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Introduction to Epithelial Tissue

Overview of the Tegmentary System and Epithelium

• The discussion begins with an introduction to the tegmentary system, skin, and its important appendages, emphasizing a focus on epithelium and histology.

• Key characteristics of epithelial tissue include tightly packed cells with minimal extracellular matrix and a basal membrane that serves as a barrier between external and internal environments.

Cellular Junctions and Glandular Structures

• Understanding cellular junctions is crucial for maintaining the integrity of epithelial barriers; glands are also derived from epithelial layers.

• Exocrine glands secrete substances externally, while endocrine glands (not covered in this session) originate from similar embryonic layers.

Classification of Epithelial Tissue

Types Based on Cell Shape

• Epithelia are classified by cell shape:

• Simple Squamous: Found in alveoli for gas exchange, Bowman's capsule, blood vessel endothelium, etc.

• Simple Cuboidal: Present in small gland ducts and kidney tubules.

Simple Columnar Epithelium

• Simple columnar epithelium has taller cells than wide ones; it is found in areas requiring secretion like the gastrointestinal tract and uterus.

Stratified Epithelia

Pseudostratified Columnar Epithelium

• Pseudostratified columnar epithelium appears layered due to varying nucleus levels but consists of a single layer; commonly found in the respiratory tract including trachea and bronchi.

Stratified Squamous Epithelium

• Stratified squamous epithelium can be keratinized (skin) or non-keratinized (esophagus, vagina); keratin provides protection against abrasion.

Rare Types of Stratified Epithelium

Stratified Cuboidal and Columnar Epithelium

• Stratified cuboidal epithelium is rare but can be seen in large gland ducts; stratified columnar is even rarer, typically found in parts of the male urethra under pressure conditions.

Transitional Epithelium (Urothelium)

• This type adapts to stretching; significant for urinary bladder function.

Urotelio y su Función Histológica

Estructura del Urotelio

• El urotelio es un epitelio que cambia de forma según la presión; se aplana bajo alta presión y se vuelve globoso con baja presión. Se encuentra desde los cálices menores hasta la vejiga.

• Las estructuras como el cáliz menor, cáliz mayor, ureter y vejiga están revestidas por este epitelio de transición, que se distiende y aplana.

Perspectiva Histológica

• Las células epiteliales actúan como una barrera, con un polo apical en la parte superior y un lado vasolateral en los costados. Esta división es crucial para sus funciones.

• La barra terminal es un conjunto de proteínas que une fuertemente las partes apical y vasolateral del epitelio, permitiendo diferentes funciones en cada área.

Uniones Celulares

• La barra apical incluye uniones estrechas (uniones ocluyentes), formadas por proteínas como ocludinas y claudinas, que crean una barrera hermética entre las células.

• Estas uniones ocluyentes son comparables a coser dos sacos de plástico: cuanto más fuerte sea la unión, más hermética será la barrera.

Transporte Celular

• La capacidad de transporte celular depende de cuán estrechas son estas uniones; algunas permiten el paso de electrolitos mientras que otras no.

• Existen también uniones adherentes laterales (célula-célula) y basales (célula-matriz), donde las primeras funcionan como remaches para mantener fuertes las conexiones entre células.

Estructuras Adicionales

• Los tonofilamentos transmiten tensión mecánica entre células a través del complejo cadherina-e-catenina, formando así la zónula adherens.

• Los desmosomas ayudan a reducir el cisallamiento al unir dos células con menos tensión mecánica mediante proteínas específicas como desmogleína.

Unión Basal

• La unión basal implica interacciones con la membrana basal a través de adhesión focal e hemidesmosomas, utilizando integrinas en lugar de cadherinas para conectar con la matriz extracelular.

Cell Adhesion and Junctions

Types of Cell Adhesions

• Hemidesmosomes form when integrins connect to collagen types 4 and 7 in the basal membrane, facilitating adhesion between cells and the extracellular matrix.

• Gap junctions, formed by six connexins that create a connexon, allow for direct communication between cells by transferring electrical signals and ions.

Importance of Cell Communication

• Connexins are crucial in cardiac muscle (myocardium) as they enable intercellular communication essential for heart function.

• Cadherins require calcium to facilitate cell-to-cell adhesion, while occludins do not have this requirement.

Homophilic vs. Heterophilic Interactions

• Homophilic interactions occur when identical proteins (e.g., cadherins or immunoglobulins) on adjacent cells bind together.

• Heterophilic interactions involve different types of molecules binding across cell membranes, such as selectins interacting with CAM (cell adhesion molecules).

Desmosomes and Integrin Functions

Structure of Desmosomes

• Desmosomes consist of tonofilaments connected through desmoglein; these structures provide mechanical strength to tissues.

• The distinction between desmosomes and hemidesmosomes is highlighted: hemidesmosomes anchor cells to the basal membrane via integrins.

Overview of Glandular Structures

• Glands are categorized into exocrine (secreting outside the body) and endocrine (releasing hormones into the bloodstream).

Classification of Exocrine Glands

• Exocrine glands can be classified based on duct structure: simple (single duct), compound (multiple ducts), or tubular/acinous based on secretory portions.

