Skeletal System - Bone Structure and Function
Overview: protection, mechanics, and bone function
Levers in the body: the lever action of a bone depends on where the attached muscle is anchored on the bone; this attachment point determines the type of movement that occurs.
Protective role of bones: major protective roles for underlying organs include:
The skull protecting the brain.
Ribs protecting the heart and lungs (also offering some protection to liver).
The spine protecting the spinal cord.
The pelvis protecting organs in the lower abdomen and reproductive structures.
Core concept introduced previously: tissue is a major mineral and fat reservoir in the body. Fat is stored not only in adipose tissue but also within bone marrow.
Fat storage in bone marrow:
Yellow marrow serves as a fat reservoir within bones, especially in the medullary (marrow) cavity of long bones.
Red marrow is where erythrocytes (red blood cells) are produced.
Erythrocyte terminology:
Red blood cell is called an erythrocyte (Latin term).
Bone marrow distribution:
Red marrow is prominent in certain regions (ends of long bones and in flat bones).
Yellow marrow predominates in the medullary cavity of long bones for fat storage.
Adult skeletal count:
There are bones in the adult skeleton.
Bone classification by location:
Axial skeleton: skull, vertebral column, rib cage, and sternum.
Appendicular skeleton: limbs and girdles (pelvic girdle, pectoral girdle).
Bone classification by shape (the major and most common method used in anatomy):
Long bones: longer than wide; include a shaft (diaphysis) and two ends (proximal and distal epiphyses).
Short bones: cube-shaped, mainly in the wrist (carpals) and ankle (tarsals).
Flat bones: thin and curved; examples include sternum, ribs, and parts of the skull (cranial bones, scapulae, iliac portion of pelvis).
Irregular bones: various shapes that don’t fit the above categories (e.g., vertebrae).
Sesamoid bones: small, rounded bones embedded in tendons (e.g., the patella).
Long bones in the body: examples include femur, tibia, fibula, metatarsals, phalanges, humerus, radius, and ulna.
Common anatomical region terms used in class:
Fibula: the lateral bone of the lower leg; region is often referred to as the peroneal region (fibular region).
Tibia: medial bone of the lower leg; articulates with the femur to form the knee joint and supports weight transmission to the foot.
Pedal region: the foot region.
Metatarsals: long bones of the foot that connect to the toes.
Phalanges: the bones of the toes (and fingers); digits contain proximal, middle, and distal phalanges; the thumb and big toe have two phalanges, while the others have three.
Carpals: short bones in the wrist; tarsals: short bones in the ankle.
Cranium, scapula, acetabulum (hip socket).
Growth and maturation context:
The growth plate (epiphyseal plate) is the region where cartilage is replaced by bone to lengthen the bone during development.
In adults, the growth plate becomes an epiphyseal line, indicating maturation and cessation of lengthwise growth.
Growth can continue into the late teens or early twenties; estimates for full ossification are around – years for complete maturity.
Joint protection and cartilage roles: cartilage acts as a cushion and reduces friction in joints. Damage to hyaline articular cartilage (found on ends of long bones) is a major factor in joint problems and knee replacement considerations.
Long bones: anatomy and key regions
Long bone regions: epiphysis, diaphysis, and metaphysis (the metaphysis is the transition region where the end of the bone begins to meet the shaft).
Epiphysis:
Each long bone has a proximal and a distal epiphysis.
Epiphyses are filled with spongy bone (also called cancellous bone).
Spongy (cancellous) bone hosts red bone marrow in many regions.
Diaphysis:
The diaphysis is the tubular shaft, the narrowest part of the bone.
Outer layer is compact bone; this is the primary region by which the bone bears and transmits mechanical stress.
Metaphysis:
The region where the epiphysis meets the diaphysis; often a site of fracture due to changes in bone geometry.
Medullary (marrow) cavity:
A hollow region within the diaphysis that houses yellow marrow (fat reservoir).
The medullary cavity is lined by endosteum (inner membrane).
Endosteum and periosteum:
Endosteum lines the inner surface of the bone within the medullary cavity and participates in growth, repair, and remodeling.
