Tissue Types (Independent Learning) PDF

TISSUE TYPES (INDEPENDENT LEARNING) Block: Foundations Block Director: James Proffitt, PhD Session Date: Wednesday, July 31, 2024 Time: 8:00 – 9:30 am Instructor: Lonnie Lybarger, PhD Department: Cellular & Molecular Medicine Email...

TISSUE TYPES (INDEPENDENT LEARNING) Block: Foundations Block Director: James Proffitt, PhD Session Date: Wednesday, July 31, 2024 Time: 8:00 – 9:30 am Instructor: Lonnie Lybarger, PhD Department: Cellular & Molecular Medicine Email: [email protected] INSTRUCTIONAL METHODS Primary Method: IM10: Independent Learning ☐ Flipped Class ☐ Clinical Correlations Resource Types: RE18: Written or Visual Media (or Digital Equivalent) INSTRUCTIONS Note: This Independent Learning session on Tissue Types should be done in preparation for the Tissue Types histology laboratory session. The notes below provide additional depth and can be used to supplement the lecture videos, if needed. There are also optional ‘Slide Guides’ - video narrations of relevant tissue sections, and an optional worksheet that accompanies the ILM content, posted with this session. These materials will guide your study and review and are strongly recommended. READINGS N/A LEARNING OBJECTIVES These Learning Objectives are also found in the notes for the Tissue Types flipped session. 1. Describe characteristics that distinguish the four tissue types from each other. Apply these features to identifying tissue types in tissue sections. 2. Apply the standard classification scheme to identifying different epithelia. Relate function of different types of epithelia to structure. 3. Explain the concept of epithelial cell polarity and discuss its importance in epithelial function. 4. Describe structure and function of the main apical, basal and lateral membrane specializations of epithelial cells. 5. Describe the composition of connective tissue proper including fibers, ground substance and cells. Relate structure to function 6. Describe the role of connective tissue in support and defense including metabolic support and inflammation. 7. List resident and itinerant cells in connective tissue. Briefly describe the function of resident cells. 8. Describe the path of white blood cell circulation, and entry into connective tissues. Block: Foundations | LYBARGER [1 of 17] TISSUE TYPES (INDEPENDENT LEARNING) 9. Compare and contrast cardiac, skeletal and smooth muscle in terms of structure and function, and distinguish these tissues in sections. 10. List components of the central and peripheral nervous systems. Identify peripheral nerves, parasympathetic ganglia, and their components in tissue sections. CURRICULAR CONNECTIONS Below are the competencies, educational program objectives (EPOs), disciplines and threads that most accurately describe the connection of this session to the curriculum. Related Related Competency\EPO Disciplines Threads COs LOs CO-01 LO #1 MK-01: Core of basic sciences Histology & N/A LO #2 and MK-02: The normal Cellular LO #3 structure and function of the Biology LO #4 body as a whole and of each of LO #5 the major organ systems; LO #6 LO #7 LO #8 LO #9 LO #10 EPITHELIUM: DISTRIBUTION OF EPITHELIA: Epithelium constitutes a diverse group of tissues that cover all body surfaces (e.g. epidermis of skin, intestinal epithelium, respiratory epithelium), and line all body cavities and blood vessels. An epithelium is made up of an uninterrupted layer of cells that both forms a barrier and mediates exchange between two compartments, usually the inside of the body and the outside world. CLASSIFICATION OF EPITHELIA: Epithelial are classified based on the shape of their cells and the number of cell layers, as follows: Squamous Cuboidal Columnar Other Simple Lining of lung alveoli, Lining of kidney Lining of GI tract, Pseudostratified some kidney tubules; tubules; many bronchioles in lung, epithelium: Endothelium (lining ducts of salivary oviduct, uterus in respiratory of blood vessels) glands and female reproductive epithelium; much of Mesothelium (lining pancreas tract male reproductive of body cavities) tract Block: Foundations | LYBARGER [2 of 17] TISSUE TYPES (INDEPENDENT LEARNING) Stratified Epidermis of skin Lining of sweat Lining of some Transitional (stratified squamous gland ducts salivary gland ducts epithelium: lining of keratinized); much of urinary Lining of oral cavity, tract, including esophagus, anal urinary bladder canal, cervix, vagina, distal urethra (stratified squamous non-keratinized) Note