Chapter 6: The Tissue Level of Organisation PDF

11/30/22, 2:22 PM 6 Chapter 6: The Tissue Level of Organisation THE TISSUE LEVEL OF ORGANISATION Figure 6.1. Micrograph of Cervical Tissue. This figure is a view of the regular architecture of normal tissue contrasted with the irregular arrangement of cancerous cells. (credit: “Haymanj”/Wikimedi...

11/30/22, 2:22 PM 6 Chapter 6: The Tissue Level of Organisation THE TISSUE LEVEL OF ORGANISATION Figure 6.1. Micrograph of Cervical Tissue. This figure is a view of the regular architecture of normal tissue contrasted with the irregular arrangement of cancerous cells. (credit: “Haymanj”/Wikimedia Commons) CHAPTER OBJECTIVES After studying this chapter, you will be able to: file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 1/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation 6.1 Identify the main tissue types and discuss their roles in the human body 6.2 Explain the functions of various epithelial tissues and how their forms enable their functions 6.3 Explain the functions of various connective tissues and how their forms enable their functions 6.4 Describe the characteristics of muscle tissue and how these enable function 6.5 Discuss the characteristics of nervous tissue and how these enable information processing The body contains at least 200 distinct cell types. These cells contain essentially the same internal structures yet they vary enormously in shape and function. The different types of cells are not randomly distributed throughout the body; rather they occur in organized layers, a level of organization referred to as tissue. The micrograph that opens this chapter shows the high degree of organization among different types of cells in the tissue of the cervix. You can also see how that organization breaks down when cancer takes over the regular mitotic functioning of a cell. The variety in shape reflects the many different roles that cells fulfill in your body. The human body starts as a single cell at fertilization. As this fertilized egg divides, it gives rise to trillions of cells, each built from the same blueprint, but organizing into tissues and becoming irreversibly committed to a developmental pathway. The term tissue is used to describe a group of cells found together in the body. The cells within a tissue share a common embryonic origin. Microscopic observation reveals that the cells in a tissue share morphological features and are arranged in an orderly pattern that achieves the tissue’s functions. Although there are many types of cells in the human body, they are organized into four broad categories of tissues: epithelial, connective, muscle, and nervous. Each of these categories is characterized by specific functions that contribute to the overall health and maintenance of the body. A disruption of the structure is a sign of injury or disease. Such changes can be detected through histology, the microscopic study of tissue appearance, organization, and function. 6.1 The Four Types of Tissues Epithelial tissue, also referred to as epithelium, refers to the sheets of cells that cover exterior surfaces of the body, line internal cavities and passageways, and form certain glands. Connective tissue, as its name implies, binds the cells and organs of the body together and functions in the protection, support, and integration of all parts of the body. Muscle tissue is excitable, responding to stimulation and contracting to provide movement, and occurs as three major types: skeletal (voluntary) muscle, smooth muscle, and cardiac muscle in the heart. Nervous tissue is also excitable, allowing the propagation of electrochemical signals in the form of nerve impulses that communicate between different regions of the body (Figure 6.2). The next level of organization is the organ, where several types of tissues come together to form a working unit. Just as knowing the structure and function of cells helps you in your study of tissues, knowledge of tissues will help you understand how organs function. The epithelial and connective tissues are discussed in detail in this chapter. Muscle and nervous tissues will be discussed only briefly in this chapter. file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 2/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation Figure 6.2. The four types of tissues in the body: nervous tissue, epithelial tissue, muscle tissue, and connective tissue. Clockwise from nervous tissue, LM × 872, LM × 282, LM × 460, LM × 800. (Micrographs provided by the Regents of University of Michigan Medical School © 2012) 6.2 Epithelial Tissue Most epithelial tissues are essentially large sheets of cells covering all the surfaces of the body exposed to the outside world and lining the outside of organs. Epithelium also forms much of the glandular tissue of the body. Skin is not the only area of the body exposed to the outside. Other areas include the airways, the digestive tract, as well as the urinary and reproductive systems, all of which are lined by an epithelium. Hollow organs and body cavities that do not connect to the exterior of the body, which includes, blood vessels and serous membranes, are lined by endothelium (plural = endothelia), which is a type of epithelium. All epithelia share some important structural and functional features. This tissue is highly cellular, with little or no extracellular material present between cells. Adjoining cells form a specialized intercellular connection between their cell membranes called a cell junction. The epithelial cells exhibit polarity with differences in structure and function between the exposed or apical facing surface of the cell and the basal surface close to the underlying body structures. The basement membrane, a mixture of glycoproteins and collagen, provides an attachment site for the epithelium, separating it from underlying connective tissue. file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 3/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation Epithelial tissues are nearly completely avascular. For instance, no blood vessels cross the basement membrane to enter the tissue, and nutrients must come by diffusion or absorption from underlying tissues or the surface. Many epithelial tissues are capable of rapidly replacing damaged and dead cells. Sloughing off of damaged or dead cells is a characteristic of surface epithelium and allows our airways and digestive tracts to rapidly replace damaged cells with new cells. 6.2.1 Generalized Functions of Epithelial Tissue Epithelial tissues provide the body’s first line of protection from physical, chemical, and biological wear and tear. The cells of an epithelium act as gatekeepers of the body controlling permeability and allowing selective transfer of materials across a physical barrier. All substances that enter the body must cross an epithelium. Some epithelia often include structural features that allow the selective transport of molecules and ions across their cell membranes. Many epithelial cells are capable of secretion and release mucous and specific chemical compounds onto their apical surfaces. The epithelium of the small intestine releases digestive enzymes, for example. Cells lining the respiratory tract secrete mucous that traps incoming microorganisms and particles. A glandular epithelium contains many secretory cells. 