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Histology Review Guide Four main types of tissues Epithelial Connective Muscle Nerve Epithelial Covering, lining, and glandular tissue General characteristics Cell walls are touching (vary in how tightly they are joined) Apical and basal surfaces Basement membrane Supported by connective tissue Avascular (epithelial tissue depends on diffusion of nutrients from connective tissue below) Innervated Regenerates well Organized by number of layers Simple – one layer Stratified – several layers (stratified is named for the top layer) And epithelial tissues are organized by shape Squamous Cuboidal Columnar Squamous – “scaly” flattened cells Simple squamous are found in the alveoli of the lungs and in the filtering structures of kidneys Stratified squamous covers the body and extends into body openings Provides protection from abrasion externally as the outer layer of skin. Internally, stratified squamous protects openings to the outside of the respiratory, digestive, and urogenital systems. Examples are the nasal passages and the esophagus. Cuboidal Simple cuboidal in kidney tubules (filtration) In endocrine glands – secretes directly into the blood stream (hormones) Pseudostratified ciliated columnar Pseudostratified ciliated columnar lines the bronchi and brings mucus and bacteria trapped in the mucus up to be swallowed. Nearby goblet cells secrete the mucus. Columnar Simple lines digestive tract Columnar may have microvilli or cilia Goblet cells are associated with simple columnar cells and secrete mucus in intestinal and respiratory passages Connective tissue Cells are separated by non-living matrix secreted by the cells ( blood cells do not secrete the matrix of blood, the plasma) Matrix includes protein fibers, for example, collagen and elastic fibers. Matrix includes proteins such as GAGs. GAGs are stored with water so the water content of matrix is generally high. Chondroitin sulfate is one example of a GAG. Connective tissue cells that are mitotic (actively dividing) and secreting matrix have the suffix – “blast”. Fibroblasts form connective tissue proper – areolar, adipose, dense, reticular and elastic connective tissue. i.e. all connective tissue types except blood, cartilage, and bone. Chondroblasts form cartilage Osteoblasts form bone When these “blast” cells mature and division slows - cyte is added instead of blast - Fibrocytes, chondrocytes, and osteocytes are mature cells that are no longer dividing and secreting matrix. Adipose Highly vascularized Very little matrix Each cell contains a large drop of triglycerides 18 – 50% of body weight within normal weight ranges Brown adipose tissue uses energy to make heat instead of ATP Brown adipose is found only in young babies and replaces shivering. Areolar or loose connective tissue Remember epithelial cells are avascular and receive nutrients by diffusion. Areolar connective tissue is the connective tissue bonded to the basement membrane of epithelial tissue. Areolar is found under the skin and mucus membrane and around organs. Areolar holds large amounts of fluid – as large a volume as the blood. Most cell types obtain nutrients and release wastes into the fluid of areolar connective tissue. The matrix of areolar connective tissue includes collagen and elastic fibers loosely associated in a viscous matrix. Areolar houses cells of the immune system – i.e. macrophages that “eat” bacteria and mast c ells that secrete histamines. Histamines increase permeability of capillaries. Dense irregular in the dermis of the skin gives skin its flexibility Dense regular – found in ligaments and tendons consists mainly of regular, wavy rows of collagen and very little matrix. Dense regular can stretch, but resists twisting. Elastic connective tissue proper is widely distributed in arteries and lungs and allows them to expand and contract. Cartilage Avascular and not innervated Hyaline – most common type of cartilage - nose, connects ribs to sternum, and between joints, growing ends of bones in children Slow recovery from injury Elastic cartilage Ears and epiglottis are high in elastic tissue. The epiglottis blocks the trachea as food passes. Fibrocartilage Fibrocartilage has high compressive and tensile strength and is found in weight-bearing areas of the body such as the intervertebral discs and the menisci which are cartilages of the knee joint. Bone Vascular Osteoblasts secrete the matrix. Osteocytes are in a pool of fluid called a lacuna. Then bone salts such as calcium phosphate are deposited Osteons are concentric layers of matrix surrounding a blood vessel Blood Blood is connective tissue because it arises from the same embryonic tissues as connective tissue Blood cells do not secrete the matrix (plasma) surrounding the cells Nervous Tissue Neurons Cell body contains the nucleus and organelles Dendrites bring nerve signals toward the cell body Axons bring nerve signals away from the cell body Muscle Skeletal muscle has striations and flattened, multiple nuclei. Skeletal muscle is voluntary Cardiac muscle has striations, is branching and has intercalated discs Intercalated discs of cardiac muscle contain gap junctions that speed communication and desmosome junctions that reinforce cardiac muscle cells Smooth muscle cells spindle-shaped are spindle-shaped and form sheets of tissue Cardiac and smooth muscle are involuntary Tissue repair When epithelial tissue is torn As platelets are attracted to the injury and attract other platelets, a clot forms. Mast cells release histamines that increase permeability of capillaries and increase flow of blood, immune cells, and clotting factors to the damaged tissues. The first tissue to regenerate is called granulation tissue. In granulation tissue, tiny nubs of regenerating capillaries cause this tissue to bleed easily. Fibroblasts multiply and migrate to the injured site and secrete collagen. Fibroblasts are contractile and pull injured tissues together. Fibroblasts secrete growth factors for regeneration of other cells. Damaged tissues produce new cells New cells may replace fibrocytes as they die and are absorbed Or fibrocytes may form scar tissue Tissues vary in their ability to regenerate. Best regeneration – Epithelial Bone Not as good Smooth muscle and ligaments and tendons Poor Skeletal muscle and cartilage Very poor to no regeneration Cardiac and nerve Where cells cannot regenerate and in severe wounds, scar tissue forms from fibroblasts Scar tissue can cause adhesions of organs after abdominal surgery and interfere with movement of intestines, of joints, of the heart In the case of non-regenerating tissues such as cardiac muscle, non-functional scar tissue replaces functional tissues.