that stratified epithelia are classified based on the shape of the most superficial layer of cells (deeper layers of stratified epithelia are usually cuboidal). Pseudostratified epithelium looks stratified because cell nuclei are at different levels and not all cells reach the cell surface. It is considered a simple epithelium, however, because all cells touch the basal lamina. Transitional epithelium (aka urothelium) is a special kind of stratified epithelium that is capable of extensive stretching. The epithelium varies in thickness depending on the degree of stretch. Membrane reservoirs in umbrella cells (aka dome cells) at the luminal surface of the epithelium contribute to the ability of the epithelium to stretch. FUNCTIONS OF EPITHELIA: Most epithelia exist as barriers between the outside world and the inside of the body. In fact, there are only two types of epithelia that do not face the outside world. These are endothelium, which lines all blood vessels and the heart, and mesothelium, which lines the pleural, pericardial, and abdominopelvic cavities. In addition to their function as barriers, epithelia conduct regulated exchange between the compartments that they separate. This includes: absorption e.g., of nutrients, fluids and electrolytes by intestinal epithelial cells; secretion, e.g., of mucus by goblet cells, digestive enzymes by pancreatic acinar cells, hormones by endocrine gland cells; and excretion, e.g. of nitrogenous wastes by kidney epithelial cells. CHARACTERISTICS OF EPITHELIA: EPITHELIAL CELL POLARITY: Block: Foundations | LYBARGER [3 of 17] TISSUE TYPES (INDEPENDENT LEARNING) Epithelial cells are polarized: they maintain characteristic non- random distributions of organelles, and their plasma membrane is differentiated into apical, lateral, and basal domains with distinct protein and lipid compositions. Epithelial cell polarity is critical to the cell’s ability to carry out directional transport across the cell. Basal Surface specializations: By definition, the basal surface of epithelial cells faces the connective tissue compartment and blood supply. The basal surface sits on a basal lamina (=basement membrane), a secreted layer of glycoproteins and other molecules that help attach the epithelium to underlying connective tissue. Adherens junctions and hemidesmosomes formed on the basal plasma membrane also contribute to attachment of the epithelium to the substrate. Lateral Surface specializations: Lateral surfaces of epithelial cells face adjacent cells, and are characterized by the presence of several different types of intercellular junction. In most epithelia the apical-most of these is the tight junction. The tight junction (aka zonula occludens) is a continuous band around the apex of each cell that functions as a seal preventing intercellular passage of fluid, ions, and other molecules across the epithelium. The tight junction is not particularly strong mechanically, and is therefore closely associated with a band-like adherens junction (zonula adherens) that attaches adjacent cells to each other and, via linker proteins, to actin filaments in the cytoskeleton. Lateral membranes also form desmosomes (aka macula adherens), which are intercellular attachment plaques that link to intermediate filaments in the cytoskeleton. Apical Surface specializations: The apical surface of most epithelia faces the outside world. The exceptions are endothelium, where the apical surface faces blood, and mesothelium where the apical surface faces a body cavity. The apical membrane of epithelial cells invariably forms some microvilli. These are fingerlike projections of the plasma membrane supported by a core of actin filaments. They function to increase surface area, usually for absorption. In some epithelial cells, e.g. intestinal epithelial cells which are very actively involved in absorption, a dense, orderly array of microvilli called a brush border, is present at the apical surface. In addition to microvili, some epithelial cells also form cilia at the apical surface. Cilia are motile surface projections with a core of microtubule “doublets” arranged in a 9+2 array, called an axoneme. Cilia bend because of the protein dynein, which, in the presence of ATP causes movement of microtubules within the axoneme relative to each other. Cilia are 7-10 um in length and beat in a synchronous rhythm to move surface fluid in a constant direction. In the Block: Foundations | LYBARGER [4 of 17] TISSUE TYPES (INDEPENDENT LEARNING) respiratory