TISSUE:%20THE%20LIVING%20FABRIC

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TISSUE THE LIVING FABRIC
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INTRODUCTION TO TISSUE

• Tissues are groups of cells that are similar in
  structure and function (tissuewoven)
• There are four primary tissues types
• Epithelial covering
• Connective support
• Nervous control
• Muscular movement
• Histology study of tissues

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EPITHELIAL TISSUE

• Epithelium (plural epithelia)
• Sheet of cells that covers a body surface or
  lines a body cavity (epithelaid on, covering)
• Occurs in the body as
• 1. Covering and lining epithelium
• Forms the outer layer of the skin, dips into and
  lines the open cavities of the cardiovascular,
  digestive, and respiratory systems, and covers
  the walls and organs of the closed ventral body
  cavity
• 2. Glandular epithelium
• Fashions the glands of the body

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EPITHELIAL TISSUE

• In its role as an interface tissue, epithelium
  accomplishes many functions, including
• 1. Protection
• 2. Absorption
• 3. Filtration
• 4. Excretion
• 5. Secretion
• 6. Sensory Reception

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Special Characteristics of Epithelium

• Has many characteristics that distinguish them
  from other tissue types
• 1. Cellularity
• Composed of closely packed cells with little
  extracellular material between
• 2. Specialized contacts
• Adjacent epithelial cells are bound together to
  form continuous sheets by specialized contacts
  such as desmosomes and tight junctions
• 3. Polarity
• Exhibits polarity by having an apical surface
  (upper free surface exposed to the body exterior
  or the cavity of an internal organ)) and a lower
  attached basal surface
• All epithelia exhibit polarity, meaning that cell
  regions near the apical surface differ from those
  near the basal surface in both structure and
  function
• Example
• Some apical surfaces have villi while the basal
  surface acts as a filter determining which
  molecules will be allowed to enter the epithelium
  
• 4. Supported by connective tissue
• Supported by the underlying connective tissue
  (reticular lamina) containing collagen fibers
• 5. Innervated but avascular
• Nourished by substances diffusing from blood
  vessels in the underlying connective tissue
• 6. Has a high regeneration capacity
• Replace lost cells rapidly by cell division

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Classification of Epithelia

• Each epithelial tissue is given two names
• The first name indicates the number of layers
  present
• Simple (one)
• Composed of a single cell layer
• Typically found where absorption and filtration
  occur and a thin epithelial barrier is desirable
• Stratified (more than one)
• Consist of two or more cell layers stacked one on
  top of the other
• Common in high-abrasion areas where protection is
  important, such as the skin surface and the
  lining of the mouth
• The second name describes the shape of the cells

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Classification of Epithelia
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Classification of Epithelia

• All epithelial cells have six (somewhat
  irregular) sides
• Apical surface view of an epithelial sheet looks
  like a honeycomb
• This polyhedral shape allows the cells to be
  closely packed
• Cells vary in height
• Three common shapes of epithelial cells nucleus
  will be the same shape
• 1. Squamous cells are flattened and scalelike
  (squamscale)
• 2. Cuboidal cells are boxlike
• Approximately as tall as they are wide
• 3. Columnar cells are tall and column shaped

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Classification of Epithelia
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Classification of Epithelia

• Simple epithelia are easy to classify by cell
  shape because all cells in the layer usually have
  the same shape
• Stratified epithelia
• Cell shapes usually differ among the different
  cell layers
• Named according the shape of the cells in the
  apical layer

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Simple Epithelia

• Concerned with absorption, secretion, and
  filtration
• Consist of a single layer and are usually very
  thin
• Protection is not one of their specialties

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Simple Squamous Epithelium
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Simple Cuboidal Epithelium
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Simple Columnar Epithelium
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Pseudostratied Columnar Epithelium
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Stratified Epithelia

• Contains two or more cell layers
• Main function is protection
• Regenerate from below
• The basal cells divide and push apically to
  replace the older surface cells
• Consequently more durable than the simple
  epithelia
• Stratified squamous epithelium is composed of
  several layers with the cells on the free surface
  being squamous-shaped and the underlying cells
  being cuboidal or columnar in shape
• Transitional epithelium forms the lining of the
  hollow organs of the urinary system that stretch
  as they fill

