 This video will cover an introduction to histology, which is the study of tissue structure. We will cover the following objectives, describe the characteristics of the four primary tissue types, epithelial, connective, muscle, and nervous tissues, describe the structure, location, and function of the four membrane types, cutaneous, mucus, cirrus, and synovial, and describe the process of tissue healing after a wound. Here are the four major tissue types. Now remember the definition of tissue is a group of cells working together to perform a common function. The four major types of tissue are nervous tissue, epithelial tissue, muscle tissue, and connective tissue. Nervous tissue is found in the brain, spinal cord, and nerves. So this is the major tissue of the nervous system that's responsible for rapidly transporting information throughout the body in order to regulate our other organs. The nervous system is regulating the muscles and muscle tissue is specialized for contraction to produce motion. There are three subtypes of muscle tissue, cardiac muscle is found in the wall of the heart, smooth muscle is found lining hollow organs like the stomach, and skeletal muscle is the muscle that attaches to bones through tendons in order to move the skeleton. Epithelial tissue forms sheets of cells that are very tightly held together with cell junctions like desmosomes and tight junctions that hold adjacent cells tightly together. Epithelial tissues line body surfaces including the exterior, the outer layer of the skin, the most superficial layer of the skin is epithelial tissue known as the epidermis. It also forms the lining of the digestive tract and respiratory tract, and epithelial tissues form glands, for example the sweat glands, salivary glands, and also endocrine glands that produce hormones as signals through the body are made from epithelial tissue. So glands produce secretions, exocrine glands like sweat glands produce secretions that exit the body, and endocrine glands produce hormones that travel through the blood to regulate other organs throughout the body. Epithelial cells have an apical surface, so a sheet of epithelial cells has an apical surface that faces the exterior or the lumen which is a space inside the body, for example the space inside of the stomach is the lumen inside of the stomach. The apical surface of an epithelium faces the lumen or with our skin the epidermis of our skin the apical surface faces the exterior. The opposite surface of the epithelium is called the basolateral surface which is attached to extracellular fibers that anchor the epithelium to the body. Epithelial tissues are avascular with no blood vessels and so there are blood vessels running through connective tissue that's underlying epithelial tissues in order to provide nutrients and carry waste away from the cells of the epithelium, but there are no blood vessels that actually travel through the epithelium. Epithelial tissues also have a high capacity to regenerate, there are stem cells found within the epithelium that can divide in order to replenish cells that are lost. The last group of tissue is connective tissue which is sort of a catch-all for a lot of different very diverse types of tissues. Connective tissues are the most structurally and functionally diverse. There are three main categories of connective tissues. Connective tissue proper, supporting connective tissue and fluid connective tissue. We see examples here of adipose tissue or fat which is a type of connective tissue proper. Then we see bone which is a type of supporting connective tissue and a tendon is another type of connective tissue proper. It's a dense, regular connective tissue. Tendons are connective tissue which connect muscles to bones. Connective tissue also includes fluid connective tissues like blood. There are specialized cells in connective tissue that are dispersed in a non-living extracellular material. The most types of connective tissue, the connective tissue proper, include cells called fibrocytes that maintain extracellular fibers of a matrix. An extracellular matrix of fibrous proteins that are found in the connective tissue proper are maintained by cells called fibrocytes. In adipose tissue and as well as some other types of connective tissue proper, there are cells called adipose sites which are specialized for storage of fat. Erythrocytes are also known as red blood cells. These are specialized cells that transport oxygen in blood. Leucocytes are the white blood cells that defend against infection. Leucocytes are found in blood but they also are found in other tissues so we can see certain types of leucocytes are residents inside of connective tissue proper. For example macrophages or basophils are cells that can wander in and out of connective tissue proper. They are also found in the fluid connective tissues like blood and lymph. Inside bone osteocytes are the mature cells that maintain the matrix of the bone. Another type of supporting connective tissue is called cartilage which is a more flexible but supportive connective tissue. The mature cells of cartilage are called chondrocytes that maintain the matrix of the cartilage. So the matrix of protein fibers surrounds cells in connective tissue and in most connective tissues there's fibrous proteins providing this matrix. In fluid connective tissue like blood the proteins are dissolved and they don't form a fibrous matrix except during blood clotting when proteins do form a fibrous matrix. The fibrous protein that is normally soluble dissolved in the plasma of blood becomes insoluble as it forms a network of fibers to prevent bleeding as a blood clot is formed. There's also a liquid surrounding the fibers and cells in connective tissue. This is called the ground substance. This extracellular liquid can vary in consistency. It could be a thin liquid like the plasma of blood or a more viscous liquid. In connective tissue proper there's a viscous liquid with the consistency of maple syrup. Or it could also be a more firm gel like extracellular material such as that found in cartilage or even a solid material as in bone. There's a calcium salt known as hydroxyapatite that gives the hardness to bone and fills the extracellular space surrounding the extracellular matrix of fibers in bone. Tissue membranes are continuous flat multicellular sheets that cover or line body parts. There are two major types, two broad categories of tissue membranes, connective tissue membranes that are composed of only connective tissue. The example shown here is a synovial membrane that lines the joint cavity and produces a synovial fluid in order to lubricate the joint. The other broad category of tissue membranes is epithelial membranes. Epithelial membranes include mucus membranes that line the digestive respiratory urinary and reproductive tracts and are coated with mucus, a secretion that comes from glands found within the epithelium that secrete mucus in order to lubricate, moisturize the surface. Serous membranes line our ventral body cavities. There are the peritoneal membrane lining the abdominal cavity, pleural membrane lining the lungs and pericardial membrane lining the heart. Serous membranes have two layers, a parietal layer and a visceral layer. The parietal layer lines the cavity