Histology and Embryology
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General Histology
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 Cells
 Smallest structures, varies in size, shape and surface.
 Cells posses properties that permit:
Excitability – nerve cells conduct an impulse
 Synthesis – aiding in the bodies function, such as glands
 Membrane transport – nutrients are transported
 Reproduction – union of sperm and ovum can lead to the
formation of an offspring

Cell Junctions
3
 Desmosomes – cell-to-cell attachments; between
ameloblasts and cells of stratified squamous epithelium that
lines the oral cavity.
 Tight junctions – cells attach to each other by fusion of
their cell membranes; adjacent odontoblasts form tight
junctions that prevent substances in the pulp from passing
into the dentin.
 Gap junctions – channel that runs between cells for
communication of cell electrical impulses and passage for
molecules; present amount some odontoblasts, allowing to
coordinate their activity.
 Hemidesmosome – attachment of a cell to a noncellular
surface; basal layer cells of stratified squamous epithelium
attach to the basement membrane by hemidesmosomes;
present in epithelial attachment of the tooth
The Cell
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The Cell
5
 Cells are surrounded by a cell membrane that separates
them from the outside environment.




1.
2.
3.
4.
cytoplasm
organelles
inclusions
nucleus
 Specialization



A) differentiation – cells that recognize one another will group
together
B) organization of chemicals – chemicals appear earlier in the
embryo. Endocrine substances are produced by one type of cell and
can affect other types of cells
C) Cells  tissues  organs  organ systems
Cell Membrane
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 Cell membrane
 A) Called a plasma membrane or plasmalemma; selectively
permeable because it controls passage of materials in and out
of the cell. Uses active transport, passive transport, or
facilitated diffusion.
 Lipids and proteins are the major components (3:2 ratio of
proteins)
 Structure is trilaminar, with bipolar membrane and a central
core of lipids between two layers of protein
 Diffusion of small lipid-insoluble substances
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Fluid Mosaic Model
8
 Shown on the previous slide
 Trilaminar structure composted of two facing layers
of lipid molecules, into which larger globular
proteins are inserted.


Lipid bilayers consist mainly of phospholipid molecules;
hydrophilic ends face the outer and inner surfaces of the cell;
the hydrophobic ends attract and face each other.
Globular proteins are integral proteins and peripheral
proteins. Integral extend through the full width of the cell
membrane and protrude and may have carbohydrate units
attached to them. Peripheral are linked or attached to the
cell membrane surface.
Cytoplasm
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 Translucent, aqueous, homogeneous gel enclosed in the
cell by the cell membrane.
 The cytoplasm has three major elements; the cytosol,
organelles and inclusions.
 All metabolic activities of the cell occur in the cytoplasm
which include:




Assimilation (digestion)
Synthesis of substances such as proteins, proteoglycans, and
glycoproteins
A transport medium in which all nutrients and metabolites are
carried from one organelle to another
Presence of enzymes and electrolytes where specific metabolic
reactions take place (glycolysis)
Nucleus
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 In cell biology, the nucleus is a membrane-enclosed organelle found
in eukaryotic cells. It contains most of the cell's genetic material,
organized as multiple long linear DNA molecules in complex with a
large variety of proteins, such as histones, to form chromosomes.
 The genes within these chromosomes are the cell's nuclear genome. The
function of the nucleus is to maintain the integrity of these genes and to
control the activities of the cell by regulating gene expression — the
nucleus is, therefore, the control center of the cell. The main
structures making up the nucleus are the nuclear membrane, a double
membrane that encloses the entire organelle and isolates its contents
from the cellular cytoplasm, and the nucleoskeleton (which
includes nuclear lamina), a mesh work within the nucleus that adds
mechanical support, much like the cytoskeleton, which supports the cell
as a whole.
Nucleus
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 Nuclear pores are required that regulate Nuclear
transport of molecules across the envelope. The pores
cross both nuclear membranes, providing a channel
through which larger molecules must be actively
transported by carrier proteins while allowing free
movement of small molecules and ions. The interior of
the nucleus does not contain any membrane-bound
sub compartments.
 The best-known of these is the nucleolus, which is
mainly involved in the assembly of ribosomes. After
being produced in the nucleolus, ribosomes are
exported to the cytoplasm where they translate mRNA.
Synthesis Activities
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 Three types of RNA are necessary for protein
synthesis:





Messenger RNA (mRNA) – copies of short segments of
deoxyribonucleic acid (DNA)
Contains all genetic information of proteins
Must pass through the ribosomes attached to the endoplasmic
reticulum
As it passes through the ribosomes, transfer RNA (tRNA)
adds the exact amino acid to the newly forming proteins
Protein synthesis can also occur on polyribosomes floating
freely in the cytoplasm; proteins synthesized on the ribosomes
attached to the ER are transported out of the cell
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Inclusions
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 Inclusions are nonliving metabolic by-products
found in the cytoplasm.
 May appear as lipid droplets, carbohydrate
accumulations, or engulfed foreign substances
Lysosomes
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 Intracellular digestion is carried out by organelles called
lysosomes. There are several contexts in which cells need
to carry out digestion.



Include the recycling of cellular organelles and the breakdown of
viruses and other cellular invaders. Single-celled organisms use
lysosomes to digest their food as they have no process for
extracellular digestion. The pH within a lysosome is very acidic and
the enzymes within work most effectively in this environment.
The components of a lysosome have evolved specific conformations
that make them resistant to break down by the enzymes within the
lysosome.
During phagocytosis, lysosomes fuse with engulfed substances to
form a secondary vesicle; the vesicle may then remain in the cell as a
residual body or discharged outside the cell
Golgi Complex
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 The Golgi apparatus receives protein and/or lipid-filled vesicles that
bud from the ER. The Golgi apparatus contains enzymes that
modify proteins and lipids. For example, it can add a chain of sugars
to proteins and lipids, thereby making them glycoproteins and
glycolipids, which are molecules found in the plasma membrane.
 The vesicles that leave the Golgi apparatus move to other parts of
the cell. Some vesicles proceed to the plasma membrane where they
discharge their contents. Because this is secretion, note that the
Golgi apparatus is involved in processing, packaging, and secretion.
Other vesicles that leave the Golgi apparatus are lysosomes.
 The Golgi complex is the storage site for newly synthesized proteins
and of course packaging and transporting many cell products. Also
produces large carbohydrate molecules and lysosomes.
Mitochondria
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 Although the size and shape of mitochondria (sing.,
mitochondrion) can vary, all are bounded by a double
membrane. The inner membrane is folded to form little
shelves called cristae, which project into the matrix, an
inner space filled with a gel-like fluid.
 Mitochondria are the site of ATP (adenosine
triphosphate) production involving complex metabolic
pathways. As you know, ATP molecules are the common
carrier of energy in cells. A shorthand way to indicate the
chemical transformation that involves mitochondria.
Mitochondria Continued
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 Mitochondria are often called the powerhouse of the
cell: Just as a powerhouse burns fuel to produce
electricity, the mitochondria convert the chemical energy
of carbohydrate molecules into the chemical energy of
ATP molecules.
 In the process, mitochondria use up oxygen and give off
carbon dioxide and water. The oxygen you breathe in
enters cells and then mitochondria; the carbon dioxide
you breathe out is released by mitochondria. Because
oxygen is used up and carbon dioxide is released, we say
that mitochondria carry on cellular respiration.
Endoplasmic Reticulum
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 The endoplasmic reticulum (ER), a complicated system of
membranous channels and flattened vesicles, is physically
continuous with the outer membrane of the nuclear envelope.
Rough ER is studded with ribosomes on the side of the membrane
that faces the cytoplasm. Here proteins are synthesized and enter
the ER interior where processing and modification begin. Some of
these proteins are incorporated into membrane, and some are for
export. Smooth ER, which is continuous with rough ER, does not
have attached ribosomes.
 Smooth ER synthesizes the phospholipids that occur in
membranes and has various other functions, depending on the
particular cell. In the testes, it produces testosterone, and in the
liver it helps detoxify drugs. Regardless of any specialized function,
ER also forms vesicles in which large molecules are transported to
other parts of the cell. Often these vesicles are on their way to the
plasma membrane or the Golgi apparatus.
Filaments and Tubules
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 Associated with contractility in cells – thread-like
structures about 7-10nm thick
