Cirvello #3: Structure and Function of Tissues

Question 1

Desmosomes

Answer 1

Spot-like points of adhesion between cells commonly found within muscle annd epithelium. Desmosomes are designed to resist shearing because of the nature of the connections made between cell membranes. The cell-to-cell connections are made between cadherins, an integral membrane protein, which homodimerizes (forms a connection with another carherin) in a Ca+2-induced process. The cadherins are anchored intot he cell membrane through an attachment plaque made up of the proteins desmoplakina nd plakoglobin. Cadherins bind to plakoglobin, which in turn binds to desmoplakin. The cellular cytoskeleton actin filaments bind to desmoplakin and plakoglobin, directly anchoring each cell membrane together.

Question 2

Adherins Junctions

Answer 2

Commonly found in epithelium and links the membrane of two cells to the actin cytoskeleton. They’re most commonly found within the basal site of epithelium-to-epithelium connections, and are known as adhesion plaques if formed as spots, and as zona adherins if formed as bands. Carherin proteins again make the cell-to-cell membrane connections. In addition, these connections include catenins, vinculins, and alpha-actinin proteins. The catenin and vinculin proteins connect the cell membrane to actin filaments, while alpha-actinin proteins connect actin filaments to each other.

Question 3

Tight Junctions

Answer 3

Made up of a branch network of sealing strands that provide a virtually impenetrable barrier between cell membranes. Tight junctions prevent the movement of ions and molecules between cells (the pericellular pathway) as well as the lateral diffusion of integral membrane proteins. Tight junctions have several functions:

• They hold cells together.
• They block the movement of solutes and molecules between cells.
• They allow for polarization of the cell membrane (i.e., the unequal distribution of integral and peripheral membrane proteins along the surface of the cell).
• They are responsible for the blood-brain barrier limiting the movement of solutes and molecules into the cerebral spinal fluid.

This means when two epithelial cells are connected with tight junctions, the only way ions and molecules can move through the cell is through both membranes by solute-specific transport processes.

Tight junctions are formed by the interaction between integral membrane proteins in each cell membrane, with the major protein types the caludins and occludins. The claudins and occludins associate with peripheral membrane proteins in each membrane, anchoring to the actin filament cytoskeleton. Tight junctions are commonly found in skin epithelium.

Question 4

Gap Junctions

Answer 4

A specialized connection that allows for the easy movement of small solutes between cells. Gap junctions directly connect two cells allowing for the easy movement of molecules and ions smaller than 4 nm in size. Each junction is made up of connexins proteins, integral membrane proteins with four transmembrane spanning segments. Six connexins come together to form a connexon, one-half of the gap junction. Gap junctions allow for the following:

• Direct electrical communication between cells (as in neurons).
• Direct chemical communication between cells for ions and small molecules.
• Direct movement of molecules smaller than 1000 Daltons between cells, but also allowing different connexons to have slightly different pore sizes and selectivities.
• No loss of ions or electrical current into the intercellular space.

Gap junctions are found within cardiac muscle where they allow the heart to contract in unison, within neurons where they allows for direct electrical connections between neurons and in the retina.

Question 5

Hemidesmosome

Answer 5

Small found structures found on the surface of keratinocytes within the skin. They look like desmosomes by electron microscopy but instead of connecting cells to cells, they connect cells to the basement membrane. The connection is made between cell adhesion proteins and the extracellular matrix. Cellular intermediate filaments connect to plecktin proteins, which are part of an attachment plaque. The plecktin proteins connect to integrin proteins, which in turn connect to extracellular matrix filaments, anchoring the cell to the basement membrane.

Question 6

Epithelium

Answer 6

Makes up about 60% of tissues in human body. Functions include secretion, absorption, and surface protection. Epithelia have unique properties that depend on cell structure combined with cell-specific proteins. The epithelium forms a major barrier preventing these potentially harmful compounds and microbes from entering the deeper tissues of the body.

Question 7

Common Characteristics Across of Epithelium

Answer 7

Epithelium consists primarily of cells with little extracellular material between cells.

Epithelium covers the external and internal body surfaces, and forms specialized structures like sweat glands.

