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Teamwork • Human body contains about 100 trillion cells. • No one cell can carry out all the necessary bodily functions by itself. So, cells aggregate and form TISSUES. • Tissue Collection of cells with a similar structure and function. • Study of tissues is known as histology Tissues • There are 4 primary tissue types in the human body: 1. 2. 3. 4. Epithelial Connective Muscle Nervous (covering/lining) (support) (movement) (control) • These tissues differ in the types and functions of their cells, and the products of those cells, and their relative distribution. • Which picture on this slide corresponds to which tissue? Epithelial Tissue • Covers the surface of the body, lines the body cavities, forms the external and internal linings of most organs, and constitutes the bulk of most glands. • Functions include: 1. Protection 2. Secretion 3. Sensory reception 4. Absorption 5. Filtration • It’s found all over the place! • Looking at the section of skin. Where would the bulk of the epithelial tissue be found? Characteristics of Epithelia • Composed almost entirely of sheets of close-packed cells – very little extracellular material. – Look at the drawing below (it represents the epithelium that lines much of the respiratory tract) . Do you see much space between these cells? Now look at the actual slide and notice the same thing. • Epithelial cells are often strongly connected to adjacent cells via tight junctions and desmosomes. – Tight junctions are protein complexes that completely encircle a cell and thus connect it to all its neighboring cells and it prevents anything from passing b/w them. Common in lining of the stomach & intestines. (Why here?) – Desmosomes – “spot weld” that holds cells together and enables a tissue to resist mechanical stress. Common in the epidermis and cervix. (Why these 2 areas?) Characteristics of Epithelia • Polarity – Different areas of epithelial cells have different structures. (If they have different structures, they can have different ___________) – The area of an epithelial cell adjacent to the exterior or to a body cavity is the apical side and the opposite area is the basal side. • Always supported by a layer of connective tissue. – It’s known as the basement membrane and is labeled by arrows in the pictures to the right. More Characteristics of Epithelia • No blood vessels – it’s avascular. – Can you pierce your skin with a needle without bleeding? • Has the capacity to regenerate. – Why is this a good thing? – Considering the locations of epithelial tissue, is it likely to get damaged? • Epithelia characteristics are based on the number of cell layers present and the shape of the cells in the apical layer. • Therefore, epithelium with a single layer of cells, is simple. If there is more than one layer, the epithelium is stratified. • There are 3 cell shapes: 1. 2. 3. Squamous = flat, scale-like Cuboidal = cube-shaped Columnar = column-shaped Epithelia Types • There are at least 6 types: 1. 2. 3. 4. 5. 6. • Simple squamous Simple cuboidal Simple columnar Stratified squamous Stratified cuboidal Stratified columnar Two others are special – transitional epithelium and pseudostratified columnar epithelium. Simple Squamous Epithelium • Microscopic Appearance – Single layer of flattened cells, shaped like fried eggs with a bulge where the nucleus is. – Nucleus is flattened in the plane of the cell. – Cytoplasm may be very thin and tough to see. – From a surface view, cells have angular contours and nuclei appear round. Above, the roundish structure is a ball of capillaries found in the kidneys known as a glomerulus. The blue arrow indicates the simple squamous epithelium surrounding it. Simple Squamous Epithelium • Locations – Air sacs (alveoli) of lungs – Glomerular capsules of kidneys – Some kidney tubules – Lining of heart and all blood vessels • This is called endothelium – Serous membranes – External lining of visceral organs Endothelium The blue arrow points directly at the nucleus of an endothelial cell. What type of cell does the yellow arrow indicate? Simple Squamous Epithelium • Functions – Thinness allows material to rapidly diffuse or be transported through the