Teamwork • The human body contains about 100 trillion cells. • However, 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. • The study of tissues is known as histology
Tissues • There are 4 primary tissue types in the human body: • Epithelial (covering/lining) • Connective (support) • Muscle (movement) • Nervous (control) • These tissues differ in the types and functions of their cells, and the products of those cells and the relative distribution of the two. • 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 4. Absorption 2. Secretion 5. Filtration 3. Sensory reception • It’s found all over the place! • On the right we have a 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 cartoon below (it represents the epithelium that lines much of your 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 makes it impossible for anything to pass btwn 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. • In the picture at the top right, what is present in the basal portion of the cell that’s not in the apical portion? • Always supported by a layer of connective tissue. • It’s known as the basement membrane and is labeled by arrows in the pictures at 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?
We classify epithelia based on the number of cell layers present and the shape of the cells in the apicallayer. • If there is only one layer of cells, the epithelium is simple. If there is more than one layer, the epithelium is stratified. • There are 3 cell shapes: • Squamous = flat, scale-like • Cuboidal = cube-shaped • Columnar = column-shaped
Epithelia Types • You can see that we’ve got at least 6: • Simple squamous • Simple cuboidal • Simple columnar • Stratified squamous • Stratified cuboidal • Stratified columnar • There are actually 2 other ones that are special – transitional epithelium and pseudostratified columnar epithelium.
Simple Squamous Epithelium • Microscopic Appearance: • Single layer of flattened cells, shaped like fried eggs with bulge where nucleus is located. • Nucleus is flattened in the plane of the cell. • Cytoplasm may be very thin and tough to see. • In 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 • Here it’s called the endothelium • Serous membranes • External lining of visceral organs • Here it’s called the mesothelium
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. • Example 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. • Secretion of a lubricating fluid by the epithelium making up the serous membranes (we’ll talk about them soon).
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? Above, we have a longitudinal section of a kidney tubule. Notice the almost square-shaped cells . One cell is highlighted for you 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 the available 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 available surface area for the absorption of nutrients. • Movement of egg and embryo in uterine tube. • Hence 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. Notice the multiple layers of clear, dead, enucleated cells.
Stratified Squamous Epithelium • Location of keratinized version: • Epidermis. Palms and soles of feet are typically heavily keratinized. • Locations of non-keratinized version: • 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 non-keratinized version have in common?
Which of these is keratinized and which is non-keratinized stratified squamous epithelium?
Stratified Squamous Epithelium • Functions: • Protection! • Keratinized version (a.k.a. dry epithelium) protects against mechanical abrasion, water loss, and pathogen entry. • Keratin is very strong, waterproof, and is bacteriostatic (prevents bacteria from reproducing). • Non-keratinized version (a.k.a., 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 To the left, we have an oocyte (egg cell) surrounded by stratified cuboidal epithelium. The oocyte is circled in blue • Functions: • Contributes to sweat secretion. • Secretion of ovarian hormones (e.g., estrogens)
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 lots of mucus-secreting goblet cells. • The mucus traps dust and bacteria • Cilia “sweep” the bacteria-laden mucus up the respiratory tract towards the pharynx where it can be swallowed. • Smoking paralyzes cilia – smokers have to cough violently to expel their mucus. Then they die!
Here, we have pathogens traveling down the pharynx trying to attack the surface cells. How can they be repelled???
The mucosal cells lining the trachea have released a flood of mucus, trapping the pathogens! Now what???
Transitional Epithelium • Microscopic Appearance: • Somewhat resembles stratified squamous epithelium, but the surface cells are rounded and often bulge above surface (dome-shaped). • Typically 5-6 cell layers thick when relaxed and 2-3 cell layers thick when stretched. • Cells may be flatter and thinner when epithelium is stretched. • Some cells are binucleate, i.e., they have 2 nuclei.
Transitional Epithelia • Locations: • Predominant epithelium lining the urinary tract • Found in part of the kidney, the ureters (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. • Originally called “transitional” because it was thought to be an intermediate between stratified squamous and stratified columnar epithelium. This isn’t true but the name has persisted.
Thyroid Gland: An endocrine gland Glands • A gland is a cell or an organ that secretes substances for use inside or outside the body. • Glands are composed predominantly of epithelial tissue. • Glands are broadly classified as: • Endocrine • Exocrine An exocrine gland Stomach: Both an exocrine and an endocrine gland
Exocrine Glands • Typically 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. • Exo = outside and crine = secrete. • Can be multicellular or unicellular. • Multicellular: • Pancreas, stomach, sweat glands, salivary glands, mammary glands, sebaceous glands, etc. • Unicellular: • Goblet cells. The parotid gland (a salivary gland) Goblet Cell
Endocrine Glands • Endo = within. • Do not secrete material into ducts. • Secrete chemical signals called hormones into the bloodstream where they travel through the body and affect other cells. • Examples include: • Thyroid, thymus, testes, ovaries, pituitary, pineal, adrenal, etc.
Exocrine Gland Structure • Exocrine glands are either: • Simple if their ducts do not branch. • Compound if their ducts do branch. • Exocrine glands are further classified by the shape of their secretory portion as: • Tubular if the secretory portion is the same diameter as the duct. • Alveolar if the secretory portion is like a round ball . • Tubuloalveolar if it’s a combination of the 2. Compound Simple
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 – referred to as the extracellular matrix. • In the diagram above, compare the density of cells in the epithelial layers and in the connective tissue layers. What’s the difference? • Most cells are not in contact with each other but are distributed throughout the extracellular matrix.
Functions of Connective Tissue • Binding of organs • Support • Physical protection • Immune protection • Movement • Storage • Heat production • Transport What do you NOT see a lot of in this micrograph of connective tissue?
Loose Connective Tissue Categories of CT • Fibrous connective tissue (a.k.a. connective tissue proper) • Supporting connective tissue • Fluid connective tissue Bone Blood
Fibrous Connective Tissue • Most diverse type of CT. • Contain extremely conspicuous fibers – hence the name, fibrous connective tissue. • The illustration below shows a 3-D model of some 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. • Of these 3, which you do suppose is typically NOT that abundant?
Cells of Fibrous CT: • Fibroblasts • 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 own body. • They activate the immune system when they encounter foreign matter called antigens. • Derived from white blood cells known as monocytes.
Cells of Fibrous CT • 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. • Plasma Cells • Certain white blood cells differentiate into plasma cells when they detect foreign agents. • Plasma cells produce and secrete antibodies (proteins that bind to foreign molecules (antigens), thus inactivating them or marking them for future destruction.
Cells of Fibrous CT • 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. • Adipocytes • Adipo = fat • Appear in small clusters in some fibroconnective tissues. • If they dominate an area, we call that area adipose tissue. • Contain huge droplets of lipids for storage.
Fibers in Fibrous Connective Tissue: • Collagenous Fibers • Reticular Fibers • Elastic Fibers Collagen Fibers as seen with a scanning electron microscope
Collagenous Fibers Close-up of a single fiber Multiple fibers arranged in the extracellular matrix • Interwoven strands of the protein collagen. • The 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, they have a white appearance, so they are sometimes called white fibers. • In stained slides, they are often pink and they usually appear quite wavy. • Tendons, ligaments, and the deep layer of the skin (the dermis) are made primarily of collagenous fibers.