1. Peripheral Nervous System
and
Circulatory System
Study Guide
Samantha Blum

2. Peripheral Nervous System

3. General Structure: Soma
• Note that the soma (cell body) is generally large, with abundant cytoplasm and centrally located nucleus.
The perikaryon is defined as that portion of the soma surrounding the nucleus.

4. Soma

5. General Structure: Neuronal Processes
• Most neurons have several processes, the axons and dendrites, extending away from the soma. Not all
processes of each cell are visible, since they do not all lie in the plane of section.
• Examine the neuronal processes of several cells (20X), noting their variation in size.
• Try to distinguish between axons and dendrites, by looking for the one process of each neuron that lacks
stained cytoplasm. In this slide, a stain that reveals Nissl Substance (see below) has been used. Nissl
substance is found in the cell body and dendrites only, and not in the axon or axon hillock (site of origin of
the axon). Therefore, the axon will not be stained.
– Again, the axon hillock does NOT have any Nissl bodies, so it will not be stained in the image!
• Be aware that each neuron has at most one axon, and that the axon may not be present in the plane of
section.
• Examine several neurons and try to identify an axon (100X).
Variation in size Axon

6. General Structure: Nucleus
• Study the nuclei of several neurons, and identify chromatin (40X; 40X)
• Identify nucleoli (100X).

7. Nucleus/Chromatin/Nucleolus
Note: nucleolus is the least stained and chromatin is darkly stained

8. General Structure: Nissl Bodies
• Note the abundant, darkly staining material in the cytoplasm, which is comprised of rough endoplasmic
reticulum and free ribosomes

9. General Structure: Neurofibrils
• Neurofibrils are bundles of aggregated neurotubules (neuronal microtubules) and neurofilaments
(neuron-specific intermediate filaments). They have an affinity for silver ions, which have been used to
stain this specimen. The bundles form artificially during specimen preparation.
• Note that neurofibrils are present throughout the neuron, in soma, dendrites and axon (50X; EM); as a
result, their visualization allows study of the general neuronal morphology. Note that you cannot resolve
individual cytoskeletal fibers, only bundles of them (why?).
Note the length and density of the cell
processes (50X) in this tissue section.

10. General Structure: Pigment
(Lipofuscin Granules)
• Examine neurons of this sympathetic ganglion for presence of a yellowish-brown pigment. Lipofuscin
granules (arrows, 100X) are found in many large neurons, and increase in quantity with age; also called
"age pigment", they are probably secondary lysosomes (residual bodies; EM).

11. Classification of Neurons
• Neurons may be classified according to
size, shape, number of processes, the name of
their discoverer (e.g. Purkinje cells), general
function (e.g. excitatory, inhibitory), specific
function (e.g. photoreceptor), the
neurotransmitter they release (e.g.
cholinergic), whether they send processes
over a long distance (e.g. a ventral horn motor
neuron that synapses on a muscle in the foot)
or only locally, etc. etc.
• One common classification scheme organizes
neurons according to the number of
dendrites, because this usually has some
functional significance. Although neurons
generally have only one axon, the number of
dendrites can range from none to many. As
seen in the diagram above, multipolar
neurons have many processes (many
dendrites and one axon), bipolar neurons
have two processes (usually one dendrite and
one axon), and pseudounipolar neurons have
a single process that bifurcates, with one
process functioning as a dendrite and the
other as an axon.

12. Classification By # Processes:
Pseudounipolar
• Study the dorsal root ganglion cells (1X; 5X; 20X) on this slide (the large cells surrounded by several
smaller, satellite cells).
• Note that these cells are pseudounipolar: they have a single process, a modified axon, that bifurcates to
form an axon and a dendrite-like process (schematic). This information about the morphology of these
cells' processes was obtained from studies of whole cells rather than sections (why?).
• Scan the slide to find an axon, or at least the initial segment (20X). You will have to examine many
cells, since not all cells have an axon that lies in the plane of section.

13. DRG Cell
(surrounded by satellite cells)

14. Classification By # Processes:
Bipolar
• Neurons of this type have two similar
processes; in general, one of the processes is
an axon, the other a dendrite. Bipolar cells
occur rarely. No good examples, in which the
shape of the cell can be clearly seen, are
available in your slide boxes. Study an atlas
light micrograph of these cells.

