Bones and Skeletal Tissue

Skeletal Cartilage

Skeletal cartilage is a resilient tissue that provides support and flexibility in the human body. It is avascular (contains no blood vessels or nerves) and is surrounded by the perichondrium, a dense irregular connective tissue that resists outward expansion.

• Types of Cartilage:

  • Hyaline Cartilage: Most abundant; provides support, flexibility, and resilience. Found in articular surfaces of long bones, costal cartilages, respiratory structures (larynx, trachea), and nasal cartilages.

  • Elastic Cartilage: Contains elastic fibers; maintains shape and flexibility. Found in the external ear and epiglottis.

  • Fibrocartilage: Highly compressible with great tensile strength due to thick collagen fibers. Found in intervertebral discs and menisci of the knee.

• Growth of Cartilage:

  • Appositional growth: Cells in the perichondrium secrete matrix against the external face of existing cartilage.

  • Interstitial growth: Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within.

Bones and Cartilages of the Human Body

The human skeleton consists of bones and cartilages that provide structure and facilitate movement.

• Axial skeleton: Bones of the skull, vertebral column, and rib cage.

• Appendicular skeleton: Bones of the upper and lower limbs, shoulder, and hip.

• Other bone types:

  • Long bones (e.g., humerus)

  • Short bones (e.g., wrist and ankle bones)

  • Flat bones (e.g., sternum, skull bones)

  • Irregular bones (e.g., vertebrae, hip bones)

  • Sesamoid bones (form within tendons, e.g., patella)

Classification of Bones: By Shape

Bones are classified according to their shapes, which relate to their functions.

• Long bones: Longer than they are wide; mostly limb bones.

• Short bones: Cube-shaped; found in wrist and ankle.

• Flat bones: Thin, flattened, and usually curved; e.g., sternum, skull.

• Irregular bones: Complicated shapes; e.g., vertebrae, hip bones.

• Sesamoid bones: Form within tendons; e.g., patella.

Function of Bones

Bones perform several vital functions in the body:

• Support: Framework that supports the body and cradles soft organs.

• Protection: Protective case for the brain, spinal cord, and vital organs.

• Movement: Provide levers for muscles.

• Mineral storage: Reservoir for minerals, especially calcium and phosphorus.

• Blood cell formation: Hematopoiesis occurs within the marrow cavities of bones.

Gross Anatomy of Bones

Bones have a complex structure with distinct regions:

• Compact bone: Dense outer layer providing strength.

• Spongy bone: Honeycomb of trabeculae filled with yellow bone marrow (fat).

Structure of Long Bone

Long bones have a tubular shaft and expanded ends.

• Diaphysis: Shaft forming the axis of long bones; contains the medullary cavity filled with yellow bone marrow.

• Epiphyses: Expanded ends; exterior is compact bone, interior is spongy bone. Joint surfaces covered with articular (hyaline) cartilage.

• Epiphyseal line: Separates the diaphysis from the epiphyses.

Bone Membranes

Bones are covered and lined by specialized membranes:

• Periosteum: Double-layered membrane covering external bone surfaces; outer fibrous layer is dense regular CT, inner osteogenic layer contains osteoblasts and osteoclasts. Richly supplied with nerves, blood, and lymphatic vessels.

• Endosteum: Delicate membrane covering internal bone surfaces.

Comparison of Different Types of Bone Cells

Bones contain several specialized cell types:

Structure of Short, Irregular, and Flat Bones

These bones consist of thin plates of periosteum-covered compact bone on the outside and endosteum-covered spongy bone (diploë) on the inside. They have no diaphysis or epiphyses, and bone marrow is found between the trabeculae.

Location of Hematopoietic Tissue (Red Marrow)

Red marrow is responsible for blood cell formation.

• Found in the medullary cavity and all areas of spongy bone in infants.

• In adults, found in the diploë of flat bones and the head of the femur and humerus.

Microscopic Anatomy of Bone: Compact Bone

Compact bone is organized into structural units called osteons (Haversian systems).

