Histology Notes

Histology

• Histology: Study of normal structures of tissues.
• Tissues: Groups of structurally and functionally related cells and their external environment that perform common functions.
• Two basic components of tissues:
  • Discrete population of cells, related in structure and function.
  • Extracellular matrix (ECM): Surrounding material.

Types of Tissues (~4)

• Four primary tissue types, defined by:
  • Kind and number of cells.
  • Amount and composition of ECM.
  • Specific functions.
• Types:
  • Epithelial tissues:
  • Tightly packed sheets of cells with no visible ECM.
  • Cover and line all body surfaces and cavities.
  • Specialized epithelia form glands that manufacture secretions (sweat, saliva, or chemical messengers like hormones).
  • Connective tissues:
  • Connect all other tissues to one another.
  • ECM is a prominent feature for most connective tissue types, with cells scattered throughout.
  • Bind, support, protect, and allow for transportation of substances.
  • Muscle tissues:
  • Capable of generating force by contracting.
  • Little ECM between cells.
  • Nervous tissues:
  • Capable of generating, sending, and receiving messages.
  • Cells that support this activity are also within a unique ECM.

Extracellular Matrix (ECM)

• ECM: Substances in liquid, thick gel, or solid form that surround cells of tissue.
• Two main components: ground substance and protein fibers.
• Functions:
  • Provides tissue with strength to resist tensile (stretching) and compressive forces.
  • Directs cells to proper positions within tissue and holds those cells in place.
  • Regulates the development, mitotic activity, and survival of cells in a tissue.

Ground Substance

• Most of ECM; extracellular fluid (ECF) or interstitial fluid with water, nutrients, ions, and three families of macromolecules:
  • Glycosaminoglycans (GAGs): Chondroitin sulfate (small) and hyaluronic acid (enormous).
  • Negative charges of sugars in GAG attract positively charged ions in ECF.
  • Ions create a concentration gradient within ECF, drawing water out of cells and blood vessels by osmosis.
  • Effectively “trap” water in ECM, helping it resist compression; Gradients Core Principle.
  • Proteoglycans: GAGs bound to protein core (resembles a bottle brush).
  • Thousands of proteoglycans bind to very long GAGs (such as hyaluronic acid), forming huge proteoglycan “aggregates”.
  • Make ECM firmer, more solid, and resistant to compression.
  • Form a barrier to diffusion of substances through ECM; protect underlying tissue from invading microorganisms.
  • Cell-adhesion molecules (CAMs): Different types of glycoproteins.
  • Adhere cell to cell and cells to surroundings; hold everything in place within ECM.
  • CAMs bind to cell surface proteins as well as protein fibers and proteoglycans, maintaining normal tissue architecture.

Protein Fibers

• Embedded within ground substance; long molecules composed of multiple fibrous subunits with ropelike structure; enormous tensile strength; three protein fiber types:
  • Collagen fibers: Make up 20–25% of all proteins in the body; composed of multiple repeating subunits; form white fibrous protein; resistant to tension (pulling and stretching forces) and pressure.
  • Elastic fibers: Composed of the protein elastin surrounded by glycoproteins; extensibility allows fibers to stretch up to one and a half times resting length without breaking; once stretched, fibers return to resting length (elasticity).
  • Reticular fibers: Thin, short collagen fibers; form meshwork or scaffold that supports cells and ground substance of many tissues; form weblike structure in organs such as the spleen, helping to trap foreign cells.

Marfan Syndrome

• Results from defects in the gene that codes for glycoprotein fibrillin-I; a component of ECM required for normal deposition of elastic fibers.
  Elastic fibers are not correctly distributed and anchored in ECM, and cannot function properly.
  Leads to characteristic signs and symptoms:
  Tall stature with long limbs and fingers; multiple skeletal abnormalities, recurrent joint dislocations, heart valve and lens (eye) abnormalities, and dilation of the aorta.
  Aortic dissection (rupture) – most lethal complication; layers of the aortic wall separate and blood flows between them; leads to aortic rupture; ensuing blood loss is nearly always fatal if not caught and treated immediately.