15. Classification By # Processes:
Multipolar
• This is the most common type of neuron (schematic). Their multiple dendrites can be arranged in a wide
variety of ways (see "cell shape", below). Neurons of the anterior (ventral) horn of the spinal cord are good
examples of multipolar neurons; in fact, the anterior horn of the spinal cord may be identified by these
characteristic, large multipolar neurons.
• Identify the central core of gray matter, which consists of neurons (including their dendrites and axons)
and supporting cells (glia). The surrounding white matter consists of neuronal axons only (i.e. no neuronal
cell bodies) and glia.
• Compare the appearances of anterior horn neurons stained by the three different methods used on these
slides (20X-Nissl stain, 40X-Nissl stain, 20X-silver stain, 40X-silver stain, 20X-H&E, 50X-H&E).
• Why do these neurons appear to have so few dendrites? (Hint: these paraffin sections are 6-8 microns in
thickness) not all the dendrites are in the plane of the section
Nissl stain (40x) Silver stain (40x) H&E stain (50x)

16. Classification By Shape:
Pyramidal
• Locate the cerebral cortex (2x). Note that many cells in the cerebral cortex have a pyramidal shape (10X).
• Identify the apical dendrite (50X), a characteristic of pyramidal cells. In cortical gray matter, the apical
dendrites all point towards the brain's (pial) surface.

17. Pyramidal Neurons

18. Classification By Shape:
Pyriform (pear-shaped)
• Locate the cerebellar cortex. Note that the cell bodies of Purkinje cells (10X) of the cerebellar cortex have
a pyriform (pear-like) shape.
• Note that these pear-shaped cells have one or more very large, branching dendrites (50X).

19. Classification By Shape:
Stellate (star-shaped)
• A star shape is common among multipolar
neurons; there are no good examples of
stellate cells in your slide sets.

20. Classification By Size
• In spinal cord, compare the large, anterior horn motor neurons (40X) with smaller multipolar neurons of
other spinal cord regions (e.g. cells of the dorsal horn 40X).
• Note that, in general, the larger the neuron, the greater the diameter of its processes.
• Note that cells located in different brain regions exhibit markedly different sizes.
• Note that many non-neuronal supporting (glial) cells are also present. Unfortunately, the processes of
these cells are not visible in H&E preparations; only their nuclei are visible.
Large anterior horn motor neurons Smaller dorsal horn cells

21. Synapse
• Communication between neurons occurs through this specialized structure, in which the terminal
branches of an axon meet a cell body or dendrite (or sometimes an axon) of another neuron.
• Find the anterior horn of the spinal cord, and examine the cell bodies of motor neurons there.
• Identify small, black, terminal boutons (50X, synaptic terminals) on the ends of terminal axonal branches.
• Look for these synaptic terminals (small black dots) on both perikarya and dendrites.
Neurons stained with
silver stain to display
the high
concentration of
cytoskeletal elements
in neurons
Terminal boutons

22. Peripheral Nerve: Axons
• In H&E-stained preparations, nerves may look moth-eaten, or extracted, look to them, because the myelin
that covers the axons has been extracted during preparation. The axon cylinder stains slightly, and is
surrounded by a white space where the myelin was, before the tissue was fixed (5X, 40X).
• Many cell nuclei are visible in nerves (20X). These belong to Schwann cells (peripheral glia) and to
fibroblasts, but nerves do not contain neuronal cell bodies. Where are those neuronal cell bodies
located? Peripheral neuron cell bodies are located in GANGLIA
• Frequently, you may find a nerve in association with an artery and vein, in a neurovascular bundle.
Nerves, arteries, and veins often run together in such structures (1X, 20X).
Neurovascular
bundle

