Anatomy and Physiology: Human Tissues

Simple Epithelia

• Single layer of cells.
• Three types, named by cell shape:
  • Simple squamous: thin, scaly cells.
  • Simple cuboidal: squarish or round cells.
  • Simple columnar: tall, narrow cells.
• Pseudostratified columnar: Not all cells reach the surface, but all reach the basement membrane.
  • Appears stratified because some cells are taller than others.
  • Goblet cells: wineglass-shaped, mucus-secreting cells found in simple columnar and pseudostratified epithelia.

Simple Squamous Epithelium

• Single row of thin cells.
• Permits rapid diffusion or transport of substances.
• Secretes serous fluid.
• Locations: alveoli, glomeruli, endothelium, and serosa.

Simple Cuboidal Epithelium

• Single layer of square or round cells.
• Function: absorption and secretion, mucus production and movement.
• Locations: liver, thyroid, mammary and salivary glands, bronchioles, and kidney tubules.

Simple Columnar Epithelium

• Single row of tall, narrow cells.
• Oval nuclei in basal half of cell.
• Brush border of microvilli, sometimes ciliated, may possess goblet cells.
• Function: absorption and secretion; secretion of mucus.
• Locations: lining of GI tract, uterus, kidney, and uterine tubes.

Ciliated Pseudostratified Columnar Epithelium

• Looks multilayered, but all cells touch the basement membrane.
• Nuclei at several layers.
• Has cilia and goblet cells.
• Function: secretes and propels mucus.
• Locations: respiratory tract and portions of male urethra.

Stratified Epithelia

• Range from 2 to 20 or more layers of cells.
• Some cells rest directly on others; only the deepest layer attaches to the basement membrane.
• Three stratified epithelia are named for the shapes of their surface cells:
  • Stratified squamous.
  • Stratified cuboidal.
  • Stratified columnar.
• Urothelium: unique to the urinary tract (previously called transitional epithelium).
• Stratified columnar is rare and found only where two other epithelial types meet.
• Stratified squamous is the most widespread.
  • Deepest cells are cuboidal to columnar and include mitotically active stem cells.
  • Daughter cells push toward the surface and become flatter as they migrate upward.
  • Finally, they die and flake off—exfoliation (desquamation).
• Two kinds of stratified squamous epithelia:
  • Keratinized (cornified): found on the skin surface, abrasion resistant.
  • Nonkeratinized: found lining internal spaces, lacks surface layer of dead cells.

Keratinized Stratified Squamous Epithelium

• Multiple cell layers; cells become flat and scaly toward the surface.
• Resists abrasion, retards water loss through the skin, and resists penetration by pathogenic organisms.
• Locations: epidermis; palms and soles are heavily keratinized.

Nonkeratinized Stratified Squamous Epithelium

• Same as keratinized epithelium without the surface layer of dead cells.
• Resists abrasion and penetration of pathogens.
• Locations: tongue, oral mucosa, esophagus, and vagina.

Stratified Cuboidal Epithelium

• Two or more cell layers; surface cells are square or round.
• Function: secretes sweat; produces sperm; produces ovarian hormones.
• Locations: sweat gland ducts, ovarian follicles, and seminiferous tubules.

Urothelium

• Multilayered epithelium with surface cells that change from round to flat when stretched.
• Allows for filling of the urinary tract.
• Locations: ureter and bladder.

Connective Tissue

• Most abundant, widely distributed, and variable tissue type in which cells occupy less space than the matrix.
• Most cells are not in direct contact with each other.
• Highly variable vascularity.
  • Loose connective tissues have many blood vessels.
  • Cartilage has no blood vessels.
• Functions:
  • Binding of organs: connects bones to each other, muscles to bones, skin to muscle, and holds organs in place.
  • Support: supports the body and its organs and forms the internal framework of organs.
  • Physical protection: protects and cushions delicate organs.
  • Immune protection: connective tissue cells attack foreign invaders.
  • Movement: bones provide a lever system for body movement; cartilages are involved in speech.
  • Storage: maintains stores of fat, calcium, and phosphorus.
  • Heat production: metabolism of brown fat generates heat.
  • Transport: blood transports gases, nutrients, wastes, hormones, and blood cells.

