Introduction to Tissues
A. Histology=the study of tissues.
B. Although studying tissues can be accomplished using a light microscope, studying cell parts often requires an electron microscope and the study of atoms and molecules can only be examined through special imaging techniques and experimental procedures.
Types of Tissues
A. Despite the fact the body is composed of trillions of cells, there are only about 200 different cell types. These cells in turn produce only four principle tissue types:
1. Epithelial tissues=covers exposed surfaces; lines internal passageways; and produces glandular secretions.
2. Connective tissues=fills internal spaces; provides structural support, and stores energy
3. Muscle tissues=contracts to produce active movements
4. Nervous tissue=conducts electrical impulses; detects, interprets, and responds to stimuli
B. Relative contribution of the four tissue types to the overall weight of the adult body.
C. Embryonic origins: There are three types of embryonic tissues from which all adult tissues are derived.
a. Endoderm=gives rise to the functional linings of the digestive and respiratory tracts as well as to the associated accessory glands and organs (i.e. liver, stomach, pancreas, etc.)
b. Mesoderm= gives rise to the components of the skeletal, muscular, and circulatory systems
c. Ectoderm= gives rise to the epidermis of skin and all of the components of the nervous system
D. Tissue Membranes
1. Mucous Membranes=composed of epithelial tissues. These membranes line body cavities that open to the exterior environment such as those of the digestive tract, respiratory tract, or urogenital tract. In all cases, these are "wet" or moist membranes because of the secretion of mucous. The moisture helps reduce friction and in many cases, facilitates absorption or secretion activities.
2. Serous Membranes=consists of a mesothelium supported by areolar tissue. These are never exposed or connected to the exterior. Serous membranes secrete transudate, or serous fluid. There are three serous membranes that line the ventral body cavity:
a. Pleura=lines the chest cavity and surrounds the lungs.
b. Pericardium=lines the pericardial cavity and surrounds the heart
c. Peritoneum=lines the peritoneal cavity and lines the surfaces of the visceral organs
3. Cutaneous Membranes=made of stratified squamous and areolar tissue reinforced by dense irregular connective tissue. In contrast to mucous and serous membranes, cutaneous membranes are dry, relatively thick, and waterproof.
4. Synovial Membranes=line mobile joint cavities but do not cover the opposing joint surfaces. Secretes synovial fluid. Although the covering of the synovial membrane is often called an epithelium, it differs from true epithelia in four respects: it develops within a connective tissue, no basal lamina is present, gaps of up to 1 mm may separate adjacent cells, and the synovial fluid and capillaries in the underlying connective tissue are continuously exchanging fluid and solutes.
Epithelial Tissues
A. Functions of Epithelial Tissues
1. Epithelia provide physical protection. Epithelial tissues protect exposed and internal surfaces from abrasion, dehydration, and destruction by chemical or biological agents.
2. Epithelia control permeability. Any substance that enters or leaves the body has to cross an epithelial tissue. Some epithelia are relatively impermeable, whereas others are permeable to compounds as large as proteins. Most are capable of selective absorption or secretion. The epithelial barrier can be regulated and modified in response to various stimuli. For example, a callus forms on your hands when you do rough work for an extended period of time.
3. Epithelia provide sensation. Sensory nerves extensively innervate most epithelia. Specialize epithelial cells can detect changes in the environment and convey information about such changes to the nervous system.
4. Epithelial cells that produce secretions are called glands. Individual gland cells are often scattered among other cell types in an epithelium that may have many other functions.
B. Location of Epithelial Tissues
1. Epithelia=forms sheets or layers of cells that line the body tubes, cavities, or coverings of the body surfaces.
2. Glands=formed of epithelial cells with secretory functions. Two types of glands are found in the human body:
a. Endocrine glands=secrete hormones (or hormonal precursors) into the interstitial fluid or bloodstream. These glands are ductless.
b. Exocrine glands=secretes non-hormonal substances (milk, wax, enzymes, oil, acids, etc.) onto external surfaces or internal passageways (ducts) that connect to the exterior.