Cell Junctions

• Another way cells bind to one another; neighboring cell’s plasma membranes are linked by integral proteins; three major types of cell junctions:
  • Tight junctions.
  • Desmosomes.
  • Gap junctions.

Tight Junctions

• (Occluding junctions) hold cells closely together; space between is impermeable to the movement of macromolecules.
• Integral proteins of adjacent cell’s plasma membranes are locked together, forming a seal around the apical perimeter of the cell.
• The seal may not be complete, allowing for leakage in some tissues.
• Example: Between cells in blood vessels; prevent blood cells from exiting vessels.

Desmosomes

• Linking integral proteins; allow for materials in extracellular fluid to pass through the space between cells.
• Increase the strength of tissue by holding cells together; mechanical stress is more evenly distributed.
• Integral “linker” proteins are attached to intermediate filaments of the cytoskeleton for structural reinforcement.
• In tissues subjected to a great deal of mechanical stress (epithelia of skin).

Gap Junctions

• Small pores formed by protein channels between adjacent cells; allow small substances to flow freely between each cell’s cytoplasm.
• In between cells that communicate with electrical signals (cardiac muscle cells).
• Gap junctions illustrate Cell–Cell Communication Core Principle.

Epithelial Tissues

• On every internal and external body surface; a barrier between the body and the external environment; line organs and fluid-filled cavities; functions:
  • Protection: Shield underlying tissues from mechanical and thermal injury.
  • Immune defenses: Form physical barriers; prevent invasion by microorganisms; specialized cells of the immune system are scattered throughout epithelial tissues to protect underlying tissues.
  • Secretion: Form glands that produce substances like hormones and oils; secreted into the blood or through ducts respectively.
  • Transport into other tissues: Selectively permeable membranes; substances are able to cross these barriers by passive or active transport and enter other tissues.
  • Sensation: Most are associated with a rich nerve supply; detect changes in internal and external environments; taste buds are examples of specialized sensory epithelial cells.

Identifying Tissues

• Key to identifying tissues is to reduce every tissue to simplest components: cells and extracellular regions containing different types of chemicals.
• Section of esophagus (tubular organ; transports food from mouth to stomach) below:
  • Each structure that contains a dark purple nucleus is a cell.
  • Ground substance either looks clear or has a slight tinge of color; protein fibers look like wavy or straight lines.
  • Collagen fibers form bundles that resemble certain cell types; the easiest way to differentiate is to look for nuclei; if bundles lack nuclei, then they are likely to be collagen fibers.

Sublingual Gland

• Next section is the sublingual gland (salivary gland that produces saliva) under the tongue.
• Contains clusters of small, light red, round discs that lack nuclei.
• Discs are red blood cells (erythrocytes); located in blood vessels; visible in many different tissue sections.

Components and Classification of Epithelia

• Epithelial tissues consist of tightly packed cells linked together by tight junctions and desmosomes.
• Make sheets of cells fairly impermeable and resistant to physical stresses and mechanical injury.
• Epithelial tissues are avascular (lack blood vessels; must obtain oxygen and nutrients by diffusion from deeper tissues); limits thickness.
  ECM is found beneath cells in a thin basement membrane; two components:
  Basal lamina: ECM synthesized by epithelial cells; consists of collagen fibers and ground substance.
  Reticular lamina: synthesized by underlying connective tissue; consists of reticular fibers and ground substance.
  Together, these two layers “glue” epithelial tissue to underlying connective tissue; anchor underlying blood vessels in place; provide a barrier between epithelia and underlying tissues.

Epithelial Tissue Classification

• Classified by two criteria: the number of cell layers and the shape of cells in those layers; both have functional significance.
• Simple epithelia: Single cell layer.
• Stratified epithelia: More than one cell layer.
• Squamous cells: Flattened.
• Cuboidal cells: Short.
• Columnar cells: Tall and elongated.