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Stratified Squamous Eputhelium
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Stratified Cuboidal Epithelium

• Rare
• Found mostly in the ducts of some of the larger
  glands
• Sweat glands
• Mammary glands

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Stratified Columnar Epithelium

• Found in limited distribution with small amounts
  in the pharynx, male urethra, and lining some
  glandular ducts
• Only its apical layer of cells is columnar

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Transitional Epithelium
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Glandular Epithelia

• A gland consists of one or more cells that make
  and secrete (export) a particular product
• This product, called a secretion, is an aqueous
  (water-based) fluid that usually contains
  proteins
• Some release lipid-rich or steroid-rich secretion
• Secretion is an active process
• Glandular cells obtain needed substances from the
  blood and transform them chemically into a
  product that is then discharged from the cell
• Notice the term secretion can refer to BOTH the
  glands product and the process of making and
  releasing that product

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Glandular Epithelia

• Classified as to
• Where they release their products
• Endocrine internally secreting
• Exocrine externally secreting
• Relative cell numbers making up the gland
• Unicellular one-celled
• Scattered within epithelial sheets
• Multicellular many-celled
• Form by invagination (inward growth) or
  evagination (outward growth) from an epithelial
  sheet

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Endocrine Glands

• Ductless glands
• Produce hormones regulatory chemicals that they
  secrete by exocytosis directly into the
  extracellular space
• From there the hormones enter the blood or
  lymphatic fluid and travel to specific target
  organs
• Each hormone prompts its target organ(s) to
  respond in some characteristic way
• Most are complex multicellular organs
• Some are individual hormone-producing cells in
  organs (intestines/brain)

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Exocrine Glands

• More numerous than endocrine glands
• Secrete their products onto body surfaces (skin)
  or into body cavities
• Unicellular glands directly (exocytosis)
• Multicellular glands via an epithelial-walled
  duct that transports the secretion to the
  epithelial surface
• Mucous, sweat, oil, salivary glands
• Liver, (bile), pancreas (digestive enzymes)

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Unicellular Exocrine Glands

• Only important example of a unicellular
  (one-celled) gland is the goblet cell
• Shaped like a goblet (drinking glass with a stem)
• (d) Sprinkled in the epithelial linings of the
  intestinal and respiratory tracts amid columnar
  cells with other functions
• In humans produce mucin complex glycoprotein
  that dissolves in water when secreted
• Once dissolved, mucin forms mucus, a slimy
  coating that both protects and lubricates
  surfaces

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Pseudostratified Columnar EpitheliumUnicellular
Exocrine Glands (Goblet Cells)
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Multicellular Exocrine Glands

• Two basic parts
• An epithelium-derived duct
• Secretory unit consisting of secretory cells
  (acini)
• In all BUT the simplest glands, supportive
  connective tissue surrounds the secretory unit
  and supplies it with blood vessels and nerve
  fibers, and forms a fibrous capsule that extends
  into the gland proper and divides the gland into
  lobes

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Multicellular Exocrine GlandsStructural
Classification

• On the basis of their duct structures
• Simple
• Unbranched duct

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EXOCRINE GLANDS
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Multicellular Exocrine GlandsStructural
Classification

• Compound
• Branched duct
• Compond mulitple branched ducts
• Further classified by their secretory units
• Tubular if the secretory cells form tubes
• Alveolar if the secretory cells form small,
  flask-like sacs (alveolussmall hollow cavity)
• Tubuloalveolar if they have BOTH types of
  secretory units (tubes and alveolar)
• NOTE acinar is used interchangeably with
  alveolar

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EXOCRINE GLANDS
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Multicellular Exocrine GlandsFunctional
Classification

• Modes of Secretion
• Merocrine Glands
• Secrete their products by exocytosis as produced
• Secretory cells are not altered in any way
• Examples
• Sweat glands
• Pancreas
• Salivary glands

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Merocrine Gland
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Multicellular Exocrine GlandsFunctional
Classification

