chapter four is gonna focus on the tissue level of organization we need to remember that tissues are made of smaller units called cells and that each tissue is composed of cells of the same structure and function there are four adult tissues in the human body epithelial tissue connective tissue muscular tissue and nervous tissue all four of these adult tissues are going to differentiate which just means arise from what we call germ tissues now when you see this picture don't worry about it you don't have to learn this information it's not an embryology class but when you see this picture basically what's happening is a sperm is fertilized in egg and forms a zygote that's a single cell but as we know we're made up of trillions of cells so in about five to six days that single cell turns into what's known as a blaster site which is basically a hollow ball of cells and the blaster site will develop into what we call the gastrula the gastrula is going to contain three different layers of germ tissue those germ tissues are what will differentiate into our four adult tissues so we can see that the three germ tissues are in the derm mesoderm and ectoderm the best thing about germ tissues is that you always know what the answer should be because it should end in derm during a job so endoderm is the innermost layer mesoderm is the middle layer and then ectoderm is the outer layer what do these tissues actually develop into so we'll start with exa derm that's the outermost layer ectoderm is going to develop into epithelial tissue as well as nervous tissue Nissa derm which is the middle layer is going to develop into most muscle in the body so cardiac smooth and skeletal it's going to develop into epithelial tissue and it's going to develop into your adult connective tissues last is endoderm that's the innermost layer the endoderm is gonna develop into epithelial tissue so questions that you might get asked is if I said what germ tissue does bone come from this bone differentiate from well bones a connective tissue so connective tissues are gonna arise from me so darn if I asked you what germ tissue and epithelial tissue came from all three answers would be correct technically epithelial tissue comes from very specific ones depending on where in the body it's found but for our understanding just knowing that it can possibly come from all three germ tissues is good so once we have our tissues developed the cells of the tissues need to actually work to help secure the tissue together so this is done by cell junctions cell junctions have two major roles support and structural by holding cells together as well as communication points so cells can actually communicate between each other so when we look in this picture that's on here right now what we'll see is that this is one cell okay here is the wall of another cell and then on this side would be another cell we can see that the cells are stuck together by the plasma membranes and we can also see that there is junctions between those two cells plasma membranes so we're going to talk about each particular Junction now there are five junctions and of these five junctions we're going to cover four so the first Junction we're going to talk about are going to be tight junctions okay all these junctions are going to have what we call transmembrane protein and all the transmembrane protein is is a protein that goes across two membranes so these things that look like pearls are the transmembrane proteins that go across two membranes now when you think about a tight Junction I want you to think about Ziploc bags now why is the pluck bags well Ziploc bags they seal a material inside of them so these transmembrane proteins are basically acting like the lock of the bag and it's keeping anything from moving between the two cells so tight junctions allow cells to seal material within them so it cannot transfer between two cells so this is important because hold on one second okay this is important sorry about that because basically we don't need material seeping between two layers so think about your stomach your stomach is very acidic and the cells that line this stomach are equipped to handle that type of acidity so the tissue layers beneath it may not be equipped to handle that low pH so these tight junctions keep the material in the stomach from seeping into underlying layers at the next Junction we're gonna cover they're going to be desmosomes okay desmosomes are going to help give strength to tissues when they twist when they get stretched anytime there's tension so desmosomes are actually going to be connected by transmembrane proteins called cad hearings so everyone go ahead and interlace your fingers together and then close them down and lock that's basically what the CAD hearings are they're interlocking transmembrane proteins and when someone tries to pull your hands apart those interlocking fingers give it extra strength so the CAD hearings that are found between the two cells and desmosomes give the tissue extra strength because the cells have basically anchors into each other we're gonna find desmosomes in cardiac muscle as well as most epithelial tissue specifically the skin the keratinized stratified squamous okay next one we're gonna cover is Hemi desmosomes Hemi desmosomes are half of desmosomes so the very similar in structure but they're transmembrane proteins are called integrins now integrins are going to actually lock the bottom of a cell into what we call the basement membrane so the only place we find Hemi desmosomes is when the epithelial tissue is locking into and securing itself into the basement membrane so think about in a classroom where there's desk a lot of this desk in the classroom don't move and that's because the base of the desk are screwed into the floor that's basically what the integrins and Hemi desmosomes do they secure the bottom layer of cells into the base of membrane so they don't tear apart the next Junction is going to be gap junctions as we can see between these two cells we there's a gap but then we have these little channels they almost look like tunnels these little tunnels are called connect songs and what these connect sounds do is they're hollow tubes that allow ions electrical signals and fluid to travel between two cells so gap junctions are used for communication we find gap junctions epithelial tissue muscular tissue and nervous tissue again the gap junctions allow communication between two different cells and it's immediate communication so just a quick recap over the junctions we win over tight junctions tight junctions seal material into the cell so they can't be transferred desmosomes they're gonna be interlocking cells together using CAD urines and desmosomes are used for strength Keamy desmosomes which are half a dozen zones they're only found locking the epithelial tissue to the basement membrane they're transmembrane protein is called an integrand so it's basically integrating into the basement membrane and then gap junctions gap junctions again allow communication between two cells using little tubes called connects ons