Skeletal System

Skeletal System Overview • The skeletal system composed of bones, cartilages, joints, and ligaments, accounts for about 20% of the body mass (i.e., about 30 pounds in a 160-pound person). o Bones make up most of the skeleton o Cartilages occur only in isolated areas, such as the nose, parts of ribs, and the joints o Ligaments connect bones and reinforce joints, allowing required movements while restricting motions in other directions. o Joints are the junctions between bones which provide for the mobility of the skeleton Skeletal Cartilages • Human skeleton initially made up of cartilages and fibrous membranes; most are soon replaced with bone • In adults, the few areas where cartilage remains are mainly where flexible skeletal tissue is needed. • Cartilage tissue consists mainly of water—approximately 80%; high water content allows cartilage to be resilient (i.e., spring back to its original shape after being compressed). • Cartilage contains no nerves or blood vessels. • Perichondrium (“around the cartilage”) is dense irregular connective tissue; surrounds the cartilage and acts like a girdle to resist outward expansion when cartilage is compressed. o Perichondrium contains the blood vessels from which nutrients diffuse through the matrix to reach the cartilage cells. This mode of nutrient delivery limits cartilage thickness. • Three types of Cartilage Tissue in body o All three have cells called chondrocytes encased in small cavities (called lacunae) within an extracellular matrix containing a jellylike ground substance and fibers. o Skeletal cartilages contain representatives from all three types. Hyaline cartilages • Looks like frosted glass • Most abundant skeletal cartilages • Their chondrocytes appear spherical • Only fiber type in their matrix is fine collagen (undetectable microscopically) • Skeletal hyaline cartilages include: o Articular Cartilages —cover ends of most bones at movable joints o Costal cartilages —connect ribs to sternum o Respiratory cartilages —form skeleton of the larynx (voicebox) and reinforce other respiratory passages. o Nasal cartilage —support the external nose Elastin cartilages • Similar to hyaline cartilage AND contain more stretchy elastic fibers • Stand up better to repeated bending • Located in only two places: o External ear o Epiglottis —flap that bends to cover the opening of the larynx each time we swallow Fibrocartilages • Highly compressible and have great tensile strength • Intermediate between hyaline and elastic cartilages • Consist of roughly parallel rows of chondrocytes alternating with thick collagen fibers • Located in places with heavy pressure and stretch: o Meniscus—padlike cartilages of the knee joint o Intervertebral disc • Growth of Cartilage o Unlike bone, which has a hard matrix, cartilage has a flexible matrix which can accommodate mitosis; ideal environment to lay down embryonic skeleton and provide new skeletal growth. o Typically, cartilage growth ends during adolescence when skeleton stops growing. o Cartilage grows in two ways Appositional growth —“growth from outside” • Cartilage forming cells in the surrounding perichondrium secrete new matrix against the external face of the existing cartilage tissue. Interstitial growth —“growth from inside” • Lacunae bound chondrocytes divide and secrete new matrix, expanding the cartilage from within. Classification of Bones • An adult human has 206 named bones divided into two groups o Axial skeleton Forms the long axis of the body Bones of skull, vertebral column, and rib cage o Appendicular skeleton Form upper and lower limbs and girdles that attach limbs to axial skeleton • Classification based on shape o Long bones Bones that are longer than they are wide All limb bones except patella, carpals and tarsals o Short bones Cube shaped Carpals and tarsals Sesamoid bones —special short bones that form in a tendon; “shaped like a sesame seed” • Patella o Flat bones Thin, flattened and usually somewhat curved Sternum, scapulae, ribs, most skull bones o Irregular bones Complicated shapes that don’t fit any other class Vertebrae, hip bones Functions of Bones • Support o Bones provide framework for body and cradle its soft organs • Protection o Bones of skull protect brain; vertebrae protect spinal cord; rib cage protects vital organs of thorax • Movement o Skeletal muscles, which attach to bones via tendons, use bones as levers to move the body and its parts. • Mineral & growth factor storage o Bone is reservoir for minerals; most importantly, calcium & phosphate o Minerals are released into blood stream as needed o Mineralized bone matrix stores various growth factors as well • Blood cell formation o Hematopoiesis—blood cell formation; occurs in the marrow cavities of certain bones • Triglyceride (fat) storage