welcome to the da vinci academy histology video course the entire video course is available on youtube and covers all of the fundamental principles of histology and relevant cell biology you can find all the videos from the course by clicking the histology playlist link in the description below and then you can access the corresponding practice questions and histology lab videos by going to our website which is also linked in the description below welcome to the last leg of epithelial tissues lecture series here we'll discuss each exocrine glandular types in more detail and wrap up the whole series with some clinical pearls let's look at all the exocrine glands one by one we'll start with a simple tubular exocrine gland as the name suggests we're talking about a gland with a single duct and the secretory unit that looks like a test tube see if you can identify one or two or more in this image we're looking at the external environment here and underlying connective tissue here in between we have the lining or covering epithelium and when we trace this covering epithelium from this side look what's happening here the covering epithelium transitions and dips into the connective tissue so this is the glandular epithelium now which then curves back up and right around here it becomes a covering epithelium again which then dips down glandular epididylium and covering epithelium again so we've cut at least two really well represented communication of the covering epithelium with a glandular epithelium so each one of these is an exocrine gland and looking at the secretory unit it looks like a test tube and this test tube is open to the external environment by this short ductal region so this is a single tubular exocrine gland at closer observation of the secretory unit we see that a lot of the cells are columnar cells with pale staining apical regions these a lot of these are goblet cells that are producing mucin and then duct is fairly short here leading out into the external environment and such glands are typically associated or found in the colon or in large intestine in general and as you saw already there are actually many of these simple tubular exocrine glands in the corn or in large intestine that are lined up right next to each other so that's what we're seeing here we saw two right next to each other but over here that plane of section is not quite as clean as say here so we didn't quite catch that clear communication of lumen with the external environment but based on this entire image we can kind of decipher that there's that there's communication and here also here also and then we have some glands that were cut in an oblique fashion that we definitely did not catch the communication with the covering epithelium or the external environment but again we're kind of extrapolating from a three-dimensional structure so we have to imagine that these are actually leading out into the external environment either in front or back of this screen also imagine what this tissue may look like if we were to cut the organ in an oblique plane like this what would that look like it'll probably look like this so we're seeing again the covering epithelium that dips down into the glandular epithelium but we're not able to trace it all the way down in a clean fashion like this instead we've cut a sliver of that tubule here also here also and then we have all these quote-unquote orphan glandular epithelia that are seemingly just kind of embedded within the connective tissue but they're connected to the covering epithelium in a different plane than what we are seeing here so based on the uniformity of these oblique sections and also this uniform spacing of the glands with an even amount of connective tissue in between that can give us a clue that each one of these oblique circles are their own individual simple tubular exocrine gland likewise down here we've actually cut more of a transfer section through the base of these simple tubular exocrine glands next is a simple branched tubular exocrine gland so we're talking about a single duct exocrine gland with two or more tubule-shaped secretory units draining into it so such a gland is found right here in this image here's the external environment here's another external environment with the covering or lining epithelium which then dips down into the connective tissue to form a secretory unit that looks like a test tube and look there's another one right next door draining into a common outlet so really short duct and you know after drawing an outline such a gland kind of looks like a legging almost and over here it looks like we've cut a sliver of the simple branched tubular exocrine gland there and it also looks like this is one legging of the simple branch tubular exocrine gland and here's another legging they will probably and potentially drain into a common outlet up there we just didn't quite catch that in this plane of section so the secretory unit are typically comprised of the columnar cells many of which are the mucins creating goblet cells so that's what forms the tubules and again it's two or more of these tubules draining into a single really short duct leading out into the external environment and such ducts are typically associated with the stomach and small intestine simple coiled tubular exocrine glands are pretty neat little structures and they're typically associated with our skin and in this image here's our lining epithelium our skin histology and within it we're seeing this kind of spirally lumen that's traveling through this is the ductal component that's leading out into the external environment and deep in the connective tissue we're seeing the continuation of that duct down here down here probably and then all of these are the different planes of sections of that coiled exocrine gland so if i were to