BIO111 Lecture 7: Tissue Organization and Classification

Structural Organization of the Human Body

Levels of Organization

The human body is organized hierarchically, from the smallest chemical components to the entire organism. Each level builds upon the previous, contributing to the structure and function of the body.

• Chemical level: Atoms, molecules, and organelles form the foundation of cellular structure.

• Cellular level: Single cells are the basic units of life.

• Tissue level: Groups of similar cells perform common functions.

• Organ level: Organs consist of two or more types of tissues working together.

• Organ system level: Organs that work closely together form organ systems.

• Organismal level: All organ systems combine to create the whole organism.

Chapter 4: Tissue - The Living Fabric

Definition and Importance of Tissues

Tissues are groups of cells similar in structure that perform common or related functions. The study of tissues is called histology. Each tissue type is specialized to maintain homeostasis and support the body's functions.

Four Basic Tissue Types

• Epithelial tissue: Covers surfaces and lines cavities.

• Connective tissue: Supports, protects, and binds other tissues.

• Muscle tissue: Contracts to produce movement.

• Nervous tissue: Enables internal communication.

Microscopy of Human Tissue

Preparation for Microscopy

To study tissues under a microscope, samples must be:

• Fixed: Preserved with a solvent.

• Sectioned: Cut into thin slices for light or electron transmission.

• Stained: Enhanced contrast using dyes or heavy metals. Light microscopy uses colored dyes, while electron microscopy uses metal coatings.

Epithelial Tissue

Overview and Functions

Epithelial tissue consists of sheets of cells covering body surfaces or lining cavities. It arises from all three embryonic germ layers and has two main forms:

• Covering and lining epithelia: Found on external and internal surfaces (e.g., skin).

• Glandular epithelia: Secretory tissue in glands (e.g., salivary glands).

Main functions include protection, absorption, filtration, excretion, secretion, and sensory reception.

Distinguishing Characteristics of Epithelial Tissue

• Polarity: Cells have apical (top) and basal (bottom) surfaces, each with distinct structure and function.

• Specialized contacts: Cells fit closely together, forming continuous sheets with tight junctions and desmosomes.

• Supported by connective tissues: The basement membrane (basal and reticular lamina) reinforces the sheet.

• Avascular but innervated: No blood vessels, but supplied by nerves; nutrients diffuse from underlying tissues.

• Regeneration: High capacity for renewal, especially in areas exposed to friction or hostile environments.

Classification of Epithelia

By Number of Cell Layers

All epithelial tissues are named based on the number of cell layers:

• Simple epithelia: Single layer, specialized for absorption, secretion, or filtration.

• Stratified epithelia: Two or more layers, specialized for protection.

By Cell Shape

• Squamous: Flattened and scale-like.

• Cuboidal: Box-like, cube-shaped.

• Columnar: Tall, column-like.

Types of Simple Epithelia

• Simple squamous epithelium: Thin, allows rapid diffusion; found in kidneys, lungs, blood vessels (endothelium), and serous membranes (mesothelium).

• Simple cuboidal epithelium: Single layer, involved in secretion and absorption; forms walls of ducts and kidney tubules.

• Simple columnar epithelium: Tall cells, some with microvilli or cilia; contains goblet cells; found in digestive tract, gallbladder, bronchi, and uterine tubes.

• Pseudostratified columnar epithelium: Appears multi-layered but is single-layered; often ciliated; found in upper respiratory tract and ducts of large glands.

Types of Stratified Epithelia

• Stratified squamous epithelium: Most widespread; protects against abrasion; keratinized in skin, nonkeratinized in moist linings.

• Stratified cuboidal epithelium: Rare; found in sweat and mammary glands.

• Stratified columnar epithelium: Limited distribution; found in pharynx, male urethra, and glandular ducts.

• Transitional epithelium: Lines urinary organs; cells change shape to allow stretching.

Glandular Epithelia

Classification of Glands

Glands are classified by site of product release and number of cells:

• Endocrine glands: Internally secreting (hormones).

• Exocrine glands: Externally secreting (sweat, oil).

• Unicellular: Single cells (e.g., goblet cells).

• Multicellular: Multiple cells (e.g., salivary glands).

Exocrine Glands

• Unicellular exocrine glands: Mucous and goblet cells; produce mucin, forming mucus for protection and lubrication.

• Multicellular exocrine glands: Composed of a duct and secretory unit; surrounded by connective tissue, forming capsules and lobes.

Modes of Secretion

• Merocrine: Secrete products by exocytosis (e.g., sweat, pancreas).

• Holocrine: Accumulate products and rupture, releasing cell fragments (e.g., sebaceous glands).

Connective Tissue

Overview and Functions

Connective tissue is the most abundant and widely distributed tissue, arising from the mesoderm. Its functions include binding, support, protection, insulation, storage, and transport (blood).

• Common embryonic origin: All arise from mesenchyme.

• Varying vascularity: Cartilage is avascular; bone is highly vascularized.

• Extracellular matrix (ECM): Cells are suspended in a protein-sugar mesh, enabling weight-bearing and tension resistance.

Structural Components of Connective Tissue

• Ground substance: Gel-like material filling space between cells; contains interstitial fluid, cell adhesion proteins, proteoglycans, and water.

• Fibers: Collagen (strongest), elastic (stretch and recoil), and reticular (branched, flexible).

• Cells: "Blast" cells (immature, secrete ECM), "Cyte" cells (mature, maintain matrix), fat cells, white blood cells, mast cells, and macrophages.

Connective Tissue Cell Types

• Fibroblasts: Found in connective tissue proper.

• Chondroblasts: Found in cartilage.

• Osteoblasts: Found in bone.

• Hematopoietic stem cells: Found in bone marrow.

• Fat cells: Store nutrients.

• White blood cells: Neutrophils, eosinophils, lymphocytes; respond to injury.

• Mast cells: Initiate local inflammatory response.

• Macrophages: Phagocytic cells that remove dead cells and microorganisms.

Additional info: Connective tissue's extracellular matrix is crucial for its mechanical properties, allowing tissues to withstand physical stress and maintain structural integrity.