reticular lamina, a layer of extracellular material containing a fine
network of collagen protein fibers that ?belong to? the underlying
connective tissue. Together the two laminae form the membrane
basement. The basement membrane reinforces the epithelial sheet,
helping it to resist stretching & tearing forces, & defines the epithelial
boundary.
An important characteristic of cancerous epithelial cells is their
failure to respect this boundary, which they penetrate to invade the
tissue beneath.
Innervated but avascular. – Although epithelium is innervated (supplied
by nerve fibers), it is avascular (contains no blood vessels). Epithelial
cells are nourished by substances diffusing from blood vessels in the
underlying connective tissue.
Regeneration. – Epithelium has a high regenerative capacity. Some
epithelia are exposed to friction & their surface cells removed by
abrasion. Others are damaged by hostile substances in the external
environment (bacteria, acid, smoke). As long as epithelial cells receive
adequate nutrition, they can replace lost cells rapidly by cell division.
Classwork (pgs. 115-118) February 16, 1999
?Stratified & Glandular Epithelia?
Stratified Epithelia
Stratified epithelia consists of two or more cell layers.
Stratified Squamous Epithelium – is the most widespread of the
stratified; found in the exterior part of the skin.
Stratified Cuboidal & Stratified Columnar – are rare; usually found in
large ducts & some glands.
Transitional Epithelium – found in the lining of urinary organs.
Transitional epithelium can change shape in order to stretch.
Glandular Epithelia
Epithelium of the glandular type is specialized for secretory activity. All
glands are classified as exocrine or endocrine.
Exocrine glands – discharge their secretory products into ducts (ex.
salivary glands)
Endocrine glands – are ductless; they discharge their secretions
directly: hormones.
Multicellular exocrine glands have two structural elements: ducts &
secretory units. On the basis of their duct structures they are either
simple glands – single unbranched ducts or compound glands – that have a
branched duct. Then they are further described according to their
secretory parts:
Tubular – forms tubes.
Alveolar – small flask like sacs.
Tubuloalveolar – both.
Functional Classifications of Exocrine Glands.
Methods by which they discharge. Three types:
Apocrine Glands – collect their products near the tips of the cell & then
they release into a duct by pinching of (ex. mammary glands).
Holocrine Glands – collect inside the cells & then they rupture (ex.
sedaceous (oil) glands).
Merocrine Glands – discharge directly through the cell membrane (ex.
salivary glands).
Homework (pgs. 119) February 16, 1999
?Unicellular Exocrine Glands?
Unicellular exocrine glands are single cells scattered in am epithelial sheet
amid cells with other functions. They have no ducts. In humans, all such
glands produce mucin, a complex glycoprotein that dissolves in water
when secreted. Once dissolved, mucin forms mucus, a slimy coating that
both protects & lubricates surfaces. The only important unicellular glands
in humans are the goblet cells found sprinkled in the columnar epithelium
cells lining the intestinal & respiratory tracts. Although unicellular glands
probably outnumber multicellular glands, unicellular glands are the less
well known of the two glands types.
Classwork (pgs. 119, 122-126) February 17, 1999
?Connective Tissue?
Connective Tissue is the most abundant tissue. Its major functions are:
Binding & Support
Protection
Insulation & Blood
Transportation
Common Characteristics of Connective Tissue
Common origin – derived from the mesoderm.
Degrees of vascularity; some are vascularized, others are not.
Extracellular matrix – this separates the living cells of the tissue.
Two Classes of Connective Tissue
The first is divided into four groups.
Loose Ordinary Tissue (Areolar) – found between other tissues or
other organs; used in connection; it is a fluid.
Adipose Tissue (Fat) – found under the skin & as padding at various
points. Used for protection, insulation, & a reserve for food.
Reticular Tissue – slender branching of reticular fibers forms the
framework for the spleen, lymph nodes, & bone marrow; look like
little strings that run in all directions.
Dense Fibrous Tissue – tendons & ligaments; they are bundles or
callagenous fibers in parallel rows in a fluid matrix; they are thicker
strings that run in one direction.
The second class of connective tissue contains cartilage – has qualities
intermediate between dense fibrous connective tissue & bone. It is
avascular (no bloods run through it) & has no nerves.
Hyaline Cartilage – is the most abundant tissue type in the body;
provides firm support with some pliability.
Elastic Cartilage – nearly identically like hyaline cartilage, but has
more elastin fibers which gives this tissue a greater tolerance for
repeated bending.
Fibrocartilage – (fibrous cartilage) often found where hyaline
cartilage meets a true ligament or tendon. Found where strong
support & ability to withstand heavy pressure are required.
Homework (pgs. 120-122) February 17, 1999
?Structural Elements of Connective Tissue?