6.2.2 The Epithelial Cell Epithelial cells are typically characterized by the polarized distribution of organelles and membrane-bound proteins between their basal and apical surfaces. Particular structures, such as cilia and microvilli, are found in some epithelial cells as adaptations to specific functions. Certain organelles are segregated to the basal sides, whereas other organelles and extensions are on the apical surface. Cilia are microscopic extensions of the apical cell membrane that are supported by microtubules. They beat in unison and move fluids as well as trapped particles. The ciliated epithelium of your airway forms a mucociliary escalator that sweeps particles of dust and pathogens trapped in the secreted mucous toward the throat. It is called an escalator because it continuously pushes mucous with trapped particles upward. In contrast, nasal cilia sweep the mucous blanket down towards your throat. In both cases, the transported materials are usually swallowed, and end up in the acidic environment of your stomach. Microvilli are cylindrical apical surface extensions of the plasma membrane of some epithelial cells. Although their small size makes it difficult to see each microvillus, their combined microscopic appearance suggests a mass of bristles, which is termed the brush border. There are an estimated 200 million microvilli per square millimeter of small intestine, greatly expanding the surface area of the plasma membrane and thus greatly enhancing absorption. 6.2.3 Cell to Cell Junctions Cells of epithelia are closely connected and separated by very little intercellular material. Three basic types of connections allow varying degrees of interaction between the cells: tight junctions, anchoring junctions, and gap junctions (Figure 6.3). file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 4/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation Fig 6.3. The Three Types of Cell Junctions At one end of the spectrum is the tight junction, which separates the cells into apical and basal compartments. When two adjacent epithelial cells form a tight junction, there is no extracellular space between them and the movement of substances through the extracellular space between the cells is blocked. This enables the epithelia to act as selective barriers. An anchoring junction includes several types of cell junctions that help stabilize epithelial tissues. Anchoring junctions are common on the lateral and basal surfaces of cells where they provide strong and flexible connections. There are three types of anchoring junctions: desmosomes, hemidesmosomes, and adherens. Desmosomes occur in patches on the membranes of cells. The patches are structural proteins on the inner surface of the cell’s membrane that attach to a similar patch on the adjacent cell. These connections are especially important in holding cells firmly together while allowing for flexibility at the same time. Hemidesmosomes, which look like half a desmosome, link cells to the extracellular matrix, for example, the basement membrane. Adherens junctions can connect cells by forming a belt-like structure inside the cell. These junctions influence the shape and folding of the epithelial tissue. In contrast with the tight and anchoring junctions, a gap junction forms an intercellular passageway between the membranes of adjacent cells to facilitate the movement of small molecules and ions between the cytoplasm of adjacent cells. These junctions allow electrical and metabolic coupling of adjacent cells, which coordinates function in large groups of cells. file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 5/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation 6.2.4 Classification of Epithelial Tissues Epithelial tissues are classified according to the shape of the cells and number of the cell layers formed (Figure 6.4). Cell shapes can be squamous (flattened and thin), cuboidal (boxy, as wide as it is tall), or columnar (rectangular, taller than it is wide). Similarly, the number of cell layers in the tissue can be one—where every cell rests on the basement membrane —which is a simple epithelium, or more than one, which is a stratified epithelium and only the basal layer of cells rests on the basement membrane. Pseudostratified (pseudo- = “false”) describes tissue with a single layer of irregularly shaped cells that give the appearance of more than one layer. Transitional describes a form of specialized stratified epithelium in which the shape of the cells can vary. Figure 6.4. Simple epithelial tissue is organized as a single layer of cells and stratified epithelial tissue is formed by several layers of cells. Simple Epithelium The shape of the cells in the single cell layer of simple epithelium reflects the functioning of those cells. The cells in simple squamous epithelium have the appearance of thin scales. Squamous cell nuclei tend to be flat, horizontal, and elliptical, mirroring the form of the cell. The endothelium is the epithelial tissue that lines vessels of the lymphatic and cardiovascular system, and it is made up of a single layer of squamous cells. Simple squamous epithelium, because of the thinness of the cell, is present where rapid passage of chemical compounds is observed. The alveoli of lungs where gases diffuse, segments of kidney tubules, and the lining of capillaries are also made of simple squamous epithelial tissue. In simple cuboidal epithelium, the nucleus of the box-like cells appears round and is generally located near the center of the cell. These epithelia are active in the secretion and absorptions of molecules. Simple cuboidal epithelia are observed lining file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 6/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation most of the kidney tubules, at the ovary surface and in the ducts of glands. In simple columnar epithelium, the nucleus of the tall column-like cells tends to be elongated and located in the basal end of the cells. Like the cuboidal epithelia, this epithelium is active in the absorption and secretion of molecules. Simple columnar epithelium forms the lining of some sections of the digestive system (stomach, intestines) and parts of the female reproductive tract (upper cervix). Ciliated columnar epithelium is composed of simple columnar epithelial cells with cilia on their apical surfaces. These epithelial cells are found in the lining of the fallopian tubes and parts of the respiratory system (bronchioles), where the beating of the cilia helps remove particulate matter. Pseudostratified columnar epithelium is a type of epithelium that appears to be stratified but instead consists of a single layer of irregularly shaped and differently sized columnar cells. In pseudostratified epithelium, nuclei of neighboring cells appear at different levels rather than clustered in the basal end. The arrangement gives the appearance of stratification; but in fact all the cells are in contact with the basement membrane, although some do not reach the apical surface. Pseudostratified columnar epithelium is found in parts of the respiratory tract, where some of these cells have cilia. Both simple and pseudostratified columnar epithelia are heterogeneous epithelia because they include additional types of cells interspersed among the epithelial cells. For example, a goblet cell is a mucous-secreting unicellular “gland” interspersed between the columnar epithelial cells of mucous membranes (Figure 6.5). Figure 6.5. Goblet Cell. (a) In the lining of the small intestine, columnar epithelium cells are interspersed with goblet cells. (b) The arrows in this micrograph point to the mucous-secreting goblet cells. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 7/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation INTERACTIVE LINK View the University of Michigan WebScope to explore the tissue sample in greater detail. Stratified Epithelium A stratified epithelium consists of several stacked layers of cells. This epithelium protects against physical and chemical wear and tear. The stratified epithelium is named by the shape of the most apical layer of cells, closest to the free space. Stratified squamous epithelium is the most common type of stratified epithelium in the human body. The apical cells are squamous, whereas the basal layer contains either columnar or cuboidal cells. The top layer may be covered with dead cells filled with keratin. Mammalian skin is an example of this dry, keratinized, stratified squamous epithelium. The lining of the mouth cavity is an example of an unkeratinized, stratified squamous epithelium. Stratified cuboidal epithelium and stratified columnar epithelium can also be found in certain glands and ducts, but are uncommon in the human body. Another kind of stratified epithelium is transitional epithelium, so-called because of the gradual changes in the shapes of the apical cells as the bladder fills with urine. It is found only in the urinary system, specifically the ureters, urinary bladder and urethra. When the bladder is empty, this epithelium is convoluted and has cuboidal apical cells with convex, umbrella shaped, apical surfaces. As the bladder fills with urine, this epithelium loses its convolutions and the apical cells transition from cuboidal to squamous. It appears thicker and more multi-layered when the bladder is empty, and more stretched out and less stratified when the bladder is full and distended. Figure 6.6 summarizes the different categories of epithelial cell tissue cells. file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 8/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation Fig 6.6 Summary of Epithelial Tissue Types INTERACTIVE LINK file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 9/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation Watch this video to find out more about the anatomy of epithelial tissues. Where in the body would one find non-keratinizing stratified squamous epithelium? Glandular Epithelium A gland is a structure made up of one or more cells modified to synthesize and secrete chemical substances. Most glands consist of groups of epithelial cells. A gland can be classified as an endocrine gland, a ductless gland that releases secretions directly into surrounding tissues and fluids (endo- = “inside”), or an exocrine gland whose secretions leave through a duct that opens directly, or indirectly, to the external environment (exo- = “outside”). The pancreas acts as both an endocrine and exocrine gland (Figure 6.7) Endocrine Glands The secretions of endocrine glands are called hormones. Hormones are released into the interstitial fluid, diffused into the bloodstream, and delivered to targets, in other words, cells that have receptors to bind the hormones. The endocrine system is part of a major regulatory system coordinating the regulation and integration of body responses. A few examples of endocrine glands include the anterior pituitary, thymus, adrenal cortex, and gonads. Exocrine Glands Exocrine glands release their contents through a duct that leads to the epithelial surface. Mucous, sweat, saliva, and breast milk are all examples of secretions from exocrine glands. They are all discharged through tubular ducts. Secretions into the lumen of the gastrointestinal tract, technically outside of the body, are of the exocrine category. Exocrine glands are classified as either unicellular or multicellular. The unicellular glands are scattered single cells, such as goblet cells, found in the mucous membranes of the small and large intestine, as well as certain regions of the respiratory tract. The multicellular exocrine glands secrete directly into an inner cavity by releasing their contents through a tubular duct. file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 10/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation Figure 6.7. Acinar cells of the pancreas secrete digestive enzymes into a duct whereas the Islets of Langerhans secrete insulin directly into the bloodstream. 6.3 Connective Tissue As may be obvious from its name, one of the major functions of connective tissue is to connect tissues and organs. Unlike epithelial tissue, which is composed of cells closely packed with little or no extracellular space in between, connective tissue cells are dispersed in a matrix. The matrix usually includes a large amount of extracellular material produced by the connective tissue cells that are embedded within it. The matrix plays a major role in the functioning of this tissue. The major component of the matrix is a ground substance often crisscrossed by protein fibers. This ground substance can range from being fluid to mineralized and solid, as in bones. Connective tissues come in a vast variety of forms, yet they typically have in common three characteristic components: cells, large amounts of amorphous ground substance, and protein fibers. The amount and structure of each component correlates with the function of the tissue, from the rigid ground substance in bones supporting the body to the inclusion of specialized cells; for example, a phagocytic cell that engulfs pathogens and also rids tissue of cellular debris. Functions of Connective Tissues Connective tissues perform many functions in the body, but most importantly, they support and connect other tissues; from the connective tissue sheath that surrounds muscle cells, to the tendons that attach muscles to bones, and to the skeleton that supports the positions of the body. Protection is another major function of connective tissue, in the form of fibrous capsules and bones that protect delicate organs and, of course, the skeletal system. Specialized cells in connective tissue defend the body file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 11/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation from microorganisms that enter the body. Transport of fluid, nutrients, waste, and chemical messengers is ensured by specialized fluid connective tissues, such as blood and lymph. Adipose cells store surplus energy in the form of fat and contribute to the thermal insulation of the body. Classification of Connective Tissues The three broad categories of connective tissue are classified