Sheet #9 Dr. Heba Kalbouneh Dr. Heba Kalbouneh Dr. Heba Kalbouneh
Elastic fibers The main function of elastic fibers is to provide elasticity. In other words these fibers are able to restore the original shape of the tissue after distortion. (Think of an elastic fiber as a rubber band, stretched when a force is applied and relaxed (back to its original shape) when the force is released. - Elastic fibers can deteriorate (be damaged) by three factors: 1-The elasticity of tissue decreases with age, ex: This effect is easily demonstrated by recruiting two volunteers, one youthful and one elderly. Pinch up a bit of skin on the back of each person's hand and then watch how quickly the skin returns to its original position when released 2- Sun exposure: the elastic fibers (like a rubber band) will lose their resilience and elasticity when exposed to sunlight for a long period of time. 3-Could be digested by pancreatic enzyme elastase. - Elastic fibers consist of individual microfibrils, which are embedded in an amorphous matrix. - Elastic fiber consists of 2 proteins: 90% of the fiber is elastin 10% of the fiber is microfibrils of fibrillin protein - main properties of elastic fibers: 1- Elasticity: due to presence of elastin protein 2- Stability: due to the presence of fibrillin microfibrils. - Remember that elastic fibers as well as other ECM components are produced by the fibroblasts (with some exceptions according to the site). - Elastic fibers are found in all types of connective tissues but in some tissues they are the predominant fibers and the tissue is called elastic tissue examples: 1- certain types of ligaments contain elastic fibers mainly like the ligamentum flavum (connects the vertebrae of the vertebral column), flavum = yellow Note: a ligament connects bone to bone and is usually (not always) made of dense collagenous connective tissue.
2- certain types of cartilages are composed mainly of elastic fibers and so they are called elastic cartilages, example epiglottis ) المزمار (لسان at the back of the tongue that closes the respiratory tract so that food can enter the esophagus. 3- another important location is in the wall of large arteries to ensure recoiling of blood vessels after dilation. - When elastic fibers are the predominant (large amount is present) they appear yellow in fresh tissue (fresh specimen) - Formation of elastic fibers Elastic fibers are produced by fibroblasts 1-what appears first are the microfibrils 2- elastin protein (characterized by being a coiled protein) deposits and accumulates over these microfibrils. Note: under EM elastin protein appears as amorphous material (with no shape). How do elastic fibers perform the elasticity function? Elastic fibers are cross-linked by a special type of link called desmosine When a certain force is applied to this elastic fiber the fiber is going to stretch When force is released the fiber will restore its original state. What prevents elastic fiber in the stretched state from over stretching? Collagen, there is always a certain amount of collagen fibers, even in elastic tissues. There is a minimal amount of collagen fibers which prevents overstretching or provides a mechanical stop. - Can you see elastic fibers under LM? Normally elastic fibers in tissues with small amount of them cannot be visualized using H&E and we need special stains in order to view them under the microscope. H&E stain doesn t view elastic fibers unless they are present in high concentration.
3- Reticular Fibers: Because of their different staining characteristics, reticular fibers were initially thought to be completely different from collagen fibers. Crossstriations with the same periodicity as in collagen fibers type 1 are visible using electron microscopy. We now know that reticular fibers consist of collagen - although the main type of tropocollagen found in reticular fibers, type III, is different from that of the collagen fibers type 1 - Reticular fibers are very delicate and form fine networks (thin and branching) instead of thick bundles Location: 1- They are found mainly in the reticular lamina of the basement membrane. 2- They are found in RES organs (Reticulo- endothelial system), which consists of blood channels supported by connective tissue rich in reticular fibers. These organs (bone marrow, liver, lymph nodes and spleen) have a meshwork of reticular fibers which acts as the framework for these organs (supportive stroma and supporting individual cells). Why it was thought to be a different type of collagen fibers? Because it is stained positively by the "PAS" stain, it has a high amount of carbohydrates. and it is also stained positively by silver nitrate (there is a method of staining called silver impregnation which is adding silver to the tissue, because these fibers have high amount of carbohydrates, and these carbohydrates are negatively charged, thus, they are going to interact with the silver and cause its reduction, which results in deposition of silver inside the tissue, so reticular fibers can be seen as black threads. All the rest of collagen fibers types contain carbohydrates but in minimal amount (about 1% only), thus, collagen fibers are considered glycoproteins, but collagen fibers type three (reticular fibers) contain much higher amount of carbohydrates (10% carbohydrates). The cells that are specialized in secreting reticular fibers are called reticular cells, which are modified fibroblasts.