 Microfilaments act as a support system for the cell
cytoskeleton
 Bundles of microfilaments form tonofibrils and
become part of the attachment apparatus between
cells (desmosomes)
Microtubules
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 Delicate tubes, 20-27 nm wide, found in cells that
are undergoing mitosis and alterations in cell shape
 They have an internal support function, especially in
long cellular processes such as neurites or
odontoblastic processes
 They have the capacity to direct intracellular
transport through the cytoplasm
Centrioles
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 Cylindrical structures composed of microtubule like
components
 Centrioles function in cell replication and the
formation of cellular extensions
Internal Environment and Homeostasis
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 Extracellular fluid
 Circulates outside and between cells
 Intracellular fluid
 Fluid located inside the cells of the body
 Homeostasis
 The delicate balance maintained between the two fluid
compositions
Transport through the Cell Membrane
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 Diffusion - is the random movement of simple atoms or
molecules from area of higher concentration to an area of lower
concentration until they are equally distributed. To illustrate
diffusion, imagine putting a tablet of dye into water . The water
eventually takes on the color of the dye as the dye molecules
diffuse.
 The chemical and physical properties of the plasma membrane
allow only a few types of molecules to enter and exit a cell by
simple diffusion. Lipid-soluble molecules such as alcohols can
diffuse through the membrane because lipids are the
membrane’s main structural components. Gases can also diffuse
through the lipid bilayer; this is the mechanism by which oxygen
enters cells and carbon dioxide exits cells. For example, consider
the movement of oxygen from the lungs to the bloodstream.
Transport Continued
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 When you inhale, oxygen fills the tiny air sacs, or alveoli,
within your lungs. Neighboring lung capillaries contain
red blood cells with a very low oxygen concentration.
Oxygen diffuses from the area of highest
concentration to the area of lowest
concentration: first through alveolar cells, then lung
capillary cells, and finally into the red blood cells.
 When atoms or molecules diffuse from areas of higher to
lower concentration across plasma membranes, no
cellular energy is involved. Instead, kinetic or thermal
energy of matter is the energy source for diffusion.
Osmosis
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 Osmosis is the diffusion of water across a plasma membrane.
Osmosis occurs whenever an unequal concentration of water
exists on either side of a selectively permeable membrane.
(Recall that a selectively permeable membrane allows water to
pass freely, but not most dissolved substances.) In a solution,
water is more concentrated when it contains fewer
dissolved substances, or solutes, (and thus is closest to pure
water).
 Water is less concentrated as solute concentration increases.
Osmotic pressure is the force exerted on a selectively
permeable membrane because water has moved from the area
of higher water concentration to the area of lower water
concentration(higher concentration of solute).
Osmosis Continued
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 Tonicity is the degree to which a solution’s concentration of
solute-versus-water causes water to move into or out of cells.
Normally, body fluids are isotonic to cells that is, there is an
equal concentration of solutes (dissolved substances)
and solvent (water) on both sides of the plasma membrane,
and cells maintain their usual size and shape.
 Medically administered intravenous solutions usually have
this tonicity. Body fluids which are not isotonic to body cells
are the result of dehydration or water intoxication. Solutions
(solute plus solvent) that cause cells to swell or even to burst
due to an intake of water are said to be hypotonic solutions. If
red blood cells are placed in a hypotonic solution, which has a
higher concentration of water (lower concentration of solute)
than do the cells, water enters the cells and they swell.
Continued
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 The term lysis refers to disrupted cells: hemolysis,
then, is disrupted red blood cells. Solutions that
cause cells to shrink or to shrivel due to a loss of
water are said to be hypertonic solutions.
 If red blood cells are placed in a hypertonic solution,
which has a lower concentration of water (higher
concentration of solute) than do the cells, water
leaves the cells and they shrink.
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Active Transport
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 Process used by the cell when large quantities of a




