acetylcholine: A neurotransmitter whose release leads to muscle contraction.
actin: A contractile protein found in muscle cells. Together with myosin, actin provides the mechanism for muscle contraction.
apocrine gland: A large sweat gland found in hairy regions of the body. Starting at puberty, apocrine glands secrete a mixture of protein and fat. Bacteria can thrive in these environments and are responsible for body odor. The glands are also thought to send sexual messages through odor. Modified apocrine glands produce mother’s milk.
basal cells: Small, round cells located in the lower portion of the skin’s epidermis.
bone marrow: A soft tissue found in the center of large bones. Bone marrow produces blood cells.
calcium: A chemical element that plays a vital role in the biochemistry of a cell. Calcium is an important part of a healthy diet. It is stored in the skeleton and released into the bloodstream as needed.
cartilage: An elastic connective tissue. Unlike bone, cartilage does not contain blood vessels and lacks the ability to regenerate.
chondrocyte: A type of cell found in cartilage.
collagen: The primary protein found in connective tissue. It gives skin its elasticity and provides strength to ligaments and tendons.
dermis: The layer of skin found beneath the epidermis. It contains many nerve endings that are responsible for the sense of touch.
diaphysis: The shaft of a long bone.
differentiation: During development, the process by which individual cells in the body take on specialized functions.
eccrine gland: A sweat gland that participates in regulating the temperature of the body. It is found in the skin over most parts of the body.
epidermis: The outermost layer of the skin.
epiphyses: The end portions of long bones where growth occurs.
fluorapatite: A fluorine-containing mineral that contributes strength to bone.
ground substance: Material in which cells of connective tissue are embedded. It is also called matrix.
hair follicle: An infolding of the epidermis that contains the root of an individual hair.
hydroxyapatite: A mineral containing calcium and phosphorous that contributes strength to bone.
joint: A location where bones meet and allow movement about that location.
keloids: An overgrowth of fibrous scar tissue caused by excessive tissue repair at the site of a skin injury.
keratin: The primary structural protein found in hair and nails.
keratinocytes: Cells of the epidermis that synthesize keratin.
ligament: Connective tissue that connects bone to bone.
melanin: A dark pigment produced in the bottom layer of the skin’s epidermis. Melanin absorbs ultraviolet light and is largely responsible for the color of the skin.
melanocyte: A type of cell found in the epidermis that produces the pigment melanin.
melanoma: A form of skin cancer that derives from a melanocyte.
myoblast: An immature muscle cell.
myosin: A contractile protein found in muscle cells. Together with actin, myosin provides the mechanism for muscle contraction.
myotube: A mature muscle cell.
organ: A part of the body that performs a specific function. The heart and lungs are examples of organs.
osteoblast: A cell that contributes to the formation of bone.
osteoclast: A cell that contributes to the breakdown and resorption of bone.
osteocyte: A branched cell found in the bone matrix. Osteocytes are derived from osteoblasts. They communicate with other cells and help the bone respond to its environment.
proteoglycans: A class of proteins with a high polysaccharide content.
sarcomere: The structural and functional unit of muscle contraction.
sarcoplasmic reticulum: The form of endoplasmic reticulum found in muscle fibers.
scleroderma: A chronic disorder characterized by hardening and thickening of the skin.
sebaceous gland: A gland that secretes an oily substance into a hair follicle to lubricate hair or skin.
shadow rule: A rule stated as, “short shadow—seek shade.” It is a convenient way to assess when protection from the sun is needed.
sliding-filament model: A model that describes how muscle cells contract using filaments made of actin and myosin.
squamous cells: A layer of flat skin cells found in the epidermis.
subcutaneous fat layer: A layer of fat cells beneath the dermis.
sweat gland: Small tubular glands found nearly everywhere in the skin that secrete perspiration through pores in the skin.
synovial fluid: A fluid found in joints that provides lubrication.
tendon: A band of connective tissue that attaches muscle to bone.
thick filament: One of two types of filaments found in myofibrils. It is mainly composed of the protein myosin.
thin filament: One of two types of filaments found in myofibrils. It is mainly composed of the protein actin.
tissue: A population of similar cells that act together to perform one or more specific functions in the body.
ultraviolet light: A portion of the electromagnetic spectrum having wavelengths shorter than visible light but longer than X-rays.
UVA: Long wavelength (320–380 nanometers) ultraviolet light.
UVB: Short wavelength (280–320 nanometers) ultraviolet light.
vitamin D: A fat-soluble vitamin needed for normal growth of bone. Vitamin D is produced when sterols in the body are irradiated by ultraviolet light.