23. Peripheral Nerve: Axons
• Nerve structure is best seen in trichrome-stained
preparations, where the connective tissue
elements are colored blue-green. Try to find
cross-sections and longitudinal sections, so that
you can identify all of the components of the
nerve (1X, 20X, 20X). Note that nerves are
frequently "wavy" in longitudinal view. Why?
• In this organ (slide O-33, penis), try to find
sensory structures that the above nerves might
be innervating.
• Identify the large nerve connecting the dorsal
root ganglion (DRG, schematic; 1X) to the spinal
cord, and identify the axons (50X). Be aware that
the staining method (silver) stains axons, but
does not stain myelin (the myelin is represented
by clear areas that surround the darkly-stained
axons).
• Note the variation in axonal diameter in the
silver-stained preparation. How does axonal Silver stain
function vary with axon diameter?
• In the cross-sectioned, osmium-stained nerve
(slide O-81B), indentify the darkly-stained myelin
(40X). Note that the axon cylinders themselves do
not stain at all. Would this preparation be useful
for finding unmyelinated axons?
Osmium stain

24. Peripheral Nerve: Schwann
Cells, Myelin & Neurilemma
• Every axon is surrounded by its own myelin sheath (in the case of myelinated axons), or embedded with
other axons within the cytoplasm of Schwann cells (in the case of unmyelinated axons). The cells
responsible for forming the sheaths are Schwann cells, which are the neuroglia of peripheral nerves.
– The whole stretch of myelin sheath between 2 nodes of Ranvier is made by 1 Schwann cell!
• Study both longitudinal (40X) and cross-section (50X) views of osmium-stained axons. In these osmic acid-
treated slides, axon cylinders are not stained, and myelin appears black.
• In the other slides, identify Schwann cell nuclei (100X; EM; EM), and large myelinated axons (50X) with
thick myelin sheaths.
longitudinal
Cross section

25. Peripheral Nerve: Schwann Cells,
Myelin & Neurilemma
• Which stain preparation would be the best for unmyelinated axons (EM; EM)?
• In slide O-76, identify axon cylinders in a peripheral nerve as it travels toward the spinal cord (1X; 20X).
• Identify nodes of Ranvier in the osmium-stained slide (O-81, 40X) and in slide E-80 (40X). These
indentations mark the point between adjacent Schwann cells; the myelin covering is absent here
(schematic). This specialization is the basis for saltatory conduction (schematic), which permits a dramatic
increase in conduction velocity in myelinated nerves.
• Area of myelin nearest each node does not pick up the stain very much
– It has a lot of cytoplasm in it end of Schwann cell myelin is lighter on images
Node of Ranvier

26. Peripheral Nerves: Endoneurium
• Identify the delicate collagenous and reticular fibers of endoneurium (1X; 20X; 100X) that lie among
individual nerve fibers (myelinated and unmyelinated). The trichrome-stained slide (E-28) reveals
connective tissue especially well.
• Note that fibroblasts are also present in the endoneurium of peripheral nerve; fibroblasts and Schwann
cells both produce endoneurium.

27. Peripheral Nerves: Perineurium
• Identify fascicles of nerve fibers bundled together by a sheath of connective tissue, the perineurium
(40X). This is the toughest of the three connective tissue ensheathments of peripheral nerve. See the
trichrome stained section.
• Note that concentric layers of flattened fibroblasts (40X) form the cylindrical sheaths, which vary in
thickness depending upon the diameter of the fascicle.
• Note that dense connective tissue fibers (40X) are also present.
• Identify small blood vessels (40X).
• Be aware that the perineurium forms the innermost layer of the external sheath (5X, 40X); in this respect
the arrangement is different from that in skeletal muscle. The outermost layer of the external sheath is the
epineurium.
Innermost layer of external sheath

28. Peripheral Nerves: Epineurium
• Identify this outermost connective tissue layer of a peripheral nerve in the trichrome-stained section. The
epineurium binds several fascicles together, forming a single nerve (5X, 20x).
• Note that the areolar C.T. of epineurium (40X) is considerably less dense than that of perineurium.
Epineurium is actually the fascia that you would grasp with forceps while gently dissecting a nerve.
• Identify fibers in epineurium (1X, 20X, 40X).
• Identify blood vessels within the epineurium.
• Identify adipocytes (1X, 10X) around nerves. What is their function?
perineurium

29. Dorsal & Ventral Roots of Spinal Cord
• Nerve fibers "enter" and "exit" the spinal cord via dorsal and ventral roots.
• Study the roots and follow the fiber tracts to the dorsal (4X, posterior) and ventral (anterior) horns of the
spinal cord.
• Identify axons, myelin, and nodes of Ranvier (50X).

