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Tissues โ A tissue (tishโฒ uฬ ) is a group of cells with similar structure and function that have similar extracellular substances located between them. โ The microscopic study of tissue structure is called histology EMBRYONIC GERMS โ Endoderm โINNER layer, forms the lining of digestive tract and its derivative โ Mesoderm โ MIDDLE layer, forms tissues such as muscle, bone, and blood vessels. โ Ectoderm โ OUTER layer, forms the skin; a portion of the ectoderm called neuroectoderm โ These are called germ layers because the beginning of all adult structures can be traced back to one of them and they give rise to all the tissues of the body EPITHELIAL TISSUE โ or epithelial tissue, covers external and internal surfaces throughout the body โ Surfaces of the body include the outer layer of the skin and the lining of cavities, such as the digestive tract, airways, and blood vessels. It also forms most glands โ Although there are some exceptions, most epithelia have a free surface , which is not in contact with other cells, โ The free surface can be smooth or lined with microvilli or cilia โ A smooth free surface reduces friction as material moves across it โ basal surface adjacent to a basement membrane, which attaches the epithelial cells to underlying tissues โ The basement membrane is secreted partly by epithelial cells and partly by the cells of the underlying tissues. โ consists of a meshwork of protein molecules โ It can function as a filter and as a barrier to the movement of cells. โ specialized type of extracellular material secreted by epithelial and connective tissue cells. โ Acts like an adhesive tile which anchor cells โ Blood vessels do not extend from the underlying tissues into epithelium, so gases and nutrients that reach the epithelium must diffuse across the basement membrane from the underlying tissues, where blood vessels are abundant. โ Microvilli are cylindrical extensions of the cell membrane that increase the free surface area โ microvilli cover the free surface of each cell involved in absorption or secretion, such as the cells lining the small intestine โ Cilia propel materials along the free surface of cells โ Intermixed with the ciliated cells are specialized mucus-producing cells called goblet cells Functions of Epithelia โ Protecting underlying structures โ Acting as a barrier. epithelium of the skin acts as a barrier to water and reduces water loss from the body โ prevents many toxic molecules and microorganisms from entering the body โ Permitting the passage of substances. oxygen and carbon dioxide are exchanged between the air and blood by diffusion through the epithelium in the lungs. โ Secreting substances. Sweat glands, mucous glands, and the enzyme-secreting portion of the pancreas are all composed of epithelial cells. โ Absorbing substances. The cell membranes of certain epithelial tissues contain carrier proteins that regulate the absorption of materials
โ Excretes waste Classification of Epithelia โ Epithelia are classified according to the number of cell layers and the shape of the cells โ Simple epithelium consists of a single layer of cells. โ Stratified epithelium consists of more than one layer of epithelial cells, with some cells sitting on top of others โ Categories of epithelium based on cell shape are squamous (flat), cuboidal (cubelike), and columnar (tall and thin) โ Simple squamous epithelium is a single layer of thin, flat cells โ the alveoli consist of simple squamous epithelium that allows oxygen from the air to diffuse into the body and carbon dioxide to diffuse out of the body into the air โ Simple squamous epithelium in the filtration membranes of the kidneys forms thin barriers through which small molecules โ Simple squamous epithelium also prevents abrasion between organs in the pericardial, pleural, and peritoneal cavities โ Diffusion, filtration, some secretion, and some protection against friction โ Location : Lining of blood vessels and the heart, lymphatic vessels, alveoli of the lungs, portions of the kidney tubules, lining of serous membranes of body cavities (pleural, pericardial, peritoneal) โ Simple cuboidal epithelium is a single layer of cubelike cells that carry out active transport, facilitated diffusion, or secretion. โ The kidney tubules have large portions of their walls composed of simple cuboidal epithelium. โ Secretion and absorption by cells of the kidney tubules; secretion by cells of glands and choroid plexuses; movement of particles embedded in mucus out of the terminal bronchioles by ciliated cells โ Location :Kidney tubules, glands and their ducts, choroid plexuses of the brain, lining of terminal bronchioles of the lungs, and surfaces of the ovaries โ Simple columnar epithelium is a single layer of tall, thin cells โ The simple columnar epithelium of the small intestine produces and secretes mucus and digestive enzymes. โ Mucus (muฬโฒkuฬs) is a clear, viscous (thick) fluid. The mucus protects the lining of the intestine, and the digestive enzymes complete the process of digesting food. โ Glands, and some ducts, bronchioles of lungs, auditory tubules, uterus, uterine tubules, stomach, intestines, gallbladder, bile ducts, and ventricles of the brain. โ Pseudostratified columnar epithelium is a special type of simple epithelium โ this type of epithelium appears stratified but is not. โ It consists of one layer of cells, with all the cells attached to the basement membrane. โ Pseudostratified columnar epithelium lines some glands and ducts, the auditory tubes, and some of the airways, such as the nasal cavity, nasal sinuses, pharynx, trachea, and bronchi โ Stratified squamous epithelium forms a thick epithelium because it consists of several layers of cells โ As these newly formed cells are pushed to the surface, they become flat and thin. โ As the cells flatten, the cytoplasm of the epithelial cells is replaced by a protein called keratin , and the cells die. โ One type of stratified squamous epithelium forms the outer layer of the skin and is called keratinized squamous epithelium โ stratified squamous epithelium of the mouth is composed of living cells with a moist surface. โ Stratified cuboidal epithelium consists of more than one layer of cuboidal epithelial cells.
