Bones: Structure, Function, and Bone Markings (Vocabulary Flashcards)

Skeletal System: Structure, Function and bone markings

• Source context: Transcript covers the structure of the musculoskeletal system, bone tissue types, cellular components, long and flat bones, the Haversian (Osteon) system, bone markings, and their functional significance.

Skeletal System Overview

• Initially made up of cartilage and fibrous membranes; replaced by bone during development.

• Skeletal system components: bones, joints, cartilages, ligaments.

• The Human Endoskeleton (selected parts shown): sternum, ribs, vertebral column, cranium, maxilla, mandible, skull, clavicle, pectoral girdle, scapula, humerus, radius, ulna, carpal bones, metacarpal bones, phalanges.

• Functions of the skeletal system (summarized):

  • Support: bones provide a supporting framework and anchor soft tissues.

  • Protection: hard bony boxes enclose delicate structures (e.g., skull for brain; sternum and ribs for lungs and heart).

  • Movement: muscles attach to bones; contraction pulls on bones to produce movement.

  • Provision for attachment: surfaces for muscle attachment.

  • Blood formation (haemopoiesis): occurs in red bone marrow.

  • Storage of minerals: calcium stored in bones; released when needed.

Structure and Components of Bone

• Bones are organs composed of several tissues:

  • Dominant tissue: bony (osseous) tissue.

  • Other tissues: nervous tissue (nerves), cartilage in articular areas, fibrous connective tissue, muscles and epithelial tissue in blood vessels.

• Bone texture:

  • Compact (cortical) bone: dense outer layer, smooth and solid.

  • Spongy (trabecular, cancellous) bone: honeycomb of trabeculae; spaces filled with red or yellow marrow.

• Spongy bone is also described as diploë in flat bones.

• Articular cartilage present at joints for smooth movement.

Long Bones: Structure in Detail

• Diaphysis: tubular shaft forming the long axis; outer thick collar of compact bone; central medullary (marrow) cavity (adults contain yellow marrow in the medullary cavity).

• Epiphysis: bone ends; more expanded than the diaphysis; exterior is compact bone covered by articular cartilage; interior contains spongy bone.

• Epiphyseal Line: growth line between diaphysis and epiphysis; remnant of the epiphyseal plate (hyaline cartilage) from which bone develops.

• Medullary Cavity: shaft cavity; contains fatty yellow marrow in adults.

• Bone Marrow: found in bones with cancellous bone; in infants red marrow forms blood cells; in adults red marrow is found in femur, ribs, pelvis, and vertebral bones.

Periosteum and Endosteum

• Periosteum: outer fibrous layer (dense, irregular connective tissue) and inner osteogenic layer (bone-forming osteoblasts + bone-destroying osteoclasts).

  • Richly supplied with blood vessels, nerves, and lymphatics.

  • Nutrient vessels enter the shaft via nutrient foramen; attached to bone by Sharpey’s fibers (collagen fibers).

  • Functions: provides insertion/anchorage points for tendons and ligaments.

• Endosteum: lines the medullary cavity, covers trabeculae of spongy bone and line canals within compact bone; contains osteoblasts and osteoclasts.

Flat Bones

• Structure: two thin plates of compact bone with spongy bone in the center (diploë).

• Bone marrow resides in the spongy layer.

• No diaphysis or epiphyses in flat bones.

Chemical Composition of Bone Tissue

• Bone tissue consists of inorganic and organic components:

  • Inorganic: $65\%$ of bone mass, mainly calcium phosphate crystals tightly packed in the matrix.

  • Organic: $35\%$ of bone mass, consisting of bone cells (osteoblasts, osteoclasts, osteocytes) and osteoid (collagen and proteins).

• Why these components matter: inorganic portion provides hardness and compression resistance; organic portion (especially collagen) provides tensile strength and toughness.

Bone Cells

• Osteogenic cells (osteoprogenitor cells): stem cells capable of differentiating into osteoblasts.

• Osteoblasts: matrix-synthesizing cells responsible for bone formation.

• Osteocytes: mature bone cells that monitor and maintain the mineralized bone matrix.