according to the characteristics of their ground substance and the types of fibers found within the matrix. Connective tissue proper includes loose connective tissue and dense connective tissue. Both tissues have a variety of cell types and protein fibers suspended in a viscous ground substance. Dense connective tissue is reinforced by bundles of fibers that provide tensile strength, elasticity, and protection. In loose connective tissue, the fibers are loosely organized, leaving large spaces in between. Supportive connective tissue—bone and cartilage—provide structure and strength to the body and protect soft tissues. A few distinct cell types and densely packed fibers in a matrix characterize these tissues. In bone, the matrix is rigid and described as calcified because of the deposited calcium salts. In fluid connective tissue, in other words, lymph and blood, various specialized cells circulate in a watery fluid containing salts, nutrients, and dissolved proteins. Connective Tissue Proper Fibroblasts are present in all connective tissue proper (Figure 6.8). Fibrocytes and adipocytes are fixed cells, which means they remain within the connective tissue. Other cells move in and out of the connective tissue in response to chemical signals. Macrophages, mast cells, lymphocytes, plasma cells, and phagocytic cells are found in connective tissue proper but are actually part of the immune system protecting the body. Figure 6.8. Connective Tissue Proper. Fibroblasts produce this fibrous tissue. LM × 400. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Cell Types file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 12/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation The most abundant cell in connective tissue proper is the fibroblast. Polysaccharides and proteins secreted by fibroblasts combine with extracellular fluids to produce a viscous ground substance that, with embedded fibrous proteins, forms the extracellular matrix. Adipocytes are cells that store lipids as droplets that fill most of the cytoplasm. There are two basic types of adipocytes: white and brown. The brown adipocytes store lipids as many droplets, and have high metabolic activity. In contrast, white fat adipocytes store lipids as a single large drop and are metabolically less active. Their effectiveness at storing large amounts of fat is witnessed in obese individuals. The number and type of adipocytes depends on the tissue and location, and vary among individuals in the population. The macrophage cell is a large cell derived from a monocyte, a type of blood cell, which enters the connective tissue matrix from the blood vessels. Macrophages are an essential component of the immune system, and act by engulfing infectious agents and cellular debris. When stimulated, macrophages can also release cytokines, small proteins that act as chemical messengers, to recruit other cells of the immune system to infected sites. The mast cell, found in connective tissue proper, has many cytoplasmic granules. These granules contain the chemical signals histamine and heparin. When irritated or damaged, mast cells release histamine, an inflammatory mediator, which causes vasodilation and increased blood flow at a site of injury or infection, along with itching, swelling, and redness you recognize as an allergic response. Like blood cells, mast cells are derived from hematopoietic stem cells and are part of the immune system. Connective Tissue Fibers and Ground Substance Three main types of fibers are secreted by fibroblasts: collagen fibers, elastic fibers, and reticular fibers. Collagen fiber is made from fibrous protein subunits linked together to form a long and straight fiber. Collagen fibers, while flexible, have great tensile strength, resist stretching, and give ligaments and tendons their characteristic resilience and strength. These fibers hold connective tissues together, even during the movement of the body. Elastic fiber contains the protein elastin along with lesser amounts of other proteins and glycoproteins. The main property of elastin is that after being stretched or compressed, it will return to its original shape. Elastic fibers are prominent in elastic tissues found in skin and the elastic ligaments of the vertebral column. Reticular fiber is also formed from the same protein subunits as collagen fibers; however, these fibers remain narrow and are arrayed in a branching network. They are found throughout the body, but are most abundant in the reticular tissue of soft organs, such as liver and spleen, where they anchor and provide structural support to the functional cells, blood vessels, and nerves of the organ. All of these fiber types are embedded in ground substance. Secreted by fibroblasts, ground substance is made of polysaccharides, and proteins. These combine to form a proteoglycan with a protein core and polysaccharide branches. The proteoglycan attracts and traps available moisture forming the clear, viscous, colorless matrix you now know as ground substance. Loose Connective Tissue file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 13/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation Loose connective tissue is found between many organs where it acts both to absorb shock and bind tissues together. It allows water, salts, and various nutrients to diffuse through to adjacent or embedded cells and tissues. Adipose tissue consists mostly of fat storage cells, with little extracellular matrix (Figure 6.9). A large number of capillaries allow rapid storage and mobilization of lipid molecules. White adipose tissue is most abundant. White fat contributes mostly to lipid storage and can serve as insulation from cold temperatures and mechanical injuries. White adipose tissue can be found protecting the kidneys and cushioning the back of the eye. Brown adipose tissue is more common in infants, hence the term “baby fat.” In adults, there is a reduced amount of brown fat. The many mitochondria in the cytoplasm of brown adipose tissue help explain its efficiency at metabolizing stored fat. Figure 6.9. Adipose Tissue. This is a loose connective tissue that consists of fat cells with little extracellular matrix. It stores fat for energy and provides insulation. LM × 800. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Areolar tissue shows little specialization. It contains all the cell types and fibers previously described and is distributed in a random, web-like fashion. It fills the spaces between muscle fibers, surrounds blood and lymph vessels, and supports organs in the abdominal cavity. Areolar tissue underlies most epithelia and represents the connective tissue component of epithelial membranes, which are described further in a later section. Reticular tissue is a mesh-like, supportive framework for soft organs such as lymphatic tissue, the spleen, and the liver (Figure 6.10). Reticular cells produce the reticular fibers that form the network onto which other cells attach. It derives its name from the Latin reticulus, which means “little net.” Figure 6.10. Reticular Tissue. This is a loose connective tissue made up of a network of file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 14/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation reticular fibers that provides a supportive framework for soft organs. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Dense Connective Tissue Dense connective tissue contains more collagen fibers than does loose connective tissue. As a consequence, it displays greater resistance to stretching. There are two major categories of dense connective tissue: regular and irregular. Dense regular connective tissue fibers are parallel to each other, enhancing tensile strength and resistance to stretching in the direction of the fiber orientations. Ligaments and tendons are made of dense regular connective tissue. Dense regular elastic tissue contains elastin fibers in addition to collagen fibers, which allows the ligament to return to its original length after stretching. The ligaments in the vocal folds and between the vertebrae in the vertebral column are elastic. In dense irregular connective tissue, the direction of fibers is random. This arrangement gives the tissue greater strength in all directions and less strength in one particular direction. The dermis of the skin is an example of dense irregular connective tissue rich in collagen fibers. Dense irregular elastic tissues give arterial walls the strength and the ability to regain original shape after stretching (Figure 6.11). Figure 6.11. Dense Connective Tissue. (a) Dense regular connective tissue consists of collagenous fibers packed into parallel bundles. (b) Dense irregular connective tissue consists of collagenous fibers interwoven into a mesh-like network. From top, LM × 1000, LM × 200. (Micrographs provided by the Regents of University of Michigan Medical School © 2012) Supportive Connective Tissues file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 15/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation Two major forms of supportive connective tissue, cartilage and bone, allow the body to maintain its posture and protect internal organs. Cartilage The distinctive appearance of cartilage is due to polysaccharides called chondroitin sulfates, which bind with ground substance proteins to form proteoglycans. Embedded within the cartilage matrix are chondrocytes, or cartilage cells, and the space they occupy are called lacunae (singular = lacuna). Cartilaginous tissue is avascular, thus all nutrients need to diffuse through the matrix to reach the chondrocytes. This is a factor contributing to the very slow healing of cartilaginous tissues. The three main types of cartilage tissue are hyaline cartilage, fibrocartilage, and elastic cartilage (Figure 6.12). Hyaline cartilage, the most common type of cartilage in the body, consists of short and dispersed collagen fibers and contains large amounts of proteoglycans. Under the microscope, tissue samples appear clear. The surface of hyaline cartilage is smooth. Both strong and flexible, it is found in the rib cage and nose and covers bones where they meet to form moveable joints. It makes up a template of the embryonic skeleton before bone formation. A plate of hyaline cartilage at the ends of bone allows continued growth until adulthood. Fibrocartilage is tough because it has thick bundles of collagen fibers dispersed through its matrix. The intervertebral discs are examples of fibrocartilage. Elastic cartilage contains elastic fibers as well as collagen and proteoglycans. This tissue gives rigid support as well as elasticity. Tug gently at your ear lobes, and notice that the lobes return to their initial shape. The external ear contains elastic cartilage. file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 16/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation Figure 6.12. Types of Cartilage. Cartilage is a connective tissue consisting of collagenous fibers embedded in a firm matrix of chondroitin sulfates. (a) Hyaline cartilage provides support with some flexibility. (b) Fibrocartilage provides some compressibility and can absorb pressure. (c) Elastic cartilage provides firm but elastic support. From top, LM × 300, LM × 1200, LM × 1016. (Micrographs provided by the Regents of University of Michigan Medical School © 2012) Bone Bone is the hardest connective tissue. It provides protection to internal organs and supports the body. Bone’s rigid extracellular matrix contains mostly collagen fibers embedded in a mineralized ground substance containing hydroxyapatite, a form of calcium phosphate. Both components of the matrix, organic and inorganic, contribute to the unusual properties of bone. Without collagen, bones would be brittle and shatter easily. Without mineral crystals, bones would flex and provide little support. Osteocytes, bone cells like chondrocytes, are located within lacunae. The histology of transverse tissue from long bone shows a typical arrangement of osteocytes in concentric circles around a central canal. Bone is a highly vascularized tissue. Unlike cartilage, bone tissue can recover from injuries in a relatively short time. Cancellous bone looks like a sponge under the microscope and contains empty spaces. It is lighter than compact bone and found in the interior of some bones and at the end of long bones. Compact bone is solid and has greater structural strength. file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 17/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation Compact Bone Compact bone is the denser, stronger of the two types of bone tissue (Figure 6.13). It can be found in the shaft and ends of long bones, where it provides support and protection. Figure 6.13. Diagram of Compact Bone. (a) This cross-sectional view of compact bone shows the basic structural unit, the osteon. (b) In this micrograph of the osteon, you can clearly see the concentric lamellae and central canals. LM × 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) The microscopic structural unit of compact bone is called an osteon, or Haversian system. Each osteon is composed of concentric rings of calcified matrix called lamellae (singular = lamella). Running down the center of each osteon is the central canal, or Haversian canal, which contains blood vessels, nerves, and lymphatic vessels. The osteocytes are located inside spaces called lacunae (singular = lacuna), found at the borders of adjacent lamellae. They can communicate with each other and receive nutrients through channels within the bone matrix known as canaliculi (singular = canaliculus). Canaliculi connect with the canaliculi of other lacunae and eventually with the central canal. This system allows nutrients to be transported to the osteocytes and wastes to be removed from them. file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 18/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation Spongy (Cancellous) Bone Like compact bone, spongy bone, also known as cancellous bone, contains osteocytes housed in lacunae, but they are not arranged in concentric circles. Instead, the lacunae and osteocytes are found in a lattice-like network of matrix spikes called trabeculae (singular = trabecula) (Figure 6.14). The spaces of the trabeculated network provide balance to the dense and heavy compact bone by making bones lighter