Periosteum is the outer membrane surrounding the bone; it contains nerves and blood vessels and serves as an attachment site for tendons and ligaments via Sharpey’s fibers.
Outer compact bone vs inner spongy bone:
Compact bone provides most of the bone’s strength and resistance to stress.
Spongy (cancellous) bone is lighter, highly vascularized, and contains trabeculae (network of bony struts) that support the bone with a porous architecture.
Epiphyseal plate and line:
Growth plate (epiphyseal plate) is cartilage in developing bones where lengthwise growth occurs.
In mature bones, this area becomes the epiphyseal line, indicating completed ossification.
Cartilage in the growth plate is eventually replaced by bone; this process can be disrupted by injury, affecting final height.
Cartilage at maturity and replacement:
In a maturing bone, cartilage is present in the growth plate and is replaced by bone tissue over time.
The growth plate is cartilage early on; as maturation occurs, it ossifies to form the epiphyseal line.
Expected age range for complete maturity:
While maturation varies, bone growth can continue until around – years of age in some individuals.
Blood supply and remodeling context:
Bones are highly vascularized to supply nutrients for growth, repair, and remodeling.
The inner regions (medullary cavity and trabeculae) host red marrow in which hematopoiesis occurs and yellow marrow which stores fat.
Clinical relevance of metaphyseal region:
Fractures often occur in the metaphyseal region due to changes in bone geometry and stress distribution.
Osteon-related concept (basic remodeling idea):
The outer compact bone bears most mechanical stress; the inner marrow regions can undergo remodeling as needed.
Bone cells and the bone matrix
Key bone cells:
Osteoblasts: build bone; actively produce bone matrix (osteoid).
Osteocytes: mature bone cells maintaining bone tissue; embedded within bone matrix.
Osteoclasts: resorb bone; break down mineral matrix when remodeling or when calcium is needed in the bloodstream.
Osteogenic cells: immature stem cells that differentiate into osteoblasts.
Bone matrix and mineralization:
The bone matrix is rich in collagen (for resilience and flexibility) and mineral salts, primarily calcium phosphate, which give hardness and strength.
The mineral salts crystallize on the collagen framework to form a hardened matrix.
Homeostasis and remodeling dynamics:
If a bone is under constant stress and loading, the outer compact bone adapts by enlarging attachment points (e.g., trochanters on the femur) to handle greater forces.
If bone is not used, osteoclast activity can dominate, leading to bone resorption and loss of density (as seen in osteoporosis).
Bone remodeling is a continuous process that maintains bone strength and mineral homeostasis.
Interplay of growth, repair, and remodeling:
Growth and repair are linked to the activity of osteoblasts and osteocytes, with osteoclasts balancing mineral release and resorption.
The remodeling process allows bone to change shape and size in response to mechanical stresses and metabolic needs.
Cartilage, connective tissues, and bone interactions:
Cartilage is a separate type of connective tissue involved in joints and cushioning; it interacts with bone via membranes and cartilage-specific vascular and nerve features.
Cartilage: types, structure, and membranes
Cartilage as a skeletal component:
Cartilage is part of the skeletal system, not just bone, and is found in ears, epiglottis, articular joints, larynx, trachea, nose, intervertebral discs, pubic symphysis, and more.
Cartilage types and examples:
Hyaline cartilage:
Found at articular surfaces of joints (articular cartilage), costal cartilage, nose, larynx, trachea.
Hyaline cartilage provides smooth, low-friction surfaces for joints and reduces wear.
Elastic cartilage:
Found in the ear (pinna) and epiglottis; highly flexible and bendable.
Fibrocartilage:
Found in intervertebral discs and the pubic symphysis; highly resistant to compression and tension.
Cartilage membranes: perichondrium
Cartilage is surrounded by a dense connective tissue layer called the perichondrium (analogous to the periosteum for bone).
The perichondrium helps with nutrient supply and growth of cartilage, and contains cells capable of differentiating into chondroblasts in some contexts.
