The Tissue Level of Organization: Part A Chapter 4

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The Tissue Level of Organization Part AChapter 4

• Lecture 5

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For our bodies to function, cells must work
together as tissues

• Tissues collections of specialized cells with
  specific functions.
• Histology the study of tissues.

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There are 4 basic types of tissues

• Epithelial tissue covers surfaces exposed to the
  environment (skin, airways, digestive tracts,
  glands)
• Connective tissue fills internal spaces, supports
  other tissues, transports materials and stores
  energy.
• Muscle tissue is specialized for contraction
  (skeletal muscle, heart muscle, walls of hollow
  organs).
• Neural tissue carries electrical signals from one
  part of the body to another.

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Tissues

• () Tissues are collections of cells and cell
  products that perform specific, limited
  functions. Four tissue types form all the
  structures of the human body epithelial,
  connective, muscle, and neural.

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II. Epithelial Tissue, p. 107

• Epithelial tissue includes
• epithelia layers of cells that cover internal or
  external surfaces.
• glands structures that produce fluid secretions.

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Epithelia

• Epithelia line digestive, respiratory, urinary
  and reproductive tracts. Also fluid or gas-filled
  internal cavities and passageways such as the
  chest cavity, inner surfaces of blood vessels and
  chambers of heart.

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Epithelia have 5 important characteristics

• Cellularity cells are tightly bound together by
  cell junctions.
• Polarity the structural and functional
  differences between the exposed (apical) and
  attached (basal) surfaces of the tissue.
• Attachment the base of the epithelia is bound to
  a basal lamina or basement membrane.
• Avascularity epithelia are avascular (lacking
  blood vessels)
• Regeneration a high rate of cell replacement by
  stem cells in the epithelium.

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Fig. 4-1, p. 108
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There are 4 basic functions of epithelial
tissues

• Provide Physical Protection from abrasion,
  dehydration, biological and chemical agents.
• Control Permeability to proteins, hormones, ions
  or nutrients.
• Provide Sensation such as touch or pressure.
• neuroepithelia are specialized for the sensations
  of smell, taste, sight, equilibrium, and hearing.
  
• Produce Specialized Secretions for physical
  protection or chemical messengers.
• gland cells are scattered among other epithelial
  cells.
• in glandular epithelium, most cells produce
  secretions.

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Specialized epithelial cells

• Individual epithelial cells may be specialized
  for
• Movement of fluid over the epithelial surface
  (protection or lubrication).
• Movement of fluid through the epithelium
  (permeability).
• Production of secretions (protection or chemical
  messengers).

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Polarity of Epithelial CellsFigure 4-1

• The apical surfaces of cells lining internal
  passageways (such as digestive and urinary
  tracts) have microvilli on their surfaces which
  increase surface area to aid in absorption,
  secretion and transport.
• Longer epithelial extensions called cilia
  (ciliated epithelium) move fluids across the
  surface of the epithelium. Cilia in the
  respiratory tract move mucus, containing
  particles such as smoke, out of the lungs.

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Maintaining the Integrity of Epithelia, p. 108

• Three factors make the epithelium an effective
  barrier intercellular connections, attachment to
  basal lamina, and maintenance and repair.

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I. Intercellular Connections

• Figure 4-2
• Cells can form permanent or temporary bonds with
  other cells or extracellular material.
• Connections between large areas of opposing cell
  membranes are formed by transmembrane proteins
  called cell adhesion molecules (CAMs).
• Adjacent cell membranes may be bonded by a thin
  layer of proteoglycans called intercellular
  cement.

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Cell junctions

• Cell junctions are specialized areas of
  attachment between cells. The 3 types of cell
  junctions are
• Tight junctions
• Gap junctions.
• Desmosomes

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Fig. 4-2, p. 109
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Tight junctions

• Tight junctions close enough to prevent water
  and solutes from passing through. Tight junctions
  can isolate destructive chemicals such as
  enzymes, acids and wastes inside tubular
  passageways called lumen. In tight junctions, the
  lipid portions of 2 cell membranes are tightly
  locked by membrane proteins, forming an adhesion
  belt.

