Histology 1 HISTOLOGY: THE FORM AND FUNCTION OF TISSUES
Objectives:
1.
2.
2.
To gain an appreciation of tissue types in terms of their functional significance.
To learn how to recognize and name the featured cell and tissue types.
To gain experience with the use of a compound microscope.
Readings and resources:
1.
2.
3.
Appendix M - Use of the Light Microscope.
Appendix H - Histology Photomicrographs PDF.
Tissues - Life: The Science of Biology, 10th edition., by Sadava, et al, pp 817-820
Grade: Post-lab quiz to be taken on your own at the end of lab. Value: 20 points. Note that this quiz
covers a lot of material, and may be difficult for some. It is highly recommended that you study the
information in this handout and both appendices thoroughly before coming to lab. Your goal is to know
the name of each tissue and cell type, its appearance, form and function by the end of lab.
INTRODUCTION
Anatomy is the branch of science that addresses the external form and internal organization of
organisms. The architectural principles upon which living organisms are built are important as is the
understanding of the structural basis for the functioning of the various parts. Developmental mechanisms
are also of fundamental importance.
We can divide the field of anatomy into two sub-disciplines; namely: gross anatomy, or the anatomy of
large structures, and microscopic anatomy, known as histology. You will become acquainted with gross
anatomy in later labs. In contrast to gross anatomy, histology involves the examination of the various
structures too small to be seen with the naked eye. The study of these very small structures requires
special optical microscope systems and specific staining and visualization techniques.
In this lab, you will observe normal mammalian tissues under the compound microscope. Review
Appendix M to make sure that you now know the parts of the microscope, which should help you to
obtain the best results. Your TF will provide a hands-on tutorial for the particular model of microscope
you will find in your lab room.
Each pair is provided a box of prepared microscope slides. Please handle them with care. Be sure to
close the cover on the box when you are done looking at slides; stains fade in the light and prolonged
exposure will result in deterioration of the specimen. Also please keep the slides in numerical order
when putting them back.
2 Remember as you study the slides that your objective is to be able to recognize each cell or tissue type
and associate it with its major characteristics. You should make sketches or notes of your observations.
Spend no more than 5-7 minutes per slide if you want to get through them all. After you have gone
through the slides once, quiz your lab partners.
STAINING:
The most commonly employed staining technique in animal histology is the use of hematoxylin and
eosin together ("H&E"). The basic dye hematoxylin stains acidic structures a purplish blue; those
structures are basophilic. Nuclei and rough endoplasmic reticulum, for example, both have a strong
affinity for this dye owing to their high content of DNA and RNA respectively. In contrast, eosin is an
acid dye which stains basic structures (therefore acidophilic structures) red or pink. Most cytoplasmic
proteins are basic and hence cytoplasm generally stains pink or pinkish red. In general, when the H&E
staining technique is applied to animal cells, nuclei stain blue and cytoplasm stains pink or red.
MAMMALIAN TISSUES: FORM AND FUNCTION
As an example of a mammal, recent studies estimate that the human body has approximately 37.2
trillion living eukaryotic cells (this excludes prokaryotes in the microflora, which vastly outnumber the
host’s larger cells). Each day a human may lose 10,000 brain cells and 50 million skin cells, with many
of these dying cells being replaced by new cells. There are about 200 types of eukaryotic cells in an
adult human, which are grouped into four main categories: epithelial tissue, connective tissue, muscle
tissue, and nerve tissue. It should be mentioned that this classification is based on mammals and related
vertebrates, and that its application to other animals is often not relevant.
The classification of animal tissues can be summarized as follows:
I. Epithelium
A. Simple epithelium (squamous, cuboidal, or columnar)
B. Stratified epithelium (stratified squamous, stratified cuboidal)
C. Others: transitional and pseudostratified ciliated columnar
II. Connective Tissue
A. Vascular tissue (blood, lymph)
B. Connective tissue proper (loose connective tissue, dense connective tissue)
C. Cartilage
D. Bone
III. Muscle
A. Skeletal
B. Smooth
C. Cardiac
IV. Nerve
Epithelial Tissue
Epithelial tissues form the covering or lining of all body surfaces, cavities and tubes, both internal and
external, in animals. Epithelial tissues function in protection and absorption, forming sheets covering a
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surface; in secretion, forming glands; and in excretion. Examples include skin (epidermis), glands, and
the linings of the digestive tract and blood vessels.
