Tissues and the Tissue Level of Organization

Introduction to Tissues and Histology

• Definition of Tissues: Tissues are collections of specialized cells and their associated cell products that work together to perform a limited number of specific functions.

• Histology: This is the formal scientific study of tissues.

• Classification of Tissues: Tissues are classified based on two primary criteria:

  • Cellular Appearance and Shape.

  • Function.

• Primary Tissue Types: There are $4$ basic types of tissue in the human body:

  • Epithelial Tissue: Covers body surfaces, lines cavities, tubular structures, passageways, and ducts, and forms glands.

  • Connective Tissue: Provides physical protection, energy storage, and structural support.

Pluripotent: Ectoderm, mesoderm, and endoderm, Capable of making almost any cell type

Multipotent: Hematopoietic stem cells, Able to make several different cell types

Totipotent: Zygote, capable of making any human cell type

Developmental Processes and Germ Layers

• Differentiation: The process by which unspecialized cells, such as stem cells, develop into specialized cells with distinct structures and functions. This process is guided by specific genes turning on and off in response to various signals. Differentiation allows cells to perform specialized roles within the body.

• Transcription factors: When proteins in the cell nucleus switch genes on and off to change the structure and function of the daughter cell.

Epithelial Tissue Characteristics and Functions

• General Characteristics:

  • Cells are bound closely together.

  • Possesses an apical (free) surface exposed to the environment or an internal space.

  • Attached to underlying connective tissue via a basement membrane.

  • Avascular: Lacks its own blood vessels.

  • Regeneration: Features continual replacement or rapid regeneration of cells.

• Primary Functions:

  • Physical Protection: Provides a first line of defense against physical, chemical, and biological wear and tear.

  • Permeability Control: Acts as a gatekeeper to regulate what enters or leaves the body.

  • Sensation: Provides sensory information about the environment.

  • Specialized Secretions: Produced by glandular epithelium.

Classification and Types of Epithelium

• Classification Criteria:

  • Number of Cell Layers: Simple ($1$ single layer) or Stratified (multiple layers).

  • Cell Shape: Squamous (thin and flat), Cuboidal (square/cube-shaped), or Columnar (rectangular/tall).

Simple Epithelia

• Simple Squamous Epithelium:

  • Appearance: Side view is thin and flat; top view looks like "fried eggs laid side by side."

  • Purpose: Diffusion and filtration.

  • Locations: Air sacs of the lungs (alveoli), capillaries, lining of the heart, blood vessels, lymphatic vessels, and serous membranes.

• Simple Cuboidal Epithelium:

  • Appearance: Side view is square with a large, round nucleus in the center; top view looks like hexagonal boxes.

  • Purpose: Secretion and absorption.

  • Locations: Thyroid glands, sweat glands, salivary glands, and kidney tubules.

• Simple Columnar Epithelium:

  • Appearance: Side view is rectangular with nuclei located near the base; top view looks like hexagonal boxes.

  • Purpose: Secretion and absorption; may contain microvilli to increase surface area for absorption.

  • Locations: Stomach, intestines, and gallbladder. Ciliated versions are found in the bronchi, uterine tubes, and uterus.

Stratified and Pseudostratified Epithelia

• Stratified Squamous Epithelium:

  • Purpose: Protection against abrasion.

  • Keratinized: Surface cells are dead and filled with the protein keratin, making the tissue hard, waterproof, and immune to infection. Found in the epidermis (skin).

  • Non-keratinized: Surface cells are living and remain moist. Found in the mouth, esophagus, and vagina.

• Stratified Cuboidal Epithelium:

  • Purpose: Protective; secretes and protects.

  • Locations: Relatively rare; found in the ducts of sweat glands, mammary glands, and salivary glands.

• Stratified Columnar Epithelium:

  • Purpose: Secretion and protection.

  • Locations: Relatively rare; found in parts of the pharynx, epiglottis, anus, and the male urethra. Note that only superficial cells are truly columnar.

• Pseudostratified Columnar Epithelium:

  • Appearance: "False stratified" appearance because nuclei are at multiple levels, but it is only $1$ cell layer thick; all cells contact the basement membrane.