• Osteoclasts: bone-resorbing cells involved in bone remodeling.

Classification of Bones by Shape

• Long bones: longer than they are broad; contain a diaphysis (shaft) and epiphyses at both ends. Examples: humerus.

• Short bones: shorter than they are broad; thin layer of compact bone surrounding a spongy interior. Examples: bones of the wrist (carpals) and ankle (tarsals).

• Flat bones: thin, flattened, curved; two parallel layers of compact bone sandwiching a layer of spongy bone (diploë). Examples: skull bones, ribs, sternum.

• Sesamoid bones: embedded within tendons; protect tendons and sometimes alter tendon mechanics. Example: patella (kneecap).

• Irregular bones: do not fit into other categories; may have a variety of shapes. Example: vertebrae, pelvic bones.

Examples of Bone Shapes (from figures)

• (a) Long bone: humerus

• (b) Irregular bone: vertebra

• (c) Flat bone: sternum

• (d) Short bone: talus

Structure of the Osteon / Haversian System (Compact Bone)

• Long bones are composed of parallel osteons (central structural units).

• Each osteon consists of:

  • A central canal (Haversian canal) with blood vessels and nerves.

  • Concentric lamellae: layers of bone matrix around the central canal.

  • Lamellae are made of collagen fibers in alternating directions.

  • Osteocytes reside in lacunae between lamellae.

  • Canaliculi: tiny channels that connect osteocytes and allow communication and nutrient/waste exchange.

• The osteon is part of the compact bone; surrounding it, spongy bone contains trabeculae and osteocytes connected by canaliculi.

• Perforating (Volkmann’s) canals run perpendicular to the long axis and connect blood vessels and nerves from the periosteum to the central canals.

• Endosteum lines the bony canals and covers trabeculae; periosteum covers the outer surface of the bone.

• Sharpey’s fibers (perforating fibers) anchor the periosteum to the bone.

Bone Markings: Projections, Depressions, and Openings

• Function: bone markings serve as attachments for muscles, conduits for nerves and vessels, and help form joints.

• Projections (sites for muscle/ligament attachment):

  • Head, trochanter, tuberosity, condyle, etc.

• Depressions (sites for vessels/nerves or at joints):

  • Notch, fossa, sinuses, groove, etc.

• Openings (passageways for nerves and vessels):

  • Foramen, meatus.

Illustrative Examples of Projections and Features

• Iliac crest, ischial spine, and other pelvic landmarks (coxal bone)

• Spinous process, facets, and articular features of vertebrae

• Trochanter, intertrochanteric line, and epicondyles in the femur

• Condyles and epicondyles at knee joints and distal femur

• Head, facets, and ramus in the mandible and other bones

• Meatus, fossa, notch, groove, and foramen in various skull and facial bones

Summary Connections and Implications

• Structure–function link: bone shape and markings correspond to mechanical roles (weight-bearing, leverage, protection) and to joint mechanics (articulation surfaces, ligaments attachment).

• Growth and remodeling:

  • Epiphyseal plate (growth plate) activity enables longitudinal bone growth during development.

  • Epiphyseal line marks the remnant of the growth plate in adulthood.

  • Periosteum and endosteum supply osteoblasts/osteoclasts for growth and remodeling.

• Clinical relevance (implied): understanding the distribution of red vs yellow marrow, the role of the nutrient foramen, and the significance of marrow changes with age in diagnostics.

Remembered Terminology Quick Reference

• Axial components: skull, vertebral column, thoracic cage (sternum, ribs).

• Appendicular components: limbs and girdles (pelvic, pectoral girdles).

• Key tissue terms: osteoblasts, osteoclasts, osteocytes, osteoid, calcified matrix.

• Key bone types: compact bone, spongy bone, diploë (in flat bones).

• Key structural units: osteon (Haversian system), central canal, lamellae, lacunae, canaliculi.

• Key growth structures: epiphyseal plate/line, metaphysis, diaphysis.

• Key coverings: periosteum, endosteum.

• Key marrow types: red marrow (blood cells in infants and in specific adult sites), yellow marrow (fat).