Histology: Epithelial Tissue

• General histology studies tissues
• Histology is the study of tissues
• Histo means "tissues"
• Ology means "study"
• A tissue is a group of cells with similar structure and function, along with extracellular substances located between them

Types of Tissues

• Epithelial
• Connective
• Muscular
• Nervous

Epithelial Tissue

• Provides the lining/covering (epithelium)
• Covers external and internal surfaces throughout the body
• Lines cavities and forms the outer layer of the skin
• Consists mostly of cells with very little extracellular material between them
• Most epithelia have a free surface not in contact with other cells
• Contains a basal surface adjacent to a basement membrane
• Attaches to underlying tissues via the basement membrane
• Can consist of a single layer or multiple layers between the free surface and the basement membrane
• The basement membrane is secreted partly by epithelial cells and partly by cells of the underlying tissues
• It is a meshwork of protein molecules with other molecules bound to them
• Substances that cross the epithelium must cross the basement membrane
• Functions as a filter and a barrier to cell movement
• For instance, if epithelial cells become cancerous, the basement membrane can prevent cancer from spreading for a time
• Blood vessels from underlying tissues do not penetrate the basement membrane
• Gases and nutrients must diffuse from the basement membrane to reach the epithelium
• Waste products from the epithelium diffuse across the basement membrane to reach the blood vessel-rich underlying tissues

Functions of the Epithelium

• Protection of underlying structures, such as protecting the oral cavity from abrasion with an epithelial lining
• Acts as a barrier; skin epithelium prevents water loss and the entrance of bacteria
• Secretes substances; includes sweat glands, mucous glands, and enzyme-secreting portions of the pancreas
• Absorption of substances, such as cell membranes of certain epithelium containing carrier proteins that regulate the absorption of materials
• Classification is according to the number of cell layers and the shape of the cells

Simple Squamous Epithelium

• Single layer of flat, hexagonal cells
• Nuclei appear as bumps in cross-section
• Substances pass through easily in this thin layer compared to others
• Found in alveoli, allowing oxygen to diffuse easily and CO2 out
• Filtration membranes of kidneys
• Prevents abrasion between organs in pleural cavities
• Outer surface of organs prevent damage from friction

Simple Cuboidal Epithelium

• Single layer of cube-like cells
• Carries out facilitated transport, active transport, or secretion
• Kidney tubules have walls composed of simple cuboidal epithelium
• Secretes waste products into tubules and reabsorbs useful material as urine is formed
• Some cells have cilia that move mucus over the free surface
• Features microvilli

Simple Columnar Epithelium

• Single layer of tall, thin cells
• Found in the small intestine, producing and secreting mucus and digestive enzymes
• Absorbs digested foods by active transport, diffusion, or facilitated diffusion
• Ciliated, moving particles out of the bronchioles of the lungs
• Ciliated, moving oocytes through the uterine tubes

Pseudostratified Columnar Epithelium

• Specific type of simple epithelium
• Epithelium appears stratified, but consists of only one layer of cells all attached to the basement membrane
• Appears to have two layers because some cells appear tall and reach the free surface, while others are short and do not reach the surface
• Some cells are almost always ciliated
• Nuclei of cells are at different levels
• Often has goblet cells that secrete mucus onto the free surface

Stratified Squamous Epithelium

• Thick, consisting of multiple layers of cells
• Deepest cells are cuboidal or columnar, capable of dividing and producing new cells
• Newly formed cells are pushed to the surface, becoming thin and flat
• Cytoplasm is replaced by keratin as cells flatten, and the cells die (keratinized)
• Dead cells provide protection against abrasion and act as a barrier against microorganisms
• Damaged cells are replaced by cells formed in the deeper layers
• Stratified squamous epithelium is different in the mouth
• Composed of living cells with a moist surface
• Water readily moves across due to it acting as a mechanical barrier

Stratified Cuboidal Epithelium

• Composed of more than one layer of epithelial cells
• Relatively rare
• Found in sweat glands, ovarian follicular cells, and salivary glands
• Functions as absorption, secretion, and protection

Stratified Columnar Epithelium

• More than one layer of epithelial cells
• Only the surface is columnar
• Deeper layers are irregular or cuboidal
• Rare
• Located in mammary gland ducts, the larynx, and a portion of the male urethra
• Functions include secretion, protection, and absorption

Transitional Epithelium

• Type of stratified epithelium that can be greatly stretched
• In its unstretched state, the epithelium contains five or more layers of cuboidal or columnar cells that are dome-shaped at the free surface
• In a stretched state, cells change to a low cuboidal or squamous shape, decreasing the number of layers
• Lines cavities that can expand greatly, such as the urinary bladder, ureters, and the superior urethra
• Accommodates fluctuations in liquid volume
• Protects against caustic effects of urine

Structural and Functional Relationships

• The number of cell layers and the shape of cells in a specific type reflect the function
• Simple epithelium is often found in organs with the primary function to move materials
• Stratified epithelium is more adapted to protective functions
• Stratified epithelium will be found on areas most likely abraded such as skin, the anal canal and the vagina
• Cells that are flat and thin when the function is diffusion
• Cells that are cuboidal or columnar in shape are mostly involved in secretion or absorption
• The shape and number of layers a cell has can change if subject to long term irritation or abnormal conditions
• People who smoke eventually experience changes in the epithelium of their airways (PCCE -> Stratified squamous epithelium)
  • Stratified squamous epithelium is more resistant to irritation

Cell Specializations

• Free surfaces are smooth or lined with microvilli or cilia, reflecting the epithelium's function
• Microvilli are cylindrical extensions of the cell membrane
• Cilia propel materials along the free surface of cells
• Goblet cells secrete mucus, intermixed with ciliated cells