epithelial layer. • Ex. Gases diffuse across the thin air sac epithelium in the lungs. • Blood traveling to the lungs is high in CO2 and low in O2. After gas exchange in the lungs, the blood leaving is high in O2 and low in CO2 • Why is thinness an asset in this situation? • In pneumonia, a build-up of mucus can increase the distance that the gases must move. Does this make it easier or harder “to breathe?” Simple Squamous Epithelium • More Functions – Blood is filtered through the epithelium of the kidney capillaries. • This begins the process of removing waste products from the blood and modifying its ionic content. – Makes up the serous membranes and secretes a lubricating fluidserous fluid Simple Cuboidal Epithelium • Microscopic Appearance – Single layer of square or round cells. – Centrally-placed spherical nuclei. – Apical layer often has microvilli. • Microvilli are small extensions of the cell membrane that increase the surface area of the cell. • You usually want a cell to have more surface area when it is involved with secretion or absorption. – WHY? This is a longitudinal section of a kidney tubule. Notice the almost square-shaped cells highlighted in red. Simple Cuboidal Epithelium • Locations – Liver – Pancreas – Thyroid, salivary, and most other glands – Most kidney tubules – Bronchioles (small tubes within the lungs) Cross-Section of a kidney tubule Simple Cuboidal Epithelium • Functions: – Absorption • Kidney tubules – Microvilli often present to increase surface area • Liver – Secretion • Kidney tubules • Liver • Pancreas • Thyroid, salivary, and most other glands Cross-section of the thyroid gland Simple Columnar Epithelium • Microscopic Appearance – Single layer of tall, narrow cells. – Oval or sausage-shaped nuclei, vertically-oriented, usually located in the basal half of the cell. – Secretory vesicles sometimes visible in apical portion of the cell. – Microvilli occasionally present. – Cilia occasionally present. • Cilia are hair-like extensions of the cell membrane that can move and sweep material across the cell surface. – Goblet cells often interspersed. • Secrete a lubricating mucus Simple Columnar Epithelium • Locations – Inner lining of stomach, intestines, and rectum. – Inner lining of gallbladder – Inner lining of uterus and uterine tubes Simple columnar epithelium lining the lumen of the gallbladder. Simple Columnar Epithelium • Functions – Absorption and secretion • Columnar cells in small intestine have microvilli to increase the surface area for the absorption of nutrients. – Movement of egg and embryo in uterine tube. • b/c of the presence of cilia. – Secretion of mucus. • Lots of goblet cells in the large intestine so as to lubricate it and ease the passage of feces. Stratified Squamous Epithelium • Microscopic Appearance – Multiple cell layers with cells becoming flatter and flatter toward surface. – In keratinized stratified squamous epithelium, the apical layers are layers of dead cells lacking nuclei and packed with the tough protein keratin. – Nonkeratinized stratified squamous epithelium lacks the layers of dead cells at the surface. This is keratinized stratified squamous epithelium from the sole of the foot. It extends the length of the blue line on the left. Note the multiple layers of clear, dead, enucleated cells. Stratified Squamous Epithelium • Location of keratinized type – Epidermis. Palms and soles of feet are typically heavily keratinized. • Locations of nonkeratinized type – Lining of oral cavity and surface of tongue – Lining of esophagus – Lining of vagina and anal canal Non-keratinized stratified squamous epithelium What do the regions that contain the nonkeratinized version have in common? Which of these is keratinized and which is nonkeratinized stratified squamous epithelium? Stratified Squamous Epithelium • Functions – Protection! – Keratinized version (i.e., dry epithelium) protects against mechanical abrasion, water loss, and pathogen entry. • Keratin is very strong, waterproof, and is bacteriostatic (i.e. prevents bacteria from reproducing). – Non-keratinized version (i.e., wet epithelium) also protects from mechanical abrasion. • Eating food, swallowing, sexual intercourse, birth, defecation. Stratified