system, the action of cilia moves mucus up to the throat for excretion. In the female oviduct, ciliary action moves the ovum toward the uterus. OTHER CHARACTERISTICS: Epithelia form a discrete layer with high cell density and very little extracellular matrix. Epithelia are avascular. Epithelial cells are dependent on the capillaries present in underlying connective tissue to supply their metabolic needs. Most glands in the body are formed from epithelium. Glands may be unicellular (e.g. mucus-secreting goblet cells of intestinal and respiratory epithelia) or multicellular (e.g. salivary gland, thyroid gland). Exocrine glands (e.g., salivary glands, sweat glands), secrete their products to the outside of the body usually via ducts which are also formed by epithelial cells. Endocrine glands (e.g., pituitary gland, thyroid gland) secrete their products into connective tissue. Endocrine secretions are picked up by capillaries in the connective tissue investing the glands, and distributed via the vascular system to the rest of the body. CONNECTIVE TISSUE: Capsules of organs and the supporting tissues within organs, as well as fascia, tendons, ligaments, cartilage, bone, blood and fat all fall into the category of connective tissue. Bone, cartilage, and blood are considered specialized connective tissues. They are discussed in the Foundations Block (blood), and in the Musculoskeletal Block (cartilage and bone). All the other connective tissues fall into the broad category of connective tissue proper, and we will examine these as part of this topic. Structurally, what all connective tissue subtypes have in common is an abundance of extracellular molecules and a relatively low density of cells compared to epithelium, muscle and nerve (though there are some very important exceptions to this, e.g. lymphoid tissue). In addition to capsules and septa of solid organs as well as fascia, tendon and ligaments, connective tissue proper includes dermis of the skin and lamina propria of hollow organs. Although sometimes considered as a special subcategory of connective Block: Foundations | LYBARGER [5 of 17] TISSUE TYPES (INDEPENDENT LEARNING) tissue, adipose tissue (fat) is also often included with connective tissue proper and we will do that here. FUNCTIONS OF CONNECTIVE TISSUE PROPER: Mechanical support. This is especially true of dense connective tissues such as the dermis of skin, and tendons, which connect muscles to bones, and ligaments which connect bones to bones. Metabolic support: Blood vessels including capillaries travel only in connective tissues. Defense: White blood cells, the main defensive cells of the body, have their effects for the most part in connective tissues. Inflammatory responses take place in connective tissue! COMPONENTS OF CONNECTIVE TISSUE PROPER EXTRACELLULAR MATRIX Like epithelium, connective tissue is made up of cells and extracellular matrix, but whereas in epithelium (where the cellular component predominates), in connective tissue, cells are usually relatively sparse and extracellular matrix is abundant. The extracellular matrix of connective tissue is composed of various kinds of fibers, and of non-fibrous molecules and the fluid bound to them (together called ground substance). The relative abundance of particular kinds of cells, fibers and ground substance is what determines the varying properties of different kinds of connective tissues. The fibers and ground substance of the extracellular matrix are secreted by fibroblasts (see below). Fibers: The fibers of connective tissue give it its tensile strength (collagen fibers) and elasticity (elastic fibers). Type I and Type III Collagen Fibers: Collagen is the most abundant fiber type in CT (and in fact is the most abundant protein in the body). There are >20 different types of collagen, of which 2 (Type I and Type III) are prominent in CT proper. Type I collagen is by far the most abundant and best studied of the different kinds of collagen, and is usually the most conspicuous component of CT in histological sections. Type I Block: Foundations | LYBARGER [6 of 17] TISSUE TYPES (INDEPENDENT LEARNING) collagen forms fibrils (visible in the EM) which combine to form fibers (visible in the light microscope), and large bundles of fibers (visible to the naked eye). Type I collagen synthesis and degradation will be discussed in more detail in other sessions. Briefly, collagen is synthesized by the RER of fibroblasts (see below) as procollagen, a triple helix with short