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 all right so this is the second lecture for connective tissue proper and in this lecture we're going to talk about the different cells you'll find in connective tissue and the different types of connective tissue so now that we've talked about some of the important fibers let's talk about some of the important cells you'll see in connective tissue the main player for sure is fibroblasts you've heard us mention it a number of times throughout this lecture it's essentially the resonant cell of connective tissue they synthesize fibers mainly collagen and elastin as we've talked about among others and then also the ground substance as well one thing i want to point out though before we get into the appearance is first you'll have what's called a fibrocyte which is the inactive form it gets activated and that becomes the fibroblast which is the active form the active form meaning that it's actually synthesizing and secreting these fibers so on light microscopy you're going to see spindle or oval shaped cells with pale eosinophilic cytoplasm on h e staining specifically the nucleus will be oval shaped sometimes elongated and it'll contain numerous euchromatin that makes sense because it's transcriptionally active it's actively synthesizing proteins and secreting them electron microscopy again this makes sense it's going to have prominent rough endoplasmic reticulum again it's synthesizing proteins that are specifically made for export out of the cell and then they also have the golgi to help facilitate that and again this is part of its synthesis of fibers so if we come down here this is a section from the pelvic bone and we can see some nice examples of fibroblasts here here's one here where the nucleus is shows a lot of euchromatin again transcriptionally active it has that elongated shape to it like this here's some more up here here's fibroblasts here then let's try to find a fibrocyte for comparison so here's one here you can see the nucleus is much thinner much more dense demonstrating heterochromatin here's another one down here and so these are two good examples here where it's in comparison and then here's another one here where as you can see the nuclei are much thinner than these these ones up here which are the more at the active form the fibroblasts so lymphocytes are definitely a type of cell you'll see in connective tissue sections we're gonna give you a very superficial overview of lymphocytes here and some of the other cells over the next few slides and we're gonna go in much more detail about lymphocytes and other white and red blood cells in the hematology and blood lectures so here we'll just give you a general overview and kind of their histological appearance especially in connective tissue so lymphocytes they're a type of white blood cells so remember there's red blood cells which are erythrocytes which carry hemoglobin with oxygen and then you have the white blood cells which are cells that mediate the immune response and inflammation and lymphocytes specifically there are three cell populations there's the t cells the b cells and the natural killer cells and here we have a very superficial description of each of these immunology is an incredibly complex subject we're not going to really get into the finer details we're just going to give you a very very basic overview so you understand the basic function so t cells a good way to remember this is they develop in the thymus so t for thymus and the thymus is a lymphoid organ within the chest and t cells they have two basic kind of subtypes you have the cd8 positive cytotoxic cells it's in the name these are responsible for killing infectious cells often virally infected cells cancer cells and then cd4 helper cells kind of more mediate immune response they can activate b cells to produce antibodies and other immune cells as well and then b cells they constitute what's called humeral immunity via antibody secretions so when b cells get activated they can secrete antibodies this can occur in response to a cd4 helper cell activation then you have natural killer cells which are another type of cytotoxic cells now appearance on histology you can't really differentiate a t cell from a b cell or a natural killer cell really the only exception of that is an activated b cell which is called a plasma cell and we'll talk about that on the next slide but an inactivated b-cell t-cell natural killer cell you can't tell them apart so here we have a section actually from a lymph node which is very abundant when liposites and you what you'll notice is they have a dark round nucleus which a very with a very thin rim of cytoplasm around them so a way you could think of it as you have the nucleus like this and then you have a very membrane is very close around it oftentimes you won't even see depending on the cut you won't actually even see the cytoplasm or a very kind of small portions of it so if we look in the in here a lot of different cells so here you can see you know the nucleus is almost the entire cell here so here if you look at this cell right here you can see it has the nucleus taking up most of the space of the cell but then you can see kind of this little rim here of cytoplasm here so here's an example here where you know it's in pretty much the entire cell at least in this cut and then here you can see a little bit of cytoplasm here and you can see it's all throughout here all these are lymphocytes throughout here for the most part so a plasma cell like we talked about is an activated b lymphocyte that produces antibodies so it's actively producing antibodies the appearance on histology is an eccentric nucleus with clumped heterochromatin remember it has heterochromatin because it's transcriptionally active making proteins because antibodies are proteins it has what's called a clock face nucleus and so you can kind of see that here if you see that here almost like the numbers on the outside of the clock like this and one way i try to remember plasma cells is it's almost like a ghost like appearance so if you have it like this the nucleus is kind of out here eccentrically placed off to the off to the side here and then you have kind of this apron like or it kind of looks like a ghost on halloween like this and so you'll see that here so here's an example here here's that clock phase nucleus with the numbers on the outside and you can see out here here's that cytoplasm extension out like this and so that's definitely also how you could differentiate it from an inactivated b cell as well there's going to be a much more thin rim of cytoplasm versus here you have this large extension of cytoplasm here and usually the cytoplasm is basophilic and then here on this section here you can see as well here's the nucleus off to