Chapter 4 – Part 1 Tissues

Epithelium Epithelial tissue covers the surface of the body and lines its cavities and tubes. One surface is free and faces either the environment or a body fluid; the other adheres to a basement membrane, a densely packed layer of proteins and polysaccharides. Cells are linked tightly together; there may be one or more layers.

There are two basic types of epithelia. Simple epithelium is a single layer of cells functioning as a lining for body cavities, ducts, and tubes. Simple epithelium functions in diffusion, secretion, absorption, or filtering of substances across the cell layer. Pseudostratified epithelium is a single layer of cells that looks like a double layer; most of the cells are ciliated; examples are found in the respiratory passages and reproductive tracts. Stratified epithelium has many layers—as in human skin.

Both simple and stratified epithelium can be subdivided into groups based on shape at the tissue surface: Squamous epithelium consists of flattened cells; examples are found in the lining of the blood vessels. Cuboidal epithelium has cube-shaped cells; examples are found in glands. Columnar epithelium has elongated cells; examples are found in the intestine.

cilia columnar cells basement membrane TYPE: Simple squamous DESCRIPTION: Friction-reducing slick, single layer of flattened cells COMMON LOCATIONS: Lining of blood and lymph vessels, heart; air sacs of lungs; peritoneum FUNCTION: Diffusion; filtration; secretion of lubricants TYPE: Simple cuboidal DESCRIPTION: Single layer of squarish cells COMMON LOCATIONS: Ducts, secretory part of small glands; retina; kidney tubules; ovaries, testes; bronchioles FUNCTION: Secretion; absorption TYPE: Simple columnar DESCRIPTION: Single layer of tall cells; free surface may have cilia, mucus-secreting glandular cells, microvilli COMMON LOCATIONS: Glands, ducts; gut; parts of uterus; small bronchi FUNCTION: Secretion; absorption; ciliated types move substances Figure 4.2: Characteristics of connective tissues.

Glands develop from epithelium. Glands are secretory structures derived from epithelium that make and release specific substances, such as mucus. Glands are classified according to how their products reach the site where they are used. Exocrine glands often secrete through ducts to free surfaces; they secrete mucus, saliva, earwax, milk, oil, and digestive enzymes for example. Endocrine glands have no ducts but distribute their hormones via the blood.

Cell Junctions Epithelial cells tend to adhere to one another by means of specialized attachment sites. Tight junctions link cells of epithelial tissues to form seals that keep molecules from freely crossing the epithelium. Adhering junctions are like spot welds in tissues subject to stretching. Gap junctions link the cytoplasm of adjacent cells; they form communication channels.

cell basement membrane protein channel plaques intermediate filaments Figure 4.6: Cell junctions. intermediate filaments protein channel plaques TIGHT JUNCTION ADHERING JUNCTION GAP JUNCTION

Connective Tissue Connective tissue binds together, supports, and anchors body parts; it is the most abundant tissue in the body. Fibrous connective tissues and specialized connective tissues are both found in the body. Fiber-like structural proteins and polysaccharides secreted by the cells make up a matrix (ground substance) around the cells that can range from solid to liquid.

Dense, irregular connective tissue Dense, regular connective tissue Loose connective tissue Dense, irregular connective tissue Dense, regular connective tissue Cartilage collagenous fiber ground substance with collagen fibers collagenous fibers collagenous fibers Figure 4.2: Characteristics of connective tissues. fibroblast fibroblast elastic fiber cartilage cell (chondrocyte)

Types and examples of fibrous connective tissue: Loose connective tissue supports epithelia and surrounds blood vessels and nerves; it contains few cells and loosely arrayed thin fibers. Dense, irregular connective tissue has fewer cells and more fibers, which are thick; it forms protective capsules around organs. Dense, regular connective tissue has bundled collagen fibers lying in parallel; such arrangements are found in ligaments (binding bone to bone) and tendons (binding muscle to bone). Elastic connective tissue contains fibers of elastin; this tissue is found in organs that must stretch, like the lungs.