o Fat is stored in bone cavities and is a source of stored energy for the body. Bone Structure • Compact Bone o Dense outer layer of bone that looks smooth and solid to the naked eye o Haversian system or Osteon —structural unit of compact bone Each osteon is an elongated cylinder oriented parallel to the long axis of the bone Functionally, osteons are tiny weight-bearing pillars • An osteon is a group of hollow tubes of bone matrix, one placed outside the next like the growth rings of a tree trunk. • Lamella —“little plate”—each matrix tube; compact bone is often called lamellar bone. • Think of an osteon’s design as a “twister resister” o All the collagen fibers in a particular lamella run in a single direction, collagen fibers in adjacent lamellae always run in different directions. o The adjacent lamellae reinforce one another to resist twisting. • Haversian canal , or central canal —running through the core of each osteon containing small blood vessels and nerve fibers. • Volkmann’s canals , or perforating canals —lie at right angles to the long axis of the bone and connect the blood and nerve supply of the periosteum to those in the central canals and medullary cavity. • Osteocytes (spider-shaped bone cells) occupy lacunae (little hollows) at the junctions of the lamellae. o Osteocytes maintain the bone matrix and act as stress or strain “sensors” in cases of bone deformation or other damaging stimuli. • Canaliculi —hairlike canals; connect lacunae to each other and to the central canal. • Spongy Bone o Internal to compact bone o Contain a honeycomb of small needle-like or flat pieces called trabeculae Trabeculae align precisely along lines of stress and help bone resist stress as much as possible. Trabeculae contain irregularly arranged lamellae and osteocytes interconnected by canaliculi. No osteons are present in the spongy bone. o In living bones the open spaces between trabeculae are filled with red or yellow bone marrow • Typical Long Bone : o Diaphysis —a tubular shaped shaft forming the long axis of the bone Constructed of a thick collar of compact bone surrounding a central medullary cavity— marrow cavity—containing fat (yellow marrow) in adults. o Epiphyses —bone ends Compact bone forms the exterior and their interior contains spongy bone. Joint surface of each epiphysis is covered with hyaline cartilage In adults, between diaphysis and epiphysis is an epiphyseal line, a remnant of the epiphyseal plate, a disc of hyaline cartilage that grows during childhood to lengthen the bone. This region is sometimes called the metaphysis. o Membranes Periosteum • A glistening white, double-layered membrane covering the external surface of the entire bone except the joint surface • Outer fibrous layer is dense irregular connective tissue • Inner osteogenic layer , abutting the bone surface, consists primarily of bone- forming cells called osteoblasts , bone-destroying cells called osteoclasts, and primitive stem cells ( osteogenic cells ) that give rise to the osteoblasts. • Periosteum is richly supplied with nerve fibers, lymphatic vessels, and blood vessels which enter the diaphysis via nutrient foramina. • Periosteum provides anchoring points for tendons and ligaments. Endosteum • A delicate connective tissue membrane covering the internal bone surfaces. • Covers the trabeculae of spongy bone and lines the canals that pass through the compact bone. • Contains osteoblasts, osteoclasts and osteogenic cells. • Short, Irregular & Flat Bones: o On outside, thin plates of periosteum-covered compact bone o On inside, endosteum-covered spongy bone o They contain bone marrow between the spaces of the trabeculae o These bones are not cylindrical; thus, they do not contain a shaft or epiphyses o In flat bones, spongy bone is called diploë and resembles a stiffened sandwich • Hematopoietic tissue in bones o Hematopoietic tissue (i.e., red marrow) o Found within trabecular cavities of spongy bone of long bones and diploë of flat bones o In newborn infants, medullary cavity of diaphysis and all areas of spongy bone contain red bone marrow o In most adult long bones, yellow bone marrow extends well into the epiphysis and little red marrow is present in spongy bone cavities. Blood cell production, in adult long bones, routinely occurs only in the heads of the femur and humerus. Bone Development • Ossification and osteogenesis = synonyms meaning the process of bone formation • Formation of Bony Skeleton o Before week 8, the skeleton of a human embryo is constructed entirely from fibrous membranes and hyaline cartilage. o Intramembranous ossification —bone developing from fibrous membranes beginning around week 8 of development. Results in the formation of cranial bones (frontal, parietal,

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