Tissues

Epithelial Tissue Overview and Core Functions

• Epithelial tissues are divided into two main functional and structural categories:

  • Covering and Lining Epithelium: Covers or lines all free ($f\,r\,e\,e$) internal and external body surfaces.

  • Glandular Epithelium: Forms the secretory tissue of exocrine and endocrine glands.

• Locations of Covering and Lining Epithelium:

  • Epidermis (outermost layer of the skin).

  • Digestive tract lining (from the oral cavity/cheeks, through the esophagus, stomach, and intestines).

  • Cardiovascular cavity lining.

  • Respiratory tract pathway lining.

• Seven Primary Functions of Epithelial Tissue:

  1. Establishing Boundaries: Forms a structural barrier separating internal body organs and tissues from external environments.

  2. Protection: Shields underlying tissues from physical abrasion, chemical damage, and external pathogens.

  3. Absorption: Facilitates the movement of nutrients across tissue layers into the body (e.g., digestive tract lining).

  4. Filtration: Filters substances passing through biological membranes (e.g., blood filtration in the kidneys).

  5. Secretion: Produces and releases metabolic products (e.g., thyroid hormone secretion by cuboidal cells in the thyroid gland).

  6. Excretion: Eliminates metabolic waste products from body cavities and fluids.

  7. Sensory Reception: Integrates specialized sensory receptors within the epithelial layer to detect environmental stimuli.

Structural Characteristics and Membranes of Epithelium

• Free Surface Recognition: In histological specimens, the presence of white space (an open lumen or free surface) adjacent to a tissue indicates covering and lining epithelial tissue.

• Cellular Polarity:

  • Apical Surface: The upper, exposed free surface facing the exterior environment or an internal body cavity/lumen.

  • Basal Surface: The lower, attached bottom surface anchored to underlying structures.

  • Polarity exists in all epithelial structures, whether single-layered or multi-layered.

• Intercellular Junctions: Covering and lining epithelial cells are tightly held together by specialized junctions:

  • Tight Junctions: Impermeable seals that prevent molecules from passing between adjacent cells.

  • Desmosomes: Anchoring junctions that provide mechanical resistance against tension and frictional forces.

• Basement Membrane Architecture: Epithelial tissue is anchored to underlying connective tissue by a basement membrane consisting of two distinct layers:

  1. Basal Lamina: The superficial layer directly adjacent to the basal cells.

  • Composed of a noncellular, adhesive sheet of glycoproteins (proteins conjugated with carbohydrate side-chains).

  • Functions as an anchoring structure and a selective filter regulating molecule movement into and out of the tissue.

  • Visualized histologically as a dark, flat line under specialized staining techniques.

  1. Reticular Lamina: The deeper layer situated beneath the basal lamina.

  • Composed of an extracellular network of collagen fibers providing structural strength and support.

• Avascularity:

  • Epithelial tissues are avascular ("a-" meaning without, "vascular" referring to blood vessels), containing no direct blood supply.

  • Oxygen and nutrients must diffuse indirectly from blood vessels situated in the underlying connective tissue through the basement membrane to nourish epithelial cells.

Classification Schemes for Epithelial Tissue

• Epithelial tissues are classified systematically using two main criteria: the number of cell layers and the shape of the cells.

Classification by Number of Cell Layers

• Simple Epithelium:

  • Composed of a single ($1$) layer of cells attached to the basement membrane.

  • Specialized primarily for absorption, secretion, and filtration.

  • Absent in areas subjected to high mechanical wear, tear, or friction.

• Stratified Epithelium:

  • Composed of two or more ($2$ to $17+$) cell layers stacked on top of one another.

  • Located in areas prone to high abrasion, providing a robust protective barrier (e.g., skin surface, digestive tract linings).

• Pseudostratified Epithelium:

  • Appears to have multiple cell layers due to nuclei positioned at varying heights, but consists of only a single ($1$) layer of cells where every cell contacts the basal lamina.

  • Found in regions requiring limited tissue movement (e.g., ciliated respiratory tract linings, oviduct/fallopian tube linings where eggs pass).

Classification by Cell Shape

• Squamous: Thin, flattened cells.

• Cuboidal: Box-like cells with roughly equal height and width.

• Columnar: Tall, column-like cells that are longer than they are wide.

• Rule for Naming Stratified Epithelium: The tissue is named according to the cell shape located exclusively at the apical surface.

• Transitional Epithelium:

  • A specialized stratified tissue featuring rounded, dome-shaped cells on its apical surface when relaxed.

  • Lines hollow urinary organs (e.g., the urinary bladder).

  • Apical cells flatten out ("transition" shape) as the organ distends with urine.

  • Allows the urinary bladder to store approximately $400\,\text{cc}$ of urine prior to initial wall stretching, expanding to accommodate an additional $400\,\text{cc}$.

Glandular Epithelium and Mechanisms of Secretion

• Definition of a Gland: A single cell or group of specialized epithelial cells that synthesize and secrete substances either into ducts or directly into the bloodstream/surrounding surfaces.

• Energy Requirements: Glandular secretory activity is an active process requiring cellular energy expenditure.

Classification by Secretory Pathway

• Exocrine Glands:

  • Possess ducts through which secretions are transported directly to specific target sites or epithelial surfaces.

  • Examples: Sweat glands, salivary glands, gallbladder, digestive enzyme-producing glands.

  • Goblet Cell: The human body's sole unicellular exocrine gland.

  • Secretes mucus.

  • Acts as a mechanical lubricant in the digestive tract to prevent scraping.

