Epithelial Tissues Flashcards
What properties are common to all epithelial cells and tissues?
1) Adherent to one another
2) Cells are arranged in one to several layers or sheets
3) Polar, or Asymmetric
- free outer or apical surface
- inner basal or basolateral surface
4) Basolateral surface connects to underlying connective tissue
5) Undergo turn-over or renewal
6) Avascular, nutrients and oxygen must diffuse through connective tissue and basal lamina to reach epithelial cells
7) Can have several different cell types within any given epithelium
Where are epithelial tissues found?
Line body surfaces, body cavities, surfaces of internal tubes, ducts, and other spaces in organs.
Main functional units of glands and several organs.
What is the basal lamina?
A sheet of extracellular material that lines and is attached to the basal surface and is also attached to elements of the underlying connective tissue.
Epithelial cells of the anterior pituitary…
secrete pituitary hormones
What functions do epithelia serve?
Barrier, selective absorption and transport from environment, selective secretion, movement of particles and mucous through passage ways, biochemical modification of molecules (liver), communication to and from other tissues and organs, sensory stimuli reception
Name some functions of intestinal epithelium
absorbs, secretes, protects, sends molecular signals to other organs and tissues
What are two distinguishable types of internal epithelia?
Endothelium - faces blood and lymph
Mesothelium - line enclosed internal spaces
What organs are composed primarily of epithelia?
liver, pancreas, kidney - epithelia is primary functional unit
From what germ layers to epithelia derive?
All three - ectoderm, mesoderm, and endoderm
Describe epithelial to mesenchymal transition.
During early development, epithelia often disassemble and move into the mesenchymal (connective) tissues; migrate to other locations to form new epithelia, or transform into distinct non-epithelial cell lineages and give rise to other tissues.
Describe epithelial tissue orientation
Apical surface faces outside world
Basal side attaches to basal lamina, which is attached to underlying connective tissue.
How is epithelia vascularized?
Indirectly. Blood vessels run through connective tissue along with nerves, diffusion gets nutrients to/from epithelia.
Epithelia directly attach to what other tissue?
Connective tissue via the basal lamina
How do epithelia connect to blood vessels, muscles, and nerve endings?
Separated from them, but attached via connective tissue and by different basal laminae that surround each tissue type.
What types of nerves actually penetrate epithelia?
Special sensory nerves, such as taste buds in the tongue.
What cell types other than special sensory nerves infiltrate epithelia?
Specialized immune system cells called dendritic cells
Describe mucosae generally, give examples
Moist internal linings that separate “outside” from “inside”.
Mouth, nose, throat, GI tract, reproductive systems
Describe the layers of mucosa
Two layers - outer epithelium and the lamina propria, the connective tissue (CT) directly underneath
What is contained in the lamina propria
lots of immune system cells and small blood vessels
Why does the lamina propria have lots of immune system cells and small blood vessels?
Survey and extract foreign or ingested materials, cells, and molecules from the environment; efficiently transport and monitor mucosa
How are deep layers of CT different from lamina propria?
Directly continuous with lamina propria, but house other tissues such as bigger vessels and muscles, nerve axon bundles, etc.
What is the deeper CT tissue layer called?
Submucosa
What is the nomenclature of the external skin?
Epithelial layer is the epidermis
CT layer is the dermis
Deep CT layer is the hypodermis
What is the basic, general, holds across all examples relationship of epithelial tissues?
Space (lumen) -> Epithelia -> epithelial basal lamina -> CT -> other CT-embedded tissues (blood vessels, muscle, nerves, all with their own basal laminae that connects them with the CT)
All tube and duct surfaces are lined by?
epithelia
How are epithelia classified?
Based on the arrangement of cell layers and on cell shape
Epithelia are classified based on ____, not ____.
Structure, function
What are the most general epithelial classifications?
Simple, stratified, and pseudostratified
Describe simple epithelia
All cells arranged in a single layer or sheet
Describe stratified epithelia
more than one layer of cells in which cells of the outer layers do not directly contact the basal lamina
Describe pseudostratified epithelia
special case where some cells do not reach the free surface (giving a stratified appearance), but all directly rest on the basal lamina
What epithelial cell shape classifications are there?
Squamus, cuboidal, columnar, and transitional
Describe the squamus cell type
flattened cells
Describe the cuboidal cell type
cube-like
Describe the columnar cell type
taller than they are wide
Describe the transitional cell type
special case - stratified cell layer in which the cells change shape when stretched from cuboidal to squamus (bladder), appearing to decrease layering. Indicates tightly adherent epithelium that is very resilient and stretchable
How are stratified epithelia named?
