Chapter 6: Bones and Skeletal Tissues
Basic Structure, Types, and Locations
Skeletal cartilages are made from cartilage, surrounded by a layer of dense irregular connective tissue called the perichondrium.
Hyaline cartilage is the most abundant skeletal cartilage, and includes the articular, costal, respiratory, and nasal cartilages.
Elastic cartilages are more flexible than hyaline, and are located only in the external ear and the epiglottis of the larynx.
Fibrocartilage is located in areas that must withstand a great deal of pressure or stretch, such as the cartilages of the knee and the intervertebral discs.
Growth of Cartilage
Appositional growth results in outward expansion due to the production of cartilage matrix on the outside of the tissue.
Interstitial growth results in expansion from within the cartilage matrix due to division of lacunae-bound chondrocytes and secretion of matrix.
Classification of Bones
There are two main divisions of the bones of the skeleton: the axial skeleton, consisting of the skull, vertebral column, and rib cage; and the appendicular skeleton, consisting of the bones of the upper and lower limbs, and the girdles that attach them to the axial skeleton.
Long bones are longer than they are wide, have a definite shaft and two ends, and consist of all limb bones except patellas, carpals, and tarsals.
Short bones are somewhat cube-shaped and include the carpals and tarsals.
Flat bones are thin, flattened, often curved bones that include most skull bones, the sternum, scapulae, and ribs.
Irregular bones have complicated shapes that do not fit in any other class, such as the vertebrae and coxae.
Functions of Bones
Bones support the body and cradle the soft organs, protect vital organs, allow movement, store minerals such as calcium and phosphate, and house hematopoietic tissue in specific marrow cavities.
Bone markings are projections, depressions, and openings found on the surface of bones that function as sites of muscle, ligament, and tendon attachment, as joint surfaces, and as openings for the passage of blood vessels and nerves.
Bone Textures: Compact and Spongy Bone
All bone has a dense outer layer consisting of compact bone that appears smooth and solid.
Internal to compact bone is spongy bone, which consists of honeycomb, needle-like, or flat pieces, called trabeculae.
Structure of a Typical Long Bone
Long bones have a tubular bone shaft, consisting of a bone collar surrounding a hollow medullary cavity, which is filled with yellow bone marrow in adults.
Epiphyses are at the ends of the bone, and consist of internal spongy bone covered by an outer layer of compact bone.
The epiphyseal line is located between the epiphyses and diaphysis, and is a remnant of the epiphyseal plate.
The external surface of the bone is covered by the periosteum.
The internal surface of the bone is lined by a connective tissue membrane called the endosteum.
Structure of Short, Flat, and Irregular Bones
Short, flat, and irregular bones consist of thin plates of periosteum-covered compact bone on the outside, and endosteum-covered spongy bone inside, which houses bone marrow between the trabeculae.
Location of Hematopoietic Tissue in Bones
Hematopoietic tissue of bones, red bone marrow, is located within the trabecular cavities of the spongy bone in flat bones, and in the epiphyses of long bones.
Red bone marrow is found in all flat bones, epiphyses, and medullary cavities of infants, but in adults, distribution is restricted to flat bones and the proximal epiphyses of the humerus and femur.
Microscopic Anatomy of Bone
The structural unit of compact bone is the osteon, or Haversian system, which consists of concentric tubes of bone matrix (the lamellae) surrounding a central Haversian canal that serves as a passageway for blood vessels and nerves.
Perforating, or Volkmann’s, canals lie at right angles to the long axis of the bone, and connect the blood and nerve supply of the periosteum to that of the central canals and medullary cavity.
Osteocytes occupy lacunae at the junctions of the lamellae, and are connected to each other and the central canal via a series of hair-like channels, canaliculi.
Circumferential lamellae are located just beneath the periosteum, extending around the entire circumference of the bone, while interstitial lamellae lie between intact osteons, filling the spaces in between.
Spongy bone lacks osteons but has trabeculae that align along lines of stress, which contain irregular lamellae.
Chemical Composition of Bone
Organic components of bone include cells (osteoblasts, osteocytes, and osteoclasts) and osteoid (ground substance and collagen fibers), which contribute to the flexibility and tensile strength of bone.
Inorganic components make up 65% of bone by mass, and consist of hydroxyapatite, a mineral salt that is largely calcium phosphate, which accounts for the hardness and compression resistance of bone.
Formation of the Bony Skeleton
Intramembranous ossification forms membrane bone from fibrous connective tissue membranes, and results in the cranial bones and clavicles.
In endochondral ossification bone tissue replaces hyaline cartilage, forming all bones below the skull except for the clavicles.