non-helical extensions at either end called propeptides. Procollagen is processed in RER and Golgi then secreted. Once outside the cell, the propeptides are cleaved; the remaining triple helix is called tropocollagen. Tropocollagen molecules self- assemble to form collagen fibrils which in turn assemble to form fibers. The tropocollagen molecules pack in a very regular staggered array that results in a characteristic banding pattern (visible in EM only) on the assembled fibrils. Vitamin C is required for synthesis of the normal collagen triple helix; vitamin C deficiency is the cause of scurvy, which manifests in a variety of symptoms including a tendency to hemorrhage because of inadequate connective tissue support of small blood vessels, and impaired wound healing. Type III collagen fibers, often called reticular fibers, group in small bundles (visible in LM only with special stains) that form a loose three-dimensional network (reticulum) or scaffold that is the main support of loose connective tissue, especially the lamina propria of hollow organs. Reticular fibers also form the 3- dimensional scaffolding that underlies the structure of solid organs such as lymph nodes, spleen and liver. Cells secreting Type III Collagen are a subset of fibroblasts called mesenchymal reticular cells or fibroblastic reticular cells. Mutations in Type III collagen cause a kind of Ehlers Danlos syndrome (Type IV), in which there are severe defects in the structural stability of organs especially blood vessels and intestines, which become prone to rupture. Elastic fibers: Elastic fibers confer elasticity on connective tissues. They are synthesized by fibroblasts and smooth muscle cells as tropoelastin, which is secreted and assembled extracellularly to form fibers (e.g. in skin) and sheets (e.g. in the walls of arteries). Assembly requires the glycoprotein fibrillin, which is incorporated into the elastic fibers and sheets. The elastin molecule is composed of short hydrophobic segments that are cross-linked to each other. The tendency of the hydrophobic segments to coil on themselves is what gives elastin its elasticity. Block: Foundations | LYBARGER [7 of 17] TISSUE TYPES (INDEPENDENT LEARNING) Marfan’s syndrome is a disease in which there is a genetic defect in fibrillin. This causes defects in elastic fibers throughout the body, and the most serious consequence in affected individuals is that the aorta (an elastic artery – more about this when we discuss blood vessels) is subject to aneurysm and/or rupture. Ground Substance: The cells and fibers of connective tissue are embedded in a gel- like matrix or ground substance composed mainly of proteoglycans and hyaluronan, and containing important adhesive glycoproteins which mediate cell migration and regulate cell differentiation. Proteoglycans: Proteoglycans are like glycoproteins in that they are synthesized in the RER and are composed of a core protein with sugar side chains. They differ from glycoproteins in the length and configuration of their sugar sidechains: in glycoproteins the sugar side chains are branched and tend to be short (oligosaccharides), whereas in proteoglycans they are very long and unbranched. These long polysaccharide chains are called glycosaminoglycans (GAGs). They are highly negatively charged and therefore repel each other, such that they stick out from the protein core like the bristles on a bottle brush. The negative charges of the GAGs attract cations which in turn attract water, thus forming a highly hydrated gel which fills volume, resists compression, and provides a space through which small molecules and cells can travel. There are a number of different kinds of GAGs, and proteoglycans are often named after the particular GAG which forms their sidechains. Dermatan sulphate proteoglycan is present in skin, aggrecan (containing chondroitin sulphate and keratan sulphate GAGs) is present in cartilage and in developing heart and brain, heparan sulphate proteoglycan is present in basal lamina. Block: Foundations | LYBARGER [8 of 17] TISSUE TYPES (INDEPENDENT LEARNING) Hyaluronan (=hyaluronic acid) Hyaluronan is an unusual GAG in that it is the only one that exists not linked to a core protein (i.e. as part of a proteoglycan) and not synthesized in the RER (rather it is synthesized on the plasma membrane of fibroblasts by enzymes secreted by the fibroblast). Hyaluronan molecules are immense. They are composed of up to 25,000 repeating disaccharide units, each carrying negative