• Modes of Secretion
• Holocrine Glands
• Accumulate their products within them until they
  rupture
• They are replaced by the division of underlying
  cells
• Secretion includes the synthesized product plus
  dead cell fragments (holoall)
• Examples
• Sebaceous (oil) glands

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Holocrine Gland
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Multicellular Exocrine GlandsFunctional
Classification

• Modes of Secretion
• Apocrine Glands
• Present in all animals but questionable in humans
• Accumulated their products just beneath the free
  surface
• Eventually, the apex of the cell pinches off
  (apofrom off), releasing the secretory granules
  and a small amount of cytoplasm
• Example controversy in humans
• Some believe mammary glands are apocrine while
  others say merocrine

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Connective Tissue

• Found everywhere in the body
• It is the most abundant and widely distributed of
  the primary tissues
• Amounts vary in particular organs
• Example
• Skin is primarily connective tissue
• Brain has very little connective tissue
• Four main classes and several subclasses
• 1. Connective tissue proper
• Includes fat and fibrous tissue of ligaments
• 2. Cartilage
• 3. Bone tissue
• 4. Blood

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CONNECTIVE TISSUE

• Does more than just connect body parts
• It has many forms and functions
• Major functions include
• Binding and support
• Protection
• Insulation
• Transportation

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Common Characteristics of Connective Tissue

• 1. Common origin All connective tissue arises
  from an embryonic tissue called mesenchyme
• 2. Degrees of vascularity Connective tissue
  ranges from avascular (cartilage) to poorly
  vascularized (dense connective tissue) to highly
  vascularized
• 3. Extracellular matrix Connective tissue is
  composed mainly of nonliving extracellular matrix
  that separates the cells of the tissue
• Enables connective tissue to withstand physical
  trauma

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Connective Tissue Origins
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EMBRYONIC CONNECTIVE TISSUE
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Structural Elements of Connective Tissue

• Three main elements
• Ground substance extracellular matrix
• Fibers extracellular matrix
• Cells
• Properties of the cells and the composition and
  arrangement of extracellular matrix elements vary
  tremendously
• Resulting in an amazing diversity of connective
  tissues
• Matrix can be delicate and fragile (soft packing
  around an organ) to rope-like (tendons and
  ligaments)

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Structural Elements of Connective Tissue

• Even though there are diverse types they still
  have a common plan
• Prototype (model) used is areolar connective
  tissue
• All other subclasses are simply variants of this
  common tissue type

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AREOLAR CONNECTIVE TISSUE
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Ground Substance

• Unstructured material that fills the space
  between the cells and contains the fibers
• Composed of
• Interstitial (tissue) fluid
• Cell adhesion proteins serves as connective
  tissue glue that allows connective tissue cells
  to attach themselves to matrix material
• Fibronectin
• Laminin
• Proteoglycans
• Consist of a protein core to which
  glycosaminoglycans are attached
• Strandlike GAGs ( chondroitin sulfate, keratan
  sulfate, hyaluronic acid) are large, negatively
  charged polysaccharides that stick out from the
  core protein like the fibers of a bottle brush
• Intertwine and trap water, forming a substance
  that varies from a fluid to a viscous gel

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PROTEOGLYCAN
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Ground Substance

• Holds large amounts of fluid and functions as a
  molecular sieve, or medium, through which
  nutrients and other dissolved substances can
  diffuse between the blood capillaries and the
  cells
• Fibers embedded make it less pliable and impede
  diffusion somewhat

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AREOLAR CONNECTIVE TISSUE
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Fibers

• Fibers of the connective tissue provide support
• Three types of fibers are found in connective
  tissue matrix
• Collagen
• Elastic
• Reticular

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AREOLAR CONNECTIVE TISSUE
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Collagen Fibers

• Strongest and most abundant
• Constructed primarily of the fibrous protein
  collagen
• Secreted into the extracellular space, where they
  assemble spontaneously into cross-linked fibers
• Collagen fibers are extremely strong and provide
  high tensile strength (ability to resist
  longitudinal stress) to the matrix
• Stress test show that collagen fibers are
  stronger than steel fibers of the same size

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AREOLAR CONNECTIVE TISSUE
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Elastic Fibers