The cells of epithelial tissues are tightly packed together, with very little intercellular materials or
spaces. Epithelial cells are customarily divided into three principal types: flattened or squamous, cubeshaped or cuboidal, (when viewed in a section perpendicular to the tissue surface), and columnar.
Epithelial tissue may be only one cell thick, in which case it is called simple epithelium, or it may be
two or more cells thick and called stratified epithelium. The various types of epithelia are named on the
basis of the cell type and the number of cell layers (it is the cells of the outermost layer that determine
the name of the stratified epithelia). Thus we can recognize simple squamous epithelium, simple
cuboidal epithelium, simple columnar epithelium, stratified squamous epithelium, etc. Epithelium,
regardless of type, is usually separated from the underlying connective tissue by an extracellular
basement membrane. The basement membrane is not penetrated by blood vessels; epithelia are thus
dependent on the diffusion of oxygen and metabolites from underlying tissues. Both epithelial and
connective tissue cells are thought to participate in the formation of the basement membrane.
Figure 1. Different shapes and arrangements of epithelia.
4 Slide 1. Squamous epithelial cells from cheek. You will use a sterile toothpick to take a scraping of
stratified squamous epithelial cells from the lining inside of your mouth, and stain the cells with
Toluidine blue, an acid loving dye.
Cheek Cell Staining Protocol:
1) Drag the flat side of a sterile toothpick along the inside of your mouth on one side,
wiggling it up and down briefly.
2) Drag the same side of the toothpick on a fresh glass slide to transfer cells onto the
surface. You should be able to see hazy, grainy patches of cells on the slide.
3) Allow the slide to dry 5-10 minutes. While this is occurring go on to look at a prepared
histology slide or two.
4) Drop 1-2 drops of Toluidine Blue solution onto a patch of cells on the slide. Allow the stain to
sit 4-5 minutes to penetrate into the cells.
5) Rinse the excess dye off using a deionized water squirt bottle, draining the slide over a waste
beaker or sink. The cells should now be visibly bluish-purple on the side of the slide where you
transferred them.
6) Blot the back of the slide, so that it’s free of water and dye. Make sure to not touch the side
with the cells. You do not need to put a cover slip over the cells. The slide is now ready to use on
your microscope.
What shape do these cells appear? Would they look different if you rotated one side toward you? Where
is the toluidine blue dye most concentrated in the cells? Why?
Slide 2. Kidney, thin section. The kidney contains two distinct types of epithelium:
Simple squamous epithelium. This tissue is found only in specific parts of the slide. Look for circular
clumps of cells near the outer or cortex part of the kidney (near one outer edge of your specimen). Each
of these structures is a spherical capillary bed called a Glomerulus which is surrounded by a fluid space
bound by a single layer of simple squamous epithelial cells called the Bowman’s Capsule. Find a
glomerulus and center it in the 10X objective’s field of view, than switch to the 40x objective. Notice
the capillary bed, the lumen (the clear space) around it and at the edge of the lumen is the simple
squamous epithelium. These cells are the viewed on their thin edge. These cells act as barriers
containing the fluid leaking from the capillaries, that will be subsequently be concentrated into urine by
the simple cuboidal epithelium.
Simple cuboidal epithelium. This tissue type is the majority of the tissue in the specimen. Simple
cuboidal epithelial cells are approximately the same dimensions in each direction or roughly cube
shaped, though in cross section the cells appear square to polygonal in shape. Nuclei are usually round
or circular in cross section. The function of the cells you see here is concentration of urine. Non-kidney
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examples of this tissue are found in salivary glands and pancreas. Simple cuboidal epithelium usually
lines small ducts and tubules which may have excretory, secretory, or absorptive functions.