Cuboidal Epithelium • Microscopic Appearance – 2 or more layers of cells. – Surface layers are square or round (cuboidal). • Locations – Some sweat gland ducts. – Ovarian follicle • Cells that surround the developing egg Stratified Cuboidal Epithelium • Functions – Contributes to sweat secretion. – Secretion of ovarian hormones (e.g., estrogens) To the left, we have an oocyte (i.e., egg cell) surrounded by stratified cuboidal epithelium. The oocyte is circled in blue Stratified Columnar Epithelium • Microscopic Appearance – 2 or more layers of cells. – Surface cells tall and narrow with basally located nuclei. • Locations – Rare. – Small portions of anal canal, pharynx, epiglottis, and male urethra. – Sometimes seen in large ducts of sweat and salivary glands. • Functions – Often seen where 2 other tissue types meet – Structural integrity of gland ducts Pseudostratified Columnar Epithelium • Microscopic Appearance – Looks multi-layered, but it’s NOT! • All cells touch the basement membrane. • In stratified epithelia, only the bottom cell layer touches the basement membrane. – Cells are of varying heights which gives the appearance of stratification. Nuclei are at several levels. – Often has goblet cells interspersed. – Cells often have cilia. Pseudostratified Columnar Epithelium • Locations: – Respiratory tract from nasal cavity to bronchi. • Ciliated • Goblet cells – Portions of male reproductive tract • Non-ciliated Ciliated pseudostratified epithelium from the respiratory tract. Do you see how it appears that there are multiple layers. Do you see the cilia (indicated by the arrow)? Pseudostratified Columnar Epithelium • Functions – In the respiratory tract there are mucus-secreting goblet cells. – The mucus traps dust and bacteria. – Cilia “sweep” the bacterialaden mucus up the respiratory tract towards the pharynx where it can be spit out or swallowed. – Smoking paralyzes and can kill cilia – smokers have to cough violently to expel their mucus. Transitional Epithelium • Microscopic Appearance – Resembles stratified squamous epithelium, but the surface cells are rounded and often bulge above surface (dome-shaped). – Readily see 5-6 cell layers when relaxed and 2-3 cell layers when stretched. – Cells may be flatter and thinner when epithelium is stretched. – Some cells are binucleate, i.e., they have 2 nuclei. • Locations – Predominant epithelium lining the urinary tract. Transitional Epithelia – Found in part of the kidney, the ureters (i.e. tubes that connect the kidney to the urinary bladder), the urinary bladder, and part of the urethra. • Functions – Stretches to allow filling of the urinary tract. – Called “transitional” because it was thought to be an intermediate between stratified squamous and stratified columnar epithelium. Not true but the name has persisted. Easy Epithelium Review Glands • A gland is a cell or an organ that produces and secretes substances for use inside or outside the body. • Predominantly composed of epithelial tissue. • Secrete substances onto a surface, into a cavity, or into the blood • Glands are broadly classified as: 1. 2. Endocrine Exocrine Thyroid Gland: An endocrine gland An exocrine gland Stomach: Both an exocrine and an endocrine gland Endocrine Glands • Ductless glands • Produce & secrete hormones Endocrine Glands • Do not secrete material into ducts. • Secrete chemicals called hormones into the bloodstream where they travel through the body and affect other cells. Examples: thyroid, thymus, testes, ovaries, pituitary gland, pineal, adrenal, etc. Endo = within Exocrine Glands • Secrete material into ducts that lead to the body surface or to one of the cavities that is continuous with the body surface, i.e., digestive, reproductive, respiratory tract. The parotid gland (a salivary gland) • Exo = outside and crine = secrete. • Can be multicellular or unicellular. – Multicellular: • Pancreas, stomach, sweat glands, salivary glands, mammary glands, sebaceous glands, etc. Goblet Cell – Unicellular: • Goblet cells. Exocrine Glands • Glands w/ ducts • produce & secrete substances onto body surfaces (internal/ external) • 3 types - holocrine, apocrine, merocrine (eccrine) Application • Carcinoma = cancerous growth of