 This video will cover part 3 of Introduction to Histology. We will cover the following objective, describe the types of connective tissue found in the body, their characteristics, location, and function. Connective tissue proper is the most common and general type of connective tissue found in the body. The cells are dispersed in a non-living matrix that includes extracellular fibers, an extracellular matrix of fibers, and a liquid ground substance with the consistency of maple syrup. Fibroblasts are found in connective tissue proper called fibroblasts, and fibroblasts mature into fibrocytes. Fibroblasts produce the fibers of the extracellular matrix. The collagen fibers are the large bundles of the protein collagen found in connective tissue proper. Fibroblasts are also smaller branching fibers called reticular fibers, which have a distinct appearance, although they are also made from the protein collagen. There are elastic fibers that provide the ability to stretch and recoil, which are made from the protein elastin. The fibroblasts produce these fibers, and fibrocytes are the mature cells that maintain the fibers of the extracellular matrix in connective tissue proper. Other cells of connective tissue proper include adipocytes that store fat and provide cushioning and leukocytes, or white blood cells that defend against infection and help to clean up debris and clean up damaged tissue. Areolar connective tissue is a type of connective tissue proper. It has a matrix with a fluid gel ground substance and collagen fibers, reticular fibers and elastic fibers. All three of the major fiber types are found in areolar connective tissue. The cell types include mesenchymal cells, which are a stem cell that produces the variety of cells in connective tissue. Mesenchymal cells can differentiate into fibroblasts that produce the fibers of the extracellular matrix, and fibroblasts then differentiate into fibrocytes. Adipocytes are also found in areolar connective tissue as well as a variety of leukocytes. Here we can see that areolar connective tissue is a very loose connective tissue proper, where there's lots of space in between the collagen fibers and the fibers are running in multiple directions. Areolar connective tissue is often found supporting epithelia, for example in serious membranes. There's areolar connective tissue found underneath the simple squamous epithelium, and in mucous membranes there's areolar connective tissue found underneath the epithelium. In the cutaneous membrane there is a layer of areolar connective tissue found directly underneath the epidermis in what's known as the papillary layer of the dermis. Adipose tissue is another type of connective tissue proper, where the predominant cell type is the adipose site that functions to store fat. The cytoplasm is filled with fat globules and the nucleus is pushed off to the side of the adipose site. The matrix is generally the same as in areolar connective tissue with a variety of extracellular matrix fibers and a ground substance with the consistency of maple syrup. The main thing that distinguishes adipose tissue is the large number of adipose sites. Adipose tissue is important as a storage of energy, as a fuel reserve, as fat storage. It also functions for insulation to help contain heat within the body and as cushioning. Adipose tissue is found under the skin in a layer deep to the dermis known as the hypodermis or subcutaneous adipose tissue just deep to the cutaneous membrane is a layer of adipose tissue. There's also adipose tissue surrounding the kidneys and in the mesenteries or in the folds of the peritoneal membrane in the abdomen and there's adipose tissue in the breasts surrounding the mammary glands. Adipose tissue forms the soft supportive skeleton of organs like the liver bone marrow lymph nodes and spleen. The distinguishing characteristic of reticular connective tissue is a large number of reticular fibers. Here's another image of reticular connective tissue where you can see the branching reticular fibers that run in multiple directions. Dense connective tissue proper forms tendons and ligaments as well as capsules that surround organs. Dense regular connective tissue has collagen fibers closely packed together running parallel. These parallel bundles of collagen fibers provide strength to resist a force. These are found in tendons and ligaments. The predominant cells are fibroblasts and fibrocytes so the distinctive feature of dense regular connective tissue is that all the bundles of collagen fiber are parallel whereas dense irregular connective tissue has collagen fibers that are randomly arranged. Bundles running a variety of different directions in order to resist multiple directions of stress. Dense regular connective tissue is found in the deep layer of the dermis of the skin known as the reticular layer of the dermis. It's also found in the joint capsules surrounding joints and fibrous capsule surrounding organs. Elastic connective tissue is another type of dense connective tissue proper where there are numerous elastic fibers in order to resist stress of stretching to allow a tissue stretch and recoil. For example the elastic artery such as the aorta the largest artery carrying blood out of the heart has a large number of elastic fibers in its wall in order to allow it to stretch and recoil. Supporting connective tissue has a firmer structure to provide structural support. For example collagen provides the structure for part of the ear the external ear is supported by cartilage the nose is supported by cartilage there's also cartilage covering the ends of the bones of the joints and cartilage attaching our ribs to our sternum and cartilage forms the larynx or a voice box. Every major types of cartilage highland cartilage is the most common type of cartilage has a glassy uniform appearance the matrix contains proteoglycans which are collagen proteins modified with conjoined sulfate which is a amino glycan a carbohydrate that has nitrogen in it so a special type of protein a proteoglycan gives the matrix of cartilage a firm and gelatinous ground substance and the cells