Bones (EX2)

Most abundant type of skeletal cartilage

Hyaline cartilage

Seven primary functions of the skeletal system

1. Support

2. Protection

3. Movement

4. Mineral and growth factor storage

5. Blood cell formation (hematopoiesis)

6. Triglyceride (fat) storage

7. Hormone production

Diaphysis

The tubular shaft that forms the long axis of a long bone.

Epiphyses

The proximal and distal expanded ends of a long bone.

Metaphysis

The transitional zone between the diaphysis and epiphysis containing the growth zone.

Epiphyseal growth plate

A disc of hyaline cartilage located at the metaphysis that permits longitudinal bone growth during development.

Epiphyseal line

The calcified osseous seam remaining in the metaphysis after longitudinal growth ceases and the cartilaginous growth zone closes.

Bone remodeling

The continuous coupled physiological cycle of matrix breakdown by osteoclasts and new tissue deposition by osteoblasts.

Osteoblasts

Bone-forming cells that synthesize and secrete unmineralized organic matrix (osteoid) and promote mineralization.

Osteocytes

Mature bone cells entrapped within lacunae that act as mechanosensors and maintain extracellular matrix integrity.

Osteoclasts

Large, multinucleated phagocytic cells derived from hematopoietic stem cells that break down calcified bone matrix through acid and enzymatic resorption.

Compact bone

Dense, solid osseous tissue organized into osteons that forms the protective external layer and shaft of bones.

Spongy (cancellous) bone

Porous, lightweight osseous tissue arranged in a lattice network of trabeculae that lines internal cavities and epiphyses.

Osteon (Haversian system)

The structural and functional unit of mature compact bone, consisting of concentric lamellae surrounding a central canal.

Lamellae

Concentric, interstitial, or circumferential rings of mineralized collagen matrix arranged in alternating directions within bone.

Central (Haversian) canal

A longitudinal channel passing through the core of each osteon containing blood vessels and nerve fibers.

Perforating (Volkmann's) canals

Transverse channels perpendicular to central canals that link neurovascular supplies from the periosteum to internal marrow cavities.

Lacunae

Small fluid-filled cavities within the mineralized bone matrix that house individual osteocytes.

Canaliculi

Hairlike microscopic canals extending between lacunae that permit metabolic exchange and gap-junction communication among osteocytes.

Trabeculae

Slender plates and interconnecting struts of mineralized tissue that form the structural framework of spongy bone.

Hematopoiesis

The biological process of blood cell formation and maturation.

Red bone marrow

Hematopoietic connective tissue located within the trabecular spaces of spongy bone that produces erythrocytes, leukocytes, and platelets.

Yellow bone marrow

Adipose-rich tissue residing within the medullary cavity of adult long bones that serves as an energy reserve.

Articular cartilage

A thin layer of hyaline cartilage covering joint surfaces of epiphyses that cushions compressive forces and minimizes friction.

Periosteum

A double-layered membrane of dense irregular connective tissue and osteogenic cells covering the external surface of bones, excluding articular regions.

Endosteum

A delicate osteogenic membrane lining the medullary cavity, trabecular surfaces, and central canals internally.

Perichondrium

A sheath of dense irregular connective tissue surrounding non-articular cartilage that provides mechanical protection and vascular supply.

Chondroblasts vs. Chondrocytes

Chondroblasts are actively secreting immature cartilage cells; chondrocytes are mature cells that maintain the surrounding cartilage matrix from within lacunae.

Ossification (osteogenesis)

The developmental and physiological process of bone tissue formation during embryonic development, growth, remodeling, and repair.

Intramembranous ossification

The pathway of bone development directly from fibrous mesenchymal sheets without using a preexisting hyaline cartilage scaffold.

Bones formed via intramembranous ossification

Cranial bones of the skull (frontal, parietal, occipital, temporal), the facial bones (including the mandible), and the clavicles.