charge. As with other GAGs, hydration of hyaluronan forms a gel- like substance. Hyaluronan is present in the cavities of joints, where it acts as a lubricant, and in the vitreous of the eye, where it allows light transmission. Hyaluronan binds aggrecan proteoglycan and forms huge aggregates that fill vast molecular domains and provide strong resistance to compression in cartilage as well as in developing heart and brain. Fibronectins are adhesive glycoproteins of the ECM that attach cells to collagens. They bind to integrins, transmembrane proteins that mediate attachment of cells to extracellular matrix. They are essential for migration of macrophages and other immune cells during inflammation and wound healing, and of many cell types during embryogenesis. BASAL LAMINA: Epithelial cells secrete a molecular layer called the basal lamina (sometimes referred to as basement membrane), which anchors the epithelium to the substrate. Major components of basal lamina include laminin, an adhesive glycoprotein which binds to integrins in the basal membranes of the epithelial cells and to Type IV collagen in the basal lamina. Type IV collagen forms a sheet-like network that binds laminin to other components of the basal lamina and to underlying extracellular matrix. It is essential for basal lamina integrity. (Note that Schwann cells in nervous tissue, skeletal muscle cells, and adipocytes also secrete a basal layer that is sometimes called a basal lamina.) CELLS OF CONNECTIVE TISSUE PROPER: RESIDENT CELLS Primitive Mesenchymal Cell: derived from embryonic mesenchyme small inconspicuous cells usually situated alongside small blood vessels thought to serve as connective tissue stem cells Block: Foundations | LYBARGER [9 of 17] TISSUE TYPES (INDEPENDENT LEARNING) Fibroblasts/fibrocytes: derive from primitive mesenchymal cell; synthesize extracellular matrix molecules including collagen fibers, elastic fibers, proteoglycans and glycosaminoglycans; a major player in wound healing and fibrosis (scarring): during wound healing and during growth fibroblasts are capable of locomotion and cell division during wound healing fibroblasts differentiate into myofibroblasts which contract and help shrink the wound Adipocytes: derive from primitive mesenchymal cell; very large cells each containing a single large lipid droplet which displaces the nucleus and other organelles into a thin rim of cytoplasm around the periphery. store and mobilize lipids based on energy needs of the body Mast Cells: derived from stem cells in the bone marrow, travel to CT via the blood reside alongside small blood vessels have abundant secretory granules containing histamine, heparin and proteolytic enzymes degranulate in response to mechanical injury, some toxins, allergens; degranulation in response to allergens is mediated by IgE antibodies, for which mast cells have receptors function in initiation of inflammatory response: increase vascular permeability (promoting edema), cause itching; histamine from mast cells is an important mediator of allergic reactions including anaphylaxis, a systemic allergic reaction which can cause death in minutes due to bronchoconstriction and severe drop in blood pressure. Plasma Cells: differentiate from antigen-stimulated B lymphocytes reside in connective tissue, usually near the site where antigen encounter by the parent B cell occurred secrete antibodies (up to 2000 molecules/second) occur in increased numbers during chronic inflammation WHITE BLOOD CELLS IN CONNECTIVE TISSUE: White blood cells (also called leukocytes), including neutrophils, eosinophils, monocytes (macrophages in tissues; aka histiocytes), and lymphocytes originate in bone marrow, circulate in blood, and function in tissues. Obviously, for this cell migration to happen there has to be a mechanism for WBCs to leave blood vessels and enter tissues. Block: Foundations | LYBARGER [10 of 17] TISSUE TYPES (INDEPENDENT LEARNING) They do this through the walls of post-capillary venules. Post- capillary venules are the leakiest part of the blood vascular system (even more leaky than most capillaries). In response to chemotactic factors (inflammatory mediators) produced in tissues, gaps form between endothelial cells lining post-capillary venules. In addition, post-capillary venule endothelial cells express surface receptors that bind to white blood cells and promote their transfer across the endothelium and into connective tissue. Neutrophils and eosinophils enter tissues and survive