C. Characteristics of Epithelial Tissues
1. Polarity=epithelial cells possess two structurally and functionally different surfaces:
a. Apical surface=free edge which faces the exterior of the body or the lumen of an internal space.
b. Basal surface=attached surface which anchors the cells to adjacent tissues.
2. Supported by a basal lamina=also known as the basement membrane, is a complex structure produced by the basal surface of the epithelial cells and the underlying connective tissue. The underlying connective tissue is composed of two things:
3. Cellularity=epithelial cells are extensively interconnected so that they create an effective barrier that behaves as if it were a single cell.
a. Occluding junctions=form a barrier that isolates the basolateral surfaces and deeper tissues from the contents of the lumen. At an occluding junction, the attachment is so tight that it prevents the passage of water and solutes between the cells.
b. Adhesion belt=locks together the terminal webs of neighboring cells, strengthening the apical region and preventing distortion and leakage at the occluding junctions. It forms a continuous band that encircles cells and binds them together.
c. Gap junctions=permits chemical communication that coordinates the activities of adjacent cells. At a gap junction, two cells are held together by interlocking junctional proteins called connexons which serve as channels that form a narrow passageway to let small molecules and ions to pass from cell to cell.
d. Desmosomes=provides firm attachment between neighboring cells by interlocking their cytoskeletons. At a desmosome, the opposing plasma membranes are very strong and resist stretching and twisting. Hemidesmosomes attach the basal surface to the basement membrane.
e. CAM=cell adhesion molecules; present in the adhesion belt and desmosomes; transmembrane proteins that bind to each other and to extracellular materials.
4. Avascular=epithelial tissues lack blood vessels; all nutrient and waste exchange occurs as a result of diffusion and osmosis from underlying tissues.
5. Highly innervated=epithelial tissues are supplied with many nerve endings
6. Regenerate rapidly=although the exact rate varies from one type of epithelia to another, most epithelial tissues regenerate within days (rather than weeks or years).
D. Naming Epithelial Tissues
1. Almost all epithelial tissues possess a two part name where the first part of their name indicates their arrangement (number of layers) while the second part of their name indicates the shape of the cells.
2. Arrangement of epithelial tissues
a. Simple=only one layer thick
b. Stratified=more than one layer thick
c. Pseudostratified= “false layers”; it looks like more than one layer but in fact its only one layer thick
3. Shape of epithelial cells
a. Squamous=thin, flat, and somewhat irregular in shape. From the surface, they look like fried eggs lay side by side. In a sectional view, they look like a pancake with a pat of butter (indicating the nucleus).
b. Cuboidal=are about as wide as they are tall; resemble hexagonal boxes with the spherical nucleus located in the center of each cell.
c. Columnar=are taller than they are wide; resemble rectangles with the elongated nuclei tend to crowd into a narrow band close to the basal lamina.
E. Diversity of Epithelial Tissues
1. Simple squamous epithelium
a. Description: single layer of flattened cells with a disc-shaped central nuclei and sparse cytoplasm.
b. Function: allows passage of materials by diffusion and filtration in sites where protection is not important. Also secretes lubricant.
c. Locations: Kidney glomeruli, air sacs of lungs, capillaries, linings of heart and lymphatic system.
2. Stratified squamous epithelium
a. Description: thick layers of flattened cells; often keratinized layer and a mitotic layer.
b. Function: protects underlying tissues in areas subject to abrasion
c. Location: non-keratinized type lines the mouth and vagina; keratinized type forms the epidermis of skin.
3. Simple cuboidal epithelium
a. Description: single layer of cube-like cells with large spherical centrally located nuclei.
b. Function: secretion and absorption
c. Locations: Kidney tubules, ducts and secretory portions of glands, ovary surface
4. Stratified cuboidal epithelium
a. Relatively rare in the human body.
b. Most common along the ducts of sweat glands, mammary glands, and other exocrine glands.
c. DO NOT NEED TO KNOW FOR THE LAB PRACTICAL!!
