THE SKELETAL SYSTEM: BONE TISSUE

 INTRODUCTION

 1. A bone is made up of several different tissues: bone or osseous tissue, cartilage, dense connective tissue,epithelium, adipose tissue, and nervous tissue.

 2. The entire framework of bones and their cartilages constitutes the skeletal system

FUNCTIONS OF BONE AND THE SKELETAL SYSTEM

 1. The skeletal system functions in support, protection, movement, mineral homeostasis, blood cell production, and triglyceride storage. 

STRUCTURE OF BONE

 1. Parts of a typical long bone are the diaphysis (shaft), proximal and distal epiphyses (ends), metaphyses,

articular cartilage, periosteum, medullary (marrow) cavity, and endosteum.

 HISTOLOGY OF BONE TISSUE

 1. Bone tissue consists of widely separated cells surrounded by large amounts of extracellular matrix.

 2. The four principal types of cells in bone tissue are osteoprogenitor cells, osteoblasts (bone-building

cells), osteocytes (maintain daily activity of bone), and osteoclasts (bone-destroying cells).

 3. The extracellular matrix of bone contains abundant mineral salts (mostly hydroxyapatite) and collagen

fibers.

 4. Compact bone tissue consists of osteons (haversian systems) with little space between them.

 5. Compact bone tissue lies over spongy bone tissue in the epiphyses and makes up most of the bone tissue

of the diaphysis. Functionally, compact bone tissue is the strongest form of bone and protects, supports,and resists stress.

 6. Spongy bone tissue does not contain osteons. It consists of trabeculae surrounding many red bone marrow–filled spaces.

 7. Spongy bone tissue forms most of the structure of short, flat, and irregular bones, and the interior of the epiphyses in long bones. Functionally, spongy bone tissue trabeculae offer resistance along lines of stress, support and protect red bone marrow, and make bones lighter for easier movement.

Blood And Nerve Supply Of Bone

 1. Long bones are supplied by periosteal, nutrient, metaphyseal, and epiphyseal arteries; veins accompany the arteries.

 2. Nerves accompany blood vessels in bone; the periosteum is rich in sensory neurons.

Bone Formation

 1. The process by which bone forms, called ossification, occurs in four principal situations: (1) the initial

formation of bones in an embryo and fetus; (2) the growth of bones during infancy, childhood, and

adolescence until their adult sizes are reached; (3) the remodeling of bone (replacement of old bone by

new bone tissue throughout life); and (4) the repair of fractures (breaks in bones) throughout life.

 2. Bone development begins during the sixth or seventh week of embryonic development. The two types

of ossification, intramembranous and endochondral, involve the replacement of a preexisting connective

tissue with bone. Intramembranous ossification refers to bone formation directly within mesenchyme arranged in sheetlike layers that resemble membranes. Endochondral ossification refers to bone formation within hyaline cartilage that develops from mesenchyme. The primary ossification center of a long bone is in the diaphysis. Cartilage degenerates, leaving cavities that merge to form the medullary cavity.

Osteoblasts lay down bone. Next, ossification occurs in the epiphyses, where bone replaces cartilage,

except for the epiphyseal (growth) plate.

 3. The epiphyseal plate consists of four zones: zone of resting cartilage, zone of proliferating cartilage,zone of hypertrophic cartilage, and zone of calcified cartilage. Because of the cell division in the epiphyseal (growth) plate, the diaphysis of a bone increases in length.

 4. Bone grows in thickness or diameter due to the addition of new bone tissue by periosteal osteoblasts around the outer surface of the bone (appositional growth).

 5. Bone remodeling is an ongoing process in which osteoclasts carve out small tunnels in old bone tissue

and then osteoblasts rebuild it.

 6. In bone resorption, osteoclasts release enzymes and acids that degrade collagen fibers and dissolve mineral salts.

 7. Dietary minerals (especially calcium and phosphorus) and vitamins (A, C, D, K, and B12) are needed for bone growth and maintenance. Insulinlike growth factors (IGFs), human growth hormone, thyroid hormones, and insulin stimulate bone growth.

 8. Sex hormones slow resorption of old bone and promote new bone deposition.

Fracture and Repair of Bone

 1. A fracture is any break in a bone. Types of fractures include closed (simple), open (compound), comminuted, greenstick, impacted, stress, Pott, and Colles.

 2. Fracture repair involves formation of a fracture hematoma during the reactive phase, fibrocartilaginous callus and bony callus formation during the reparative phase, and a bone remodeling phase.

Bone’s Role in Calcium Homeostasis

 1. Bone is the major reservoir for calcium in the body.

 2. Parathyroid hormone (PTH) secreted by the parathyroid glands increases blood Ca2 level. Calcitonin

(CT) from the thyroid gland has the potential to decrease blood Ca2 level. Vitamin D enhances absorption of calcium and phosphate and thus raises the blood levels of these substances.

Exercise and Bone Tissue

 1. Mechanical stress increases bone strength by increasing deposition of mineral salts and production of collagen fibers.

 2. Removal of mechanical stress weakens bone through demineralization and collagen fiber reduction.

Aging and Bone Tissue

 1. The principal effect of aging is demineralization, a loss of calcium from bones, which is due to reduced osteoblast activity.

 2. Another effect is decreased production of extracellular matrix proteins (mostly collagen fibers), which makes bones more brittle and thus more susceptible to fracture.