According to their outermost layer
Where are the cell junctions of epithelial cells located?
lateral surfaces, generally toward the apical side
Name the four types of epithelial cell junctions
Tight junctions
Adherence junctions
Desmosomes
Gap junctions
Describe tight junctions
AKA zonula occludens; highly selective barrier that limits or prevents diffusion of substances between epithelial cells
What are the key core proteins of tight junctions?
occludins and claudins
What do tight junctions (barriers) ensure?
substances absorbed or secreted must pass through the epithelial cell by specific transport pathways
Describe adherence junctions
AKA zonula adherens; promote attachment, polarity, morphological organization and stem cell behavior within epithelial sheet
What are cadherins?
Cadherins are transmembrane proteins with extracellular domains that interact with each other. They have cytoplasmic tails that bind adapters and actin filaments
Where are cadherins found?
Cadherins are found in adherence junctions, where they link to actin filaments and other adapter/signaling proteins in the cytoplasm.
Describe desmosomes
AKA macula adherens; promote mechanical strength and resist shearing forces, promote structural organization of the epithelial sheet
What are the core components of desmosomes?
A different class of cadherins that link to intermediate filaments and other adapter proteins than those in tight junctions
Describe gap junctions
Channels between epithelial cells that promote rapid communication through diffusion of ions and small molecules
Describe the polarity of simple epithelia
polarized with distinct regions of plasma membrane and cytoplasm.
Which surface does the apical domain face?
free surface
Which surface does the basolateral domain face?
basal lamina
Describe epithelial cytoplasm polarization
The cytoskeleton (particularly microtubules) is asymmetric or polar in orientation. Organelles are distributed in a precise polarized pattern. Secretory vesicles in the apical domain are usually different from those in the basal domain, and move in specific directions.
Describe epithelial cell plasma membrane polarization
Distinct membrane proteins and phospholipids located in the apical domain verse the basal domain.
Why are epithelial cells polarized?
Allow unidirectional secretion and/or absorption of molecules to or from one side of the epithelium. Also necessary for “transepithelial” transport of ions and macromolecules from the apical to basal surfaces (and vice versa).
What is transcytosis?
The endocytosis of substances from one membrane region, followed by trans-cellular transport of the vesicles and their exocytosis from another membrane region.
How are the apical surfaces of epithelial cells modified?
Microvilli and cilia
Describe epithelial microvilli
Cell surface extensions (protrusions) that contain actin bundles connected to cytoskeletal elements in the cell interior.
What is the primary function of epithelial microvilli?
Increase surface area, which greatly increases the rate/efficiency of membrane transport and secretion.
Where are stereocilia found?
Epididymis and in sensory cells in the ear
What are stereocilia?
Extremely long, actin-filled microvilli that are NOT related to cilia.
What do stereocilia in the ear do?
Function in the reception of sound.
Describe epithelial cilia
Microtubule-containing extensions (protrusions).
Name three kinds of epithelial cilia
Primary cilium
Motile cilia
Sensory cilia
What is an epithelial primary cilium?
A primary cilium is a single, one cell, non-motile microtubule-based extension found on many different epithelial cell types.
What do epithelial primary cilia do?
Primary cilia organize and promote signal transduction systems (receptors and effectors) that control epithelial cell division, fate (differentiated state), and function.
What are epithelial motile cilia?
Motile cilia are related microtubule extensions that move.
Where are epithelial motile cilia found?
Specific epithelial cell types, such as in the respiratory tract and in the oviduct (assist in moving ova towards the uterus).
What do epithelial motile cilia do?
Wave like a boat oar to move mucous and other materials along passageways.
Tell me about epithelial sensory cilia
Highly specialized, non-motile, and function in sensory reception. Specific variations of primary cilia, connected directly to the central nervous system.
Give some examples of epithelial sensory cilia
Vestibular hair cells of the ear, specialized cilia for touch and taste
What are ciliopathies?
Diseases resulting from mutations in proteins common to cilia or their support structures.
What is the basal lamina?
A thin sheet of extracellular material that underlies the basal surface of each epithelial tissue.
Where is basal lamina found?
Basal laminae are found underlying the basal surface of each epithelial tissue, and also surrounding many other cell and tissue types, including the endothelium of blood vessels, muscle, and nerve tissue.
What forms basal laminae?
Basal laminae are formed by a special type of network-forming collagen (a fibrous protein).
Describe the collagen found in basal laminae
Forms sheets of thin fibers that are interwoven with a variety of other extracellular glycoproteins.