Slide 3. Stratified squamous epithelium, thin section. The major function of this type of epithelium is
protection against mechanical abrasion. This section shows several layers of epithelial cells and the
basement membrane. Cells closest to the basement membrane appear cuboidal, and here the tissue’s
(adult type) stem cells are actively dividing. However the outermost layer of epithelia consists of
squamous shaped cells, so this epithelium overall is named stratified squamous epithelium, even though
the innermost cell layers are not squamous in shape. This type of tissue is found in the oral cavity,
pharynx, esophagus, anal canal and vagina, and the oral version is what you collected with a toothpick to
make your Slide #1.
Note: ALWAYS switch back to your 10X objective lens before changing slides. The
end of the 40X objective will be very close to the slide, and you risk damaging the
lens or slide if you change slides with the 40X lens in the working position. Also,
because of its wider field of view the 10x objective is much more appropriate for
searching around on a new slide.
Slide 4. Simple columnar epithelium, thin section (duodenum, cross section). At 10x magnification,
search for an area of the slide that has finger-like projections surrounded by a lumen - these are the villi.
Switch to 40x. Cells are tall and columnar, at right angles to the basement membrane. The nuclei are
elongated and may be arranged toward the base, the center or occasionally the apex. Simple columnar
epithelium is most often found on highly absorptive surfaces such as in the small intestine, although it
may constitute the lining of highly secretory surfaces such as that of the stomach.
Slide 5. Pseudostratified ciliated columnar epithelium (thin section) gives one the erroneous
impression that there is more than one layer of cells. However, all the cells rest on the basement
membrane (although not all cells extend to the luminal surface), so it is a simple epithelium. The nuclei
are disposed at different levels, thus creating the illusion of cellular stratification. Notice the border of
cilia on the luminal side (luminal is the adjective form of the noun lumen). This type of tissue is almost
exclusively confined to the larger airways of the respiratory system (i.e., trachea) and therefore often is
referred to as respiratory epithelium. What do you suppose is the function of this tissue?
Slide 6. Stratified squamous keratinizing epithelium, thin section. This specialized form of epithelial
tissue constitutes the surface layer of the skin and is adapted to withstand constant abrasion and
desiccation; the outer cells of the skin have become engorged with the protein keratin, they lack nuclei,
appear scale-like, and are actually dead cells. Deeper in the tissue are cells of this tissue with lesser
6 amounts of keratin, and pick up more blue stain. They have nuclei and were alive before sample
harvesting. In contrast, the tough non-living surface of skin is composed of the protein keratin, and the
remnants of degenerate epithelial cells. The dead cells rub off into the environment, but are replaced by
stem cells living in pockets close to the tissue’s basement membrane. The stem cells divide by mitosis,
pushing the maturing epithelial cells out toward the surface.
Slide 7. Transitional epithelium, thin section (cross section of a ureter). This form of stratified
epithelium is observed mostly to the urinary tract in mammals (superior urethra, bladder, ureters, but
also in the prostate in male mammals), where it is highly specialized to accommodate a great degree of
stretch and to withstand the mild toxicity of urine. Transitional epithelium is so-named because over
time it exhibits cell shapes from cuboidal to squamous in shape, depending on the amount of pressure
the lumen is experiencing. Under high pressure the lumen will expand and the cells flatten and stretch
out into a squamous shape. In the relaxed state, as seen in most of the prepared ureter slides, this tissue
appears to be stratified, approximately 4-5 cells thick. Basal cells are roughly cuboidal, intermediate
cells are more polygonal, and the luminal surface cells may be large and rounded. Notice that the plasma
membranes of the surface cells are quite thick to provide a permeability barrier. Notice the two layers of
smooth muscle surrounding the epithelia; the inner layer is longitudinal and the outer layer is circular.
The next layer of the section is loose connective tissue and the outermost layer is adipose tissue (fat
cells). NOTE: You will look at slide #7 again for adipose ( #11).
Connective Tissue
Connective tissue, with its many varieties, is the most widespread and abundant tissue in the body. It
surrounds other tissues, encases internal organs, sheathes muscles, wraps bones, encloses joints,
composes the blood, and forms the supportive framework for all organs. Structures made of connective
tissue differ widely. Delicate tissue-paper webs, strong tough cords, rigid bones, liquid blood - all are
made of connective tissue. By definition connective tissue consist of three elements: cells, extracellular
matrix (material outside of and between cells), and fibers (also extracellular, these are large proteins that
help give the tissue rigidity or flexibility). You will see one exception to this definition in adipose tissue.