• Long, thin fibers that form branching networks in
  the extracellular matrix
• Contain a rubberlike protein, elastin, that
  allows them to stretch and recoil like rubber
  bands
• Connective tissue can stretch only so much before
  its thick, ropelike collagen fibers become taut
• When the tension lets up, elastic fibers snap the
  connective tissue back to its normal length an
  shape
• Found where elasticity is needed skin, lungs,
  blood vessel walls

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AREOLAR CONNECTIVE TISSUE
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Reticular Fibers

• Fine collagenous fibers (form and chemically
  different) and are continuous with collagen
  fibers
• Branch extensively forming delicate networks
  (reticulnetwork) that surround small blood
  vessels and support the soft tissue of organs
• Abundant where connective tissue abuts other
  tissue types
• Example
• Basement membrane of epithelial tissues
• Around capillaries

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AREOLAR CONNECTIVE TISSUE
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Cells

• Each major class of connective tissue has a
  fundamental cell type that exists in immature and
  mature forms
• The undifferentiated cells, indicated by the
  suffix blast (bud, sprout, forming), are actively
  mitotic cells that secrete the ground substance
  and the fibers characteristic of their particular
  matrix
• The primary blast cell types by connective tissue
  class are
• 1. Fibroblast connective tissue proper
• 2. Chondroblast cartilage
• 3. Osteoblast bone
• 4. Hematopoietic stem cell blood

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Connective Tissue Origins
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Cells

• Once they synthesize the matrix, the blast cells
  assume their less active, mature mode, indicated
  by the suffix cyte
• Mature cells maintain the health of the matrix
• If the matrix is injured, they can easily revert
  to their more active state to repair and
  regenerate the matrix

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Connective Tissue Origins
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Cells

• Additionally, connective tissue is home to an
  assortment of other cell types
• Fat cells nutrient-storing cells
• Mobile cells that migrate into the connective
  tissue matrix from the bloodstream
• White blood cells neutrophils, eosinophils,
  lymphocytes
• Cell types that respond to injury mast cells,
  macrophages
• Antibody-producing plasma cells

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AREOLAR CONNECTIVE TISSUE
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Mast Cells

• Mast cells and macrophages are very important to
  overall body defense
• Oval mast cells typically cluster along blood
  vessels
• Act as sensitive sentinels to detect foreign
  substances (e.g., bacteria, fungi)
• Initiate local inflammatory responses against
  foreign substances
• Cytoplasm contains conspicuous secretory granules
  (maststuffed full of granules) containing
  several chemicals that mediate inflammation,
  especially in severe allergies
• Heparin anticoagulant chemical that prevents
  blood clotting when free in the bloodstream
• Histamine substance that makes capillaries leaky
• Proteases protein-degrading enzymes

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AREOLAR CONNECTIVE TISSUE
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Macrophages

• Mast cells and macrophages are very important to
  overall body defense
• Macrolarge phagoeat
• Large, irregularly shaped cells that avidly
  phagocytize a broad variety of foreign materials
• Foreign molecules
• Bacteria
• Dust particles
• Dead tissue cells
• Active in the immune system
• May attach to connective tissue fibers (fixed) or
  may migrate freely through the matrix

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AREOLAR CONNECTIVE TISSUE
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Types of Connective Tissue

• Mesenchyme forms during the early weeks of
  embryonic development from the mesoderm layer and
  eventually differentiates into other connective
  tissues

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EMBRYONIC CONNECTIVE TISSUE
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Connective Tissue Proper

• Two Subclasses
• Loose Connective Tissue
• Areolar
• Adipose
• Reticular
• Dense Connective Tissue
• Dense Regular
• Dense Irregular
• Elastic
• Except for bone, cartilage, and blood, all mature
  connective tissues belong to this class

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Types of Connective Tissue

• Areolar connective tissue serves to bind body
  parts together while allowing them to move freely
  over one another, wraps small blood vessels and
  nerves, surrounds glands, and forms the
  subcutaneous tissue

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AREOLAR CONNECTIVE TISSUE
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Types of Connective Tissue

• Adipose (fat) tissue is a richly vascularized
  tissue that functions in nutrient storage,
  protection, and insulation

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ADIPOSE CONNECTIVE TISSUE
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Types of Connective Tissue

