Long Bone Anatomy and Growth

Overview

This lecture covers the gross and microscopic anatomy of long bones, the processes of bone development (ossification), and the mechanisms of bone growth in length and width.

Long Bone Anatomy

• The diaphysis is the middle shaft of the long bone, mainly made of compact bone.
• The medullary cavity, inside the diaphysis, contains yellow bone marrow, which stores fats and lipids.
• Epiphyses are the ends of long bones, comprised of a thin compact bone layer around spongy bone with red bone marrow inside.
• Articular cartilage is a thin layer of hyaline cartilage covering bone ends, reducing friction at joints.
• The periosteum is a fibrous connective tissue membrane covering bones and is continuous with tendons (muscle to bone) and ligaments (bone to bone).
• The endosteum is a thin connective tissue lining the medullary cavity; both periosteum and endosteum contain bone-forming cells.

Microscopic Structure of Bone

• Osteons are the structural units of compact bone, composed of concentric circles called lamellae.
• Bone matrix consists of collagen (gives flexibility) and hydroxyapatite (calcium and phosphate, provides hardness).
• Central canals run through osteons, containing blood vessels and nerves, allowing bone repair.
• Perforating canals connect central canals across osteons.
• Lacunae are small cavities housing bone cells (osteocytes).
• Canaliculi are tiny channels connecting osteocytes for communication via gap junctions.

Bone Development and Growth (Osteogenesis)

• Ossification is the process of converting connective tissue to bone.
• Intramembranous ossification forms bone directly from mesenchymal connective tissue, producing mostly spongy bone initially.
• The periosteum then forms, and compact bone fills in the outer areas.
• Intramembranous ossification makes flat, irregular, short, and sesamoid bones (not long bones).

Endochondral Ossification (Long Bone Formation)

• Begins with a hyaline cartilage model replaced by bone.
• Chondrocytes in the cartilage model turn into osteoblasts to start bone formation.
• Ossification spreads from the center towards the epiphyses.

Bone Growth in Length

• The epiphyseal (growth) plate is a cartilage band separating the epiphysis and diaphysis, allowing length growth.
• New cartilage forms in the growth plate and is gradually pushed toward the diaphysis, elongating the bone.
• The zone of calcification beneath the growth plate converts cartilage to bone.
• When ossification overtakes the growth plate, bone growth stops.

Bone Growth in Width

• Osteoblasts in the periosteum lay down new bone material, forming grooves around blood vessels.
• As new bone wraps around vessels, new osteons are formed, increasing bone thickness.

Key Terms & Definitions

• Diaphysis — Shaft of a long bone.
• Epiphysis/Epiphyses — End(s) of a long bone.
• Medullary cavity — Central cavity in diaphysis containing yellow marrow.
• Periosteum — Outer membrane covering bone.
• Endosteum — Membrane lining the medullary cavity.
• Osteon — Structural unit of compact bone.
• Lamellae — Concentric rings in osteons.
• Lacuna — Small cavity containing an osteocyte.
• Canaliculi — Tiny channels connecting osteocytes.
• Ossification — Conversion of connective tissue to bone.
• Intramembranous ossification — Bone formation from mesenchyme.
• Endochondral ossification — Bone formation from hyaline cartilage.
• Epiphyseal plate — Growth plate of cartilage in long bones.

Action Items / Next Steps

• Review diagrams of long bone anatomy and microscopic bone structure.
• Study the steps of intramembranous and endochondral ossification.
• Understand the roles of the periosteum and endosteum in bone growth.
• Be prepared to identify key structures in bone cross-sections.