Epithelium cells are polarized, with an asymmetrical distribution of cytoplasmic content, organelles, and proteins. The apical surface faces the external environment and doesn’t contact other cells or the basement membrane. The apical surface also commonly has cilia for movement and microvilli for absorption, which are absent from all other surfaces of the cell. The lateral (side) surfaces connect one epithelial cell to another epithelial cell through expression of specialized proteins. The basal surfaces are connected to the basement membrane. Together, the basal and lateral surfaces are called the basolateral surface. The basement membrane is a specialized extracellular matrix (composed of proteins and sugars) secreted by epithelial cells and connective tissue cells. The basement membrane provides sites of attachment to epithelial cells and acts as a signaling system that affects epithelial cell function (division of stem cells and movement of cells during repair). There are exceptions to these generalities: Epithelium found in lymphatic capillaries or liver sinusoids do not attach to basement membranes, whereas epithelium in endocirne glands don’t have an apical or basal surface.

Epithelium has specialized cell-to-cell contacts, such as tight junctions, adherens junctions, hemidesmosomes, and desmosomes that bind adjacent cells and limit the movement of solutes between cells and is a major part of its protective role.

Epithelium is avascular (tissue lacks blood vessels). Blood capillaries don’t penetrate the basement membrane so nutrients must diffuse through the basement membrane, to reach the epithelium. For epithelium several cell layers thick, the most metabolically active cells are closest to the basement membrane. Epithelium can be richly innervated and is important source of sensory info.

Epithelium contains stem cells that divide and replace damaged and dead cells throughout a person’s life.

Question 8

Simple Epithelium

Answer 8

Consists of a single cell layer. The basal surface of each cell is anchored to the basement membrane and the apical surface is exposed to the environment. These cells are typically found in areas specialized for diffusion, such as the gastrointestinal tract or the inside of the lungs. They’re not found in areas that in direct contact with outside threats, like the skin.

Question 9

Stratified Epithelium

Answer 9

Consists of multiple cell layers. Only one layer is connected to the basement membrane. Stratified epithelium is found in the outer layer of the skin and provides up to 30 layers of cells.

Question 10

Pseudostrafied Epithelium

Answer 10

Appears to be stratified but is not, hence pseudo in the name. It consists of a single cell layer, with the basal surface attached to the basement membrane. The cells are relatively tall and have a free apical surface. Not all cells reach the surface because they have different heights. These cells are typically found in airways (nasal cavity, trachea, and bronchi), are ciliated, and are coated with mucus. The mucus traps particle debris, and the cilia move the debris and mucus of of the body.

Question 11

Transitional Epithelium

Answer 11

Contains cells that are flattened and cells that are cuboidal, hence the name transitional. Transitional epithelium is found in the bladder and in the first part of the ureters as they leave the kidneys. Transitional epithelium has the capacity to stretch and flatten so the bladder can expand whereas maintaining its functional properties. The surface and underlying cells are either cuboidal or columnar when relaxed but become squamous-like when stretched, and the apparent number of layers decreases with stretch. For this to occur, the epithelial cell layers move relative to one another to condense multiple layers into two or three layers.

Question 12

Basic Epithelium Cell Shapes

Answer 12

Squamous - cells are scale like or flat.

Cuboidal - cells are cube shaped and as wide as they are tall.

Columnar - cells are taller than they are wide and look like columns.

Question 13

Epithelium - Protection

Answer 13

Epithelium protects underlying tissues. The type of protection depends on the connections between cells, cell shape, and the number of cell layers. If protection against mechanical or chemical stress is more important than diffusion or absorption, a stratified epithelium is much more effective. As cells are lost from the outer layer of the stratified epithelium, stem cells dividing int he lowest layer next to the basement membrane replace them. Stratified epithelium is found in areas where mechanical abrasion occurs, such as the skin, mouth, throat, esophagus, anus, and vagina. The cell surface of stratified or simple epithelium can be smooth, contain microvilli, be ciliated, or be folded.

Question 14

Epithelium - Barrier

Answer 14

The ability of epithelium to prevent diffusion of solutes (including water) depends on proteins found in the apical membrane and the cell-to-cell connections. In simple epithelium, the presence or absence of transporter proteins int he apical and basal membranes determines the permeability of the epithelium to specific solutes. In the absence of those transporters, water-soluble solutes cannot pass through the epithelial layer–hence the barrier function. The tight junction between epithelial cells prevents the movement of water-soluble solutes between cells (pericellular pathway).

