BIOL 2401 Chapter 5: The Tissue Level of Organization
Overview of Tissues and Histology
Definition of Tissue: A collection of similar cells working together to perform a common function.
Definition of Histology: The microscopic study of tissues.
Human Cellular Diversity: The human body contains approximately distinct cell types that combine to form four primary tissue classes.
Four Primary Tissue Types
Epithelial Tissue: Covers external body surfaces, lines hollow organs, cavities, and ducts, and forms glandular structures.
Connective Tissue: Supports and protects the body and its internal organs.
Muscle Tissue: Specialized for active contraction and force generation; categorized into three types: skeletal muscle, cardiac muscle, and smooth muscle tissue.
Nervous Tissue: Detects internal and external environmental changes and responds by generating electrical signals called action potentials (nerve impulses) that stimulate muscle contractions and glandular secretions.
Key Tissue Distinctions: The four primary tissue types are differentiated by:
The specific types and functions of their constituent cells.
The characteristics and composition of the matrix (extracellular substance) surrounding the cells.
The relative amount of space occupied by cells versus the extracellular matrix.
Epithelial Tissue Functions and General Characteristics
Epithelial Tissue Organization: Cells are arranged in continuous sheets as single or multiple layers.
Epithelia: Cell layers that cover external surfaces or line internal cavities and passageways (e.g., skin, and lines of the digestive, respiratory, reproductive, and urinary tracts).
Glandular Epithelium: Structures (glands) specialized for fluid secretion.
Four Essential Functions of Epithelial Tissue:
Physical Protection: Shields underlying tissues from physical abrasion, infection, and dehydration.
Permeability Control: Regulates absorption, filtration, and secretion of nutrients and metabolic wastes.
Sensation Provision: Extensively innervated with rich nerve supplies that act as touch and temperature receptors.
Specialized Secretion: Produces specialized secretions such as saliva, mucus, or sweat.
Five Fundamental Characteristics of Epithelial Tissue:
Cellularity: Composed almost entirely of tightly packed cells with minimal extracellular matrix.
Polarity: Demonstrates structural and functional differences between exposed (apical) and attached (basal) cell surfaces.
Intercellular Connections: Firmly attach one epithelial cell to another, maintaining strong connections between adjacent epithelial cells and the underlying basement membrane.
Avascularity: Completely lacks blood vessels; cells must obtain nutrients via diffusion or absorption from nearby underlying tissues.
Regeneration: Cells located on surfaces are easily damaged and undergo continuous replacement through continual division of stem cells located near the basement membrane.
Surface Polarity and Intercellular Junctions
Structural Polarity:
Apical Surface: Exposed surface facing the external environment or an internal space (lumen).
Microvilli: Plasma membrane extensions that increase the ability of the surface to absorb nutrients.
Cilia: Motile extensions that provide movement of materials along the cell surface.
Basal Surface: The base of the basolateral surface that always attaches to underlying tissues via the basement membrane.
Intercellular Connections and Junctions:
Tight Junctions: Formed by the fusion of plasma membranes of adjacent cells, creating an extremely tight barrier between the outside environment (or lumen) and underlying tissues. They prevent water and solutes from passing between cells.
Gap Junctions: Form small protein channels between adjacent cells that allow ions and other small solutes to freely move from one cell to another.
Desmosomes: Button-like connections constructed from cell adhesion molecules (CAMs) that connect adjacent cells to one another; very strong and capable of resisting stretching and twisting forces.
Hemidesmosomes: Connect epithelial cells to an underlying basement membrane rather than to an adjacent cell.
Classification Scheme for Epithelial Tissues
Epithelial tissue classification is based upon two distinct criteria:
Shape of Cells:
Squamous: Thin and flat.
Cuboidal: Square with equal width and length.
Columnar: Rectangular; taller than they are wide.
Layers of Cells:
Simple Epithelium: Consists of only one layer of cells covering the basement membrane. Thin and fragile; provides little mechanical protection. Ideal for absorption or secretion as it reduces the time required for material exchange. Lines internal compartments and passageways (e.g., intestine, some body cavities, blood vessels).
Stratified Epithelium: Consists of several layers of cells covering the basement membrane. Ideal for providing protection from mechanical or chemical assaults (e.g., skin and mouth).
Detailed Categorization of Specific Epithelia
Squamous Epithelia:
Simple Squamous Epithelium:
Function: Thinnest possible barrier to allow for rapid diffusion and filtration; also performs secretion.
Location: Alveoli of lungs, glomeruli of kidneys, lining of heart and blood vessels, inner lining of serous membranes of stomach and intestines, and mesothelium of pleurae, pericardium, peritoneum, and mesenteries.
Stratified Squamous Epithelium:
Function: Protection against physical abrasion and chemical attacks.
Keratinized Stratified Squamous Epithelium: Surface layers consist of compact, dead squamous cells that lack nuclei and contain keratin. Functions in protection of underlying tissue from abrasion and pathogen invasion, and prevents water loss. Located in the epidermis of skin.
Non-Keratinized Stratified Squamous Epithelium: Compact, live squamous cells that lack keratin and retain visible nuclei. Functions in protection of underlying tissue from abrasion and pathogen invasion. Located in the lining of the oral cavity, part of pharynx, esophagus, vagina, and anus. Will dry out and deteriorate unless kept moist.
