CELL JUNCTIONS ( in Chapter 4)
tight junctions (form fluid-tight seals between cells), are found in epithelial cells that line stomach, intestines and bladder; they prevent leaks. Very tight junctions are found in capillaries in the brain and testes.
anchoring junctions (fasten cells to one another or the matrix) found
where there is friction and stretching - outer layer of skin, heart muscle, neck
of uterus, and G.I. tract.
A desosome is the most common anchoring junction-like a spot weld
made up of intermediate filaments attached to a dense area of protein. Found in
the heart and muscles of digestive tract.
gap junctions (permit electrical or chemical signals to pass between cells). important in heart and GI muscle, and some parts of the nervous system. Membranes of adjacent cells come within 2-3nm, and proteins called connexons form tiny fluid filled channels. Ions and small molecules can pass directly from one cell cytosol to the other.These are lacking in cancer cells.
Epithelial tissue or Epithelium (plural is epithelia) comes in two basic types:
Covering and lining epithelium and
Glandular epithelium.
Epithelial tissue also combines with nervous tissue to make up special sense
organs for hearing, smell, vision and touch.
Some general characteristics of epithelium are:
Epithelium consists mostly of closely packed cells with little extracellular material.
Epithelial cells are arranged in continuous sheets, in single or multiple layers
Epithelial cells have an apical (free) surface, which is exposed to body cavity, lining of internal organ or exterior of body, and a basal surface which is attached to a basement membrane.
Cell junctions are plentiful, providing secure attachments among cells.
Epithelia are avascular, blood vessels are located in nearby connective tissue; exchange of materials occurs by diffusion.
Epithelia adhere firmly to nearby connective tissue by means of the basement membrane which is made of two layers: the basal lamina contains collagen, protein and proteoglycans secreted by the epithelium.The reticular lamina contains reticular fibers, proteins and glycoproteins secreted by the connective tissue. For physical support, cell attachment, as a filter in the kidney, and a guide for development and tissue repair.
Epithelia have a nerve supply.
Epithelia have a high capacity for renewal (high mitotic rate) since it is subjected to wear and tear.
Epithelia are derived from all three primary germ layers : ectoderm, mesoderm and endoderm.
Epithelial functions include: protection, filtration, lubrication, secretion,
digestion, absorption, excretion, transportation, sensory reception, and
reproduction.
Epithelial layers are arranged as:
1) simple epithelium (one layer), where diffusion, osmosis,
filtration, secretion and absorption occur.
2) stratified epithelium (several layers), protects underlying tissue
from wear
3) pseudostratified epithelium (one layer that appears as several)
because nuclei at different level, not all cells reach the surface, those that
do have cilia or secrete mucus.
Cell shapes include:
1) squamous (flat) for diffusion
2) cuboidal (cubelike) - produce secretions and function in
absorption
3) columnar (rectangular) - protect underlying tissue, may have
cilia, secretion or absorption.
4) transitional (variable) - change in shape from flat to columnar
due to distention, expansion or movement of body parts.
Simple Epithelial Tissue:
Simple squamous epithelium consists of a single layer of flat cells. It is adapted for diffusion, osmosis and filtration and is found in lungs and kidneys. Endothelium lines the heart, blood vessels, and lymph vessels and forms the walls of capillaries. Mesothelium lines the thoracic and abdominopelvic cavities and covers the organs within them.
Simple cuboidal epithelium consists of a single layer of cube-shaped cells. It is adapted for secretion and absorption. It is found covering ovaries, lining the kidney tubules, and lining some glandular ducts.
Simple columnar epithelium comes in two forms:
Nonciliated simple columnar epithelium consists of a single layer of
nonciliated rectangular cells. Also functions in secretion and absorption. It
lines most of the gastrointestinal tract. Specialized cells containing
microvilli perform absorption. Goblet cells secrete mucus, which
lubricates, traps particles, and protects cells ( from digestive enzymes, etc.).
Goblet cells are found in the digestive, respiratory, reproductive and most of
the urinary tract.
Ciliated simple columnar epithelium consists of a single layer of
ciliated rectangular cells. Aids in movement. It is found in a few portions of
the upper respiratory tract where it moves foreign particles trapped in mucus
out of the respiratory tract, and in Fallopian tubes, uterus and central canal
of the spinal cord.
Pseudostratified columnar epithelium has only one layer but gives the
appearance of many. All cells are anchored to the basement membrane, but some do
not reach the surface.
1) Psuedostratified ciliated - has cilia and goblet cells - most upper
respiratory structures. Functions in secretion and movement.
2) Pseudostratified nonciliated has no cilia or goblet cells.
It lines larger excretory ducts, part of male urethra, and epididymis.
Stratified Epithelial Tissue - at least two layers of cells. Named by
shape of the cells in the superficial layer.