5. Simple columnar epithelium
a. Description: single layer of tall cells with round to oval nuclei; some cells bear cilia; may contain goblet cells that produce mucus; may contain microvilli.
b. Function: absorption; secretion of mucus and enzymes; cilia propel substances.
c. Location: non-ciliated type lines digestive tract, gallbladder, and ducts from glands; ciliated type lines small bronchi, uterine tubes, and uterus.
6. Stratified columnar epithelium
a. Relatively rare in the human body.
b. Most often found lining large ducts such as those of the salivary glands and pancreas.
c. DO NOT NEED TO KNOW FOR THE LAB PRACTICAL!!
7. Pseudostratified columnar epithelium
a. Description: single layer of cells of differing heights so that nuclei are a differing levels; may contain goblet cells and bear cilia.
b. Function: secretion, propulsion by ciliary action.
c. Location: non-ciliated type lines male reproductive ducts; ciliated type lines much of respiratory tract.
8. Transitional epithelium
a. Description: resembles both stratified squamous and stratified cuboidal. Basal cells are cuboidal or columnar; surface cells are dome shaped.
b. Function: stretches readily and permits distension.
c. Location: Lines uterus, bladder, and urethra
F. Glandular Epithelia are Specialized for Secretion
1. Endocrine glands= “ductless” glands that produce hormones. Secrete directly into interstitial fluids or bloodstream. Examples: pituitary gland, adrenal gland, thyroid gland, etc.
2. Exocrine glands=glands possessing ducts. Exocrine glands secret their substance either on the body surfaces or within ducts. They general demonstrates one of two different modes secretion:
a. Merocrine=secrete products from secretory vesicles by exocytosis. Most common type. Example: salivary glands of the oral cavity
b. Holocrine=accumulate products until the cell ruptures. Destroys the cell and must be replaced by cell division. Example: sebaceous glands of the skin
c. Apocrine=products accumulate within the cells then the apex of the cell pinches off packets that contain the secretion. Example: mammary gland of the breast
3. Exocrine glands are unicellular or multicellular.
a. Unicellular=goblet cells that produce mucin which mixes with water to form mucus.
b. Multicellular=two structural classes:
i. Simple=a single duct that does not branch on its way to the secretory cells (examples: gastric glands, sebaceous glands)
ii. Compound= duct divides one or more times on its way to the secretory cells (examples: duodenal glands, mammary glands and salivary glands)
Connective Tissues: Supports and Protects
A. Location of Connective Tissues
1. Most abundant tissue in the body.
2. Never exposed to the outside environment.
B. Characteristics of Connective Tissues
1. All types of connective tissue originate from mesenchyme.
2. Connective tissues vary widely in appearance and function but all forms share three basic components:
a. Specialized cells=the cells present in each type of connective tissue helps to distinguish the various types from one another. A few of the cells are listed here:
i. Fibroblast cells=produce connective tissue proper
ii. Chondrocytes=produce cartilage
iii. Osteocytes=produce bone
iv. Hemocytoblast cells=produce blood
b. Extracellular proteins fibers=three primary fibers are produced in connective tissues
i. Elastic fibers=slender, straight, and very stretchy. They recoil to their original length after stretching or distortion.
ii. Collagen fibers=thick, straight or wavy, and often forms bundles. They are very strong and resist stretching.
iii. Reticular fibers=strong fibers that form a branching network or scaffolding
c. Ground substance=material that fills the space between cells and surrounds the extracellular fibers. In some connective tissues the ground substance is gel-like while in others it is liquid based and in others it is rigid or calcified. Ground substance and extracellular fibers make up the matrix of connective tissues.
3. Many types of connective tissue are highly vascular and contain sensory receptors that detect pain, pressure, temperature, and other stimuli.
