Bone, Cartilage, and Muscle Histology

Histology, Lecture 1. This lecture covers the microscopic structure of the three major connective tissues of the musculoskeletal system: bone, cartilage, and skeletal muscle.

BONE HISTOLOGY

Bone is a specialised connective tissue consisting of cells embedded in a calcified extracellular matrix of collagen (type I, ~90% of organic matrix) and hydroxyapatite (Ca10(PO4)6(OH)2 — the mineral phase). This combination provides both tensile strength (collagen) and compressive strength (hydroxyapatite).

Bone cell types: (1) Osteoblasts: derived from mesenchymal stem cells; responsible for bone formation — synthesise and secrete osteoid (unmineralised matrix, principally type I collagen); express alkaline phosphatase (marker of osteoblast activity); regulate mineralisation. (2) Osteocytes: mature osteoblasts trapped within lacunae in the mineralised matrix; maintain bone and act as mechanosensors; communicate via dendritic processes through canaliculi (the lacunocanalicular network). (3) Osteoclasts: derived from monocyte-macrophage lineage; multinucleated cells that resorb bone; attach to bone via integrins forming a sealing zone; acidify the resorption lacuna via carbonic anhydrase II and proton pumps; secrete cathepsin K to degrade organic matrix. Regulated by RANKL (expressed on osteoblasts/stromal cells) binding to RANK on osteoclast precursors; OPG (osteoprotegerin) is a decoy receptor that inhibits osteoclastogenesis. (4) Bone lining cells: quiescent, flat osteoblasts covering resting bone surfaces.

Compact (cortical) bone: the outer dense layer of long bones. Organised into osteons (Haversian systems): concentric lamellae (rings of mineralised matrix) around a central Haversian canal containing blood vessels and nerves. Between osteons lie interstitial lamellae (remnants of older osteons). Volkmann's canals run perpendicularly, connecting adjacent Haversian canals and the periosteum.

Cancellous (trabecular/spongy) bone: inner network of trabeculae (interconnected plates and rods); spaces filled with bone marrow. More metabolically active than cortical bone; site of haematopoiesis in red marrow.

Ossification: (1) Intramembranous ossification — bone forms directly from mesenchymal tissue (condensation→osteoblast differentiation→osteoid→mineralisation). Forms flat bones (skull, clavicle). (2) Endochondral ossification — bone replaces a pre-existing hyaline cartilage template. Long bone growth: cartilage at the epiphyseal growth plate undergoes sequential zones — resting, proliferating, hypertrophying, calcifying cartilage — replaced by bone from the metaphysis. Growth plates close at the end of puberty (under sex hormone influence).

CARTILAGE HISTOLOGY

Cartilage is avascular, aneural connective tissue; cells (chondrocytes) reside in lacunae surrounded by an abundant extracellular matrix (ECM) of collagen and proteoglycans (primarily aggrecan, which binds hyaluronic acid to form large aggregates that trap water, giving cartilage its compressive resistance). Nourished by diffusion from perichondrium or synovial fluid. Three types:

1. Hyaline cartilage: most common; glass-like appearance; type II collagen fibres (not visible by light microscopy); chondrocytes in isogenous groups (cell nests). Found: articular surfaces, costal cartilages, tracheal rings, epiphyseal growth plate. Has limited repair capacity — damaged articular cartilage replaced by fibrocartilage (inferior mechanical properties), hence the challenge of osteoarthritis.

1. Fibrocartilage: contains abundant type I collagen fibres visible under light microscopy (hence opaque); dense, strong. No perichondrium. Found: intervertebral discs (annulus fibrosus), pubic symphysis, menisci of the knee, TMJ disc.

1. Elastic cartilage: contains elastin fibres in addition to type II collagen; more flexible. Found: epiglottis, external ear (auricle), Eustachian tube.

SKELETAL MUSCLE HISTOLOGY

Skeletal muscle fibres are multinucleated cells (syncytia), formed by fusion of myoblasts. Nuclei are peripherally located (unlike cardiac muscle where nuclei are central, and unlike regenerating fibres or leiomyoma where nuclei can be central). Each fibre contains hundreds to thousands of myofibrils, arranged in parallel along the length of the fibre.

Sarcomere structure: the functional unit of contraction. Under electron microscopy: A band (anisotropic — dark; contains thick myosin filaments and thin actin filaments in the overlap zone); I band (isotropic — light; contains only actin; bisected by the Z-line); H zone (lighter central region of A band — myosin only, no actin overlap); M line (centre of H zone — links myosin tails). Z-lines define the boundaries of each sarcomere.

T-tubules (transverse tubules): invaginations of the sarcolemma that penetrate deep into the fibre at the A-I band junction (in skeletal muscle). They transmit the action potential from the surface membrane deep into the fibre. Each T-tubule is flanked by two terminal cisternae of the sarcoplasmic reticulum (SR) to form a triad.

Fibre types: Type I (slow-twitch, oxidative): abundant mitochondria and myoglobin (red), fatigue-resistant, sustained posture. Type IIa (fast-twitch, oxidative-glycolytic): intermediate. Type IIx (fast-twitch, glycolytic): large, fast, powerful, fatigable; few mitochondria.

Smooth muscle and cardiac muscle: Smooth muscle cells are spindle-shaped, uninucleate, no striations, no T-tubules (or poorly developed ones). Found in vessel walls, gut, uterus. Cardiac muscle: striated, involuntary, branching fibres with CENTRAL nuclei; connected at intercalated discs (contain fascia adherens, desmosomes, and gap junctions — which transmit the electrical signal). Gap junctions (connexons) allow cardiac muscle to act as a functional syncytium.