Gap junctions allow molecules up to approximately 1000 Mw in size and ions to pass between neighboring cells and coordinate cellular activity in epithelial layers. The channels provide a direct intercellular communication pathway. These channels have short half-lives and complex assembly and degradation pathways. Gap junctions are found in epithelium in heart muscle, where they allow the rapid movement of electrical depolarization between cells.

Question 15

Epithelium - Exchange

Answer 15

Exchange across epithelium can occur by two mechanisms.

1) Direct diffusion of hydrophobic compounds through the cell layer, such as the gases O2 and CO2
2) Through solute-specific transport processes that bring compounds into the cell, like the transport of glucose.

Epithelium optimized for exchange is always simple epithelium and not stratified. The lining of the gut and lungs is composed of a single layer of epithelium to maximize diffusion and specific transport processes. Because it’s only one cell layer thick, simple epithelium doesn’t impede the diffusion (lungs), filtration (kidneys), absorption (intestine), or secretion (mouth). Simple epithelium does protect against nonspecific permeability of a wide range of solutes because of tight cell-to-cell contacts. In these tissues, protection against mechanical stress is not as important as maximizing the diffusion of gases and regulating solute permeability.

In contrast with epithelium designed to promote diffusion, absorptive epithelium is typically characterized by columnar shape and the presence of microvilli. The columnar shape increases cytosolic volume and space for organelles that are required for different types of transport.

Question 16

Microvilli

Answer 16

Tremendously increase surface area of epithelium. Dramatically increase the absorptive surface area and are especially useful in the gastrointestinal tract, where they aid in the absorption of nutrients.

Question 17

Cilia

Answer 17

Useful in moving fluid or material over the surface of the cells and are especially useful int he bronchial airways, where they act to remove dust and debris that is brought with each breath.

Transitional epithelial tissues are excellent for those tissues that expand and contract, such as the stomach or the bladder.

Question 18

Epithelium - Secretion

Answer 18

Involves the release of material inside the cell to the extracellular space through the apical membrane or by exocytosis of vesicle contents. Glandular epithelium is composed of secretory cells found in many tissues with a supporting network of connective tissue. Glandular epithelium usually develops from an in folding or out-folding of an embryonic epithelium and is commonly columnar or cuboidal in shape. This increases the cytoplasmic volume, making more space for organelles required for secretion, such as rough endoplasmic reticulum, ribosomes, Golgi, and secretory vesicles. Secretory tissue can also be more complex, with glandular epithelium organized into secretory acini within lobules, which are organized within lobes that are separated by connective tissue capsules. A secretory acini is composed of glandular epithelium that forms an acinus and is connected to a duct that carries secreted material to other parts of the body.

Question 19

Exocrine Gland

Answer 19

Type of glandular epithelium. Secretes fluid into a duct. These ducts are commonly found in the linings of the intestinal tract, stomach, and mouth.

Commonly, exocrine glands have more than one cell type (multicellular gland) or a single cell type (unicellular gland). The pancrease is a multicellular exocrine gland; goblet cells are unicellular exocrine glands (mucus secreting) that line the respiratory airways.

Multicelluar exocrine glands that have ducts with few branches are simple glands; those with many branches are compound glands.

The ending of ducts also varies among exocrine glands: Some ducts end in small tubes (tubules) some end in acini (a grapelike or small, saclike structure) and other ducts end in alveoli (hollow sacs).

Tubular glands are either straight or coiled, but most are simple and straight, simple and coiled, or compound and coiled. From a structural (and functional) standpoint, the duct is designed to most efficiently deliver the secretory product to its destination.

Question 20

Endocrine Gland

Answer 20

Type of glandular epithelium. Has no duct, and secretions travel from extracellular space into blood supply or are directly secreted into the blood, like the adrenal glands, kidney, or heart.

Question 21

Merocrine Secretion

Answer 21

Way exocrine glands secrete substances. Cell products are released from secretory vesicles that fuse with the cell membrane during exocytosis. This is the most common type of exocrine secretion, and an example is provided by the secretion of mucus (mucin and water) that coats respiratory and digestive passageways.

Question 22

Apocrine Secretion

Answer 22

Way exocrine glands secrete substances. Cell products are released through secretory vesicles, along with the loss of cytoplasm. An example is provided by the thick, sticky underarm perspiration that results from apocrine secretion.