Cartilage composition and water content:
Cartilage contains a matrix rich in proteoglycans and collagen with a high water content, making it resilient and capable of absorbing compressive forces.
Cartilage growth and repair:
Cartilage has limited regenerative capacity compared to bone; repair is slower and less complete, which is why joint injuries can lead to long-term problems.
Cartilage in joints:
Articular cartilage refers specifically to hyaline cartilage covering the ends of bones in synovial joints.
Joint surfaces are cushioned by articular cartilage; damage to this cartilage is a major factor in osteoarthritis and joint pain.
Key bone markings and their functional significance
Bone markings reflect attachment, articulation, and passage of vessels/nerves:
Projections: trochanters, tuberosities, tubercles – sites for muscle, tendon, and ligament attachments; they adapt in size with stress and help anchor large muscles (e.g., hip muscles attaching at the greater/lesser trochanters).
Grooves (sulci): indicate paths for blood vessels or nerves.
Depressions and indentations: often mark joints or passageways for vessels/nerves.
Functional principle:
The shape and features of bone markings are shaped by mechanical stress and the bone’s functional demands.
Common terminology (without exhaustive memorization):
Head: rounded end that often forms a joint surface.
Groove (sulcus): a groove that a tendon or nerve may run through.
Tuberosity vs tubercle vs trochanter: different-sized projections for muscle attachments.
Emphasis for labs:
Students will learn these terms in lab; the important takeaway is that markings indicate function and stress adaptation, not just decoration.
Growth, aging, and clinical implications
Growth and development:
Growth plate (epiphyseal plate) is cartilage and grows in length as the bone elongates during development; after maturation, it becomes the epiphyseal line.
Cartilage in the growth plate is gradually replaced by bone (ossification) as growth proceeds.
Age-related changes:
Full ossification can occur by around – years, though this varies among individuals.
With aging, bones can become more brittle due to loss of mineral density and organic matrix, increasing fracture risk (e.g., osteoporosis).
Clinical considerations:
Cartilage damage in joints, particularly hyaline articular cartilage, can lead to pain and degenerative changes; knee replacements replace joint surfaces to restore smooth movement.
Growth plate injuries can disrupt longitudinal growth, prompting careful evaluation in children after falls or injuries.
Fractures often occur at transitional regions like the metaphysis where changes in bone geometry create weaker zones.
Quick reference: common terms and ideas to remember
Regions and terms:
Acetabulum: hip socket formed by three pelvic bones; the femoral head sits within this socket.
Acetabular region and the femoral head form the hip joint; stress is transmitted through this joint.
Patella: a sesamoid bone embedded in the quadriceps tendon to facilitate knee movement.
Pedal: relating to the feet; brachial: relating to the arm; femoral: relating to the thigh; sural: relating to the calf.
Regions and bones in the limb:
Forelimb: humerus (brachial region), radius (lateral/wrist side in anatomical position), ulna (medial/pinky side in anatomical position).
Lower limb: femur (femoral region), tibia (shin), fibula (lateral side; peroneal region).
Regional anatomy memory aids (as discussed in class):
Radius is lateral to the ulna in the standard anatomical position; radial side is near the thumb (radial pulse site).
Peroneal region refers to the fibula area, not the fibula itself.
Pedal and brachial terms help locate bones in the feet and arms respectively.
Summary: integrated view of bone structure and cartilage health
Bones are dynamic structures that protect organs, transmit mechanical forces as levers, store minerals (notably calcium and phosphorus), and host marrow for blood cell formation (red marrow) or fat storage (yellow marrow).
The long bones’ exterior compact bone provides strength and resists stress, while the interior spongy bone with trabeculae reduces weight and houses red marrow.
Bone remodeling, driven by osteoblasts, osteocytes, osteoclasts, and osteogenic cells, maintains calcium homeostasis and adapts to mechanical demands.
Cartilage provides cushioning in joints, supports growth (growth plate), and, in its mature state (hyaline, fibrocartilage, elastic), contributes to joint function and overall skeletal health.
Understanding bone markings and regional anatomy helps explain movement, joint mechanics, and why injuries in certain regions are clinically significant.