Describe bone tissue, long bone structure, ossification, growth and remodeling, classify joints, and identify the major bones of the axial and appendicular skeleton

Learning goal: Describe the skeletal system as bones, cartilage, joints and ligaments; identify the four cell types of bone tissue and the osteon of compact bone; classify bones by shape and label a typical long bone; name the major bones of the axial and appendicular skeleton and important bony features; classify joints by structure and function and describe a synovial joint; explain the processes of hematopoiesis, ossification, bone growth and remodeling and the functions they serve; use measurements of the femur to estimate height; and explain homeostatic imbalances such as osteoporosis, malnutrition, fractures, ACL injuries and arthritis.

What the skeletal system contains. The skeletal system is composed of bones, cartilage, joints and ligaments. An adult has 206 bones, which make up most of the skeleton; cartilage is found where a firm but flexible or smooth surface is needed, in the nose, the outer ears, the costal cartilages that join the ribs to the sternum and the surfaces of joints; ligaments are bands of dense regular connective tissue that connect bone to bone and stabilize joints. Its functions are support for the body, protection of soft organs (the skull around the brain, the ribs around the heart and lungs, the vertebrae around the spinal cord), movement, because skeletal muscles attach to bones and use joints as levers, storage of minerals, especially calcium and phosphate, and of fat in the yellow marrow, and formation of blood cells, hematopoiesis, in the red marrow.

Bone tissue and its four cell types. Bone is a connective tissue whose matrix is collagen fibers hardened by crystals of calcium phosphate; the collagen resists pulling and gives some flexibility, the mineral resists compression. Four cell types build and maintain it, each with a specific function. Osteogenic cells are stem cells in the periosteum and endosteum that divide to produce osteoblasts. Osteoblasts build bone: they secrete collagen and then deposit mineral on it. When an osteoblast becomes surrounded by the matrix it made, it becomes an osteocyte, a mature cell that sits in a small space called a lacuna, maintains the matrix around it and senses mechanical stress. Osteoclasts are large cells with many nuclei that dissolve bone by secreting acid and enzymes, releasing calcium into the blood. Building by osteoblasts and dissolving by osteoclasts happen throughout life; their balance decides whether bone mass rises or falls.

Compact and spongy bone. The microscopic anatomy of compact bone is the osteon: a cylinder of concentric layers (lamellae) of matrix around a central canal that carries blood vessels and nerves. Osteocytes lie in lacunae between the lamellae and reach one another through tiny channels called canaliculi, which allow nutrients to pass from the central canal cell to cell, because the hard matrix itself is impermeable. Osteons lie parallel to the long axis of a bone, which is why bone resists forces along its length best. Spongy bone has no osteons; it is a lattice of thin struts called trabeculae, oriented along the lines of stress, with red marrow in the spaces. Spongy bone is lighter and absorbs shock, so it fills the ends of long bones and the interior of flat bones.

Bones classified by shape, and the long bone. Long bones are longer than wide: the femur, humerus and even the small phalanges. Short bones are roughly cube-shaped: the carpals of the wrist and tarsals of the ankle. Flat bones are thin and often curved: the sternum, ribs, scapulae and most skull bones, built as two plates of compact bone with spongy bone between. Irregular bones are the rest: the vertebrae and hip bones. Sesamoid bones form within tendons; the patella is the largest. A typical long bone has a shaft, the diaphysis, a tube of compact bone around the medullary cavity, which holds yellow marrow in adults; two expanded ends, the epiphyses, of spongy bone covered by a thin shell of compact bone and, at the joint surface, by articular cartilage (hyaline cartilage) that reduces friction; and between shaft and end the metaphysis, which in a growing bone contains the epiphyseal plate, a disc of hyaline cartilage where lengthening occurs, replaced after growth stops by a bony epiphyseal line. The outer surface is covered by the periosteum, a fibrous membrane with an inner layer of osteogenic cells that also anchors tendons and ligaments; the internal surfaces are lined by the endosteum.