epithelial tissue. Endocrine Glands • Do not secrete material into ducts. • Secrete chemicals called hormones into the bloodstream where they travel through the body and affect other cells. • Types of glands include: – Thyroid, thymus, testes, ovaries, pituitary, pineal, adrenal, etc. Endo = within Connective Tissue • Most abundant, widely distributed, and histologically variable of the 4 primary tissue types. • Consists of cells that are typically widely separated by lots of extracellular material (i.e., extracellular matrix). – From the diagram, compare the density of cells in the epithelial layers and in the connective tissue layers. Do you see a difference? • Most cells are not in contact with each other, but are distributed throughout the extracellular matrix. Functions of Connective Tissue 1. Binding of organs 2. Support 3. Physical protection 4. Immune protection 5. Movement 6. Storage 7. Heat production 8. Transport What do you NOT see a lot of in this micrograph of connective tissue? Categories of CT 1. Fibrous connective tissue (i.e., connective tissue proper) 2. Supporting connective tissue 3. Fluid connective tissue Blood Tendon Bone Fibrous Connective Tissue • Most diverse type of CT. • Contain extremely conspicuous fibers – hence the name, fibrous connective tissue. • The illustration below shows typical CT fibers, typically made of multiple strong filamentous proteins twisted about one another. • Fibrous CT consists of cells, fibers, and something called ground substance. Cells of CT: 1. Fibroblasts – – – 2. Fibro = fat, blast = making Large, flat cells with tapered ends; produce fibers and ground substance. Inactive ones are known as fibrocytes. Macrophages – – – – Macro = large, phage = eating Large phagocytic cells that wander through connective tissue, where they engulf and destroy bacteria, other foreign particles, and dead or dying cells of our body. They activate the immune system when they encounter foreign matter called antigens. Derived from white blood cells known as monocytes. Cells of CT 3. Leukocytes – Leuko = white, cyte = cell – White blood cells that crawl out of the bloodstream and spend the majority of their time in the CT. Many are phagocytes that wander in search of pathogens. 4. Plasma Cells – Certain white blood cells differentiate into plasma cells when they detect foreign agents. – Plasma cells produce and secrete antibodies (i.e. proteins that bind to foreign molecules (i.e. antigens), thus inactivating them or marking them for future destruction. Cells of CT 5. Mast Cells – Often found in CT adjacent to blood vessels. – Secrete a chemical called heparin, which is an anti-coagulant and a chemical called histamine which is a vasodilator. 6. Adipocytes – Adipo = fat – Appear in small clusters in some fibrous connective tissues. – If they predominate in an area adipose tissue. – Contain lipid droplets for storage. Fibers in Connective Tissue 1. Collagen Fibers 2. Reticular Fibers 3. Elastic Fibers Collagen Fibers as seen with a scanning electron microscope Collagenous Fibers Close-up of a single fiber • Interwoven strands of the protein collagen. – Most abundant protein in the human body. • Thick fibers with great tensile strength – i.e., it’s tough to pull them apart. • In fresh tissue preps, they have a white appearance, sometimes called white fibers. • In stained slides, often pink and they usually appear quite wavy. • Tendons, ligaments, and the deep layer of the skin (the dermis) contain primarily collagenous fibers. Multiple fibers arranged in the extracellular matrix Reticular Fibers • A thinner collagen fiber coated with glycoproteins. – Stained black in the adjacent micrograph of the liver. • These fibers can branch extensively and form networks or frameworks for certain organs. Elastic Fibers • Made primarily of a protein called elastin, whose coiled structure allows it to stretch and snap back like a rubber band. • Contributes to the ability of the lungs, arteries, and skin to spring back after they are stretched. • Fresh elastic fibers are yellowish and thus often In this slide, “A” is an elastic fiber – what do you suppose “B” is? Ground Substance • Gelatinous material that occupies the space between the cells and the fibers in connective tissues. Imagine some Jell-o that a contains carrots and grapes. The carrots are like fibers, the grapes like cells, and the Jell-o itself is the ground substance. Types of Fibrous Connective Tissue • 2 types based on the relative abundance of fibers. – – Loose Connective Tissue • Lots of ground substance and few cells. Fewer fibers. • Empty spaces consisting of ground substance predominate in tissue sections. Dense Connective Tissue • Fibers occupy the most space. Much lower number of cells and less ground substance. • Appears densely packed in tissue sections. 