• Reticular connective tissue forms the internal
  framework of the lymph nodes, the spleen, and the
  bone marrow

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RETICULAR CONNECTIVE TISSUE
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Types of Connective Tissue

• Dense connective tissue is one of the two
  subclasses of connective tissue proper
• Dense regular connective tissue contains closely
  packed bundles of collagen fibers running in the
  same direction and makes up tendons and ligaments
• Dense irregular connective tissue contains thick
  bundles of collagen fibers arranged in an
  irregular fashion, and is found in the dermis

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DENSE REGULAR CONNECTIVE TISSUE
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DENSE IRREGULAR CONNECTIVE TISSUE
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Cartilage

• Stands up to both tension (stretching) and
  compression
• Qualities intermediate between dense connective
  tissue (very flexible) and bone
• Tough but flexible, providing a resilient
  rigidity to the structures it supports
• Lacks nerve fibers and is avascular
• Receives its nutrients by diffusion from blood
  vessels located in the connective tissue membrane
  (perichondrium) surrounding it
• Ground substance contains large amounts of
• GAGs glycosaminoglycans (Chrondroitin sulfate,
  Hyaluronic acid)Intertwine and trap water,
  forming a substance that varies from a fluid to a
  viscous gel
• Contains firmly bound collagen fibers and in some
  cases elastic fibers
• Matrix contains an exceptional amount of tissue
  fluid (up to 80 water)
• Movement of tissue fluid in its matrix enables
  cartilage to rebound after being compressed and
  also helps to nourish the cartilage cells

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Cartilage

• Because cartilage is avascular and aging
  cartilage cells lose their ability to divide,
  cartilages heal slowly when injured
• During later life, cartilages tend to calcify or
  even ossify (become bony)
• In such cases, the chondrocytes are poorly
  nourished and die

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Types of Connective Tissue

• Three varieties of cartilage
• Hyaline
• Elastic
• Fibrocartilage

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Types of Connective TissueHyaline Cartilage

• Hyaline cartilage (gristle) is the most abundant
  cartilage providing firm support with some
  pliability
• Blue-white color
• Hyalinglass
• Articular (ends of long bones)

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HYALINE CARTILAGE
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Types of Connective TissueElastic Cartilage

• Elastic cartilage is found where strength and
  exceptional stretchability are needed, such as
  the external ear
• More elastin fibers
• Found where strength and exceptional
  stretchability are needed

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ELASTIC CARTILAGE
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Types of Connective TissueFibrocartilage

• Fibrocartilage is found where strong support and
  the ability to withstand heavy pressure are
  required, such as the intervertebral disks
• Often found where hyaline cartilage meets a true
  ligament or a tendon
• Perfect intermediate between hyaline cartilage
  and dense regular connective tissues
• Found where strong support and the ability to
  withstand heavy pressure are required

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FIBROCARTILAGE
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Types of Connective TissueBone

• Bone (osseous tissue) has an exceptional ability
  to support and protect body structures due to its
  hardness, which is determined by the additional
  collagen fibers and calcium salts found in the
  extracellular matrix
• Provides cavities for fat storage and synthesis
  of blood cells
• Matrix is similar to that of cartilage but is
  harder and more rigid because, in addition to its
  more abundant collagen fibers, bone has an added
  matrix elementinorganic calcium salts (bone
  salts)
• Blood is classified as a connective tissue
  because it developed from mesenchyme, and
  consists of blood cells and plasma proteins
  surrounded by blood plasma

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Types of Connective TissueBone

• Osteoblasts immature bone cells
• Produce the organic portion of the matrix then
  bone salts are deposited on and between the
  fibers
• Osteocytes mature bone cells
• Reside in the lacunae (cavity in bone or
  cartilage) within the matrix they have made
• Unlike cartilage, the next firmest connective
  tissue
• Vascularized

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BONE
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Types of Connective TissueBlood

• Fluid within blood vessels
• Most atypical connective tissue
• Does not connect things or give support
• Classified as connective tissue because it
  develops from mesenchyme and consists of blood
  cells, surrounded by a nonliving fluid matrix
  called blood plasma
• Fibers of blood are soluble protein molecules
  that become visible only during blood clotting