Covering and Lining Epithelia

• On inner and outer body surfaces; each cell shape may be in varying thicknesses in broad, flat sheets; termed membranes when basement membrane is included:
  • Simple epithelia: One cell-layer thick; adapted for transportation of substances between different tissues.
  • Some have microvilli for increased surface area; some have cilia (move substances through hollow organs); Structure-Function Core Principle.

Four Types of Simple Epithelia

• Simple squamous epithelium: Very thin single layer of cells; “fried egg” appearance; adapted for rapid diffusion of substances (oxygen, carbon dioxide, fluids, and ions); in air sacs of the lung, specific segments of kidney tubules, and lining blood vessels.
• Simple cuboidal epithelium: Single layer of cube-shaped cells with large central nucleus; thin enough for rapid substance diffusion; in segments of renal tubules, respiratory passages, ducts of many glands, and thyroid gland.
• Simple columnar epithelium: Single layer of rectangular-shaped cells with nuclei in basal portion of the cell; often microvilli (small intestine) or cilia (uterine tubes and segments of the respiratory tract) on the apical plasma membrane.
• Pseudostratified columnar epithelium: Appears layered because nuclei are at various heights; only one cell-layer thick with basal plasma membranes firmly in contact with the basement membrane; in segments of respiratory tract and nasal cavity; ciliated.

Transport Across Simple Epithelia

• Occurs via two routes:
  • Paracellular transportation: Substances leak between cells in the epithelial membrane; this is limited due to tight junctions that make spaces between cells nearly impermeable.
  • Transcellular transportation: A substance enters a cell by crossing the plasma membrane; diffuses across the cytosol; exits the cell through the plasma membrane at the opposite side.

Stratified Epithelium

• More than one layer of cells; best as protective barriers where subjected to high degrees of mechanical stress; types include:
  • Keratinized stratified squamous epithelium: Apical cellular layers are dead; lack nuclei; filled with protein keratin; makes tissue tough and resistant to friction; well adapted for outer layers of skin.
  • Nonkeratinized stratified squamous epithelium: Apical cellular layers retain nuclei; are still alive; in regions subjected to mechanical stress where the surface must remain moist; mouth, throat, esophagus, anus, and vagina.
  • Stratified cuboidal epithelium: rare in humans; two cell layers; lines ducts of sweat glands.
  • Stratified columnar epithelium: also rare in humans; few layers; the apical layer is columnar and basal cell layer is cuboidal; male urethra, cornea of the eye, and ducts of certain glands such as salivary glands.
  • Transitional epithelium: Only in the urinary system; lines the interior of the kidney, ureters, urinary bladder, and urethra; cuboidal basal cell layers and dome-shaped apical cell layers in relaxed tissue; the ability of apical cells to flatten allows tissues to stretch.

Glandular Epithelia

• Gland: Structure of epithelial origin; synthesizes and secretes a product from designated secretory cells.
  • Arises from epithelial tissue that migrated into deeper connective tissue instead of remaining at the surface.
  • Classified either by shape or by how products are released.
  • Products released by two mechanisms:
  • Endocrine.
  • Exocrine.
• Endocrine glands: Secrete products, usually hormones, directly into the bloodstream without the use of ducts:
  • Allow products to have systemic effects (on distant cells).
  • Example of Cell–Cell Communication Core Principle.
• Exocrine glands: Release products onto apical surfaces of the epithelium (external surface of the body) or lining of a hollow organ (opens to the outside of the body):
  • Products, secreted from the gland through an epithelial-lined duct, have only local effects on cells in the gland’s vicinity.
  • Glands vary in complexity from single cells to large multicellular glands with branching ducts and many secretory units.

Goblet Cells

• Most common unicellular exocrine gland; in digestive and respiratory tracts; secrete mucus, a thick sticky liquid that protects the underlying epithelium.

Multicellular Exocrine Glands

• Classified according to the structure of ducts and the shape of clusters of secretory cells:
  • Duct structure:
  • Simple glands have ducts that don’t branch
  • Compound glands have branched ducts.
  • Clusters of secretory cells have three possible configurations:
  • Tubular: long and straight or coiled.
  • Acinar: Spherical.
  • Tubuloacinar: Both tubular and acinar sections.