Fibrous Connective Tissue

• Classified together because fibers are so conspicuous.
• Components:
  • Cells:
    • Fibroblasts: produce fibers and ground substance of the matrix.
    • Macrophages: arise from monocytes; phagocytize foreign material and activate the immune system when they sense foreign matter (antigens).
    • Leukocytes (white blood cells, WBCs): function in immune defense.
      • Neutrophils attack bacteria.
      • Lymphocytes react against bacteria, toxins, and other foreign agents.
    • Plasma cells: arise from lymphocytes and synthesize antibodies.
    • Mast cells: secrete heparin to inhibit clotting and histamine to dilate blood vessels.
    • Adipocytes (fat cells): clustered in some fibrous tissues; where they dominate, called adipose tissue.
  • Fibers:
    • Collagenous fibers: made of collagen.
      • Tough, flexible, and stretch-resistant.
      • Called white fibers due to appearance in fresh tissue.
      • Tendons, ligaments, and the deep layer of the skin are mostly collagen.
      • Less visible in the matrix of cartilage and bone.
    • Reticular fibers:
      • Thin collagen fibers coated with glycoprotein.
      • Form framework of spleen and lymph nodes; part of basement membranes under epithelia.
    • Elastic fibers:
      • Thinner than collagenous fibers; made of protein elastin.
      • Allows stretch and recoil like a rubber band.
  • Ground substance: featureless substance surrounding cells, usually gelatinous or rubbery.
    • Absorbs compressive forces; contains large molecules—GAGs, proteoglycans, and glycoproteins.
    • Glycosaminoglycans (GAGs):
      • Long polysaccharides composed of amino sugars and uronic acid.
      • Regulate water and electrolyte balance of tissues.
      • Examples: chondroitin sulfate, heparin, and hyaluronic acid.
    • Proteoglycans:
      • Gigantic molecules shaped like bottle brushes.
      • Form gravy-like colloids that hold tissues together.
    • Adhesive glycoproteins:
      • Protein–carbohydrate complexes.
      • Bind components of a tissue together.

Types of Fibrous Connective Tissue

• Two broad categories: loose and dense connective tissue.
  • Loose connective tissue: mostly ground substance in the space surrounding cells.
    • Types: areolar and reticular.
  • Dense connective tissue: mostly fibers in the space surrounding cells.
    • Types: dense regular and dense irregular.

Areolar Tissue

• Loosely organized fibers, abundant blood vessels.
• Possesses all six cell types and fiber types.
• Underlies epithelia, in serous membranes, between muscles, and passageways for nerves and blood vessels.

Reticular Tissue

• Mesh of reticular fibers and fibroblasts.
• Forms supportive stroma (framework) for lymphatic organs.
• Found in lymph nodes, spleen, thymus, and bone marrow.

Dense Regular Connective Tissue

• Densely packed, parallel collagen fibers.
• Compressed fibroblast nuclei.
• Elastic tissue forms wavy sheets in some locations.
• Tendons attach muscles to bones, and ligaments hold bones together.

Dense Irregular Connective Tissue

• Densely packed, randomly arranged collagen fibers and few visible cells.
• Withstands unpredictable stresses.
• Locations: deeper layer of skin; capsules around organs.

Adipose Tissue

• Adipose tissue (fat): tissue in which adipocytes are the dominant cell type.
• Space between adipocytes is occupied by areolar tissue, reticular tissue, and blood capillaries.
• Fat is the body’s primary energy reservoir.
• The quantity of stored triglyceride and the number of adipocytes are quite stable in a person.
• Fat is recycled continuously—new triglycerides synthesized while old molecules are hydrolyzed and released to the blood.
• Two types in humans: white (or yellow) fat and brown fat.
  • White adipose tissue (WAT, or white fat) is the most abundant and significant type in adults.
    • Provides thermal insulation.
    • Cushions organs such as eyeballs and kidneys.
    • Contributes to body contours—female breasts and hips.
    • Secretes hormones that regulate metabolism.
  • Brown adipose tissue (BAT, or brown fat) is found mainly in fetuses, infants, and children.
    • Adults have smaller deposits of it.
    • Color comes from blood vessels, mitochondria, and mitochondrial enzymes.
    • Functions as a heat-generating tissue.

White Adipose Tissue

• Empty-looking cells with thin margins; the nucleus is pressed against the cell membrane.
• Function: energy storage, insulation, cushioning.
• Brown fat produces heat.

Cartilage

• Stiff connective tissue with a flexible, rubbery matrix.
• Gives shape to the ear, tip of nose, and larynx.
• General features:
  • Chondroblasts: cartilage cells that produce the matrix.
  • Chondrocytes: cartilage cells that are trapped in lacunae (cavities).
  • Perichondrium: sheath of dense irregular connective tissue that surrounds elastic and most hyaline cartilage (not articular cartilage).
    • Contains a reserve population of chondroblasts that contribute to cartilage growth throughout life.
  • No blood capillaries (avascular).
    • Diffusion brings nutrients and removes wastes.
    • Heals slowly.
  • The matrix is rich in GAGs and contains collagen fibers.
• Types of cartilage vary with fiber composition.
  • Hyaline cartilage.
  • Fibrocartilage.
  • Elastic cartilage.