What glycoproteins are commonly included in the collagen of basal laminae?
laminins and entactin, among others common to different basal laminae
What type of collagen is network-forming?
Type IV
Where are the components of basal lamina synthesized?
Synthesized and secreted by epithelial cells. Some extracellular components of the connective tissue made by fibroblasts bind to and possibly organize elements in the basal lamina.
Name seven important functions of basal laminae
1) Mediate attachement of epithelia to underlying CT
2) Often contribute to selective filtrations of substances diffusing to or from the epithelia
3) Necessary for the establishment and maintenance of epithelial cell polarity
4) Serve as specific “highways” for the migration of cells through connective tissue
5) Provide a barrier to movement of invading microbes or cancerous cell into other tissues
6) Control the gene expression of cells to affect their proliferation or development
7) Control the development, morphogenesis, and organization of epithelial cells
Name a critical function of the basal laminae
In controlling the development, morphogenesis, and organization of epithelial cells, they provide a sort of “tissue scaffolding” function. Thus they are critical to the repair of epithelial tissue following damage by injury or disease.
How does the basal lamina relate to epithelial cells?
Basal laminae both separate epithelial cells from the underlying connective tissue and attach epithelial cells to the extracellular matrix of connective tissue.
How do epithelial cells attach to basal laminae?
Direct connection by attachment of hemidesmosomes and focal adhesions on the basal surface of the epithelial cell to components of the basal lamina.
What class of proteins form hemidesmosomes and focal adhesions?
Integrins
What is the origin of integrins?
Distinct proteins from the cadherins that mediate cell-cell linkage.
Where do the integrins of hemidesmosomes connect?
Intermediate filaments in the epithelial cell
Where do the integrins of focal adhesions connect?
Actin filaments inside the epithelial cell
Which units of epithelial cells control polarity?
Focal adhesions regulate epithelial polarity and function through signaling mechanisms.
Describe epithelial stem cells
Epithelial stem cells are capable of cell division and serve to self-renew (regenerate stem cells with each division) and produce differentiated cell types specific to each epithelia.
How do stem cells produce differentiated cell types?
Cell division coupled with specific pathways of cell specification imposed on some of the daughter cells
What are transit amplifying cells?
Stem cell daughter cells that also proliferate, often at faster rates
What happens to transit amplifying cells?
The produce differentiated cells, either directly or through several steps of specification.
What is a cell lineage?
A specific stem cell type, its intermediate progeny, and their differentiated progeny.
How quickly do stem cells typically divide?
Very slowly, or infrequently in some tissues.
Describe stem cell abundance.
Much less abundant relative to differentiated offspring.
How are division and differentiation in stem cell lineages regulated?
Tightly regulated by cell to cell communication and cell signaling pathways, activated or inhibited by physiological and developmental cues.
Describe a typical cell signaling pathway for stem cells
1) Extracellular ligand (signal) secreted by signaling cells
2) Receptor in receiving cells that binds and is activated/inactivated by the ligand
3) Downstream effector proteins in the cytoplasm and nucleus
4) Numerous modulator proteins promote or suppress pathway components
What are the possible origins of cell signaling in epithelial stem cells?
Could be local, involving ligands secreted by cells within the same epithelia, or by cells in neighboring connective tissue.
Describe long distance signals in epithelia
Typically secreted from endocrine glands into the blood stream, or produced by the nervous system or other tissues.
What are the important signaling systems that control epithelial stem cell renewal and differentiation?
Wnt, Sonic Hedgehog, TGF-beta, Notch, receptor tyrosine kinase family (RTK, i.e., EFG receptors), and FGF receptor pathways
Epithelial stem cells share signaling pathways with which other processes?
Embryogenesis, fetal development, and childhood
What are the key principles of the epithelial stem cell signaling pathways?
1) Each pathway is used by multiple, very distinct stem cell systems in different organs/tissues.
2) A single signaling pathway often triggers different developmental outcomes in different stem cell lineages.
What contributes to differences in a single signaling pathway across different stem cell lineages?
a) Different developmental histories of each stem cell type and cytoplasmic state generated earlier in development
b) Different environments in which the stem cells reside (i.e., different combinations of signaling inputs)
c) Differences in the levels of the extracellular ligands, receptors, and/or downstream components.
Why are stem cell signaling pathways prime targets for drug therapies?
Defects in regulation, expression, or structure of signaling components (ligands, receptors, effectors) lead to disease, particularly cancer.
Name an example of an anti-tumor agent and describe how it works.
Tarceva (erlotinib) inactivates EGF receptor and is commonly used to treat lung and pancreatic cancers.