Methods of Product Secretion by Exocrine Glands

• Merocrine: Used by the majority of exocrine glands, including salivary and sweat glands; products are packaged in secretory vesicles for release by exocytosis into ducts.
• Holocrine: Used by sebaceous glands in the skin to secrete sebum; secretory cells accumulate product in the cytosol; only release product when the cell ruptures and dies; cells are replaced by mitosis at the gland base.
• Apocrine: A rare type of secretion; portions of cytoplasm are pinched off with the product being secreted; observed during lipid droplet secretion in lactating mammary glands of many mammal species.

Carcinogens and Epithelial Tissues

• Epithelia cover all body surfaces; are more subject to injury than most other tissues.
• Carcinogens: Agents that induce DNA changes (injury); can lead to cancer.
• Carcinoma: Term for epithelial cancer; common examples:
  • Lung adenocarcinoma.
  • Ductal and papillary carcinoma (cancer of breast).
  • Basal cell carcinoma (cancer of skin).

Basement Membrane

• Provides a barrier to prevent/slow the spread of carcinomas; cancers that have not invaded other tissues are termed “pre-malignant”; cancer cells produce enzymes that degrade the basement membrane, facilitating spread.

Connective Tissues

• Divided into two basic groups; differ in cell types and ECM components:
  • Connective tissue proper.
  • Specialized connective tissue.

Connective Tissue Functions

• Connecting and binding: Anchor tissue layers in organs and link organs together.
• Support: Bone and cartilage support the weight of the body.
• Protection: Bone tissue protects internal organs; cartilage and fat provide shock absorption; components of the immune system are throughout connective tissues.
• Transport: Blood is fluid connective tissue; the main transport medium in the body.

Connective Tissue Composition

• Consists of cells and ECM:
  • Cells are surrounded by protein fibers and embedded in ground substance.
  • ECM is a characteristic feature; plays an extensive role in the function of the connective tissue type.

Connective Tissue Proper (General Connective Tissue)

• Widely distributed in the body.
• Connects tissues and organs to one another.
• Components of the internal architecture of some organs.

Cells of Connective Tissue Proper

• Resident cells permanently inhabit the tissue. Migrant cells migrate to areas of the body depending on the situation

Resident Cells

• Fibroblasts.
• Adipocytes.
• Mast cells.

Migrant Cells

• Phagocytes.
• Other immune system cells.

Fibroblasts

• Most common resident cell:
  • Mature cells with properties of immature “blast” cell.
  • Make protein fibers and ground substance (components of ECM); continually produce collagen proteins.

Adipocytes (Fat Cells)

• In many different connective tissues; cytoplasm filled with a single large lipid inclusion.

Mast Cells

• Largest resident cell:
  • Immune system cells filled with cytosolic inclusions (granules) of inflammatory mediators (histamine).
  • Release mediators (degranulate) when stimulated, causing inflammation (protective response that activates the immune system).

Phagocytes:

• Immune system cells; ingest foreign substances, microorganisms, and dead or damaged cells by phagocytosis; include macrophages (resident or migrant) and neutrophils (migrant cells).
• Other immune system cells migrate in and out of connective tissues depending on the body’s needs.

Four Basic Types of Connective Tissue Proper

• Loose connective tissue.
• Dense connective tissue.
• Reticular tissue.
• Adipose tissue.

Loose Connective Tissue

• (Areolar tissue): Mostly ground substance; contains all three types of protein fibers, fibroblasts, and occasionally adipocytes, suspended in ground substance:
  • Beneath the epithelium of skin, in membranes lining body cavities, and within the walls of hollow organs.
  • Contains and supports blood vessels vital to avascular epithelial tissues; houses immune system cells.