Cuboidal Epithelia:
Simple Cuboidal Epithelium:
Features: Simple layer of uniformly shaped cells with a centrally located spherical nucleus.
Function: Absorption and secretion; production of a protective mucous layer.
Location: Kidney, liver, thyroid, mammary, salivary, and other glands.
Stratified Cuboidal Epithelium:
Features: Two or more layers of cells; superficial cells are cuboidal in shape.
Function: Protection and secretion.
Location: Along the ducts of sweat glands, larger ducts of mammary glands, male urethra, and ovarian follicles.
Columnar Epithelia:
Simple Columnar Epithelium:
Features: Single layer of cells taller than wider; oval-shaped nucleus located in the lower of the cell.
Non-Ciliated Simple Columnar Epithelium: Contains microvilli that form a brush border; interspersed with Goblet cells that secrete mucin (which forms mucus when mixed with water). Function: Absorption and secretion. Location: Inner lining of stomach and intestines.
Ciliated Simple Columnar Epithelium: Cilia project from apical surface to move mucus along. Function: Secretion and movement of mucus along apical surface; movement of oocytes. Location: Bronchioles of the lungs, uterine tubes.
Stratified Columnar Epithelium:
Features: Two or more layers of cells; cells at the apical surface are taller than they are wide.
Function: Protection and secretion.
Location: Parts of the pharynx, male urethra, anal canal, ducts of larger glands, and conjunctiva.
Two Unique Types of Epithelial Tissue:
Transitional Epithelium:
Features: Resembles stratified squamous epithelium, but surface cells are rounded; usually or layers thick.
Function: Stretches to allow filling of the urinary tract.
Location: Urinary bladder, renal pelvis, ureters.
Ciliated Pseudostratified Columnar Epithelium:
Features: Single layer of cells with varying heights; all cells connect to the basement membrane, but not all cells reach the apical surface; contains goblet cells and cilia.
Function: Protection, secretion of mucin, and movement of mucus along apical surface of epithelium by cilia.
Location: Respiratory tract from nasal cavity to bronchi, portions of male urethra.
Glandular Epithelia and Secretion Mechanisms
Definition of Glands: Collections of epithelial cells that produce secretions.
Two Main Types of Glands:
Exocrine Glands: Use ducts to secrete substances onto a surface (e.g., sweat, mammary, and tear glands, and enzymes entering the digestive tract).
Endocrine Glands: Have no ducts (ductless glands); release secretions directly into the blood or surrounding tissues to produce hormones (e.g., thyroid gland, thymus, and pituitary gland).
Classification of Exocrine Glands by Mode of Secretion:
Merocrine Secretion: Merocrine glands package their contents into secretory vesicles and then release those contents by exocytosis. Most common method of exocrine secretion (e.g., merocrine sweat glands, salivary glands).
Apocrine Secretion: Apical portion of apocrine glands actually pinches off and disintegrates, releasing cytoplasm and secretory contents (e.g., mammary glands).
Holocrine Secretion: Cell becomes packed with secretory vesicles and then bursts, releasing the secretion, but killing the cell. Must constantly produce new secretory cells.
Connective Tissue Structural Components and Properties
Overview: The most abundant and widely distributed of the four primary tissues. It represents a diverse group of supporting tissues that connects epithelium to the rest of the body. Composes ligaments, tendons, fat, bone, and blood.
Three Essential Components:
Specialized cells.
Protein fibers.
Ground substance.
Note: The specific types of cells, fibers, and makeup of ground substance may differ, but these three components are usually present.
Ground Substance Properties:
Substance that fills the space between the cells and fibers of connective tissue.
Acts as a protective insulation for the cells, trapping bacteria and slowing the spread of pathogens.
Extracellular Matrix (ECM):
The combination of protein fibers and ground substance that surrounds the cells.
Typically accounts for most of the volume of connective tissue.
Vascularity and Innervation:
Unlike epithelial tissue, most connective tissues are highly vascular (contain many blood vessels), with the exception of cartilage.
Connective tissue contains sensory receptors that detect pain, pressure, temperature, and other stimuli.
Functions and Classification of Connective Tissue
Six Specific Functions of Connective Tissue:
Support and Structural Framework: Bones and cartilage support the body and its parts.
Transport: Blood transports gases, nutrients, wastes, hormones, and blood cells.
Physical Protection: Skull, ribs, sternum, and fatty cushions protect organs.
Binding of Organs: Tendons, ligaments, fat, and fibrous tissue bind structures together.
Storage: Fat acts as an energy reserve; bones store calcium and phosphorus.
Immune Protection: Provides a battlefield for immune cells to attack pathogens.
Classification of Connective Tissue by Physical Properties:
Connective Tissue Proper: Connects and protects other tissues and organs, fills spaces, and provides structural framework. Includes:
Loose Connective Tissue: Loose arrangement of cells and fibers with abundant ground substance.
Dense Connective Tissue: Fibers are densely packed together, with fewer cells and less ground substance.
Fluid Connective Tissue: Functions in transport of substances, immune defense, and homeostasis.
Supporting Connective Tissue: Protects other tissues and organs, providing significant strength and rigidity.