extrapolate what this tissue might look like or gland might look like it would be this coily duct up here which dips down into the connective tissue and then just starts to coil like so and then we've cut a sliver kind of plane of section like this so we've cut an area where the tubule is cut in transfer section so we see a proper circle or we've actually cut through a section of the coil or tubule that is in the midst of coiling like here and here's probably an oblique section more transverse sections you get the idea so with the coil tubular exocrine gland that histology may be quite complex that requires a little more mental gymnastics in our head to extrapolate the three-dimensional picture another thing to notice at this lomag image is that some of these epithelial tissues are staining lighter but here this one this one and that one are staining a little more darker and that has to do with the secretory versus the ductal component that's all coiled up there together so the lighter staining regions are the secretory units and the darker staining regions indicate the dots because the ducts are made up of smaller cells there's more nuclear crowding therefore the basophilia the secretory units themselves are actually comprised of the stratified cuboidal epithelium as opposed to a simple columnar epithelia which we kind of used as a generalization earlier on so this is an exception where the tubular gland is comprised of the stratified cuboidal epithelium we can see that when we zoom in onto this particular area and if we were to kind of outline one of these cross sections of secretory unit we can see he's a lumen here and then one layer of cells down here some with some without nuclei in this plane of section and then we have the apical compartment with more cuboidal cells with proper spherical nuclei so on and so forth so stratified cuboidal epithelium with the cytoplasm that's a little more plump compare that to a ductal component of this coil tubular exocrine gland this duct is also made up of stratified cuboidal epithelium but as we said earlier the cells themselves are smaller and with much smaller cytoplasm as well so there's nuclear crowding therefore the ductal epithelium looks a little more basophilic in staining such glands are associated with our skin these are our sweat glands that are releasing this sweat onto the external environment in order to help us temperature regulate simple acenar exocrine gland we're talking about a spherical secretory unit draining into a single duct and that's exactly what we have here here's the lining epithelia and here is the glandular epithelium that is kind of spherical that is draining into this short ductal area and this is actually an exception to that that general rule again where we said that the spherical acenar exocrine units are comprised of pyramidal cells beforehand but in this case it's actually the columnar goblet cells that are forming these spherical units and the ducts as we saw just now are fairly short and almost negligible and these type of exocrine glands are fairly unique to the urethral glands along the urethra particularly in male spongy urethra is where we'll find a simple a center exocrine glands secreting mucin into the lumen then simple branched acinar exocrine glands are the ones with number of spherical asynar draining into a single duct that's exactly what we see here here's the lining epithelium which then kind of invaginates into the connective tissue to form the glandular epithelium and this is the ductal portion and then we have the secretory cells lighter staining cells and are forming these spherical acini all draining into a common duct here a single duct and then the ductal glandular epithelium comes back up and continues on as the covering or lining epithelium again so that's an exocrine gland with number of spheres draining into a single duct upon closer observation of the secretory unit what we find is that the secretory cells are these columnar looking cells that are packed right up next to each other with little to no ecm in between typical of the epithelial tissue and interestingly enough the cytoplasm of these cells kind of appear almost bubbly because there are a lot of these small round vesicles that are non-staining or pale staining packed inside these cuboidal cells and these vesicles contain oily products that don't stain well with h nor e and that's why we're seeing all these bubbly looking stuff and again number of these spheres are draining into a single duct and in this case this what appears to be a long duct here is actually a hair follicle so that's a whole different story we'll cover in integumentary system lecture later but imagine a short duct that is draining into the hair follicle and this type of glands are exclusively found in the sebaceous glands these are the glands associated with hair follicles that produce a lot of oily substance to coat the hair as well as the skin surface now on to the compound tubular exocrine gland whenever we hear or see compound it should immediately indicate that we're now talking about a much larger exocrine gland that has many ductal systems that drain an appropriately shaped secretory unit and in this case this tubular secretory unit that's what we see here here is a group of secretory units that are all packed together we call that a labio smaller grouping of secretory units and here's another these lobules are fairly well demarcated in terms of how they're separated away from the connective tissue here's another lobule a bunch of these lobules in compound exocrine glands are structurally organized into a bigger kind of grouping called the lobes so it may be easier to kind of imagine a a bunch of grape let's say that this is a main stem and off of the main stem we have a smaller stem and off of that we have smaller stem still