Dense Connective Tissue (Fibrous Connective Tissue)

• Mostly protein fibers; three classes:
  • Dense irregular connective tissue:
  • Predominantly disorganized collagen bundles.
  • Strong; resists tension in all three planes of movement.
  • In high tension areas such as the dermis (deep to skin) and surrounding organs and joints.
  • Dense regular connective tissue:
  • Organized into parallel collagen bundles.
  • In tendons and ligaments; subjected to tension in one plane of movement.
  • Dense regular elastic connective tissue (elastic tissue):
  • Mostly parallel-oriented elastic fibers with randomly oriented collagen fibers.
  • In walls of organs that must stretch to function (large blood vessels and certain ligaments).

Reticular Tissue

• Mostly reticular fibers produced by fibroblasts (reticular cells):
  • Forms fine networks that support small structures such as blood and lymphatic vessels.
  • Also in lymph nodes and spleen; forms weblike nets that trap old and foreign cells.
  • Forms part of the basement membrane; supports all epithelia and the internal structure of liver and bone marrow.

Adipose Tissue (Fat Tissue)

• Fat-storing adipocytes and surrounding fibroblasts and ECM; adipocytes can increase in size to a point where fibroblasts and ECM are scarcely visible; functions:
  • Fat storage (the major energy reserve of the body).
  • Insulation (retains warmth).
  • Shock absorption and protection.

White Adipose Tissue

• Predominant fat tissue; appears white; adipocytes with one large lipid inclusion in cytosol; deep to skin as subcutaneous fat, and in abdomen, breasts, hips, buttocks, and thighs; visceral fat surrounds the heart and abdominal organs.

Brown Adipose Tissue

• Less common; has a brown appearance due to numerous mitochondria in the cytoplasm and a vast blood supply; contains multiple lipid inclusions; more readily converted to energy to produce heat in cold temperatures.

Adipose Tissue and Obesity

• Obesity: Condition of excess adipose tissue in proportion to lean body mass; two forms:
  • Hypertrophic: Lipid inclusions accumulate fatty acids; increase in size up to 4´ normal; the number of adipocytes remains unchanged.
  • Hypercellular: Generally severe; the number of adipocytes increases; correlates with the development of obesity in infancy or early childhood (not adulthood); adult adipocytes lack the ability to divide to form new cells.
• Both increase the risk of health problems; the development of related disorders is complex; depends on the distribution of adipose tissue and genetic factors.

Specialized Connective Tissues

• More specific functions; include three types of tissue:
  • Cartilage: in joints between bones, the ear, the nose, and segments of the respiratory tract.
  • Bone tissue (osseous tissue): supports the body; protects vital organs; provides attachments for muscles that allow for movement; stores calcium; and houses bone marrow (which produces blood cells and stores fat).
  • Blood: Unique connective tissue with liquid ECM (plasma); consists of mostly water, dissolved solutes, and proteins.

Cartilage

• Tough, flexible tissue; absorbs shock and resists tension, compression, and shearing forces; ECM consists of collagen and elastic fibers, proteoglycans, and glycosaminoglycans.
• Populated with two cell types:
  • Chondroblasts: Immature cells; divide by mitosis and make most of ECM.
  • Chondrocytes: Eventually surround themselves in small cavities (lacunae) in ECM to become mature, largely inactive chondrocytes.

Avascular Nature

• Essentially avascular, unlike most connective tissues; the blood supply is limited to the outer sheath (perichondrium) of dense irregular collagenous connective tissue.
• Oxygen and nutrients must diffuse from blood vessels in the perichondrium through ECM to supply chondroblasts and chondrocytes; this limits the thickness of living cartilage.

Classes of Cartilage

• Can be further divided into three classes by ECM composition:
  • Hyaline cartilage: Most abundant cartilage:
  • ECM is mostly ground substance made of small bundles of fine collagen, giving the tissue a glossy bluish-gray appearance.
  • On ends of bones in joints (articular cartilage), linking the sternum to ribs, framing sections of the respiratory tract, and in the nose.
  • Most of the fetal skeleton is hyaline cartilage; this is replaced with bone during development.
  • Fibrocartilage: Filled with bundles of collagen fibers; little room for ground substance in ECM:
  • Fibroblasts reside in tissue; also chondroblasts and chondrocytes; fill ECM with collagen and some elastic fibers.
  • Tissue has great tensile strength with some degree of elasticity.
  • In between bones of fibrous joints; forms articular discs that improve the fit of bones in joints.
  • Elastic cartilage: Mostly elastic fibers in ECM:
  • Allows tissue to vibrate.
  • In a limited number of structures; the external ear assists with the detection of sound in the air; the larynx assists with the production of sound.

