Epithelial Tissue Flashcards
Epithelial Tissue
- Protective coverings of surfaces both inside and outside of the body
- Includes the skin surface and the linings of the heart, blood vessels, digestive tract, respiratory tract, urinary tract and body cavities
- Includes secretary cells and glands
- Function is predicted by the cell organisation/histology
Functions of epithelium
- Physical protection
- Protect exposed and internal surfaces from abrasion, dehydration, and destruction by chemical or biological agents
- Outer layer of skin, lining of the gut, respiratory tract - Control permeability
- Allow water, gases and nutrients to cross into and out of the body
- Prevent toxins and microorganisms from entering - Provide sensation
- Provide information on the external and internal environment - Produce specialized secretions
- Epithelial cells that produce secretions are called gland cells
- Hormones, sweat, mucous and joint fluid
Classification of Epithelia
- Based on shape
- Squamous epithelia: thin and flat
- Cuboidal epithelia: square shaped
- Columnar epithelia: tall, slender rectangles
- Based on layers
- Simple epithelium: single layer of cells
- Stratified epithelium: two or more layers of cells
- Based on surface specializations
Examples of Columnar epithelium
Intestine
Examples of Simple Squamous epithelium
Capillary and alveoli
Examples of Cuboidal epithelium
Sweat duct
Simple Squamous epithelium
- Role in secretion and absorption
- Endothelium – lines blood vessels and heart
- Mesothelium – lines body cavity
Simple Squamous epithelium Locations
Mesothelia lining pleural, pericardial, and peritoneal cavities; endothelia lining heart and blood vessels; portions of kidney tubules (thin sections of nephron loops); inner lining of cornea; alveoli of lungs
Simple Squamous epithelium Functions
Reduces friction; controls vessels permeability; performs absorption and secretion.
Stratified Squamous epithelium Location
surface of the skin; lining of the mouth, throat, oesophagus, rectum, anus and vagina.
Stratified Squamous epithelium Functions
Provides physical protection against abrasion, pathogens and chemical attack
Simple Cuboidal epithelium Locations
Glands; Ducts; portions of kidney tubules; thyroid gland
Simple Cuboidal epithelium Functions
limited protection, secretion and absorption
Stratified cuboidal epithelium Functions
protection, secretion and absorption
Stratified cuboidal epithelium Locations
lining of some ducts (rare) e.g. sweat gland duct
Stratified columnar epithelium Functions
Protection
Simple columnar epithelium Locations
Lining of stomach, intestine, gallbladder,uterine tubes and collecting ducts of kidneys
Simple columnar epithelium Functions
protection, secretion and absorption
Pseudostratified columnar epithelium Locations
Lining of the nasal cavity, trachea and bronchi; portions of male reproductive tract
Stratified columnar epithelium Locations
Small areas of the pharynx, epiglottis, anus, mammary glands, salivary gland ducts and urethera
Pseudostratified columnar epithelium Functions
Protection, secretion and move mucus with cilia
Transitional epithelium Locations
Urinary bladder; renal pelvis; ureters
Transitional epithelium Functions
Allows repeated expansion and recoiling after stretching
Returns to previous shape without damage
Application – GIT Epithelia (Epithelium)
changes with function; protection, secretion and absorption:
* Stratified squamous at mouth, oesophagus and anus
* Simple columnar in stomach and intestine
Application – GIT Epithelia (Gland Cells)
found among epithelial cells, submucosal and in accessory structures (salivary glands, pancreas and liver)
Gland cells in GIT may be mucous, hormone or serous (enzyme secreting)
Application – GIT Epithelia (Microvilli)
increase surface area for absorption
Glandular Epithelia
- Epithelia containing gland cells are specialised for secretion- these are
known as glandular epithelia - Epithelial cells can produce secretions:
- Goblet cell – secretions by a single cell
- Gland – secretions by a group of cells
Types of Glands
Endocrine Glands
Exocrine Glands
Endocrine Glands
- Produce endocrine secretions – called hormones
- Hormones enter bloodstream for distribution throughout body
- Ductless glands – secretions not released into ducts
- Hormones regulate or coordinate activities of tissues, organs, and organ systems.
- E.g. Insulin hormone
Exocrine Glands
- Release secretions directly onto epithelia
surface or into passageways (ducts) that
open onto epithelia surface - e.g. Sweat Gland releases secretions to
surface of body
Goblet Cells
Mucous Glands
in the GIT and respiratory tract secrete mucous.
Secretion protects the surface cells.
In respiratory tract mucous moistens and warms the incoming air and traps any dust particles.