substance are needed inside the cell and only a small
amount of the substance is present in the extracellular
fluid.
Pumps the substance against its concentration gradient.
ATP
Sodium pump; important for the transmission of nerve
impulses
Almost all monosaccharides are actively transported into
the body.


Phagocytosis – movement of a solid particle into the cell
Pinocytosis – movement of fluid into a cell, the cell invaginates
around fluid
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The Cell Cycle
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33
Cell Replication - Mitosis
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 Interphase:
 During interphase, the cell carries on its regular
activities, and it also gets ready to divide if it is going to
complete the cell cycle. For these cells, interphase has
three stages, called G1 phase, S phase, and G2 phase.
 G1 Phase - it is best to think of G as standing for
“growth.” Protein synthesis is very much a part of these
growth phases. During G1, a cell doubles its organelles
(such as mitochondria and ribosomes) and accumulates
materials that will be used for DNA synthesis.
Continued
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 S Phase Following G1, the cell enters the S (for “synthesis”)
phase. During the S phase, DNA replication occurs. At the
beginning of the S phase, each chromosome is composed of
one DNA double helix, which is equal to a chromatid. At the
end of this phase, each chromosome has two identical DNA
double helix molecules, and therefore is composed of two
sister chromatids. Another way of expressing these events is
to say that DNA replication has resulted in duplicated
chromosomes.
 G2 Phase During this phase, the cell synthesizes proteins
that will assist cell division, such as the protein found in
microtubules. The role of microtubules in cell division is
described later in this section.
Prophase
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 Several events occur during prophase that visibly
indicate the cell is about to divide. The two pairs of
centrioles outside the nucleus begin moving away from
each other toward opposite ends of the nucleus. Spindle
fibers appear between the separating centriole pairs, the
nuclear envelope begins to fragment, and the nucleolus
begins to disappear.
 The chromosomes are now fully visible. Spindle fibers
attach to the centromeres as the chromosomes continue
to shorten and thicken. During prophase, chromosomes
are randomly placed in the nucleus.
Prophase Continued
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 At the end of prophase, a cell has a fully formed
spindle. A spindle has poles, asters, and fibers. The
asters are arrays of short microtubules that radiate
from the poles and the fibers are bundles of
microtubules that stretch between the poles. (A
spindle resembles a lopsided bicycle wheel; the
asters are the “spokes.”) Centrioles are located in
centrosomes, at opposite poles of the cell.
Centrosomes are believed to organize the spindle.
Metaphase
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 During metaphase, the nuclear envelope is
fragmented, and the spindle occupies the region
formerly occupied by the nucleus.
 The paired chromosomes are now at the equator
(center) of the spindle. Metaphase is characterized
by a fully formed spindle, and the chromosomes,
each with two sister chromatids, are aligned at the
equator
Anaphase
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 At the start of anaphase, the sister chromatids separate.
Once separated, the chromatids are called
chromosomes.
 Separation of the sister chromatids ensures that each cell
receives a copy of each type of chromosome and thereby
has a full complement of genes.
 During anaphase, the daughter chromosomes move
to the poles of the spindle. Anaphase is characterized
by the movement of chromosomes toward each pole and
thus, to opposite sides of the cell.
Telophase
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 A nuclear membrane forms around each set of
chromosomes
 Centrioles replicate in each cell
Dental Tissues
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 Organelles play an important role in providing
energy (mitochondria) which helps in calcifying
dental tissues
 Cell organelles help maintain tissues after the
initial formation by the cell; fibroblasts contain
increased numbers of cell organelles; these
additional organelles aid fibroblasts in their
synthesizing and secretory functions
Basic Tissues
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 Individual cells multiply and differentiate to perform
specialized functions; groups of cells with similar
characteristics and functions come together and
form tissues.
 Tissue components



Cells
Intercellular substance – product of living cells, passageway
Tissue Fluid – blood plasma that transports
Tissues in the Human Body
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 1.
epithelial tissues (covering/lining);
 2. connective tissues (support);
 3. muscle tissues (movement);
 4. nervous tissues (control).
Epithelial Tissue
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 Many epithelial tissues are classified according to
their shape and the number of layers they possess:
 Some terms used to describe epithelia include:
a.
b.
c.
d.
e.
than wide);
simple = single layer of cells;
stratified = many layers of cells;
squamous = flattened cells;
cuboidal = square-shaped cells;
columnar = elongated cells (i.e. taller
Types of Simple Epithelium
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 Simple squamous epithelium:
 a single layer of flattened cells;
 generally allows for easy passage (diffusion) of
substances;
 Locations:
1.
2.
3.
4.
lining air sacs of lungs,
lining capillaries,
lining body cavities,
covering ventral organs;
Simple Cuboidal Epithelium
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 Simple cuboidal epithelium:
• a single layer of square-shaped cells with large centrally located
nuclei;
• Functions:
1.
secretion
2.
absorption;
 Locations:
1.
lining kidney tubules,
2.
lining ducts of glands,
3.
covering surface of ovary;
Simple Columnar Epithelium
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 A single layer of elongated cells with basally located nuclei (near
basement membrane);
1.
2.
3.
protection,
absorption,
secretion;
1.
2.
lining small intestine,
lining uterus;
 Free Surface Modifications:
1.
2.
microvilli (increase surface area)
goblet cells (secrete protective mucus);
Pseudostratified Columnar Epithelium
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 Pseudostratified columnar epithelium: a single layer of
elongated cells with scattered nuclei (i.e. look stratified but are not);
all cells touch the basement membrane
 Functions:
1.
secretion,
2.
protection;
Locations:
1.
lining trachea,
2.
lining fallopian tube;
 Free surface modifications:
1.
cilia (trap debris and aid in passage of mucus up and
out of airway);
2.
goblet cells (produce mucus which coats cilia and
helps trap debris).
Stratified Epithelium
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 Stratified squamous epithelium: many layers
of flattened cells;
 Function = protection;
 Locations:
 Non-keratinized:
1.
lining mouth,
2.
lining throat,
 Keratinized – epidermis of the skin
Stratified Cuboidal Epithelium
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 Stratified Cuboidal epithelium: 2-3 layers of
cuboidal cells.
 Locations
1.
2.
3.
4.
mammary glands
sweat glands
salivary glands
pancreas
Stratified Columnar Epithelium
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 Stratified columnar epithelium: 2-3 layers of
elongated cells
 Locations
1.
2.
vas deferens
part of male urethra
 Transitional epithelium:



many layers of cells that change shape in response to tension;
Function = distensibility (i.e. stretches easily to allow urine to fill
bladder);
Location = lining urinary bladder and ureters.
Glandular Epithelium
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Glandular Epithelium
a.
usually simple cuboidal columnar ET;
b.
Function = secretion;
c.
two major types:
 Exocrine glands secrete products into a duct, which
opens onto:
 an external surface (i.e sweat gland) or
 an internal space/lumen (i.e. gastric gland);
Glandular Epithelium Continues
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Endocrine glands secrete hormones into the blood.
d.
Exocrine glands structure varies tremendously
 Single cells (unicellular) – goblet cells
 Many cells (multicellular)
 Simple – unbranched
 Compound – branched
 Tubular – tube-like
 Alveolar – sac-like
e.
Exocrine glandular secretions are
classified according to whether they consist of cellular
products or portions of glandular cells:
Continued
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 Three types of secretions.

Merocrine: secrete fluid through cell
membranes into a duct with no loss of glandular cells.
Example = salivary glands.

Apocrine: lose small portion of cells with
secretion. Example = mammary glands;

Holocrine: release entire cells into
secretion. Example = sebaceous glands in skin (oil).
Connective Tissue
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 Ground Substance:
a.
amorphous material that fills the space
between cells and fibers;
b.
Functions as a molecular "sieve" through
which nutrients and gases can diffuse between cells
and blood capillaries.
Major Cell Types
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 Major Cell Types:
1.
fixed cell in each CT type: maintains
constant numbers
a.
fibroblasts in CT proper
b.
osteocyte in bone,
c.
chondrocyte in cartilage.
d.
blast cells = undifferentiated cells that
secrete matrix; fibroblast in CT proper, chondroblast
in cartilage; osteoblast in bone;
Wandering Cells
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 wandering cells; are not always there
a.
migrating white blood cells that
respond to tissue damage (i.e. inflammation) ; 2
types:
 mast cells:
Secrete heparin to prevent excessive
blood clotting
 Secrete histamine to promote inflammation

macrophages or phagocytes:
 Eat foreign material
See Fig 5.14, page 142.
Connective Tissues
58
 Connective Tissue Fibers = 3 types:
1.
collagen.
a.
b.
 2.
Collagen fibers are composed of the protein
provide high tensile strength to matrix;
stain pink.
Elastic fibers are composed of the protein elastin.
a.
provide rubbery resiliency to matrix;
b.
stain purple;
c.
found in skin, lungs, and blood vessels.
Continued
59
 3. Reticular fibers are fine collagenous fibers.
a.
b.
c.
form delicate networks;
found in basement membranes;
stain purple.
Categories of Connective Tissues
60
 Embryonic CT = mesenchyme: from mesoderm
a.
b.
Location = embryo;
Function = gives rise to all other types of CT;
 2. Connective
Tissue Proper – All CT with a semifluid ground substance