the side here and then you see the cytoplasm again kind of the ghost like appearance here where you have this extension of the cytoplasm out this way so eosinophils they're the primary cell responsible for fighting parasitic infections so if a patient with a parasitic infection their complete blood count is going to have elevated eosinophils and that's going to be a marker of parasitic infection clinically also eosinophils along with mast cells and basophils will help mediate allergic reactions and then their appearance on histology is pretty unique they have eccentric bilobe nucleus so their nucleus kind of looks like this it kind of looks like almost like a boomerang if you will and so it has kind of these two lobes to it like this or a v shape to it and then here's a good example of it right here so you can see the nucleus that's off to the side it's eccentric it's got these two lobes like this and then the other big hallmark is the red cytoplasm hence the name eosinophil eosinophilic cytoplasm here's another good example of that cytoplasm right here nice and bright red like this this is not a great example of the nucleus it's probably just the cut of the tissue but again again that good look of it right here we have this kind of bilobe nucleus like that so another component of connective tissue is the ground substance which is basically this amorphous gelatinous material that's present within the interstitial space and it surrounds cells and extracellular fibers and it's basically like this gel medium where a lot of these cells and these support fibers and structures exist to support the parenchyma and so they enable diffusion of nutrients and gases between the cells and blood vessels so let's say you have a blood vessel like this and then you have your you know parenchyma here like this and then so through here let's say you know you got like o2 coming in like this it would diffuse through this gelatinous medium here which is the ground substance which would also contain you know the extracellular matrix and then other cells as well and specifically it contains the following specialized fibers that help provide support to tissues and those would be glycosyl aminoglycans or gags and then glycoproteins so glycoaminoglycans or gags these are polysaccharide polymers so sugars that consist of repeating disaccharide units so remember disaccharide is two monosaccharides bound together so examples of that are like sucrose or lactose and so you have these sugar units like this so this would be your disaccharide and it's basically a repeat of these so you have these long polymers or polysaccharides and then one of these disaccharide units is sulfated and examples of this would be heparin heparin sulfate keratin sulfate and hyaluronic acid which is actually not sulfated and all gags except for hyaluronic acid bind to a core protein that forms proteoglycans that then would form large negatively charged molecules to attract water so you have kind of this core protein like this and then you have these repeating units like this attached to it all along this core protein on all sides it's a very crude drawing but you get the idea and by doing this this entire structure is negatively charged to help attract water to form the gel that forms ground substance so glycoproteins their main function is to facilitate the binding of cells to the extracellular matrix so let's come down here we'll draw a cell like this we'll draw the lipid bilayer like this we've got these two lines representing that so the first proteins we talk about are integrins and so they're a trans membrane protein which means they're going to come down like this and cross the lipid bilayer like that and what they do is they act to link the cytoskeleton within the cell to the extracellular basal lamina so we'll you know draw some actin fibers and other cytoskeletal fibers like this and then it crosses the cell membrane and links up with the basal lamina which will abbreviate bl then you have another protein which is known as laminin which is produced by the epithelial cells to form an attachment between the cell and the basal lamina via binding collagen type 4. remember collagen type 4 is found within the basal lamina so you'll have collagen type 4 and then you have this protein which we'll draw in blue here called laminin and so that links the collagen type 4 in the basal lamina to the integrins and then lastly here we have fibronectin which helps mediate adhesion and migration of cells so you have a cell like this you have a protein like this across the cell membrane and let's say it helps with adhering to another cell or it would help with say adhering to some kind of extracellular matrix structure and so that's what the function of fibronectin is neutrophils these are granulocytes that enter connective tissue during acute inflammation they're actually a marker or hallmark of acute inflammation and they're essentially the first phase of the cleanup crew they come in to clean up damaged tissue or infectious agents via phagocytosis so they'll engulf tissue debris from tissue damage or their villain golf infectious agents such as bacteria their appearance on histology they have a multi-segmented nucleus that's kind of their hallmark you can see that here so here these here's these different segments here and then you have this like bridging bridging portion here to this other segment another segment down here so very complex nucleus structure a lot of different segments and then they'll have kind of a pale or pink cytoplasm and you can see that here this is a section from a blood smear so this is a peripheral blood smear so that's why there's all these red blood cells here and then a mast cell so this is a granulocyte that mediates the inflammatory reactions mainly in response to allergens so if someone has say like a peanut allergy and they have an allergic reaction to it mast cells would be the main cells mediating that reaction to the peanut in that particular person they contain granules full of histamine and heparin and that makes sense because these mediate inflammatory responses such as significant vasodilation their appearance on histology is they have a very round nucleus and you see that here here's a great example so this round nucleus right here and then they have numerous bright red eosinophilic granules you can see that here all along their cytoplasm and one way you can distinguish these from eosinophils because they also have a pretty bright red cytoplasm is mainly the nucleus shape so here it's a round nucleus like this and a mast cell and then remember in eosinophil it's kind of that bilobe shape like this that v shaped if you will like that so on this slide we just want to mention the two types of embryonic connective tissue you should be aware of so first