TISSUE LEVEL OF ORGANIZATION
All organ systems work together. They are easier to understand if they are studied
separately. As cells differentiate, they become specialized and perform limited
functions. One cell cannot perform all the functions humans needs. Humans have
trillions of cells but only 200 different types which carry out all vital functions.
These cells combine to form tissues or collections of specialized cells that perform
a limited number of functions. There are four basic tissue types-Epithelial,
Connective, Muscle & Neural. Epithelial tissues cover body surfaces and line
cavities, connective tissue supports the body and the organs, muscle tissue is
specialized to contract and to generate force and nervous tissue detects changes in
conditions both inside and outside the body.
Tissues are groups of cells working together as a unit. Therefore these cells must
be connected to each other. Connections help epithelial tissue to be an effective
barrier and protect underlying tissues. In order to grow and divide normally cells
must be anchored to one another and to the matrix. Intercellular connections attach
cells firmly to each another. There are several types of cell connections including
tight junctions, adherens, desmosomes, hemidesmosomes and gap junctions.
Tight Junctions are found at the most apical part of the cell. They completely
encircle an epithelial cell near its apex and joins it tightly to the neighboring cells.
They fuse 2 adjacent membranes with fibrous connections, like strapping tape.
This junction prevents passage of molecules and ions between cells. The
attachment is so tight that water and solutes cannot pass. Materials must enter by
diffusion or active transport. If an epithelium forms a tube, the space in that tube is
called the lumen. The presence of tight junctions ensures that the contents of the
lumen are isolated from the basolateral surfaces of the cell. In this way such things
as digestive enzymes cannot harm delicate underlying tissues.
Gap Junctions allow cells to communicate with one another. Gap junctions are
intercellular channels of 1.5-2 nm diameter which permit passage of ions and small
molecules to a MW of 1000 daltons. Gap junctions are comprised of pore-like
transmembrane proteins called connexons. Because ions can flow through these
junctions changes in membrane potential can pass from cell to cell. In this way
action potentials can be sent simultaneously to all heart cells to produce rhythmic
contractions of the heart. They are most abundant in cardiac and smooth muscle to
coordinate muscle cell contraction.
Desmosomes are localized patches that hold cells together. These junctions allow
tissues to resist mechanical stress. Desmosomes are very strong and resist twisting
and stretching. They stabilize cell shapes and are most abundant in superficial skin
layers. The desmosomes links are so strong that dead skin cells are shed in thick
sheets and not individually.
Hemidesmosomes look like half of a desmosome. They are made of proteins called
integrins while those in desmosomes are cadherins. These anchor cells to the
basement membrane.
Adherens are dense layers of proteins on the inside of a membrane. They serve to
attach membrane proteins to the microfilaments of the cell’s cytoskeleton.
Epithelial Tissue
Epithelial tissue consists of flat sheets of contiguous cells which line body surfaces
and cavities. Epithelial tissues cover every exposed surface-skin and all
passageways-digestive, reproductive, urinary and respiratory that communicate
with the outside world.
There are several characteristics off all epithelia tissue. 1. Cellularity-epithelial
tissue is made almost entirely of cells bound by cell junctions. Cells are packed
together tightly with little extracellular space. 2. Polarity- cytoplasmic components
of the cells comprising epithelial tissues are not evenly distributed. The cells have
one exposed face either to the external world or to a lumen called the apical
surface and a base or basal surface which faces the underlying connective tissue.
3. Attachment-the bottom row of cells of an epithelium are bound to the basement
membrane. 4. Avascularity- no direct contact of epithelial cells with blood vessels.
They obtain nutrition via diffusion or absorption from underlying tissues. 5.
Regeneration-eptithelia are found on protective surfaces and therefore must be
able to repair and renew themselves. Stem or germinative cells are found in the
deepest layer of epithelium near the basement membrane. These cells continually
divide to make cells which are lost via exposure to digestive enzymes, toxic
chemicals, pathogenic bacteria, and/or mechanical abrasion.
Functions
Epithelial tissues have several important functions determined by their structure.
One function is physical protection. They protect underlying cells from abrasion,
dehydration and destruction. These tissues control permeability. Anything entering
or leaving the body must cross an epithelium. They selectively absorb or secrete
substances. Another function of epithelial tissue is to provide sensation. Some
detect environmental changes and relay information to the nervous system such as
the skin and touch. Neuroepithelium is the termed given to epithelium with special
sensory function. Finally epithelial tissues produce special secretions. This is the
primary function of glandular epithelium.
