Comprehensive Study Guide for Bones and Bone Structure
Functions and Components of the Skeletal System
The skeletal system is a complex structural framework that consists not only of the bones of the skeleton but also of cartilages, ligaments, and several other connective tissues that serve to stabilize and interconnect the bones. This system provides five major functions critical to human physiology. First, it offers structural support for the entire body, providing a framework for the attachment of soft tissues and organs. Second, it serves as a storage site for minerals and lipids, specifically maintaining reserves of calcium and phosphate ions while storing energy in the form of lipids within yellow bone marrow. Third, the skeletal system is the location of blood cell production, where red blood cells, white blood cells, and other blood elements are produced in the red bone marrow. Fourth, it provides protection for many soft tissues and internal organs, surrounding them with hard, calcified structures. Finally, the system provides leverage for movement, acting as levers that can change the magnitude and direction of the forces generated by skeletal muscles.
Classification of Bones by Shape and Structure
Bones are classified according to their shape and structure into six specific categories. Sutural bones, also known as Wormian bones, are small, flat, and irregularly shaped bones found specifically between the flat bones of the skull. Their borders are jagged like pieces of a jigsaw puzzle, and their number varies significantly between individuals, with sizes ranging from as small as a grain of sand to as large as a quarter. Irregular bones possess complex shapes with short, flat, notched, or ridged surfaces; primary examples include the vertebrae of the spinal column, the pelvic bones, and several bones within the skull. Short bones are boxy in appearance, with examples including the carpal bones of the wrists and the tarsal bones of the ankles. Flat bones feature thin, parallel surfaces and are found in the roof of the skull (such as the parietal bone), the sternum or breastbone, the ribs, and the scapulae or shoulder blades. These bones provide essential protection for underlying soft tissues and offer a broad surface area for the attachment of skeletal muscles.
Long bones are characterized by being relatively long and slender, located in the arms, forearms, thighs, legs, palms, soles, fingers, and toes. The femur, which is the long bone of the thigh, is noted as being the largest and heaviest bone in the human body. Finally, sesamoid bones are typically small, round, and flat. They develop within tendons and are most commonly found near the joints of the knees, hands, and feet. While the number and location of sesamoid bones vary by individual, every person possesses sesamoid patellae, commonly referred to as kneecaps or small shallow dishes. Bone markings, or surface features, further classify bone anatomy based on their function. Projections occur where muscles, tendons, and ligaments attach or where bones articulate with one another, while openings and depressions are present to allow the passage of blood vessels and nerves.
Anatomy of Long and Flat Bones
A representative long bone consists of three distinct regions: the diaphysis, the epiphysis, and the metaphysis. The diaphysis is the tubular shaft that forms the long axis of the bone. Its wall is composed of dense compact bone that surrounds a hollow internal space called the medullary cavity, or marrow cavity. The epiphyses are the expanded ends of the bone that articulate with adjacent bones; they are composed primarily of spongy bone, also known as trabecular bone. The metaphysis is the narrow zone representing the point of connection between the diaphysis and the epiphysis. In contrast, the structure of a flat bone resembles a sandwich, consisting of a core of spongy bone situated between two layers of compact bone known as the cortex. Within the cranium, this central layer of spongy bone is specifically referred to as the diploë.
Histology of Bone Tissue and the Extracellular Matrix
Bone tissue, or osseous tissue, is a dense connective tissue characterized by specialized cells surrounded by a solid extracellular matrix. The matrix is composed of protein fibers and deposits of calcium salts. Approximately two-thirds of the bone matrix consists of calcium phosphate, . This mineral reacts with calcium hydroxide, , to form crystals of hydroxyapatite, . These crystals incorporate other calcium salts, such as calcium carbonate (), and various ions including sodium (), magnesium (), and fluoride (). While these crystals are extremely hard and brittle, allowing the bone to resist compression, they would shatter if used alone. To counter this, one-third of the matrix weight is composed of collagen fibers, which provide a strong and flexible organic framework. A bone that lacks a calcified matrix will look normal but will be highly flexible and unable to support weight. The specialized cells of the bone, though they make up only of the total bone mass, reside in pockets called lacunae. These lacunae are connected to one another and to blood vessels by narrow passageways called canaliculi, which facilitate the exchange of nutrients, wastes, and gases.