Connective tissues have three main elements: ground substance, fibers, &
cells. Ground substances make up the extracellular matrix. (Note: that the
term matrix indicates the ground substance.)
Ground Substance
Ground Substance – is an amorphous (unstructured) material that fills the
space between the cells & contains the fibers. It is composed of instertitial
fluid, cell adhesion proteins, & proteoglycans. Cell adhesion proteins, a
group that includes fibronectin & lamina, sever mainly as a connective
tissue glue that allows connective tissue cells to attach themselves to
matrix elements. The proteoglycans consists of a protein core to which
glycosaminoglycans (GAGs) are attracted. The strand-like GAGs which are
large, negatively charged polysaccharides, stick out from the core protein
like the fibers of a bottle brush. Important examples of GAGs in
connective tissues are chondroitin sulfate, keratan sulfates, & hyaluronic
acid. The GAGs intertwine & trap water, forming a substance that varies
from a fluid to a semi-stiff hydrated gel. The relative amounts & kinds of
GAGs help determine the properties of the matrix. Example – The higher
the GAG content, the stiffer the ground substance is.
The ground substance holds fluids & functions as a molecular sieve, or
medium, through which nutrients & other dissolved substances can diffuse
between the blood capillaries & the cells. The fibers embedded in the
ground substance makes it less pliable & impede diffusion somewhat.
Fibers
The fibers of the connective tissue provide support. Three types of fibers
are found in connective tissue matrix:
Collagen Fibers – (white fibers), are constructed primarily of the
fibrous protein collagen. Collagen molecules are secreted into the
extracellular space, where the are assembled spontaneously into
cross-linked fibers. Collagen fibers are extremely tough & provide high
tensile strength to the matrix. Stress test show that collagen fibers are
stronger than steel fibers of the same size. Collagen fibers are the
most abundant.
Elastic Fibers – (yellow fibers), are formed largely from another fibrous
protein, elastin. Elastin has a randomly coiled structure that allows it
to stretch & recoil like a rubber band. The presence of elastin in the
matrix gives it a rubbery, or resilient, quality. Connective tissue can
stretch only so much before its thick, rope-like collagen fibers become
taut. Then, when the tension lets up, elastic fibers snap the connective
tissue back to its normal length & shape. Elastic fibers are found where
greater elasticity is needed (ex. skin, lungs, & blood vessel walls).
Reticular Fiber – are fine callagenous fibers & are continuous with
collagen fibers. They branch extensively, forming delicate networks
that surround small blood vessels & support the soft tissue of organs.
They are particularly abundant where connective tissue abuts other
tissue types, for example, in the basement membranes of epithelial
tissues, & around capillaries, where they form fuzzy ?nets.?
Cells
Each major class of connective tissue has a fundamental cell type that
exists in immature & mature forms. The undifferentiated cells, indicated
by the suffix blast, are actively mitotic cells that secrete the ground
substance & the fibers characteristics of their particular matrix. The
primary blast cell types by connective tissue class are:
Connective tissue proper: fibroblast.
Cartilage: chondroblast.
Bone: osteoblast.
Blood: hemocytoblast or hematopoietic stem cell.
Once they synthesize the matrix, the blast cells assume their less active,
mature mode, indicated by the suffix cyte. The mature cells maintain the
health of the matrix. However, if the matrix is injured, they can easily
revert to their more active state to repair & regenerate the matrix. (The
hemocytoblast, the blood-forming stem cell found in bone marrow, always
remains actively mitotic.)
Additionally, connective tissue is home to an assortment of other cell
types, such as nutrient-storing fat cells & mobile cells that migrate into the
connective tissue matrix from the bloodstream. The latter include white
blood cells (neutrophils, eosinophils, lymphocytes) & other cell types
concerned with tissue response to injury, such as mast cells,
macrophages, & antibody-producing plasma cells. This wide variety of
cells is particularly obvious in our prototype, areolar connective tissue.
The oval mast cells are typically found clustered in tissue spaces deep to
an epithelium or along blood vessels. These cells act as sensitive sentinels
to detect foreign substances (ex, bacteria, fungi) & initiate local
inflammatory responses against them. In the mast cell cytoplasm are
conspicuous secretory granules containing:
Heparin – an anticoagulant (a chemical that prevents the blood clotting)
when free in the bloodstream, but its significance in human mast cells
is uncertain
Histamine – is released during inflammatory reactions, makes the
capillaries leaky.
Macrophages are large, irregularly shaped cells that avidly phagocytize a
broad variety of foreign materials, ranging from foreign molecules to
entire bacteria to dust particles. Macrophages also engulf & dispose of
dead tissue cells, & they are central actors in the immune system. In
connective tissues, they may be attached to connective tissue fibers or
they may migrate freely through the matrix. Macrophages are peppered
throughout loose connective tissue, bone marrow, & lymphatic tissue.
