TISSUE: THE LIVING FABRIC
INTRODUCTION TO TISSUE
• Tissues are groups of cells that are
similar in structure and function
(tissue=woven)
• There are four primary tissues types:
– Epithelial: covering
– Connective: support
– Nervous: control
– Muscular: movement
• Histology: study of tissues
EPITHELIAL TISSUE
• Epithelium (plural: epithelia)
• Sheet of cells that covers a body surface or
lines a body cavity (epithe=laid 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
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
Special Characteristics of Epithelium
•
•
Has many characteristics that distinguish them from other tissue types:
1. Cellularity:
–
•
2. Specialized contacts:
–
•
Composed of closely packed cells with little extracellular material between
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:
–
•
4. Supported by connective tissue:
–
•
Supported by the underlying connective tissue (reticular lamina) containing collagen fibers
5. Innervated but avascular:
–
•
Some apical surfaces have villi while the basal surface acts as a filter determining which molecules will be allowed
to enter the epithelium
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
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
Classification of Epithelia
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 (squam=scale)
2. Cuboidal cells are: boxlike
– Approximately as tall as they are
wide
•
3. Columnar cells are: tall and
column shaped
Classification of Epithelia
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
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
Simple Squamous Epithelium
Simple Cuboidal Epithelium
Simple Columnar Epithelium
Pseudostratied Columnar Epithelium
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
Stratified Squamous Eputhelium
Stratified Cuboidal Epithelium
• Rare
• Found mostly in the ducts of some of the
larger glands
– Sweat glands
– Mammary glands
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
Transitional Epithelium
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
gland’s product and the process of making and
releasing that product
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
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)
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)
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
Pseudostratified Columnar Epithelium
Unicellular Exocrine Glands (Goblet Cells)
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
Multicellular Exocrine Glands
Structural Classification
• On the basis of their
duct structures
– Simple:
• Unbranched duct
EXOCRINE GLANDS
Multicellular Exocrine Glands
Structural 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 (alveolus=small hollow
cavity)
• Tubuloalveolar: if they have
BOTH types of secretory
units (tubes and alveolar)
– NOTE: acinar is used
interchangeably with
alveolar
EXOCRINE GLANDS
Multicellular Exocrine Glands
Functional 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
Merocrine Gland
Multicellular Exocrine Glands
Functional 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
(holo=all)
• Examples:
– Sebaceous (oil) glands
Holocrine Gland
Multicellular Exocrine Glands
Functional 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 (apo=from
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
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
CONNECTIVE TISSUE
• Does more than just connect body parts
– It has many forms and functions
• Major functions include:
– Binding and support
– Protection
– Insulation
– Transportation
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
Connective Tissue Origins
EMBRYONIC CONNECTIVE
TISSUE
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)
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
AREOLAR CONNECTIVE TISSUE
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
PROTEOGLYCAN
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
AREOLAR CONNECTIVE TISSUE
Fibers
• Fibers of the
connective tissue
provide support
• Three types of fibers
are found in
connective tissue
matrix:
– Collagen
– Elastic
– Reticular
AREOLAR CONNECTIVE TISSUE
Collagen Fibers
• Strongest and most
abundant
• Constructed primarily of the
fibrous protein collagen
• Secreted into the extracellular
space, where they assemble
spontaneously into crosslinked 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
AREOLAR CONNECTIVE TISSUE
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
AREOLAR CONNECTIVE TISSUE
Reticular Fibers
• Fine collagenous fibers
(form and chemically
different) and are continuous
with collagen fibers
• Branch extensively forming
delicate networks
(reticul=network) 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
AREOLAR CONNECTIVE TISSUE
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
Connective Tissue Origins
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
Connective Tissue Origins
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
AREOLAR CONNECTIVE TISSUE
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 (mast=stuffed 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
AREOLAR CONNECTIVE TISSUE
Macrophages
•
•
•
Mast cells and macrophages
are very important to overall
body defense
Macro=large; phago=eat
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
AREOLAR CONNECTIVE TISSUE
Types of Connective Tissue
• Mesenchyme forms during the early
weeks of embryonic development from the
mesoderm layer and eventually
differentiates into other connective tissues
EMBRYONIC CONNECTIVE
TISSUE
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
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
AREOLAR CONNECTIVE TISSUE
Types of Connective Tissue
• Adipose (fat) tissue is a richly
vascularized tissue that functions in
nutrient storage, protection, and
insulation
ADIPOSE CONNECTIVE TISSUE
Types of Connective Tissue
• Reticular connective tissue forms the
internal framework of the lymph nodes,
the spleen, and the bone marrow
RETICULAR CONNECTIVE
TISSUE
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
DENSE REGULAR CONNECTIVE
TISSUE
DENSE IRREGULAR
CONNECTIVE TISSUE
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
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
Types of Connective Tissue
• Three varieties of cartilage:
– Hyaline
– Elastic
– Fibrocartilage
Types of Connective Tissue
Hyaline Cartilage
• Hyaline cartilage (gristle): is the most
abundant cartilage providing firm support
with some pliability
• Blue-white color
• Hyalin=glass
• Articular (ends of long bones)
HYALINE CARTILAGE
Types of Connective Tissue
Elastic 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
ELASTIC CARTILAGE
Types of Connective Tissue
Fibrocartilage
• 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
FIBROCARTILAGE
Types of Connective Tissue
Bone
• 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 element—inorganic
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
Types of Connective Tissue
Bone
• 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
BONE
Types of Connective Tissue
Blood
• 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
BLOOD
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
COVERING and LINING MEMBRANES
Cutaneous Membrane (a)
• Cutis=skin
• 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
MEMBRANES
COVERING and LINING MEMBRANES
Mucous 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
MEMBRANES
COVERING and LINING MEMBRANES
Mucous Membranes (b)
•
Most mucosae contain either stratified squamous or simple columnar
epithelia
– Underlain by a layer of loose connective tissue called the lamina propria
(one’s 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
COVERING and LINING MEMBRANES
Serous 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
MEMBRANES
COVERING and LINING MEMBRANES
Serous 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
MEMBRANES
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
NERVE TISSUE
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
SKELETAL MUSCLE
CARDIAC MUSCLE
SMOOTH MUSCLE
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
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
TISSUE REPAIR
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
TISSUE REPAIR
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
TISSUE REPAIR
TISSUE REPAIR
TISSUE REPAIR
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
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
EMBRYO TISSUE
CANCER