Specialized Bone Cells and Their Functions
There are four primary types of cells found within bone tissue. Osteogenic cells, or osteoprogenitor cells, are mesenchymal stem cells that divide to produce daughter cells that differentiate into osteoblasts. These cells are located in the inner cellular layers of the periosteum and the endosteum and are vital for fracture repair. Osteoblasts are immature bone cells responsible for osteogenesis, the production of new bone matrix. They secrete an initial organic matrix called osteoid and subsequently promote the deposition of calcium salts to convert the osteoid into bone. Once an osteoblast becomes completely surrounded by the bone matrix it has created, it matures into an osteocyte. Osteocytes are mature bone cells that reside in lacunae and do not divide. They possess thin cytoplasmic extensions that pass through the canaliculi to communicate with other cells. Their two major functions are to maintain the protein and mineral content of the surrounding matrix and to participate in the repair of damaged bone. The fourth cell type is the osteoclast, which is a large, multinucleate cell derived from the same stem cells that produce monocytes and macrophages. Osteoclasts are responsible for osteolysis, the process of dissolving bone matrix and releasing stored minerals through the secretion of acids and protein-digesting enzymes. These cells are not related to the osteogenic lineage.
Structural Comparison of Compact and Spongy Bone
Compact bone is organized into functional units called osteons. At the center of each osteon is a central canal (Haversian canal) that contains blood vessels and runs parallel to the bone surface. Perforating canals, or Volkmann’s canals, run perpendicular to the surface, carrying blood vessels into deeper bone tissues and the medullary cavity. The bone matrix is arranged in layers called lamellae: concentric lamellae form the nested cylinders of the osteons, interstitial lamellae fill the spaces between osteons, and circumferential lamellae are found at the extreme inner and outer surfaces of the bone. In contrast, spongy bone does not contain osteons. Its matrix is arranged into a network of fiber-like struts called trabeculae. Spongy bone lacks a dedicated internal blood supply within the matrix; instead, nutrients reach the osteocytes via diffusion along canaliculi from the blood vessels in the surrounding marrow. Spongy bone lightens the skeleton, enables the bone to withstand stress from multiple directions, and supports either red bone marrow (for blood cell production) or yellow bone marrow (for fat storage).
Bone Stress Distribution and Surface Coverings
The coordinated functions of compact and spongy bone are best illustrated by the femur. The femur receives the weight of the body at the hip joint, which is positioned off-center relative to the shaft. The trabeculae in the proximal epiphysis transfer this weight from the pelvis across the joint to the compact bone of the shaft. As a result of this off-center loading, the medial side of the femoral shaft is placed under compression, while the lateral side is placed under tension and must resist bending. The outer surface of the bone is covered by the periosteum, a membrane with an outer fibrous layer and an inner cellular layer. The periosteum isolates the bone from surrounding tissues, provides a route for blood vessels and nerves, and participates in bone growth and repair. It is attached to the bone by perforating fibers (Sharpey’s fibers), which are collagen fibers that become incorporated into the bone tissue. The inner surfaces of the bone, including the medullary cavity, the trabeculae of spongy bone, and the central canals, are lined by the endosteum, an incomplete cellular layer that is active during bone growth, repair, and remodeling.
Bone Formation: Endochondral and Intramembranous Ossification
Ossification, or osteogenesis, is the process of bone formation, whereas calcification is the deposition of calcium salts into a tissue. Endochondral ossification occurs when bone replaces an existing hyaline cartilage model. The process begins in the center of the shaft at the primary ossification center as chondrocytes enlarge and the matrix calcifies, leading to the death of the chondrocytes. Blood vessels then grow around the cartilage, and cells of the perichondrium differentiate into osteoblasts, forming a superficial layer of bone. Fibroblasts migrate into the center and differentiate into osteoblasts to produce spongy bone. As development continues, remodeling creates a medullary cavity, and the shaft thickens. Secondary ossification centers then form in the epiphyses, which eventually fill with spongy bone. A thin layer of epiphyseal cartilage (the epiphyseal plate) remains at the metaphysis to allow for further growth. At puberty, ossification accelerates and eventually overtakes the cartilage production, leading to epiphyseal closure and the formation of a visible epiphyseal line. Interstitial growth refers to this increase in bone length, while appositional growth refers to the increase in bone diameter, where osteoblasts of the periosteum add circumferential lamellae and osteoclasts enlarge the medullary cavity.