Hyaline Cartilage

• Clear, glassy appearance because of the fineness of collagen fibers.
• Eases joint movement, holds the airway open, moves vocal cords, and allows for the growth of juvenile long bones.
• Locations: articular cartilage, costal cartilage, trachea, larynx, and fetal skeleton.

Elastic Cartilage

• Cartilage containing an abundance of elastic fibers.
• Covered with perichondrium.
• Provides flexible, elastic support.
• Locations: external ear and epiglottis.

Fibrocartilage

• Cartilage containing large, coarse bundles of collagen fibers.
• Resists compression and absorbs shock.
• Locations: pubic symphysis, menisci, and intervertebral discs.

Bone

• Bone (osseous tissue) is a calcified connective tissue that composes the skeleton.
• Two forms of osseous tissue:
  • Spongy bone: delicate slivers and plates give it a spongy appearance.
    • Found in heads of long bones and in the middle of flat bones such as the sternum.
  • Compact (dense) bone: denser calcified tissue with no visible spaces.
    • Forms external surfaces of all bones.

Features of Compact Bone

• Arranged in cylinders called osteons.
• Central (osteonic) canal: passage running longitudinally along bone shaft; contains blood vessels and nerves.
• Concentric lamellae: ringlike layers of bone surrounding the central canal.
• Osteon: central canal and its surrounding lamellae.
• Osteocytes: mature bone cells within lacunae between lamellae.
• Canaliculi: delicate canals radiating from each lacuna to its neighbors, allowing osteocytes to contact each other.
• Periosteum: tough fibrous connective tissue covering the whole bone.

Compact Bone

• Cylinders that surround central (osteonic) canals that run longitudinally through shafts of long bones.
• Blood vessels and nerves travel through the central canal.
• Bone matrix deposited in concentric lamellae.

Blood

• Blood: fluid connective tissue that travels through tubular blood vessels.
• Transports cells and dissolved matter from place to place.
• Contains formed elements suspended in a liquid ground substance called blood plasma.
• Formed elements include:
  • Erythrocytes (red blood cells, RBCs): transport $O<em>2$ and $CO</em>2$.
  • Leukocytes (white blood cells, WBCs): defend against infection and disease.
    • Examples: neutrophils, eosinophils, basophils, lymphocytes, monocytes.
  • Platelets: cell fragments involved in clotting.

Blood Smear

• Erythrocytes appear as pale pink discs with light centers.
• Leukocytes are slightly larger and have variously shaped nuclei.
• Platelets are small cell fragments with no nuclei.

Nervous and Muscular Tissues

• Excitable Tissues
• Excitability: ability to respond to stimuli by changing membrane potential.
• Developed to the highest degree in nervous and muscular tissues, so they are called excitable tissues.
• Excitation is founded in charge difference (voltage) across the membrane called the membrane potential.
• In nerve cells, changes in voltage result in rapid transmission of signals to other cells.
• In muscle cells, changes in voltage result in contraction, shortening of the cell.

Nervous Tissue

• Specialized for communication by electrical and chemical signals.
• Consists of:
  • Neurons (nerve cells): detect stimuli, respond quickly, and transmit coded information rapidly to other cells.
  • Neuroglia (glial cells): protect and assist neurons; are the “housekeepers” of the nervous system.
• Parts of a neuron:
  • Cell body: houses the nucleus and other organelles; controls protein synthesis.
  • Dendrites: short, branched processes that receive signals from other cells and transmit messages to the cell body.
  • Axon (nerve fiber): sends outgoing signals to other cells; can be more than a meter long.

Cellular Junctions

• Connections between two cells.
• Most cells are anchored to each other or their matrix.
• Cells communicate with each other, resist mechanical stress, and control what moves through the gaps between them.

Tight Junctions

• Zipperlike, interlocking linkage between two adjacent cells by transmembrane cell-adhesion proteins.
• Encircles an epithelial cell near its apical surface.
• Joins epithelial cell with neighboring cells.
• Seals off the intercellular space, making it difficult or impossible for substances to pass between cells.
• Example: tight junctions in the stomach and intestines prevent digestive juices from seeping between epithelial cells and digesting underlying connective tissue.

Desmosomes

• Patch that holds cells together, like a clothing snap.
• Keeps cells from pulling apart—resists mechanical stress.
• Hook-like, J-shaped proteins arise from the cytoskeleton.
• Common in epidermis and other epithelia.
• Hemidesmosome: half-desmosome that anchors basal cells of an epithelium to an underlying basement membrane.
  • Epithelium cannot easily peel away from underlying tissues.

Gap Junctions

• Gap (communicating) junction: channel between cells formed by a ringlike connexon.
• Connexon consists of six transmembrane proteins arranged like segments of an orange around a water-filled channel.
• Ions, nutrients, and other small solutes pass between cells.
• Located in cardiac and smooth muscle, embryonic tissue, lens, and cornea.