Question 23

Holocrine Secretion

Answer 23

Way exocrine glands secrete substances. Cell products are released when the secretory cell is destroyed. During holocrine secretion, the entire cell becomes filled with secretory vesicles until the cell bursts. Further secretion depends on the replacement of secretory epithelium by the division of stem cells. Sebaceous glands associated with hair follicles produce an oily secretion through holocrine secretion.

Question 24

Epithelium - Absorption

Answer 24

The absorptive surfaces of the body are lined with simple columnar epithelium, which is polarized and has distinct apical and basolateral surfaces. The proteins and lipids found in each membrane are not the same. Proteins reach these surfaces by two pathways. Newly made proteins can travel from the Golgi directly to the apical or basolateral surface (through the addition of a cellular zip code sequence). Alternatively, proteins can reach one surface, generally the basolateral, and then be endocytosed and trancytosed to the opposite surface.

Trancytosis is the process by which particles and compounds are endocytosed at one side of the cell (basolateral membrane), transported through the cytoplasm, and then exocytosed at the other side of the cell (apical membrane). Transcytosis occurs universally in all epithelium, and in some, epithelium is the only pathway for apical delivery of proteins. Transcytosis can also transport molecules from apical membrane to the basolateral membrane.

The consequences of a polarized membrane are that one membrane (typically the apical) can be optimized for transport or absorption of solutes, whereas the other membrane can be optimized for other tasks. The apical cell membrane commonly has microvilli that dramatically increase the surface area. Absorption depends on the epithelium acting as a barrier to the nonspecific movement of solutes through the cell layer, coupled with the expression of proteins in the apical membrane that selectively move solutes into the cell, where they can be absorbed into the body.

Question 25

Mucous Membrane

Answer 25

Mucous membranes (mucosae) line passageways that communicate with the outside environment (in digestive, respiratory, urinary and reproductive tracts).
- The epithelial surfaces are moist (lubricated) to reduce friction, or facilitate absorption and excretion.
- The areolar tissue in mucous membranes is the lamina propria.

Question 26

Serous Membrane

Answer 26

A simple squamous epithelium (also known as the mesothelium), a basement membrane, and a delicate layer of connective tissue. Serous membranes have a visceral and parietal surface; the parietal surface is the outer surface, and the visceral surface is directly associated with underlying tissues (i.e., viscera). Serous membranes line cavities that don’t open to the outside (the pericardial, pleural, and parietal cavities). Serous membranes are aglandular: They don’t have glandular epithelium, but they do produce a thin coating called serous fluid. The serous fluid acts as a lubricant and aids in the movement of internal organs by greatly reducing friction. Serous membranes also act to provide structural support to membranes, and as selectively permeable barriers that limit fluid and ion flow into the cavities.

Serous membranes line cavities that are not open to the outside environment. Serous membranes are thin but strong. They are lubricated with a fluid transudate to reduce friction.

• Each serous membrane has a parietal portion covering the cavity surface, and a visceral portion (serosa) covering the organs.
  pleural membrane lines pleural cavities and covers the lungs.
  peritoneum lines the peritoneal cavity and covers abdominal organs.
  pericardium lines the pericardial cavity and covers the heart.

Question 27

Cutaneous Membrane

Answer 27

Cutaneous membrane is the skin that covers the surface of the body.
- It is thick, waterproof and dry.
- It consists of stratified squamous epithelium, areolar tissue, and dense irregular connective tissue.

Question 28

Connective Tissue

Answer 28

Abundant and found in each organ. Acts to connect different cell types. Connective tissue is defined both by the cells found within it and make up of the extracellular matrix, the types of ground substances and fibers (collagen, elastic, reticular). The physiologic functions of connective tissue are equally dependent on properties of the specific cell and extracellular matrix proteins. Connective tissue includes adipose, cartilage, blood, bone, and dense and loose connective tissues.

Composed of cells and the extracellular matrix proteins that they secrete. The cell type and the types of proteins that make up the extracellular matrix identify each connective tissue. There are different cell types within each connective tissue.

• Cells with the suffix blast within their name produce extracellular matrix.
• Cells with the suffix cyte within their name maintain extracellular matrix.
• Cells with suffix clast within their name break extracellular matrix down and remodel it.

Question 29

Connective Tissue Physiologic Functions

Answer 29

Enclosure and separation. Sheets of connective tissue form capsules around organs, like the kidney and liver. Connective tissue also separates tissues within a specific organ or body area; for example, within the arm, connective tissue separates muscle from blood vessels and nervous tissues.

