Histology: Tissue Types & Functions - Chapter 4

• Chapter 4 focuses on the tissue level of organization.
• The chapter's aims are to explore tissue types, cell connections, tissue comparisons, membrane structure/function, and tissue repair.

Introduction to Tissues

• Tissues are groups of cells with a shared embryonic origin that perform specific functions.
• Tissue types range from hard tissues like bone, to semisolid tissues like fat, and liquid tissues like blood.

Tissue Study - Histology

• Histology is the study of cells and tissues at a microscopic level.
• Histology is a branch of pathology.
• The human body contains 10 trillion cells, none of which can be called "typical".
• A trained histologist can identify over 200 types of human cells microscopically.
• Histologists can differentiate cells from various tissues, like pancreatic versus skin.
• Pathologists are medical doctors who diagnose illnesses by examining cells using microscopic techniques.

Four Basic Tissue Types

• The body has four basic tissue types:
• Epithelial
• Connective
• Muscular
• Nervous

Intercellular Junctions

• Tissues form as cells group using intercellular junctions.
• Intercellular junctions connect adjacent cells mechanically via membranes or cytoskeletal elements.
• Five types of intercellular junctions: tight, adherens, desmosomes, hemidesmosomes, and gap.

Tight Junctions

• Tight junctions create leakproof seals between cells.
• They prevent substance leakage from organs like the stomach and bladder.
• Tight junctions feature transmembrane protein strands that fuse cells and seal passageways.

Adherens Junctions

• Adherens junctions create adhesion belts to prevent tissue separation during stretching and contraction.
• These belts encircle the cell's circumference.
• Cadherin, a transmembrane glycoprotein, extends to neighboring cells.
• Plaque, an intracellular protein complex, links cadherins to microfilaments.
• Adherens junctions allow intestinal epithelial cells to remain attached.

Desmosomes

• Desmosomes act like "spot welds" using transmembrane cadherins to connect cells
• Plaque connects cadherins to intermediate filaments, such as keratin.
• Intermediate filaments add structural stability by extending across the cell at the plaque.
• Desmosomes connect skin epithelium and cardiac muscle cells.

Hemidesmosomes

• Hemidesmosomes are "half-welds" attaching cells to the basement membrane.
• They connect cells to non-cellular structures like the epithelium's basement membrane.
• Integrin is found instead of cadherin.
• Integrin binds to laminin, an extracellular component of the basement membrane.

Gap Junctions

• Gap junctions are pores, known as connexons, that allow small substances such as ions to pass between cells.
• Their main function is communication between adjacent cells.
• These junctions can seal when a cell is damaged, preventing harm to other cells.

Epithelial vs. Connective Tissue

• Major structural differences exist between epithelial and connective tissues.
• These involve the number of cells relative to the extracellular matrix.
• Epithelial tissue is avascular.

Basic Tissues: Epithelial

• Epithelial tissues cover surfaces, form glands, and line hollow organs, cavities, and ducts.

Basic Tissues: Connective

• Connective tissues support and bind organs.
• Adipose tissue (fat) is a type of connective tissue which stores energy.
• Blood cells and platelets are connective tissue.

Basic Tissues: Muscle

• Muscular tissues generate physical force for body movement and heat.

Basic Tissues: Nervous

• Nervous tissues detect environmental changes, assess them, and trigger responses.

Tissue Development

• Body tissues arise from three primary germ layers: endoderm, mesoderm, and ectoderm.
• Epithelial tissues originate from all three germ layers.
• Connective and muscle tissues originate from the mesoderm.
• Nervous tissue originate from the ectoderm.

Epithelium Overview

• Epithelium lines surfaces and forms protective barriers.
• Epithelium secretes mucous, hormones, and other substances.
• Epithelial tissues have a free apical surface and a basal surface attached to a basement membrane.
• Epithelium is avascular but supplied by nerves.
• Epithelium always has a free apical surface.

Epithelium and Lamina

• The epithelium's basal layer secretes a basal lamina.
• Connective tissue secretes a reticular lamina.
• Together, these form a non-cellular basement membrane.

Epithelium: Naming Conventions

• Epithelial tissues are named based on their cell shape and layer arrangement.
• Epithelial Layers are of the following arrangements:
• Simple
• Pseudostratified
• Stratified
• Epithelial shapes are named:
• Squamous
• Cuboidal
• Columnar

Epithelium Name Combinations

• Three cell shapes multiplied by three arrangements yields nine possibilities, but two are not used.
• With the addition of transitional cells, there are eight possible combinations.
• When different shapes appear in layers, the epithelium is named after the shape in the apical, or outermost, layer.

Simple Squamous Epithelium

• Single layer of flat cells is found in:
• Air sacs of lungs
• Lining of blood vessels, heart, and lymphatic vessels (endothelium)
• All capillaries, including kidney capillaries
• Major part of serous membranes (mesothelium); key for filtration/diffusion

Simple Cuboidal Epithelium

• Single layer of cube-shaped cells is often found lining:
• Kidney tubules
• Many glands
• Functions in secretion and absorption

Simple Columnar Epithelium

• Single layer of column like cells, ± cilia, ± microvilli, ± mucous (goblet cells).
• Goblet cells are simple columnar cells differentiated to secrete mucous.
• Functions in GI tract for secretion & absorption

Pseudostratified Columnar Epithelium

• Tissue appears layered because nuclei are at different depths, but all cells attach to the basement membrane in a single layer and some do not extend to the apical surface.
• Ciliated tissue has goblet cells that secrete mucus.

Stratified Squamous Epithelium

• Apical layer is made of squamous (flat) cells.
• Other layers have different shapes, but are named based on the apical layer.
• The many layers are ideal for protection against strong friction forces.

Stratified Cuboidal Epithelium

• Tissue has an apical surface of two or more layers of cube-shaped cells
• Is located in:
• Sweat glands
• Part of the ♂ urethra.

Stratified Columnar Epithelium

• Very rare.

Transitional Epithelium

• Cells change shape depending on tissue stretch.
• Apical "dome cells" in the top layer signify an empty bladder.
• Cells flatten when the bladder are full.

Epithelial Localization

• While epithelia are found throughout the body, certain types are associated with specific locations.
• Stratified squamous is prominent in the outer skin layers.
• Simple squamous makes up epithelial membranes and lines the blood vessels.
• Columnar is common in the digestive tract.
• Pseudostratified ciliated columnar is characteristic of the upper respiratory tract.
• Transitional is found in the bladder.
• Cuboidal lines ducts and sweat glands.

Covering and Lining Epithelium: Endothelium

• Endothelium is a specialized simple squamous epithelium that lines the entire circulatory system from the heart to the smallest capillary.
• It reduces blood turbulence.

Covering and Lining Epithelium: Mesothelium

• Mesothelium is found in serous membranes like the pericardium, pleura, and peritoneum.
• Unlike other epithelial tissue, both are derived from embryonic mesoderm (middle layer of the 3 primary germ layers of the embryo).