The axial skeleton (80 bones). The axial skeleton forms the body's long axis: the skull, the vertebral column and the thoracic cage, plus the hyoid and the ear ossicles. The skull has 22 bones: eight cranial bones enclosing the brain (frontal, two parietal, two temporal, occipital, sphenoid and ethmoid) and fourteen facial bones (including the two maxillae, two zygomatic bones, two nasal bones and the mandible, the only freely movable skull bone). The six ear ossicles and the hyoid bone in the neck, which anchors the tongue and attaches to no other bone, bring the head region to 29. The vertebral column has 26 bones: seven cervical vertebrae, of which the first two, the atlas and the axis, allow the head to nod and turn; twelve thoracic vertebrae that carry the ribs; five large lumbar vertebrae; the sacrum, formed from five fused vertebrae; and the coccyx, from four fused. Between the vertebrae are intervertebral discs of fibrocartilage. The thoracic cage has 25 bones: the sternum and twelve pairs of ribs. Important bony features to know include the foramen magnum, the opening in the occipital bone through which the spinal cord passes, the mastoid process of the temporal bone behind the ear, and the spinous processes of the vertebrae, felt as bumps down the back.

The appendicular skeleton (126 bones). The appendicular skeleton comprises the limbs and the girdles that attach them. The pectoral girdle (four bones) consists of two clavicles and two scapulae. Each upper limb has 30 bones: the humerus, whose head fits the glenoid cavity of the scapula and whose olecranon fossa receives the ulna at the elbow; the radius (lateral, on the thumb side in anatomical position) and ulna (medial, with the olecranon process forming the point of the elbow); eight carpals; five metacarpals; and fourteen phalanges. The pelvic girdle consists of two hip bones, each formed from a fused ilium, ischium and pubis, which meet at the pubic symphysis in front and join the sacrum behind; its cup-shaped acetabulum receives the head of the femur. Each lower limb has 30 bones: the femur, the longest and strongest bone, with its head, neck and the greater trochanter that muscles attach to; the patella; the tibia (medial, weight-bearing, with the medial malleolus at the ankle) and the fibula (lateral, with the lateral malleolus); seven tarsals, including the calcaneus (heel) and talus; five metatarsals; and fourteen phalanges. The diagram shows the bone counts by region.

Joints. Joints can be classified by structure or by function. By structure: fibrous joints are held by dense connective tissue with no cavity, as in the sutures of the skull; cartilaginous joints are joined by cartilage, as at the epiphyseal plate or the pubic symphysis and intervertebral discs; synovial joints have a fluid-filled cavity. By function: synarthroses are immovable (sutures), amphiarthroses slightly movable (intervertebral discs, pubic symphysis), diarthroses freely movable (all synovial joints). The general structure of a synovial joint is the same in the knee, hip, shoulder and knuckle: the bone ends are covered by articular cartilage; a joint cavity separates them; an articular capsule encloses the joint, with an outer fibrous layer continuous with the periosteum and an inner synovial membrane that secretes synovial fluid, which lubricates and nourishes the cartilage; ligaments reinforce the capsule. Some joints add fibrocartilage pads (the menisci of the knee) and fluid-filled bursae that reduce friction where tendons cross. Synovial joints are named by movement: hinge (elbow: flexion and extension; the knee is usually called a modified hinge, because besides flexion and extension it allows slight rotation when bent), ball-and-socket (shoulder, hip: movement in all planes plus rotation), pivot (atlas on axis: rotation), condyloid (wrist), saddle (base of thumb) and plane (between carpals: gliding). Movements have names: flexion decreases a joint angle and extension increases it; abduction moves a limb away from the midline and adduction toward it; rotation turns a bone on its axis; circumduction traces a cone; pronation turns the palm backward and supination forward; dorsiflexion lifts the foot and plantar flexion points it.