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Fig. 4-2ab, p. 109
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Gap junctions

• Gap junctions allow rapid intercellular
  communications. Cells are held together by
  channel proteins (junctional proteins) call
  connexons. Small molecules and ions pass from
  cell to cell through the channels. Gap junctions
  in cardiac muscle tissue coordinate contractions.

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Fig. 4-2ac, p. 109
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Desmosomes

• Desmosomes durable structural connections which
  allow tissues to stretch, bend and twist.
• The 2 types of desmosomes are
• Button desmosomes discs connected to
  intermediate fibers, which stabilize cell shape.
• Hemidesmosomes attach a cell to extracellular
  filaments in the basal lamina

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Fig. 4-2ad, p. 109
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Fig. 4-1, p. 108
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Fig. 4-2ae, p. 109
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II. Attachment to the Basal Lamina

• The inner surface of the epithelium is attached
  to a 2-part basal lamina.
• Lamina lucida, the thin layer closest to the
  epithelium, acts as a barrier to proteins and
  other large molecules. Contains glycoproteins and
  a layer of fine protein filaments.
• Lamina densa, the deeper layer, gives the
  basement membrane its strength and filters
  substances entering from adjacent tissues.
  Contains bundles of coarse protein fibers.

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III. Maintenance and Repair

• Epithelial cells are exposed to toxic chemicals,
  pathogens and mechanical abrasion.
• An epithelial cell of the small intestine may
  survive only a day or two before it is destroyed.
  
• New epithelial cells are produced by division of
  stem cells (germinative cells) located near the
  basal lamina.

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Classification of epithelia

• Table 4-1
• Epithelia are sorted into categories by cell
  shape (squamous flat, cuboidal square or cube
  shaped, columnar tall) and number of cell
  layers.
• One cell layer is simple epithelium, more than
  one layer is stratified epithelium.

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I. Squamous Epithelia Figure 4-3a

• Simple squamous epithelium
• Mesothelium simple squamous epithelium lining
  ventral body cavities (pleura, peritoneum,
  pericardium).
• Endothelium simple squamous epithelium lining
  heart and blood vessels.

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Fig. 4-3a, p. 112
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2. Stratified squamous epithelium

• Figure 4-3b
• Stratified squamous epithelium forms many layers
  which protect against chemical and physical
  attacks. It is found lining the mouth, esophagus
  and anus, and on exposed body surfaces.

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Stratified squamous epithelium

• Figure 4-3b
• 2. Stratified squamous epithelium
• Keratinized stratified squamous epithelium
  (packed with the fibrous protein keratin), found
  in apical layers of skin cells, is tough and
  water resistant.
• Nonkeratinized stratified squamous epithelium
  resists abrasion but dries out and must be
  lubricated (e.g. oral cavity, pharynx, esophagus,
  anus, vagina).

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Fig. 4-3b, p. 112
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II. Cuboidal Epithelia

• Figure 4-4a
• 1. Simple cuboidal epithelium occurs where
  secretion or absorption takes place (e.g. lining
  of kidney tubules).
• Figure 4-4b
• 2. Stratified cuboidal epithelia are relatively
  rare, found in ducts of sweat glands and mammary
  glands.

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Fig. 4-4a, p. 113
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Fig. 4-4b, p. 113
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III. Transitional epithelia

• Figure 4-4c
• Transitional epithelia tolerate repeated cycles
  of stretching without damage (e.g. urinary
  bladder). It is called transitional because cell
  layers change appearance (from stratified to
  simple) as they stretch.