1 Types of Loose CT 1. Areolar CT 2. Reticular Tissue 3. Adipose Tissue 3 2 Areolar CT Microscopic Appearance • Loose arrangement of collagenous and elastic fibers. Some reticular fibers. (All 3 fiber types.) • Scattered Cells. All 6 types can be present. • Abundant ground substance. • Numerous blood vessels. (Highly vascular.) Areolar CT • Locations – Underlying nearly all epithelia. – Surrounding blood vessels, nerves, trachea, and esophagus. – B/w muscles. – W/in mesenteries, and the visceral layers of the pericardium and the pleura. • Functions: – Loosely binds epi. to deeper tissues. – Allows passage of nerves and blood vessels through to other tissues. – Provides an arena for immune defense. – Blood vessels provide nutrients and waste removal for overlying epithelia. Areolar CT Reticular Tissue • Microscopic Appearance – Loose network of reticular fibers and a type of fibroblast known as the reticular cell. – Infiltrated with numerous white blood cells. – Often appears dark purple or black. • Locations: – Lymph nodes, spleen, thymus, and bone marrow. Reticular Tissue • Functions: – The branching network of reticular fibers will form a scaffold-like framework for lymphatic organs. • Spleen, thymus, and lymph nodes. • Such a framework is known as a stroma. • The functional tissue of these organs is known as the parenchyma. Adipose Tissue • Microscopic Appearance – Dominated by adipocytes – large, empty-looking cells with thin margins. – Nucleus usually pressed against the cell membrane – signet ring appearance. – Often pale. – Blood vessels often present. Adipose Tissue • Locations – Subcutaneous fat beneath skin. – Breast. – Around Heart. – Cushioning organs • Kidneys • Eyes Adipose Tissue • Functions – Energy storage. – Thermal insulation. – Shock absorption. – Protection – Cushioning for some organs. Types of Dense CT 1. Dense regular 2. Dense irregular 2 1 Dense Regular CT • Microscopic Appearance – Densely packed, parallel, often wavy collagenous fibers. – Slender fibroblast nuclei compressed between bundles of collagenous fibers. – Scanty open space (i.e. little ground substance) – Scarcity of blood vessels. Dense Regular Connective Tissue • Locations – Tendons. – Ligaments. NOTE the waviness of the fibers. What function could this structural aspect provide? Dense Regular CT • Functions – Ligaments bind bone tightly to other bones. Resist stress. – Tendons attach skeletal muscles to bone and transfer muscular tension to bones. Dense Irregular CT • Microscopic Appearance – Densely packed, collagenous fibers running in random directions. Compare this to dense regular CT. – Scanty open space (i.e. ground substance). – Few visible cells. – Blood vessels present. Dense Irregular CT • Locations – Deeper portion of dermis of skin. – Capsules around visceral organs, such as, the liver, spleen, and kidneys. – Fibrous sheaths around cartilages and bones. Dense Irregular CT • Functions – Provides a durable, hard to tear structure that can withstand stresses placed in unpredictable directions. – Why aren’t tendons and ligaments made like this? Supporting Connective Tissue • They provide the majority of the structural support of the human body. • 1 2 types – Cartilage. 