30. Afferent Neuron: Doral Root Ganglia
• Note that these ganglia (4X) contain the cell bodies of pseudounipolar afferent (sensory, primary afferent)
neurons.
• Identify large, darkly staining cell bodies with vesicular nuclei and prominent nucleoli, and convince
yourself that these are pseudounipolar cells. Why are the single processes so hard to find?
• Identify satellite cells (40X), which are the neuroglial cells of the ganglia (equivalent to Schwann cells).
• Note that satellite cells (40X) are small cells with round nuclei, and that they completely surround the
afferent neuron cell bodies, forming a capsule.
• Identify nerve fibers (20X) in the ganglion, and note that they occur in bundles, separated by large groups
of cells bodies.
• Be aware that there are no synapses in these ganglia (they are unusual in this regard).

31. Afferent Neuron: Spinal Ganglia
• Compare this afferent ganglion with a dorsal root ganglion.
• These afferent neurons are bipolar, although their shape is not obvious in this preparation.
• Note the density of neuronal cell bodies (40X) in the ganglion.

32. Efferent Neuron: Sympathetic Ganglia
(paravertebral and prevertebral)
• Synapses between pre- and postsynaptic sympathetic neurons are present in these ganglia.
• Identify multipolar cell bodies of sympathetic neurons (50X).
• Note that they are considerably less regular in outline than are the neurons of the dorsal root ganglion.
• Note that the nucleus (50X) is often eccentrically placed.
• Note the presence of Schwann cells (50X, satellite cells) that incompletely surround the efferent neuron
cell bodies.

33. Efferent Neuron: Enteric Ganglia
(parasympathetic?)
• In the alimentary canal, these ganglia lie between the inner and outer muscular walls (10X); they regulate
the contraction of gut smooth muscle, generating rhythmic waves of muscular activity called peristalsis.
• Examine these sections for myenteric (Auerbach's) plexus (50X). This region is best seen in slide E-13 (due
to better fixation).
• Slide O-56 is a wholemount preparation, in which the layer containing Auerbach's plexus is flattened and
mounted onto a slide. This technique reveals the overall architecture of the nerve network particularly
well, as well as individual ganglia (arrows, 1X). Each ganglion contains neurons with several different
shapes (20X), most likely reflecting different functions. (Note that this slide allows you to focus through
the tissue.)
• Note that the enteric ganglia of the plexus are considerably smaller than a sympathetic ganglion.
• Identify ganglionic neurons, satellite cells (50X), and nerve fibers.
Ganglion containing neurons of several different shapes

34. Neuromuscular Junction
• At the motor endplate, the axon invaginates into the muscle membrane to form a series of synapses
(schematic). Identify these clusters of contacts in preparations stained with silver (10X, 20X).
• Release of acetylcholine ultimately results in the contraction of myofibrils within the myofiber
(schematic).
• How does acetylcholine cause the muscle to contract?
– Opens sodium channels Depolarization Causes intracellular calcium release Muscle contraction
Silver stain

35. Muscle Spindle
• The muscle spindle is probably the most easily seen end organ in your slide boxes. This end organ is a
modified skeletal muscle structure that receives both afferent and efferent nerve fiber terminations.
• "Fusiform" means "spindle-shaped". Therefore, intrafusal muscle fibers are inside the spindle, while
extrafusal muscle fibers are normal muscle fibers outside the spindle.
• Find a muscle spindle (40x), which will be located in either endomysium or perimysium. Look for modified
skeletal muscle fibers (intrafusal fibers) encapsulated by connective tissue.

36. Meissner’s Corpuscles
• Note that these complex, encapsulated, tactile corpuscles occur primarily in glabrous (hairless) skin of the
fingertips, palms, and soles of the feet.
• Identify these oval structures, which are composed of flattened connective tissue cells surrounded by a
connective tissue capsule (1X, , 10X, 20X). Two or more myelinated fibers are distributed to each
corpuscle.
• Note their presence in the connective tissue of the dermal papillae. These receptors mediate fine touch.