• Osteon: Weight-bearing, column-like matrix tubes composed mainly of collagen.

• Central (Haversian) canal: Channel containing blood vessels and nerves.

• Perforating (Volkmann's) canals: Channels lying at right angles to the central canal, connecting blood and nerve supply of the periosteum to the central canal.

• Lacunae: Small cavities in bone that contain osteocytes.

• Canaliculi: Hairlike canals that connect lacunae to each other and the central canal.

Chemical Composition of Bone

Organic Components

• Osteoblasts: Bone-forming cells.

• Osteocytes: Mature bone cells.

• Osteoclasts: Large cells that resorb or break down bone matrix.

• Osteoid: Unmineralized bone matrix composed of proteoglycans, glycoproteins, and collagen.

Inorganic Components

• Mineral salts: Sixty-five percent of bone by mass; mainly calcium phosphates.

• Responsible for bone hardness and resistance to compression.

Bone Markings

Bone markings are projections, depressions, and openings that serve as sites of attachment for muscles, ligaments, and tendons, as well as conduits for blood vessels and nerves.

• Projections (Sites of Muscle and Ligament Attachment):

  • Tuberosity – large rounded projection

  • Crest – narrow, prominent ridge

  • Trochanter – large, blunt, irregularly shaped process

  • Line – narrow ridge

  • Tubercle – small rounded projection

  • Epicondyle – raised area above a condyle

  • Spine – sharp, slender projection

  • Process – any bony prominence

• Projections That Help to Form Joints:

  • Head – bony expansion carried on a narrow neck

  • Facet – smooth, nearly flat articular surface

  • Condyle – rounded articular projection

  • Ramus – armlike bar of bone

• Depressions and Openings:

  • Meatus – canal-like passageway

  • Sinus – cavity within bone

  • Fossa – shallow, basinlike depression

  • Groove – furrow

  • Fissure – narrow, slitlike opening

  • Foramen – round or oval opening through bone

Bone Development

Ossification (osteogenesis) is the process of bone tissue formation, which leads to the formation of the bony skeleton in embryos, bone growth until early adulthood, and bone thickness, remodeling, and repair.

• Formation of the Bony Skeleton: Begins at week 8 of embryonic development.

• Intramembranous ossification: Bone develops from a fibrous membrane; forms most flat bones of the skull and clavicles.

• Endochondral ossification: Bone forms by replacing hyaline cartilage; forms most bones of the body.

Stages of Intramembranous Ossification

• Ossification center appears in the fibrous CT membrane.

• Bone matrix is secreted within the fibrous membrane.

• Bone and periosteum form.

• Bone collar of compact bone forms, and red marrow appears.

Stages of Endochondral Ossification

• Formation of bone collar.

• Cavitation of the hyaline cartilage.

• Invasion of internal cavities by the periosteal bud, and spongy bone formation.

• Formation of the medullary cavity; appearance of secondary ossification centers in the epiphyses.

• Ossification of the epiphyses, with hyaline cartilage remaining only in the epiphyseal plates.

Postnatal Bone Growth

After birth, bones grow in length and thickness.

• Growth in length of long bones occurs at the epiphyseal plate.

• Cartilage on the side of the epiphyseal plate closest to the epiphysis is relatively inactive.

• Cartilage abutting the shaft of the bone organizes into a pattern that allows fast, efficient growth.

• Epiphyseal plate consists of functionally different zones: growth, transformation, and osteogenic.

Functional Zones in Long Bone Growth

• Growth zone: Cartilage cells undergo mitosis, pushing the epiphysis away from the diaphysis.

• Transformation zone: Older cells enlarge, the matrix becomes calcified, cartilage cells die, and the matrix deteriorates.

• Ossification/osteogenic zone: New bone formation takes place.

Long Bone Growth and Remodeling

Bones grow in length as cartilage continually grows and is replaced by bone. Remodeling involves bone resorption and addition by appositional growth.

