this fibrous structure ensures that impulses cannot cross the atrioventricular junction except via the specialized conduction system, thereby coordinating sequential atrial and ventricular contraction.^[1]^[2]^[4] The exclusive pathway for conduction across the cardiac skeleton is provided by the atrioventricular (AV) bundle of His, which penetrates the skeleton at the right fibrous trigone within the central fibrous body. This penetration site allows the electrical impulse from the AV node to reach the ventricles in a controlled manner, with the bundle then dividing into left and right bundle branches that emerge inferior to the central fibrous body and distribute along the interventricular septum. By confining conduction to this discrete route, the skeleton maintains orderly impulse transmission and avoids haphazard spread that could disrupt ventricular depolarization.^[1]^[15] This insulating function plays a key role in preventing re-entrant arrhythmias by blocking direct atrial-to-ventricular electrical transmission, which would otherwise allow premature impulses to create self-sustaining circuits and lead to uncoordinated contractions. In normal physiology, the barrier enforces a delay at the AV node, ensuring that ventricular activation follows complete atrial emptying. Failure of this isolation, such as the presence of accessory pathways that bypass the skeleton, can permit aberrant conduction and predispose to rapid supraventricular rhythms, highlighting the skeleton's essential role in electrophysiological stability.^[7]^[16]

Development

Embryonic Origins

The formation of the cardiac skeleton begins during weeks 4 to 5 of human gestation, coinciding with the development of endocardial cushions in the atrioventricular (AV) canal and outflow tract (OFT). These cushions arise from the cardiac jelly, an acellular matrix secreted by endocardial cells, and serve as precursors to the fibrous components of the skeleton, including the primitive annuli fibrosi and central fibrous body.^[17] The primary tissue sources for the cardiac skeleton include extracardiac mesenchyme derived from the epicardium (via the proepicardial organ), cardiac neural crest cells, and contributions from endocardial cushions. Epicardial-derived mesenchymal cells migrate into the subepicardial space, providing fibroblasts that form the fibrous annuli of the AV valves and parts of the ventricular myocardium's interstitial framework. Neural crest cells, originating from the dorsal neural tube, migrate through pharyngeal arches to populate the OFT cushions and contribute to the aortopulmonary septum, which integrates into the skeletal framework at the base of the aorta and pulmonary trunk. Endocardial cushions in the AV region supply mesenchymal cells that form the AV rings and a portion of the membranous interventricular septum.^[1]^[18]^[19] Sequential assembly of the cardiac skeleton involves the fusion of these cushions to establish primitive annuli, followed by cellular migration that shapes the trigones and membranous septum. In the AV canal, superior and inferior cushions fuse midline by week 6, creating a platform for valve leaflets while their fibrous remnants coalesce into the central fibrous body; sulcus tissue (extracardiac mesenchyme) invaginates to reinforce the AV rings. In the OFT, neural crest-populated cushions and ridges align and merge with the muscular ventricular septum, forming the intervalvar fibrous tissue and membranous septum through directed cell migration and matrix deposition. The dorsal mesocardium contributes to the atrial aspect of the central fibrous body via ingrowth during this remodeling phase.^[18]^[20] Key processes driving this development include epithelial-mesenchymal transition (EMT) and apoptosis. EMT occurs in endocardial cells overlying the cardiac jelly, triggered by signals such as transforming growth factor-β, enabling these cells to delaminate, invade the matrix, and differentiate into mesenchymal progenitors that populate the cushions and form fibrous elements. Subsequent apoptosis sculpts the cushions, removing excess tissue to refine the annuli and septa, ensuring proper alignment and preventing overgrowth that could disrupt septation. Epicardial EMT similarly generates mesenchymal cells for annular support.^[21]^[17] The cardiac skeleton's assembly integrates concurrently with the development of the conduction system, particularly the AV node and bundle of His. The AV node emerges from slow-conducting myocardium in the AV junction around week 5, with its fibers penetrating the forming central fibrous body; the His bundle penetrates the primitive annulus via myocardial sleeves, establishing electrical insulation as the skeleton matures. This temporal overlap ensures the skeleton's fibrous tissue isolates atrial and ventricular impulses, with neural crest contributions aiding alignment of the penetrating bundle.^[22]^[20]

Postnatal Changes

Following birth, the cardiac skeleton undergoes rapid remodeling to support the transition from fetal to postnatal circulation, characterized by increased hemodynamic demands. Cardiac fibroblasts proliferate extensively in the neonatal period, doubling in number and shifting from approximately 10% to 70% of the total cardiac cell population within the first two weeks in mice, driving de novo collagen deposition to form the fibrous skeleton. This process involves heightened synthesis of fibrillar collagens I and III, which constitute the primary components of the extracellular matrix (ECM) scaffold, enabling structural reinforcement and adaptation to elevated ventricular pressures. The ECM transitions from a compliant, proteoglycan-rich network to a stiffer collagen-dominated structure by around 30 days postnatal, aligning with cardiomyocyte hypertrophy and loss of regenerative capacity to meet circulatory needs.^[23]^[24] With advancing age, the cardiac skeleton experiences progressive dystrophic calcification, beginning subtly in the third and fourth decades and accelerating thereafter, alongside increased fibrosis and diminished elasticity. Calcification accumulates in the fibrous rings and valve annuli, driven by inflammatory and osteogenic processes, with prevalence rising to 26% for aortic valve sclerosis by age 65 and higher in women for mitral annular involvement. Fibrosis intensifies due to elevated collagen deposition, raising myocardial collagen content from about 3.9% in young adults (20-25 years) to 5.9% in the elderly (67-87 years), which reduces tissue compliance and contributes to overall skeletal rigidity by age 60 and beyond. Collagen cross-linking, mediated by advanced glycation end-products, further augments this stiffness, impairing the skeleton's flexibility without invoking pathological states.^[25]^[26]^[27] Hormonal and environmental factors modulate these changes; estrogen exerts protective effects in premenopausal females by suppressing excessive collagen I and III remodeling in cardiac fibroblasts, resulting in lower fibrosis compared to males until menopause, after which female collagen accumulation rises. Hypertension accelerates matrix turnover independently of age, promoting collagen deposition and degradation imbalance via upregulated matrix metalloproteinases, which heightens skeletal remodeling and stiffness. Quantitatively, collagen cross-linking elevates myocardial stiffness by 20-50% in aged models, while valvular calcification progresses at rates such as a median of 10.1 Agatston units per year in those with prevalent mitral annular calcification. These alterations culminate in functional consequences such as reduced valve leaflet mobility in senescence, limiting efficient opening and closure without direct disease linkage.^[28]^[29]^[30]^[31]