One of the major differences between connective and epithelial tissues is the proportionate relationship
between cells and extracellular material. Cells predominate in epithelial tissues, with very little material
outside the cells. In connective tissues proper the reverse is true: there is a larger amount of extracellular
matrix and fiber material and a small proportion of cells by volume.
The most common cells in connective tissues are fibroblasts. They are responsible for the maintenance
of the integrity of many connective tissues by repairing damage over time, slowly replacing extracellular
fibers and the gelatinous matrix between them.
Histology 7 Extracellular Matrix is composed of the intercellular substances secreted by the connective tissue cells.
This matrix allows for the diffusion of metabolites and is an important barrier to the spread of
microorganisms. Matrices are composed of ground substances, such as hyaluronic acid, structural
glycoproteins and interstitial fluid. Together with the embedded extracellular fibers the extracellular
matrix of a connective tissue may be liquid, semisolid, or solid.
The most common types of connective tissue fibers are collagen and elastin. Collagen is the principal
fiber type found in the matrix of most connective tissues and is the most abundant protein in the body. It
is found in loose fibrous connective tissue, skin, tendons, ligaments and bone, in various arrangements
from loose to dense. Parallel collagen fibers are often arranged into strong bundles which confer great
tensile strength to the tissue those bundles are sometimes visible under the light microscope, but
sometimes they are packed so close together they are not individually distinguishable. Collagen is
secreted into the matrix in its precursor form of tropocollagen; in the matrix, the tropocollagen
molecules polymerize to form collagen. Elastin is a rubber-like material which is arranged as fibers and
discontinuous sheets in the matrix particularly of skin, lung, and blood vessels. Elastin fibers impart
elasticity (or stretchiness sand springiness) to connective tissues. Like collagen, elastin is synthesized by
fibroblasts in a precursor form known as tropoelastin which undergoes polymerization after secretion.
Connective Tissue with Liquid Matrices:
Slide 8. Human blood, smear. Blood and lymph are good examples of connective tissues with liquid
matrices. In blood, the erythrocytes, or red blood cells, and the leukocytes, or white blood cells, are
floating in the plasma, which is the liquid matrix. The fibers in blood are not collagen and elastin, but
the soluble protein fibrinogen. When clotting occurs, a part of each fibrinogen molecule is cleaved off
by the protease thrombin. The cleaved protein the polymerizes, subunits joining together into long,
insoluble fibrin protein fibers, which contribute to the structure of the blood clot. Notice the cells are
predominantly erythrocytes with a few purple-stained leukocytes randomly interspersed. (You'll notice
that the leukocytes have different appearances - there are several classes of them.) Also, small platelets
can be seen, which appear as tiny purple dots between cells, platelets are also involved in blood clot
formation. Question: why don't erythrocytes pick up the purple stain?
Connective Tissue with Semisolid Matrices:
Slide 9. Loose fibrous connective tissue or areolar tissue (spread) is widespread throughout the body,
functioning to bind together the individual cells of muscles and nerves, to bind organs together and hold
them in place, etc. In other words, it acts as a biological packing material between other tissues of more
specific functions. The matrix includes some large, tough collagen fibers (pink stained) and some
thinner elastin fibers (blue stained); both types of fibers are produced by the purple stained fibroblasts.
Notice that only the nuclei of the fibroblasts stain. The cytoplasm of these cells resists staining, thus
most of their cell bodies remain invisible in this slide. As you can see, in loose connective tissue, the
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fibers are not packed tightly together and they are oriented in many different directions. However, this is
a spread of tissue, not a thin section so the fibers are closer together in a live mammal than in the
prepared slide.
