The Crosstalk Between Cartilage and Bone in Skeletal Growth

Abstract

While the flat bones of the face, most of the cranial bones, and the clavicles are formed directly from sheets of undifferentiated mesenchymal cells, most bones in the human body are first formed as cartilage templates. Cartilage is subsequently replaced by bone via a very tightly regulated process termed endochondral ossification, which is led by chondrocytes of the growth plate (GP). This process requires continuous communication between chondrocytes and invading cell populations, including osteoblasts, osteoclasts, and vascular cells. A deeper understanding of these signaling pathways is crucial not only for normal skeletal growth and maturation but also for their potential relevance to pathophysiological processes in bones and joints. Due to limited information on the communication between chondrocytes and other cell types in developing bones, this review examines the current knowledge of how interactions between chondrocytes and bone-forming cells modulate bone growth.

Keywords: cell–cell interactions; chondrocytes; endochondral ossification; growth plate; osteocyte; osteogenesis.

Figure 1

Membranous ossification vs. endochondral ossification. Mesenchymal stem cells (MSCs) present in the bone marrow can differentiate to become either chondrocytes or osteoblasts. Osteoblasts build up bone directly through a process called intramembranous ossification (in red), while chondrocytes proliferate, hypertrophy, and mineralize, and then new bone is deposited onto the cartilaginous matrix through a process called endochondral ossification (in blue). Endochondral ossification forms the bones of the limbs and long bones, while membranous ossification forms the bones of the axial skeleton. In this process, various factors and precursors are involved, from mesenchymal cell differentiation to hypertrophic chondrocyte. the green arrows indicate the differentiation of precursor cells into osteoclasts, which are cells involved in bone resorption. M-CSF and RANK/RANKL induce the differentiation of osteoclast precursors into mature osteoclasts (green arrow). Created using BioRender.com.

Figure 2

Histological sections of the tibial epiphysis of a rat of 35 days of age. (A) A paraffin section stained with Alcian blue/acid fuchsin showing the cartilage of the growth plate (stained in blue) and the bone tissue (stained in fuchsia) in a typical endochondral ossification process where the cartilage is progressively replaced by bone. (B) A magnification of figure (A) showing that longitudinal septa of the growth plate serve as a scaffold upon which osteoblasts deposit mineralized osseous matrix. (C) A semi-thin section of a rat tibia showing a group of osteoblasts secreting bone matrix on a bone trabecula in a representative membranous ossification process.

Figure 3

Schematic diagram of a longitudinal section of the epiphyseal growth plate. The growth plate is a cartilage-like structure situated between the metaphysis and the diaphysis of all long bones. It consists of hyaline cartilage. The growth plate is histologically made up of 4 zones. The epiphysis is above the reserve zone, followed by the proliferative and prehypertrophic zones; finally, the metaphysis is below, called the hypertrophic zone. Created using BioRender.com.

Figure 4

Structure and components of long bones. Long bones consist of a long shaft (the diaphysis) plus two articular (joint) surfaces, called epiphyses. They are composed mostly of compact bone, but they also contain spongy or trabecular bone and marrow in the hollow center (the medullary cavity). The diagram shows the main structural features of bone as well as a magnified view showing some of the finer details of trabecular (A) and cortical bones (B) and the growth plate (C). Created using BioRender.com.

Figure 5

Molecular expression of the main factors involved in the dynamics of the postnatal growth plate. Solid colored bars indicate the protein’s expression in that specific region. Bars with only oblique stripes (no solid color) indicate areas where the protein is not expressed but performs its function through signaling or other indirect means. Bars with both color and oblique stripes represent regions where the protein is actively expressed and the cells in that zone also possess receptors for that protein, allowing direct autocrine or paracrine effects. RZ: resting zone; PZ: proliferative zone; PHZ: prehypertrophic zone; HZ: hypertrophic zone; THZ: terminal hypertrophic zone; OB: osteoblasts; OCL: osteoclasts; OC: osteocytes; 24,25(OH)2D3: 24,25-dihydroxyvitamin D3; 1,25(OH)2D3: 1,25-dihydroxyvitamin D3; VDR: vitamin D receptor; 1α-hydroxylase: 1-alpha hydroxylase; ERα: estrogen receptor alpha; ERβ: estrogen receptor beta; GPR30: G-protein coupled receptor 30; TRα/β1/β2: thyroid hormone receptors α and β1/2; GR: glucocorticoid receptor; GC: glucocorticoids, CaSR: calcium-sensing receptor; PTH: parathyroid hormone; PTH1R: parathyroid hormone 1 receptor; Sik2/3: salt-inducible kinase 2/3; HDAC4/5: histone deacetylase 4/5; GH: growth hormone, GHR: growth hormone receptor; IGF1R: insulin-like growth factor 1 receptor; IGFbp2: insulin-like growth factor binding protein 2; IGF1: insulin-like growth factor 1; JAK2: Janus kinase 2; STAT5b: signal transducer and activator of transcription 5b; SOCS2: suppressor of cytokine signaling 2; Sox: SRY-box transcription factor; Foxa2: forkhead box A2; RUNX2: runt-related transcription factor; Osx: Osterix or Sp7; Ihh: Indian hedgehog; Ptch1: patched-1; Smo: smoothened; Gli1/Gli2: GLI family zinc finger 1 and 2; mTOR: mechanistic target of rapamycin; PTHrP: parathyroid hormone-related protein; Zfp521: zinc finger protein 521; FGF: fibroblast growth factor; FGFR1/2/3/4: fibroblast growth factor receptors 1, 2, 3, and 4; TGFβ1: transforming growth factor beta 1; BMP: bone morphogenetic protein; Smad1/5: SMAD family members 1 and 5; Shn2/Shn3: Schnurri-2 and Schnurri-3; CNP: C-type natriuretic peptide; NPr2: natriuretic peptide receptor 2; NKCC1: Na-K-Cl cotransporter 1; AQP1: aquaporin 1; Col II: type II collagen; ACAN: cartilage-specific proteoglycan core protein; MMP9/13: matrix metalloproteinase 9/13; Col X: type X collagen; ALP: alkaline phosphatase, OPN: osteopontin; VEGF: vascular endothelial growth factor; Col I: type I collagen; BGLAP: bone gamma-carboxyglutamate protein or osteocalcin; DMP1: dentin matrix protein 1; CTSK: cathepsin K; Wnt: Wnt family proteins; Frz: frizzled receptors; LRP5: low-density lipoprotein receptor-related protein 5; RANK: receptor activator of nuclear factor kappa-B; RANKL: RANK ligand; cRaf: Raf kinase; pERK: phosphorylated extracellular signal-regulated kinase.

Figure 6

Schematic representation of cellular interactions in the growth plate environment. The diagram illustrates the different stages of chondrocytes from resting chondrocytes (RCs) through proliferative chondrocytes (PCs), prehypertrophic chondrocytes (PHCs), hypertrophic chondrocytes (HCs), and terminal hypertrophic chondrocytes (THCs). Key signaling pathways and molecular mediators influencing cell differentiation, including parathyroid hormone-related protein (PTHrP), Indian hedgehog (Ihh), vascular endothelial growth factor (VEGF), insulin-like growth Factor 1 (IGF1), RANK Ligand (RANKL), Bone Morphogenetic Proteins (BMP), and Fibroblast Growth factor 18 (FGF18), are shown with black and red arrows. Interactions between chondrocytes and surrounding cells, including osteoblasts (OBs), osteoclasts (OCs), and vascular endothelial cells (VECs), highlight factors like IGF1, FGF23, and Wnt signaling in growth plate regulation. Feedback inhibition pathways are also indicated, such as SOST.