- The function of reticular fibers they form the stroma of certain organs (like lymph nodes and spleen) and support individual cells. They also allow the movement of cells and molecules. Ex: lymphoid tissue, like in lymph nodes, is filled with cells called lymphocytes. So, these fibers form spaces in which these cells lymphocytes " can move freely within the tissue). - The structures that are stained positively with silver are called Argyrophilic, so the reticular fibers are called Argyrophilic (silver loving) Summary Fibers of connective tissue: 1- Collagen fibers: the main function is to give strength. They appear as pinkish bundles with H&E. 2- Elastic fibers: give elasticity. Usually we use special stains like Orcein and resorcin (elastin stain). 3- Reticular: form a network. Delicate network (meshwork) of fibers, and they provide the structure of certain organs. - Its stained positively with PAS stain as well as Silver stain. In any connective tissue we can find the 3 types of fibers, but the proportions of these components differ. If the tissue is composed mainly of collagen type 1, we call it collagenous connective tissue. If it's mainly composed of elastic fibers, we call it elastic connective tissue. If it's mainly composed of reticular, we call it reticular connective tissue. All what have been explained earlier is the components of the connective tissue. Now, we are going to consider the classification of it.
The classification of the connective tissue, the standard classification scheme is based on composition 1- Connective Tissue Proper o o Loose Connective Tissue Dense Connective Tissue 1) Dense Irregular Connective Tissue 2) Dense Regular Connective Tissue 2- Special Connective Tissue (has similar characteristics of the connective tissue) Blood (cells and extracellular matrix with no fibers and stabilizing macromolecules (gags), that s why it's fluid), bone (cells and mineralized extracellular matrix, that s why it's hard), cartilage, reticular tissue, elastic tissue, adipose tissue. Note: when we refer to one type of special connective tissue, we refer to it by its name. 3- Embryonic connective tissue Connective tissue proper This is the type that we talked about in the previous lectures. - It contains the components that were mentioned (fibers, cells, ground substance), with different proportions. - When we say connective tissue we mean connective tissue proper. *if the main component of tissue is collagen fibers then it is called dense collagenous connective tissue *if the main component is cells or ground substance (or equal proportions) then it is called loose connective tissue
Loose connective tissue Cells and/or ground substances predominate, but still there are some fibers (thin or delicate fibers) Function: 1. Diffusion (medium for transport) because it has a lot of spaces filled with ground substance, Found around blood vessels, directly under epithelium layer, highly vascular (high amount of capillaries in order for the molecules to pass from the blood vessels to the epithelium and backward. 2. Second line of defense below basement membrane, because it has inflammatory cells (ex macrophages) 3. Fills spaces inside any organ ** Found in lamina propria of mucus membrane below epithelium (special name for the loose connective tissue of mucus membranes=mucosa) ** It is a flexible structure, because it has high amount of ground substance, high amount of water. The extracellular matrix is viscous. ** Not resistant to stress because it has low amount of collagen, flexible but not very strong (the function is diffusion and defense). ** It has more regenerative power than dense connective tissue because it has more cells and more vascularized 2-dense irregular connective tissue (collagenous) *mostly composed of collagen fibers *collagen fibers are arranged haphazardly (randomly) Collagen fibers add strength to the connective tissue so it's very resistant to compressional forces and tensile forces *provides powerful resistance in many directions (according to the orientation of the fibers)
