Meniscus, articular cartilage and nucleus pulposus: a comparative review of cartilage-like tissues in anatomy, development and function

Abstract

The degradation of cartilage in the human body is impacted by aging, disease, genetic predisposition, and continued insults resulting from daily activity. The burden of cartilage defects (osteoarthritis, rheumatoid arthritis, intervertebral disc damage, knee replacement surgeries, etc.) is daunting in light of substantial economic and social stresses. This review strives to broaden the scope of regenerative medicine and tissue engineering approaches used for cartilage repair by comparing and contrasting the anatomical and functional nature of the meniscus, articular cartilage (AC), and nucleus pulposus (NP). Many review papers have provided detailed evaluations of these cartilages and cartilage-like tissues individually, but none have comprehensively examined the parallels and inconsistencies in signaling, genetic expression, and extracellular matrix (ECM) composition between tissues. For the first time, this review outlines the importance of understanding these three tissues as unique entities, providing a comparative analysis of anatomy, ultrastructure, biochemistry, and function for each tissue. This novel approach highlights the similarities and differences between tissues, progressing research toward an understanding of what defines each tissue as distinctive. The goal of this paper is to provide researchers with the fundamental knowledge to correctly engineer the meniscus, AC, and NP without inadvertently developing the wrong tissue function or biochemistry.

Osteoarthritis and intervertebral disc degeneration: Quite different, quite similar

“…The NP and AF are unique structures in humans and other mammals, but as other vertebrates like birds and reptiles develop articular joints between vertebrae instead of intervertebral discs, more similarity is suggested than one may expect from looking at anatomy and embryology alone . For a more extensive description of the anatomy and embryonic development of human intervertebral discs we refer to …”

“…For a more detailed description of the cell‐types in the intervertebral disc and articular joint, we refer to more extensive reviews on this subject . However, considering the resemblance in some features of cell behavior after birth (eg, produce the same type of ECM proteins), the local environment of the cell (ie, ECM and mechanobiological cues) may have more influence on its behavior than its embryonic origin …”

“…28,34 For a more extensive description of the anatomy and embryonic development of human intervertebral discs we refer to. 32,33,[35][36][37] The formation of arms and legs occurs slightly later in human embryonic development, in week 6 ( Figure 2B). Limbs derive from the limb bud, a structure of mesenchyme covered with ectoderm.…”

“…49,50 Besides proteoglycans, the ECM of the NP and AC contains collagen, mainly collagen type II. 51 The ratio of aggrecan to collagen is more than five times higher in the NP than in AC, 35,52,53 and it is this high ratio of aggrecan that creates a high intradiscal pressure in the intervertebral disc (ie, 0.43-0.50 MPa in relaxed standing 54 ), whereas the pressure inside AC is much lower (ie, AE 300 kPa in 30% compression 45,55 ). Toward the outer regions of the intervertebral disc, the AF becomes a more fibrous structure of mainly collagen type I, 51,56 which is able to resist tensile stresses caused by the intradiscal pressure.…”

Biomaterials for intervertebral disc regeneration: Current status and looming challenges

“…In the healthy lumbar IVDs, in vivo pressures of the NP range from 91 to 2,300 kPa depending on the positions of the trunk; pressures are highest when flexing forward while holding a weight and lowest when lying prone (Newell, Little, Christou, et al, ). The mature NP is a relatively acellular tissue (with the average density of 4 × 10 3 cells/mm 3 ) and its primary components are water (80% of wet weight), proteoglycans (14% of wet weight), and collagen (4% of wet weight; Chen, Fu, Wu, et al, ). An increased rate of cell apoptosis or necrosis, altered homeostasis of extracellular matrix (ECM), and elevated levels of inflammation are the major inducers for NP degeneration.…”