Mucositis
- Inflammation of the mucosa - the mucous membranes that line the mouth and gastrointestinal tract
- Common side effect of chemotherapy or radiation cancer therapy
Submucosal Exocrine Glands
- In some structures where more protection is required submucosal glands are found below the surface but attached to the surface by a duct (tube)
- Allow increased secretions
Exocrine gland structure
-Unicellular gland
-Multicellular glands
Unicellular exocrine glands
- Single cells called goblet cells that are specialised for secretion
- Goblet cells secrete mucin – mixes
with water to form a sticky lubricant
called mucus - individual secretory cells in epithelia that have independent, scattered gland cells
Multicellular exocrine glands
- include glandular epithelia and aggregations of gland cells that produce exocrine or endocrine secretions into an inner compartment e.g., salivary glands
produce mucins and digestive
enzymes
Characteristics when describing the structure of multicellular exocrine glands
- Structure of the duct:
- Simple
- Compound
- Shape of the secretory portion of the gland
- Tubular
- Alveolar or acinar
- Tubulo-alveolar or tubulo-acinar
- Relationship between ducts and glandular area
- Branched (refers to glandular areas
and not to the duct)
- Branched (refers to glandular areas
Exocrine Gland – modes of secretion
Merocrine secretion
Apocrine secretion
Holocrine secretion
Merocrine secretion
- Product is released from an exocrine cell by secretory vesicles through exocytosis
- Most common method of exocrine secretion
- E.g., mucin is a merocrine secretion: mucous secretions of the salivary glands coat food and reduce friction during
swallowing; merocrine sweat glands in the skin produce perspiration that cools body on a hot day
Apocrine secretion
- Involves the loss of cytoplasm as well as the secretory product
- Apical portion of cytoplasm is packed with secretory vesicles and then shed
- E.g. milk production in the mammary glands involves merocrine and apocrine
secretions
Holocrine secretion
- Superficial cell in a stratified glandular epithelium becomes packed with secretory vesicles and then burst – releasing secretion but also killing the cell
- Further secretion dependent on replacing destroyed gland cells by division of underlying stem cells
- E.g., sebaceous glands of hair follicles produce oily hair coating by holocrine secretion
Mammary Gland
- Mammary gland cells produce and store milk
- Have an outer epithelial cells layer with some contractile proteins
- MYO – refers to muscle function
- The muscle proteins form part of the lower layer of cells just above the basement membrane
Hormones
- Chemical messengers released from endocrine glands
- Transported in the bloodstream
- Reach target cell/organ(s)
- Alter metabolism of target cell
Insulin secretion – the pancreas
The pancreas has islets of Langerhans that produce insulin (and other hormones)
Insulin secretion
- Food is converted into glucose in the stomach and intestinal tract
- Glucose then enters the bloodstream
- Glucose reaches the pancreas, resulting in
the pancreas releasing insulin - Insulin will cause glucose uptake in target
cells (liver, muscle, adipose tissue) - Glucose is stored as glycogen in liver and
muscle, stored as fat in adipose tissue
Reasons for Epithelial cells specialisation
- The movement of fluids over the epithelial surface, providing protection and lubrication
- The movement of fluids through the epithelium to control permeability
- The production of secretions that provide physical protection or act as chemical messengers
Specialised epithelial cells types
- Apical surface – cell is exposed to internal or external environment
- Lateral surface – side surfaces, where the cell contacts its neighbours
- Basal surface – cell attached to underlying epithelial cells or deeper tissues
Apical Surface Specialisations
- Cell is exposed to internal or external environment
- At surface of layer of cells – in contact with exterior surface (air), lumen (tube interior) e.g. blood, chyme, urine, or body cavitiesn
- The epithelial cells surfaces may be
* smooth
* may have cilia
* may have microvilli - Some skin surfaces have a protein called keratin in the cells that provides additional strength
Cilia
- move substances (fluids or secretions) over epithelial surface
- Each ciliated cell may have up to 250 cilia that beat in a co-ordinated fashion
- Behave like escalator:
* In the trachea cilia or hairs on the
surface of the epithelial cells move the
mucous along the respiratory tract
* Moving mucus up from lungs, mucus
traps particles such as pollen,
pathogens, dust
* Ciliated epithelium carries mucus to
throat; mucus is swallowed or expelled
by coughing - Smoking one cigarette immobilises the cilia for one hour
- The epithelium of the trachea is a classified as a ciliated pseudostratified epithelium with goblet cells.
- also found in the fallopian tubes
Microvilli
- increase the surface area of the epithelial cells
- Increase the capacity for absorption by a factor of 20
- Important in the jejunum of the gut for absorption of nutrients
- Important in the early part of the kidney tubules (PCT) for absorption
Keratin
- produced by the epithelial cells of skin (keratinocytes)
- a protein that forms granules in the cytoplasm
- Keratinised epithelia are found in the oesophagus in some animals – depends on the diet
Tight (Occluding) junctions
- between two plasma membranes
- Adhesion belt forms and attaches to terminal web
- Prevents passage of water and solutes
- Isolates wastes in the lumen
Lateral Surface Specialisations
May be:
* Tight junctions
* Gap junctions
* Anchoring junctions (desmosomes)
Gap junctions
- allow rapid communication
- Held together by channel proteins (junctional proteins, connexons)
- Allow ions to pass so cells can communicate
- Allow the coordination of cilia beating or coordinating muscle cell contractions
Anchoring junctions (desmosomes)
- Very strong, can resist stretching and twisting
- At a desmosome CAMs and proteoglycans link opposing plasma membranes
- Spot desmosomes:
* Small disks connected to bands of
intermediate filaments
* Tie cells together
* Allow bending and twisting - Hemidesmosomes
* attach cells to the basal lamina
* Helps stabilise position of epithelial cell
CAMs – cell adhesion molecules, dense areas, and intercellular cement
Basal lamina
- closer to the epithelium
- functions as selective filter – restricts movement of proteins from underlying connective tissue
Epithelium repair/dead
- Epithelia may last a few days before being worn off or destroyed
- E.g., exposed to enzymes, pathogenic bacteria, physical distortion
- Cells lost are replaced by division of stem cells
- Stem cells are found in the basal layer of stratified epithelia
Reticular lamina
- Gives the basement membrane strength
- Attachments between fibres of basal lamina and reticular lamina hold the two layers together
- Acts as filter determining substances that can diffuse between epithelium and adjacent tissues
- Hemidesmosomes attach epithelial cells to basement membrane