37. Pacinian Corpuscles
• These lamellated structures are found in deeper subcutaneous tissue of skin, organs, and around joints,
where they detect deep or heavy pressure, and vibration. Note their presence in the hypodermis of skin
(10X), where they are large enough to be visible to the naked eye.
• Identify Pacinian corpuscles within the pancreas (10X, 40X).

38. Circulatory System

39. • The blood vascular system is comprised of a
series of blood vessels of various sizes, and the
heart. All of the components are hollow
structures that share a basic architectural plan.
The lymphatic vascular system is composed of
lymphatic vessels that allow movement of
interstitial fluid from extracellular spaces back to
the blood vascular system. Vessels of this system
have extremely thin walls compared to blood
vessels.

40. Blood Vascular System
• In general, blood vessels have three layers, or tunics:
• The tunica intima (inner tunic) consists of a simple
squamous epithelial layer (endothelium) that lines
the lumen, and a thin subendothelial connective
tissue layer. The intima of arterial vessels also
contains a thick elastic layer (internal elastic
lamina).
• The tunica media (middle tunic) contains smooth
muscle cells, elastic and collagenous fibers, and an
external elastic lamina.
• The tunica adventitia (outer tunic) contains loose
connective tissue, blood vessels that supply the
blood vessel itself, and nerves that innervate its
muscular wall.
• The larger blood vessels (larger than pre- and post
capillary vessels) contain all 3 vascular coats. In
arterial vessels, the tunica media is the thickest
layer; in venous vessels, the tunica adventitia is the
thickest layer.
• Blood vessels normally occur in companion pairs (an
artery and a vein); in this case, the lumen of the vein
will generally be larger in diameter than the lumen
of the artery. Also, for an artery and a vein with the
same size lumen, the artery will have the thicker
wall.

41. Elastic Arteries
• The largest arteries contain many concentric layers of elastic membrane, alternating with layers of smooth
muscle; as a result, the terms "large artery" and "elastic artery" are used interchangeably.
• Note the presence of concentric, tubular, fenestrated elastic membranes (10X, 10X, 40X, 50X, 100X, 100X)
that range from 40-60 in number (the fenestrations are not visible in these preparations).
• Study the spaces between elastic membranes; identify scattered smooth muscle cells (40X, 50X, 100X)
and collagenous fibers (100X).
• Note that the tunica intima is thicker than in small and medium arteries, and that the tunica adventitia is
relatively thin and may contain small blood vessels called vasa vasorum (20X). What is the function of
these small vessels?
– Vasa vasorum= “vessels of vessels”; supply the elastic arteries

42. Elastic Artery: Elastic Fibers & Smooth
Muscle Nuclei
Elastic fibers

43. Muscular Arteries
• Muscular arteries derive their name from the presence of many layers of smooth muscle within the
middle tunic. Elastic fibers and fenestrated sheets are also present, but are not as common as they are in
elastic arteries. Muscular arteries are also called "small-to-medium arteries".
• Examine the tunica intima, and identify three distinct layers: endothelium (40X, 50X, 100X), intermediate
subendothelium (100X), and internal elastic membrane (40X, 50X, 100X) closest to tunica media.
• Look for elastic fibers (1X, 10X). Note that the artery has a prominent internal elastic lamina (IEL), while
the vein does not.
• Observe that the tunica adventitia (50X) may be as thick as the tunica media. As a general
rule, however, an artery's media is thicker than its adventitia; the opposite is generally true of veins.
• Identify flattened fibroblasts (40X, 100X) in the adventitia; these are responsible for producing the
collagenous and elastic fibers there.
• What cells produce the collagenous and elastic fibers in the tunica media?
– Smooth muscle cells (there are NO fibroblasts present in the tunica media)

44. Arterioles
• Identify tunica intima (40X, 100X, 100X). Note that it may be thrown into folds in larger arterioles (Why?).
• Note that almost all arterioles have an internal elastic membrane (50X, 100X, 100X, 100X). However, the
smallest arterioles, with a single layer of smooth muscle, usually do not (EM).
– On EM, the elastic of the internal elastic membrane looks like black blobs under the endothelial cells.
• Identify tunica media, with smooth muscle cells arranged in concentric layers (40X, 50X, 100X, 100X, 100X,
100X, 100X, EM). By definition, arterioles have up to five complete layers of smooth muscle cells.
• Note the presence of varying amounts of elastic and collagenous fibers interspersed among smooth
muscle cells.
• Identify tunica adventitia (50X, 100X, 100X, 100X, 100X), the connective tissue layer. It may be thin and
indistinguishable from surrounding C.T., or as thick as the tunica media (small and large arterioles,
respectively).
• Note the absence of an external elastic membrane.

