right as we begin our study on tissues I want to start with cell Junctions cell Junctions are ways that cells are connected to each other if your cells are going to combine to form a tissue then they have to have a way to connect together so when you're talking about cells they're surrounded by this extracellular Matrix this is basically a fluid gel-like substance has lots of carbohydrates in it some proteins um but this extracellular Matrix helps do a lot of things that have to happen in your SE in your body uh regulates your cell growth movement and differentiation or in other words the ability of stem cells to turn into very specific mature cells um there are a lot of different macromolecules found in The Matrix collagen elastin and fibronectin these are connective uh sort of fibrous types of proteins you have proteoglycans hyaluronic acid and you also have cells called fiberblast that are important in tissue repair now cell to cell adhesions are the way cells are held together they're points of Junction between cells and they allow the cells to group together to form tissues and organs now whenever you're going to have a um group of cells they're going to combine together in a certain manner you're going to have a basil surface if it's a free tissue like an epithelial tissue you may have an open surface here called an apical surface and then where cells go together or the lateral surface now if you're going to have an open surface here you're also going to have a basement membrane and this is to allow the tissue to connect so in this case this is epithelial tissue up here this is connective tissue down here and the epithelial tissue is connected through a basement membrane we'll talk more about that shortly now cells are held together through what are called cams or cell adhesion molecules these are proteins on the surface of the cell so they're embedded in the lipid B layer but they help hold cells together and to connect cells to The extracellular Matrix there are many different types of protein families you don't have to identify them from this diagram I just want you to get an idea of what they're doing um but you have things like integrin caterin selectins there's also an imun globulin super family uh fibron neatin there's others present as well but this is how they function they hold the two cells together by their membranes now you do in some cases have specialized cell Junctions these also hold the cells together and they're combinations of all those cell adhesion molecules but they provide very specific types of connections depending on what the tissue is um and there are s several types here and we're going to talk about all of these in particular and what their function is so ad herin Junctions I recognize these because they look kind of like a belt around the two cells holding them together and then you have these cadherin or these cams in between um adherance Junctions are found in most of your epithelial tissues and they hold the two cells together because you have these cads or cams these cell adhesion molecules that are bound to these in cellular proteins or plaques um where they're stuck through the lipid B layer into these proteins that help hold the cells together epithelial tissue are lining tissues we're going to talk about those next but it's really important that you have good connections between your epithelial tissues those are that forms things like Skin So you want the cells of your skin to be held together pretty tightly desmosomes are similar but you don't have a belt on the inside in fact it looks more like a little button uh but you're going to have these cell adhesion molecules and they're connected to a network of proteins inside that hold it together um is just more of a button type uh desmosomes are very strong uh you're going to find these in epithelial tissue but also uh holding your cardiac muscle cells together in your heart uh your bladder and your GI tract as well we'll have desmosomes Hemi desmosomes are very similar except you only have half a wood him meaning half and so you only have it on in this case the epithelial cell and that connects the epithelial tissue to the basement membrane underneath it uh so your epithelial tissue doesn't just slide off you wouldn't want your skin to just slide off of your tissue underneath um so that's the purpose of your hemidesmosomes tight junctions are a little bit different type Junctions are going to be found wherever you do not want anything to slide between the cells notice it looks almost like quilting between your cells um so there's very little space between the cells and so that prohibits the movement of water or any fluid between the cells uh so you'll see them in the kidney the GI tract and on the skin Gap Junctions are kind of almost the opposite with gap Junctions you have very they're they're holding the cells together very well but they have openings between the cells so things are not moving between the cells but they move from one cell through the tunnel to the next cell and these proteins are called conexon and the purpose of these particularly in cardiac muscle cell but you find them in other cells as well is you have ions in here that transmit an electrical impulse and they move directly through these tunnels to the next cell so it speeds up the transmission of signals so particularly in heart muscle your heart can contract all at once because the signal