Epithelial Tissues Flashcards

Here is a detailed summary of epitheliums: ## Introduction and Definitions * An epithelium is a polarized set of cells, joined and organized in one or more layers. It facilitates transition and separation between the organism and its external environment or internal compartments. * All epitheliums are based on connective tissue, usually via a basal membrane. * Connective tissue provides nutrition to epitheliums through nutrient diffusion from blood vessels and controls their growth. * Blood vessels are located in the connective tissue, not within the epitheliums themselves. * Two primary types of epitheliums are defined by their main function: * Lining epitheliums: protection or exchange functions * Glandular epitheliums: secretion function ## Lining Epitheliums ### General Information Lining epitheliums cover: * The body's surface * The lining epithelium is known as the epidermis, with the underlying connective tissue called the dermis. Together with the hypodermis, they form the skin. * Cavities that connect with the outside environment * The connective tissue below the epithelium is called the chorion. The entire epithelium/connective tissue is referred to as the mucous membrane, found in: 1. The digestive tract 2. The respiratory system 3. The urinary tract 4. The genital tract * Closed cavities * The lining epithelium is the mesothelium and the entire epithelium/connective tissue structure, called the serous membrane, which lines: 1. Pleural cavities 2. Peritoneal cavities 3. Pericardial cavities * Cardiovascular cavities * The lining epithelium is called endothelium, with the entire epithelium/connective tissue known as the intima in vessels and endocardium in the heart. ## Classification of Epitheliums Epitheliums are classified based on: * The shape of the surface cells * The number of cell layers The combination of these features helps identify different types of epithelium. * Squamous epithelium: flattened cells, wider than they are tall * Columnar/prismatic epithelium: taller cells than they are wide * Cuboidal epithelium: cells are as tall as they are wide * Polymorphic epithelium: cells of varying sizes and shapes * Simple epithelium: a single layer of cells where all cells rest on the basal membrane * Stratified epithelium: multiple cell layers where only the cells in the deepest layer rest on the basal membrane * Pseudo-stratified epithelium: cell nuclei are at different heights, but all cells rest on the basal membrane. Only some cells reach the surface. ### Types of Epithelium 1. Simple Squamous Epithelium * Locations: * Cardiovascular cavities (endothelium) * Peritoneal cavities (mesothelium) * Pulmonary alveoli, inner and middle ear, descending limb of the loop of HENLE (endotheliform epithelium) 2. Simple Columnar Epithelium * Common in the body. * Additional feature for classification: differentiation of surface cells. * Examples: * Ciliated: In uterine tubes * Striated border and goblet cells: Intestine 3. Simple Cuboidal Epithelium * Additional feature for classification: differentiation of surface cells. * Examples: * Ovaries * Terminal bronchioles (lungs) * Proximal convoluted tubule (kidney) * With a brush border: cells have microvilli to increase the surface area for exchange 4. Stratified Squamous Epithelium (Malpighian) * Additional feature for classification: differentiation of surface cells. * Types: * Non-keratinized: surface cells are alive and nucleated (e.g., oral cavity, esophagus, vagina, anal canal, cornea) * Keratinized: surface cells are dead and anucleated (e.g. epidermis) 5. Stratified Columnar/Prismatic Epithelium * Rare * Locations: urethra, palpebral conjunctiva 6. Stratified Cuboidal Epithelium * Locations: excretory ducts of sweat glands and salivary glands 7. Pseudo-stratified Columnar Epithelium * Additional feature for classification: differentiation of surface cells. * Types: * Ciliated: trachea, bronchi * Stereocilia: epididymis 8. Polymorphic Pseudo-stratified Epithelium (Urothelium or Transitional Epithelium) * Characteristics: * Extensible to withstand stretching in the bladder * Impermeable thanks to junction complexes, membrane interdigitations, and uroplakins. * Locations: bladder, ureters ## Roles of Lining Epitheliums Epitheliums have varied roles based on their location. ### Protection * Mechanical: Malpighian epithelium and urothelium. * Chemical: Gastric epithelium produces mucus to protect against gastric acidity. * Radiation: Epidermis. ### Exchange * Gas exchange: Alveolar epithelium. * Nutrient exchange: Endothelium and intestinal epithelium. * Active absorption and excretion: Renal tubule epithelium. * Excretion: Biliary epithelium. ### Motility * Cilia (respiratory, uterine tube epithelium) facilitate substance transport. ### Sensory Function * Touch: Epidermis. * Taste: Sensory gustatory epithelium. * Hearing: Sensory auditory epithelium. ## Glandular Epitheliums ### Exocrine Glandular Epitheliums Exocrine glandular epithelium: * Consists of clusters of exocrine glandular cells specialized in secretion released to the external environment. * Predominantly serves a secretory function. * Forms exocrine glands. * Are simple epitheliums. * Exocrine glands have two main parts: * Cul-de-sac responsible for secretion products. * Excretory Duct for transporting secretetd products to the lumen. Glands can be located: * In the wall of an organ: * Within lining epithelium known as intra-epithelial. * Or, deeper as intramural glands. * Or, grouped to form an exocrine organ. ### Classification of Exocrine Glands Based on specific morphological criteria: * Shape of the secretory part (spherical, tubular, or alveolar/acinar) * The complexity of the duct system (simple or compound) * The mode of secretion (merocrine, holocrine, or apocrine) ### Secretion There are three primary modes of classified secretion, dependant on how the secretory cell releases the product: * Merocrine * Exocytosis of vesicles containing secretory products, and preserves cell integrity (most exocrine glands). * Holocrine * Cells accumulate secretion. Cell rupture releases product while destroying the cell (sebaceous glands). * Apocrine * Apical portion of the cell containing secretion vacuoles detaches (mammary glands) ### Biochemical Glandular exocrine cells mainly produce: * Proteins (serous glands) * Mucus (mucous glands) Some glands produce other products: * Hydro-electrolytic: (HCI) in the stomach * Lipidic: (milk, or sebum). ### Associated Helper Cells In glands derived from the ectoderm: Glands sudoripares, glandes mammaires, glandes lacrymales et glandes salivaires: * Myoepithelial cells facilitate product expulsion. * These cells positioned between basal membrane and glandular cells and contract in response to oxytocin. ### Endocrine Glandular Epitheliums Endocrine glandular epithelium features: * Clusters of endocrine glandular cells release secretions into the circulation (hormones). * Forms endocrine glands without excretory ducts, releasing hormones directly into the bloodstream * Endocrine glands are highly vascularized and endocrine secretion is of the merocrine type. #### Endocrine glands can * Exist as small clusters of cells within an organ * Themselves form an organ #### Some organs (glands) have both exocrine and endocrine functions. * By the same cell (liver). * Differnt cell (pancreas). ### Architecture of Endocrine Glands * Follicular * Are composed of cells are arranged surrounding a central space filled with colloid. * Trabecular * Cells arranged in cords separated by thin connective tissue