Specializations of the Apical Surface
Epithelia cells exhibit polarity which means cells are divided into 2 functional
regions: 1) apical surface which is exposed to a lumen or the external environment
and 2) basal surface at the base. There are several specializations of the apical
surface. Microvilli are finger-like projections which increase the surface area 20X.
Epithelia with many microvilli is specialized for absorption & secretion. Cilia can
also be found on the apical surface. Cilia are generally longer with a larger
diameter than microvilli. They beat in a coordinated fashion and function in
movement of fluids across and through epithelia.
Types of Epithelia
Epithelia is classified according to arrangement of layers and cell shapes. There
are three types of arrangement of layers-simple, pseudostratified and stratified..
One layer of cells covering the basement membrane is termed simple. Each cell
rests on the basement membrane and one surface faces either the lumen or the
outside world. Cells are typically fragile and do not provide much protection
against mechanical damage. Simple epithelium therefore is found only internally in
areas of absorption or secretion. More than one layer of cells covering the
basement membrane is called stratified. The basal layer of cells rests on the
basement membrane but subsequent layers do not. They are stacked on top of the
basal layer. Cells of only the most superficial layer have a free surface. Stratified
epithelium is found in areas subjected to mechanical or chemical stresses such as
the skin and the lining of the mouth. Pseudostratified epithelium looks as if there
are several layers. But on closer look even though all of the nuclei are at different
levels, all cells rest on the basement membrane but are not all the same height. The
epithelium appears layered but is not. This type of epithelium often contains cilia
and goblet cells.
There are four types of cell shapes-squamous, cuboidal, columnar and
transitional. Squamous cells are very flat and irregularly shaped. They are often
so thin that the flattened nucleus bulges at the cell surface. Cuboidal cells are about
as tall as wide. They look like cubes or hexagonal boxes. The nucleus is usually
round and not flattened. Columnar cells are taller than they are wide and look like
columns. The nucleus usually is elongated and found in the long axis of the cell.
Transitional cells change shape. They go from squamous to cuboidal and back
again as an organ changes shape. These can be found in places where stretching or
expanding is needed such as the urinary bladder.
Epithelium is named by combining the cell shape with the number of cell layers.
Simple Squamous epithelium therefore is one layer of squamous cells. This type of
epithelium is the most delicate and is found in protected regions where filtration or
diffusion is a priority or where slick, slippery surfaces are needed to reduce
friction.
Stratified Squamous epithelium consists of several layers of squamous cells. The
surface cells look squamous whereas the lower ones appear more cuboidal or
columnar in shape. This type of epithelium is well suited for protection and is
found where the body experiences severe mechanical stresses. Cells are worn away
quickly and replaced rapidly by mitosis in the lower layers. The outer layer of the
skin or epidermis is composed of stratified squamous cells. Here mechanical stress
and dehydration of the superficial layers is aided with the protein keratin. Keratin
is comprised of protein filaments that are tough & water resistant. The skin is said
to be keratinized. Non-keratinized stratified squamous epithelium will resist
abrasion but will dry out unless kept moist. This can be found in the mouth,
pharynx, and esophagus.
Simple Cuboidal epithelium consists of one layer of cuboidal cells. This type is
specialized for secretion and absorption and can be found in the secretory portion
of glands. Some cells may have a dense border of microvilli and can be found in
kidney tubules, the pancreas, and salivary glands.
Stratified Cuboidal epithelium is comprised of typically only 2 cell layers of
cuboidal cells. There is not a great quantity found in the human body; only in the
large ducts of sweat and mammary glands.
Simple Columnar epithelium consists of one layer of columnar cells. The cells
look rectangular. This epithelium is found where absorption and secretion take
place such as the small intestine. In the small intestine this epithelium has goblet
cells which secrete mucus to protect and lubricate. Simple columnar epithelium
can be found with cilia in the oviducts and respiratory tract.
Pseudostratified Columnar epithelium looks at first glance like stratified
columnar. This type of epithelium often contains cilia and goblet cells. It is found
lining most of the respiratory tract.
Stratified Columnar epithelium is very rare. It can be found where 2 other types of
epithelia meet such as in some large ducts, and in the pharynx, epiglottis, anus and
urethra.
Transitional epithelium is a thicker, multilayered epithelium which can be found
facing the lumen of the bladder. This epithelium tolerates a great deal of stretching.
The surface cells are more muffin-shaped. The cells are rounded when the organ is
not filled and then flattens as the organ fills in this way it increases the holding
capacity of the bladder and other urinary organs.