for hours or days (depending on their level of activity) before they die there and become part of the debris of inflammation. Monocytes enter tissues and become macrophages; as macrophages they live for several months (or less if they are called into action), then die without returning to the blood circulation. A small subset of tissue macrophages travel from the tissues via lymphatic capillaries and larger lymphatic to lymph nodes, where they present antigen then die, but they don’t reenter the blood. For these white blood cells, travel through the body is from their site of origin in the bone marrow, into the blood vascular system where the cells circulate until they are stimulated by chemotactic factors to exit the blood. WBC’s exit the blood at post-capillary venules to enter tissues, where they function and die. Lymphocytes can also follow this route from blood circulation to connective tissues. Further, they have a unique pattern of circulation between lymphatics and the blood that will be discussed later with the immune system. Block: Foundations | LYBARGER [11 of 17] TISSUE TYPES (INDEPENDENT LEARNING) MUSCLE TISSUE OVERVIEW There are three main types of muscle in the body: skeletal, cardiac, and smooth muscle. All can contract and generate tension, but they differ in several important aspects of their structure and function. In the Foundations Block, we will compare and contrast the three types of muscle at the level of cells and tissues. Structural and functional features of these tissues will be presented in more detail in later blocks. SMOOTH MUSCLE Smooth muscle provides relatively weak, slow contractions, not under voluntary control. Smooth muscle is found in the walls of hollow organs – the gastrointestinal tract, portions of the reproductive and urinary tracts, in the walls of blood vessels, in respiratory passages. Smooth muscle strengthens the walls of hollow organs. In the gut, its contraction drives peristalsis. In blood vessels and bronchi, its contraction regulates blood pressure and flow and air flow, respectively. Smooth muscle is also present in skin, where it causes erection of hairs. Smooth muscle normally contains few blood vessels, although it is always bordered by connective tissue that contains more blood vessels, and nutrients from those vessels can reach smooth muscle cells by diffusion. Smooth muscle cells' metabolic needs are less than those of skeletal or cardiac muscle, so they can tolerate this more limited blood supply. Structure as seen at low magnification: Smooth muscle fibers are elongated cells, wider in the central portion and with tapered ends. Each cell contains a single nucleus, located near the center of the cell. Cells in groups are arranged parallel with each other, generally with the tapered ends of one cell nestled between the enlarged central portions of its neighbors. Because of the shapes and alignment of the cells, when smooth muscle is cut in cross section, individual cells show different apparent diameters, and nuclei can be seen in some cells but not others. This is typical of smooth muscle and different from skeletal or cardiac muscle. Structure as seen at high magnification: Smooth muscle cells contract, but their contractile proteins are not regularly organized into sarcomeres, so the cells have no striations – thus, the name "smooth" muscle. Actin filaments in Block: Foundations | LYBARGER [12 of 17] TISSUE TYPES (INDEPENDENT LEARNING) smooth muscle cells attach to each other and to the plasma membrane at structures called "dense bodies," which are visible in the electron microscope, but not the light microscope. Regeneration Smooth muscle contains no special cells with stem cell-like properties, but when smooth muscle is damaged, differentiated smooth muscle cells can reenter the cell cycle, proliferate, and then re-differentiate to repair the damage. SKELETAL MUSCLE Skeletal muscle provides rapid, powerful contractions, under voluntary control. A skeletal muscle is a gross-anatomical structure – an organ – that has an origin and insertion, innervation, and blood supply, and is surrounded by a dense, collagenous sheath of connective tissue. Most muscles are composed of multiple fascicles that run parallel to each other along the length of the muscle. Large blood vessels and nerves penetrate the outer sheath of the muscle, and then branch within the connective tissue between fascicles and around individual fibers. Microscopic organization A muscle fascicle is composed of