From what tissue do glands of the body derive?
Epithelial tissue
What is the primary function of glands?
Secretion of either specific bioactive molecules (e.g., hormones), complex fluids (e.g., sweat, mucous), or both
What are the two major types of epithelial glands?
Exocrine and endocrine
What do exocrine glands do?
Secrete materials onto epithelia-lined surfaces or the outside world
What do endocrine glands do?
Secrete substances into the blood stream
What are the two general ways that glands secrete their substances?
1) Exocytosis (common)
2) Total cell disintegration (less common)
What are glands that secrete by exocytosis called?
Merocrine or apocrine glands
What are glands that participate in total cell disintegration called?
Holocrine glands
Give an example of a holocrine gland secretion
oily sebum onto hair and skin
Onto what surface do exocrine glands secrete materials?
The apical side of epithelial surfaces
Describe multicellular exocrine glands
Two components:
1) Secretory units - clumps of secretory epithelial cells which produce and secrete the bulk of the secretion
2) Ducts - tubular structures that emanate from the secretory units and function as passageways to conduct secretions to their destinations.
How are the secretory units of exocrine glands organized?
Classified as either bowl or flask-shaped lobules called alveoli or acini, called alveolar or acinar glands. Alternatively, the lobules may be organized into tubes and be called tubular glands.
What is a tubuloalveolar gland?
A gland having both tubular and alveolar character
Discuss the ducts of an exocrine gland
Function as a passageway to conduct secretions to a destination. Duct epithelia may also modify the secretion content by secretory and ion transport properties of duct cells.
How are ducted glands differentiated?
Simple glands possess a single duct, while compound glands possess multiple branched ducts.
Further subclassifications reflect whether glands are coiled and whether the secretory units are multilobed or not.
What are the three general types of exocrine glands of the body tubes?
1) mucous - produce viscous glycoprotein-rich fluid
2) serous - produce watery fluid containing salts and some specific proteins
3) mixed - produces both kinds of secretions
How do endocrine glands secrete substances?
Directly into the blood stream, without ducts.
What do endocrine glands produce?
Specific hormones that act over relatively long distances to control tissue function.
How are endocrine glands generally organized?
As clumps or chords of cells that are embedded with and surrounded by connective tissue containing extensive capillary networks.
Where are blood vessels relative to endocrine glands?
Embedded in CT on the other side of the epithelial basal lamina surrounding each clump.
How do hormone molecules get from an endocrine gland into the blood stream?
Hormones must cross the basal surface and basal lamina of the epithelium and the basal lamina and endothelial layer of the capillary to reach the blood stream.
From which surface do endocrine glands secrete their molecules?
The basolateral membrane
What regulates secretions from both exocrine and endocrine glands?
The autonomic nervous system (i.e., by direct neuronal stimulation), hormones from blood, or both
Identify a distinguishing characteristic of exocrine versus endocrine secretions
Exocrine glands often secrete continuously at a low rate with intermittent triggers to greatly increase secretion volume. Endocrine secretions tend to be tightly regulated by hormonal or neuronal stimulation.
Gives some general examples of epithelial diseases
Diabetes, various liver and kidney diseases, GI disease (gastric ulcers, IBD) skin diseases, hearing diseases, ciliopathies.
Describe ulcerative colitis
Epithelial disease of the GI tract in which extensive ulceration and destruction of the absorptive epithelium occurs
Describe pemphigus
an autoimmune disease in which antibodies are produced against components of desmosomes in skin, leading to extensive blistering.
Why do epithelial tissues require stem cell driven renewal?
Because of the high rate of turnover of cells and tissues
Why are the most common cancers derived from epithelial tissues?
High rate of exposure to UV, pathogens, environmental factors coupled to a high rate of cellular turnover.
What are cancers of epithelial origin called?
Carcinomas
What are cancers derived from glandular epithelium called?
Adenocarcinomas
Where do tumors usually first develop?
Within an epithelial sheet, but can subsequently become invasive and metastatic
How is the diagnosis and treatment of different carcinomas aided?
Carcinomas often retain some properties of their tissue of origin. Treatment is aided by histological appearance and knowledge of epithelial biology.
What regulatory pathways are targeted in epithelial cancer treatments?
Signaling systems directly controlling development (Wnt, EGF, Notch), internal cell cycle control factors, and factors controlling DNA damage repair and apoptosis.
When does scar tissue arise in skin epithelial injury?
When the basal skin layers and basal laminae are severely compromised. Severe burns may require skin grafts to replace a damaged basal lamina and restore a population of basal skin epithelial stem cells.