Cartilage, bone, adipose tissue, and blood are specialized connective tissues. Cartilage contains a dense array of fibers in a rubbery ground substance; cartilage can withstand great stress but heals slowly when damaged. Hyaline cartilage has many small fibers; it is found at the ends of bones, in the nose, ribs, and windpipe. Elastic cartilage, because of its elastin component, is able to bend yet maintain its shape, such as in the external ear. Fibrocartilage is a sturdy and resilient form that can withstand tremendous pressure such as in the disks that separate the vertebrae.

Bone tissue is composed of osteocytes trapped in a matrix of collagen, ground substance, and calcium salts. Minerals thicken bone so it is capable of supporting and protecting body tissues and organs. Cartilage tissue is composed of chondrocytes trapped in a similar matrix as bone but there is more collagen than minerals so it is softer than bone. Adipose tissue contains adipocytes trapped in a fat matrix.

compact bone tissue nucleus blood vessel cell bulging with fat droplet bone cell (osteocyte) Figure 4.2: Characteristics of connective tissues. TYPE: Bone tissue DESCRIPTION: Collagen fibers, matrix hardened with calcium COMMON LOCATIONS: Bones of skeleton FUNCTION: Movement, support, protection TYPE: Adipose tissue DESCRIPTION: Large, tightly packed fat cells occupying most of matrix COMMON LOCATIONS: Under skin, around heart, kidneys FUNCTION: Energy reserves, insulation, padding

Blood is a fluid connective tissue involved in transport; plasma forms the fluid “matrix” and blood proteins, blood cells, and platelets compose the “fiber” portion of the tissue.

Muscle Tissue: Movement Muscle tissue contracts in response to stimulation, then passively lengthens; movement is a highly coordinated action. There are three types of muscle: Skeletal muscle tissue attaches to bones for voluntary movement; long muscle cells are bundled together in parallel arrays, which are enclosed in a sheath of dense connective tissue. skeletal muscle

Smooth muscle tissue contains tapered, bundled cells that function in involuntary movement; it lines the gut, blood vessels, and glands. Cardiac muscle is composed of short cells that can function in units due to the signals that pass through special junctions that fuse the cells together; cardiac muscle is only found in the wall of the heart. smooth muscle cardiac muscle

Nervous Tissue: Communication Nervous tissue consists mainly of cells, including neurons (nerve cells) and support cells; nervous tissue forms the body’s communication network. Neurons carry messages. Neurons have two types of cell processes (extensions): branched dendrites pick up chemical messages and pass them to an outgoing axon. A cluster of processes from different neurons is called a nerve.

Neuroglia are support cells. Glial cells (neuroglia) make up 90 percent of the nervous system. Neuroglia provide physical support for neurons. Other glial cells provide nutrition (astrocytes), clean-up, and insulation services (Schwann cells make myelin).

Membranes Membranes contain epithelial and connective tissue. Mucous membranes line the tubes and cavities of the digestive, respiratory, and reproductive systems where embedded glands secrete mucus. Serous membranes such as those that line the thoracic cavity occur in paired sheets and do not contain glands. Cutaneous membrane better known as skin. Synovial membranes line the sheaths of tendons and the capsule-like cavities around certain joints.

cutaneous membrane (skin) Figure 4.7: Examples of membranes in the human body. mucous membrane serous membrane cutaneous membrane (skin) synovial membrane

Organs and Organ Systems An organ is a composite of the four main types of tissues that act together to perform one or more functions. Two or more organs that work in concert form an organ system. Organ systems work cooperatively together within an organism. An organism such as the human body contains five major cavities : cranial, spinal, thoracic, abdominal, and pelvic.

cranial cavity spinal cavity thoracic cavity abdominal cavity Figure 4.8: (a) The major cavities in the human body. (b) Directional terms and planes of symmetry for the human body. Notice how the midsagittal plane divides the body into right and left halves. The transverse plane divides it into superior (top) and inferior (bottom) parts. The frontal plane divides it into anterior (front) and posterior (back) parts. abdominal cavity pelvic cavity

Organs and Organ Systems Eleven organ systems (integumentary, nervous, muscular, skeletal, circulatory, endocrine, lymphatic, respiratory, digestive, urinary, and reproductive) contribute to the survival of the body. Organ failure may lead to organ system shutdown which may lead to the organism’s death. No body dies until massive cell death occurs. If we can keep the cells alive then the tissues survive and the organs survive and the organ system functions properly to keep the organism alive.

The Body Has Eleven Organ Systems