and then off of the smaller stem we have these grapes hanging off so then you can imagine these grapes as the secretory units and this smaller grouping of grapes as a lobule and then a bunch of these lobules coming together and draining into this common bigger stem as a lobe of secretory units and imagine this happening throughout the grape bunches like so like so and of course this being a three-dimensional structure we have to imagine another one of these lobul and lope organization kind of going into the screen in three dimension and perhaps another bunch coming out from the screen towards us and then imagine making a single transfer section through it and then imagine what that cut section might look like so that's why compound exocrine glands are a bit complex to look at but with practice it'll get easier and easier at any rate the duct as you can imagine in this kind of grape bunches analogy that they become progressively larger coming off of the lobule to then draining the lobe to this main stem which then becomes continuous with the lining epithelium eventually to drain out into the external surface the compound tubular exocrine glands are fairly rare in our body and are limited to a small section in our small intestine called the duodenum so they're called the duodenal glands where sometimes they're also called the bruners glands which you may actually have heard of before it's an eponym named after someone in the world of histology and embryology we're now trying to move away from using the eponyms and going with the terms that are more anatomically and histologically indicative compound acenar exocrine gland is another type of large exocrine gland with impressively complex ductal systems and lobules and lobes and the acini that comprise the secretory units so in this low magnification image we're seeing a lot of the field what we see are these lobular organization of the secretory units so lots of lobules which are then collectively organized into a lobe and within the lobe and even lobules were kind of seeing a potential duct or two and these look like larger ducts in the middle so we won't know whether these are indeed docked or not until we start to zoom in but we're making again the best possible guess before we start to zoom in at the lowest possible magnification in terms of the secretory units these are more typical of the general rule that we discussed earlier in that the ace and i are typically comprised of the pyramidal cells that produce serous secretion so here's one sphere or acins here's another here's another a little out of focus but here's another so these secretory units are really kind of packed tightly in there in a given lobule and looking at this one aciness the lumen in the middle is not quite as apparent and that may have something to do with the plane of section but i'm just going to draw one drawn a lumen and then the glandular epithelium then is comprised of those typical serous secreting cell morphology this pyramidal cell with spherical nucleus the basal basophilia due to a large amount of rrna in the ribosomes and the apical compartment staining kind of granular and eosinophilic due to protein-filled vesicles that's packed up there in terms of the ductal systems they're quite complex but they do become progressively larger as we get closer to the external environment and within this lobule we've cut this one this is a duct that is made up of a simple columnar epithelium because these cells are tall columnar cells compound acinar exocrine gland comprise our parotid salivary glands these are these large salivary glands that sit on our lateral sides of the jaw just in front of our ears and this these are the glands that produce a lot of watery saliva when we see a bowl of strawberry or bunches of grapes or something and our our mouth gets all watery that's parotid salivary glands that produce lots of watery secretions with amylase the protein in it the enzyme that can break down the carbohydrates in addition to parotid glands i should write down here that pancreas in our body is also comprised of the compound acenar exocrine gland and lastly the compound tubuloacinr exocrine gland is just as dimensionally complex as the other compound exocrine glands but this time the secretory units are actually the mixed type with the tubular as well as the acinar component comprising this entire gland so macroscopically or low magnification we're seeing the lobular and low bar organization with these smaller lobules forming a lobe and things like that and with perhaps progressively larger ducts interspersed in there the secretory units as we start to zoom in we'll be able to see some tubular components that are made up of so you can see these columnar cells and the apical components of these columnar cells are staining pretty pale and that has to do with the fact that these are a lot of goblet cells that are forming these tubules and then we have the next door a secretory unit that is staining a little more eosinophilic throughout which is indicative of the aciness and here's another asymus here and then unique to the compound tubular acinar exocrine glands are these things called the demi loons demi refers to have loons refers to a moon so we're talking about a half a moon literally shaped like half a sphere or half a circle that's capping the end of a tubule so that's what we're seeing here we see a lot of demi loons that's included within this secretory units so if i were to identify additional demi loons there's one there's one and like the other compound exocrine glands the compound tubular exocrine glands have the system of ducts that become progressively larger here's a duct that is most likely a simple columnar epithelial tissue and these compound tubular acenar exocrine glands comprise our submandibular and sublingual salivary