Tejido epitelial

Tejido Epitelial: Conceptos Fundamentales

Introducción al Tejido Epitelial

• El soldado Pérez presenta el tema del tejido epitelial, parte de la cátedra de teología y enología en la Universidad Nacional de Rosario.

• Se define el tejido epitelial como una asociación de células unidas con escasa o nula sustancia intercelular, sin vasos ni nervios.

Estructura y Función del Tejido Epitelial

• El tejido epitelial se asocia a un tejido conectivo laxo que proporciona soporte, nutrición e innovación.

• Entre el epitelio y el tejido conectivo se establece una membrana basal compuesta por lámina basal (glicoproteínas) y lámina reticular (fibras de retícula).

Nutrición y Variedades del Tejido Epitelial

• Los tejidos epiteliales se nutren por difusión desde los vasos del tejido conectivo a través de la membrana basal.

• Se identifican diferentes tipos de epitelio: revestimiento (cubre superficies), glandular (produce secreciones), germinativo (divisiones mitóticas), sensoriales (órganos de los sentidos).

Clasificación del Tejido Epitelial

• El epitelio de revestimiento protege superficies internas y externas, además puede realizar funciones como transporte transcelular y absorción.

• La clasificación se basa en la cantidad de capas: monoestratificado (una capa) o estratificado (varias capas).

Morfología del Tejido Epitelial

• Las formas celulares determinan su clasificación: plano simple, cúbico, cilíndrico o pseudoestratificado.

• El epitelio estratificado puede ser plano con queratina para resistencia; si no tiene queratina es más susceptible al desgaste.

Características Especiales del Epitelio Transicional

• El epitelio estratificado puede ser también transicional, caracterizado por células que aumentan en tamaño hacia la superficie apical.

Histological Classification of Epithelial Tissues

Overview of Epithelial Types

• The discussion begins with the classification of epithelial tissues based on cell arrangement and shape, highlighting simple versus stratified epithelium.

• Simple squamous epithelium is characterized by a single layer of flattened cells, observed in arterial tissue sections under microscopy.

• Simple cuboidal epithelium features cube-shaped cells that are similar in height and width, typically lining ducts and glands.

• The structure of simple columnar epithelium is described, noting its taller cells which may appear cylindrical; this type often lines organs involved in secretion.

• Pseudostratified columnar epithelium presents multiple nuclei at varying heights, giving an illusion of stratification while still being a single layer.

Characteristics and Functions

• Columnar epithelial cells can vary in height and width; their arrangement plays a role in absorption and secretion functions within various organs.

• Observations suggest specialized structures at the apical surface of certain epithelial types, potentially indicating adaptations for specific functions.

• Pseudostratified epithelia are noted for their unique appearance due to nuclei positioned at different levels; they serve important roles in respiratory pathways.

• Stratified epithelia consist of multiple layers providing protection; most commonly found as stratified squamous epithelium with variations depending on location (e.g., keratinized vs. non-keratinized).

Special Cases and Variations

• Transitional epithelium is discussed as an exception to typical classifications, adapting between different states (e.g., bladder expansion).

• The presence of keratinization is highlighted as a protective feature in some stratified squamous epithelia, particularly those exposed to friction or abrasion.

• Images illustrate differences between keratinized and non-keratinized stratified squamous epithelia based on cellular characteristics at the surface level.

• The importance of connective tissue papillae is emphasized for nutrient supply to superficial layers through vascularization.

Epithelium Types and Characteristics

Stratified Squamous Epithelium with Papillae

• Superficial epithelial layers have fewer nutrients; however, the presence of papillae allows for better nutrient absorption due to shorter distances between capillaries and basal membranes.

• The stratified squamous epithelium is characterized by flattened cells that are less likely to change shape due to their horizontal arrangement.

Transitional Epithelium

• Transitional epithelium features rounded cells that can stretch horizontally without breaking, adapting to volume changes in the urinary system.

• The name "transitional" reflects the ability of these cells to change shape in response to organ volume fluctuations.

Cellular Structure and Junctions

• Epithelial cells are tightly connected through various junctional complexes, which include zonula occludens (tight junctions), zonula adherens, and desmosomes.

• Gap junctions facilitate communication between adjacent cells but do not form part of the primary adhesion complex.

Cellular Adhesion Mechanisms

Types of Cell Junctions

• Tight junctions prevent interstitial fluid movement, while other junction types allow for some permeability for interstitial fluid circulation.

• Adherens junctions provide strong connections between cell membranes using proteins linked to the cytoskeleton.

Structural Details

• Desmosomes anchor intermediate filaments within cells, providing structural integrity against mechanical stress.

Specializations of Epithelial Cells

Apical Surface Modifications

• The apical surface may feature two types of modifications: cilia for movement and microvilli for increased absorption capacity.

Microvilli Characteristics

• Microvilli increase surface area for absorption; they can vary in height and structure (e.g., brush border vs. striated border).

Cilia Functionality

Understanding Cellular Structures and Functions

The Role of Cilia and Microtubules

• Cilia function like a whip, generating coordinated movements that create a peristaltic force to mobilize elements on the surface.

• Triplets of microtubules form the core structure of cilia, with nine pairs surrounding a central pair, all interconnected with proteins, anchoring them to the cytoskeleton.

• A three-dimensional electron microscopy image illustrates how these ciliary movements resemble peristaltic waves, creating rhythmic motions for effective transport.

Differences in Epithelial Structures

• Microvilli are distinct from cilia; they consist of actin microfilaments rather than microtubules and primarily serve to increase absorption surface area rather than propel substances.

• The arrangement of actin filaments within microvilli allows for movement but is not designed for propulsion; instead, it enhances absorption efficiency.

Types of Epithelial Tissue

Glandular Epithelium Classification

• Glandular epithelium synthesizes and secretes substances; it can be classified as exocrine (secreting onto surfaces or ducts) or endocrine (releasing into interstitial fluid or blood).

• Exocrine glands release their products through ducts to external surfaces while endocrine glands secrete hormones directly into the bloodstream.

Structural Variations in Glands

• Glands can be unicellular (single-celled glands embedded in epithelial layers) or multicellular (composed of multiple cells), affecting their secretory functions.

• Multicellular exocrine glands can have various structures: tubular (elongated), acinar (rounded), or alveolar (dilated).

Secretion Mechanisms in Glands

Types Based on Secretion Methodology

• Tubular glands have uniform diameters along their length, while acinar structures feature rounded shapes with varying lumen sizes compared to ducts.

• Apocrine secretion involves loss of part of the cell during secretion, where vesicles at the apical surface release stored substances by shedding part of their cytoplasm.

Examples and Functional Implications

Cellular Structures and Functions in Glandular Tissue

Overview of Glandular Cell Function

• The apical border of glandular cells is crucial for secretion; for instance, sebaceous glands have cells that store secretions until they burst, losing the cell in the process.

• A mother cell theoretically undergoes cellular renewal, synthesizing and storing secretions before being replaced by dividing cells.

Types of Epithelial Structures

• Simple cylindrical epithelium can form elongated tubular glands; variations in cutting angles reveal different structural forms.

• Certain structures appear pale or pink due to a lack of specific staining properties, indicating their composition rich in glycosaminoglycans.

Alveolar Structures and Their Functions

• Alveoli are rounded structures with a small central lumen, which differ from other gland types based on their diameter.