Secretion Types

• Secretions can be mucinous, serous, or mixed; glandular secretion methods include:

• Merocrine: Secretion without loss of cellular material.

• Apocrine: Partial loss of cell during secretion.

• Holocrine: Entire cell disintegrates to release its contents.

Specific Examples

• A sweat gland is an example of a simple tubular gland with serous secretion using merocrine method.

Histological Classification of Glands

Tubular Glands

• The simplest gland type is tubular, characterized by a long structure with a single duct. An example includes sweat glands.

• Most sweat glands are coiled tubular glands; however, some are simple tubular found in the gastrointestinal tract, particularly in the small intestine.

Branched Glands

• Gastric secretory glands are classified as branched due to their multiple secretory branches and can be found in the gastric epithelium.

Acinar Glands

• Acinar glands appear bag-like and have a single duct. They can also be located within the gastrointestinal tract and male reproductive system (e.g., prostate).

• Sebaceous glands exemplify branched acinar types, featuring multiple secretion units that converge into one duct leading to hair follicles.

Compound Gland Types

Compound Classification

• Compound glands possess multiple main ducts and can be categorized into tubular, acinar, or tubuloacinar types.

Examples of Compound Glands

• The pancreas is an example of a large compound gland with both acini and tubules.

• Mammary glands represent compound acinar types while prostate secretions are primarily from compound tubular glands.

Glandular Secretion Mechanisms

Types of Secretion

• Eccrine glands release only components or substances without cellular material.

• Apocrine glands release portions of cells along with their contents, resulting in thicker secretions compared to eccrine ones.

Characteristics of Secretions

• Apocrine sweat (e.g., from axillary regions) contains additional components like pheromones, responding to sexual hormones unlike eccrine sweat which responds to adrenergic receptors.

Skin Structure Overview

Layers of Skin

• The skin consists mainly of two layers: the epidermis (outer layer marked in blue), separated from the dermis by a basal membrane (marked in black).

Epidermis vs. Dermis

• The epidermis originates from ectodermal tissue while the dermis comes from mesodermal tissue; epidermis is stratified squamous epithelium whereas dermis is connective tissue rich in collagen and inflammatory cells.

Subdivisions of Dermis

• The dermis has two parts: papillary dermis (upper part with projections into the epidermis), and reticular dermis (lower part containing collagen bundles).

Epidermal Layer Breakdown

Epidermal Layers

• The epidermis comprises several layers: basal layer (stem cells for regeneration), spinous layer (keratinocyte maturation), granular layer (keratohyalin granules present), lucid layer (only on palms/soles), and cornified layer (dead keratinocytes forming protective barrier).

Keratinization Process

• Keratinocytes undergo differentiation over approximately 28 days before shedding at the surface as they accumulate keratin.

Epidermis and Dermis Structure

Skin Epithelium Replacement

• The skin epithelium undergoes a replacement cycle, primarily composed of keratin, which serves as a protective lipoprotein barrier to prevent heat loss and moisture retention.

Components of the Dermis

• The dermis contains sweat glands, sebaceous glands, nerve endings (e.g., free nerve endings for pain), and various connective tissues including collagen and fibroblasts. It also houses blood vessels that nourish both the dermis and epidermis.

Temperature Regulation Mechanism

• Blood vessels in specific areas like fingertip pads form glomic apparatuses that regulate temperature by changing color based on heat exposure (red when hot, pale when cold). This mechanism is crucial for thermoregulation rather than nutrition.

Cellular Composition of the Epidermis

Keratinocytes Dominance

• Over 80% of epidermal cells are keratinocytes responsible for keratin production, essential for skin structure and function.

Melanocyte Functionality

• Melanocytes originate from neural crest tissue; they extend dendritic processes into the stratum spinosum to distribute melanin granules to keratinocytes, influencing skin pigmentation. The primary factors affecting skin color are melanin levels, vascularization, and skin thickness.

Skin Pigmentation Factors

Variability in Melanin Production

• All individuals possess an equal number of melanocytes; however, variations in melanin production lead to different skin phenotypes ranging from albinism to deep pigmentation. This spectrum includes all shades influenced by genetic factors.

Impact of Vascularization on Coloration

• Skin can exhibit red spots due to underlying blood vessels or appear paler based on varying thickness; thus both vascularity and epidermal thickness play roles in overall pigmentation appearance.

Immune Defense Cells in the Epidermis

Antigen-Presenting Cells

• The epidermis contains antigen-presenting cells such as Langerhans cells (macrophages) with Birbeck granules that help identify them as immune defenders within the tissue structure. These cells play a critical role in immune response against pathogens entering through the skin barrier.

Hair Follicle Anatomy

Hair Unit Structure

• Each hair follicle consists of a sebaceous gland that moisturizes hair with sebum; this prevents breakage while maintaining hair health through hydration mechanisms facilitated by these glands. Additionally, each follicle is associated with an arrector pili muscle responsible for hair erection during temperature regulation or stress responses in animals.

Muscle Functionality

• The arrector pili muscle contracts causing hair to stand upright (piloerection), serving dual purposes: regulating body temperature and providing defense mechanisms in animals by making them appear larger when threatened. This physiological response is significant across species for survival strategies.