CTO - Lecture 6 Epithelium

What is the role of epithelia in contractility?

(contract to clear secretions from glands) there are specializations of epithelial cells for contractility such as myelin epithelial cells that help move secretions from some glands

What are the functions (4) of basement membrane?

1) limit growth and invasion of the connective tissue by the epithelium 2) provide a point of contact and scaffolding for the epithelium 3) allow diffusion of nutrients from underlying connective tissue, and 4) giving epithelial cells various molecular signals involved in growth and maintenance of the epithelium

What are 3 different mechanisms of secretion used by exocrine glands?

1) merocrine secretion - vesicles fuse with apical membranes, release contents via exocytosis (used for proteinacious or watery secretions) 2) aprocrine secretion - includes budding off of cytoplasm from surface of secretory cell, buds contain vesicles 3) holocrine secretion is where the cell disintegrates and contributes all of its contents to the secretion, these secretions are very oily and complex.

In how many days does epithelium renew itself? why is this the case?

15-30 days, very mitotically active; apical surface is exposed to outside and to damage - thus it needs to have a way to replace cells constantly, does so by being very mitotically active. The surface layers are thick and designed to slough off, with constant replacement from below.

How many connexons are required for a complete channel?

2; although one junction can have several hundreds of connexon pairs

What is the composition of cilia?

9 doublets +2 arrangement of microtubules, basal body made up of 9 triplets (same as centriole)

Question 65. What are the functions of epithelia?

Answer 65. The functions of epithelia include protection, absorption, secretion and sensory reception.

Question 66. What are the two sides of an epithelium called?

Answer 66. Epithelia are polarized with an apical domain on the side facing the surface, and a basal (basolateral) domain facing underlying connective tissue.

Question 67. What are microvilli?

Answer 67. These are projection of the cell surface with an actin core that increase surface area for absorption. Actin filaments are anchored in the terminal web. They are not motile in themselves, although if the terminal web is moved, the microvilli spread apart.

Question 68. What cell surface structure is motile?

Answer 68. The vast majority of ciilia are motile (microvilli and stereocillia are not). These have a (9+2 microtubules organization; axoneme). Dynein is critical for moving the arms. There are some "primary cilia" that are not motile (and are lacking the central microtubules). Some cells (such as sperm cells) have a single flagella that has a similar structure to a large, long cillium).

Question 69. What is a stereocillia?

Answer 69. These are not motile and are essentially long microvilli. They are found in the inner ear and the epididymus.

Question 70. What does the junctional complex of epithelial cells consist of?

Answer 70. The junctional complex is comprised of three different junctions. The one that is closest to the apical surface of the epithelial cell is the zonula occludens (aka tight junction). This goes all the way around the cell and is a site where the plasma membranes of adjacent cells are tightly opposed. The tight junction limits the ability of materials to pass between adjacent epithelial cells (paracellular pathway). The main channel proteins that connect the adjacent cells are occludin and claudin. The zonlula adherens is the next junction; actin filaments of adjacent cells are linked with transmembrane proteins of the cadherin family. Desmosomes are the third junction that are located a little further from the surface of the cell. These are "spotwelds", where cadherin proteins are liked to intermediate filaments.

Question 71. What is the basement membrane?

Answer 71. All epithelia rest on a basement membrane. It is a layer between epithelium and connective tissue that stains well with PAS because it has lots of glycoproteins. It has type IV collagen fibers, laminin, proteoglycans and many other components. It is important for support and migration of epithelial cells.

Question 72. What are hemidesmosomes?

Answer 72. These are structures that bind intermediate filaments to the basement membrane with integrins.

Question 73. What is the definition of a simple epithelia and what are the types?

Answer 73. A simple epithelium has every cell resting on the basement membrane. The types include: simple squamous, with cells of flattened shape; simple cuboidal, with box-shaped cells; simple columnar, with tall cells, usually having microvilli or cilia; pseudostratified, where every cell touches the basement membrane even though there appear to be several rows of nuclei.

Question 74. What are the types of stratified epithelia?

