1. Tissues consist of cells and cell products organized to perform specific functions. Histology is the study of tissues.
2. There are four primary tissue types: epithelia, connective tissues muscle, and nervous tissue.
1. An epithelium forms a barrier with specific permeability characteristics. Epithelia cover surfaces and line internal cavities.
2. Gland cells are epithelial cells that produce exocrine or endocrine secretions.
Functions of Epithelia
1. Epithelia control permeability, provide sensitivity, provide physical protection, and produce specialized secretions.
Specializations of Epithelial Cells
1. Epithelial cells are interconnected by junctional complexes, proteoglycan cement, membrane interlocking, and attachment to a common basement membrane.** 2. Epithelia are avascular, and derive their nutrients by diffusion from underlying blood vessels. Diffusion times may be reduced by passage through a network of interfacial canals.
3. Epithelial cells may show polarity, the uneven distribution of organelles within the cytoplasm.
4. Mitoses by germinative cells (stem cells) replace functional cells on a continual basis.
Specializations of Epithelial Cell Membranes
1. Epithelia may contain cilia or microvilli on their exposed surfaces. Cilia move overlying fluids; microvilli provide increased membrane surface area for absorption and secretion.
A Classification of Epithelia
1. Epithelia may be classified by the number of cell layers (simple or stratified) and the shape of the cells exposed at the surface (squamous, cuboidal, or columnar).
1. Gland cell secretions may be apocrine, merocrine, or holocrine. The gland cells may exist as individuals or as part of a multicellular gland.
2. Multicellular exocrine glands are categorized according to the organization of the gland cells and whether or not the duct branches.
1. Connective tissues contain cells, fibers, and ground substance.
Connective Tissue Proper
1. Connective tissue proper contains fixed and wandering cells, collagen, elastic, and/or reticular fibers, and a ground substance containing hyaluronic acid.
2. Connective tissue proper includes loose connective tissue, adipose tissue, dense regular and irregular connective tissues, elastic tissue, and reticular connective tissues.
Blood and Lymph
1. Blood and lymph are connective tissues containing cells in a fluid matrix containing dissolved fibers.
2. Blood contains red blood cells, white blood cells, and platelets. The fluid of blood is called plasma.
3. In capillaries fluid and dissolved materials leave the blood and enter the interstitial fluid. Lymphatics return interstitial fluids to the circulation. Lymph contains large numbers of lymphocytes.
1. In cartilage, chondrocytes are surrounded by a dense matrix containing proteoglycans (chondroitin sulfates) and a variety of fibers.
2. The properties of hyaline cartilage, elastic cartilage, and fibrocartilage vary due to differences in the types and numbers of fibers present. Hyaline cartilage is stiff and somewhat flexible, elastic cartilage is resilient and very flexible, and fibrocartilage is very tough.
3. Cartilage is surrounded by a fibrous and cellular perichondrium.
4. Cartilage grows by interstitial and appositional mechanisms. Imperfect repairs are made following damage.
5. Chondrocytes rely upon diffusion through the avascular matrix. If the matrix calcifies the cells will be killed.
1. In bone, or osseous tissue, the matrix contains crystallized mineral deposits of calcium phosphate and calcium carbonate.
2. The crystals form around a framework of collagen fibers.
3. Osteocytes within lacunae depend upon diffusion through canaliculi for nutrient intake and waste removal.
4. Bone is surrounded by a fibrous and cellular periosteum.
5. Bone is highly vascular, and has extensive abilities to repair itself after an injury.
1. Membranes form barriers or interfaces.
2. Mucous, serous, and cutaneous membranes include epithelia and underlying connective tissues.
3. Synovial membranes are formed by modification of the loose connective within a joint.
1. Muscle tissue is specialized for contraction. Contraction depends upon interactions between actin and myosin microfilaments.
2. Muscle tissue is categorized according to the appearance (striated or nonstriated) and the degree of voluntary control exercised (voluntary or involuntary).
3. Skeletal muscle is striated voluntary, cardiac muscle is striated involuntary, and smooth muscle is nonstriated, involuntary muscle tissue.
1. Neural tissue is specialized for the conduction of information or instructions in the form of electrical impulses.
2. Cells in neural tissue are either neurons or neuroglial cells.
3. A typical neuron has dendrites, a soma, and an axon that ends at a synapse.
4. Information is carried in the form of a conducted change in the transmembrane potential of the axon. When that change reaches the synapse, the axons releases a neurotransmitter that affects the cell membrane on the other side of the synapse.
HOMEOSTASIS AT THE TISSUE LEVEL
The Inflammatory Response
1. Inflammation produces swelling, redness, heat, tenderness, and a loss of function at the inflamed site. An infection is an inflammation produced by an invading organism, such as a bacterium.
2. The inflammatory response begins with the release of histamine and heparin by mast cells. The basic sequence of events is: homeostasis disturbed --> mast cells release chemicals --> blood flow and permeability increases --> clot formation isolates area ---> phagocytes remove debris and microorganisms --> homeostasis returns.