C. Functions of Connective Tissues
1. Establish a structural framework for the body.
2. Transport fluids and dissolved materials.
3. Protect delicate organs.
4. Support, surround, and interconnect other types of tissue.
5. Store energy reserves, especially in the form of triglycerides.
6. Defend the body from invading microorganisms.
D. Diversity of Connective Tissues
1. Connective Tissue Proper=includes connective tissues with many types of cells and extracellular fibers in a gel-like ground substance.
a. Loose Connective Tissues – fibers created a loose, open framework
i. Areolar tissue=most common form of connective tissue proper in adults. It is the general packing material in the body. Attaches skin to underlying body parts and is sometimes called the superficial fascia. All of the cell types found in other forms of connective tissue proper can be found in areolar.
ii. Adipose tissue=found deep to the skin, especially at the flanks, buttocks, and breasts. It also forms a layer that provides padding within the orbit of the eyes, in the abdominopelvic cavity, and around the kidneys. The distinction between areolar tissue and adipose is the larger number of adipocytes (fat cells).
iii. Reticular tissue=found in the liver, kidney, spleen, lymph nodes, and bone marrow, where it forms a tough, flexible network that provides support and resists distortion. In reticular tissue, reticular fibers create a complex supporting network known as a stroma. Fixed macrophages and fibroblasts are present but these cells are seldom visible. DO NOT NEED TO KNOW FOR THE LAB PRACTICAL!!
b. Dense Connective Tissues – fibers are densely packed together
i. Dense regular=all collagen fibers are oriented parallel to each other providing strength along the axis of the collagen fibers. Found in cords (such as tendons) or sheets (ligaments). Tendons connect muscle to bones. Ligaments connect bones to bones.
ii. Dense irregular=collagen fibers are non-parallel forming an interwoven network. These tissues provide strength in many directions and are particularly important in areas subjected to stress from many directions such as the dermis of the skin.
iii. Elastic=when elastic fibers outnumber collagen fibers, the tissue has a springy, resilient nature that allows it to tolerate cycles of extension and recoil. This elastic tissue is bound between the vertebrae of the spinal column and the erectile tissues of the penis. DO NOT NEED TO KNOW FOR THE LAB PRACTICAL!!
2. Fluid Connective Tissues=have distinctive populations of cells suspended in a watery matrix that contains dissolved proteins. NOT ON LAB PRACTICAL!
a. Blood – flows within the cardiovascular system
3. Supporting Connective Tissues=differ from connective tissue proper in have a less diverse cell population and a matrix containing much more densely packed fibers. Supporting connective tissues protect soft tissues and support the weight of part or all of the body.
a. Cartilage – solid, rubbery matrix containing chondrocytes. All cartilage is surrounded by a membrane of connective tissue called the perichondrium.
i. Hyaline cartilage=found connecting the ribs to the sternum, covering the articular surfaces of long bones, supporting the respiratory passageways such as the trachea, and forming the tip of the nose and part of the nasal septum. Has an amorphous matrix with few visible fibers. It provides stiff but somewhat flexible support and reduces friction between bony surfaces.
ii. Elastic cartilage=found in the ear and epiglottis. Has many more elastic fibers within the matrix and is therefore more flexible.
iii. Fibrous cartilage=found within the intervertebral discs, the meniscus of the knee, and pubic symphysis. Has many more collagen fibers within its matrix and is therefore very strong.
b. Bone – solid, crystalline matrix containing osteocytes. All bone is surrounded by a membrane of connective tissue called the periosteum. NOT ON LAB PRACTICAL!
c. Comparison of cartilage and bone.
Muscle Tissue in Motion (discussed in detail in Chapter 10-11) NOT ON LAB PRACTICAL!
A. Highly vascularized muscular tissue is comprised of elongated cells (called fibers) containing myofilaments (actin and myosin proteins).
B
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