Connective Tissue โ Connective tissue is found throughout the body. โ It is usually characterized by large amounts of extracellular material that separates cells from one another โ The extracellular matrix, has three major components: โ protein fibers โ ground substance consisting of nonfibrous protein and other molecules โ Fluid. โ Three types of protein fibers help form most connective tissues. โ Collagen fibers , which resemble microscopic ropes, are flexible but resist stretching. โ Reticular fibers are very fine, short collagen fibers that branch to form a supporting network. โ Elastic fibers have a structure similar to that of coiled metal bed springs; after being stretched, they can recoil to their original shape. โ Ground substance is the shapeless background against which cells and collagen fibers can be seen when using a light microscope โ Proteoglycans resemble the limbs of pine trees, with proteins forming the branches and polysaccharides forming the pine needles. โ Connective tissue cells are named according to their functions. โ Cells whose names contain the suffix -blast (germ) produce the matrix; โ cells ending in -cyte (cell) maintain it; โ and cells ending in -clast (break) break it down for remodeling. โ Macrophages are large white blood cells that are capable of moving about and ingesting foreign substances, including microorganisms, in the connective tissue. โ Mast cells are nonmotile cells that release chemicals, such as histamine, that promote inflammation Functions of Connective Tissue โ Enclosing and separating other tissues. โ Connecting tissues to one another. Tendons are strong cables, or bands, of connective tissue that attach muscles to bone, and ligaments are connective tissue bands that hold bones together. โ Supporting and moving parts of the body. โ Storing compounds โ Cushioning and insulating โ Transporting. โ Protecting Classification of Connective Tissue โ Loose connective tissue consists of relatively few protein fibers that form a lacy network, with numerous spaces filled with ground substance and fluid. โ Three subdivisions of loose connective tissue are areolar, adipose, and reticular โ Areolar has an extracellular matrix consisting mostly of collagen fibers and a few elastic fibers. โ The most common cells in loose connective tissue are the fibroblasts, which are responsible for producing the matrix โ Loose connective tissue is widely distributed throughout the body and is the โloose packingโ material of most organs and other tissues; โ it attaches the skin to underlying tissues and provides nourishment for the structures with which it is associated โ injury repair, phagocytosis, inflammatory response โ Fibroblasts, histiocytes, mast cells โ Adipose tissue consists of adipocytes, or fat cells, which contain large amounts of lipid for
energy storage. โ Adipose tissue also pads and protects parts of the body and acts as a thermal insulator. โ Reticular tissue forms the framework of lymphatic tissue, such as in the spleen and lymph nodes, as well as in bone marrow and the liver โ Dense connective tissue has a relatively large number of protein fibers that form thick bundles and fill nearly all of the extracellular space. โ Dense collagenous connective tissu e has an extracellular matrix consisting mostly of collagen fibers โ In tendons and ligaments, the collagen fibers are oriented in the same direction, and so the tissue is called dense regular โ dermis and in organ capsules, the fibers are oriented in many different directions, and so the tissue is called dense irregular โ Dense elastic connective tissue has abundant elastic fibers among its collagen fibers. The elastic fibers allow the tissue to stretch and recoil. Supporting Connective Tissue โ Cartilage is composed of chondrocytes, or cartilage cells, located in spaces called lacunae within an extensive matrix โ Collagen in the matrix gives cartilage flexibility and strength. โ Cartilage is resilient because the proteoglycans of the matrix trap water, which makes the cartilage relatively rigid and enables it to spring back after being compressed โ Cartilage provides support, but if bent or slightly compressed, it resumes its original shape. โ Cartilage heals slowly after an injury because blood vessels do not penetrate it โ is composed of cartilage cells (chondrocytes) within an extensive and relatively rigid matrix; chondrocytes located in lacunae โ Hyaline cartilage is the most abundant type of cartilage and has many functions. โ covers the ends of bones where they come together to form joints. โ Forms smooth, resilient surfaces that can withstand repeated compression. โ Hyaline cartilage also forms the cartilage rings of the respiratory tract, the nasal cartilages, and the costal cartilages, which attach the ribs to the sternum โ Fibrocartilage has more collagen than does hyaline cartilage, and bundles of collagen fibers can be seen in the matrix โ withstanding compression, it is able to resist pulling or tearing forces. โ is found in the disks between the vertebrae (bones of the back) and in some joints, such as the knee and temporomandibular (jaw) joints