Connection. Connective tissue provides a physical connection between two different tissues. In some cases, this is obvious, as when tendons connect muscle to bone. In other cases, it’s less obvious; blood is a connective tissue allowing contact between different tissues.

Support and motion. Connective tissue provides support, like the bone that supports the shoulder. It also allows for motion in the tissues where it provides support.

Storage. Connective tissue can store energy (adipose) or essential minerals (in bone).

Cushion and insulation. Adipose tissue provides for both insulation (against chances in temperature), and cushions tissues and organs against mechanical stress.

Transportation. Blood is a connective tissue that transports gases, nutrients, hormones, and waste products throughout the body and acts to interconnect all cells.

Protection. Within blood, immune cells provide protection against microorganisms that cause disease. Bones protect underlying soft tissues; the ribs protect the lungs and heart.

Question 30

Extracellular Matrix

Answer 30

Connective tissue extracellular matrix is a combo of fibrous and nonfibrous proteins, additional organic molecules, and fluid. The extracellular matrix structure gives connective tissue most of its functional characteristics: mobility, tensile strength, resistance to compression, and elasticity.

The extracellular matrix is composed of many different proteins. They’re produced by connective tissue cells and secreted into the extracellular space. An extracellular protein structure contains sequences that induce the protein to assemble into large macromolecular structures. The macromolecular structure provides the physiologic function of the extracellular matrix. Mutations in monomers can lead to serious health problems. At least 50 proteins have been identified in extracellular matrix.

Question 31

Connective Tissue Proper

Answer 31

Contains collagen, elastic, and reticular fibers. Connective tissue proper contains undifferentiated mesenchymal cells (even in adults) that are precursors for other connective tissue cells, fibroblasts, adipocytes, macrophages, and small numbers of other cells (melanocytes, mast cells, and some blood cells). Fibroblasts are responsible for fiber and ground substance production. Adipocytes are responsible for support and protection against mechanical stresses, and energy storage. Macrophages are responsible for the removal of debris and foreign cells.

Question 32

Mesenchyme

Answer 32

Or embryonic connective tissue. Made up of irregularly shaped fibroblasts surrounded by a semisolid extracellular matrix that has many thin collagen fibers. Mesenchyme forms in the embryo during the third and fourth weeks from mesoderm and neural crest cells.

All adult connective tissue develops from mesenchyme. By the 8th week of embryonic development, the mesenchyme has begun to divide and differentiate into the adult forms of connective tissue found in muscle, blood vessels, and joints.

A special mesenchyme called mecous connective tissue, or Wharton’s jelly, or, is found in the umbilical cord and is also a source of primitive stem cells. The umbilical cords can be saved and used as a source of stem cells for research and treatment.

Question 33

Loose Connective Tissue

Answer 33

Composed of aereolar, adipose, and reticular tissue. Loose connective tissue is composed of protein fibers that form extensive networks with fluid-filled spaces.

Loose connective tissue is a “loose-packing” material found in many organs, tissues, and in the dermal regions of the skin. It contains fibroblasts that secrete the extracellular matrix proteins, macrophages that engulf cell debris, mast cells that regulate inflammation, and white blood cells involved in immune surveillance.

The loose packing of this type of connective tissue is important in areas that need to balance support and protection with flexibility and extensibility (such as in adipose tissue).

Question 34

Adipose Tissue

Answer 34

Made up of adipocytes with very little extracellular matrix. Adipocytes have large lipid droplets that take up most of the cell volume and represent stored chemical energy. Adipocytes are arranged in clusters separated by small amounts of loose connective tissue. The sparse extracellular is composed of loosely arranged collagen and reticular fibers with scattered elastic fibers. In contrast with other connective tissues, adipose tissue is highly vascularized and has an extensive blood supply, suggesting that this tissue carries out a variety of tasks. The unique structure of adipose tissue gives it both connective tissue properties and nonconnective tissue properties. The conenctive tissue properties function as an insulator and protector of soft tissues from mechanical or physical trauma. Nonconnective tissue properties are its ability to store energy for the body’s needs.

Adipose appears white or yellow on physical examination, with yellow much more common. Adipose is originally white at birth but becomes yellow with the accumulation of food pigments. A 3rd type of adipose tissue found in babies, brown adipose tissue, is found only in the axillaie (underarms), neck, and near the pigments. Brown adipose is more prevalent in babies than in adults, and is specialized to generate heat from the metabolism of lipids. It helps infants regulate body temperature before they learn to shiver.