45. • In pulmonary arteries/arterioles, the lumen
can be larger and the outer layers can be
thinner
– Lower pressure in lungs compared to other parts
of the body

46. Arteriole

47. Microcirculation
• Nutrient and waste exchange between blood and bodily tissues takes place across the walls of capillaries.
Capillaries, along with the small arterial vessels that supply them and the small venous vessels that drain
them, are collectively referred to as the microcirculation.
• As arterioles approach the capillary bed, they gradually lose their smooth muscle covering, becoming
metarterioles (pre-capillary arterioles). The remaining smooth muscle cells often cluster at branch points
into the capillary bed; these pre-capillary sphincters function to control blood flow into the capillary bed
and, therefore, also control blood pressure.
• As blood flows from the capillary bed, it drains into larger, but still capillary-like, vessels called post-
capillary venules. These gradually fuse to form larger venules, which eventually drain into veins.

48. Capillaries
• Capillaries are simple endothelial tubes surrounded by a basal lamina.
• Some tissues require a higher rate of transfer of nutrients across the endothelium; the structure of
capillary walls varies to meet the needs of the specific tissue.
• In continuous capillaries, the endothelial cells are firmly joined to one another to form a continuous
sheet. Nutrient transfer then takes place across the wall of the endothelial cell.
• Depending on the need for permeability, capillaries may have openings within the endothelial cell walls (in
fenestrated capillaries), may have small or large spaces between adjacent endothelial cells, or may even
have incomplete endothelial coverings with very large spaces (in sinusoids).

49. Capillaries
• The smallest, and structurally simplest, blood vessels are the capillaries. These vessels are comprised
solely of tunica intima. In the slides listed, scan the field of view for these very small vessels, which have a
diameter approximately that of an erythrocyte (which is... ?). Look for them first in loose connective
tissue, especially in areas with abundant adipocytes. Then look for them in areas where they are less
obvious. Identify capillaries in both longitudinal (100X, 100X, 100X) and cross-sections (100X, 100X), and
note the thin rim of endothelial cell cytoplasm.
• In cross-section (100X, 100X, 100X), you will most likely see 1-3 endothelial cells forming the wall of the
vessel. Look for endothelial nuclei bulging into the capillary lumen.
• The capillary is ensheathed by delicate collagenous and reticular fibers. A basal lamina is present between
endothelial cells and the C.T. sheath (EM).
Cross-section Longitudinal

50. Capillaries
• Note that continuous capillaries (EM) and fenestrated capillaries (EM; EM) can only be distinguished by
electron microscopy, because the pertinent structural features are below the resolution limit of the light
microscope.
• Leukocytes roll along the luminal surface of capillaries by interacting with capillary endothelial cells. These
interactions are mediated by integrins on the surface of the leukocytes, and selectins on the endothelial
cell surface (schematic). Damage or infection in the surrounding connective tissue causes the release of
cytokines, which signal the circulating cells to migrate across the capillary wall and move to the site of
injury (animated movie).
Continuous capillary Fenestrated capillary

51. Pre- and Post-Capillary Vessels
• The classification of these vessels as part of the arterial system (pre-capillary arterioles) or the venous
system (post-capillary venules) depends on whether they have smooth muscle cells or connective tissue
external to the endothelium. The slide suggested for study here is a spread (rather than a section) of
mesentery. Thus, focusing "up and down", using the Virtual Microscope's focusing feature, will allow you
to study the 3-dimensional structure of these vessels. Also, you will be able to follow vessels for long
distances, and therefore observe transitions from one vessel type to another.