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Fig. 4-4c, p. 113
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IV. Columnar Epithelia

• Figure 4-5a
• 1. Simple columnar epithelium is found where
  absorption or secretion occur (e.g. stomach,
  small intestine, large intestine). Secretions
  protect against chemical stress.
• Figure 4-5b
• 2. Pseudostratified columnar epithelium appears
  stratified but is actually simple. Cilia-bearing
  cells found in portions of the respiratory tract
  (e.g. nasal cavity, trachea and bronchi) and
  portions of the male reproductive tract.
• Figure 4-5c
• 3. Stratified columnar epithelia are relatively
  rare. They protect portions of the pharynx,
  epiglottis, anus and urethra.

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Simple columnar

• Simple columnar epithelial cells of the intestine
  (B)
• Goblet cells from the lining of the trachea (A)

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Fig. 4-5a, p. 115
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Fig. 4-5b, p. 115
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Fig. 4-5c, p. 115
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Glandular Epithelia, p. 114

• Glands are cells, or collections of cells,
  specialized for secretions ranging from sweat to
  hormones.
• Endocrine glands
• Exocrine glands

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Glandular Epithelia, p. 114

• Endocrine glands (endo in) release hormonal
  secretions into interstitial fluids.
• The blood stream carries hormones throughout the
  body.
• Hormones control specific tissues, organs and
  organ systems.
• Examples of endocrine glands are the thyroid
  gland and pituitary gland.
• Endocrine glands have no ducts.

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Glandular Epithelia, p. 114

• Exocrine glands (exo out) release secretions
  into ducts which carry the secretions onto an
  epithelial surface such as the skin, or an
  internal passageway that communicates with the
  outside environment.
• Examples of exocrine secretions are digestive
  enzymes, sweat, tears and milk.

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There are 3 methods of glandular secretion
merocrine, apocrine and holocrine.

• 1. Merocrine secretion.
• 2. Apocrine secretion
• 3. Holocrine secretion

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1. Merocrine secretion

• Merocrine secretion is the most common.
• Merocrine secretions are released from secretory
  vesicles by exocytosis.
• Examples are the mucus-producing secretion mucin,
  and merocrine sweat glands which produce the
  watery secretions that cool you when you are hot.

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2. In apocrine secretion

• In apocrine secretion, part of the cell cytoplasm
  is released along with the secretory product.
• Milk production involves both apocrine and
  merocrine secretions.

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3. Holocrine secretion

• Holocrine secretion fills a gland cell and causes
  it to burst, killing the cell.
• Holocrine cells must be replaced by stem-cell
  division.
• An example of holocrine secretion is the
  sebaceous gland which produces oil in hair
  follicles.

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Fig. 4-6, p. 116
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Fig. 4-6a, p. 116
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Fig. 4-6b, p. 116
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Fig. 4-6c, p. 116
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Exocrine glands can also be categorized by 3
types of secretions

• 1. Serous glands produce watery secretions
  containing enzymes.
• Example parotid salivary glands
• 2. Mucous glands secrete mucins.
• Examples sublingual salivary glands, submucosal
  glands of small intestine
• 3. Mixed exocrine glands produce both serous and
  mucous secretions.
• Example submandibular salivary glands.

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Exocrine glands can also be classified by
structure

• either unicellular (one cell) or multicellular
  (many cells).
• 1. The only unicellular exocrine glands are
  goblet cells, which secrete mucins.
• Goblet cells are scattered among other epithelial
  cells.
• Examples linings of trachea, small and large
  intestines.
• 2. All other exocrine glands are multicellular
  exocrine glands.

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Three characteristics describe the structure of
multicellular exocrine glands

• 1. Structure of the duct
• simple (undivided)
• compound (divided)
• 2. Shape of secretory portion of the gland
• tubular (tube shaped)
• alveolar (blind pockets)
• acinar (chamber-like)
• 3. The relationship between ducts and glandular
  areas
• branched (several secretory areas sharing one
  duct)

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Fig. 4-7, p. 117
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Fig. 4-7, top, p. 117
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Fig. 4-7, bottom, p. 117