Slide 10. Tendon, long section, thin section. Tendons, which
connect muscles to bones and ligaments, which connects bones
to other bones, are composed of dense fibrous connective
tissue. This tissue differs from loose fibrous connective tissue
in that its elastin and collagen fibers are tightly packed together
and are all oriented in the same direction, thus increasing
strength and elasticity in that direction. These fibers are wavy
in shape, and are closely packed in bundles, giving the tissue a
characteristic wavy appearance. Between the bundles may be
seen the flattened fibroblasts arranged in rows.
Figure 2. Slide 10, Tendon (350X).
Number 11. !"#$"%&'($)*$+,-.//$/(,0&.1$.2$3$4-(0(-56$07(-($&/$89$"%&'($)::;$2%%3)4%5)$67)89":%#7)
%;)$67)%<$75)79'7#)%4)$67)#:7=">7;?)485$67#$)45%>)$67)&<>7;)%4)$67)<57$75@)Adipose or fat tissue is a
unique type of connective tissue. Unlike all the other connective tissues in this lab, adipose tissue is
almost all cellular, with very little extracellular collagen in its tiny amount of extracellular matrix. It is
grouped with other connective tissue because it plays a similar function in mammals, acting to connect
dissimilar tissue types. Often adipose encases parts of organs, such as here around the ureter, where it
acts as a protective, cushioning sheath, helping to prevent the ureter from being crushed. Under the
microscope, cross sections of adipose cells (adipocytes) appear to have a ring-like shape. The
triglycerides in adipocytes are an important energy reservoir for the organism. Before being dissolved
away in the manufacturing process for your histology slide, triglycerides (lipids, fats) occupied a large
part of the contents of each adipocyte, so that only a small margin of cytoplasm, cell membrane and the
cell’s nucleus can be seen. The nucleus will appear to be missing in many adipocytes. Why might this
be?
Connective Tissues with Solid Matrices:
Slide 12. Hyaline cartilage, thin section. (Look at the center of the section.) This type of cartilage is
the most abundant form in the body. It has a firm rubbery texture and is composed mostly of
extracellular materials: many tightly packed collagen fibers, some elastin fibers, and a matrix high in the
polymerized glycosaminoglycan hyaluronic acid or hyaluronan. Cartilage's cells are named
chondrocytes, a specialized sort of fibroblast are located in small spaces called lacunae, which are
scattered throughout the matrix. Cartilage varies in its texture, color, and elasticity. It is found in the
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body in such places as the nose, larynx, trachea, ear, intervertebral discs, and many parts of the skeleton.
From where in the body do you suppose this slide was taken?
Slide 13. Elastic cartilage, thin section. (This may be from a monkey ear; focus in the center of the
section). This slide has been stained so that elastin fibers
appear dark blue to black. The higher proportion of
elastin to collagen distinguishes elastic cartilage from
hyaline cartilage, and allows the elastic type to be more
flexible than hyaline cartilage. Your slide might contain
hair follicles located on either side of the cartilage. In
mammals, this type of cartilage is found in the external
ear, in the walls of the external auditory and eustachian
tubes, and in the epiglottis.
Figure 3. Slide 13, Elastic cartilage (160X).
Slide 14. Developing cartilage bone, thin section. The immature skeleton of the fetus is initially made
up of cartilage. Through the complex process known as ossification, the cartilage becomes entirely
calcified and replaced by bone. What you see in the slide is the interface between the diaphysis, the
calcified shaft of this fetal bone and one of its rounded cartilaginous ends, or an epiphysis. (A complete
developing bone has two epiphyses, one at each end of its shaft.) The diaphysis is composed of marrow
space, which contains haemopoietic stem cells that give rise to dark purple staining white blood cells
and red blood cells, surrounded by newly formed compact bone which stains pink to red. The cartilag in
the epiphysis stains a light purple-gray. The epiphysis contains the growth plate, or zone of
proliferation, where chondrocytes proliferate by mitosis, and make new cartilage, pushing the epiphysis
along as the developing bone elongates near its ends. In the Osteogenic zone chondrocytes give way to
osteoprogenitor cells, and these osteoblasts begin deposition of calcium phosphate to form new
compact bone tissue on the outside of the leading edge of the diaphysis. Some spicules of bone are also
formed in the marrow space inside the diaphysis.