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BLOOD
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COVERING and LINING MEMBRANES

• Membranes that incorporate BOTH connective tissue
  and epithelial tissues
• Three types
• Cutaneous
• Mucous
• Serous
• Essentially they all are continuous multicellular
  sheets composed of at least two primary tissue
  types
• An epithelium bound to an underlying layer of
  connective tissue proper
• Hence a simple organ
• Synovial membranes line joint cavities and
  consist of connective tissue ONLY

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COVERING and LINING MEMBRANESCutaneous Membrane
(a)

• Cutisskin
• Cutaneous membrane, or skin, is 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, the cutaneous
  membrane is exposed to the air and is a dry
  membrane

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MEMBRANES
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COVERING and LINING MEMBRANESMucous Membranes (b)

• Mucosae
• Mucous membranes line body cavities that open to
  the exterior, such as those of hollow organs of
  the digestive, respiratory, and urogenital tracts
• They are ALL wet (moist) membranes bathed by
  secretions or, in the case of urinary mucosa,
  urine
• Name mucosa refers to the location of the
  membrane, NOT its cell composition, which varies

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MEMBRANES
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COVERING and LINING MEMBRANESMucous Membranes (b)

• Most mucosae contain either stratified squamous
  or simple columnar epithelia
• Underlain by a layer of loose connective tissue
  called the lamina propria (ones own layer)
• In some mucosae, the lamina propria rests in a
  third (deeper) layer of smooth muscle cells
• Often adapted for absorption and secretion
• Most secrete mucus (digestive and respiratory)
• Urinary tract does NOT

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COVERING and LINING MEMBRANESSerous Membranes (c)

• Serosae
• Moist membranes found in closed ventral body
  cavities
• Serosa moist membrane found in closed ventral
  body cavities
• Visceral serosa the part of the double-layered
  membrane that lines the outer surfaces of organs
  within the ventral body cavity
• Parietal serosa the part of the double-layered
  membrane that lines the walls of the ventral body
  cavity
• Serous membranes consist of simple squamous
  epithelium (mesothelium) resting on a thin layer
  of loose connective (areolar) tissue

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MEMBRANES
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COVERING and LINING MEMBRANESSerous Membranes (c)

• Mesothelial cells enrich the fluid that filters
  from the capillaries in the associated connective
  tissue with hyaluronic acid (GAGs)
• Result is the thin, clear serous fluid that
  lubricates the facing surfaces of the parietal
  (lines the walls of the ventral body cavity) and
  visceral layers (lines outer surface of the
  organ), so that they slide across each other
  easily
• Named according to their site and specific organ
  associations
• Example
• Pleura serosa lining the thoracic wall and
  covering the lungs
• Pericardium serosa lining the heart
• Peritoneums serosa ling the abdominopelvic
  cavity and viscera

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MEMBRANES
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NERVOUS TISSUE

• Nervous tissue is the main component of the
  nervous system (brain, spinal cord, and nerves),
  which regulates and controls body functions
• Nervous tissue is composed of two types of cells
• Neurons are specialized cells that generate and
  conduct electrical impulses
• Supporting cells are nonconductive cells that
  support, insulate, and protect the neurons

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NERVE TISSUE
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MUSCLE TISSUE

• Muscle tissues are highly cellular,
  well-vascularized tissues responsible for
  movement
• Muscle cells (muscle fibers) possess
  myofilaments
• Elaborate versions of the actin and myosin
  filaments that bring about movement or
  contraction in all muscle cell types
• There are three types of muscular tissue
• Skeletal muscle is packaged by connective tissue
  sheets into organs called skeletal muscles that
  are attached to the skeleton and produces
  voluntary body movement
• Cardiac muscle is responsible for the involuntary
  movement of the heart
• Found ONLY in the walls of the heart
• Smooth muscle is found in the walls of the hollow
  organs (digestive and urinary tract organs,
  uterus, and blood vessels)
• No striations
• Acts to squeeze substances through these organs
  by alternately contracting and relaxing
• Involuntary

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SKELETAL MUSCLE
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CARDIAC MUSCLE
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SMOOTH MUSCLE
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TISSUE REPAIR