Clinical and Historical Aspects

Pathological Conditions

Calcific degeneration of the cardiac skeleton manifests primarily as mitral annular calcification (MAC), a degenerative process involving calcium deposits along the fibrous mitral annulus, which can extend to the valve leaflets and myocardium, leading to mitral stenosis through annular narrowing and restricted leaflet mobility.^[32] This calcification similarly affects the tricuspid annulus, though less frequently, contributing to tricuspid stenosis by impeding valve opening.^[33] Key risk factors include advanced age and chronic kidney disease, which disrupt calcium-phosphorus metabolism and promote dystrophic calcification.^[32] Prevalence reaches up to 10% in the general elderly population, rising to over 40% in those with cardiovascular comorbidities.^[34] Congenital defects of the cardiac skeleton arise from incomplete fusion of the endocardial cushions during embryogenesis, resulting in atrioventricular septal defects (AVSD) characterized by malformations at the atrioventricular junction, including a common AV orifice and abnormal valve septation.^[35] These defects directly involve the developing fibrous skeleton, leading to shunting between atrial and ventricular chambers and potential valve incompetence.^[36] AVSD is strongly associated with Down syndrome (trisomy 21), occurring in approximately 15-20% of affected individuals due to genetic disruptions in mesenchymal tissue contributions and septation processes.^[37] Inflammatory conditions can compromise the cardiac skeleton through rheumatic heart disease, which induces chronic inflammation leading to fibrosis and scarring of the mitral annular ring, exacerbating valve stenosis and restricting annular flexibility.^[38] Infective endocarditis further erodes the skeleton, particularly the intervalvular fibrosa, via enzymatic degradation of connective tissue by bacterial abscesses, causing structural destruction and paravalvular complications.^[39] The arrhythmogenic potential of the cardiac skeleton stems from accessory pathways, such as the bundle of Kent, which consist of anomalous myocardial tissue bridging the insulating fibrous skeleton between atria and ventricles, bypassing the atrioventricular node's delay and enabling pre-excitation.^[40] These pathways facilitate reentrant circuits, contributing to supraventricular tachycardias in Wolff-Parkinson-White (WPW) syndrome.^[40] In surgical contexts, the integrity of the cardiac skeleton is crucial for valve replacement procedures, as the fibrous annuli and intervalvular fibrosa provide the primary anchoring sites for prosthetic valves; damage from degeneration or infection necessitates reconstruction with patches (e.g., autologous pericardium) to ensure stable prosthesis fixation and prevent leaks or migration.^[39]

Historical Recognition

The earliest references to a bony structure within the heart, known as the os cordis, date back to the 2nd century AD, when Galen described it in animals such as elephants during his anatomical studies.^[41] This observation was later misinterpreted as indicating the presence of an os cordis in human hearts, a misconception that persisted in medical literature until the 19th century, when detailed dissections clarified that humans lack such a bone, with occasional calcifications in the fibrous trigones being mistaken for it.^[41] During the Renaissance, Andreas Vesalius advanced the understanding of cardiac anatomy in his seminal 1543 work De Humani Corporis Fabrica, where he illustrated and described the fibrous rings encircling the heart valves, providing one of the first accurate visual representations of these structures, though he offered no insight into their functional significance.^[42] In the late 17th century, Richard Lower contributed to the evolving knowledge of cardiac structure in his 1669 treatise Tractatus de Corde, including descriptions of heart fibers and their role in contraction.^[43] The 19th century saw more detailed mapping of the cardiac skeleton's components. William G. MacCallum further refined this in 1900, offering precise anatomical delineations of the trigones and their connections to the valve annuli through meticulous dissections.^[43] A key milestone came in 1918 with the publication of Gray's Anatomy, which definitively rejected the notion of an os cordis in humans, attributing previous reports to misidentified calcified fibrous elements.^[44] In the 20th century, embryological correlations emerged with Adrianus C.G. Wenink's work in the 1980s, which linked the development of the fibrous skeleton to endocardial cushions and extracardiac tissues, integrating historical anatomy with developmental biology.^[45] The modern era brought computational approaches, as Charles S. Peskin and David McQueen developed biomechanical models in the 1990s to simulate the cardiac skeleton's boundaries and their role in electromechanical function.^[46] Recent imaging studies in the 2020s, utilizing advanced techniques like micro-CT, have confirmed and expanded upon these histological details, revealing variations in the skeleton's composition across species and highlighting its conserved role in cardiac integrity.^[47]

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