Hormonal Regulation of Bone Growth During Youth

Bone growth is regulated by hormones:

• Growth hormone stimulates epiphyseal plate activity in infancy and childhood.

• During puberty, testosterone and estrogens promote adolescent growth spurts and masculinization/feminization of specific parts of the skeleton.

• Later, epiphyseal plate closure ends longitudinal bone growth.

Bone Remodeling

Bone remodeling involves the coordinated activity of osteoblasts and osteoclasts at periosteal and endosteal surfaces.

• Units deposit and resorb bone at periosteal and endosteal surfaces.

• Deposit occurs where bone injury or added strength is required.

• Remodeling is rich in vitamins C, D, A, manganese, magnesium, calcium, and phosphorus.

Bone Deposition

Bone deposition occurs where bone is injured or added strength is needed.

• Requires a diet rich in protein, vitamins C, D, and A, calcium, phosphorus, magnesium, and manganese.

• Alkaline phosphatase is essential for mineralization of bone.

• Sites of new matrix deposition are revealed by the presence of the osteoid seam (unmineralized band of bone matrix).

Bone Resorption

Osteoclasts break down bone matrix, releasing minerals into the blood.

• Osteoclasts secrete lysosomal enzymes and acids to digest bone matrix.

• Calcium is released into the bloodstream.

Importance of Ionic Calcium in the Body

Calcium is essential for several physiological processes:

• Transmission of nerve impulses

• Muscle contraction

• Blood coagulation

• Secretion by glands and nerve cells

• Cell division

99% of body calcium is found in bone; the remainder is in blood and cells.

Control of Bone Remodeling

Bone remodeling is controlled by hormonal and mechanical factors.

• Hormonal mechanism:

  • Blood calcium levels trigger the thyroid to release calcitonin, which stimulates calcium salt deposit in bone.

  • Low blood calcium levels stimulate the parathyroid glands to release parathyroid hormone (PTH), which stimulates osteoclasts to degrade bone matrix and release calcium into the blood.

• Mechanical and gravitational forces: Bones remodel in response to the demands placed upon them (Wolff's law).

Bone Fractures

Fractures are classified by their position, completeness, and whether the bone ends penetrate the skin.

• Types of fractures:

  • Nondisplaced – bone ends retain their normal position

  • Displaced – bone ends are out of normal alignment

  • Complete – bone is broken all the way through

  • Incomplete – bone is not broken all the way through

  • Linear – fracture is parallel to the long axis of the bone

  • Transverse – fracture is perpendicular to the long axis

  • Open (compound) – bone ends penetrate the skin

  • Closed (simple) – bone ends do not penetrate the skin

• Common types:

  • Comminuted – bone fragments into three or more pieces

  • Compression – bone is crushed

  • Spiral – ragged break due to twisting forces

  • Epiphyseal – separates epiphysis from diaphysis

  • Depressed – broken bone portion is pressed inward

  • Greenstick – incomplete break; one side breaks, other bends

Stages in the Healing of a Bone Fracture

Bone healing occurs in several stages:

• Hematoma formation – blood vessels break, forming a clot

• Fibrocartilaginous callus formation – capillaries grow, phagocytic cells clean debris, fibroblasts produce collagen fibers

• Bony callus formation – new spongy bone forms

• Bone remodeling – compact bone is laid down, restoring shape

Developmental Aspects of Bones

Bone formation begins in the embryo and continues through life.

• Embryonic mesenchymal cells produce membranes and cartilage that form the initial skeleton.

• Most bones are well ossified by birth, except for some fontanelles.

• In old age, bone resorption predominates, leading to decreased bone mass and increased risk of osteoporosis.

• Genetic factors influence bone mass accumulation and osteoporosis risk.

Example: The process of endochondral ossification is responsible for the formation of most bones in the body, such as the femur and humerus, by replacing hyaline cartilage with bone tissue.

Additional info: Bone remodeling is a lifelong process, with about 5-7% of bone mass recycled weekly. Hormones such as PTH and calcitonin play key roles in calcium homeostasis and bone health.