Those in certain sites are given specific names. They are called histiocytes
in loose connective tissue, Kupffer cells in the liver, & microglial cells in the
brain. Although all these cells are phagocytes, some have selective
appetites. For example, macrophages of the spleen primarily dispose of
aging red blood cells, but they will not turn down other ?delicacies? that
come their way.
Classwork (pgs. 128-134) February 18, 1999
?Bone, Blood, Membranes, Nervous, & Muscle Tissues?
Bone (osseous) – due to its rock hardness it has the ability to support
& to protect softer tissue.
Blood – the fluid within blood vessels; functions as the transport vehicle
in the cardiovascular system.
Membranes – a continuous multicellular sheet composed of at least
two primary tissue types: an epithelium bound to an underlying layer of
connective tissue proper.
Cutaneous Membranes – are your skin.
Mucous Membranes – lines cavities open to the exterior.
Serous Membranes – are moist membranes found in closed ventral
body cavities (ex. pericardium, pleura).
Nervous Tissue – Has two major cell types.
Neurons – specialized cells that generate & conduct nerve
impulses.
Neuralgia – are supporting cells.
Muscle Tissue – made up of muscle fibers. Muscle cells possess
mylofilaments.
Skeletal Muscle – attached to bone, voluntary or stratified (lines);
form the flesh of the body.
Cardiac Muscle – occurs in the heart, it is striated & contains
intercalated discs ( junctions of branching cells).
Smooth Muscle – visceral or involuntary; found in hallow internal
organs.
Homework (pgs. 136) February 18, 1999
?Steps of Tissue Repair?
Tissue repair requires that cells divide & crawl, activities that are initiated
by growth factors (wound hormones) released by injured cells. It occurs in
two major ways: by regeneration – the replacement of destroyed tissue
with the same kind of tissue- & by fibrosis – involves proliferation of
fibrous connective tissue called scar tissue. Each of these occurs depends
on:
The types of tissue damaged.
The severity of the injury.
In skin, the tissue we will use as our example, repair involves both
activities.
Inflammation sets the stage. The process begins while the
inflammatory reaction is still going on. Let us briefly examine what has
happened up to this point. Tissue injury sets the following
inflammatory events into motion. First, because of the release of
histamine & other inflammatory chemicals by injured tissue cells,
macrophages, mast cells, & others, the capillaries dilate & become very
permeable. This allows white blood cells & plasma fluid rich in clotting
proteins, antibodies, & other substances to seep into the injured area.
Then the leaked clotting proteins construct a clot, which stops the loss
of blood, holds the edges of the wound together, & effectively walls off,
or isolates, the injured area, preventing bacteria, toxins, or other
harmful substances from spreading to surrounding tissues. The
portion of the clot exposed to the air quickly dries & hardens, forming a
scab. The inflammatory events leave excess fluid, bits of destroyed
cells, & other debris in the area. Most of this material is essentially
removed from the area via lymphatic vessels or phagocytized by
macrophages. At this point, the first step of tissue repair,
organization, begins.
Organization restores the blood supply. During organization the
temporary blood clot is replaced by granulation tissue. Granulation
tissue is a delicate pink tissue composed of several elements. Thin,
extremely permeable capillaries bud from intact capillaries nearby &
enter the damaged area, laying down a new capillary bed; they
protrude nub-like from the surface of the granulation tissue, giving it a
granular appearance. These capillaries are fragile & breed freely, as
demonstrated when someone ?picks at? a scab. Also present in
granulation tissue are scattered macrophages & fibroblasts that
synthesize new collagen fibers to bridge the gap permanently. As
organization continues, macrophages digest & remove the original
blood clot. The granulation tissue, destined to become scar tissue (a
permanent fibrous tissue patch), is highly resistant to infection
because it produces bacteria-inhibiting substances.
Regeneration &/or fibrosis effects permanent repair. While
organization is going on, the surface epithelium begins to regenerate.
Epithelial cells migrate across the granulation tissue just beneath the
scab, which soon detaches. As the fibrous tissue beneath matures &
contracts, the regenerating epithelium thickens until it finally
resembles than of the adjacent skin. The end result is a fully
regenerated epithelium, & an underlying region of scar tissue. The scar
may be invisible, or visible as a white thin line, depending on the
severity of the wound.
Classwork (pgs. 189-203) February 24, 1999
?Skeletal System?
The adult Skeleton has 206 separate bones. There are two main divisions:
Axial Skeleton – has 80 bones; the upper axis has 74; the inner ear has
6. Contains the skull. There are two major divisions: