Reviewer in Biology science, Study notes of Chemistry

Download Reviewer in Biology science and more Study notes Chemistry in PDF only on Docsity! CHAPTER 4 TISSUES TISSUE A tissue is a group of cells with similar structure and function, plus the extracellular substance surrounding them. Histology is the microscopic study of tissues. TYPES OF TISSUES There are four tissue types in the human body: Epithelial – a covering or lining tissue. Connective – a diverse primary tissue type that makes up part of every organ in the body. Muscle – a tissue that contracts or shortens, making movement possible. Nervous – responsible for coordinating and controlling many body activities. EPITHELIAL TISSUES Epithelium, or epithelial tissue, covers and protects surfaces, both outside and inside the body. Included under the classification of epithelial tissue are the exocrine and endocrine glands. EPITHELIAL TISSUE CHARACTERISTICS 1. Mostly composed of cells 2. Covers body surfaces Distinct cell surfaces 3. Cell and matrix connections 4. Nonvascular 5. Capable of regeneration FUNCTIONS OF EPITHELIAL TISSUES 1. Protects underlying structures 2. Acts as a barrier 3. Permits passage of substances 4. Secretes substances 5. Absorption of substances CLASSIFICATION OF EPITHELIAL Epithelial tissues are classified primarily according to the number of cell layers and the shape of the superficial cells. The cell layers can be simple, stratified, or pseudostratified. The cell shapes can be squamous, cuboidal, columnar, or a special transitional shape, that varies with the degree of stretch. Simple epithelium consists of a single layer of cells, with each cell extending from the basement membrane to the free surface. Stratified epithelium consists of more than one layer of cells, but only the basal layer attaches the deepest layer to the basement membrane. Pseudostratified columnar epithelium is a special type of simple epithelium, that appears to be falsely stratified. It consists of one layer of cells, with all the cells attached to the basement membrane. Due to variations in the shape of the cells, the epithelia appears stratified. There are three types of epithelium based on idealized shapes of the epithelial cells: 1. Squamous cells are flat or scalelike. 2. Cuboidal cells are cube-shaped—about as wide as they are tall. 3. Columnar cells tend to be taller than they are wide. SIMPLE SQUAMOUS EPITHELIUM Simple squamous epithelium is a single layer of thin, flat cells. Some substances easily pass through this thin layer of cells, but other substances do not. The lungs, simple squamous epithelium, allows for gas exchange. The kidneys, simple squamous epithelium, helps filter wastes from the blood while keeping blood cells inside the blood vessels. SIMPLE CUBOIDAL EPITHELIUM Simple cuboidal epithelium is a single layer of cube-like cells that carry out active transport, facilitated diffusion, or secretion. They have a greater secretory capacity than simple squamous epithelial cells. Gap junctions are small channels that allowsmall molecules and ions to pass from one epithelial cell to an adjacent one. Most epithelial cells are connected to one another by gap junctions, and researchers believe that molecules or ions moving through the gap junctions act as communication signals to coordinate the activities of the cells. GLANDS Glands are secretory organs that secrete substances onto a surface, into a cavity, or into the bloodstream. Glands are composed primarily of epithelium, with a supporting network of connective tissue. Glands with ducts are called exocrine glands. Both the gland and its ducts is lined with epithelium. Endocrine glands are ductless glands; they secrete their products (termed hormones) into the bloodstream. *Most exocrine glands are multicellular, comprised of many cells. Some exocrine glands are composed of a single cell, like goblet cells, that secrete mucus. Multicellular exocrine glands can be classified according to the structure of their ducts and secretory regions. Simple glands have a single, non-branched duct, some have branched ducts. *Compound exocrine glands have multiple, branched ducts. Glands with secretory regions shaped as tubules (small tubes) are called tubular, whereas those shaped in saclike structures are called acinar or alveolar. Tubular glands can be straight or coiled. Glands with a combination of the two are called tubuloacinar or tubuloalveolar. *Exocrine glands can also be classified according to how products leave the cell. Merocrine secretion involves the release of secretory products by exocytosis. Apocrine secretion involves the release of secretory products as pinched-off fragments of the gland cells. Holocrine secretion involves the shedding of entire cells. CONNECTIVE TISSUE Connective tissue is a diverse primary tissue type that makes up part of every organ in the body. Connective tissue differs from the other three tissue types in that it consists of cells separated from each other by abundant extracellular matrix. Connective tissue is diverse in both structure and function. Connective tissue is comprised of cells, protein fibers, and an extracellular matrix. FUNCTIONS OF CONNECTIVE TISSUE 1. Enclose and separate other tissues 2. Connecting tissues to one another 3. Supporting and moving parts of the body 4. Storing compounds 5. Cushioning and insulating 6. Transporting 7. Protecting CONNECTIVE TISSUE CELLS The specialized cells of the various connective tissues produce the extracellular matrix. The name of the cell identifies the cell functions. Osteoblasts form bone, osteocytes maintain it, and osteoclasts break it down. Fibroblasts are cells that form fibrous connective tissue, and fibrocytes maintain it. Chondroblasts form cartilage and chondrocytes maintain it. Found in connective tissue are cells associated with the immune system, such as white blood cells. Macrophages are large cells that are capable of moving about and ingesting foreign substances, including microorganisms in the connective tissue. Mast cells are nonmotile cells that release chemicals, such as histamine, that promote inflammation. EXTRACELLULAR MATRIX The extracellular matrix of connective tissue has three major components: protein fibers, ground substance, and fluid. 