Initially, osteoblasts secrete osteoid, creating a bone collar around the diaphysis of the hyaline cartilage model.
Cartilage in the center of the diaphysis calcifies and deteriorates, forming cavities.
The periosteal bud invades the internal cavities and spongy bone forms around the remaining fragments of hyaline cartilage.
The diaphysis elongates as the cartilage in the epiphyses continues to lengthen and a medullary cavity forms through the action of osteoclasts within the center of the diaphysis.
The epiphyses ossify shortly after birth through the development of secondary ossification centers.
Postnatal Bone Growth
Growth in length of long bones occurs at the ossification zone through the rapid division of the upper cells in the columns of chondrocytes, calcification and deterioration of cartilage at the bottom of the columns, and subsequent replacement by bone tissue.
Growth in width, or thickness, occurs through appositional growth due to deposition of bone matrix by osteoblasts beneath the periosteum.
Hormonal Regulation of Bone Growth
During infancy and childhood, the most important stimulus of epiphyseal plate activity is growth hormone from the anterior pituitary, whose effects are modulated by thyroid hormone.
At puberty, testosterone and estrogen promote a growth spurt, but ultimately induct the closure of the epiphyseal plate.
Bone Homeostasis: Remodeling and Repair
In adult skeletons, bone remodeling is balanced bone deposit and removal, bone deposit occurs at a greater rate when bone is injured, and bone resorption allows minerals of degraded bone matrix to move into the blood.
Control of Remodeling
The hormonal mechanism is mostly used to maintain blood calcium homeostasis, and balances activity of parathyroid hormone and calcitonin.
In response to mechanical stress and gravity, bone grows or remodels in ways that allow it to withstand the stresses it experiences.
Fractures are breaks in bones, and are classified by: the position of the bone ends after fracture, completeness of break, orientation of the break relative to the long axis of the bone, and whether the bone ends penetrate the skin.
Repair of fractures involves four major stages: hematoma formation, fibrocartilaginous callus formation, bony callus formation, and remodeling of the bony callus.
Homeostatic Imbalances of Bone
Osteomalacia and Rickets
Osteomalacia includes a number of disorders in adults in which the bone is inadequately mineralized.
Rickets is inadequate mineralization of bones in children caused by insufficient calcium or vitamin D deficiency.
Osteoporosis refers to a group of disorders in which the rate of bone resorption exceeds the rate of formation (pp. 193-195, Fig. 6.14).
Bones have normal bone matrix, but bone mass is reduced and the bones become more porous and lighter increasing the likelihood of fractures.
Older women are especially vulnerable to osteoporosis, due to the decline in estrogen after menopause.
Other factors that contribute to osteoporosis include a petite body form, insufficient exercise or immobility, a diet poor in calcium and vitamin D, abnormal vitamin D receptors, smoking, and certain hormone-related conditions.
Paget’s disease is characterized by excessive bone deposition and resorption, with the resulting bone abnormally high in spongy bonIt is a localized condition that results in deformation of the affected bone.
Developmental Aspects of Bones: Timing of Events
The skeleton derives from embryonic mesenchymal cells, with ossification occurring at precise times. Most long bones have obvious primary ossification centers by 12 weeks gestation.
At birth, most bones are well ossified, except for the epiphyses, which form secondary ossification centers.
Throughout childhood, bone growth exceeds bone resorption; in young adults, these processes are in balance; in old age, resorption exceeds formation.
Clinical Advances in Bone Repair
Electrical stimulation - increases healing by (??) inhibiting PTH stimulation of osteoclasts or stimulating production of growth factors that affect osteblasts.
Ultrasound - reduces healing time of broken arm bones and shinbones by 35-35%; seems to stimulate callus formation by cartilage cells.
Free vascular fibular graft - grafts normal blood vessels along with pieces of fibula to replace missing or severely damaged bone; problem in kids because they end up with a "limb length discrepancy", however, for knee replacement candidates the self-extending endoprosthesis (Figure 16) seems to help.
Vascular endothelial growth factor - stimulates growth of blood vessels, formation of osteoblasts and bone proteins at sites of repair.
Nanobiotechnology - synthetic fibers mimic collagen fibers and stimulate mineral deposits, may speed healing.
Bone substitutes - used like putty, derived from cadaver bone or synthetics. Coral coated with bone morphogenic protein avoids problems of HIV, HBV, or inflammation. Artificial bone - TCP, a ceramic is easily shaped and biodegradable but not very strong; Norian SRS is a bone cement made from calcium phosphate, can be injected as a paste and hardens to be more compressible than spongy bone but can't replace compact bone because it can't resist twisting and flexing (good for epiphysis but not diaphysis).