  • Features: Often contains goblet cells which secrete mucus and cilia to move mucus or reproductive cells.

  • Locations: Trachea and much of the upper respiratory tract.

• Transitional Epithelium:

  • Appearance: Changes shape with stretching. In an empty bladder, outermost cells appear cuboidal; in a full bladder, they appear flattened.

  • Purpose: Allows urinary organs to expand and stretch.

  • Locations: Urinary bladder, ureters, and part of the urethra.

Intercellular Junctions in Epithelia

• Tight Junctions: There is no extracellular space between cells. They form a zipper-like closure and an impermeable barrier. Example: Lining of the urinary bladder to prevent urine leakage.

• Gap Junctions: Form an intercellular passageway (via connexons composed of connexins) between the membranes of adjacent cells. This allows for the sharing of electrical currents or small molecules. Example: Between heart muscle cells.

• Anchoring Junctions: Provide strong and flexible connections. Commonly found in tissues prone to mechanical stress, like the skin.

  • Desmosomes: Connect adjacent cells via transmembrane glycoproteins (cadherin) and intermediate filaments (keratin).

  • Hemidesmosomes: Use integrins to attach cells to the basal lamina (basement membrane).

  • Adherens: Use cadherin and actin filaments for connection.

Glandular Epithelium and Secretion

• Gland Classification:

  • Endocrine Glands: "Ductless" glands that release secretions (hormones) directly into surrounding tissues and fluids.

  • Exocrine Glands: Secretions leave through a duct that opens to the external environment (directly or indirectly).

• Modes of Exocrine Secretion:

  • Merocrine Secretion: The most common mode. Product is released from secretory vesicles by exocytosis. Example: Salivary glands. Mucin is a product that mixes with water to produce mucus.

  • Apocrine Secretion: Secretory vesicles are packed into the outer portion of the cytoplasm. Release involves shedding a portion of the cytoplasm along with the vesicles. The cell then repairs itself. Example: Mammary glands (secretion of lipid droplets) and certain sweat glands.

  • Holocrine Secretion: The entire cell becomes packed with vesicles and then bursts, releasing the secretion and killing the cell. Gland cells are replaced by stem cells. Example: Sebaceous glands (oil glands of the skin).

• Types of Secretions:

  • Serous Glands: Produce watery secretions containing enzymes (e.g., parotid salivary gland).

  • Mucous Glands: Produce mucins that form mucus (e.g., sublingual salivary gland).

  • Mixed Glands: Produce both serous and mucous secretions (e.g., submandibular salivary gland).

Tissue Membranes

• Definition: Physical barriers that line or cover portions of the body, consisting of an epithelium supported by connective tissue.

Connective Tissue Membranes

• Synovial Membranes: Line freely moving, articulating joint cavities. They consist primarily of areolar tissue, protect bone ends, and produce viscous synovial fluid for lubrication.

Epithelial Membranes

• Mucous Membranes (Mucosae): Line passageways that open to the exterior (digestive, respiratory, urinary, reproductive tracts). Surfaces are kept moist by mucous secretions. The connective tissue layer is called the lamina propria.

• Serous Membranes: Line cavities that do not open to the outside. Consist of simple epithelium supported by areolar tissue. They have two portions:

  • Parietal Layer: Lines the inner surface of the cavity (metaphorically like a "jacket").

  • Visceral Layer (Serosa): Covers the outer surface of organs (metaphorically like a "shirt" under the jacket).

  • Serous Fluid: Watery fluid that reduces friction between these layers.

  • Specific Serous Membranes: 3 types

    • Pleura: Lines pleural cavities and covers lungs (Visceral pleura on lungs; Parietal pleura on the outer layer).

    • Peritoneum: Lines the peritoneal cavity and covers abdominal organs.

    • Pericardium: Lines the pericardial cavity and covers the heart (Visceral pericardium on the heart; Parietal pericardium on the outer lining).

• Cutaneous Membrane (Skin): Covers the body surface. Consists of stratified squamous epithelium and underlying connective tissues (areolar and dense irregular connective tissue). It is thick, relatively waterproof, and usually dry. It is a stratified squamous epithelial membrane resting on top of connective tissue.