โ Elastic cartilage contains elastic fibers in addition to collagen and proteoglycans. โ The elastic fibers appear as coiled fibers among bundles of collagen fibers. โ Elastic cartilage is able to recoil to its original shape when bent. โ The external ear, epiglottis, and auditory tube contain elastic cartilage. โ Bone is a hard connective tissue that consists of living cells and a mineralized matrix โ Bone cells, or osteocytes are located within holes in the matrix, which are called lacunae โ Compact (outer) Cancellous/Spongy (inner) โ Mineral salts: especially calcium and phosphorus โ HARD โ TEETH โ Dentin โ related to bone in structure but is harder and denser. Light brown โ Enamel โ outer covering of the crown with white appearance โ Blood is unique because the matrix is liquid, enabling blood cells to move through blood vessels
โ Nervous tissue consists of neurons and support cells. โ The neuron , or nerve cell, is responsible for conducting action potentials โ The cell body contains the nucleus and is the site of general cell functions โ Dendrites usually receive stimuli leading to electrical changes that either increase or decrease action potentials in the neuronโs axon. โ axons are nerve cell processes โ Action potentials usually originate at the base of an axon where it joins the cell body and travel to the end of the axon โ Neuroglia are the support cells of the nervous system; they nourish, protect, and insulate the neurons. Tissue Membranes โ A membrane is a thin sheet or layer of tissue that covers a structure or lines a cavity โ Mucous membranes consist of various kinds of epithelium resting on a thick layer of loose connective tissue โ The functions of mucous membranes vary, depending on their location, but they include protection, absorption, and secretion โ Serous membranes consist of simple squamous epithelium resting on a delicate layer of loose connective tissue โ Serous membranes secrete serous fluid, which covers the surface of the membranes. โ The pleural membranes are associated with the lungs; the pericardial membranes are associated with the heart; and the peritoneal membranes are located in the abdominopelvic cavity โ pericarditis and peritonitis refer to inflammation of the pericardial membranes and peritoneal membranes, respectively. Pleurisy is inflammation of the pleural membranes. Tissue Damage and Inflammation โ Inflammation (flamma, flame) occurs when tissues are damaged. โ Inflammation mobilizes the bodyโs defenses and isolates and destroys microorganisms, foreign materials, and damaged cells so that tissue repair can proceed โ chemical mediators are released or activated in the injured tissues and adjacent blood vessels. โ The mediators include histamine and prostaglandins โ Some mediators cause dilation of blood vessels, which produces redness and heat, similar to what occurs when a person blushes โ Chemical mediators of inflammation also increase the permeability of blood vessels, allowing materials and blood cells to move out of the vessels and into the tissue, where they can deal directly with the injury โ Edema , or swelling, of the tissues results when water, proteins, and other substances from the blood move into the tissues โ One type of blood cell that enters the tissues is the neutrophil , a phagocytic white blood cell that fights infections by ingesting bacteria. โ Neutrophils die after ingesting a small number of bacteria; the mixture of dead neutrophils, other cells, and fluid that can accumulate is called pus. โ Chronic, or prolonged, inflammation results when the agent responsible for an injury is not removed or something else interferes with the healing process Tissue Repair โ Tissue repair is the substitution of viable cells for dead cells. โ Tissue repair can occur by regeneration or by fibrosis.
โ regeneration , the new cells are the same type as those that were destroyed, and normal function is usually restored โ fibrosis , or replacement, a new type of tissue develops that eventually causes scar production and the loss of some tissue function โ In these cases, regeneration is accomplished primarily by stem cells. Stem cells are self-renewing, undifferentiated cells that continue to divide throughout life โ The clot contains the threadlike protein fibrin, which binds the edges of the wound together and stops the bleeding. โ The surface of the clot dries to form a scab , which seals the wound and helps prevent infection. โ A s econd type of phagocytic cell , called a macrophage, removes the dead neutrophils, cellular debris, and the decomposing clot. โ The result is fibrosis, during which the clot is replaced with granulation tissue, a delicate, granular- appearing connective tissue that consists of fibroblasts, collagen, and capillaries. โ Much more granulation tissue forms, and wound contracture, a result of the contraction of fibroblasts in the granulation tissue, pulls the edges of the wound closer together โ Tissue repair involves clot formation, inflammation, the formation of granulation tissue, and the regeneration or fibrosis of tissues. In severe wounds, wound contracture can occur.
