Anatomy/Physiology
Chapter 4 Tissues
Epithelial Tissue: (epithelium)
A sheet of cells that covers a body surface or lines a body
cavity. (epithe = laid on or covering)
1. Covering and lining epithelium: forms 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 cavity.
2. Glandular: covers the glands of the body
Epithelia form boundaries between different environments.
Ex. epidermis of the skin separates the inside from the outside
of the body. Also, the epithelium lining in the bladder
separates the bladder wall from the urine.
As an interface tissue epithelium has many functions:
1. Protection
2. Absorption
3. Filtration
4. Excretion
5. Secretion
6. Sensory reception
Epithelial tissue characteristics:
1. Polarity: all epithelia have an apical surface, an upper free
surface exposed to the body exterior or the cavity of an
internal organ, and a lower attached basal surface. All
epithelia exhibit apical/basal polarity, the cell regions near
the apical surface differ from those near the basal surface
in both structure and function. Most apical surfaces have
microvilli which give them a “fuzzy” appearance (brush
border). Some epithelia (lining of the trachea) have cilia.
Lying next the basal surface is the a supporting sheetbasal lamina.
2. Specialized contacts: epithelial cells fit close together to
form continuous sheets (except for glandular epithelia).
Adjacent cells are bound together at many points by
lateral contacts, including tight junctions and
desmosomes. Tight junctions help keep proteins in the
apical region of the plasma membranes from diffusing into
the basal region helping it to maintain its polarity
3. Supported by connective tissue: all epithelial tissue rests
upon and is supported by connective tissue. Just deep to
the basal lamina is the reticular lamina (layer of extracellular material containing a fine network of collagen
fibers. Together the two lamina make up the basement
membrane which reinforces it to resist stretching and
tearing forces defining the epithelial boundary.
4. Avascular but innervated. (contains no blood vessels but
does have nerves). It is nourished by substances diffusing
from blood vessels in the underlying connective tissue.
5. Regeneration: epithelium has high regenerative capacity.
If and when their apical-basal polarity and their lateral
contacts are destroyed, they begin to regenerate
themselves rapidly through cell division.
Classification of epithelia:
Simple epithelia: composed of a single layer of cells (usually
very thin), concerned with absorption, secretion and filtration
Stratified epithelia: consisting of 2 or more layers of cells
(stacked on top of one another) common in high abrasion areas
where protection is important. They regenerate from below,
the basal cells divide and push apically to replace the older
surface cells. It is more durable than simple epithelia. Major
role is protection.
3 common shapes: (both simple and stratified)
Squamous, flattened or scale-like (squam= scale) nucleus is flat
and disk like
Cuboidal: are boxlike – approx. as tall as they are wide. The
nucleus is spherical
Columnar: are tall and column shaped. Nucleus is elongated
and near the base of the cell
Transitional: These “transitional cells” can change their shape
(transition) to allow for greater elasticity. They form the lining
of hollow urinary organs which stretch as they fill with urine.
To help identify the cell types keep the nucleus shape in mind.
(see pages 120-124)
Glandular Epithelia:
A gland consists of one or more cells that make and secrete
(export) a particular product. The product is called a secretion
is a water based (aqueous) fluid that usually contains proteins
but can release lipids or steroids.
Glands are classified as endocrine (internally secreting) or
exocrine (externally secreting) depending on where they
release their product.
Endocrine glands:
They eventually lose their ducts and are called “ductless
glands” secreting hormones by exocytosis directly into the
extracellular space. The hormones then enter the blood or
lymphatic system and travel to their target organs.
Exocrine Glands:
They secrete their products directly onto the body surfaces
(skin) on into the body cavities. They include: mucous, sweat,
oil, and salivary glands, the liver (bile), the pancreas (digestive
enzymes) and others.
Modes of secretion: (most multicellular exocrine glands)
Merocrine glands secrete their products by exocytosis. The
secretory cells are not altered in any way. The pancreas, sweat
and salivary glands belong to this class.
Holocrine glands accumulate their products within them until
they rupture (they are replaced by underlying cells). They “die
for their cause” ex. Sebaceous (oil) glands in the skin
Connective Tissue:
Found everywhere in the body. It is the most abundant and
widely distributed of the primary tissues.
4 main classes:
1. connective tissue (proper) which includes fat and fibrous
tissue of ligaments
2. cartilage
3. bone tissue
4. blood
Major functions:
1. binding and support
2. protection
3. insulation
4. transportation (blood) of substances within the body
Common Characteristics:
1. Common origin: connective tissue arises from
mesenchyme (embryonic tissue) so they have a common
kinship
2. Degrees of vascularity, cartilage is avascular, dense
connective tissue is poorly vascularized while other types
have a rich supply of blood.
3. Extracellular matrix, connective tissues are largely
composed of extracellular matrix (non-living). Because of
this connective tissue can bear weight, withstand great
tension, and endure abuses (physical trauma and abrasion)
Structural elements of connective tissue:
3 main elements:
1. ground substance
2. fibers
3. cells
Ground substance and fibers make up the extracellular matrix.
The properties of the cells and the arrangement of the matrix
elements vary tremendously providing an amazing diversity of
connective tissues. The common structural plan is called
areolar connective tissue.
Ground substance is unstructured material that fills space
between the cells and contains fibers. It is composed of
interstitial (tissue) fluid, cell adhesion proteins, and
proteoglycans.
Fibers provide support. 3 types;
1. Collagen fibers, made of collagen, provide high tensile
strength to the matrix they have a white appearance
“white fibers”
2. Elastic fibers, made up of a rubber like protein elastin.
Yellowish appearance “yellow fibers” stretch like a rubber
band. Found in the skin and lungs and blood vessel walls.
3. Riticular fibers, are short fine collagenous fibers (slightly
different chemistry and form) they are continuous with
collagen fibers. They surround the small blood vessels and
support the soft tissue of organs.
Cells: each major class of connective tissue has a fundamental
cell type that exists in immature and mature forms.
Undifferentiated cells, indicated by the suffix “blast” are
actively mitotic cells that secrete the ground substance and
fibers of their particular cells. The primary blast cell types:
1. Connective tissue proper – fibroblast
2. Cartilage – chondroblast
3. Bone – osteoblast
4. Blood – hematopoietic stem cell (this does not form the
matrix like the others)
Once they synthesize the matrix, they mature and become less
active. They are identified by the suffix “cyte”
Connective tissue is also home to other cells:
White blood cells: neutrophils, eosinophils, lymphocytes
Mast cells and macrophages; concerned with tissue response
to injury
Mast cells act as sentinels to detect foreign substances
(bacteria, fungi) and initiate local inflammatory response
against them.
Macrophages: (macro = large, phago = eat)
Phagocytize foreign materials and dead tissue cells. They can be
“fixed” in connective tissue or migrating throughout the matrix
Types of connective tissue:
All classes of connective tissue consist of living cells surrounded
by a matrix. Mature connective tissues come from a common
embryonic tissue called Mesenchyme
Connective Tissue Proper: has 2 subclasses:
Loose connective tissue: areolar, adipose, and reticular
Dense connective tissue: dense regular, dense irregular, and
elastic. Except for bone, cartilage, and blood, all mature
connective tissues belong to this class
Areolar Connective Tissue:
Functions: (share by some but not all connective tissues)
1. Supporting and binding other tissues (the job of the fibers)
2. Holding body fluids (the ground substance role)
3. Defending against infection (via activity of WBC’s and
macrophages)
4. Storing nutrients as fat (in fat cells)
Areolar connective tissue is the most widely distributed
connective tissue in the body. It serves as a kind of universal
packing material between other tissues.
Adipose (fat) Tissue:
Similar to areolar tissue in structure and function but has a
greater nutrient storage capacity. Usually accumulates in
subcutaneous tissue and acts as a shock absorber, insulation,
and energy storage sites.
Reticular Connective Tissue:
Limited to certain sites, forms a labyrinth-like stroma (“bed”or
“mattress”) an internal framework that supports free blood
cells (mostly lymphocytes) in the lymph nodes, spleen, and
bone marrow.
Dense Regular Connective Tissue (fibrous connective tissue)
Enormous tensile strength, they form tendons which attach
muscles to bones, muscles to other muscles (aponeuroses) and
fascia (fibrous membrane that wraps around muscles, groups of
muscles blood vessels, and nerves. Like plastic wrap.
Dense Irregular Connective Tissue:
Found in the skin (dermis), forms fibrous joint capsules and
coverings that surround some organs (kidneys, bones,
cartilages, muscles and nerves)
Cartilage:
Tough and flexible, providing resilient rigidity to the structures
it supports. Lacks nerve fibers and is avascular. Composed of
up to 80% water, this enables the cartilage to rebound after
compression.
3 types of Cartilage:
Hyaline Cartilage (most abundant in body) covers the ends of
long bones, tip of the nose, connects ribs to sternum, and
supports most of the respiratory system passages. Most of the
embryonic skeleton before bone is formed.
Elastic Cartilage: elastic fibers, forms the “skeleton” of the
external ear and the epiglottis. Exceptional strength and
stretchability
Fibrocartilage: compressible and tension resistant
Intervertebral disks and spongy cartilage of the knee
Bone (osseous tissue) support and protect body structures.