wall and the visceral layer lines the organ. There is a serous fluid secreted in between those two layers. For example we have a parietal layer of the pleural membrane lining the thoracic cavity surrounding the lung, the surface of the lung is covered with the visceral layer of the pleural membrane and then there is fluid in between those two layers inside the pleural cavity. Similarly there is a visceral layer of the pericardium which covers the surface of the heart and a parietal layer of the pericardium which lines the walls of the pericardial cavity and there is a parietal layer of the peritoneum lining the walls of the abdominal cavity and a visceral layer of the peritoneum lining the surface of the intestines. The epithelium found in serous membranes is a simple squamous epithelium that means there is just one layer of flat cells. So a very thin layer of flat cells also known as a mesothelium is the specific name for the simple squamous epithelium found in a serous membrane and then there is some loose connective tissue proper in between the simple squamous epithelium supporting the simple squamous epithelium and anchoring it to the other surfaces of the body and this is an areolar connective tissue which is a loose connective tissue proper that provides some space for blood vessels to travel through. The cutaneous membrane is the skin and the cutaneous membrane has a stratified squamous epithelium that contains keratin. So stratified squamous epithelium has multiple layers that's why it's stratified and it's squamous meaning that the surface cells are flat. So stratified squamous epithelium has multiple layers of cells and the cells at the surface of the epithelium are flat. The cells facing the apical surface are flat or squamous. So stratified squamous epithelium is forming the epidermis that is the superficial layer of the cutaneous membrane and then underneath that there is connective tissue forming the dermis. As we study the integumentary system we'll see the types of connective tissue in the layers of the dermis. There's a papillary layer of areolar connective tissue and a reticular layer of dense irregular connective tissue found underneath the stratified squamous epithelium of the epidermis. The epidermis also contains keratin which is a protein that's produced as an intermediate filament inside of the epithelial cells and as the epithelial cells mature and move towards the surface they produce a large amount of keratin. Eventually those epithelial cells die and the remaining keratin provides strength to the cutaneous membrane to resist abrasion. As we study epithelia in more detail we'll see different types of epithelia that are found in mucus membranes that the mucus membrane in the intestines has a simple columnar epithelium that is just one layer of tall cells whereas there's a stratified squamous epithelium in the esophagus and oral cavity. So mucus membranes can have a variety of different types of epithelia whereas the cutaneous membrane has a stratified squamous epithelium with keratin and the cirrus membranes have a simple squamous epithelium. Now we're going to take a look at the process of wound healing to see how a membrane we're focused on a wound in the cutaneous membrane that is the epidermis endermis or the skin and how a wound in the skin is healed. The first step in the process of healing a wound in the skin is the process of clotting or hemostasis after the blood vessels are ruptured and bleeding occurs in order to stop bleeding there are clotting proteins found in the blood there's a protein called fibrinogen that's normally dissolved in the plasma of the blood that becomes a network of fibrous proteins called fibrin and this traps blood cells and binds the edges of the tear in the blood vessel in order to stop bleeding the platelets or also known as thrombocytes are cells that participate in hemostasis by stimulating the cascade of clotting that leads to the formation of the fibrin clot and platelets stick in the fibrin clot and produce clotting factors in order to stimulate the production of the clot and the growth of the clot until bleeding stops. Inflammation is the next response of the body or simultaneous response to damage inflammation is a response that works to increase the number of leukocytes or white blood cells in the tissue in order to defend against infection and remove damaged tissue there are inflammatory chemicals that are released in response to tissue damage and these inflammatory chemicals attract leukocytes and they also stimulate an increase in blood flow to the tissue there are four cardinal signs of inflammation that can be recognized in inflammation redness, heat, swelling and pain inflammatory chemicals stimulate vasodilation that is widening of blood vessels which leads to increased blood flow the blood vessels also become leaky allowing leukocytes and fluid to move from the blood into interstitial extracellular space inside of the tissue surrounding cells space inside of the connective tissue here of the dermis the next step after leukocytes have cleaned up damaged tissue and destroyed any foreign cells that could cause infection next step is proliferation and migration where cells divide and move in in order to replace damaged tissue fibroblasts from the surrounding connective tissue fill in the wound with a network of extracellular fibers and then blood vessels will grow through forming what's known as granulation tissue then epithelial cells grow in from edges along the damaged surface and cover over the granulation tissue forming a new layer of epidermis over time the next step is maturation or remodeling where the fibers of the connective tissue become reorganized in order to maximize strength and minimize scarring initially there may be a bulge from the thick layer of connective tissue that was produced and if that connective tissue isn't remodeled and covered with epithelial tissue there can be a scar fibrosis is this scar where a damaged tissue is replaced by dense connective tissue some tissues heal better than other tissues or will fully regenerate so regeneration is when a damaged tissue is replaced by the same type of tissue for example here regeneration would replace the epidermis that's been damaged with a new layer of stratified squamous epithelium with keratin a new layer of epidermis will replace the damaged epidermis and a new layer of dermis the connective tissue would grow in underneath and the fibers would be remodeled until they're oriented along the stress lines that provide the most strength to that tissue the tissues that regenerate easily are the skin for example most epithelial tissues regenerate fairly easily so our cutaneous membrane would regenerate easily connective tissue proper regenerates easily and bone is also able to regenerate easily muscle and cartilage regenerate poorly and then some tissues will not fully regenerate instead will be replaced with scar tissue for example dense connective tissue is the scar tissue that's going to be laid down to replace the cardiac muscle tissue or nervous tissue that's been damaged which cannot be regenerated instead it will be replaced with scar tissue with dense connective tissue and this process is called fibrosis when a damaged tissue is replaced with a scar tissue