Enchondroma

Excerpt

Bone is a unique connective tissue type, being one that undergoes mineralization. Bone's inorganic component consists of calcium hydroxyapatite, which imparts strength and toughness. This mineral is also the body's main calcium (99%) and phosphate (85%) reservoir and stores 65% of its sodium and magnesium.

Meanwhile, bone's organic component is comprised of bone cells and matrix proteins. The bone cells include the following:

1. Osteoprogenitor cells: pluripotent mesenchymal stem cells in bone surfaces that growth factors can stimulate to differentiate into osteoblasts

2. Osteoblasts: initiate mineralization and produce, transport, and arrange matrix proteins. Osteoblast actions are regulated by parathyroid hormone, vitamin D, estrogen, growth factors, cytokines, leptin, and low-density lipoprotein-related protein 5. Osteoblasts may become osteocytes or surface-lining cells of bones.

3. Osteocytes: the most numerous cells in bone. Osteocytes participate in key bodily functions like calcium and phosphate homeostasis and cyclic adenosine monophosphate activation.

4. Osteoclast: hematopoietic progenitor cell-derived bone cells responsible for bone resorption

Bone matrix proteins include type 1 collagen and noncollagenous osteoblast-derived proteins.

Homeobox genes encode the skeletal morphogenesis regulators. Mesenchyme from the following regions gives rise to different parts of the skeleton:

1. Cranial neural crest: craniofacial skeleton

2. Paraxial mesoderm: axial skeleton

3. Lateral plate mesoderm: appendicular skeleton

In the embryo, craniofacial and clavicular bones undergo intramembranous ossification, whereby osteoblasts form bone directly from mesenchyme. These bones enlarge only when new bone deposits on the surface—a process called "appositional growth." Meanwhile, most other bones begin as a cartilage anlage, undergoing enchondral ossification around the 8th week of gestation. Bones that form this way increase in length due to the development of the physis or growth plate at their ends. Growth plates have a cartilaginous core covered by a layer of bone (primary spongiosa). Enchondral ossification within growth plates increases bone length and diameter.

Hyaline-cartilage bone tumors arise either within the medullary cavity or on the bone surface. Enchondromas are benign medullary cavity hyaline-cartilage tumors occurring in bones of endochondral origin. These growths are usually solitary, central, metaphyseal tubular bone lesions. In contrast, subperiosteal or juxtacortical chondromas form on the bone surface.

Enchondromas arise most commonly from the hand and foot bones but may occasionally appear in the femur and humerus. These neoplasms are also the most common primary bone tumors of the hand, where they commonly appear in the proximal phalanges. Other common sites include the middle phalanges, metacarpals, and distal phalanges.

Enchondromatous tumors typically begin and grow in childhood, arising from rests of growth plate cartilage or chondrocytes that initially proliferate but stop developing normally and persist throughout adulthood. These tumors are most frequently noted in the 3rd and 4th decades of life. The small finger is the most commonly affected digit. Hand enchondromas are unique, as they may also demonstrate cellular atypia, resembling chondrosarcoma on histopathological examination.

Enchondromas have malignant potential and may transform into chondrosarcoma. However, malignancies arising from enchondromas are low-grade and rarely metastasize to other body regions.