Question 35

Reticular Connective Tissue

Answer 35

Composed of a network of reticular fibers and fibroblast cells. Fibroblast cells produce the extracellular matrix proteins that create the fibrils and stay attached to them. The meshlike structure of this connective tissue provides a physical framework for the attachment and growth of other cell types. Reticular tissue is common in the immune system organs, where it provides support for the growth and differentiation of white blood cells.

Question 36

Dense Connective Tissue

Answer 36

Found in areas that require a more organized connective tissue that fills up the available space. The protein fibrils of dense connective tissue form thick bundles and occupy a greater % of the extracellular matrix than loose connective tissue fibrils. The major cell type in dense connective tissue is spindle-shaped fibroblasts that become fibrocytes when encased in extracellular matrix. Dense connective tissue can be divided into regular or irregular connective tissue.

Question 37

Dense Regular Connective Tissue

Answer 37

Has abundant collagen fibrils arranged in predominately one orientation, giving this tissue a white color. The arrangement of fibrils in one direction gives a great deal of strength along the same axis and resists stretching. Dense regular connective tissue is found in tendons that connect muscles to bones and ligaments that connect bone to bone, giving these tissues strong, cablelike properties, Tendons and ligaments aren’t identical. Ligament collagen fibrils are neither as compact nor as parallel as they are in tendons, and ligaments are more flattened and form sheets or bands of tissues.

Question 38

Dense Irregular Connective Tissue

Answer 38

Contains collagen fibrils arranged either in a random manner or with one layer oriented in one direction and the layer below it oriented at right angles. This type of connective tissue is not as strong in one direction as dense is, but it provides strength in many directions. This type of connective tissue is especially useful in skin dermis. The dermis is composed mainly of dense irregular tissue and is well supplied with blood vessels, lymphatic vessels, and nerves. Cutaneous receptors, glands, and hair follicles reside within the dermis.

Question 39

Dense Regular Elastic Connective Tissue

Answer 39

Consists of parallel collagen fibrils with many elastin fibrils. The elastin fibrils give this connective tissue a yellowish color and allow the tissue to stretch and their return to its original form. This tissue is found in the ligaments that hold up the head and in the vocal folds of the larynx.

Question 40

Dense Irregular Elastic Connective Tissue

Answer 40

Contains randomly aligned collagen fibrils with elastin fibrils. This connective tissue has the ability to stretch in any direction and return to its original shape. This tissue is found in walls of elastic arteries, like the aorta.

Question 41

Cartilage

Answer 41

Synthesized by chondroblasts that secrete and surround themselves with an extracellular matrix. Chondrocytes differentiate from stem cells and are capable of cell division within the cartilaginous matrix for growth or replacement. Chondrocytes produce randomly oriented type II collagen, which provides tensile strength, and proteoglycans. Aggrecan, the major component of cartilage, is a chondroitin sulfate proteoglycan that binds hyaluronic acid and provides a firm base for the elastic matrix. Aggrecan is very sensitive to proteolysis and proteolysis of aggrecan has been thought to play a role in the development of arthritis. Several other proteoglycans with specialized functions (often related to formation of complexes and cellular attachments) are also present in cartilage.

Chondroblasts and chondrocytes exist within a rigid walled cavity called the lacuna. Catilage is the second firmest connective tissue in the body (bone is the firmest). The rigid framework contains protein fibrils, ground substance, and fluid. The protein fibrils are predominantly collagen, but there are also elastic fibrils. The ground substance is composed of chondronectin, proteoglycans, adhesive proteins, and other compounds. The proteoglycans form aggregates with hyaluronic acid and are hygroscopic, trapping water within the framework. The presence of water helps cartilage resist compression (water is noncompressible) and makes it rebound to its original shape after compression.

The cartilage surface is covered with a dense irregular connective tissue known as perichondrium. Cartilage cells arise from the perichondrium and secrete proteins to produce the extracellular matrix. Once completely surrounded by matrix, the chondroblasts become chondrocytes and they’re found within lacunae. Cartilage is avascular and has no nervous tissue, but the perichondrium has blood vessels and nerve cells. Once damaged, cartilage is very slow to heal because it has no blood supply, so cells and nutrients necessary for tissue repair are slow to reach the area.