multiple cells, each of which is called a muscle fiber, or myofiber. Individual muscle fibers are long and unbranched. Some may extend the full length of the muscle. Skeletal muscle fibers vary in diameter from muscle to muscle, but in most muscles they are very large (about 80-100 µm diameter). In normal skeletal muscle, all the fibers in a given muscle are about the same size. Each muscle cell (fiber) has multiple nuclei. You can see this most clearly in longitudinal sections. In normal muscle, almost all of the nuclei are located at the periphery of the cell, just under the plasma membrane. You can see this most clearly in cross sections. The presence of multiple nuclei reflects the development of the muscle fiber: muscle fibers form by the fusion of multiple precursor cells (the precursors are called myoblasts). Longitudinal sections normally reveal regularly spaced cross- striations within the fibers. These reflect the highly ordered arrangement of the cell's contractile proteins into repeating units called sarcomeres. The molecular structure and function of sarcomeres will be discussed in more detail in the Musculoskeletal Block. Block: Foundations | LYBARGER [13 of 17] TISSUE TYPES (INDEPENDENT LEARNING) Each muscle fiber is surrounded by a sheath that includes both a basal lamina surrounding each cell and a fine, loose connective tissue. A network of small blood vessels runs in this connective tissue and surrounds each muscle fiber. Fine branches of nerves also run in the connective tissue. In electron micrographs of skeletal muscle, you sometimes see small, flat cells that sit inside a muscle fiber’s basal lamina but separate from the muscle fiber; they usually sit in a slight depression in the muscle fiber’s surface. These are satellite cells. Satellite cells in muscle have stem cell-like properties. Satellite cells normally are metabolically inactive, but damage to the muscle can activate them, causing them to grow, fuse with damaged myofibers to help repair them or with each other to produce new fibers. Intense exercise can also activate satellite cells. CARDIAC MUSCLE Cardiac muscle provides regular, powerful contractions, not under voluntary control. Cardiac muscle is found only in the heart and the portion of the pulmonary veins where they join the heart. Cardiac muscle constitutes the bulk of the heart. Comparison to skeletal muscle Cardiac muscle bears several similarities to skeletal muscle: - individual muscle fibers are long - groups of muscle fibers are organized into fascicles - individual muscle fibers appear striated and contain sarcomeres Cardiac muscle displays several differences from skeletal muscle, as follows: - individual cardiac muscle fibers can be branched - each cardiac muscle fiber has only 1 or 2 nuclei - nuclei of cardiac muscle fibers are commonly located near the center of the cell rather than on the periphery - cardiac muscle fibers are connected end-to-end via intercalated discs Striations Cardiac muscle cells normally look striated in longitudinal section, like skeletal muscle. As in skeletal muscle, the striations reflect the highly ordered arrangement of the cell's contractile proteins into sarcomeres. Intercalated discs Intercalated discs are found throughout cardiac muscle tissue. They are located where cardiac muscle fibers abut each other Block: Foundations | LYBARGER [14 of 17] TISSUE TYPES (INDEPENDENT LEARNING) end-to-end. Intercalated discs are unique to cardiac muscle, so if you see an intercalated disc, you know you are looking at cardiac muscle! Intercalated discs perform three major functions: 1) Anchor sarcomeres (adherens junctions within the disc anchor actin filaments). 2) Provide intercellular adhesion. They attach abutting myofibers to each other via desmosomes. 3) Allow rapid communication of electrical and chemical signals between muscle fibers via gap junctions. This allows coordinated contraction of the muscle fibers, and ensures that cells respond to hormonal regulation uniformly. Structural components of intercalated discs include: 1) A fascia adherens, which resembles a zonula adherens between epithelial cells – it both anchors sarcomeres and contributes to cell-cell adhesion. 2) Desmosomes, like those found in epithelia – it also contributes to cell-cell adhesion. 