Glandular Epithelia
A gland is a cell or organ that secretes substances for use elsewhere in the body or
releases them for elimination from the body. Glands are composed primarily of
epithelia tissue.
Glands are classified as endocrine or exocrine. Endocrine glands are ductless and
release hormones into the interstitial fluid. Hormones regulate or coordinate the
activity of other tissues, organs and organ systems. Exocrine glands are ducted
and release their secretions into passageways or ducts which empty onto the skin or
other epithelial surfaces. Exocrine glands produce enzymes and perspiration.
Exocrine glands are classified as unicellular or multicellular. Unicellular glands
are composed as the name suggests of one cell and multicellular ones are
comprised of more than one cell. Multicellular glands are further classified as
branched or unbranched and by the shape of the secretory part of the gland.
Exocrine glands are classified as simple if they have an unbranched duct and
compound if they have a branched duct. Glands with a tubular secretory part are
called tubular glands and those with rounded secretory parts are termed acinar.
Tubuloacinar glands have both a tubular and a rounded secretory part.
Glands also have a functional classification based upon their mode of secretion,
i.e. how the secretion is released. These classifications include merocine or eccrine,
apocrine and holocrine. Merocrine glands are the most common. They include
sweat and mucus secreting glands. They release their product via exocytosis. In
apocrine glands the glandular product accumulates at the apical end and this end is
pinched off when the gland secretes. The rest of the gland repairs itself. Holocrine
glands accumulate secretions in their cytosol. The entire cell becomes packed with
secretory product. The cell bursts releasing the secretion and in so doing kills the
cell. Further secretion depends on replacement of the gland cell. Sebaceous or oil
glands associated with hair follicles are examples of holocrine glands.
Connective Tissue
Connective tissue is found widely spread throughout the body. It is the most
diverse tissue type and is never exposed to the outside environment. Connective
tissue is highly vascularized that is blood vessels are present with the exception of
cartilage which has no blood vessels and tendons which have few.
Connective Tissue Functions
Connective tissue functions to connect organs to one another and to support tissues
and organs. Specific functions include providing a structural framework-binds
muscle to bone, fat holds kidneys in place and fibrous tissues bind skin to
underlying muscle. Connective tissue-bone supports the body and cartilage
supports ears, nose, trachea and bronchi. Connective tissues provide protection for
delicate organs such as the brain and lungs. They provide immune protection
defending the body from microorganisms. They are involved in transporting fluids
& dissolved materials through the body. Connective tissues allow movement-bones
provide levers for body movement. The connective tissue fat is important in
storing energy and generating heat.
Connective Tissue Composition
Connective tissues are comprised of 2 basic components: cells and extracellular
matrix. The matrix is mad e of protein fibers and ground substance.
Each type of connective tissue has specialized cells at different stages of maturity.
Juvenile cells actively secrete matrix and have the suffix blast. Mature cells have
the suffix cyte. Destructive cells are called clasts. The prefix is different for
different types of connective tissues. Connective tissue proper has the prefix fibro,
for cartilage it is chondro, for bone osteo, and for blood hemo. Protein fibers and
ground substance make up most of the volume of connective tissues. Together they
are called the extracellular matrix. The matrix provides strength which is
important in bones, ligaments & tendons. Collagen fibers are long, straight,
unbranched, and very strong. Each fiber consists of a bundle of fibrous protein
subunits wound together like strands of rope. Elastic fibers contain elastin. They
are able to stretch and recoil without damage. Reticular fibers are fine collagen
fibers; made of the same protein subunits as collagen but arranged differently to
form a tough, flexible branching framework.
The ground substance is amorphous like gelatin. It takes the shape of whatever
container it is put in. It fills the spaces between cells and surrounds connective
tissue fibers.
Classification of Connective Tissue
There are two major classification types of connective tissues- embryonic and
mature. Embryonic connective tissue consists of mesenchyme and mucous types
and is found in the embryo from the third gestational month to birth. It is the tissue
from which all connective tissue originates. Mature connective tissue consists of
loose, dense, cartilage, bone and liquid types.
Loose Connective Tissue
In loose connective tissue the fibers are arranged loosely between cells. This type
of connective tissue can be thought of as packing material. It fills spaces between
organs, cushions and stabilizes cells in organs and supports epithelia. It surrounds
and supports blood vessels and nerves and stores lipid. This type of connective
tissue includes areolar, adipose and reticular tissue.