Answer 74. Stratified squamous epithelia are found in places wehre there is exposure to abrasion or other physical trauma. In moist places it is non-keratinized, while in dry places it is keratinized. Stratified cuboidal is quite rare, though it is found in certain large ducts. Stratified columnar is very rare, only appearing in epithelial to mucous membrane transitions (anus).

Question 75. Which type of epithelium is most likely to be ciliated.

Answer 75. Pseudostratified columnar is usually ciliated, being most common in the respiratory system.

Question 76. Which type of epithelium is found in the urinary bladder?

Answer 76. Transitional epithelium (urothelium) can change shape depending on stresses placed upon it. Stretching of the organ flattens cells, while a relaxed organ results in pillow-shaped cells.

Question 77. What are the two main types of glands?

Answer 77. Exocrine (secrete onto a surface or into a duct) and endocrine (secrete into the circulation) glands.

Question 78. What are the modes of secretion of exocrine glands?

Answer 78. Merocrine glands are the most common. Vesicles fuse and expel contents of the vesicle. Apocrine glands secrete by budding out entire vesicles (e.g., mammary glands). Holocrine glands secrete by disruption of entire cells, with release of all of the cell contents (sebaceous glands).

Question 79. What is the most common unicellular gland?

Answer 79. Goblet cells are common in the GI and respiratory tracts. They secrete mucinogen (will make mucus in presence of water).

Question 80. What is a simple gland?

Answer 80. A simple gland consists of a secretory portion (e.g. tubular, acinar) and one unbranched duct.

Question 81. What is a compound gland?

Answer 81. If the duct of a gland is branched, then it is a compound gland. The secretory portion can be of the tubular type, alveolar type (aka, acini; a ball of secretory cells) or a combination of both (tubuloalveolar - one of the more common architectures)

What do simple columnar epithelia look like? where are they found?

Columnar cells are tall. Usually the nucleus is located somewhat closer to the basal surface of the cell, depending on the cellular contents. This kind of epithelium lines many internal structures including smaller bronchi and the intestines.

What is the role of epithelia in sensory reception?

Epithelia are specialized for sensory reception in many parts of the body. This includes many of the special senses including vision, smell, taste, and hearing. Since epithelia cover the body, many nerves enter and allow for tactile perception (touch), as well.

All the glands of the body are comprised of _______________ cells.

Epithelium has specializations for secretion. Here you can see the gland with the duct leading onto a surface lined with epithelium.

What are exocrine and endocrine glands?

Exocrine glands release secretions into duct, on surface of epitheluim. Endocrine glands release secretions into the circulatory system. They are formed as an ingrowth from epithelium.

What attaches the basal layer to the basement membrane?

Hemidesmosomes and (focal adhesions)integrins (transmembrane proteins) attach to fibronectin and laminin on basement membrane. Other integrins attach to the actin cytoskeleton

What is the anal transition zone?

In anus, there are multiple types of stratification and cell types (transitioning from one to next) - simple columnar --> stratified columnar --> stratified squamous - this is very rare

What are junctions?

Lateral surface has connections with adjacent cells.

What is the medical term for desmosomes? what type of protein do they require interaction with?

Macula adherens (strongest one) aka spot welds; The cadherens in desmosomes are anchored to intermediate filaments through additional sets of binding proteins. Because intermediate filaments are much stronger than actin filaments, these provide strong attachment points for cells.

Is the tight junction/zona occludens completey tight?

No - not completely water tight, some passage through this barrier is possible and movement of substances this way has been termed the paracellular pathway. The amount and size of substances that can pass this way is regulated by the density and organization of the integral membrane proteins.

What can stain (to visualize) the basement membrane?

PAS - stains proteoglycans in basement membrane and can be seen as a pink line; H and E won't work

What are serous cells?

Secretory cells that release lots of protein (enzymes), have many ribosomes and stain blue in H and E staining.

What allows cilia to move?

The doublet MTs have dynein arms that reach out to adjacent doublets and participate in movement - requires energy.

How can microvilli move?

