Anatomy and Physiology - The Human Tissues

The Human Tissues

Primary Tissue Classes

• Four primary tissue classes:
  • Epithelial
  • Connective
  • Nervous
  • Muscular
• Differ in:
  • Cell types and functions
  • Matrix characteristics
  • Space occupied by cells and matrix
• Matrix composition:
  • Fibrous proteins
  • Ground substance (tissue fluid, ECF, interstitial fluid)
    • Water, gases, minerals, nutrients, wastes, hormones

Embryonic Tissues

• Three primary germ layers:
  • Ectoderm (outer): epidermis and nervous system
  • Endoderm (inner): mucous membrane lining digestive and respiratory tracts, digestive glands
  • Mesoderm (middle): mesenchyme (cartilage, bone, blood)
    • Wispy collagen fibers and fibroblasts in gel matrix

Intercellular Junctions

• Tight junctions:
  • Membranes merge and fuse.
  • Located among cells that form linings, and sheetlike layers.
  • Example: Blood-brain barrier
• Desmosomes:
  • Form "spot welds" between cells.
  • Structural reinforcement.
  • Located among outer skin cells.
• Gap junctions:
  • Tubular channels between cells.
  • Molecules can move between cells.
  • Located in cardiac muscle cells.

Epithelial Tissue

• Sheet of cells, one or more cells thick.
• Covers body surfaces and lines body cavities.
• Constitutes most glands.
• Avascular.
• Functions:
  • Protection
  • Secretion
  • Excretion
  • Absorption
  • Filtration
  • Sensation
• Structure:
  • Cells close together, small amount of extracellular material.
  • Avascular, nourished by underlying connective tissue.
  • High rate of mitosis.
  • Basement membrane: anchors epithelium to connective tissue.
• Surfaces of epithelial cells:
  • Basal surface: faces the basement membrane.
  • Apical surface: faces away from the basement membrane.
  • Lateral surface: between the basal and apical surface.
• Classified by number of cell layers:
  • Simple: each cell anchored to basement membrane.
  • Stratified: cells on top of others, not all contact basement membrane.

Simple Epithelia

• Single layer of cells.
  • 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.
  • Goblet cells: mucus-secreting cells in simple columnar and pseudostratified epithelia.

Stratified Epithelia

• Range from 2 to 20 or more layers of cells.
• Stratified squamous.
• Stratified cuboidal.
• Stratified columnar.
• Urothelium: unique to the urinary tract.
• Stratified columnar: rare, in places where two other epithelial types meet.
• Stratified squamous: most widespread.
  • Deepest cells are cuboidal to columnar, include mitotically active stem cells.
  • Daughter cells flatten as they migrate upward.
  • Exfoliation (desquamation): cells die and flake off.
  • Keratinized (cornified): on skin surface, abrasion resistant.
  • Nonkeratinized: lining internal spaces, lacks surface layer of dead cells.

Connective Tissue

• Abundant, widely distributed, and variable.
• Cells occupy less space than matrix.
• Most cells not in direct contact.
• Highly variable vascularity.
• Functions:
  • Binding of organs
  • Support
  • Physical protection
  • Immune protection
  • Movement
  • Storage
  • Heat production
  • Transport

Fibrous Connective Tissue

• Components:
  • Cells:
    • Fibroblasts: produce fibers and ground substance.
    • Macrophages: phagocytize foreign material and activate immune system.
    • Leukocytes (white blood cells): immune defense.
    • Plasma cells: synthesize antibodies.
    • Mast cells: secrete heparin and histamine.
    • Adipocytes (fat cells): clustered in some tissues.
  • Fibers:
    • Collagenous fibers: tough, flexible, and stretch-resistant.
    • Reticular fibers: thin collagen fibers coated with glycoprotein.
    • Elastic fibers: thinner than collagenous fibers, made of elastin.