52. Pre-Capillary Arterioles
(Metarterioles)
• These are intermediate in size, and located between arterioles (40X, 40X) and capillaries. Identify a pre-
capillary arteriole by its incomplete coat of smooth muscle cells (50X, 50X, 100X), and note that the
elongated smooth muscle cells are arranged circularly (circumferentially) around the cylindrical structures.
In contrast, post-capillary venules are surrounded by cells (not muscle cells) that are oriented
longitudinally.
• Compare the sizes of pre-capillary arterioles and capillaries. Note that the luminal diameters of these
vessels are easier to determine when erythrocytes are present.
– Very dark area in image on right is a PRE-CAPILLARY SPHINCTER (point at which capillary meets metarteriole)

53. Post-Capillary Venules
• These vessels have scattered C.T. elements oriented longitudinally; smooth muscle is absent
(40X, 40X, 50X). Identify fibroblast (50X) nuclei external to the endothelial cells.
• Compare (40X, 50X) the diameter of this vessel with the diameters of capillaries and pre-capillary
arterioles.

54. Venules
• Post-capillary venules drain into venules; these are larger vessels, up to 1mm in diameter, with a covering
of smooth muscle ranging from incomplete to 1-3 complete layers. The lumen of a venule is much larger
than that of its corresponding arteriole. The easiest way to find a venule, therefore, is to first find an
arteriole and then identify its companion vessel.
• Locate the tunica intima (100X), and note that it consists solely of endothelium. No subendothelium is
present.
• Examine the tunica media with its 1 to 3 layers of smooth muscle (40X, 40X, 50X, 50X, 100X, 100X). Note
that the muscle cells are more widely spaced than they are in arterial tunica media.
• Observe that the tunica adventitia is the most pronounced layer, as is true for all venous vessels of this size
and larger (40X).

55. Small & Medium Veins
• From venules, blood drains into veins of increasing diameter, classified as small, medium, and large. Small-
to-medium veins have a diameter of 1-9mm.
• Note that the tunica intima lacks an internal elastic membrane (particularly clear in SCPM033, which has
been stained for elastic fibers.
• Note that the tunica media is thin (20X, 50X); identify collagenous (50X) and elastic fibers (40X) between
muscle cells.
• Note that the tunica adventitia is well developed, with thick longitudinal bundles of collagenous fibers.
Some smooth muscle (20X, 50X, 50X) may also be present (more so in medium veins).

56. Large Veins
• The large venous vessels that drain into the heart are large veins. In large veins, tunica media is very thin,
while the adventitia is thick and contains bundles of smooth muscle oriented longitudinally (1X, 5X); it is
also rich in elastic fibers and laminae (1X, 10X).
• The largest veins, especially in the lower limbs, contain valves. These are formed by infoldings of intima
with a core of elastic fibers, and function to prevent backflow of blood in these large, relatively low-
pressure vessels.
• Identify vasa vasorum and smooth muscle in the adventitia (10X). What is the function of the vessels that
comprise the vasa vasorum?

57. Vasa Vasorum
• Identify vasa vasorum (20X, 100X) in tunica adventitia. Note that these "vessels of vessels" are present
only in larger arteries and veins, such as the carotid artery in slide E-038A (1X, 40X).

58. Vasa Vasorum
Vasa Vasorum

59. Specialized Blood Vessels:
Sinusoids in Liver & Spleen
• Note that the lumen is larger and more irregular than that of a capillary (50X, 100X). In the liver, the walls
of these sinusoids contain many large openings (arrow, EM) that permit large amounts of plasma to cross
the wall, to bathe the cells (hepatocytes) around the vessel.

60. Heart
• A modified blood vessel that functions in
propulsion of blood, the heart contains a greatly
proliferated tunica media, now called the
myocardium and containing cardiac muscle
rather than smooth muscle.
The endocardium of the heart is equivalent to
the tunica intima of arteries, and the epicardium,
which contains blood vessels and nerves, is
equivalent to the tunica adventitia of arteries.

61. Heart: Endocardium
• Identify this innermost layer, or tunica intima of the heart (40X); note that it is comprised of an
endothelial lining (100X) and underlying supporting connective tissue.
• Examine the simple squamous cells (100X) of the endothelium.
• Examine the delicate fibroelastic connective tissue (100X) that underlies the endothelium.

