Epithelia I,II,III Flashcards
What are some common properties of epithelial cells?
Adherent to one another; arranged in layers; polarized (apical and basal surfaces); basal lamina; high turnover (driven by epithelial stem cells); avascular (nutrients and O2 diffuse through connective tissue and through basal lamina to reach epithelial cells); highly diverse
What is the basal lamina
A sheet of extracellular material. Lines and is attached to the basal surface and is also attached to elements of the underlying connective tissue
What are some functions of epithelia?
Barrier; absorption; transport; secretion; movement; biochemical modification; communication; reception
What are 2 tissue types that are similar to epithelia but different? How are they different?
Endothelium (tisue that faces blood and lymph). Mesothelium (line enclosed internal spaces of the body cavities)
What is the epithelial to mesenchymal transition?
A process by which epithelial cells lose their cell polarity and cell-cell adhesion, and gain migratory and invasive properties to become mesenchymal stem cells. They may migrate and form new epithelia or may transform into non-epithelial cell lineages that give rise to other tissues.
Where are the blood vessels and many nerves that service the epithelia?
In the connective tissue (CT)
What are some exceptions to the general rule that epithelia is separate from blood/lymph/nerve etc?
Taste buds in the tongue and other specific sensory nerve cells that make intimate contact with specialized epithelial cells. Dendritic cells (from the immune system) that can migrate from epithelia to connective tissue to blood/lymph
What is the lamina propria?
AKA the CT direcly underneath the outer epithelium of the mucosae
What is the submucosa?
Deeper layers of CT. Has different properties and houses other tissues (bigger vessels and muscles, nerve axon bundles, etc.)
What are the analagous relationships for mucosa and skin?
Epithelium = epidermis. Lamina propia = dermis. Deeper CT = hypodermis
Describe the arragemet of simple epithelia
All cells arranged in a single layer or sheet
Describe the arragemet of stratisfied epithelia
More than one layer of cells in which cells of the outer layers do not directly contact the basal lamina
Describe the arragemet of pseudostratisfied epithelia
A special case where some cells do not reach the free surface (giving a stratified appearance), but all directly rest on the basal lamina
Describe the arragemet of squamous epithelia
Flattened cells
Describe the arrangement of cuboidal epithelia
Cells are cube-like
Describe the arragemet of columnar epithelia
Taller than they are wide
Describe the arrangement of stratified epithelia
Named according to their outermost layer (e.g. stratified squamous epithelia have a squamous outer layer, though inner cells are often cuboidal)
Describe the arragnement of transitional epithelia
Found around the bladder. A special case: these epithelia are stratified, but when stretched change their shape from cuboidal to squamous, and appear to decrease the layering: this is indicative of a tightly adherent epithelium that is very resilient and stretchable.
Describe tight junctions. What are the AKA?
AKA zonula occludens. Provide a highly selective barrier that limits or prevents diffusion of substances between epithelial cells. Also limit/control diffusion of membrane proteins through the plasma membrane bilayer. Ensure that substances absorbed or secreted must pass through the epithelial cell by specific transport pathways.
What are key core proteins of tight juncitons
occludins and claudins
Describe adherence junctions. What are they AKA?
AKA zonula adherens. Promote attachment but also polarity, morphological organization and stem cell behavior within the epithelial sheet. Contain cadherins
What do cadherins (in adherence junctions) do?
Link to actin filaments and other adapter/signaling proteins in the cytoplasm. Are transmembrane proteins with extracellular domains that interact with each other, and cytoplasmic tails that bind adapters and actin filaments. Some cadherin-associated proteins (e.g. beta catenin and protein kinases) control various aspects of epithelial polarity, development, and function.
Describe desmosomes. What are the AKA?
AKA macula adherens. Promote mechanical strength and resist shearing forces and promote the structural organization of the epithelial sheet. Contain a different class of cadherins
What do cadherins (in desmosomes) do?
Link to intermediate filaments and other adapter proteins
Describe gap junctions
Promote rapid communication between epithelial cells through diffusion of ions and small molecules
Which two part of the epithelia are polarized? What does this mean?
1: Plasma membrane (different surface have different transporters, ion channels, receptors, proteins, etc) 2: Cytoplasm (cytoskeleton; organelles and secretory vessels all have specific arrangements)
The tight junctions are located closer to which surface? What implications does this have for polarity?
Located closer to the apical surfaces. This means that the lateral/basal surfaces are often (though not always) very similar to each other
What is the function of epithelial cell polarity (3)?
Crucial to allow unidirectional secretion and/or absorption of molecules to or from one side of the epithelium. Also necessary for “trans-epithelial” transport of ions and macromolecules from the apical to basal surfaces (or visa versa). Last, important for localizing and orienting intercellular signaling either among epithelial cells or between epithelial cells and other cell types.
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.
What are two important apical surface modifications?
Microvilli and cilia
What are microvilli?
Cell surface extensions (protrusions) that contain actin bundles connected to cytoskeletal elements in the cell interior. Size and abundance varies with cell type
What is the primary function of microvilli? What does this do?
To increase surface area. This greatly increases the rate/efficiency of membrane transport and secretion
What are sterocilia?
Actually a microvilli (NOT cilia). Found in the epididymis and in sensory cells in the ear. Extremely long, actin-filled microvilli that function in the reception of sound
What are cilia?
Microtubule-containing extensions (protrusions).
What are the 3 types of cilia?
primary ciliia; motile cilia; sensory cilia
What are primary cilia? What do they do?