Cell Connections

• Epithelial cells connected to each other via:
• Tight junctions
• Desmosomes
• Hemidesmosomes
• Gap junctions

Tight Junctions

• Bind adjacent cells to each other, forming permeability barriers by preventing the passage of materials between cells
• Found in the lining of the intestine and most simple epithelia

Desmosomes and Hemidesmosomes

• Mechanical links that bind cells together
• Hemidesmosomes anchor cells to the basement membrane
• Both found in epithelia subjected to stress (Stratified Squamous Epithelium)

Gap Junctions

• Small channels that allow small molecules and ions to pass from one epithelial cell to an adjacent one
• Coordinates the activities of cells

Glands

• A structure that secretes substances onto a surface, into a cavity, or into the blood
• Most glands are composed primarily of epithelium and are multicellular, with the exception of unicellular goblet cells
• Exocrine glands have ducts
• Endocrine glands have no ducts and empty their secretions into the blood

Exocrine Glands

• Simple have ducts but no branches
• Compound have ducts and may have many branches
• The end of a duct can be tubular (straight or coiled) or acinus/alveolar (grape-like or sac-like)
• Some compound glands have both a tubular and an acinus component (tubuloalveolar/tubuloacinar)
• Secretions travel through tubes onto the surface

Exocrine glands are classified according to how products leave the cell:

• Merocrine: Products are released, but no actual cellular material is lost; most common type
  • Secreted products actively transported or packaged in vesicles released by exocytosis
• Apocrine: Secretory products are released as fragments of the gland cell, milk secretions for example
• Holocrine: Involves the shedding of entire cells such as sebaceous (oil) glands of the skin

Endocrine Secretion

• Glands have no ducts and secrete their products, hormones, into the blood

Connective Tissue

• Found throughout the body
• Characterized by large amounts of extracellular material that separates cells from one another
  • Exists in the extracellular matrix, which has three major components: protein fibers, ground substance, and fluid
    • Collagen is abundant, flexible, and resists stretching
    • Reticular are very fine, short collagen fibres that branch to form a supporting network
    • Elastic have the ability to be stretched and then recoil
• Cells named according to function
  • Suffix -blast produces the matrix
  • Suffix -cyte maintains the matrix
  • Suffix -clast breaks down the matrix for remodeling

Functions of Connective Tissue

• Enclosing and separating other tissues
• Connecting tissues to one another
• Supporting and moving parts of the body
• Storing compounds
• Cushioning and insulating
• Transport of materials
• Protection

Connective Tissue Classification

• Connective tissue types blend into one another,
  • Major Categories:
    • Embryonic
    • Adult

Adult Connective Tissue

• Connective tissue proper: Loose types like areolar, adipose and reticular / Dense types like regular collagenous, regular elastic, irregular collagenous, and irregular elastic
• Supporting : Cartilage or Bone
• Fluid: Blood and Hemopoietic Tissue

Connective Tissue Proper

• Loose connective tissue has few protein fibers that form a lacy network
• Its numerous spaces are filled with ground substance and fluid
• It is loose packing material of most organs and tissues

Types of Connective Tissue Proper

Areolar Loose Connective Tissue

• ECM mostly has collagen fibers rather than collagen, and very few elastic fibers
• The most common cells are fibroblasts, responsible for producing the matrix
• Also contains macrophages and lymphocytes
• Function: Loose packing for support and nourishment and substance on which the epithelial basement membrane rests

Adipose Tissue

• Consists of adipocytes/fat cells
• Contains large amounts of lipid for energy storage
• Unlike other connective tissue types, adipose tissues contain large cells and a smaller amount of ECM
• Functions as a packing material, thermo insulator, and in energy storage.
• Functions for protection of organs against injury

Reticular Connective Tissue

• Forms framework of lymphatic tissue in spleen, lymph nodes, bone marrow, and liver
• Provides a superstructure for lymphatic and hematopoietic tissues

Dense Connective Tissue

• Large number of protein fibers that form thick bundles and fill nearly all of the extracellular space
• These protein fibers are produced by fibroblasts
• 2 major sub categories:
• Dense collagenous connective tissue
• Dense elastic connective tissue

Dense collagenous connective tissue

• ECM mostly has collagen fibres
• Structures made up of it include tendons (attaches muscle to bone), ligaments (attaches bone to bone), and dermis (connective tissue of the skin)
• Collagen fibers are oriented in the same direction in tendons and bones
• Collagen fibers oriented in many directions can be found in the dermis

Dense Elastic Connective Tissue

• Has abundant elastic fibers among its collagen fibers
• Elastic fibers allow the tissue to stretch and recoil
• Present in vocal cords with elastic fibers being oriented in the same direction
• Elastic ligaments with elastic fibers oriented in the same direction
• Walls of large arteries where elastic fibers are oriented in different directions

Supporting Connective Tissue

• Cartilage; composed of chrondrocytes/cartilage cells in spaces called lacunae
• Collagen in matrix provides flexibility and strength
• Cartilage is made resilient with proteoglycans that trap water, making it rigid and able to spring back from compression
• Cartilage heals slowly because blood vessels do not penetrate

Types of Cartilage

• Hyaline is the most abundant, covers end of bones, where they join, and has smooth surfaces for repeated compression
• Fibrocartilage has more collagen than hyaline, is able to resist tearing forces, and is found in in disks between vertebrae, and some joints (knee and temporomandibular joint)
• Elastic cartilage has elastic fibers in addition to collagen fibers and proteoglycans; bends over; allows to recoil when bent; found in external ear, auditory tube, and epiglottis

Histology is the study of tissues. Four main types of tissues exist in the human body: epithelial, connective, muscular, and nervous. Epithelial tissue provides the lining and covering of organs and body surfaces.