mesenchyme this is a gelatinous connective tissue consisting mostly of ground substance with sparse amounts of reticular fibers collagen type 3 and then mesenchymal cells and so if we come down here this is a section of mesenchyme found here and these mesenchymal cells you can distinguish them by seeing that they have this very oval-shaped nucleus with very fine chromatin very pale staining and then they have a very pale staining cytoplasm as well and so you can see these cells throughout this section here again so you know oval-shaped nucleus here and here and again very pale cytoplasm mesenchyme is derived from the mesoderm and the neuroectodermal neural crest origin so you have these germ layers the mesoderm being one of them and then also the neuro ectoderm slash neural crest and this gives rise to your mesenchyme and then mesenchyme differentiates into adult connective tissues so both connective tissue proper which has been the focus of this lecture and then other types of connective tissue as well such as cartilage bone adipose tissue and so on the other type of embryonic connective tissue you want to be aware of is called wharton's jelly and this is a mucoid connective tissue found both in the umbilical cord and the subdermal connective tissue of the embryo it contains mostly ground substance with very small amounts of reticular fibers and fibroblasts so histologically not very exciting even less exciting than this so we're going to round out this lecture by talking about the types of adult connective tissue so you have connective tissue proper and then you have special connective tissue so over the next few slides we're going to feature each of these types of connective tissue proper loose connective tissue dense irregular connective tissue and then dense regular connective tissue and then the special connective tissue so we talked a little bit about elastic tissue reticular tissue there's also adipose cartilage bone hematopoietic which includes blood and bone marrow and then lymphatic so all of these are also considered types of connective tissue and we actually have lectures dedicated to each of these so first loose connective tissue this is characterized by loosely arranged collagen fibers among abundant ground substance so the ratio of ground substance to collagen fibers favors ground substance so it's much as it's appropriately named it's much looser so you can see this here in this section right here the papillary dermis of the thick skin so you can see here's the epithelial layer here the stratified squamous epithelial layer of the skin and this would be the outer portion of the skin the outer surface of the skin here and then in here in the you know all throughout here this kind of pinky lightly stained tissue here would be loose connective tissue that provides support for the epithelial layer here and as you can see here it's much loosely arranged and this the reason it's stained pink and pale like this is because of the collagen fibers because because remember collagen is a protein and protein stains eosinophilic and so examples of where it's found are the laminopropia spaces between blood vessels and lymphatics so you can see this this is a section from a neurovascular bundle here's the artery dominantly shown here and then as you can see here there's kind of these loosely arranged collagen fibers here staining in this eosinophilic pinkish reddish color and then you can see in white here which is where you'd see the abundant ground substance then you have dense irregular connective tissue so the ratio flips here where you have a larger ratio of fibers to cells and ground substance and so as you can see here in this section here from the tunica albuginea of the testicle much more densely packed with collagen fibers as you can see here staining in this nicely bright red here and so one thing to note though as far as dense irregular versus dense regular is in dense irregular the collagen fibers are not uniformly oriented in one direction so they have two or more directions and you can see that here so here kind of down here you have this wavy direction here and then you can see up here where some of them are kind of shooting up this way and some of them are kind of going this way and so it's not a one uniform direction they're kind of oriented in multiple directions the fibroblast is the most common cell found in dense irregular connective tissue and then it functions as a structural support for the parenchyma and because of that it's often found on as the outer tissue of many organs such as the organ capsule the dermis of the skin and then also the epineurium which is a thin layer of connective tissue around nerves the submucosa which is again connective tissue below that epithelial layer the mucosa layer and then the periosteum which is a connective tissue layer around bone and so you can see that here's a great example of it here if i outline it here this whole section right here so you have the muscle layer of the artery shown here and then this whole section here this outer layer which is called the tunica adventitia is kind of this outer covering of the artery or protective covering of the artery and this is a great example of dense irregular connective tissue so you can see it's very densely packed with collagen fibers but the fibers are not oriented in one direction they're oriented in multiple directions and then lastly here we have dense regular connective tissue so again densely packed collagen fibers but the key is they're oriented in a uniform direction and by being oriented in a uniform direction that structure enables support for significant forces and stress so dense regular connective tissue is found very frequently in the musculoskeletal system specifically in tendons ligaments and epineurosis because these structures have to withstand a significant amount of force to support muscular contraction fibroplasts are usually the only cell found and so down here we have a section from skeletal muscle tissue so here down here this is this is skeletal muscle and you can see the striations here that's one way you can tell skeletal muscle from dense regular connective tissue such as a tendon is that you don't see the striations here because these are collagen fibers and notice how like we say in the definition they're oriented in one direction to help provide that support and significant force and then notice they're staying brightly red again densely packed with collagen fibers collagen fibers are made of protein so it makes sense they're going to stain brightly red eosinophilic and again you don't see too many cells you see one here and here and these would all be fibroblasts which are responsible for synthesizing those collagen fibers 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 foreign [Music] you