The terminal web (actin) can be pulled on by myosin which can result in movement of the attached microvilli. Contraction of terminal web (actin pulled on by myosin) decreases the diameter of the apical surface, causing microvilli to move apart. This allows microvilli to be exposed to more extracullar components.

What are the 4 types of tissue?

There are four tissue types in the body: epithelium, connective tissue, muscle tissue, and nervous tissue.

How do ZO, ZA, and D (MA) look like in ECM of an intestinal epithelial cell?

This arrangement of the junctional complex, including zonula occludens, zonula adherens and desmosome is very easily seen in this electron micrograph of an intestinal epithelial cell On the right.

Image of coiled tubular glands

This is another example showing how a coiled tubular gland will look. You can also see the docs that are extending from the skin down towards these deeper glands. Even these are cut obliquely. You can imagine from the figure on the left how many different shapes one could see why cutting such a coiled tubular gland.

What is metaplasia?

When cells exposed to unusal stress adapt by differentiating into a cell type that better equips them to withstand an environmental stress. For example, pseudostratified cells in chronically irritated respiratory passages can change to squamous. Similarly, the non-keratinized stratified squamous cells of the lower esophagus can change to columnar gastric cells if exposed to chronic acid from reflux disease. Unfortunately, this metaplasia often predisposes cells to become cancerous. Cancerous epithelial cells are termed carcinomas. It's no wonder that carcinomas represent the largest fraction of cancers.

What is a tissue?

When groups of cells that are similar in structure come together and perform a common or related function, the group of cells is called a tissue.

What is the medical term for adherent junctions? what type of protein do they require interaction with?

Zonula Adherens, cadherins (calcium dependent transmembrane proteins) are anchoring proteins that attach to actin filaments on their intracellular domain

What is the medical term for tight junctions? what type of protein do they require interaction with?

Zonula Occludens, most apical junction, require interaction with integral membrane proteins on adjacent epithelial cell membranes

What is the basement membrane and what is it made up of?

acellular protein layer between epithelium and connective tissue, made up of ECM proteins - proteoglycans, type IV collagen, fibronectin, laminin, and entactin. It's made by epithelial cell and fibroblasts in connective tissue, but there are no cells in the basement membrane layer

What are the acinus and duct portions of gland?

acinus - secretory portion, duct - conducting portion

What are gap junctions and what do they do?

allows communication between cells by sharing small molcules - allow small ions and molecules to pass (small water-soluble ions and molecules) between cell and adjacent cell.

What are desmosomes and what do they do?

anchors one cell to other and also attaches intermediate filaments in one cell to those in a neighboring cell (IFs are much stronger than actin, so this provides a large amount of the strength of attachment between cells).

Epithelial cells are polarized, with each cell having an _______ component that is close to or touching a surface, and a ________ aspect that is contacting underlying connective tissue. _________ surfaces of the epithelial cells contact other similar cells.

apical; basal;

What mode of exocrine secretion is used by the mammary glands to secrete lipids?

apocrine

What is the basement membrane?

basal surface contacts basement membrane just under it. Think of a floor for a home.

Why does the terminal web look like an eosinophilic line under the microvilli?

because of its concentrated protein content

What could result from the failure of connections on apical or basal surface? (junctional complexes/hemidesmosomes/focal adhesions)

blistering diseases (can be caused by congenital abnormalities or by immune distraction) arising from separation of epithelium from the basement membrane or of the basement membrane from underlying connective tissue

microvilli is known as __________ in kidney tubule cells.

brush border

What are carcinomas?

cancerous epithelial cells; represent the largest fraction of cancers (metaplasia/unusual environmental stresses can predispose epithelial cells to become cancerous)

What kinds of cells are gap junctions especially critical in? think about their function...

cells that need to coordinate with one another - cardiac muscle cells (pass both nutrients and molecular signals this way)

what cells of the primitive node are essential for establishing the left-right axis of the body during early phases of gastrulation?

cilia

Mitosis occurs in the __________ layers of the epithelium and entire skin replaces itself atleast monthly.

deepest

What are compound glands?

ducts usually branch, secretory cells can be tubular, acinar, or tubuloacinar (both)

What are simple squamous cells that line blood vessels called?

endothelium - are in connective tissue beneath epithelium; are known as mesothelium in mesentaries (a fold of the peritoneum that attaches the stomach, small intestine, pancreas, spleen, and other organs to the posterior wall of the abdomen) around internal organs

What are epithelial tissues and why are they important?

epithelial tissues come in 2 configurations - sheets that line body surface and cavities and glands that secrete materials. Epithelia are important because they provide the main barriers between the outside world, including the contents of the gut, and the delicate internal environment of the body. They are also extremely important clinically because they are the origin of carcinomas.