*location: 1- Deeper layer of dermis of skin: below the loose connective tissue layer (exposed to many forces in different directions) First layer (superficial) of connective tissue is loose, the next layer (deeper) is dense irregular connective tissue (to give strength for loose and epithelium) Epithelium of the skin is epidermis, and the connective tissue of the skin is dermis 2- Organ capsules, capsules that surround any organ are whitish because they have high amount of collagen but the orientation of the collagen fibers is irregular. 3- submucosa (mucosa=mucous membrane= epithelium and thin layer of loose connective tissue (lamina propria)) Dense regular connective tissue (collagenous) *fibers arranged in bundles (oriented in certain direction) *provides resistance to stress in one direction *fewer cells and ground substance and most of the cells found are fibroblasts not inflammatory cells *ex; Ligaments: connect bone to bone, usually are collagenous (whitish in color) dense regular type of connective tissue except certain types of ligaments (elastic/yellowish in color). Tendons: connect muscle to bone. Aponeurosis: flat tendon (sheet like) instead of cord like structure
-connective tissue has variable regeneration power according to the type; loose has a higher regeneration power than dense connective tissue because it has higher amount of cells and capillaries in comparison with dense connective tissue (fibers and little amount of cells), so if a tendon gets injured usually the regeneration is poor because it's less vascularized, and repair of damaged tendons is very slow Reticular tissue consists of reticular cells (modified fibroblasts) and a network of reticular fibers formed by them forms the structural framework in which the cells of the organ are suspended in the liver, bone marrow, lymph nodes and the spleen Embryonic connective tissue: The connective tissue that presents in embryo. Embryonic connective tissue: It contains star or spindle shaped mesenchymal cell (stem cells), gel- like ground substance with fine fibers (more ground substance than fibers). If you go back to embryo you will see three germinal layers; ectoderm, mesoderm and endoderm The mesoderm differentiates into a tissue called mesenchyme or embryonic connective tissue Mesenchyme forms the filling material of the early embryo and it is the common origin of all other connective tissue types With fetal development, mesenchyme forms the connective tissue between and within the developing tissues and organs -Remember that epithelium is derived from the three germinal layers which is a characteristic for it, while connective tissue is derived from mesenchyme (or mesoderm). -The mesenchymal cells of this tissue (mesenchyme) can form bone, blood or connective tissue proper.etc. So different types of connective tissue are produced by the mesenchymal cells. Mesenchymal tissue or embryonic tissue is a tissue found only in embryo while after birth we can t find mesenchymal tissue because all mesenchymal
tissues have differentiated into different types of connective tissue. EXCEPT in two places; in the pulp of the tooth (be careful we talk now about a tissue not individual stem cells) and the umbilical cord - Mesencymal cells can differentiate into osteoblasts which form the bone or fibroblasts which form the connective tissue proper or adipoblasts which form the adipose tissue or etc. So these cells are stem cells which can differentiate into many types of connective tissue cells (pluripotent). - Stem cells mean undifferentiated cells that can become a variety of cells - The extra cellular matrix of mesenchymal c.t contains more ground substance than fibers (gel like structure because it has large amount of hyaluronic acid, chondriotin sulfate (sulfated GAG) and water) - Mesenchymal stem cells are characterized by having a little amount of cytoplasm with fine processes and large oval or rounded nuclei. - The mesenchymal c.t of the umbilical cord is called also mucoid connective tissue (also called Wharton s jelly ), because its ground substance is mucous like, it s made up of mucopolysaccharides (hyaluronic acid and chondroitin sulfate) - Mucoid connective tissue forms a compliant cushion around the vessels of the umbilical cord Umbilical cord banking simply involves collecting blood left in newborn's umbilical cord and storing it for future medical use (cord blood and/or tissue) (regenerative medicine).