Introduction

The cartilage is solely composed of cells known as chondrocytes. Chondrocytes maintain the extracellular matrix (ECM) and produce the cartilage matrix. Surrounded by collagenous fibers, chondrocytes release substances to make cartilage strong yet flexible. In general, chondrocytes are found within intervertebral discs and in any form of articular cartilage (AC). Chondrocytes play a crucial role in maintaining homeostasis within the AC joints that provide cushioning in joint movements. Like cells within other specialized tissues, chondrocytes distance themselves from each other by the cartilage matrix.[1] Chondrocytes are also responsible for chondral repair; due to their reconstructive nature, they respond to outside trauma in case of tissue damage. Because of their healing capability against degenerative conditions, chondrocytes are under active research for implantation and other reconstructive procedures.[2]

Structure

With a long axis parallel to the cellular surface, juvenile chondrocytes are elliptic in shape at the periphery of cartilages. As the frame shifts inward, the shape takes a round form. Chondrocytes may also appear in isogenous groups of up to eight cells. Mitotic cell division of individual chondral cells leads to cellular grouping. During histological development, chondrocytes and their matrix shrink, which retracts the cells from the capsule and produces the irregular shape present within cartilage. Chondrocytes uniformly fill the oblong spaces, lacunae, within tissues and act as factories of collagen production.

Chondrogenesis initiates in early development. Bone morphogenic proteins, GDF5 (growth factor), HOX gene proteins, beta transforming growth factors (TGF-b), and some other signaling molecules contribute to the endogenous development of chondrocytes. Beta-catenin levels dictate the portions of lineage commitment to endogenous chondrogenesis and osteogenesis. Specialization and development of chondrocytes are further driven by beta-catenin in the canonical Wnt signaling process.[3]

Function

Chondrocytes are mainly responsible for the production of collagen and the extracellular matrix that will lead to the maintenance of cartilaginous tissues within joints. Initial cartilage is composed of the mesenchyme during the fifth week of development. The mesenchyme activates in areas of chondral development and condenses to make chondrification centers. The mesenchymal cells develop into prechondrocytes, which later become chondroblasts; chondroblasts secrete collagenous fibrils and extracellular matrix. Consequently, collagenous and elastic fibers are stored within the intercellular matrix. The type of matrix composed contributes to cartilaginous typecast; there are mainly three types of cartilage:

1. Hyaline cartilage: This type of cartilage is the most widespread and is usually present within joints.
2. Fibrocartilage: This type of cartilage is part of the intervertebral disc.
3. Elastic cartilage: This type of cartilage is in the Eustachian tubes.

Endochondral ossification is crucial for skeletal development, specifically in preexisting cartilaginous models. The primary ossification center in long bones first appears in diaphysis (a portion of a long bone between two ends) that develops into the shaft of a bone. Chondrocytes undergo hypertrophy and enlarge at the sites of ossification. At these sites, the matrix becomes calcified and cellular necrosis appears. Osteochondral ossification is further regulated by the SOX9 transcription factor and the CARM1 (coactivator-associated arginine methyltransferase 1). Long bone elongation takes place at the diaphyseal-epiphyseal junction. Elongation directly correlates to cartilage plates made out of chondral matrix and collagen that proliferate and participate in endochondral osteogenesis and postnatal development.[4]

Mechanism

Articular cartilages (AC) obtain very few blood capillaries; chondrocytes, therefore, usually function under low oxygen tension. Whereas most body cells utilize aerobic respiration to produce energy for cellular work, hyaline chondrocytes use lactic acid fermentation for energy production. Hyaline chondrocytes metabolize glucose through anaerobic glycolysis and generate lactic acid at the terminal level. The nutrients required for glycolysis cross the perichondrium to the deeply stored chondrocytes. Two main mechanisms influence the transfer of nutrients: 1) diffusion; 2) intermittent cartilage compression and decompression pumping. These mechanisms set a limit to maximum cartilage width.

Proper endocrine balance is also crucial for chondral functioning. The hypophyseal growth hormone, known as somatotropin, mainly dictates cartilage growth. This hormone indirectly contributes to chondral growth by stimulating the production of somatomedin C in the liver. Somatomedin C directly promotes chondral growth. Testosterone, growth hormone (GH), and thyroxin expedite the production of chondral proteoglycans comprised of sulfated glycosaminoglycans (GAGs). GAG production can become inhibited via hydrocortisone, estradiol, and cortisone.