Intramembranous ossification, also called dermal ossification, occurs when bone develops directly within mesenchymal or fibrous connective tissue, typically in the deeper layers of the dermis. This process produces dermal bones, such as the flat bones of the skull, the mandible, and the clavicles. It begins when mesenchymal cells cluster together and differentiate into osteoblasts, which secrete osteoid that subsequently mineralizes into bone matrix. The developing bone grows outward from the ossification center in small struts called spicules. As these spicules interconnect, they trap blood vessels within the bone. Continuous deposition by osteoblasts results in the formation of a plate of spongy bone, which is later remodeled to produce the diploë and the outer layers of compact bone that define flat bone structure.
Vascularization and Physiological Regulation of Bone
Bones are highly vascularized organs supplied by several sets of vessels: the nutrient artery and vein (supplying the diaphysis), metaphyseal vessels (supplying the epiphyseal cartilages), and periosteal vessels (supplying superficial osteons). The periosteum also contains a network of lymphatic vessels, and sensory nerves innervate the periosteum, endosteum, medullary cavity, and epiphyses. Bone remodeling is a continuous process of recycling and renewing the matrix, involving a balance between osteocyte, osteoblast, and osteoclast activity. This process allows bones to adapt to physical stress; heavily stressed bones become thicker and stronger through exercise, while inactivity leads to rapid loss of bone mass. Nutrition is critical for this process, requiring minerals such as calcium, phosphorus, magnesium, fluoride, iron, and manganese. Vitamins also play key roles: Calcitriol (synthesized from Vitamin D) is essential for calcium and phosphate absorption; Vitamin C is required for collagen synthesis; and Vitamins A, K, and stimulate osteoblast activity and protein synthesis. Hormonally, growth hormone and thyroxine stimulate bone growth, while sex hormones (estrogen and testosterone) stimulate matrix production and eventual epiphyseal closure.
Calcium Homeostasis and Hormonal Control
Calcium is the most abundant mineral in the human body, with of it stored within the skeleton. A chemical analysis of bone shows that organic compounds (mostly collagen) make up of bone weight, while inorganic components make up . The inorganic portion is composed of approximately calcium, phosphate, carbonate, sodium, magnesium, and potassium. Normal calcium ion levels are vital for the proper function of neurons and muscle cells. Parathyroid hormone (PTH), produced by the parathyroid glands, increases blood calcium levels by stimulating osteoclast activity, increasing intestinal calcium absorption via calcitriol secretion, and decreasing renal calcium excretion. Conversely, calcitonin, secreted by C cells of the thyroid, decreases blood calcium levels by inhibiting osteoclasts and increasing renal calcium excretion. Clinical conditions related to mineral levels include osteomalacia, where bones become weak and flexible due to poor mineralization, and rickets, a form of osteomalacia caused specifically by Vitamin D deficiency.
Fractures and the Aging Process
Fractures are cracks or breaks in bones resulting from physical stress. They are categorized as open (compound) if they project through the skin, or closed (simple) if they are internal. Specific types include transverse, displaced, compression, spiral, epiphyseal, comminuted, greenstick, Colles, and Pott’s fractures. The repair process involves four major steps: first, the formation of a fracture hematoma (a large blood clot); second, the formation of calluses, where an internal callus of spongy bone develops in the medullary cavity and an external callus of cartilage and bone stabilizes the fracture edge; third, the replacement of the external callus cartilage with spongy bone by osteoblasts; and finally, remodeling of the bone to form compact bone, which may leave the repaired area slightly thicker than before.
As the body ages, the skeletal system undergoes predictable changes. Osteopenia is the inadequate ossification and reduction of bone mass that typically begins between the ages of and . During this time, women lose approximately of their bone mass per decade, while men lose about . The most affected areas include the epiphyses, vertebrae, and jaw bones. When the loss of bone mass is severe enough to compromise normal function, the condition is termed osteoporosis. This affects of women and of men over the age of . The decline in sex hormones with age contributes to this loss, and in women, it accelerates significantly after menopause. Osteoporosis can also occur as a secondary effect of some cancers that release osteoclast-activating factors.