Slide 15. Compact bone, ground (from the human femur). Bone has a hard, rigid matrix made of
collagen fibers which are impregnated with hydroxylapatite, a type of calcium phosphate. Compact
bone consists of numerous structural units called Haversian systems. Most Haversian system appear as
a round area with a Haversian canal in the middle which runs lengthwise through the bone. Some
Haversian canals in your specimen may be filled with bone dust from the slide manufacturing and be
black, while others will be empty. The matrix is arranged around the canal in concentric layers called
lamellae. The lamellae are perforated by lacunae where the osteocytes, the fibroblasts of bone cells, are
located. (The lacunae are visible as flat dark spots.) Numerous fine canaliculi interconnect lacunae and
Haversian canals to provide the resident osteocytes with fluid and metabolites and gas exchange. In a
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live mammal compact bone acts not like a static mineral, but is very much a living tissue. Small
circulatory system vessels (arterioles, venules, lymph vessels), and nerves that supply the tissue run
through the Haversian canal. Calcium is taken from or deposited to bone, depending on the body’s needs
for calcium, load bearing on the skeleton, etc. Compact bone is continuously remodeled over the whole
adult life of mammals, so you may see some Haversian systems, that have been partially dissolved or
resorbed only to be replaced by ossification forming new Haversian systems.
Figure 4. Slide 15, Compact bone (260X)
Muscle or Contractile Tissue
The cells of muscle tissues are usually elongate, and have a greater capacity for contraction than do
other cells. Muscles are responsible for most movements in higher animals. In mammals three principal
types of muscle are recognized: skeletal muscle, which is responsible for most voluntary movement
controlled by nerves; smooth muscle, which is involved in most involuntary movements of internal
organs; and cardiac muscle, the type found exclusively in the heart and is also under involuntary
control. Cardiac and Skeletal Muscle types exhibit depolarizing action potentials, flows of ions along
their membranes. An action potential stimulates cardiac or skeletal muscle cells to contract. In contrast,
in Smooth Muscle contraction may occur without action potentials or nerve stimuli.
Figure 5. Slides 16-18: The three types of Muscle tissues (600X).
Histology 11 Slide 16. Skeletal muscle, cross section, thin section. Skeletal muscle (also called Striated Muscle) is
composed of very long, unbranched, cylindrical cells, called fibers. Each fiber may be up to several
centimeters in length, while their diameter is approximately 20-100 µm. Because these cells develop
from a syncitium of many myoblast cells joining together to become one cell, each fiber is
multinucleate, containing a great many flattened, peripherally-located nuclei and mitochondria at fairly
regular intervals. Each fiber is also crossed perpendicularly by numerous, regularly-spaced, alternating
light and dark bands called striations. Also called sarcomeres, these bands are the result of the
arrangement of the contractile proteins actin and myosin into myofibrils, which are responsible for the
contractile capability of the cells, and the peripheral displacement of the nuclei. The fibers of skeletal
muscle are bound together into large bundles of cells by connective tissues. Most skeletal muscle is
attached by tendons to bones and can move the bones by voluntary contractions. Each skeletal muscle
fiber is controlled by a synapse with a motor neuron originating in the central nervous system.
Slide 17. Smooth muscle (or visceral muscle), thin section. (From an intestine cross section. Look
around the periphery of the section away from the lumen). Smooth muscle is found encapsulating
arteries, and on the outside of larger structures as the intestinal tract, the uterus, ureters and the bladder.
Note the absence of striations; actin and myosin are not organized into sarcomeres in this muscle cell
type. Each cell is teardrop shaped: elongate, pointed at each end, and contains a single centrally located
rod-shaped nucleus. Smooth muscle fibers interlace to form sheets of muscle rather than bundles. It
might appear to you that smooth muscle looks like schools of fish swimming next to each other.