Mechanical stimulation causes chondrocytes to either add or remove the extracellular matrix. The cells can also change the structure of proteoglycans in the extracellular matrix, which changes the function of the tissue.

Question 42

Hyaline Cartilage

Answer 42

Has a high concentration of collagen fibrils and proteoglycans. The collagen fibrils are evenly dispersed throughout the ground substance, and the matrix of hyaline cartilage has a very smooth appearance. Hyaline cartilage gives strong support and limited flexibility, and is found in the rib cage, trachea, and bronchi. It also covers the ends of growing bones. Hyaline cartilage forms most of the fetal skeleton and is eventually replaced by bone.

Question 43

Fibrocartilage

Answer 43

Has more collagen fibrils than proteoglycans, increasing its strength at the expense of flexibility. Fibrocartilage is slightly compressible and very tough, and is found in areas subjected to great levels of pressure, such as the knee joint and between the vertebrae.

Question 44

Elastic Cartilage

Answer 44

Has more elastic fibrils, as well as collagen fibrils and proteoglycans. More elastin decreases the strength but increases the flexibility of structures. It’s found in the external ears.

Question 45

Bone

Answer 45

A very hard connective tissue that consists of living cells and a mineralized matrix. Bones represent the other end of the continuum from loose connective tissue or blood; they have sacrificed flexibility and elasticity for strength and rigidity. Bones are a composite material, made up or cells, collagen fibrils, organic molecules, and inorganic minerals. The mineral in bone is hydroxyapatite, which contains Ca+2 and PO4-2. Bone can withstand large loads without breaking or sustaining damage because it’s a composite material (made up of two different materials with different properties) and because bone changes in response to external forces. Over time, the more weight a bone bears, the stronger it becomes. Bones resist both compression and tensile stresses, and are an ideal connective tissue to support, protect, and provide movement for body tissues and organs.

Bone is produced by osteoblasts, cells that secrete collagen proteins and other organic molecules and form hydroxyapatite. Osteocytes are bone cells that maintain bone by the synthesis of collagen and other proteins. Osteoclasts resorb (dissolve) bone. There are two main types of bone tissues: Cancellous, or spongy, bone has spaces between bone trabeculae and resembles a sponge; compact bone has a solid, continuous matrix. Bone is a more vascular tissue than cartilage, and it heals more quickly than cartilage when injured.

The spaces and cavities or large bones are filled with marrow, one type of which is responsible for the synthesis of blood cells.

Question 46

Blood

Answer 46

Connective tissue fluid. At the other end of the liquid to hard continuum from bone. It has a fluid extracellular matrix. The cells of other connective tissues are relatively stationary within their matrix, but blood cells are free to move throughout the body. White blood cells can leave blood vessels and move through other tissues. Red blood cells (erythrocytes) move through the blood and act as O2 carriers. The advantage of a liquid matrix is that free cell mobility, coupled with a liquid extracellular matrix, makes it an ideal means by which solutes and other compounds can be transported throughout the body. Because blood connects distant tissues to one another, it’s known as a connective tissue. Like other connective tissues, blood, by allowing for the movement of immune cells, acts to protect the body, and carrying O2 and nutrients supports cellular metabolism.

In contrast with other connective tissues, the blood matrix is not produced by blood cells, but by cells in other tissues. Blood cells themselves are produced from stem cells (hemopoietic tissue), found within bone marrow, the soft connective tissue inside bonds. There are two types of marrow: yellow and red. Yellow marrow consists of adipose tissue, and the red marrow consists of blood stem cells in a reticular fiber meshwork.

Question 47

Muscle Tissue

Answer 47

Has many structural and functional attributes that are not shared with other cell types. 3 Types: Skeletal, smooth, and cardiac.

Skeletal muscle cell fibers are large, multinucleated cells (formed by the fusion of individual myocytes) with high concentrations of specifically arranged actin and myosin filaments. Functionally, skeletal muscle is capable of converting chemical energy (ATP) into mechanical work, the generation of a force sufficient to move a load. Skeletal muscle generates mechanical force by themovement of actin and myosin filaments.

The contraction of muscle cells generates force to pull on bones, causing the bones (and attached body parts) to move. These 3 types of muscle differ in their structure and activity.