What are pseudostratified epithelium?

found in upper respiratory airways; look like they are stratified but are actually a single layer; all of the cells contact the basement membrane. Therefore, is not truly stratified. Typically found as ciliated

What do tight junctions look like on ECM?

fused

What do simple cuboidal epithelia look like?

have much more cytoplasm and round nucleus

What are simple glands?

have single unbranched duct, secretory cells are at the deep end of duct and can branch or coil or just line the duct, can be tubular or acinar

What mode of exocrine secretion is used by the sebaceous glands of hair follicles?

holocrine secretions - contribute to oil that coats hairs

Which integral membrane proteins are involved in tight junctions (zonula occludens) and what structure do they form?

integral membrane proteins occludin, claudin form tight junctions by interacting with similar proteins in adjacent cell membranes. These form a rim around the entire apical portion of lateral border - you can liken this to a zip lock structure between adjacent cells to prevent passage of water-soluble molecules.

What are the functions of junctional complexes on the lateral surface of epithelial cells?

involved in cell to cell adhesion, critical for ability of epithelial cells to stay together and also prevent molecules from passing between cells.

What are cilia?

involved in movement, made up of microtubules, long hair-like projections covered by plasma membrane and can be motile or immotile. This picture is from the trachea where they move inhaled substances up towards the mouth

what is a primary cilium?

involved in sensing the environment and sending signals to nucleus, is a sensory receptor in the olfactory epithelium and retina; Many cells have a single, immotile primary cilium. It lacks the central microtubular doublet of other cilia and can be described as having a "9+0" arrangement.

stratified squamous epithelium can be characterized as either _______________ or non-______________.

keratinized

What are flagella?

large cilia found in spermatazoa, one per cell - permit movement. The central cilia is surrounded by mitochondria. Image shows spermatozoa with flagella associated with ciliated epithelium of a uterine tube.

What is the urothelium (transitional epithelium)?

lines bladder and ureters; has the appearance of a stratified epithelium however, it can appear almost squamous when the organ is stretched. Notice that when this is relaxed the cells are almost circular including the dome shaped cells at the surface. Urothelium consists of approximately 3-5 cell layers and is able to stretch significantly to accommodate large volumes of urine.

What are the functions of epithelial tissues?

lining of surface or body cavities, glandular secretion

How can you tell the difference between cilia and microvilli?

look at the size, density, and and length, microvilli are typically less than 1 um, cilia are between 2 to 10 um.

What are simple squamous epithelia and where are they found?

look like a fried egg, bulge at nucleus and cytoplasm almost looks invisible - you can see epithelium lining the kidney structure in pic (renal corpuscle)

What are microvilli?

made up of actin, they are fingerlike projections of cytoplasm covered by membrane; increase surface area, this is designed to increase absorption

What is a multicellular gland? what are they connected to?

made up of multiple cells, are connected to ducts with their secretory products being released onto the surface of the epithelium. This could be into the GI or respiratory tract or it could be onto the surface of the body (sweat)

What is a unicellular gland?

made up of one cell, classic example is goblet cell, which manufactures and releases mucin granules via exocytosis- look like goblets/wine-glasses in cell (cool). They are found in respiratory and GI tracts where they are involved in lubrication and protection

What are adherent junctions and what do they do?

made up of transmembrane proteins that join cells together and also join actin filaments between adjacent cells (joins an actin bundle in one cell to a similar bundle in a neighboring cell, joining the cytoskeltons). This provides some degree of continuity of the terminal web and gives some tensile strength.