62. Heart: Myocardium
• The tunica media of heart (10X, 40X), or myocardium, is a highly modified layer. Note that it contains
cardiac muscle (100X), rather than smooth muscle.
• Look for bundles of Purkinje fibers, clearly visible in slide E-71 (20X, 40X). Compared to normal cardiac
muscle myofibers, these modified cardiac muscle cells contain fewer intracellular contractile
myofilaments. What is the function of Purkinje fibers?
– Conducting fibers of the heart that are embedded in the endocardium
– NOTE: Purkinje fibers may also be shown as longitudinal sections appear in space between cardiac muscle and show some contractile
fibers

63. Heart: Epicardium
• The outermost layer of the heart, also called visceral pericardium (10X), consists solely of a serous
membrane; the simple squamous epithelium present is a mesothelium (40X).
• Beneath the epicardium, and between it and the myocardium, lies a layer of connective tissue, the sub-
epicardial connective tissue. Note the abundance of adipocytes (20X), blood vessels and nerves (20X) in
this layer.

64. Cardiac Skeleton
• In addition to the three basic tunics, the heart
has a skeleton of dense fibrous connective
tissue which contributes to the characteristic
shape of the heart, and to which cardiac
muscle, and valves, attach. The annulus
fibrosis and interventricular septum are
generally not present in your slides, however.
Review these structures in your text.

65. Lymph Vascular System

66. Lymph Vascular System
• Lymphatic vessels allow movement of
interstitial fluid from extracellular spaces back
to the blood vascular system.
• Vessels of this system have extremely thin
walls compared to blood vessels, and have an
incomplete endothelial lining, which makes
them highly permeable.

67. Lymph Capillaries
• Lymphatic capillaries look very much like blood capillaries, and therefore are difficult to find in most
tissues. The small intestine, however, contains a readily-visible population of lymphatic capillaries called
lacteals. Look for lacteals in the loose connective tissue (lamina propria) of the small intestine; they
resemble capillaries but do not contain red blood cells (10X, 100X).
– Easiest way to identify lacteals is to find a capillary (filled with blood) and see that the lacteals are the same size but empty

68. Lympathic Vessels of Various Sizes
• Look in the lymph node capsule for lymphatic vessels (40X, 50X).
• Identify a lymphatic valve (40X, 50X).
• What is the function of lymphatic valves?
• Identify lacteals (10X, 50X) in the lamina propria of the duodenum (E-013).

69. Clinical Correlation
1. Atherosclerosis/Thrombosis of a coronary artery
– Atherosclerosis is a disease in which a thickening
develops in one portion of an arterial wall. The
thickening, or plaque, forms from large numbers of
abnormal smooth muscle cells, intracellular and
extracellular cholesterol deposits, and dense layers
of connective tissue (schematic).
– The plaque blocks the artery, reducing the flow of
blood. Often, a blood clot (thrombosis) forms at the
site of the plaque; together, the plaque and the
blood clot may close the artery completely; if the
vessel is a coronary artery (1X), then the loss of
blood supply to the heart muscle causes a heart
attack.
2. Myocardial infarct
– An infarct is an area of cell and tissue death
(necrosis) caused by blockage of the arterial supply
to the affected organ. Half of the deaths in the
United States result from myocardial or cerebral
infarction.
– Find an area in which the normal cardiac muscle
structure seems to be disrupted (5X); this is
particularly noticeable at the outer edge of the
infarct, where normal fibers can be found (20X).
– Within the infarct, look for fragments of cardiac
muscle cells, recognizeable by their striations (20X).

70. Clinical Correlation
3. Aortic dissection
– In this condition, the layers of the
tunica media separate, with the
formation of a blood-filled channel
within the aortic wall (diagram).
This channel may rupture
outwards, causing massive
hemorrhage and death. Aortic
dissection is observed primarily in
males with high blood pressure, or
in individuals with a congenital
weakness of elastic fibers (Marfan
Syndrome).
4. Filariasis/Elephantiasis
– When normal drainage of the
lymph vessels is blocked, lymph
accumulates in the affected limb.
The parasitic infection filariasis
causes scarring of the lymph
vessels in the inguinal region,
blocking drainage from the leg.
The resulting extreme swelling
(image) has led to the name
elephantiasis.