Most of the muscle in the body is skeletal and associated with bones, and is called striated muscle because of the striations (stripes) within the muscle fibers. The heart is made of cardiac muscle, which is also a striated muscle but differs in structure and function. Smooth, nonstriated, muscle is associated with blood vessels, hollow organs, and other tissues, and differs from striated muscle in both structure and function. Skeletal muscle is under voluntary control, whereas smooth and cardiac aren’t.

Question 48

Nervous Tissue

Answer 48

Has a unique structure that allows it to have a unique function not shared by other tissue types, namely the ability to use electrical currents to send messages between cells. nervous tissue has the ability to generate currents across the cell membrane and to propagate this current flow over great distances. This action potential, or nerve impulse, is part of a system for cell-cell communication. Nervous tissue is found in the brain, spinal cord, and in nerves. Nervous tissue is composed of neurons that carry action potentials and supporting cells called neuroglia. Neruoglia supports, protects, nourishes, and insulates neurons. The typical neuron is composed of a cell body, dendrites, and axons. The cell body contains the nucleus, and is the site of protein synthesis and cell metabolism. Dendrites are extensions of the chemical or electrical changes in the cell membrane. Both the chemical and electrical changes act to modify cell membrane ion channels and change ion flux across the membrane. The axon is a long extension of the neuron where info is sent out to the next cell. Info travels to the end of the axon to the synapse, where it’s converted into another form (chemical or electrical) for transport to another cell; this propagation is known as synaptic transmission.

Neurons have a wide variety of shapes depending on the number and structure of dendrites and the number and structure of the axon. Neuroglia are support cells that protect, nourish, and insulate neurons.

Question 49

Synovial membranes

Answer 49

line articulating (moving) joint cavities and produce the synovial fluid which lubricates the joint.

protect the ends of bones and allow free movement.

consist of areolar tissue, collagen fibers, proteoglycans and glycoproteins.

do not have a true epithelium.

Question 50

Fascia

Answer 50

layers that surround and support organs. There are 3 types of these layers:

1. Superficial fascia or subcutaneous layer (sub = below, cutis = skin) is the areolar tissue and fat that separates the skin from underlying tissues.
   
   • allows independent movement.
   • pads and insulates deep tissues.
2. Deep fascia is a strong, fibrous network of dense irregular connective tissue which ties structural elements together.
   
   • internal organs are anchored to deep fascia.
3. Subserous fascia is areolar tissue that separates the deep fascia of muscles from serous membranes, allowing independent movement.

Question 51

Interstitial and Appositional Growth

Answer 51

Cartilage grows by 2 mechanisms:

(1) Interstitial growth increases cartilage size from the inside.
- Chondrocytes divide and produce new matrix.

(2) Appositional growth increases the outer size of a cartilage by adding new layers.

Question 52

Inflammation

Answer 52

The restoration of homeostasis after a tissue has been injured involves 2 processes: inflammation and regeneration.

1. Inflammation is the tissue’s first response to injury. Signs of inflammatory response include swelling, redness, heat, and pain at the site of the injury.

The presence of harmful bacteria (pathogens) in a tissue (an infection) also causes an inflammatory response.

The process of inflammation occurs in several stages:

• Damaged cells release prostaglandins, protein and potassium ions into the surrounding interstitial fluid.
• As the cell breaks down, lysosomes release enzymes that destroy the injured cell and attack surrounding tissues. Tissue destruction is called necrosis.
• Necrotic tissues and cellular debris (pus) accumulate in the wound. (Pus trapped in an enclosed area is an abscess.)
• The injury stimulates mast cells in the tissue to release histamine, heparin, and prostaglandins, which trigger changes in the surrounding blood vessels.
• Dilation (widening) of blood vessels increases blood circulation in the area, causing warmth and redness.
• Plasma diffuses into the area, causing swelling and pain.
• Increased blood flow brings more nutrients and oxygen to the area, and removes wastes.
• Phagocytic white blood cells clean up the area.

Question 53

Regeneration

Answer 53

The restoration of homeostasis after a tissue has been injured involves 2 processes: inflammation and regeneration.

2. When the injury or infection has been cleared up, the regeneration or healing phase begins.
   - Fibroblasts move into the necrotic area, laying down collagen fibers that bind the area together (scar tissue).
   - New cells migrate into the area, or are produced by mesenchymal stem cells.
   - Not all tissues can regenerate. Epithelia and connective tissues regenerate well. Cardiac cells and neurons do not regenerate.