Bone

• ECM is about 35% organic components (collagen fibers and ground substance (osteoid)); the remaining 65% are inorganic calcium phosphate crystals, making bone one of the hardest substances in the body.
• Dynamic tissue capable of remodeling; occurs in regions of stress or inactivity; tension increases osteoblast activity and bone deposition; compression increases osteoclast activity and bone resorption.

Bone Cells

• Osteoblasts:
  • “Bone-builders” on the outer surface of bones; are closely associated with a dense irregular collagenous connective tissue covering (periosteum).
  • Carry out the process of bone deposition; synthesize and secrete organic ECM and chemicals involved in trapping calcium in ECM.
• Osteocytes:
  • Osteoblasts that have surrounded themselves with ECM in lacunae; mature cells, mostly inactive; continue to make and secrete substances important for bone maintenance.
• Osteoclasts:
  • Large, multinucleated bone destroyers; carry out the process of bone resorption; secrete hydrogen ions and enzymes that break down both inorganic and organic ECM.

Blood

• Unique; ECM is fluid.

Plasma Proteins

• Not like fibers found in other connective tissues; smaller with a variety of functions, including the transport of substances and blood clotting.
  • Erythrocytes (red blood cells) bind to and transport oxygen throughout the body.
  • Leukocytes (white blood cells) function in immunity.
  • Platelets are cell fragments and play a major role in blood clotting.

Osteoarthritis and Glucosamine Supplements

• Osteoarthritis is caused by age, joint trauma, genetic disorders, and infection.
• Develops as the hyaline cartilage lining joints degenerates.
• Leads to destruction of proteoglycan and collagen fibers; may continue until bone is exposed.
• Bones grind painfully together as motion occurs.
• Chondroblasts use glucosamine in the synthesis of proteoglycans; further studies are needed to determine if glucosamine supplementation will slow osteoarthritic degeneration of joints.

Muscle Tissues

• Specialized for contraction; three types share the ability to turn the chemical energy of ATP into mechanical energy of movement.
• Walking, breathing, heart beating, and propulsion of substances through hollow organs all result from contractions of different muscle tissues.
• The main component of muscle tissue is the muscle cell (myocyte); excitable (responds to electrical or chemical stimulation).

Components of Muscle Tissue

• Two forms of muscle cells based on the arrangement of myofilaments (protein bundles) in the cytoplasm:
  • Striated: Myofilaments arranged in alternating light and dark regions; appear striped (striated) under a microscope.
  • Smooth: Myofilaments arranged in irregular bundles instead of repeating light and dark regions.
• Endomysium: a small amount of ECM that surrounds muscle tissue; helps hold muscle cells together in tissue.

Types of Muscle Tissue

• Three types of muscle tissue feature different structural and functional characteristics:
  • Skeletal muscle.
  • Cardiac muscle.
  • Smooth muscle.

Striations

• Skeletal and cardiac muscle tissues are striated; smooth muscle tissue is not striated.

Skeletal Muscle Tissue

• Mostly attached to the skeleton; contraction produces body movement:
  • Skeletal muscle must be stimulated by the nervous system to contract; under voluntary (conscious) control.
  • Most skeletal muscle cells (muscle fibers) are long, extending to almost the entire length of a whole muscle.
  • Forms by the fusion of embryonic myoblasts, resulting in cells with more than one nucleus (multinucleate); useful for nearly constant synthesis of enzymes, structural proteins, and contractile proteins.