Processes: ossification, growth, remodeling, hematopoiesis. Ossification is the formation of bone. Flat bones of the skull form by intramembranous ossification, in which osteoblasts deposit bone directly within a sheet of embryonic connective tissue. Most other bones form by endochondral ossification: a model of hyaline cartilage in the embryo is gradually replaced by bone from the center of the shaft outward, and later from centers in each epiphysis, leaving the epiphyseal plate between. Growth in length happens at the epiphyseal plate, where cartilage cells divide on the epiphysis side and are replaced by bone on the diaphysis side, so the plate keeps its thickness while the shaft lengthens; growth stops when the plate itself ossifies, in the late teens to early twenties, under the influence of sex hormones. Growth in width happens under the periosteum, where osteoblasts add layers to the outside while osteoclasts widen the medullary cavity inside. Remodeling continues for life: osteoclasts remove matrix and osteoblasts replace it, so that bone responds to the loads placed on it, thickening where stressed and thinning where unused, and so that blood calcium can be kept steady. Parathyroid hormone raises blood calcium by increasing osteoclast activity when calcium is low; calcitonin from the thyroid can lower it. Hematopoiesis, the production of red cells, white cells and platelets, occurs in red marrow, found in adults mainly in the flat and irregular bones (skull, sternum, ribs, vertebrae, hip bones) and the upper ends of the femur and humerus. Fracture repair follows a set sequence: a blood clot (hematoma) forms at the break; a soft callus of fibrocartilage bridges it; osteoblasts convert this into a hard bony callus; and remodeling over months restores the shape.

Comparisons. A female pelvis is generally wider and shallower than a male pelvis with a larger inlet and a wider angle beneath the pubic symphysis, adaptations for childbirth. A child's skeleton has open epiphyseal plates, visible on X-ray as dark lines, and some bones that later fuse, so a child has more separate bones than an adult. Other vertebrates show structure matched to function: a bird's long bones are hollow with internal struts and connected to air sacs, keeping a flying animal light; a frog's radius and ulna are fused into a single radioulna and its tibia and fibula into a tibiofibula, with elongated ankle bones, giving a stiff lever for jumping and landing.

Femur length and height. The femur makes up a roughly constant fraction of standing height in adults, about 26 to 27 percent, so measuring the femur allows an estimate of height. Forensic scientists use separate published equations for different sexes and populations because the fraction varies; in a classroom, students can measure their own femur length externally (from the greater trochanter, felt at the side of the hip, to the lateral line of the knee joint) and their height, plot height against femur length, and fit a line. Graphing the results by sex and by age group shows whether the fraction differs between groups. The external measurement only approximates the true bone length: the landmarks are felt through skin and soft tissue, and the greater trochanter does not mark the top of the femoral head, so the measured value can differ from the bone's maximum length by a centimeter or more in either direction; that is a stated limit of the method.

Homeostatic imbalances. Osteoporosis is a loss of bone mass in which osteoclast removal outpaces osteoblast deposition, leaving thin trabeculae and porous compact bone that fracture easily at the hip, wrist and spine; risk rises with age, with the fall in estrogen after menopause, with low calcium and vitamin D intake and with inactivity, and weight-bearing exercise and adequate calcium during youth build the reserve that slows later loss. Malnutrition affects bone directly: without enough vitamin D, calcium is poorly absorbed and bones soften, bowing in growing children (rickets) or weakening in adults (osteomalacia); without vitamin C, collagen cannot be made properly. Fractures are breaks; a closed fracture leaves the skin intact, an open fracture pierces it, a greenstick fracture in a child bends and cracks the flexible bone on one side only, and a compression fracture crushes a vertebra weakened by osteoporosis. The anterior cruciate ligament (ACL) inside the knee stops the tibia sliding forward on the femur; it tears in twisting injuries with the foot planted, common in sports, and because ligaments have a poor blood supply it heals badly and often needs surgical reconstruction. Arthritis is joint inflammation: osteoarthritis is the wearing away of articular cartilage with age and use, so bone rubs on bone; rheumatoid arthritis is an autoimmune attack on the synovial membrane. None of this is medical advice; each imbalance is explained by the structure or process that has failed.

Investigation: a cardstock bone model. Design a model to meet a parameter, strength per gram. Build three columns from equal sheets of cardstock: a flat folded stack, a solid rolled cylinder taped tight, and a hollow tube. Make all three the same height, weigh each, then stand each upright on a level surface under a small board and add weighed loads (for example bags of sand of known mass) one at a time, centered, until it buckles, keeping hands clear. Record the load at failure in kilograms, divide by the column's mass and repeat each test. A hollow tube that carries more load per gram than the flat stack supports the claim that a tubular diaphysis gives strength with less material, which is why long bones are hollow. The model cannot show the internal struts of spongy bone, the collagen-mineral composite or the way living bone remodels under load.