Areolar is the least specialized. It consists of an open framework. Ground
substance accounts for most of its volume. It forms a soft-pliable-packing material
around other tissues. It surrounds muscles, wraps blood vessels and glands. It
functions to absorb shock. Its loose organization allows it to distort without
damage. The presence of elastic fibers makes the connective tissue able to return
to its original shape after external pressure is relieved. It forms a layer separating
the skin from deeper structures.
Adipose tissue or fat is composed mainly of adipocytes which account for most of
its volume. There is little matrix. Cells have large vacuoles filled with fat in a
single, enormous lipid droplet. Cells look hollow because the fat droplet
compresses the cytoplasm around the edges of the cell. The organelles are
squeezed to the side. Adipose tissue serves as insulation. It slows heat loss through
the skin and serves as a shock absorber around organs.
Reticular connective tissue consists of a network of reticular fibers and cells found
in the spleen, lymph nodes, and liver. It creates a 3-D network in the stroma which
supports functional cells.
Dense connective tissue has a clear, viscous ground substance due to the presence
of proteoglycans and glycoproteins. Collagen fibers make up most of the volume
and gives tissue strength without give. There are three types: dense regular, dense
irregular and elastic connective tissues.
Dense regular has collagen fibers regularly arranged in parallel. This type of
connective tissue forms ligaments which connect bone to bone and tendons which
connect muscle to bones. Dense irregular connective tissue has collagen fibers in
irregular arrangements forming interwoven meshworks. It provides strength and
support for areas subjected to stress from many directions. It is found in skin where
it gives strength to the lower layer. It forms a sheath around cartilagesperichondrium and bones-periosterum. It also forms a thick, fibrous capsule around
internal organs such as the liver, kidney and spleen. Elastic connective tissue is a
type of dense regular which contains a great deal of elastin fibers. These give tissue
flexibility. It is found in the vocal cords and the ligaments which connect
vertebrae.
Cartilage and bone are types of supporting cartilage which form the framework or
support for the body. Cartilage is strong, yet flexible and found throughout the
body. Its matrix consists of a firm gel containing chondroitin sulfate. The only
cells in the matrix are chondrocytes that are found in chambers or lacunae.
Cartilage is avascular, blood cells do not grow into it. Chondrocytes make
chemicals-antiangiogenesis factors discouraging formation of blood vessels. Cells
receive nutrition from surrounding tissues. In many cases there is perichondrium
(peri = around, chondr = cartilage) of dense irregular connective tissue surrounding
the cartilage to provide nutrition.
Cartilage is not very regenerative; once damaged recovery is slow. There are three
types of cartilage: hyaline, elastic and fibrocartilage. Hyaline cartilage is the most
common. It covers the ends of long bones. Much of the skeleton started as hyaline
cartilage and then became bone. Hyaline cartilage is covered by a dense
perichondrium except inside joint cavities. The matrix consists of closely packed
collagen fibers which makes it tough and flexible. It can be found connecting the
ribs to the sternum, nasal cartilages, supporting cartilages-respiratory tract and as a
cover in opposing bone surfaces in joints such as the knees & elbows.
Elastic cartilage is like hyaline but with more elastin fibers making it flexible and
resilient. The epiglottis is made of elastic cartilage as is the ear pinna.
Fibrocartilage looks like dense regular connective tissue. Its matrix is dominated
by collagen fibers that are densely interwoven making it durable and tough and
more compressible than other types of cartilages. It is found between spinal
vertebrae as intervertebral discs. It can also be found in the menisci of the knees,
between pubic bones in the pelvis, around or in joints and tendons. It resists
compressions, absorbs shocks and prevents bone to bone contact.
Bone is also called osseous tissue. It is for support and protection, fat storage and
blood cell formation. Volume of ground substance is small. The matrix is like
cartilage but more rigid because of calcium salts. It is calcified. 2/3 consists of Ca
salts-primarily CaPO4. The remainder is made of collagen fibers. Ca salts make
this tissue hard and brittle. Collage fibers make it strong and flexible. Minerals are
organized around the collagen fibers in such a fashion to make bone strong and
flexible making it resistant to shattering-like steel and reinforced concrete. Bone
cells are called osteocytes. They are found in lacunae and organized around blood
vessels that branch through the matrix. The matrix does not allow for diffusionosteocytes communicate with each other and blood vessels by slender cytoplasmic
extensions called canaliculi or little canals. The bone surface is sheathed by a
periosteum except in joint cavities where it is covered by hyaline cartilage. The
periosterum is made of a fibrous outer layer which aids in attachment of bone to
surrounding tissues, tendons and ligaments and a cellular inner layer which
functions in appositional bone growth and participates in repair after injury. Bone
is completely remodeled throughout life. It responds to stress which makes it
thicker and stronger. Inactivity makes bone thin & brittle.