Provides cavities for fat storage and blood cell synthesis. Along
with collagen fibers the matrix also contains (inorganic) calcium
salts.
Osteoblasts produce the organic portion of the matrix.
Osteocytes (mature bone cells) reside in the lacunae (small
space within bone or cartilage) within they have made.
Blood:
The most atypical connective tissue. It is classified as
connective tissue because it comes from mesenchyme and
consists of blood cells, surrounded by a non-living fluid matrix
called blood plasma. The vast majority of blood cells are RBC’s
the fibers of the blood are soluble protein molecules (aide in
blood clotting). Function of blood: transportation of nutrients,
wastes, respiratory gases, and other substances throughout the
body.
Nervous tissue:
The main component of the nervous system (brain, spinal
column, nerves which regulate and control body functions.
Neurons: highly specialized nerve cells that generate and
conduct nerve impulses. Typically, they are branching cells
with extensions or processes. These allow them to respond to
stimuli (role of the dendrites) and to transmit electrical
impulses over substantial distances in the body (role of the
axons). These processes can be very long and myelinated
(covered with a fatty sheath – think insulated wire) that
increases the speed of transmission.
Muscle Tissue:
Highly cellular, well-vascularized tissues responsible for most
types of body movement. They possess myofilaments (actin
and myosin filaments) that cause movement or contraction in
all cell types.
3 Types of muscle tissue:
Skeletal Muscle: attached to bones of the skeleton. Make up
the flesh of the body.. when they contract, they pull on the
bones or skin causing movement.
Skeletal muscle cells (also called muscle fibers) are long, multinucleated cylindrical cells. They have a striated (banded)
appearance reflecting the precise alignment of their
myofilaments.
Cardiac Muscle:
Found only in the wall of the heart. They are striated (like
skeletal muscle) in appearance and help propel the blood
though the blood vessels. They only have one nucleus, are
branching cells that fit together tightly at junctions called
intercalated discs.
Smooth Muscle:
Named because their cells have no visible striations.
They are spindle shaped with one nucleus. Found mainly in the
walls of hollow organs (other than the heart). It functions to
squeeze substances through these organs by contracting and
relaxing.
Voluntary muscle: Skeletal muscle because we have conscious
control over them.
Involuntary muscle: cardiac and smooth
Covering and lining membranes:
3 types: Cutaneous, mucous, serous
All are continuous multicellular sheets composed of epithelium
fused to a sheet of connective tissue proper. These
membranes are simple organs.
Cutaneous membranes: consists of 2 layers, the epidermis
(keratinized squamous epithelium) attached to the dermis
(dense irregular connective tissue). It is exposed to the air and
is a dry membrane.
Mucous Membrane: (mucosae) line the body cavities that open
to the exterior (hollow organs of the digestive, respiratory and
urinary tracts). They are “wet” membranes bathed in
secretions. Adapted for absorption or secretion.
Serous Membrane: (serosae) moist membranes found in closed
ventral cavities. They are named for their site and specific
organ associations: Pleura- lining the thoracic cavity and lungs;
Pericardium – enclosing the heart; peritoneums – lining the
abdominopelvic cavity and organs.
Tissue repair:
3 steps:
1. Inflammation – the release of mast cells and macrophages
and inflammatory chemicals. This causes blood vessels to
dilate and diffuse WBC’s and clotting proteins and
antibodies to the injured area.
2. Organization – blood clot is replaced by granulation
tissue. A delicate pink tissue composed of capillaries that
grow in from nearby areas. Granulation tissue becomes
scar tissue is highly resistant to infection because it
produces bacteria inhibiting substances. Once enough
matrix has accumulated in the area, fibroblasts revert to a
resting state or undergo apoptosis
3. Regeneration and fibrosis – during organization, the
epithelium starts to regenerate (growing under the scab).
As fibrous tissue matures and contracts, it thickens until it
resembles the surrounding skin. The end result is a fully
regenerated epithelium and an underlying scar which may
or may not be visible.
Regenerative capacity of different tissues:
Epithelial tissues, bone, areolar connective tissue, dense
irregular connective tissue, and blood forming regenerate
extremely well. Smooth muscle, and dense regular connective
tissue have moderate capacity to regenerate, and skeletal and
cartilage have weak capability for regeneration. Cardiac and
nervous tissues have virtually no functional regenerative
capacity and are routinely replaced with scar tissue.
Developmental aspects of tissues:
In embryonic development, one of the first events is the
formation of three primary germ layers (lie on top of one
another like pancakes) Ectoderm, mesoderm and endoderm.
These primary layers specialize to form the 4 primary tissues
from which all body organs are derived.