Worked long bone labeling: From end to end, a femur has: articular cartilage on the head; the epiphysis of spongy bone with red marrow; the metaphysis, with the epiphyseal line (in an adult) marking where the growth plate was; the diaphysis of compact bone surrounding the medullary cavity with yellow marrow; the periosteum on the outside and the endosteum lining the inside; the second metaphysis and epiphysis at the knee end. Bony features: the head, neck and greater trochanter at the hip end, and the condyles at the knee end. Each label is either a region, a tissue, a membrane or a feature, and the answer says which.

Worked bone count: Axial skeleton: skull 22, ear ossicles 6, hyoid 1, vertebral column 26 (7 + 12 + 5 + sacrum + coccyx), thoracic cage 25 (sternum + 24 ribs); total 22 + 6 + 1 + 26 + 25 = 80. Appendicular skeleton: pectoral girdle 4, two upper limbs at 30 each = 60, pelvic girdle 2, two lower limbs at 30 each = 60; total 4 + 60 + 2 + 60 = 126. Whole skeleton: 80 + 126 = 206. One upper limb: humerus 1, radius 1, ulna 1, carpals 8, metacarpals 5, phalanges 14 = 30. One lower limb: femur 1, patella 1, tibia 1, fibula 1, tarsals 7, metatarsals 5, phalanges 14 = 30.

Worked height estimate from femur length: A hypothetical femur measures 45 centimeters. Using the classroom fraction of 27 percent: height ≈ 45 ÷ 0.27 = 166.7, about 167 centimeters. Using 26 percent: 45 ÷ 0.26 = 173.1, about 173 centimeters. The two estimates differ by about 6 centimeters, which shows the uncertainty in the method and why a class should fit its own line from data rather than trust one fraction. If the femur was measured externally, the measured length may differ from the true bone length by a centimeter or more, which adds a further uncertainty of about 4 centimeters to each height estimate (1 ÷ 0.27 ≈ 3.7).

Worked joint classification and movement: The elbow: structurally synovial (cavity, capsule, synovial fluid, articular cartilage), functionally a diarthrosis, by movement a hinge, allowing flexion (bending, angle decreases) and extension (straightening, angle increases). The sutures of the skull: structurally fibrous, functionally synarthroses, no movement. The pubic symphysis: structurally cartilaginous (fibrocartilage), functionally an amphiarthrosis, slight movement. Recording a jump shot: the shoulder (ball-and-socket) flexes and the elbow (hinge) extends to release the ball; the hip and knee extend and the ankle plantar flexes to leave the ground.

Worked cardstock model data: Hypothetical results: flat stack 12 grams, failed at 2 kg; solid roll 12 grams, failed at 6 kg; hollow tube 12 grams, failed at 9 kg. Load per gram: 0.17, 0.5 and 0.75 kg per gram. The hollow tube met the strength parameter best with the same material, supporting the design principle of the hollow diaphysis.

Representation task: Create an illustration of a long bone with every structure labeled (diaphysis, epiphyses, metaphysis, epiphyseal line or plate, medullary cavity, compact bone, spongy bone, articular cartilage, periosteum, endosteum, red and yellow marrow) and, beside it, a labeled diagram of a synovial joint (articular cartilage, joint cavity, capsule, synovial membrane, synovial fluid, ligament). On a skeleton outline, name and label at least twenty bones, ten axial and ten appendicular, and four bony features.

Independent performance: Carry out the femur-and-height investigation and the cardstock bone model, and write a combined report. Success criteria: (a) femur length and height are measured for the class with the method stated, plotted, fitted with a line and compared by sex and age group with the difference described honestly; (b) the report states the fraction your class data give and compares it with the 26 to 27 percent classroom estimate, naming the limit of the external measurement; (c) three cardstock columns of equal mass are tested to failure with load per gram calculated and the result related to long bone structure; (d) two homeostatic imbalances from osteoporosis, malnutrition, fractures, ACL injury and arthritis are each explained by the cell, tissue or process that has failed; (e) one comparison between the human skeleton and another vertebrate, or between juvenile and adult, links a structure to a function.