Cardiac Muscle Tissue

• Only in the heart; composed of cardiac muscle cells; striated like skeletal muscle cells:
  • Involuntary brain does not have conscious control over contraction.
  • Cells are short, branched; usually only one nucleus (uninucleate).
  • Intercalated disc-dark line separating individual cardiac muscle cells; not in skeletal muscle; contains gap junctions and modified tight junctions; allows heart muscle to contract as a unit.

Smooth Muscle Tissue

• Consists of smooth muscle cells; contractions are involuntary like cardiac muscle:
  • In walls of nearly every hollow organ, blood vessels, eyes, skin, and ducts of certain glands.
  • Flattened cells with one centrally located ovoid nucleus.
  • In most smooth muscle tissue, the plasma membranes of neighboring cells are linked together by gap junctions.

Nervous Tissues

• Makes up the majority of the brain, spinal cord, and nerves; two main cell types and their surrounding ECM:
  • Neurons: Capable of sending and receiving messages.
  • Neuroglial cells: Perform various functions; support neuron activities.
• ECM is unique: ground substance with unique proteoglycans not found in other tissues; contains very few protein fibers.

Neurons

• Excitable (like muscle cells); once mature, no longer divide by mitosis; three main components:
  • Cell body (soma): Biosynthetic center of neuron; location of nucleus and most organelles.
  • Axon: Solitary axon extends from one end of soma; responsible for moving the nerve impulse from the soma to the target cell (another neuron, muscle cell, or gland); Cell–Cell Communication Core Principle
  • Dendrites: Other extensions protruding from the soma; are typically short with multiple branches; receive impulses from axons of neighboring neurons; deliver impulses to the soma.

Neuroglial Cells

• A diverse group of smaller cells; support the activity of neurons:
  • Functions:
  • Anchoring neurons and blood vessels in place.
  • Monitoring the composition of the extracellular fluid.
  • Speeding up the rate of nerve impulse transmission.
  • Circulating fluid surrounding the brain and spinal cord.
  • Able to divide by mitosis (unlike neurons).

Tissues in Organs

• Organ: Two or more tissues that combine structurally and functionally:
  • Simple organ example: Skeletal muscle:
  • Two main tissues: skeletal muscle and dense irregular collagenous connective.
  • Each has a distinct functional role; skeletal muscle tissue allows contraction; the surrounding connective tissue binds muscle cells together and supports them so that their activity produces contraction of the whole organ.
  • Trachea: A more complex organ; consists of many different tissue types:
  • Hollow; provides a passageway through which air passes on the way into/out of the lungs.
  • Consists of tissues from superficial to deep that all contribute to the overall function of conducting air.

Membranes

• Thin sheets of one or more tissues; line a body surface or cavity:
  • Most consist of a superficial epithelial layer resting on a connective tissue layer; sometimes contain smooth muscle.
  • Functions: anchor organs in place, serve as barriers, function in immunity, and secrete various substances.
• True membranes: Serous and synovial membranes; fit the above structural and functional definitions.
• Membrane-like structures: Mucous and cutaneous membranes; don’t fit the above structural and functional definitions; perform many of the same functions.

True Membranes

• Do not open to the outside of the body; two examples:
  • Serous membranes or serosae: Line the pericardial, peritoneal, and pleural body cavities:
  • Consist of mesothelium (thin layer of simple squamous epithelium), associated basement membrane, and layer of connective tissue.
  • Fold over themselves; appearance of two layers; the outer parietal layer lines the body wall; the inner visceral layer covers the organ within the body cavity.
  • Mesothelial cells produce thin, watery serous fluid, which fills the space between the parietal and visceral layers; reduces friction created when organs (heart or lungs) move within respective membranes.
  • Synovial membranes: Line cavities surrounding freely moveable joints (knee or shoulder); two connective tissue layers without a layer of epithelial cells:
  • The outer layer is usually composed of a mixture of loose and dense irregular connective tissue.
  • The inner layer consists of synoviocytes (modified fibroblasts) that secrete synovial fluid, a watery, slippery fluid; primarily functions to lubricate the joint.