What does skin epithelium look like?

many layers

What does "epithelial cells are polarized" mean?

means that they have an apical, basal, and lateral component (a = first letter = top, b = bottom, lateral = side)

What is the most common type of exocrine secretion used by most organs including digestive glands, salivary glands, and most sweat glands?

merocrine

What is made up of a core of actin filaments anchored to villin (at the top/apex) and terminal web (at the base)?

microvilli

What 3 specific components/specializations are on the apical surface?

microvilli, cilia, and sterocilia

How are signals carried from primary cilium to cell nucleus aka IFT (intraflagellar transport)?

motor proteins drag particles up and down the cilium and carry signals processed at the cilium's top into the cell nucleus, were they can influence cell growth and development

What is the function of cilia?

move materials (like secretions) along the surface of the epithelium; there is a slow, active bending of cilium in one direction with a passive recoil

What are sterocilia?

much larger microvilli found in a few discrete locations - immotile, found in epididymis and make up inner ear hair cells.

Can microvilli move on their own or change shape?

no! Contraction of terminal web (actin pulled on by myosin) decreases the diameter of the apical surface, causing microvilli to move apart.

Basal bodies are seen an ________________ line beneath the cilia

profound eosinophilic staining, you can see cilia are longer than microvilli

What are the five functions of epithelium?

protection, absorption, secretion, sensory reception, and contractility

What is the category of epithelial lining of respiratory airways? How about GI tract? How about mouth and skin? How about air sacs in lungs?

pseudostratified columnar, simple columnar, stratified squamous, simple squamous

What are non-keratinized stratified squamous cells?

resist abrasion, classified by shape of cells in surface layer, the deeper cells near the basement membrane seem cuboidal but flat cells at surface make this stratified squamous (esophagus). Food moving past this region could erode the flat squamous cells on surface but would leave the deeper layers below intact

Where are cilia located?

respiratory tract, middle ear cavity, brain ventricles, and uterine tubes

What are seromucous cells?

secrete both protein and mucus

What are mucus cells?

secrete mucus, these cells appear foamy because mucin is usually dissolved in typical histologic preparation

what are acini/what is acinar?

secretory cells in glands appearing as a hollow ball of cells.

what are simple epithelia? stratified epithelia?

simple are single layer; stratified are multiple layer

What does small intestinal epithelium look like? How is its structure conducive to its functions?

small intestine epithelium is optimized for nutrient absorption, it is a single cell layer thick - this is the absorptive surface that nutrients must pass through without introducing harmful intestinal contents into the bloodstream.

what are squamous cells? cuboidal? columnar?

squamous are flat, cuboidal are nearly equal in legnth and width and/or height, columnar are tall

microvilli is known as __________ in intestinal cells.

striated

There are few stratified cuboidal cells - where are they found?

sweat ducts

Where is basal body located? (base of cilium)

terminal web at the apex of apithelial cell, basal body anchors cilia to cytoskeleton

What are keratinized stratified squamous epithelia?

the top layer of cells is dead, organelles are replaced by keratin (IFs), resists abrasion and is impermeable to water; found in dry areas such as skin surface

How do epithelial tissue obtain nutrients?

there are no blood vessels within the epithelium, instead they rely on diffusion of nutrients from blood vessels within the supporting connective tissue.

What and where are tight junctions? (zonula occludens)

they are very close to the apical surface. These seal the cells together and prevent leakage of molecules between the cells.

____________ junctions can also affect the distribution of membrane proteins by defining/separating domains of the cell - and preventing movement of these proteins between sides/domains

tight (zona occludens)

What are 4 types of junctional complexes on the lateral surface of an epithelial cell?

tight, adherent, desmosome, gap

What are the 3 general features of epithelial tissue?

tightly packed cells, free surface, and are supported by connective tissue

What type of protein do gap junctions interact with?

transmembrane proteins - 6 connexins - form channels called connexons (organized around central pore/channel that can be opened or closed). Connexons of cells align to form a pathway for passage of molecules.

Image of gland in microscope (cross-section)

you may or may not see duct connecting to acinus, and may not see lumen at the center of hollow ball of cells (acini)