  • Acts as a sticky trap in the respiratory tract to capture inhaled foreign particulates.

• Endocrine Glands:

  • Ductless glands that secrete chemical messengers called hormones directly into the extracellular fluid or bloodstream.

  • Hormone: A signaling molecule produced in one anatomical location that travels via circulation to alter cellular activity in distant target cells.

  • Example: Thyroid gland.

• Dual-Function Glands:

  • Possess both exocrine and endocrine functions.

  • Example: Pancreas.

  • Exocrine portion secretes digestive enzymes into the pancreatic duct destined for the small intestine.

  • Endocrine portion (Islets of Langerhans) secretes hormones (insulin and glucagon) into the blood to regulate systemic glucose homeostasis.

Classification by Functional Mechanism of Secretion

• Merocrine Glands:

  • Secretory products are synthesized, packaged by the Golgi apparatus into vesicles, and discharged via exocytosis into ducts.

  • Secretory cells remain intact and undamaged during the secretory process.

  • Examples: Salivary glands, sweat glands, exocrine portions of the pancreas.

• Holocrine Glands:

  • Secretory products accumulate inside the cell as cellular inclusions.

  • The entire cell detaches, enters the duct, and ruptures (bursts open) to release its stored contents, resulting in cell death.

  • Disintegrated cells must be continuously replaced via mitotic division of stem cells.

  • Example: Sebaceous (oil) glands coating skin and hair.

Connective Tissue Properties and Matrix

• Embryonic Origin:

  • All connective tissues originate from a common embryonic tissue called mesenchyme.

  • Mesenchyme derives from the middle germ layer, the mesoderm, formed during gastrulation when the hollow blastula stage invaginates into three distinct germ layers (endoderm, ectoderm, mesoderm).

• Four Primary Classes of Connective Tissue:

  1. Connective tissue proper (e.g., loose/areolar, adipose, reticular, dense tissues).

  2. Cartilage.

  3. Bone (a solid, hard connective tissue).

  4. Blood (the body's only liquid connective tissue).

• Three Structural Components of All Connective Tissues:

  1. Ground Substance (Extracellular Matrix Background):

  • Unstructured fluid matrix composed of interstitial fluid and proteoglycans.

  • Proteoglycan Structure: Resembles a test tube brush, featuring a central protein core ("wire") with branching glycosaminoglycan carbohydrates ("bristles"). The carbohydrate side-chains of adjacent proteoglycans adhere to one another to bind tissue components.

  • Variation across tissue types: Ground substance is liquid plasma in blood, whereas it becomes mineralized calcium salts in bone.

  1. Extracellular Fibers:

  • Elastin Fibers: Thin, rubber-like fibers that provide elastic stretch and recoil.

  • Collagen Fibers: Thick, robust fibers providing high tensile strength; collagen is the most abundant protein in the human body.

  • Reticular Fibers: Fine, branching fibers forming web-like networks (cobwebs) to support soft tissues and organs.

  1. Specialized Cell Types:

  • Fibroblasts in connective tissue proper (e.g., areolar connective tissue, historically termed loose connective tissue).

  • Chondrocytes in cartilage.

  • Osteocytes in bone.

• Gelatin Metaphor for Connective Tissue: Connective tissue matrix resembles a gelatin mold dish, where the gelatin gel acts as the ground substance/matrix, and inserted fruit or nut pieces represent fibers and specialized cells.

Muscle Tissue Classification and Cytology

• Core Functions: Composed of specialized cells modified for contraction (shortening and thickening). Muscle contraction produces physical movement and generates body heat.

• Specialized Muscle Cytological Terminology:

  • Sarcolemma: The specialized plasma membrane of a muscle cell (analogous to the neurolemma in nerve cells).

  • Sarcoplasm: The specialized cytoplasm of a muscle cell.

• Three Muscle Tissue Types:

  1. Skeletal Muscle:

  • Histology: Characterized by prominent, highly visible cross-striations; long, cylindrical cells running parallel to one another.

  • Nuclei: Multinucleated (contains multiple nuclei per individual cell).

  • Control: Voluntary (under conscious neural control).

  1. Cardiac Muscle:

  • Histology: Striated (less pronounced than skeletal muscle); branching cell structure.

  • Nuclei: Uninucleate (contains one single nucleus per cell).

  • Control: Involuntary (under subconscious neural/autonomic control).

  • Intercalated Discs: Specialized intercellular junctions joining adjacent cardiac cells. Contain gap junctions (connexons) allowing rapid ionic/electrical communication so the heart contracts as a single coordinated syncytium.

  1. Smooth Muscle:

  • Histology: Non-striated/unstriated (lacks cross-striations); spindle-shaped cells with sausage-shaped nuclei.

  • Nuclei: Uninucleate (one single central nucleus per cell).

  • Control: Involuntary (under subconscious control).

  • Locations: Walls of hollow internal organs, digestive tract (propels food via peristalsis), and uterus (contracts during labor).

Nervous Tissue Composition

• Nervous tissue consists of two distinct functional cell populations, divided equally in proportion:

  1. Neurons ($50\%$ of nervous tissue):

  • Specialized conducting cells that detect physical or chemical stimuli.

  • Convert stimulus energy into electrical nerve impulses (action potentials) conducted across the body.

  1. Neuroglia ($50\%$ of nervous tissue):

  • Derives from the Greek word meaning "nerve glue".

  • Non-conducting supportive cells that surround, protect, nourish, and structurally insulate neurons.