Histochemistry and Cytochemistry

Photomicrographs of histological sections of various sample cartilages have previously been stained by Alcian blue or with specific antibodies against proteoglycans (decorin, biglycan, and aggrecan) chondroitin (chondroitin-6-sulfate, chondroitin-0-sulfate, and chondroitin-4-sulfate), and keratan sulfate, or type I and II collagen. Researchers have also used the SAM (significance analysis of microarrays) technique to mark the chondrocytes sample with low chondrogenic capacity. These samples displayed higher levels of catabolic genes (aggrecanase 2, matrix metalloproteinase 2, etc.) and insulin-like growth factor 1. High chondrogenic capacity holding chondrocytes displayed high levels of cell-matrix or cell-cell contacting genes (CD49f, CD49c, etc.). According to flow cytometry analysis, CD44, CD49c, and CD151 showed significant distribution in higher chondrogenic capacity holding chondrocytes. The analysis further indicated that CD151 and CD44 hold the capability to detect more chondrogenic clones. Chondrocytes put under brighter CD49c or CD44 signal expression yielded tissues with higher amounts of GAG/ DNA (1.4-fold max) and type II collagen mRNA (3.4-fold max) than did non-brightened cells.[5]

Pathophysiology

As mentioned earlier, chondrocytes maintain homeostasis between the creation and destruction of extracellular matrix components. Chondrocytes, influenced by external stimuli, polypeptide growth factors, and cytokines, create such components and the enzymes which break them down.[6] A disruption in homeostasis leads to osteoarthritis. Researchers are still unsure of the initial point of degradation. A microfracture from any trauma may cause the production of enzymes, leading to “wear” particle synthesis and its macrophage-caused destruction. Eventually, “wear” particle production inhibits the systematic degradation of such particles, mediating inflammation and influencing chondrocytes to secrete degradative enzymes. Collagen and proteoglycan metabolism yield particles that cause proinflammatory cytokine releases, like TNF-alpha, IL-6, and IL-1. These cytokines can attach to chondrocyte receptors, which releases metalloproteinases and promotes inhibition of type II collagen assembly, thus proliferating cartilage degradation. Homeostatic disruption increases the water content and decreases the proteoglycan content of the extracellular matrix (ECM). This modification weakens the collagen network due to a drop in type II collagen synthesis and increases the breakdown of pre-existing collagen. Chondral apoptosis is also visible.

An increased anabolic and catabolic activity mark a patient with osteoarthritic cartilage. Compensatory mechanisms such as increased production of matrix molecules (collagen, hyaluronate, and proteoglycans) and the spread of chondrocytes in deep chondral layers manage the integrity of the articular cartilage. Chondrocyte loss, however, along with transformations in the extracellular matrix predominates, causing osteoarthritic conditions.

Thinning cartilage, degrading cartilage thickness, and fibrillation of superficial layers are all caused by the initial degradations. These changes get worst over time, and articular cartilage (AC) thins to the point of complete destruction. This condition exposes the underlying subchondral bone plate and characterizes these changes as chondropathy.[7] Current investigations about the heterogeneity of cellular reaction patterns that characterize osteoarthritic cartilage degeneration highlighted apoptotic chondrocyte death and its underlying mechanisms.[8]

Clinical Significance

Chondrocyte regeneration has contributed to the development of reconstructive procedures. Direct defects on articular cartilage (AC) cause severe pain; such acute and chronic pain could only be minimized with long-term treatments just three decades ago. Those conditions can now be permanently eliminated using a surgical procedure called autologous chondrocyte implantation (ACI).

Initially performed on soccer players by Swedish orthopedic surgeons in the early 90s, ACI attained US FDA authorization on August 22, 1997. The common pre-operational scenario includes a cracked hole on the joining surface of the articular cartilage. These defects may appear in patients aged 60 to younger patients (median age 31.3). Usually performed at the distal end of the femur, ACI has been successfully performed for kneecap deficiencies and other joint issues. In general, ACI divides into two phases: cell regeneration and open procedure. The patient’s MRI procedure first confirms a surgery-worth defect. A chondrocyte sample is taken via an arthroscopic procedure. New chondrocytes are produced via the enzymatic treatment of the collected sample for three to four months. After the production of over a million cells, the orthopedic surgeon proceeds to the second phase, open procedure. After an incision, the cultured chondrocytes are injected underneath a sewn patch, leading to chondral adhesion to the patient’s cartilage for natural regeneration. A weight-bearing restriction for a maximum of 8 weeks is common after the procedure.[9][10]