YC

ANHB1101 Tissues Notes

Introduction to Tissues & Epithelia

Learning Objectives

  • Name the cell types that make up nervous tissue.
  • Describe the general characteristics of epithelium.
  • Describe how epithelia are classified into subtypes.
  • Describe the characteristics, function, and locations of each of the common epithelia found in the human body.
  • Emphasis is on recognizing tissue types and subtypes and relating structure to function.

Organisation of the Body

  • The organization of the human body is hierarchical:
    • Atom
    • Molecule
    • Macromolecule
    • Organelle
    • Cell
    • Tissue
    • Organ
    • Organ system
    • Organism
  • A tissue is defined by its specific structural and functional characteristics.

Primary Tissue Types

  • There are four primary tissue types:
    • Nervous: Communication and control.
    • Epithelia: Covering and lining body surfaces.
    • Connective Tissue: Support and protection.
    • Muscle: Movement and generation of heat.

Nervous Tissue

  • Makes up the nervous system.
  • Excitable (responds to stimuli).
  • Concerned with communication and control.
  • Highly cellular.
  • Two major cell types:
    • Neurons: Generate and conduct nerve impulses (excitable).
    • Supporting cells: Aid in the function of neurons.

Epithelium

  • Epithelium (singular); Epithelia (plural).

Characteristics of Epithelium

  • Covers body surfaces, lines body cavities and tubes, forms the secretory portion of glands.
  • Functionally, it acts as a selective barrier with roles in secretion, absorption, transport, protection, and receptor function.

Microanatomy of Epithelium

  • Polarity: Epithelial cells have distinct domains.
    • Apical Domain
    • Lateral Domain
    • Basal Domain
  • All three domains contribute to the epithelium’s function as a barrier.
  • Each surface has specific biochemical characteristics.

Lateral Domain of Epithelia

  • Cells are very closely packed with little extracellular matrix and tightly bound via cell junctions:
    • Tight junctions
    • Desmosomes
    • Gap junctions (for cell-to-cell communication)
  • Functions of these junctions:
    • Resist stress
    • Control what can enter the cells (barrier function)
    • Facilitate communication between cells

The Basal Domain of Epithelia - Basement Membrane

  • All epithelium is in direct association with a basement membrane.
  • Basement membrane: Thin layer of intercellular material (50-100nm thickness).
  • Manufactured by the epithelium and comprised of various proteins.
  • Basement membrane (BM) = basal lamina (seen via Transmission Electron Microscopy - TEM).
  • Hemidesmosomes + underlying layer of connective tissue fibres attached to BM.
  • Functions of the basement membrane:
    • Structural attachment: Provides binding sites for cell adhesion molecules.
    • Tissue organization during development.
    • Guides cellular differentiation.
    • Inhibits or promotes cell proliferation and migration.
    • Semi-permeable selective barrier.

Classification of Epithelia

  • Classification is based on:
    • Cell shape
    • Number of layers
  • Exceptions exist:
    • Pseudostratified columnar (simple)
    • Transitional epithelium (stratified)
  • Structure is closely related to function.

Types of Epithelia

  • Simple Squamous
  • Simple Cuboidal
  • Simple Columnar
  • Stratified Squamous
  • Stratified Cuboidal
  • Pseudostratified

Simple Squamous Epithelium

  • Location examples:
    • Lines blood vessels (endothelium)
    • Body cavities (mesothelium)
    • Kidneys (part of the glomerular capsule)
    • Lungs (forms alveoli)
  • Functions:
    • Simple barrier
    • Rapid transfer/exchange of fluids and gases

Simple Cuboidal Epithelium

  • Location examples:
    • Secretary portions and ducts of small glands
    • Surface of the ovary
    • Kidney tubules
    • Thyroid follicles
  • Functions:
    • Barrier
    • Absorption
    • Secretion

Simple Columnar Epithelium

  • May have microvilli (brush border) or cilia on the apical surface.
  • Location examples:
    • Stomach
    • Small & large intestines
  • Functions:
    • Barrier (e.g., from stomach acid)
    • Absorption
    • Secretion

Pseudostratified Columnar Epithelium

  • Location examples:
    • Trachea & bronchial tree
    • Vas deferens & epididymis of the male reproductive tract
  • Functions:
    • Barrier
    • Absorption
    • Secretion
    • Conduit

Stratified Squamous Epithelium

  • Two types.
  • Location examples:
    • Epidermis
    • Oral cavity & esophagus
    • Vagina
  • Functions:
    • Barrier
    • Protection against abrasion

Other Stratified Epithelia

  • Stratified Cuboidal (e.g., sweat glands, anorectal junction, large ducts of exocrine glands).
  • Stratified Columnar (e.g., anorectal junction, largest ducts of exocrine glands).

Transitional Epithelium

  • Functions:
    • Stretches to allow distension of the urinary tract.
    • Protects the underlying tissues from osmotic damage from urine (barrier).
  • Locations:
    • Parts of the kidney, ureters, bladder, part of the urethra.
  • Surface cells are the largest some cells may be bi-nucleate (2 nuclei)

Epithelium Function

  • Columnar epithelium of the stomach and gastric glands: secretion.
  • Columnar epithelium of intestines & cuboidal epithelium in parts of the nephron in the kidneys: absorption.
  • Pseudostratified columnar: Transport of materials along the surface of the epithelium.
  • Simple squamous: Trans-epithelial transport.
  • Stratified squamous & transitional: Protection.
  • Taste buds, olfactory epithelium, retina: Receive and transduce external stimuli.

Glands

  • Gland = cell or organ that secretes substances for use elsewhere in the body.
  • The gland itself may produce a product (secretion) OR the product may be something removed from tissues and modified by the gland (excretion).
  • Two types of gland:
    • Exocrine: Maintains contact with the surface by way of a duct (a tube of epithelium that conveys secretion to the surface). Examples = salivary glands, sweat glands, mammary glands.
    • Endocrine: No contact with surface (lost during development), so no ducts; product (= hormones) is secreted directly into the blood. Examples = pituitary, thyroid, adrenal glands.
  • Some organs have both exocrine and endocrine functions, e.g., pancreas.
  • Some glands are unicellular, e.g., goblet cells.

Exocrine Gland Classification

  • Simple = single unbranched duct.
  • Compound = branched ducts.
  • If duct and secretory portion are of uniform diameter = tubular.
  • If secretory portion forms a dilated sac = acinar (acinus = berry).

Modes of Secretion

  • Merocrine = product released by exocytosis, sometimes called eccrine (e.g., salivary glands).
  • Apocrine = droplets covered by cell membrane and a very thin layer of cytoplasm bud off from cell surface (e.g., milk fats).
  • Holocrine = cells accumulate a product and then the cell disintegrates, thereby becoming the product (e.g., sebaceous glands).

Types of Secretion

  • Serous – thin watery secretion, e.g., tears, sweat, digestive juices.
  • Mucous – secrete mucin (glycoprotein), mixes with H_2O to produce mucus, e.g., goblet cells.
  • Mixed – contain both serous and mucous cells, e.g., some salivary glands.
  • Etymology: Mero – fraction, part; Apo – off, away, asunder; Holo- whole, entire

Metaplasia

  • Metaplasia – a change from one mature tissue type to another.