1) Stratified squamous epithelium consists of several layers of cells
in which the top layer is flat, bottom layers vary from cuboidal to columnar.
Its function is protection. Basal layers continually replicate. Cells move up,
lose blood supply, cells become shrunken and lose junctions.
a) Nonkeratinized variety lines the mouth, vagina and anus; remains
moist
b) Keratinized variety forms contains protein keratin - outer layer
of skin. waterproof, resistant to friction, helps repel bacteria.
2) Stratified cuboidal epithelium consists of several layers of cells in which the top layer is cube-shaped. Fairly rare. Function is mainly protective. It is found in the larger ducts of the mammary glands, sweat glands, salivary glands and pancreas. Also lines ovarian follicles and seminiferous tubules, and a portion of male urethra.
3) Stratified columnar epithelium consists of several layers of cells in which the top layer is rectangular. Also uncommon. Usually basal layers are shortened, irregular polyhedral cells. It protects and secretes. It is found in a portion of male urethra and vas deferens, parts of the pharynx, and part of the conjunctiva of the eye.
4) Transitional epithelium consists of several layers of cells whose appearance is variable, wether stretched or relaxed. It lines the urinary bladder, and parts of the ureters and urethra. It is capable of stretching. Tight junctions in theses areas prevent leaks and diffusion.
Glandular Epithelium
A gland is a single cell or a mass of epithelial cells adapted for
secretion. Usually found in columnar or cuboidal epithelium.
There are two types depending on how the cells release the substances they
produce: Exocrine glands and endocrine glands.
Exocrine glands secrete into ducts or directly onto a free surface.
Their secretions include mucus, sweat, oil, ear wax and digestive enzymes.
Structural classification of exocrine glands :
multicellular glands - most glands, have a distictive appearance.
These include sweat, oil (sebaceous) and salivary glands. Communicates with the
surface (internal or external) of the body.
unicellular glands - single celled. goblet cells. No ducts. Found in
digestive, respiratory, urinary, and reproductive systems.
Functional classification of glands - depends on wether the secretion
is a product of the cell, or consists of partial or entire cells themselves.
holocrine - accumulate secretions in cell, cell dies, and is
discharged with its contents as a glandular secretion. The discharged
cell is replaced by a new cell. ex. sebaceous or oil gland in skin.
merocrine - most exocrine glands, form secretion and discharge.For
example - salivary glands.
apocrine glands - secretions accumulate near the top, which pinches
off to form secretion. Cell repairs itself and repeats. For example: mammary
glands.
Endocrine glands - called "ductless glands" discharge their secretions into the intracellular fluid, where it diffuses into the blood stream. These secretions are hormones, or chemical messengers, which regulate many bodily functions. You will learn about these in A&P 2.
Connective tissue is the most abundant and widely distributed tissue in the body .
All connective tissues have the same
embryonic origin- from mesodermal cells called mesenchyme.
The functions of connective tissue are:
1. binding, attachment and support (bones, tendons and
ligaments, organs)
2. protection- bones and adipose tissue
3. insulation - adipose tissue
4. storage - of energy and fat soluble substances
5. transportation - blood
Some general characteristics of connective tissue are:
1) It consists of three basic elements :
1)cells
2)ground
substance
3)fibers
ground substance and fibers are extracellular substances called the matrix
2) does not occur on free surfaces such as linings, exept for the joint
cavities which are lined
by areolar connective tissue.
3) Except for cartilage, it has a nerve supply.
4) Usually highly vascular, except for cartilage (avascular) and tendons
which have a small blood supply.
5) matrix may be fluid, semifluid, gelatinous, fibrous, or calcified.
It is usually secreted by the connective tissue cells (except for blood) and adjacent cells, and determines the qualities of the tissue: liquid, flexible or hard.
Cells of Connective Tissue:
Immature cell names end in - blast.
These cells can reproduce and form the matrix.
Mature cell names end in -cyte.
These cells have a reduced ability to divide
and form matrix.
Fixed cells - appear in tissues in stable
numbers
Wandering cells - found in tissues only in
response to infection or injury
Fibroblasts - most common type of fixed cell. They secrete the matrix;
particularly the fibers of the matrix.
Macrophages develop from monocytes.
Phagocytize bacteria or cell debris. Wandering macrophages leave the
blood and wander to infected tissues , while fixed macrophages remain in
certain tissues and organs. Important in infection and immunity.
Plasma cells - develop from WBC' s called B
cells or B lymphocytes. They secrete antibodies and are found primarily in
the blood stream and the spleen, lymph nodes and gastrointestinal tract.
Mast cells are found along blood vessels and
produce histamine, which dilates small blood vessels during inflammation. They
contain heparin, an anticoagulant. Also involved in allergic responses.