Slide 18. Cardiac muscle, thin section. The cells of cardiac muscle branch and interdigitate, forming a
complex three dimensional network. Like skeletal muscle, the sarcomeres of cardiac muscle form visible
striations across the cell, perpendicular to their length, but unlike skeletal muscle, cardiac muscle cells
rarely fuse together and generally have only one nucleus. You’ll see in the slide that where adjacent
cardiac muscle cells meet there are darker staining bands. These bands which look like extra thick
sarcomeres are actually not sarcomeres at all, but specialized intercellular junctions called intercalated
discs. The intercalated discs have gap junctions that permit the rapid flow of ions between cells,
spreading depolarization directly from one cell to another, resulting in nearly simultaneous contractions
of large portions of the heart. Intercalated discs also have anchorage sites for the actin proteins in the
nearest sarcomeres. The one (or occasionally two) nuclei are round or ovoid and are centrally located.
Nerve Tissue
All mammalian cells possess a resting potential, a negative charge to the cytoplasm relative to the
outside of the cell membrane, but nerve tissue (sometimes called conductive tissue) is highly specialized
for the capacity for action potentials. Similar to the action potentials in skeletal and cardiac muscle,
nerve cells depolarize, but nerve cells function to use action potentials to transmit signals to other cells,
to gather information from specialized nerve endings, and to process or analyze these signals into
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perception. The nerve cells, or neurons, are easily stimulated to transmit action potentials, impulses of
ions flowing along their cell surfaces very rapidly, and in some cells, over very long distances.
Slide 19. Motor neurons, squash (from spinal cord). Each Neuron consists of a cell body (or soma)
and several fiber-like processes extending from the cell body. The soma contains the nucleus and is
located inside the central nervous system (CNS). Thin tubes of the cell branch off each soma: dendrites,
which connect to other neurons, and a longer axon. The purple dots throughout the section are the nuclei
of glial cells, the nutritive and support cells that surround neurons in the CNS. The axon of a motor
neuron may be several feet long, and the axon exits the CNS to send an outgoing signal to a synapse
with a skeletal muscle cell. Afferent neurons by contrast, have an axon that receives an incoming signal
to the CNS from a sensory ending.
Figure 6. Slide 19, Neurons and glial cells (160X).
Slide 20. Peripheral nerve. Peripheral nerves exit the central nervous system of a mammal to
communicate with the rest of the body. A peripheral nerve consists of a large number of axons, each
insulated by Schwann cells, and grouped together into cable-like bundles. There are three levels of
connective tissues organizing these bundles of axons in a peripheral nerve: endoneurium, perineurium
and epineurium. Endoneurium is next to the Schwann cells and contains connective tissue and capillary
blood vessels. The perineurium forms collagen-rich connective tissue layers wrapping bundles of axons,
and epineurium is the outermost connective tissue layer surrounding the many bundles of axons in the
whole structure of a peripheral nerve. After looking at the larger structures with your 10x objective,
examine the finer structures with in the bundles of axons with the 40x objective. Look for purple dots
represent Schwann cell nuclei. These cells wrap around individual nerve fibers creating insulated
pockets. Your may be able to resolve well enough to see the spirals of the Schwann cells’ cytoplasm as
they wrap their axons. An individual nerve cell’s fibers may be efferent channel (outgoing signal to a
muscle cell) or afferent channel (incoming signal from sensory endings).
Slide 21. The Small neurovascular bundle is composed of an artery, vein and nerve. The vessels
supplying and draining a particular area of tissue tend to pass together, frequently accompanied by a
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peripheral nerve and surrounded by connective tissue which forms an ill-defined protective sheath. This
slide shows three components bound by loose fibrous connective tissue (areolar) and are surrounded by
a more dense collagenous sheath. You may also see some adipose tissue in the bundle of most
specimens. The artery has a rigid wall and the lumen of the artery often has a scrunched appearance
compared to the flat and smooth lumen in the vein. This is because the inner walls of the artery are have
a large proportion of elastin and smooth muscle to provide for the expansion and recoil necessary for the
maintenance of the blood pressure. In many specimens you may see a darkly staining layer right at the
edge of the arterial lumen; this is elastin-rich endothelium. In many arteries and arterioles, the diameter
of the lumen is regulated by smooth muscle in the arterial walls to control local blood flow and pressure,
under the control of the sympathetic nervous system. Veins by contrast have a grainy appearance to their
wall structure, and are under very low pressure compared to arteries.
Figure 7: Slide 20, Peripheral nerve (65X).