A single (one/cell) non-motile microtubule-based extension found on many different epithelial cell types. Primary cilia organize and promote signal transduction systems (receptors and effectors) that control epithelial cell division, fate (differentiated state), and function.
What are motile cilia? What do they do? Where are they found?
Related microtubule extensions that move and are found only on specific epithelial cell types. They wave like a boat oar to move mucous and other materials along passageways. Tend to be found in epithelia of the respiratory tract and in the oviduct (to assist in moving ova towards the uterus).
What are sensory cilia? What do they do?
Non-motile and appear to function in sensory reception (e.g. in vestibular hair cells of the ear). These sensory cilia are likely specific variations of primary cilia specialized to house sensory (touch, taste) systems that are connected to the central nervous system.
What are ciliopathies?
Mutations in proteins common to cilia or their support structures that result in a large set of diseases
What are some basolateral surface modifications?
There are some infolds and outfolds but they are not as organized as microvilli/cilia. These folds increase SA and likely to be seen in cells that transport heavily to or from their basolateral surface.
What is the basal lamina?
A thin sheet of extracellular material that underlies the basal surface of each epithelial tissue. Also surround many other cell and tissue types (endothelium of blood vessels/muscles/nerve tissue).
What makes up the basal laminae?
Formed by a special type of network-forming collagen. Forms sheets of thin fibers that are interwoven with a variety of other extracellular glycoproteins (e.g. laminins and entactin)
Are all glycoproteins in the basal laminae the same? Or nah?
Some are common to most basal laminae (laminins and entactin). Many are specific to different basal laminae
What synthesizes the basal laminae components?
Most are 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.
What are functions of basal laminae?
Mediate attachment of epithelia to the underlying connective tissue; contribute to selective filtration of substances diffusing to or from the epithelia; necessary for the establishment and maintenance of epithelial cell polarity; serve as specific �highways� for the migration of cells through connective tissue; provide a barrier to movement of invading microbes or cancerous cell into other tissues; control the gene expression of cells to affect their proliferation or development; �tissue scaffolding� function and thus are critical to repair of epithelial tissue
What do hemidesmosomes and focal adhesions do?
They are where epithelial cells directly connect to basal laminae. They are on the basal surface of the epithelial cell to components of the basal lamina.
What are integrins?
Key class of proteins that form the hemidesmosomes and focal adhesion connections
What do integrins of hemidesmosomes do?
Connect to intermediate filaments in the epithelial cell and provide strength to the epithelial-connective tissue attachment
What do integrins of focal adhesions do?
Connect to actin filaments inside the epithelial cell.
What do focal adhesions do?
Regulate epithelial polarity and function through signaling mechanisms. Important during re-establishment of an epithelium following injury or disease, or during the normal process of cell replacement (turn-over).
What are transit ampliflying cells?
Transitional intermediate cells. Arise from stem cells and divide/specialize until they become differentiated
What is a cell lineage?
A specific stem cell type, its intermediate progeny and their differentiated progeny
How may cell signaling of epithelial stem cells occur?
May be local (from cells w/in same epithelia or from neighboring CT); or long distance (from endocrine glands or nervous system)
What are 2 key principles for signaling pathways?
- Each pathway is used by multiple, very distinct stem cell systems in different organs/tissues (Most pathway components are shared by several distinct stem cell lineages). 2. A single signaling pathway often triggers different developmental outcomes in different stem cell lineages (result of diff. development histories; diff. environments; diff. in levels)
What does tarceva (erlotinib) do?
Inactivates the EGF receptor and is commonly used to treat lung and pancreatic cancers
What are the two major types of epithelial glands?
Exocrine and endocrine glands
What do exocrine glands do?
Secrete materials onto epithelia-lined surfaces or the outside world (onto the apical side). Can be multicellular (most) or unicellular (goblet cells)
What do endocrine glands do?
Secrete substances into the blood stream
What are the two main components of multicellular exocrine glands?
Secretory units and ducts
What are secretory units? What varieties are there?
Clumps of secretory epithelial cells that produce and secrete the bulk of the secretion. Can be bowl or flask shaped (alveali or acini) and called alveolar or acinar. If they are tubule they are called tubule glands. Can have tubuloalveolar. Can have multiple secretory units or single
What are ducts?
Tubular structures that emanate from the secretory units. Function as passageways to conduct secretions to their destinations and their epithelia can also modify the secretion content by secretory and ion transport properties of duct cells. Can have single duct (simple glands) or multiple branched ducts (compound glands).
What are the 3 main types of exocrine glands? What do they secrete?
Mucous (secrete a viscous glycoprotein-rich fluid); Serous (secrete a watery fluid w/salts and some proteins); Mixed.
What is the general structure of endocrine glands?
No ducts. Generally organized as clumps or chords of cells that are embedded with and surrounded by connective tissue w/ extensive capillary networks. Each clump surrounded by a basal lamina.
What structures must hormones secreted by endocrine glands cross? What does this imply for where they are secreted from?
Hormone molecules must cross the basal surface and basal lamina of the epithelium (and of course basal lamina and endothelial layer of the capillary) to reach the blood stream. For this reason, most endocrine cells secrete these molecules from the basolateral membrane.
How are glandular secretions regulated?
By the autonomic nervous system; hormones; or both. (Exocrine often secrete continuously at a low rate but can be stimulated to increase)
What are carcinomas?
Cancers of epithelial origin
What are adenocarcinomas?
Cancers derived from glandular epithelium
