skeletal histology

Skeletal histology:

There are about two hundred six bones in the body. Extra bones may exist. The two typical kinds of extra bone are sesmoid bones, which are tiny, seed-like specks located mostly in the tendons of the hands and feet, and wormian bones, or sutural bones, flat bones formed in sutures

The skeleton is divided into two main parts. The first is the axial skeleton, the bones that form the longitiudal axis of the body. The second is the appendicular skeleton, made up of the bones of the limbs and girdles. In addition to bones, the skeletal system also includes joints, cartilages and ligaments

The bones have five major functions:

Support-- the skeleton forms all internal framework, as well as the anchor to which all soft tissue organs are attached
Protection-- bones protect the soft organs underneath. Some examples would be the skull, the spine and the ribcage
Movement-- skeletal muscles are attached to bones via tendons, and are responsible for voluntary movement
Storage-- fat is stored in the internal cavities of the middle sections of bones. The bone matrix itself is a storehouse for calcium, phosphorus and others. A small amount of calcium ions (2+) must be constantly present in the blood for the nervous system to transmit needed information
Hematopoiesis-- blood cells are formed toward the ends of the bones' cavities, in the marrow

Classification of bones, or osseous tissue: Compact bone is dense and (duh) compact, smooth and homogenous in appearance Spongy bone is made up of small, needle-like pieces of bone with a great deal of open space Long bones are mostly longer than they are wide. They have a shaft, with a head at either end. For the most part, long bones are compact bone and make up the limbs, except for the wrist and ankle bones, which are short bones Short bones are, well, short. They are generally cuboidal, mostly made up of spongy osseous tissue Flat bones are thin, flattened, and usually curved. They contain two layers of compact bone sandwiching one of spongy bone. Most of the bones of the skull, the ribs, and sternum are flat bones Irregular bones do not fit any of the three categories. Some irregular bones are the vertebrae and the pelvis

The surface of bones are not totally smooth. They are scarred and pitted with bone markings. They fall into two major categories, being either projections (processes) or depressions (cavities). All terms beginning with 'T' are projections, and all that begin with 'F' (except facet) are depressions:

Projections that are sites of muscle attachment:

Tuberosity-- a large, rounded projection, sometimes rough in texture
Crest-- a narrow ridge of bone, usually prominent
Line-- a also a narrow ridge of bone, but smaller and less prominent than a crest
Tubercle-- a small, rounded process
Epicondyle-- a raised area on or above a condyle (see below)
Spine-- a sharp, slender, often pointed projection

Projection that help form joints:

Head--a bony expansion carried on a narrow neck of bone
Facet-- a smooth, nearly flat area
Condyle-- a rounded articular projection
Ramus-- an arm-like bar of bone
Trochanter-- a very large, blunt, irregular process, found only on the femur, at the hip joint

Depressions and openings to allow passage of blood vessels and nerves:

Meatus-- a canal-like passageway
Sinus-- a cavity within the bone, filled with air and lined by a mucosae
Fossa-- a shallow, basin-like depression, often serving as an articular surface
Groove-- a furrow
Fissure-- a narrow, slit-like opening
Foramen-- a round or oblong opening through a bone

Common fracture types:

Simple-- the bone breaks cleanly, without breaking the skin; sometimes referred to as a closed fracture
Compound-- broken bone penetrates the skin; an 'open fracture', causes a serious threat of bone infection, or osteomyelitis, that requires massive doses of antibiotics
Comminuted-- bone breaks into many pieces; most common in the elderly, whose bones have become brittle
Compression-- bone is crushed; common in porous bones
Depression-- broken portion is pressed inward; typical of a skull injury
Impacted-- ends of the broken bone are forced together; occurs often when one attempts to use an outstretched hand to break a fall or in a hip fracture
Spiral-- broken by twisting; a common athletic fracture
Greenstick-- the break is incomplete; common in children, whose bones have a greater amount of collagen in their matrix and are more flexible

A fracture is treated by reduction; setting, wiring or pinning the broken bone. In closed reduction, the skin is not opened, while the opposite is true of open reduction. The bone is then immobilized, and heals in six to eight weeks, or longer for an elderly person. The healing process has three major steps: Blood vessels are ruptured when the bone breaks, and a hematoma is formed. Bone cells cut off from the blood flow die The break is splinted by a fibrocartilage callus, and new capillaries (granulation tissue) is reformed into the blood clot, disposing of dead tissue via phagocytes. Connective tissue repair the break, forming a fibrocartilage callus, containing some cartilage matrix and bony matrix, closing the gap More osteoblasts and osteoclasts enter the picture, and replace the fibrocartilage with a bony callus, stronger than the original bone