Types of Fibrous Connective Tissue

• Loose connective tissue: mostly ground substance.
  • Areolar
  • Reticular
• Dense connective tissue: mostly fibers.
  • Dense regular
  • Dense irregular

Adipose Tissue

• Adipocytes are the dominant cell type.
• Body’s primary energy reservoir.
• Two types:
  • White adipose tissue: most abundant in adults, thermal insulation, cushions organs, contributes to body contours, secretes hormones.
  • Brown adipose tissue: mainly in fetuses, infants, children; heat-generating tissue.

Cartilage

• Stiff connective tissue with flexible, rubbery matrix.
• Chondroblasts: produce the matrix.
• Chondrocytes: trapped in lacunae.
• Perichondrium: sheath of dense irregular connective tissue.
• Avascular.
• Matrix rich in GAGs and collagen fibers.
  • Hyaline cartilage.
  • Fibrocartilage.
  • Elastic cartilage.

Bone

• Calcified connective tissue that composes the skeleton.
• Spongy bone: delicate slivers and plates.
• Compact bone: dense calcified tissue with no visible spaces.

Compact Bone

• Arranged in cylinders called osteons.
• Central (osteonic) canal: contains blood vessels and nerves.
• Concentric lamellae: ringlike layers of bone.
• Osteocytes: mature bone cells within lacunae.
• Canaliculi: canals radiating from lacunae, allowing osteocyte contact.
• Periosteum: fibrous connective tissue covering the bone.

Blood

• Fluid connective tissue that travels through blood vessels.
• Formed elements suspended in blood plasma.
  • Erythrocytes (red blood cells): transport $O<em>2$ and $CO</em>2$.
  • Leukocytes (white blood cells): defend against infection.
  • Platelets: cell fragments involved in clotting.

Nervous and Muscular Tissues

• Excitability: ability to respond to stimuli by changing membrane potential.
• Nervous tissue: communication by electrical and chemical signals.
  • Neurons: detect stimuli, respond quickly, and transmit coded information.
  • Neuroglia: protect and assist neurons.
  • Parts of a neuron:
    • Cell body: houses nucleus and organelles.
    • Dendrites: receive signals from other cells.
    • Axon (nerve fiber): sends outgoing signals.
• Muscular tissue: specialized to contract when stimulated.
  • Skeletal
  • Cardiac
  • Smooth

Skeletal Muscle

• Made of muscle fibers—long thin cells.
• Most skeletal muscles attach to bone.
• Contains multiple nuclei.
• Striations—alternating dark and light bands.
• Voluntary—conscious control.

Cardiac Muscle

• Limited to the heart wall.
• Cardiomyocytes are short and branched with one centrally located nucleus.
• Intercalated discs join cardiomyocytes end to end.
• Striated and involuntary.

Smooth Muscle

• Made of short, fusiform-shaped cells.
• Cells have one central nucleus, no striations.
• Involuntary function.
• Most is visceral muscle—making up parts of walls of hollow organs.

Cellular Junctions, Glands, and Membranes

Membranes

• Sheets of cells.
• Epithelial membranes: epithelial and connective tissue.
  • Serous membranes: line body cavities that do not open to the outside of the body.
    • Simple squamous epithelium + areolar connective tissue.
    • Secrete serous fluid for lubrication.
  • Mucous membranes: line cavities and tubes that open to the outside of the body.
    • Epithelium + areolar connective tissue.
    • Goblet cells secrete mucus.
  • Cutaneous membrane: covers body surface (skin).
• Synovial membranes: composed entirely of connective tissue, line joint cavities.

Glands

• Exocrine glands: maintain contact with surface of epithelium by a duct.
• Endocrine glands: no ducts, secrete hormones directly into blood.

Types of Secretions

• Serous glands: produce thin, watery secretions.
• Mucous glands: produce glycoprotein, mucin, which absorbs water to form mucus.
• Mixed glands: contain both serous and mucous cell types.

Modes of Secretion

• Merocrine secretion: vesicles release secretion by exocytosis.
• Apocrine secretion: lipid droplet covered by membrane and cytoplasm buds from cell surface.
• Holocrine secretion: cells accumulate a product until they disintegrate.