Histology Notes -Part 1: Epithelial Tissues

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1 Introduction Group of cells w/ similar structure & function = TISSUE Four Basic Tissue Types 1. Epithelial-covers 2. Connective-supports 3. Muscular*-produces movement (will discuss in the muscular system unit) 4. Nervous*-controls (will discuss this in the nervous system unit) Tissue Components -Cells specialized to carry out a particular function -Extracellular Matrix = non-living -Supports cells -Transports signals from cells outside matrix to those inside Epithelial Tissues or epithelium A sheet of cells that covers a body surface or lines a body cavity. General Characteristics of epithelial tissues Locations: 1. Covers the surface of the body 2. Covers and lines internal organs 3. Forms lining of cavities and hollow organs 4. Forms glands Functions: 1. Protection- covers body and lines all body cavities; provides a protective shield for underlying tissues 2. Absorption- in some parts of the body it takes in certain materials (digestive tract microvilli of small intestine) 3. Filtration-urine, sweat, CO2 4. Excretion- in the kidney cells rid the body of materials 5. Secretion-Endocrine glands 6. Sensory reception-sense of touch in skin External surface- protects body from: 1. drying out 2. injury & invasion 3. abrasion Internal surface -specialized functions: 1. Digestive tract secretes mucus 1

2 Apical-basal polarity Special Characteristics of epithelial tissues 1. Polarity Apical surface = upper free surface exposed to the body exterior or the cavity of an internal organ (always has one free surface) -Apical surface can have: -Smooth- surfaces reducing friction (blood vessels) -Microvilli- increase surface area involved in secretion and absorption Ex: intestine or kidney tubules -Cilia- aid movement (propelling particles) Ex: trachea -Modified for specialized functions (secretion, movement) Basal surface (basal lamina) acts as a selective filter determining which molecules are allowed to enter the epithelium. The basal surface of epithelium lies on a thin sheet called a basal lamina, which is part of the basement membrane. Deep to the basal lamina is a layer of reticular fibers belonging to the underlying connective tissue. Together the reticular fibers and the basal lamina form the basement membrane. Good visual below: 2. Specialized Contacts -Most epithelial cells fit close together forming continuous sheets -Bound together by tight junctions and desmosomes-keeps proteins in the apical region from diffusing into the basal region. Maintaining epithelial polarity. -Adjacent cells bound tightly -Regulate passage of materials -Epithelia subjected to stress -Little extracellular matrix between them 2

3 3. Supported by Connective Tissue (basement contacts) -Thin, non-living layer, anchoring epithelial tissue -Rests upon and supported by connective tissue -Defines epithelial boundary -Resists stretching and tearing forces 4. Avascular- (no blood vessels) but is Innervated (supplied with nerve cells) -Epithelial tissues are supplied with nerve cells (innervated) so they can contribute to sensory function -Epithelial tissues contain no blood vessels -Epithelial tissue receive nutrients by substances diffusing from blood vessels in the underlying connective tissue layers 5. Regeneration -High regenerative capacity -If nourished adequately, can replace lost cells rapidly by cell division Classification Classified by: -Cell layer number -Simple (one cell layer) -found where absorption & filtration are needed -Stratified (two or more cell layers) -protective or high abrasion areas -identified by shape of cells on the apical layer CELL LAYER Simple Stratified 3

4 -Cell shape (based on height b/c all have same basic shape) -Squamous = flattened or scale like (nucleus is flattened) -Cuboidal = boxlike-as tall as they are wide (spherical nucleus) -Columnar = tall and column shaped (nucleus elongated & at bottom) CELL SHAPE Squamaous Cuboidal Columnar Two Types of Epithelial Tissue: Simple Epithelia (all cells in the layer have the same shape) 1. Simple squamous 2. Simple cuboidal 3. Simple columnar -Ciliated -Non-ciliated 4. Pseudostratified -Ciliated -Non-ciliated Stratified Epithelia *In stratified epithelia, shape can vary in each layer, so the cell is named according the shape in the apical layer 1. Stratified squamous 2. Stratified cuboidal 3. Stratified columnar 4. Transitional LET S CHECK IT OUT 4

5 Simple Epithelia FUNCTION: Involved in absorption, secretion, or filtration processes Simple squamous epithelium - Cells are flattened laterally, and cytoplasm is sparse - Disc-shaped nuclei - Function: where rapid diffusion is priority and protection is not; lubricating - Location: kidney, lungs, lining of heart, blood vessels and lymphatic vessels, lining of ventral body cavity - Two special simple squamous epithelia are based on locations -Endothelium: lining of lymphatic vessels, blood vessels, and heart -Mesothelium: serous membranes in the ventral body cavity Simple cuboidal epithelium Single layer of cube-like cells Large, spherical centralized nuclei Function: secretion and absorption Location: walls of smallest ducts of glands, many kidney tubules, ovary surface Simple columnar epithelium Single layer of tall, closely packed cells -Some cells have microvilli, and some have cilia -Some layers contain mucus-secreting goblet cells - Round to oval nuclei Function: absorption and secretion of mucus, enzymes, and other substances -Ciliated cells move/propel mucus Location: non-ciliated-lines digestive tract (stomach anal canal), gallbladder, ducts of some excretory glands ciliated- bronchi, uterine tubes and uterus Pseudostratified columnar epithelium Cells vary in height-some not reaching the free surface; appear to be multilayered and stratified, but tissue is in fact single-layered simple epithelium - Pseudo means false - Many cells are ciliated Function: secretion, particularly of mucus, and movement of mucus via ciliary sweeping action Location: non-ciliated- tubules in testes (sperm carrying ducts) ducts of large glands ciliated- mostly in upper respiratory tract and lines trachea 5

6 Stratified Epithelia Stratified epithelial tissues Involve two or more layers of cells New cells regenerate from below; Basal cells divide and migrate toward surface More durable than simple epithelia because protection is the major role Stratified squamous epithelium -Most widespread of stratified epithelia -Free surface is squamous, with deeper cuboidal or columnar layers Function: protects underlying tissues in areas subjected to abrasion Location: areas of high wear and tear (example: skin) -Keratinized cells: forms epidermis (skin) -Non-keratinized cells found in moist linings of esophagus, mouth and vagina Stratified cuboidal epithelium -Quite rare -Typically only two cell layers thick Function: protection Location: some sweat glands, salivary glands, and mammary glands Stratified columnar epithelium -Also very limited distribution in body -Only apical layer (superficial layer) is columnar Function: protection and secretion Location: Small amounts in pharynx, in male urethra, and lining some glandular ducts Usually occurs at transition areas between two other types of epithelia Transitional epithelium -Basal layer cells are cuboidal or columnar -Surface cells are dome shaped-depending on degree of organ stretch Function: Ability of cells to change shape when stretched allows for increased flow of urine and, in the case of bladder, more storage space Location: Forms lining of hollow urinary organs; bladder, ureters, and urethra Glandular Epithelia -Consist of cells specialized to secrete substances into ducts or fluids -Classification: -Exocrine-secrete products into ducts that open onto surfaces Ex: Mucus, oil, sweat, saliva, etc -Endocrine- secrete products into tissue fluids or blood Ex: Produce hormones 6