can jump from one cell to another through these Gap Junctions it's also important that cells be able to communicate with each other because that's how you maintain homeostasis in the body or keep everything uh physiologically at uh its regular level they also regulate growth and differentiation coordinate functions of one system with another um there are different types of communication you have paracrine autocrine endocrine and neurotransmitters so paracrine signaling is where hormones are secreted from one cell and they act on a cell that's nearby um so nearby cells will have a receptor for whatever hormone they release I andan we say they're local hormones because they're very chemically unstable so they don't go very far from where they're secreted but this is how one cell communicates with its neighboring cells autocrine signaling is where a hormone is secreted but it comes back and binds the same cell that secreted it um and auto meaning self so autocrine is also another type of local signaling endocrine signaling is a bit different with endocrine signaling hormones are synthesized and they're secreted into the bloodstream where they travel throughout the body to cells all the way through the body um the only cells however that will respond to that hormone are the ones that have the particular receptor for it um but this is long distance signaling because it goes all across the body now neurotransmitters are also local these are small chemicals released from one neuron to move across a very small space between the neurons to the next one and that's called a synapse or a synaptic Clift so the neurotransmitters are released by what we call the pre synaptic neuron they travel across that space and they bind to receptors on the dendrites of the post synaptic neuron or the receiving neuron um so we call this a chemical transmission or a chemical synapse because it uses chemicals in the form of neurotransmitters now sometimes you have to have a method called signal transduction this is the way to get these chemical signals into a cell because the cells the chemicals were buing on the receptor on the outside now there are some types of endocrine hormones that can actually penetrate the lipid bil layer and bond re inside the cell but that's not the usual way usually it's a aquous something that's dissolves in water because blood plasma interstitial fluid that's mostly water so these hormones dissolve well in water but they won't cross the lipid bilayer instead they bind to receptors on the outside of the cell so when we say signal transduction that is the way that you get that message into the cell um we have first Messengers which which are the molecules that bind to The receptors on the outside of the cell then you have second messengers these are messengers inside the cell that send the signal through the cell um there are two main Pathways you can have calcium and then you can have something called CYCC cyclic adenosine monophosphate looks very similar to ATP except it only has one phosphate on it and it's in a circle so the way this happens is you have some hormon out here it binds to a receptor well that receptor goes across the membrane it's a transmembrane protein and it's bound to a a uh something inside the cell called a g protein so when the hormone binds to the receptor the receptor changes shape which then activates this G protein this G protein is coupled to an enzyme called ad denate cyclas cyclas meaning Circle so what it does is it takes at p and converts it to cyclic cyclic is your second messenger and that's how the signal gets to the cell the hormone binds out here the receptor changes shape which activates the G protein which activates a denate cyclas which converts ATP to cyclicamp cyclic then does a whole variety of stuff inside the cell um it activates protein kinases they're protein enzymes in the cell kinas means ads of phosphate so the enzyme's job is to put phosphates on things so cyclic activates those kinases which then add phosphates to proteins now when that there's millions of different things in your cells that can happen when you phosphorate proteins sometimes you put a phosphate on them it turns a protein off an enzyme off sometimes you put a phosphate on it it turns it on doesn't really matter you don't need to know unless you're studying one of those in particular but that's how the second messenger gets in in there you go through this process to add phosphates to proteins to either turn them on or turn them off and that's what we call signal transduction now as we jump into um the rest of this unit or section we're going to look at um the four main tissue types you have epithelial tissue which are tissues that line surfaces so like your skin and the interior lining of things like your blood vessels and your G tract we have connective tissue um connective tissue is things that connect but basically if it doesn't fall in one of the three other types it's going to be a connective tissue there are many different types and they're quite diverse um notice here though the epithelial tissue the cells are really close together in connective tissue your cells are very far apart with lots of space in between then we have muscle tissue and nervous tissue so if it's not n on a nervous tissue if it's not a muscle tissue if it's not a lining tissue it's going to be a connective tissue and we'll talk about each of those individually