3) Gap junctions – they allow rapid communication between neighboring cells. Satellite cells and regeneration Cardiac muscle contains very few satellite cells or stem cells, so damaged cardiac muscle tissue does not regenerate to any significant extent. This inability is obviously very important clinically. NEURAL TISSUE OVERVIEW CNS vs. PNS The nervous system is composed of two major structural subdivisions, the Central Nervous System (CNS), consisting of the brain and spinal cord, and the Peripheral Nervous System (PNS). The PNS includes peripheral nerves and localized clusters of neuron cell bodies that are called ganglia. Nervous tissue will be studied in detail in the Nervous System block. The Foundations block will mainly consider two components of the PNS that you can see in other organs: peripheral nerves and parasympathetic ganglia. Peripheral nerves contain the axons of neurons whose cell bodies are located in the CNS or in clusters of neurons (ganglia) outside the CNS (e.g., dorsal root ganglia or sympathetic chain ganglia), as well as the (non-neuronal) glial cells that support them. Glial cells of the PNS are called Schwann cells. Block: Foundations | LYBARGER [15 of 17] TISSUE TYPES (INDEPENDENT LEARNING) GANGLIA Any collection of nerve cell bodies outside the central nervous system is called a ganglion. There are three main types of ganglia in the PNS: Dorsal root ganglia, situated on the spinal nerves where they enter the spinal column, contain the cell bodies of sensory nerves carrying sensory stimuli from the periphery into the spinal cord. Sympathetic ganglia, containing cell bodies of neurons that make up the sympathetic arm of the autonomic nervous system are situated along the length of the spinal column in the sympathetic chain of ganglia. Parasympathetic ganglia, on the other hand, are mainly situated within the organs that they innervate. Parasympathetic ganglia are easiest to find in organs of the digestive tube, since they are consistently present within the myenteric plexus, which is located between the longitudinal and circular layers of smooth muscle that make up the muscularis propria (aka muscularis externa). The neurons of the ganglion innervate the surrounding smooth muscle and maintain the peristaltic activity of the alimentary canal. Like all peripheral nervous tissue, dorsal root ganglia, sympathetic ganglia and parasympathetic ganglia contain numerous Schwann cells, which support the neurons' function. PERIPHERAL NERVE The extension of a neuron that it uses to transmit signals to other neurons or other target cells is called an axon. Many axons can bundle together to form a small nerve or nerve fascicle, and several fascicles can bundle to form a nerve. Nerve fascicles are surrounded by a specialized layer of connective tissue called the perineurium. This is composed of modified fibroblasts that form a continuous sheath sealed cell to cell by tight junctions. It is an essential structure for allowing maintenance of an appropriate ionic environment within the nerve fascicle. Peripheral nerves range in size from nearly an inch in diameter to fine, nearly invisible threads. Some are sensory, others are motor, and some are mixed function. They can contain myelinated axons, unmyelinated axons, or a mixture of the two. Schwann cells in peripheral nerves Peripheral nerves contain many nuclei, and nearly all of them belong to Schwann cells, which reside within the nerve fascicle in close association with neuronal cell processes. Each Schwann Block: Foundations | LYBARGER [16 of 17] TISSUE TYPES (INDEPENDENT LEARNING) cell surrounds a small segment of a neuronal cell process, and there are many Schwann cells present along each neuron, such that the neuron travels in a continuous sheath of surrounding Schwann cells. This is the case for both myelinated and unmyelinated nerve fibers. In myelinated nerve fibers, Schwann cells form the myelin sheath. They do this by wrapping multiple times around the neuron like a jelly roll, forming many layers of plasma membrane - collectively called myelin - that provide electrical insulation for the segment of neuron covered by the sheath. The narrow gap between myelin segments is called a "node of Ranvier." Nodes of Ranvier have a critical function in conduction of action potentials that will be discussed in the Nervous System block. They also have a distinctive appearance in the microscope that can help you identify peripheral nerve in tissue sections. In the case of unmyelinated nerves, the function of Schwann cells is thought to be related to maintaining the proper environment surrounding the axon rather than increasing conduction velocity. Block: Foundations | LYBARGER [17 of 17]