glands so they produce saliva that are the mixture between mucus and kind of watery and proteinaceous fluid as for how the secretary products in these secretory cells are released out into the external environment there are three different ways this can happen one is by the merocrine secretion this is an exocytosis of the secretory vesicles that just means that in the cell with secretory vesicles in the apical compartment some of these vesicle membranes will fuse with the apical cell membrane like this and then whatever used to be in the vesicles will then be released into the external environment so that's the merocrine secretion and the majority of the exocrine glandular cells will release their products via the merocrine mode of secretion and then we have the epicrin secretion and this is when the apical compartment of the cell gets packed full of the secretory product so something like this an apical compartment gets filled with product and then that compartment of the cell gets pinched off like this and then that pinched off part gets released out into the external environment and and if i were to diagram that would look like this here's the nucleus and the rest of the cell cytoplasm and then the apical compartment getting packed full of secretory products and then that entire compartment getting kind of lopped off of the rest of the cell and this is the part that gets released into the lumen and then the what's remaining of the cells will then continue to produce more secretory vesicles and get taller with apical compartment packed full of stuff which will then get pinched off and then that cycle will continue and this happens in the mammary glands that produce milk and lastly we have the holocrine mode of secretion and this is when the entire cell gets packed with the product and then the entire cell is then shed into the lumen so we've seen this before this is the sebaceous gland asynus comprised of these cuboidal cells and at the base and from the look of it this is the base and this is where we're getting closer to the duct and from the basal compartment of this asanas we're seeing these cuboidal cells with proper and big and spherical nuclei and the cytoplasm that's looking kind of foamy or or bubbly because they're starting to pack their cytoplasm with this oil filled vesicles and then as we approach the ductal compartment we're starting to see the cytoplasm looking more and more pale because it's now getting more and more packed with lipid-filled vesicles and the nuclei that's starting to condense and kind of fragment and actually die away so here are some cells with a fragment of dead nucleus and some cells actually entirely without any nuclei and these are the dead cells packed full of lipid vesicles that are now being released into the duct so that's holocrine secretion and this happens in the sebaceous glands so these are the three modes of secretion in the exocrine glands there are many clinical considerations with epithelial tissues there's a concept of metaplasia where one type of epithelial tissue becomes changed into another type of normal epithelium and although this new tissue is normal it's in an inappropriate location for that tissue so that's the problem a prime example of metaplasia is barrett's esophagus this happens in the lower esophageal lining where the typical epithelium is the non-keratinized stratified squamous epithelium but when this region of the epithelium gets constant exposure to the stomach acid which is a stressor it can change the epithelium into the simple columnar epithelium that's comprised of mostly goblet cells and goblet cells will then release mucin in order to protect this area from the continued stomach acid exposure so it sounds as if this is protective and adaptive but unfortunately such metaplegia is typically pre-cancerous condition and continued exposure to the stomach acid and the stressor will then progress from metaplasia to dysplasia where the cells will actually become dysplastic or cancerous and certainly metaplasia happens most commonly in the epithelial tissues but this can also happen in the other general tissue types as well like in the connective tissues where one type of connective tissue changes into another type of connective tissue that's inappropriate for that location now speaking of dysplasia most cancers in the body arise from the epithelial origin consider why it's because of the fact that epithelial tissues are comprised of the renewing cell population that have high mitotic index in order to divide and continuously replace the cells that are shedding on a continuous basis so with high mitotic rate there's more opportunity to acquire mutations and accumulate them to the point where those mutations are no longer repairable and then we are developing cancer from there on the slow growing or benign form of epithelial derived tumors are called papillomas if they arise from the covering epithelium and adenomas if they arise from the glandular epithelium and once the tumors become more fast-growing and malignant type then we call them carcinomas or adenocarcinomas if they arise from the glandular epithelium and for this type of malignancies consider what histological features would indicate that a particular tumor is of epithelial origin and that goes all the way back to the tissue composition we've come a full circle where the epithelial derived cancer would have self-rich and ecm poor composition and if they're still maintaining some of the epithelial tissue characteristics there may even be a level of polarity thank you for watching this video from the davinci academy histology video course which is completely available on youtube to access the corresponding practice questions and histology lab videos go to our website using the link in the description below [Music] foreign [Music] you