so that muscles can move them more easily. In addition, the spaces in some spongy bones contain red marrow, protected by the trabeculae, where hematopoiesis occurs. Figure 6.14. Diagram of Spongy Bone. Spongy bone is composed of trabeculae that contain the osteocytes. Red marrow fills the spaces in some bones. Fluid Connective Tissue Blood and lymph are fluid connective tissues. Cells circulate in a liquid extracellular matrix. The formed elements circulating in blood are all derived from hematopoietic stem cells located in bone marrow (Figure 6.15). Erythrocytes, red blood cells, transport oxygen and some carbon dioxide. Leukocytes, white blood cells, are responsible for defending against potentially harmful microorganisms or molecules. Platelets are cell fragments involved in blood clotting. Some white blood cells have the ability to cross the endothelium that lines blood vessels and enter adjacent tissues. Nutrients, salts, and wastes are dissolved in the liquid matrix and transported through the body. Lymph contains a liquid matrix and white blood cells. Lymphatic capillaries are extremely permeable, allowing larger molecules and excess fluid from interstitial spaces to enter the lymphatic vessels. file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 19/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation Figure 6.15. Blood: A Fluid Connective Tissue. Blood is a fluid connective tissue containing erythrocytes and various types of leukocytes that circulate in a liquid extracellular matrix. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) INTERACTIVE LINK View the University of Michigan Webscope to explore the tissue sample in greater detail. INTERACTIVE LINK Visit this link to test your connective tissue knowledge with this 10-question quiz. Can you name the 10 tissue types shown in the histology slides? Visit this link for go over some details of connective tissue and follow up with a quiz. 6.4 Muscle Tissue Muscle tissue is characterized by properties that allow movement. Muscle cells are excitable; they respond to a stimulus. They are contractile, meaning they can shorten and generate a pulling force. When attached between two movable objects, in other words, bones, contractions of the muscles cause the bones to move. Some muscle movement is voluntary, which means it is under conscious control. For example, a person decides to open a book and read a chapter on anatomy. Other movements are involuntary, meaning they are not under conscious control, such as the contraction of your pupil in bright light. Muscle tissue is classified into three types according to structure and function: skeletal, cardiac, and smooth (Table 6.1). file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 20/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation Table 6.2 Comparison of Structure and Properties of Muscle Tissue Types Tissue Histology Function Long cylindrical fiber, Skeletal striated, many Voluntary movement, produces heat, protects peripherally located organs nuclei Location Attached to bones and around entrance points to body (e.g., mouth, anus) Short, branched, Cardiac striated, single central Contracts to pump blood Heart nucleus Short, spindle-shaped, Smooth no evident striation, single nucleus in each fiber Involuntary movement, moves food, involuntary control of respiration, moves secretions, regulates flow of blood in arteries by contraction Walls of major organs and passageways Skeletal muscle is attached to bones and its contraction makes possible locomotion, facial expressions, posture, and other voluntary movements of the body. Forty percent of your body mass is made up of skeletal muscle. Skeletal muscles generate heat as a byproduct of their contraction and thus participate in thermal homeostasis. Shivering is an involuntary contraction of skeletal muscles in response to perceived lower than normal body temperature. The muscle cell, or myocyte, develops from myoblasts during fetal development. The cells are multinucleated as a result of the fusion of the many myoblasts that fuse to form each long muscle fiber. Myocytes and their numbers remain relatively constant throughout life. Skeletal muscle tissue is arranged in bundles surrounded by connective tissue. Under the light microscope, muscle cells appear striated with many nuclei squeezed along the membranes. The striation is due to the regular alternation of the contractile proteins actin and myosin. Cardiac muscle forms the contractile walls of the heart. The cells of cardiac muscle, known as cardiomyocytes, also appear striated under the microscope. Unlike skeletal muscle fibers, cardiomyocytes are single cells typically with a single centrally located nucleus. A principal characteristic of cardiomyocytes is that they contract on their own intrinsic rhythms without any external stimulation. Cardiomyocytes attach to one another with specialized cell junctions called intercalated discs. Intercalated discs have both anchoring junctions and gap junctions. Attached cells form long, branching cardiac muscle fibers that are, essentially, a mechanical and electrochemical syncytium allowing the cells to synchronize their actions. The cardiac muscle pumps blood through the body and is under involuntary control. Smooth muscle tissue contraction is responsible for involuntary movements in the internal organs. It forms the contractile component of the digestive, urinary, and reproductive systems as well as the airways, arteries and glands. Each cell is spindleshaped with a single nucleus and no visible striations (Figure 6.16). file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 21/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation Figure 6.16. Muscle Tissue. (a) Skeletal muscle cells have prominent striation and nuclei on their periphery. (b) Smooth muscle cells have a single nucleus and no visible striations. (c) Cardiac muscle cells appear striated and have a single nucleus. From top, LM × 1600, LM × 1600, LM × 1600. (Micrographs provided by the Regents of University of Michigan Medical School © 2012) INTERACTIVE LINK Watch this video to learn more about muscle tissue. In looking through a microscope how could you distinguish skeletal muscle tissue from smooth muscle? 6.5 Nervous Tissue Nervous tissue is characterized as being excitable and capable of sending and receiving electrochemical signals that provide the body with information. Two main classes of cells make up nervous tissue: the neuron and neuroglia (Figure 6.17). Neurons file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 22/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation propagate information via electrochemical impulses, called action potentials, which are biochemically linked to the release of chemical signals. Neuroglia play an essential role in supporting neurons. Figure 6.17. The Neuron. The cell body of a neuron, also called the soma, contains the nucleus and mitochondria. The dendrites transfer the nerve impulse to the soma. The axon carries the action potential away to another excitable cell. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Neurons display distinctive morphology, well suited to their role as conducting cells, with three main parts. The cell body includes most of the cytoplasm, the organelles, and the nucleus. Dendrites branch off the cell body and appear as thin extensions. A long “tail,” the axon, extends from the neuron body and can be wrapped in an insulating layer known as myelin, which is formed by accessory cells. The synapse is the gap between nerve cells, or between a nerve cell and its target, for example, a muscle or a gland, across which the impulse is transmitted by chemical compounds known as neurotransmitters. The second class of neural cells comprises the neuroglia or glial cells. A glial cell is one of a variety of cells that provide a framework of tissue that supports the neurons and their activities. Several types of glial cells are found, such as astrocytes, microglia, oligodendrocytes and Schwann cells . Although glial cells are smaller than neurons, they are the most abundant cells in the human brain and spinal cord (Figure 6.18). file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 23/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation Figure 6.18. Nervous Tissue. Nervous tissue is made up of neurons and neuroglia. The cells of nervous tissue are specialized to transmit and receive impulses. LM × 872. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) INTERACTIVE LINK Watch this video for an overview of muscle and nervous tissue and then take a quiz yourself. Key Terms adipocytes lipid storage cells adipose tissue specialized loose connective tissue rich in stored fat anchoring junction mechanically attaches adjacent cells to each other or to the basement membrane apical that part of a cell or tissue which, in general, faces an open space areolar tissue (also, loose connective tissue) a type of connective tissue proper that shows little specialization with cells dispersed in the matrix basement membrane in epithelial tissue, a thin layer of fibrous material that anchors the epithelial tissue to the underlying connective tissue bone hard, dense connective tissue that forms the structural elements of the skeleton canaliculi (singular = canaliculus) channels within the bone matrix that house one of an osteocyte’s many cytoplasmic extensions that it uses to communicate and receive nutrients file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 24/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation cardiac muscle heart muscle, under involuntary control, composed of striated cells that attach to form fibers, each cell contains a single nucleus, contracts autonomously cell junction point of cell-to-cell contact that connects one cell to another in a tissue central canal longitudinal channel in the center of each osteon; contains blood vessels, nerves, and lymphatic vessels; also known as the Haversian canal chondrocytes cells of the cartilage collagen fiber flexible fibrous proteins that give connective tissue tensile strength compact bone dense osseous tissue that can withstand compressive forces connective tissue type of tissue that serves to hold in place, connect, and integrate the body’s organs and systems connective tissue proper connective tissue containing a viscous matrix, fibers, and cells. dense connective tissue connective tissue proper that contains many fibers that provide both elasticity and protection elastic cartilage type of cartilage, with elastin as the major protein, characterized by rigid support as well as elasticity elastic fiber fibrous protein within connective tissue that contains a high percentage of the protein elastin that allows the fibers to stretch and return to original size endocrine gland groups of cells that release chemical signals into the intercellular fluid to be picked up and transported to their target organs by blood endothelium tissue that lines vessels of the lymphatic and cardiovascular system, made up of a simple squamous epithelium epithelial tissue type of tissue that serves primarily as a covering or lining of body parts, protecting the body; it also functions in absorption, transport, and secretion exocrine gland group of epithelial cells that secrete substances through ducts that open to the skin or to internal body surfaces that lead to the exterior of the body fibroblast most abundant cell type in connective tissue, secretes protein fibers and matrix into the extracellular space fibrocartilage tough form of cartilage, made of thick bundles of collagen fibers embedded in chondroitin sulfate ground substance fluid connective tissue specialized cells that circulate in a watery fluid containing salts, nutrients, and dissolved proteins file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 25/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation gap junction allows cytoplasmic communications to occur between cells goblet cell unicellular gland found in columnar epithelium that secretes mucous ground substance fluid or semi-fluid portion of the matrix histamine chemical compound released by mast cells in response to injury that causes vasodilation and endothelium permeability histology microscopic study of tissue architecture, organization, and function hyaline cartilage most common type of cartilage, smooth and made of short collagen fibers embedded in a chondroitin sulfate ground substance lacunae (singular = lacuna) small spaces in bone or cartilage tissue that cells occupy loose connective tissue (also, areolar tissue) type of connective tissue proper that shows little specialization with cells dispersed in the matrix matrix extracellular material which is produced by the cells embedded in it, containing ground substance and fibers muscle tissue type of tissue that is capable of contracting and generating tension in response to stimulation; produces movement. myocyte muscle cells nervous tissue type of tissue that is capable of sending and receiving impulses through electrochemical signals. neuroglia supportive neural cells neuron excitable neural cell that transfer nerve impulses osteocyte primary cell in mature bone; responsible for maintaining the matrix osteon (also, Haversian system) basic structural unit of compact bone; made of concentric layers of calcified matrix pseudostratified columnar epithelium tissue that consists of a single layer of irregularly shaped and sized cells that give the appearance of multiple layers; found in ducts of certain glands and the upper respiratory tract reticular fiber fine fibrous protein, made of collagen subunits, which cross-link to form supporting “nets” within connective tissue reticular tissue type of loose connective tissue that provides a supportive framework to soft organs, such as lymphatic tissue, spleen, and the liver file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 26/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation simple columnar epithelium tissue that consists of a single layer of column-like cells; promotes secretion and absorption in tissues and organs simple cuboidal epithelium tissue that consists of a single layer of cube-shaped cells; promotes secretion and absorption in ducts and tubules simple squamous epithelium tissue that consists of a single layer of flat scale-like cells; promotes diffusion