Membrane-Like Structures

• Cutaneous membranes and mucous membranes are membrane-like structures:
  • Mucous membranes (mucosae): Line all passages opening to the outside of the body; respiratory passages, mouth, nasal cavity, digestive tract, and male and female reproductive tracts:
  • Layer of epithelium and basement membrane (connective tissue called lamina propria) and occasionally a thin layer of smooth muscle.
  • Contain glands with goblet cells; produce and secrete mucus; serve several functions, primarily protection.
  • Cutaneous membrane refers to the skin: is the largest organ of the body and consists of:
  • An outer layer of keratinized stratified squamous epithelium (epidermis); a tough, continuous protective surface that protects structures deep to it.
  • Dermis, a layer of loose connective tissue beneath the epidermis plus a deeper layer of dense irregular connective tissue, is home to many blood vessels; provides means for oxygen and nutrients to diffuse into avascular epidermis.

Friction Rubs

• Inflammation of serous membranes of the pleural or pericardial cavities may result from a viral or bacterial infection.
• Serous fluid secreted for lubrication becomes inadequate to reduce friction; layers rub together as organs contract and expand.
• Friction rub results in a grating sound and can be heard with a stethoscope.
• Causes chest pain; worsens with inhalation, body movement, and swallowing.
• Usually resolves with treatment of the underlying condition.

Tissue Repair

• Also known as wound healing; dead and damaged cells are removed and replaced with new cells or tissues; fills gaps for the maintenance of homeostasis; the process differs with different tissues:
  • Some tissues are capable of regeneration; dead and damaged cells are replaced with cells of the same type; when the process is finished, the tissue returns to a normal functional level.
  • Other tissues are not capable of full regeneration; fibroblasts fill gaps left from injury by a process called fibrosis:
  • Fibroblasts divide by mitosis and produce collagen that fills the gap; the tissue loses some functional ability.
  • The end result of fibrosis is the development of scar tissue, a dense irregular connective tissue.

Capacity for Tissue Repair

• The capacity of specific tissue for repair is largely dependent on the resident cells’ ability to undergo mitosis.

Epithelial Tissues

• Typically undergo regeneration:
  • Skin and digestive tract lining are subjected to a great deal of stress; must have a mechanism for replacing dead, damaged, or worn-out cells; new cells are derived from stem cells (immature cells capable of mitosis).
  • Other epithelial tissues (liver and blood vessels): Mature cells in the vicinity of injuries are capable of replacing dead and damaged cells by dividing.

Connective Tissues

• Most heal by regeneration:
  • Connective tissue proper, bone, and blood regenerate easily through the division of resident immature cells.
  • Cartilage is the exception; resident cells have limited capacity for regeneration; heals by fibrosis.

Muscle Tissue

• Smooth muscle tissue usually regenerates; cardiac and skeletal muscle tissues generally heal by fibrosis:
  • Smooth muscle cells retain the ability to undergo mitosis; readily heal by regeneration.
  • Mature skeletal muscle fibers and cardiac muscle cells have lost the ability to divide by mitosis; cannot regenerate:
  • Satellite cells in skeletal muscle tissue can divide and mature into skeletal muscle cells, providing a limited degree of regeneration.
  • No satellite cells are associated with cardiac muscle tissue; injuries are healed by fibrosis.

Nervous Tissue

• Generally undergoes fibrosis; neurons have lost the ability to divide by mitosis; are incapable of regeneration:
  • Neuroglial cells retain the ability to divide by mitosis and replace dead and damaged neurons with scar tissue.
  • Axons of neurons outside the brain and spinal cord can regenerate given the right conditions.

Other Factors Affecting Tissue Repair

• Include nutrition and blood supply:
  • Tissue repair involves the production of large quantities of protein (collagen) and requires an adequate supply of amino acids.
  • Vitamin C is required by fibroblasts to produce functional collagen.
  • Blood supply to the injured region must be adequate to deliver oxygen, nutrients, and cells of the immune system (critical for tissue repair); the reason people with diseases of the arteries often have nonhealing wounds.