Adipocytes - store triglycerides. Insulate and cushion
Cells specific to a particular type of connective tissue:
Osteocytes (osteoblasts) - bone
Chondrocytes (chondroblasts) - cartilage
leukocytes and erythrocytes ( hemocytoblast)- blood
Fibers:
Collagen fibers made of collagen. Collagen fibers
are tough and only slightly elastic.. They often occur in bundles with the
fibers parallel to one another, which gives great tensile strength. Collagen is
found in most connective tissues, esp. bone, tendons, and ligaments.
Elastic fibers are composed of a protein called
elastin and glycoprotein framework. They are very stretchy and branch and join
to form a network. They provide strength to tissues, but allows the tissue to be
flexible and stretchy. They are found in skin, blood vessels, and lungs.
Reticular fibers are composed of collagen and
glycoprotein. They provide support and form loose networks of fibrous tissue.
They are found around fat cells, nerve fibers, and skeletal and smooth muscle
cells. They also form the stroma or supporting framework of many soft,
nonmuscular organs such as the spleen and lymph nodes. These fibers also help
form the basement membrane.
Ground substance can contain a variety of large
molecules. Some are proteoglycans and glycosaminoglycans, which are
complex combinations of polysaccharides and proteins. Some of these are
hylauronic acid , chondroitin sulfate, and keratan sulfate.
Other molecules in the matrix are adhesion proteins. Adhesion
proteins interact with receptors on plasma membranes to anchor cells in position
and to provide traction for movement of cells.
The ground substance supports and binds cells together, and provides a medium for the exchange of materials.
Types of Connective Tissues:
Loose Connective Tissues:
areolar connective tissue - It is loosely
arranged, it is found wrapping organs and under skin, and holds and conveys
tissue fluid. Contains primarily fibroblasts, but also macrophages,
adipocytes and some blood cells. It contains all three fiber types, randomly
arranged. The ground substance is fluid, semifluid or gelatinous. This tissue
contains hyaluronic acid, which is thick and may slow the passage of some drugs.
Hylauronidase is an enzyme produced by wbc's, sperm and some bacteria.
Injection of hyaluronidase can speed the passage of drugs and fluid through the
tissue.
adipose tissue- cells are adipocytes, and
specialize in storing fat. Adipocytes are tightly packed, and very little matrix
. They accumulate under the skin and yellow marrow of long bones. Functions in
energy reserves, insulation, protection, and support.
reticular connective tissue - consists of
reticular fibers and fibroblasts and forms the supporting framwork for cells in
the liver, spleen, lymph nodes and bone marrow.
Dense connective tissue has fibers as the dominant component.
Dense regular connective tissue- collagen fibers
are arranged in parallel bundles which makes this tissue flexible, but resistant
to stretching. Fibroblasts are found in rows between the fibers. Found in
tendons and ligaments. Blood supply is poor.
Dense irregular connective tissue - also has
collagen fibers and fibroblasts, but the collagen fibers are thicker and
arranged irregularly. This tissue usually forms sheets, and resists pulling in
many directions. It is found in heart valves, around cartilage, bone, muscles,
dermis of skin, and around some organs.
Elastic connective tissue consists of elastic
fibers and fibroblasts. Very elastic. It is found in the lungs, walls of
arteries, bronchial tubes and in the attachments between the vertebrae.
Cartilage has a dense network of collagen
fibers, which gives strength, and elastic fibers. The matrix contains
chondroitin sulfate, which is rubbery and gives cartilage resilience. Cells are
chondrocytes, and are found in lacuanae. Surface of cartilage is
surrounded by perichondrium, where blood vessels are located.
Hyaline cartilage -
(gristle) most abundant cartilage, and weakest. It has very fine
collagen fibers and a resiliant gel as its ground substance. It is found in the
embryonic skeleton, at the ends of bones , in the nose, and in respiratory
structures. It is flexible, allows movement, reduces friction, absorbs shock and
provides support. Important in bone growth and repair.
Elastic cartilage - has condrocytes in a network
of elastic fibers. It maintains the shape of organs such as the epiglottis of
the larynx, auditory (Eustachian) tubes, and external ear.
Fibrocartilage contains visible bundles of
collagen fibers and combines strength and rigidity. Also acts as a shock
absorber. It is found in the pubic symphysis, intervertebral discs, and menisci
(cartilage pads) in joints. It is the strongest cartilage.
Bone (osseous tissue) consists of collagen fibers (flexibility) , mineral salts (that contribute to the hardness of bone) and cells called osteocytes. It is covered by the periosteum and lined by the endosteum.. It can be compact or spongy depending on how the matrix and the cells are organized. The basic unit of compact bone is the osteon or Haversian system. It supports, protects, helps provide movement, stores minerals, and houses blood-forming tissue.
Blood (vascular tissue) consists of plasma and formed elements (erythrocytes, leukocytes, and platelets). Functionally, its cells transport oxygen and carbon dioxide, carry on phagocytosis, participate in allergic reactions, provide immunity, and bring about blood clotting.