found in cartilage the mature cells are called chondrocytes and the chondrocytes are found within little chambers within the matrix called lacuni the lacuni are little spaces that surround chondrocytes inside of cartilage and so highland cartilage provides a flexible structure to the nose also protects the ends of the bones and forms coastal cartilage coastal cartilage that connects the ribs to the sternum and there are rings of highland cartilage in the trachea that help support the airway to prevent the airway from collapsing under the negative pressure as we draw air into our lungs elastic cartilage is similar to highland cartilage but has numerous elastic fiber bundles the elastic fibers are still surrounded by the proteoglycans in order to give a firm but flexible structure however with a larger amount of elastic fibers elastic cartilage is able to be very flexible in order to bend and spring back to shape elastic cartilage is found in the external ear and also the eustachian tube which is a passageway a tube that connects between the middle ear inside of the temporal bone of the cranium down into the nasopharynx the region of the throat that connects to the nasal cavity and another place elastic cartilage is found is the epiglottis which is a flap on the top of the larynx the voice box that closes over the airway to prevent food or drink from entering into the lower respiratory tract when we swallow so all three of those locations begin with an e the external ear the eustachian tube in the epiglottis are three locations where you can find elastic cartilage in the body fibro cartilage is another type of cartilage but has a high number of collagen fiber bundles this large amount of collagen enables fibro cartilage to be compressible and to withstand large amounts of pressure to flex but not break fibro cartilage is found in the inter vertebral discs there are rings of fibro cartilage in between the bodies of the vertebrae the bones of the spine and these discs have to withstand the pressure of our of our body weight that's put on our spine and prevent the bodies of the vertebrae from contacting one another the pubic synthesis in the pubic region where the pelvic bones join the two pubic bones come together and a pad of fibro cartilage holds together the bones this is the pubic synthesis there's also fibro cartilage forming meniscus a meniscus is a ring a or a C shaped pad of fibro cartilage found in joints for example in the knee there's medial and lateral meniscus that cushion the knee joint and help increase the the fit between the femur and the tibia to stabilize the knee joint here's a light micrograph of hyaline cartilage where you can see that there's a translucent lightly stained matrix and large lacunae containing chondrocytes here's an image of fibro cartilage where you can see there's a similar appearance to hyaline cartilage but with numerous bundles of collagen giving a darker streaking stain pattern in the matrix here's another example of fibro cartilage this image shows smaller lacunae with chondrocytes and a darker staining of collagen fiber bundles here's an image showing elastic cartilage with thick bundles of elastic fibers in the matrix surrounding large lacunae with chondrocytes inside bone is another type of supporting connective tissue in the matrix of bone there are collagen fibers surrounded by solid calcium salts called hydroxyapatite the mature cells of bone are called osteocytes and they're found in chambers called lacunae similar to the chambers of cartilage called lacunae where chondrocytes are found osteocytes the mature cells of bone are found within lacunae the function of bone is to provide structure for the body and protection and the the bones of the skeleton are the primary organs where this osteos tissue bone tissue is found we'll study bone tissue in more detail as we study the skeletal system fluid connective tissues are another type of connective tissue where the matrix is a thin liquid and the matrix does not contain a network of fibers blood is an example of fluid connective tissue the liquid in blood is called plasma which is mostly water with some dissolved nutrients minerals proteins and the cells of blood are erythrocytes or red blood cells that are specialized for transporting oxygen leukocytes which are white blood cells specialized for defending against infection and thrombocytes are the platelets that are specialized for preventing us from bleeding for forming clots to prevent us from losing our blood they're specialized for hemostasis the blood clotting and so the primary function of blood is to transport materials throughout the body for example transporting oxygen from the lungs to cells throughout the body or transporting waste away from cells throughout the body and bloods found within the blood vessels and heart throughout the cardiovascular system another type of fluid connective tissue is called lymph which is the fluid inside lymphatic vessels which has a similar consistency to blood plasma and there are leukocytes also found in lymph however there are no erythrocytes only leukocytes found in in lymph and the function of of the lymph is to drain extracellular fluid and transport materials for example fat is transported from the digestive system in lymphatic vessels and lymphatic vessels drain the extracellular material extracellular fluid all over the body and return fluid from the extracellular space back to drain into the blood to return into the veins and so this extracellular fluid originally forms from the blood plasma at capillaries as fluid leaks out of the capillaries and then lymphatic capillaries drain extracellular fluid into lymphatic vessels that eventually return this blood into the veins return this liquid this lymph into the blood of the veins here's a light micrograph showing blood where the majority of the cells we see these small red cells are erythrocytes and then the one large cell in the center here that has a nucleus with multiple lobes is it a type of leukocyte that specific type of leukocyte is called a neutrophil it's the most common type of leukocyte found in a blood smear