1. Ground substance consists of non-fibrous protein and other molecules. The structure of the matrix is responsible for the functional characteristics of connective tissues—for example, they enable bones and cartilage to bear weight. 1. MATRIX PROTEIN FIBERS Three types of protein fibers—collagen, reticular, and elastic—help form most connective tissues. A. Collagen fibers, which resemble microscopic ropes, are very flexible but resist stretching. B. Reticular fibers are very fine, short collagen fibers that branch to form a supporting network. C. Elastic fibers have the ability to return to their original shape after being stretched or compressed, giving tissue an elastic quality. 2. MATRIX GROUND SUBSTANCE The ground substance consists of non-fibrous molecules and is shapeless. It consists of proteoglycans, which are large molecules that consist of a protein core attached to many long polysaccharides. Proteoglycans trap large quantities of water between the polysaccharides, which allows them to return to their original shape when compressed or deformed. TYPES OF CONNECTIVE TISSUE The two main types of connective tissue are embryonic and adult connective tissue. By eight weeks of development, most of the embryonic connective tissue has become specialized to form the types of connective tissue seen in adults. 1. Loose connective tissue consists of relatively few protein fibers that form a lacy network, with numerous spaces filled with ground substance and fluid. Three subdivisions of loose connective tissue are areolar, adipose, and reticular. A. Areolar connective tissue primarily consists of collagen fibers and a few elastic fibers. The most common cells in loose connective tissue are the fibroblasts. B. Adipose tissue consists of adipocytes, or fat cells, which contain large amounts of lipid for energy storage. Adipose tissue pads and protects parts of the body and acts as a thermal insulator. C. 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. Dense connective tissue has a relatively large number of protein fibers that form thick bundles and fill nearly all of the extracellular space. There are two major subcategories of dense connective tissue: collagenous and elastic. Smooth muscle forms the walls of hollow organs; it is also found in the skin and the eyes. Smooth muscle is responsible for a number of functions, such as moving food through the digestive tract and emptying the urinary bladder. Smooth muscle cells are tapered at each end, have a single nucleus, and are not striated. NERVOUS TISSUE Nervous tissue forms the brain, spinal cord, and nerves. It is responsible for coordinating and controlling many body activities. Nervous tissue consists of neurons and support cells, termed glial cells. The neuron is responsible for conducting action potentials. It is composed of three parts: a cell body, dendrites, and an axon. TISSUE MEMBRANES A tissue membrane is a thin sheet or layer of tissue that covers a structure or lines a cavity. Most membranes consist of epithelium and the connective tissue on which the epithelium rests. There are four tissue membranes in the body: cutaneous, mucous, serous, and synovial. The skin, termed the cutaneous membrane, is an external body surface membrane. The mucous membranes line cavities that open to the outside of the body, such as the digestive, respiratory, and reproductive tracts. Mucous membranes consist of epithelial cells, their basement membrane, and a thick layer of loose connective tissue. Many, but not all, mucous membranes secrete mucus. The functions of mucous membranes include protection, absorption, and secretion. Serous membranes line cavities that do not open to the exterior of the body, such as the pericardial, pleural, and peritoneal cavities. Serous membranes consist of three components: a layer of simple squamous epithelium, its basement membrane, and a delicate layer of loose connective tissue. Serous membranes do not contain glands, but they secrete a small amount of fluid called serous fluid, which lubricates the surface of the membranes. Synovial membranes line the cavities of freely movable joints. They are made up of only connective tissue and consist of modified connective tissue cells. Synovial membranes produce synovial fluid, which makes the joint very slippery, thereby reducing friction and allowing smooth movement within the joint. INTERNAL MEMBRANES a. Mucous membrane: Respiratory Digestive b. Serous membranes Pleural Peritoneal c. Synovial membranes Patellar Movable joints TISSUE INFLAMATION Inflammation is usually a beneficial process occurring when tissues are damaged. When viruses infect epithelial cells of the upper respiratory tract, inflammation and the symptoms of the common cold are produced. The inflammatory process occurs in stages. Inflammation mobilizes the body’s defenses and isolates and destroys microorganisms, foreign materials, and damaged cells so that tissue repair can proceed. Inflammation produces five major symptoms: redness, heat, swelling, pain, and disturbance of function. TISSUE REPAIR Tissue repair involves substitution of dead cells for viable cells. Tissue repair can occur by regeneration or by fibrosis. In regeneration, the new cells are the same type as those that were destroyed, and normal function is usually restored. In fibrosis, or replacement, a new type of tissue develops that eventually causes scar production and the loss of some tissue function. Regeneration can completely repair some tissues, such as the skin and the mucous membrane of the intestine. In these cases, regeneration is accomplished primarily by stem cells. Stem cells are self-renewing, undifferentiated cells that continue to divide throughout life. Tissue repair occurs in sequential steps. EFFECTS OF AGIING ON TISSUES Appearance of the skin changes as people age, approx. 30-35 years 1. Cells divide more slowly 2. Collagen fibers become more irregular 3. Reduced flexibility and elasticity