Skeletal system The skeletal system consists of connective tissue>Complete Study Guide_ Guaranteed Success.

The skeletal system consists of connective tissue - The skeletal system comprises three types of connective tissue—bones, ligaments, and cartilage. • Bones are the hard elements of the skeleton with which we are most familiar. • Ligaments consist of dense fibrous connective tissue—they bind the bones to each other. • Cartilage is a specialized connective tissue consisting primarily of fibers of collagen and elastic in a gel-like fluid called ground substance. Cartilage has several functions, including reducing friction in joints. Bones are the hard elements of the skeleton - Most of the mass of bones consists of nonliving extracellular crystals of calcium minerals that give bones their hard, rigid appearance and feel. But bone is actually a living tissue that contains several types of living cells involved in bone formation and remodeling, plus nerves and blood vessels. Indeed, bones bleed when cut during orthopedic surgery or when they break. Bones perform five important functions. • The first three—support, protection, and movement—are the same as the functions of the skeleton overall, which is, after all, primarily bone. The rigid support structure of bones is what allows us to sit and to stand upright. The bones of the skeleton also support, surround, and protect many of our soft internal organs, such as the lungs, liver, and spleen. The attachment of bones to muscles makes it possible for our bodies to move. • The fourth and fifth functions of bones blood cell formation and mineral storage. Cells in certain bones are the only source of new red and white blood cells and platelets for blood. Without this production and supply function we would die within months. Bones also serve as an important long-term storage depot for two important minerals, calcium and phosphate. These two minerals can be drawn from bone when necessary, though excessive withdrawal may have consequences for bone composition and strength. Extracellular matrix: - The bone, cartilage, tendons, and ligaments of the skeletal system are all connective tissues. Their characteristics are largely determined by the composition of their extracellular matrix. - The matrix always contains collagen, ground substance, and other organic molecules, as well as water and minerals. But the types and quantities of these substances differ in each type of connective tissue. • Collagen is a tough, ropelike protein. • Proteoglycans are large molecules consisting of polysaccharides attached to core proteins, similar to the way needles of a pine tree are attached to the tree’s branches. The proteoglycans form large aggregates, much as pine branches combine to form a whole tree. Proteoglycans can attract and retain large amounts of water between their polysaccharide “needles.” 1. The extracellular matrix of tendons and ligaments contains large amounts of collagen fibers, making these structures very tough, like ropes or cables. 2. The extracellular matrix of cartilage contains collagen and proteoglycans. Collagen makes cartilage tough, whereas the water-filled proteoglycans make it smooth and resilient. As a result, cartilage is relatively rigid, but it springs back to its original shape after being bent or slightly compressed. It is an excellent shock absorber. 3. The extracellular matrix of bone contains collagen and minerals, including calcium and phosphate. The ropelike collagen fibers, like the reinforcing steel bars in concrete, lend flexible strength to the bone. The mineral component, like the concrete itself, gives the bone compression (weight-bearing) strength. Most of the mineral in bone is in the form of calcium phosphate crystals called hydroxyapatite. Bone contains living cells - A typical long bone, so called because it is longer than it is wide, consists of a cylindrical shaft (called the diaphysis) with an enlarged knob called an epiphysis at each end. - Dense compact bone forms the shaft and covers each end. A central cavity (medullary cavity) in the shaft is filled with yellow bone marrow. Yellow bone marrow is primarily fat that can be utilized for energy. - The outer surface of the bone is covered by a tough dense layer of connective tissue, the periosteum, which contains specialized bone- forming cells and consists of two layers and contains blood vessels and nerves. The surface of the medullary cavity is lined with a thinner connective tissue membrane, the endosteum. If an epiphysis of a long bone forms a movable joint with another bone, the joint surface is covered by a smooth layer of cartilage that reduces friction. Inside each epiphysis is spongy bone. - Spongy bone is less dense than compact bone, allowing the bones to be light but strong. Spongy bone is a latticework of hard, relatively strong trabeculae (from Latin, meaning “little beams”) composed of calcium minerals and living cells. - In certain long bones, most notably the long bones of the upper arms and legs (humerus and femur, respectively), the spaces between the trabeculae are filled with red bone marrow. Special cells called stem cells in the red bone marrow are responsible for the production of red and white blood cells and platelets. - Osteocytes nearest the center of an osteon receive nutrients by diffusion from blood vessels that pass through a central canal (Haversian canal). called lacunae. However, the osteocytes remain in direct contact with each other via thin canals called canaliculi. Histology of bone - The periosteum and endosteum contain osteoblasts (bone-forming cells), which function in the formation of bone, as well as in the repair and remodeling of bone. - When osteoblasts become surrounded by matrix, they are referred to as osteocytes (bone cells). - Osteoclasts (bone-eating cells) are also present and contribute to bone repair and remodeling by removing existing bone. - Bone is formed in thin sheets of extracellular matrix called lamellae (lă-mel′ ē; plates), with osteocytes located between the lamellae trapped within spaces called lacunae (lă-koo′ nē; a hollows). - Cell processes extend from the osteocytes across the extracellular matrix of the lamellae within tiny canals called canaliculi (little canal). - Bone tissue found throughout the skeleton is divided into two major types, based on the histological structure. Compact bone is mostly solid matrix and cells. Spongy bone, or cancellous bone, consists of a lacy network of bone with many small, marrow-filled spaces. Compact Bone Compact bone forms most of the diaphysis of a long bone and the thinner surfaces of all other bones. - Compact bone is made up largely of extracellular deposits of calcium phosphate enclosing and surrounding living cells called osteocytes (from the Greek words for “bone” and “cells”). Osteocytes are arranged in rings in cylindrical structures called osteons (sometimes called Haversian systems). - Compact bone has a predictable pattern of repeating units. These units are called osteons. Each osteon consists of concentric rings of lamellae surrounding a central canal, or Haversian canal. - Osteocytes are located in lacunae between the lamellae of each osteon. Blood vessels that run parallel to the long axis of the bone are located in the central canals. Osteocytes are connected to one another by cell processes in canaliculi. The canaliculi give the osteon the appearance of having tiny cracks within the lamellae. - Nutrients leave the blood vessels of the central canals and diffuse to the osteocytes through the canaliculi. Waste products diffuse in the opposite direction. The blood vessels in the central canals, in turn, are connected to blood vessels in the periosteum and endosteum. Within the canaliculi, extensions of the cell cytoplasm of adjacent osteocytes are joined together by gap junctions. (Gap junctions are channels that permit the movement of ions, water, and other molecules between two adjacent cells.) By exchanging nutrients across these gap junctions, all the osteocytes can be supplied