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.