2 – Bone. Cartilage • Supportive CT with a flexible, rubbery matrix. • Cells called chondroblasts secrete the matrix and surround themselves in it until they become trapped in little cavities known as lacunae (lacuna is Latin for “lake”). • Once enclosed in lacunae, cells are called chondrocytes. Cartilage • Cartilage is avascular and chondrocytes depend on the diffusion of nutrients through the stiff, viscous matrix. • – Thus, their metabolism and rate of division (i.e. mitosis) is low and healing of torn cartilage is a long process. – It’s avascular b/c chondrocytes produce a chemical called antiangiogenesis factor that, like its name suggests, prevents the growth of blood vessels. • Based on this, why do you suppose shark cartilage has been touted as a possible aid in the fight against cancer? • The matrix consists of collagenous fibers that range in thickness from invisibly fine to conspicuously coarse. 3 Cartilage • 3 types (classified based on fiber differences): 1. 2. 3. 2 Hyaline Cartilage Elastic Cartilage Fibrocartilage. 1 Hyaline Cartilage • Microscopic Appearance – Clear, glassy matrix, often stained light blue or pink. • Hyalos is Greek for glass. – Fine, dispersed collagenous fibers, not usually visible. – Chondrocytes often in small clusters of 3-4 cells within a single lacuna (known as cell nests or isogenous groups). – Covered by a perichondrium – a fibrous sheath made of dense irregular connective tissue. Hyaline Cartilage • Locations – Forms the majority of the fetal skeleton. – Forms boxlike structure around larynx and supportive rings around trachea and bronchi. – Attaches ribs to the sternum. – Forms a thin articular cartilage over the ends of bones at moveable joints. Close-up of Hyaline Cartilage. Notice the 2 cells in the single lacuna Hyaline Cartilage • Functions: – Eases joint movements. – Keeps airways open (i.e. patent). – Moves vocal cords. – Precursor of bone in the fetal skeleton. – Structural attachment. Elastic Cartilage • Microscopic Appearance – Elastic fibers form web-like mesh amid lacunae. – Always covered by a perichondrium. • Locations – External ear. – Epiglottis – flap of tissue that covers the trachea when you swallow to prevent food/liquid from going down the “wrong pipe.” – Eustachian tube – connects the ear to the nasopharynx. Elastic Cartilage • Functions – Provides flexible, elastic support. – What happens when you bend and release your ear? A – Chondrocyte B – Matrix w/ blackish purple elastic fibers C -Lacuna Fibrocartilage • Microscopic Appearance – Parallel collagenous fibers similar to those of tendon. – Rows of chondrocytes in lacunae between collagenous fibers. • Chondrocytes are fewer and smaller and are not in isogenous groups. – Never has a perichondrium. • Locations – Pubic symphysis – the anterior joint between the 2 halves of the pelvic girdle. – Intervertebral discs that separate the bones of the spinal column. – Menisci (i.e. shock-absorbing pads of cartilage) in the knee joint. – At points where tendons insert on bones near articular hyaline cartilage. • Functions – Resists compression and absorbs shock in some joints. – Often a transitional structure between dense connective tissue and hyaline cartilage. • For example, at some tendon-bone junctions. Fibrocartilage More Connective Tissues • Bone is the other supporting connective tissue. • Blood is a fluid connective tissue. • Both will be discussed in detail later. Muscle and Nervous Tissue • Nervous tissue – Consists of 2 cell types: Neurons and neuroglia. – Detects stimuli, integrates information, and transmits signals. • Muscular tissue – 3 types: skeletal, cardiac, and smooth. – Specialized to contract and exert forces on other tissues. – Major function is the creation of movement. Nervous Tissue • Characteristics - specialized to produce & conduct nerve impulses (electrical events) - function: regulate & control body activities - location: brain, spinal cord, nerves 5/1/2017 85 Nervous Tissue • Characteristics - vascular - loss of mitotic ability by neurons 5/1/2017 86 Nervous Tissue • Structure: - 2 major nerve cells 1. neuron – conduct & produce impulses consist of dendrites, cell body, axons 2. neuroglia – numerous supporting cells surround the neurons 5/1/2017 87 Nervous Tissue Muscle Tissue • Characteristics: - specialized to contract (shorten) -contains contractile proteins - vascularized - decreased mitotic ability - 3 kinds of muscle tissue (skeletal, cardiac & smooth) 5/1/2017 89 Muscle Tissue • Skeletal Muscle: - struc: long fiber-looking cells (shorten), striated (light/ dk. bands), multinucleated - voluntary (conscious control) - location: lies on bones 5/1/2017 90 Skeletal Muscle Tissue Muscle Tissue • Smooth Muscle: - struc: spindle-shaped (wide in center, tapered ends), no striations, uninucleated - involuntary (no conscious control) - location: wall of hollow organs (uterus, bladder & stomach) 5/1/2017 92 Smooth Muscle Tissue Muscle Tissue • Cardiac Muscle Tissue: - struc: cylindrical-looking-fiber and highly branched, striations, 1 nuclei, intercalated disks - involuntary (no conscious control) - location: wall of heart 5/1/2017 94 Cardiac Muscle Tissue Intercalated disk Membranes • Thin sheet or layer of tissue that covers a structure or lines a cavity (epithelium with underlying connective tissueepithelial membrane). • Two major categories: – Mucous- epithelium on thick loose connective tissueline cavities that open to the outside of the body (eg. digestive, respiratory, excretory and reproductive tracts). – Serous- simple squamous on thin layer of loose connective tissuelines trunk cavities and cover organs located within cavities (eg. pleural, pericardial and peritoneal). Secrete serous fluid Epithelial Membranes The majority of the body’s structures lined by epithelial membranes. – 1. 2. 3. Serosa covering the heart Such structures include: body cavities, tracts, external surfaces of organs, and the external surface of the body itself Mucous membranes Serous membranes and even cutaneous membranes Mucosa lining the duodenum (1st part of the small intestine Not an epithelial membrane! • Line passageways (body cavities) that open to the external environment. – Digestive, respiratory, urinary, and reproductive tracts. • Wet membranes that are bathed by their own secretions or, in the case of the urinary tract, urine. • Mucosa have 2 or occasionally 3 layers: – Lining epithelium (simple columnar or nonkeratinized stratified squamous) is adjacent to the lumen. – Deeper is a layer of areolar CT called the lamina propria. – Below this, often times, is a layer of smooth muscle called the muscularis mucosae. Mucous Membranes (i.e. Mucosa) • Have absorptive, secretory & protective functions. Mucous Membranes • Often covered with mucus secreted by goblet cells, multicellular mucous glands, or both. – Mucus is often involved in trapping foreign particles (including bacteria) or providing lubrication. – The presence of mucus does not define a mucous membrane however – e.g., the mucous membrane lining the urinary tract lacks mucus. • Cells modified for absorption are present in the small intestine • Cells modified for protection are present mainly at those positions of the tracts closest to the exterior. Normal stomach mucosa Serous Membranes (i.e. Serosa) • Moist membranes found in closed ventral body cavities – not open to the exterior. – Line the insides of the peritoneal, pleural, and pericardial cavities; and line the outer surfaces of some of the viscera. • Consist of a simple squamous epithelium lying upon a thin layer of areolar CT. • Secrete a thin, watery fluid that arises from the blood. It’s called serous fluid. Serosa • Provide an efficient means of lubricating cavity walls and organ exteriors so as to reduce the friction associated with movement. – Why is this essential? • The serosa lining the pleural cavity and the lung exterior are the parietal and visceral pleurae, respectively. Those of the heart are the parietal and visceral pericardium and those of the abdomen are the parietal and visceral peritoneum. Cutaneous Membrane • Another name isSkin • It’s an organ system consisting of a keratinized squamous epithelium (epidermis) firmly attached to a thick layer of dense irregular connective tissue (dermis). • Unlike other epithelial membranes, it is exposed to the air and is a dry Other Membrane Types – Synovial-connective tissue only; lines the inside of joint cavities – Periosteum-connective tissue only; surrounds bone. Synovial Membranes • Another type of membrane without an epithelial layer – Lines specialized cavities called joint cavities – Composed of areolar CT and adipose CT with collagen fibers – Secretes synovial fluid lubricates the end of bones as they move at joints, nourishes the cartilage covering bones, and removes microbes and debris from the joint cavity. Periosteum