a. Loose Areolar CT:
• gel-like matrix with fibroblasts, macrophages, mast cells and collagen
and elastic fibers;
• Location = beneath epithelium, covering ventral organs;
• Functions = diffusion of nutrients and gases; wraps & cushions organs.
Continued
61
 Connective Tissue Proper
b.
Adipose Tissue: closely packed
adipocytes (fat-cells) with nuclei pushed to one side
within matrix (resemble signet rings);
 Location = under skin (as subcutaneous layer),
around kidneys and eyeballs, breasts;
 Functions = energy store, insulation, protection;
Continued
62
 Reticular CT:
 network of reticular fibers within loose ground substance and
reticulocytes. Location = basement membranes and lymphatic
organs (i.e. lymph nodes, thymus, spleen); Function = support;
 Dense Regular CT (White Fibrous CT): primarily collagen
fibers (pink) with few fibroblasts (you can only see nuclei!);
Location = tendons, ligaments; Functions = attachment,
tensile strength; Poor blood supply = slow to no healing;
 Dense Irregular CT: primarily collagen fibers randomly
arranged; Location = dermis of skin, heart valves; Function =
provides tensile strength;
 Elastic CT: primarily elastin fibers (purple);Location = lung
tissue, wall of aorta; Function = durability with stretch;
Special Connective Tissue
63
 Hyaline cartilage: amorphous (chondroitin and
glucosamine) matrix that surrounds cells =
chondrocytes (within lacunae); Locations = embryonic
skeleton, costal cartilages, cartilage of the nose, trachea,
and larynx; Function = support; Avascular = no healing.
 Elastic cartilage: same as above plus elastic fibers
(purple); Locations = external ear, epiglottis; Functions
= maintenance of shape plus flexibility;
 Fibrocartilage: less firm than above; Locations =
intervertebral discs, pubic symphysis; Functions =
tensile strength plus shock absorber;
Continued
64
 Bone: hard, calcified matrix ([Ca3(PO4)2.(OH)2] =
rigidity), with collagen fibers (tensile strength) and
cells = osteocytes (within lacunae); Location =
bones of the skeleton; Functions = protection,
support, movement, calcium store and
hematopoiesis;Highly vascular = fast healing;
 Blood: red cells (erythrocytes), white cells
(leukocytes), and platelets (thrombocytes) in a fluid
matrix called plasma; Location = within heart and
blood vessels;Function = transport of gases,
nutrients, wastes.
Types of Muscle Tissue
65
1. Skeletal Muscle Tissue
 a. Structure: long thin cells (fibers) with many nuclei;
alternating areas of light & dark (striations); Location:
attached to bones; Function: move bones of skeleton;
Control: voluntary = conscious.
 2. Cardiac Muscle Tissue
 a. Structure: network of cells with one centrally located
nucleus; intercalated discs (where 2 cells meet);
striations; Location:
heart; Function: to pump
blood from heart -----> lungs; to pump blood from heart
-----> body; Control: involuntary = unconscious.
Continued
66
 3. Smooth Muscle Tissue
 a.
Structure: spindle-shaped cells with one
centrally located nucleus; no striations; Location:
walls of hollow visceral organs; walls of blood
vessels; attached to hair follicles in the dermis
Function: movement of food through digestive tract;
vasoconstriction; Control: involuntary =
unconscious.
Nerve Tissue
67
 1. Primary cells = neurons which respond to
changes in their surroundings (stimuli);
 2. neurons are surrounded by neuroglia
(supporting cells);
 B.
Locations: Brain, Spinal Cord, Nerves
 C. Function: Coordination or integration of body
parts (i.e. to transmit signals from body parts to
brain and from brain back to body parts); No
reproduction of neurons, only neuroglia can
divide.
Epithelial Membranes
68
 DEFINITION: An epithelial membrane is a continuous multicellular
sheet composed of at least two primary types of tissue: an
epithelium bound to a discrete underlying CT tissue.
B.
Three Common Types:
1.
Cutaneous Membrane:
a.
skin; consists of keratinized stratified squamous ET
firmly attached to a thick layer of dense irregular CT.
2.




Mucous Membranes (mucosae):
a.
line body cavities that open to the outside;
b.
include lining of digestive, respiratory and urinary tract;
c.
are "wet" or moist membranes (through secretions of
mucus);
d.
consist of a layer of epithelium (varies depending upon
location) firmly attached to a layer of loose areolar CT.
Three Common Types
69
 Serous Membranes (serosae):
a.
are found in closed ventral body cavities;
b.
consist of two layers with a potential space (cavity)
between them: visceral membrane surrounds an organ; parietal
membrane lines a body cavity;
c.
secrete a thin watery fluid called serous fluid into the
cavity between the membranes; function = lubrication;
d.
each membrane consists of a thin layer of simple
squamous ET resting on a thin layer of areolar (loose) CT;
e.
are named for the organs that occupy each cavity:
 pleural = lungs;
 pericardial = heart;
 peritoneal = abdominal organs.