Unit 4, part I Bones: Structure and Function

The Skeletal System The skeletal system consists of bones, cartilages, ligaments and joints. The skeleton itself can be subdivided into two parts: 1.The Axial skeleton, which includes the bones that form the longitudinal axes of the body. 2.The Appendicular skeleton which is the bones of the limbs and girdles.

Functions of Bone Bones perform five major functions for the body: 1.Support—bones form the internal framework that supports the body. 2.Protection—bones protect soft body organs. 3.Movement—skeletal muscles use bones as levers to move the body. 4. Storage—bone is an important storehouse for minerals, especially calcium and phosphorus. 5. Blood cell formation— occurs in the marrow of certain bones.

Classification of Bones There are two basic types of bone tissue: o Compact bone is dense and looks smooth and homogenous. o Spongy bone has lots of open space with small pieces of bone. Bones themselves are classified into four groups by shape (see next slide).

Common Bone Shapes 1.Long bones—mainly compact bone that is longer than it is wide. Composes the limbs. 2.Short bones—cube- shaped, mainly spongy bones. Includes wrist and ankle bones. 3.Flat bones—thin, flattened and usually curved; consist of 2 layers of compact bone with a layer of spongy bone between. Include the skull, ribs and sternum. 4. Irregular bones—bones with an irregular shape that don’t fit into the other categories (e.g. vertebrae).

Bone Structure—Gross Anatomy  A long bone (as an example) consists of a compact bone shaft, known as the diaphysis, with mostly spongy epiphyses at each end.  The diaphysis is covered by a fibrous connective tissue membrane known as the periosteum.  Epiphyses are covered with glassy, hyaline cartilage known as articular cartilage, which reduces friction at joints.

Long Bone Gross Anatomy (cont.)  Adult long bones have an epiphyseal line that is a remnant of the juvenile epiphyseal plate, composed of hyaline cartilage to allow bone growth.  In adults, the shaft has yellow marrow, which is adipose tissue (fat).  In infants and young children, the shaft has red marrow, which forms blood cells. In adults, red marrow only occurs in spongy bone of flat bones and some epiphyses.  Bone markings are irregularities in exterior bone surfaces caused by attachment of muscles, tendons, and ligaments, and by the passage of blood vessels and nerves.

Bone Structure: Microscopic Anatomy  Mature bone cells, or osteocytes, are found in cavities within the matrix known as lacunae.  The lacunae are arranged in circles called lamellae around central (Haversian) canals.  Each system of lamellae and central Haversian canal together is known as a Haversian system or osteon.  Tiny canals, called canaliculi radiate out to all lacunae and provide nutrients throughout the hard bone matrix.  Perforating (Volkmann’s) canals run from the outside of the bone to the interior. Thus, bone is well nourished, allowing it to heal quickly.

Bone Formation and Growth  In human embryos, the skeleton is made mainly of hyaline cartilage, but by birth or shortly after, most cartilage has been converted to bone.  The process of bone formation, or ossification involves two steps: 1.Hyaline cartilage is totally covered with bone matrix by bone-forming cells called osteoblasts. 2. The hyaline cartilage is digested away, opening up a medullary cavity within the new bone.

Bone Remodeling Bones are remodeled throughout life in response to two factors: 1. Changing calcium levels in the blood—when blood Ca+ levels drop, parathyroid glands release parathyroid hormone (PTH) into the blood. This activates osteoclasts (bone destroying cells) to break down bone matrix and release Ca+. When blood calcium levels are too high, calcium is deposited into bone matrix. 2. The pull of gravity and muscles on the skeleton—bones become thicker and form projections to increase their strength in areas where large, bulky muscles are attached. Conversely, bones of inactive or bedridden people tend to lose mass because they are not subjected to weight-bearing stress.

Bone Fractures  Bones are susceptible to fractures or breaks throughout life.  In older people, bones are weaker and fractures are more common (especially if osteoporosis is a factor).  Fractures are treated by reduction, which is realignment of the broken bone ends, followed by immobilization while natural healing occurs. Below is a diagram showing the stages of bone healing (“remodeling”):