There are two types of liquid or fluid connective tissues-blood and lympth. Each
have distinctive cell types suspended in a watery matrix containing dissolved
proteins. Blood is formed of blood cells (RBCs-½ the volume, WBCs-leukocytes,
neutrophils, basophils, eosinophils, and lymphocytes) and fragments of cells or
platelets called formed elements. The formed elements are suspended in a liquid
matrix called plasma which contains protein fibers that are important in blood
clotting. Lymph forms as interstitial fluid. It enters lymphatic vessels or small
passageways that return it to cardiovascular system.
Membranes
Membranes are physical barriers composed of epithelia supported by connective
tissue. They cover and protect other tissues. There are 4 types: Mucous, Serous,
Cutaneous & Synovial.
Cutaneous membranes cover the body surface. It is the largest membrane in the
body and is known as skin. It consists of stratified squamous epithelium + a layer
of areolar connective tissue reinforced by an underlying dense connective tissue.
This membrane is thick, relatively water proof and usually dry.
The two principle types of internal membranes are mucous and serous. Mucus
membranes line cavities in communication with the outside such as the digestive,
urinary, reproductive and respiratory tracts. A mucosa consists of two to three
layers-an epithelium, an areolar connective tissue layer (the lamina propia) and
sometimes a layer of smooth muscle called the muscularis mucosae. Mucus
membranes have absorptive, secretory and protective functions. They help keep
epithelial surfaces moist with a surface covered with mucus made by goblet cells.
Serous membranes line sealed internal parts such as ventral body cavities and are
composed of a simple squamous epithelium resting on a thin layer of areolar
connective tissue. Serous membranes produce watery serous fluid. The pleura
lines the pleural cavity and covers the lungs, the peritoneum lines the peritoneal
cavity and covers internal organs and the pericardium lines the pericardial cavity
covering the heart. Each of these serous membranes are thin, attached to the body
wall and attached to the underlying organs. Each can be divided into a parietal part
which lines the inner surface of a cavity and a visceral or serosa part which covers
the outer surface of organs. The membranes are relatively permeable so tissue fluid
can enter to keep it moist and slippery.
Synovial membranes surround joint cavities. Joints are articulations for bones.
They allow for movement. Joints are surrounded by a fibrous capsule consisting of
extensive areolar tissue with a matrix of interwoven collagen fibers, proteoglycans
and glycoproteins. The space is filled with synovial fluid, a solution like the
ground substance of loose connective tissue.
Muscle Tissue
Muscle tissue is specialized for movement and contraction. There are 3 types:
skeletal, cardiac and smooth. All contract alike but have different internal
organizations. Skeletal muscles have cells called fibers which are long and thin.
They are multinucleated often containing several hundred nuclei. They are striated
or striped due to repeating groups of cellular proteins actin and myosin-responsible
for contraction. Skeletal muscle cells cannot divide. New cells are made by the
division of satellite cells. Cells contract when stimulated by nerves. They are said
to be under voluntary control and can be called striated voluntary muscle. Cardiac
muscle is found only in the heart. It is striated like skeletal and arranged in the
same way. It is uninucleate-may have 1-5-centrally located. The cardiocyte is
smaller than skeletal m. cells. Cells are connected to one another via darkened
bands between them called intercalated discs. These are special areas locked
together by desmosomes, gap junctions and intercellular cement. Cardiac muscle
cells cannot divide. There are no satellite cells therefore once heart muscle is
damaged it cannot regenerate. The heart is not under voluntary control and
therefore is called striated involuntary muscle. Smooth muscles cells are small,
spindle shaped with tapering ends. They contain actin and myosin but not arranged
in a striated fashion. The cells are uninucleate. Smooth muscle is found in digestive
and urinary organs, uterus, and blood vessel walls. It can divide after injury.
Smooth muscle contracts on own but can be controlled by the nervous system. It is
not under voluntary control and therefore is often called non-striated involuntary
muscle.
Nervous Tissue
Nerve tissue consists of neurons (nerve cells) and neuralgia cells which protect and
assist the neurons. Neurons are specialized to detect stimuli, respond quickly and
transit information. Each nerve cell has a soma or cell body and one long processthe axon that transmits messages and many smaller projections-dendrites that
receive information.