Four stages of intramembranous ossification

1. Development of ossification centers

2. Calcification of the osteoid

3. Formation of woven trabeculae

4. Development of the periosteum and compact bone collars

Endochondral ossification

The pathway of bone formation where a preexisting model of hyaline cartilage is progressively broken down and replaced by osseous tissue.

Six stages of endochondral ossification

1. Formation of the hyaline cartilage model

2. Cartilage growth and central matrix calcification

3. Primary ossification center formation in the diaphysis

4. Medullary cavity hollowed out by osteoclasts

5. Secondary ossification centers emerge in epiphyses

6. Retention of articular cartilage and epiphyseal growth plates

Primary vs. Secondary ossification centers

The primary center develops first in the center of the diaphysis; secondary centers appear later around the time of birth in the epiphyses.

Role of Vitamin A in bone metabolism

Regulates osteoblast and osteoclast activity to ensure coordinated remodeling during longitudinal bone growth.

Role of Vitamin C in bone metabolism

Acts as an essential cofactor for the enzymatic hydroxylation of proline and lysine during collagen synthesis.

Role of Vitamin D in bone metabolism

Facilitates active intestinal absorption of dietary calcium and phosphate essential for matrix mineralization.

Role of Vitamin K in bone metabolism

Serves as a cofactor for gamma-carboxylation of osteocalcin, enabling high-affinity binding of calcium ions to matrix.

Rickets vs. Osteomalacia

Rickets is pediatric defective mineralization causing soft, deformed bones; osteomalacia is the adult equivalent resulting in poorly mineralized, painful, fracture-prone bone.

Effects of Growth Hormone (GH) and IGF-1 on bone

GH stimulates hepatic synthesis of insulin-like growth factors (IGFs), which stimulate chondrocyte mitosis in the epiphyseal plate and matrix secretion by osteoblasts.

Gigantism vs. Acromegaly

Gigantism results from GH hypersecretion in childhood prior to epiphyseal plate fusion (causing extreme stature); acromegaly results from adult GH hypersecretion post-fusion (causing appositional thickening of facial bones, hands, and feet).

Parathyroid hormone (PTH)

A hypercalcemic hormone released by parathyroid glands that increases blood calcium by stimulating osteoclast resorption, increasing renal calcium reabsorption, and promoting calcitriol synthesis.

Calcitonin

A hypocalcemic hormone released by thyroid parafollicular cells that lowers blood calcium by inhibiting osteoclast activity and reducing bone resorption.

Calcitriol

The physiologically active hormonal metabolite of vitamin D ($1,25\text{-(OH)}_2\text{D}_3$), synthesized in the kidneys, that stimulates active intestinal calcium transport.

Four stages of bone fracture repair

1. Hematoma formation

2. Fibrocartilaginous (soft) callus formation

3. Bony (hard) callus formation

4. Bone remodeling

Osteoporosis

A metabolic bone disease characterized by an uncoupling of remodeling where osteoclastic resorption outpaces osteoblastic formation, leading to low bone mineral density and increased skeletal fragility.

Type I vs. Type II collagen in cartilage

Type I collagen provides high tensile strength and is abundant in fibrocartilage. Type II collagen is the main collagen in hyaline and elastic cartilage.

Medullary cavity

The hollow central cavity in the diaphysis of a long bone that contains bone marrow, usually yellow marrow in adults.

Thyroid hormone and bone growth

Thyroid hormones support normal skeletal growth, bone maturation, and remodeling. Too little can delay growth and skeletal development.

Sex hormones and the pelvis

Estrogen and testosterone contribute to the pubertal growth and maturation of bones. Estrogen helps close the epiphyseal plates, and the female pelvis typically becomes wider with a broader pelvic inlet and outlet

Exercise and bone tissue

Weight-bearing exercise and mechanical stress stimulate bone remodeling and help maintain bone density and strength.

Aging and bone tissue

With aging, bone resorption can exceed bone formation, decreasing bone density and increasing the risk of fractures and osteoporosis.