The shaft of a lining bone is called the diaphysis. The diaphysis is covered in a protective periosteum, a connective tissue sheath, attached by hundreds of Sharpey's fibers, also of connective tissue. Running down the center of the diaphysis is the yellow marrow, or medullary, cavity. The cavity is lined by the endosteum. It is filled, in adults, with adipose tissue. In children, this area contains red marrow, producing blood cells. In adulthood, the red marrow is restricted to the epiphysis. The epiphyses are the rounded ends of the long bone. They consists of two thin layers of dense bone sandwiching one thicker of spongy tissue. Articular cartilage takes the place of the periosteum in sheathing the bone at the epiphyses. Made of glassy hyaline cartilage, it reduces friction at the joints. In adult bones, a line of bony tissue spans the epiphysis that looks a bit different. It is the epiphyseal line, the last remnant of the epiphyseal plate. After the ossification of the fetal skeleton, the epiphyseal plate remains cartilage, causing the lengthwise growth of the bone. During puberty, these plates finally ossify

Dense bone is riddled with passages to allow for blood vessels and nerves. While the tissue appears to be solid and uniform to the naked eye, it contains many structures:

Osteocytes-- the mature bone cells, found in tiny cavities within the matrix called lacunae
Lacunae-- tiny cavities arranged in concentric circles, called lamellae
Lamellae-- circles of lacunae and osteocytes about the central Haversian canal
Haversian canals-- central canals carrying the blood vessels and nerves. The Haversian canals run lengthwise through the bone
Canaliculi-- radiate outward from the Haversian canal to all of the lacunae. The canaliculi connect all bone cells to the nutrient supply, keeping the well-supplied in spite of their hard matrix material
Haversian system, or osteon-- each complex of a Haversian canal and it's matrix rings
Volkmann's canals-- the compliment to the Haversian canals, running at a right angle to them

Bones are formed of some of the strongest materials known to man. The matrix is made up of hydroxyapatite; a hardening agent; calcium carbonate, magnesium, a few other mineral salts, collagen fibers for flexibility and tensile strength, and very little water. The skeleton also has four types of cells:

Osteoporgenetors-- the only cell with mitotic potential, and can become osteoblasts. They are located in the inner periosteum, the endosteum, and blood vessel canals
Osteoblasts-- build bone and secrete matrix, located on the surface
Osteocytes-- mature bone cells found in the lacunae
Osteoclasts-- break down bone and secrete alkaline phosphates, located on the surface of the bone

Fetal bone has more osteoblasts and collagen fibers, and their pattern is more random. In the more orderly adult bone, there are fewer osteoblasts and more matrix. Ossification requires collagen to crystallize nuclei, and minerals to harden the bone

When calcium level drop too low, the parathyroid gland produce and secrete their hormone into the blood. This stimulates the osteoclasts. Calcium is deposited when calcium levels are too high (hypercalcemia)

Rickets is a disease children get when the bones fail to calcify, causing bowing. Mostly, it is a problem where calcium-rich foods are difficult to get on a regular basis

Stressing the bones makes them stronger. Not stressing them, however, can lead to osteoporosis, a condition in which calcium is lost and bones become brittle. Pathological fracture, that is, those without any apparent cause, are common in those with osteoporosis. Joints also suffer from osteoarthritis. As the bone breaks down, bone spurs grow around the margins of the eroded cartilage that has been broken down. The bone spurs restrict movement. OA is hardly ever crippling, and affects most often the hands

Rhumatoid arthritis is a chronic inflammatory disorder. It affect three times as many women as men, usually in a symmetrical manner. This type of arthritis is an autoimmune disease. The immune system attempts to destroy it's own tissues. RA begins with inflammation of the synovial membranes, which thickens and swells as synovial fluid accumulates. Inflammatory cells-- white blood cells and some others-- enter the joint and produce pannus, an abnormal tissue that clings and erodes the articular cartilage. Scar tissue forms and joins the ends of the bones, which eventually ossifies, a process called ankylosis. The bones are often deformed as a result

Gouty arthritis, or gout, is a disease in which uric acid builds up in the blood and deposited in crystal form in the soft tissue of the joints. The result is acutely painful attacks of usually a single joint, often in the great toe. Untreated, the bone ends will fuse. Colochicine and some other drugs are available and patients are told to avoid food high in nucleic acids such as liver and other organ meats, as well as alcohol and excessive vitamin C

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