and filtration across surface skeletal muscle usually attached to bone, under voluntary control, each cell is a fiber that is multinucleated and striated smooth muscle under involuntary control, moves internal organs, cells contain a single nucleus, are spindle-shaped, and do not appear striated; each cell is a fiber stratified columnar epithelium tissue that consists of two or more layers of column-like cells, contains glands and is found in some ducts stratified cuboidal epithelium tissue that consists of two or more layers of cube-shaped cells, found in some ducts stratified squamous epithelium tissue that consists of multiple layers of cells with the most apical being flat scale-like cells; protects surfaces from abrasion striation alignment of parallel actin and myosin filaments which form a banded pattern tight junction forms an impermeable barrier between cells tissue group of cells that are similar in form and perform related functions transitional epithelium form of stratified epithelium found in the urinary tract, characterized by an apical layer of cells that change shape in response to the presence of urine vasodilation widening of blood vessels Games file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 27/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation Revision Questions 1. In observing epithelial cells under a microscope, the cells are arranged in a single layer and look tall and narrow, and the nucleus is located close to the basal side of the cell. The specimen is what type of epithelial tissue? columnar stratified squamous transitional file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 28/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation 2. Which of the following is the epithelial tissue that lines the interior of blood vessels? columnar pseudostratified simple squamous transitional 3. Which type of epithelial tissue specializes in moving particles across its surface and is found in airways and lining of the oviduct? transitional stratified columnar pseudostratified ciliated columnar stratified squamous 4. Connective tissue is made of which three essential components? cells, ground substance, and carbohydrate fibers cells, ground substance, and protein fibers collagen, ground substance, and protein fibers matrix, ground substance, and fluid 5. Under the microscope, a tissue specimen shows cells located in spaces scattered in a transparent background. This is probably ________. loose connective tissue a tendon bone hyaline cartilage 6. Which connective tissue specializes in storage of fat? tendon adipose tissue reticular tissue dense connective tissue 7. Ligaments connect bones together and withstand a lot of stress. What type of connective tissue should you expect ligaments to contain? areolar tissue adipose tissue dense regular connective tissue dense irregular connective tissue file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 29/30 11/30/22, 2:22 PM Chapter 6: The Tissue Level of Organisation 8. In bone, the main cells are ________. fibroblasts chondrocytes lymphocytes osteocytes 9. Striations, cylindrical cells, and multiple nuclei are observed in ________. skeletal muscle only cardiac muscle only smooth muscle only skeletal and cardiac muscle 10. The cells of muscles, myocytes, develop from ________. myoblasts fibroblasts chondrocytes osteocytes 11. Skeletal muscle is composed of very hard working cells. Which organelles do you expect to find in abundance in skeletal muscle cell? nuclei striations golgi bodies mitochondria 12. The cells responsible for the transmission of the nerve impulse are ________. Schwann cells neurons oligodendrocytes microglia 13. The nerve impulse travels down a(n) ________, away from the cell body. dendrite collagen fiber microglia axon Submit Answers Clear Answers file:///C:/Users/m1110/AppData/Local/Kotobee Author/epub2969/EPUB/xhtml/epubb4bwk/EPUB/xhtml/raw/swbw04.xhtml 30/30 Chapter 6 The Tissue Level of Organization cancerous 1 Outline 6.1 The four types of tissues 6.2 Epithelial tissue  Functions of epithelial tissue  Cell junctions  Classification of epithelial tissue  Glandular epithelium 6.3 Connective tissue  Functions of connective tissue  Classification of connective tissue  Connective tissue proper  Supportive connective tissue  Fluid connective tissue 6.4 Muscle tissue 6.5 Nervous tissue Human Biology (BIOL-102), Health track 2 Objectives After studying this chapter, students will be able to:  Identify the main tissue types and discuss their roles in the human body  Explain the functions of various epithelial tissues and how their structure enables their functions  Explain the functions of various connective tissues and how their structure enables their functions  Describe the characteristics of muscle tissue and how these enable function  Discuss the characteristics of nervous tissue and how these enable information processing Human Biology (BIOL-102), Health track 3 Lesson 1 Human Biology (BIOL-102), Health track 4 6.1 The Four Types of Tissues The Four Types of Tissues  The human body has over 200 types of cells that are organized in what is known as tissues.  Tissue: group of cells found together that perform related functions.  Histology is the microscopic study of tissue architecture, organization and function.  There are four categories of tissues, each with a specific function:  Epithelial: covers body exterior, lines interior cavities, forms certain glands.  Connective: binds cells and organs, provides protection and support.  Muscle: contracts to provide movement.  Nervous: propagation of nerve impulses for communication all over the body. Human Biology (BIOL-102), Health track 5 6.1 The Four Types of Tissues is Figure 6.2. The four types of tissues in the body: nervous tissue, epithelial tissue, muscle tissue, and connective tissue. 6 6.2 Epithelial Tissue Epithelial Tissue  Highly cellular, avascular, with little or no extracellular material between cells.  Epithelial tissue is polarized with two distinct surfaces:    Apical surface (exposed to surface or lumen)  Basal surface (close to the underlying body structures). All epithelial tissue rests on a basement membrane which  is composed of glycoproteins and collagen.  acts as an attachment site for the epithelium.  separates epithelium from underlying connective tissue. Epithelial cells are joined through specialized intercellular connections known as cell junctions. Human Biology (BIOL-102), Health track 7 6.2 Epithelial Tissue  Epithelial Tissue Epithelial cells can have certain structures on their apical surface that helps them achieve specific functions such as cilia and microvilli.  Cilia: microtubule structures that beat in a coordinated pattern to move fluids and trapped particles. For example: cilia found on the epithelium of airways.  Microvilli: finger-like extensions of the plasma membrane that greatly expand the apical surface area. For example: microvilli found on intestinal cells. A B TEM of (A) cilia projecting from respiratory epithelium and (B) a section through intestinal tissue showing microvilli (rod-like structures in two rows) in the apical membrane of intestinal epithelial cells (https://fineartame