• When tissue injury occurs, the responses usually
  take place in connective tissue
• Tissue repair occurs in two ways
• Regeneration
• Replacement of destroyed tissue with the same
  kind of tissue
• Fibrosis
• Involves proliferation of fibrous connective
  tissue called scar tissue

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Steps of Tissue Repair

• Three steps are involved in the tissue repair
  process
• 1. Inflammation (a)
• Tissue trauma causes injured tissue cells,
  macrophages, mast cells, and others to release
  inflammatory chemicals, which cause the
  capillaries to dilate and become very permeable
• Allows white blood cells (neutrophils,
  monocytes,) and plasma fluid rich in clotting
  proteins, antibodies, and other substances to
  seep into the injured area
• The leaked clotting proteins construct a clot
• Stops the loss of blood
• Holds the edges of the wound together isolating
  the injured area preventing bacteria, toxins, or
  other harmful substances from spreading to
  surrounding tissues
• Part exposed to the air quickly dries and hardens
  forming a scab
• Leaves excess fluid, bits of destroyed cells, and
  other debris in the area, which are eventually
  removed via lymphatic vessels or phagocytized by
  macrophages

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TISSUE REPAIR
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Steps of Tissue Repair

• 2.Organization restores the blood supply (b)
• Blood clot is replaced by granulation tissue
• A delicate pink tissue composed of several
  elements
• Contains capillaries that grow in from nearby
  areas and lay down a new capillary bed
• Granulation tissue is actually named for these
  capillaries, which protrude nublike from its
  surface, giving it a granular appearance
• Proliferating fibroblasts produce growth factors
  as well as new collagen fibers to bridge the gap
• Some fibroblasts have contractile properties that
  pull the margins of the wound together
• Macrophages digest the original blood clot
• Collagen fiber deposit continues
• Granulation tissue, destined to become scar
  tissue (a permanent fibrous patch), is highly
  resistant to infection because it produces
  bacteria-inhibiting substances

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TISSUE REPAIR
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Steps of Tissue Repair

• 3.Regeneration and fibrosis effect permanent
  repair (c)
• Surface epithelium begins to regenerate (b)
• Growing under the scab, which soon detaches
• As the fibrous tissue beneath matures and
  contracts, the regenerating epithelium thickens
  until it finally resembles that of the adjacent
  skin (c)
• End result is a fully regenerated epithelium, and
  an underlying area of scar tissue
• May be invisible, or visible as a thin white
  line, depending on the severity of the wound
• The repair process described (1,2,3) follows
  healing of a wound (cut, scrape, puncture) that
  breaches an epithelial barrier
• In pure infections (pimple or sore throat),
  healing is solely by regeneration
• Usually no clot or scarring
• Only severe (destructive) infections lead to
  scarring

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TISSUE REPAIR
116
TISSUE REPAIR
117
TISSUE REPAIR
118
Regenerative Capacity of Different Tissues

• The generative capacity of tissues varies widely
  among the tissue types
• Epithelial, bone, areolar connective tissue,
  dense irregular tissue, and blood clotting tissue
  regenerate extremely well
• Smooth muscle and dense regular connective tissue
  have moderate capacity for regeneration
• Skeletal muscle and cartilage have a weak
  regeneration capacity
• Cardiac muscle and nervous tissue of the brain
  and spinal cord have virtually no functional
  regenerative capacity
• Hence, routinely replaced by scar tissue
• Scar tissue is strong (mostly collagen fibers) ,
  but it lacks the flexibility and elasticity of
  most normal tissue
• Cannot perform the normal functions of the tissue
  it has replaced

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DEVELOPMENTAL ASPECTS OF TISSUES

• Embryonic and Fetal Development of Tissues
• Primary germ layer formation is one of the first
  events of embryonic development
• Ectoderm is the most superficial of the layers
• Mesoderm is the middle layer
• Endoderm is the deepest layer
• The primary germ layers specialize to form the
  four primary tissues
• With increasing age, epithelia become thin, the
  amount of collagen fibers in the body decreases,
  and bone, muscle, and nervous tissue atrophy

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EMBRYO TISSUE
121
CANCER