Southern Illinois University


Nerve & Muscle Tissue

Questions & Answers

These are questions submitted by one class of students as they began their study histology.  Some of these questions relate to a problem-case which is no longer in the curriculum.  Click on a question to see the answer or explanation.

13.  What part of the connective tissue is affected by Marfan syndrome, and is there a certain area that is damaged more severely?

12.  Can the surface of the eye tan or burn from exposure to sun, like skin?

10.  How are individual nerve fibers are arranged inside a nerve?

9.  How is inflammation related to scleroderma?

8.  Does the number of cells in an inflammatory infiltrate indicate degree of tissue damage?

7.  What effect(s) does histamine have on smooth muscle?

6.  What is the difference between chronic and acute inflammatory infiltrate?

5.  Is extracellular matrix associated only with connective tissue?

4.  How do nutrients from blood reach reach cells in the upper layers of the epidermis?

3.  Do all cells belong to one tissue type or another?  

2.  How does one calculate the size of microscopic structures?

1.  How much histology should we be trying to learn?

13.  How does Marfan's syndrome affect connective tissue?  Is there a certain area that is damaged more severely?  Marfan's syndrome is caused by a defect in the gene FBN1, which affects the connective tissue matrix protein fibrillin-1.  A major consequence is aortal aneurysm, due to damage of the vessel lining and weakening of its wall. 

12.  Can the surface of the eye tan or burn from exposure to sun, like skin?  Short answer:  Tanning, No.  Burning, Yes.  

Tanning -- Normal conjunctival epithelium does not contain melanocytes, so this epithelium does not "tan."  The melanocytes of deeper, uveal tissues (choroid and iris, see eye) do not provide any adjustable protection for surface tissues.  Instead, they provide intense pigmentation to make the the eyeball light-proof, so the only light reaching the retina is what passes through the cornea, pupil, and lens. 

Burning ("ultraviolet keratitis," "snowblindness," "flash burn" from welding)-- Although the eye is fairly well protected from exposure to direct sunlight by eyelids, eyebrows, and behavior that avoids aiming the eyes in the direction of the sun, conjunctival tissues can be damaged by excess exposure to radiation, especially by strong indirect light reflected from snow, sand or water.  "The ocular equivalent of sunburn, . . . acute, i.e., short term, exposure . . . is characterized by reddening (inflammation) of the eyeball, gritty feeling of severe pain, tearing, photophobia (avoidance of light) and blepharospasm (twitching).  Frequently diagnosed in skiers as 'snowblindness,' photokeratitis is also seen in beach-goers and others involved in outdoor recreation." [source of quote]

More:  Ultraviolet keratitis from eMedicine

10.  How are individual nerve fibers arranged inside a nerve?  (For example:  Do motor predominate on the inside and sensory on outside?   No.  Do axons arrange themselves to be on their anatomically correct side for splitting?  Possibly, to some extent; see below.  Since the anterior interosseous nerve splits off from the median nerve, do the motor axons that enter the anterior interosseus nerve run along the median nerve on the appropriate side before they split off, or is the arrangement of axons random?)

As far as I know, there is no within-nerve arrangement of axons which is consistent and reliable.

There probably is some tendency toward a non-random arrangement of axons within a nerve, at least in the immediate vicinity of a branch point.   Thus you might expect that axons which travel together in a distal nerve would also be grouped together, more-or-less, proximal to the point where that nerve branches from its trunk.  This might matter if damage to a nerve were somehow confined to one side of the nerve, immediately proximal to a branch point.  The result could then be symptoms more-or-less consistent with the distribution of the distal branch, even though the damage was proximal to the point of branching.  On the other hand, any such "anatomically correct" bundling of axons probably becomes increasingly scrambled with increasing distance from the branch point. 

Note that this comment applies to PERIPHERAL nerves. In contrast, in the SPINAL CORD, and elsewhere in the central nervous system, there are many definite "pathways" in which axons of similar functionality are reliably bundled together (e.g., ascending touch pathway, ascending pain and temperature pathway, various descending motor pathways).  And within a pathway, axons tend, at least approximately, to retain positional organization (e.g.., to be organized "somatotopically").

9.  Is the presence of an inflammatory infiltrate, sufficient to say that there is an inflammation/inflammatory response?  Can our patient [in a former PBL case] have an inflammatory infiltrate and yet not exhibit any of the 4 signs of inflammation?

By definition, yes.  But certain sites (such as tonsils and appendix) have a normal appearance which resembles inflammation.

The four signs of inflammation reflect mechanisms whose biological function is the mobilization of white blood cells (WBCs).  If WBCs are mobilized without externally obvious signs of inflammation, this process is still characterized as inflammation.  (This is pretty much a matter of definition -- transferring an older meaning to a more recent understanding of underlying cell-signalling and immunological mechanisms.)

Scleroderma biopsies can be difficult to interpret with any confidence.  What we get are hints that are consistent with scleroderma, which must be diagnosed on the basis of many signs besides the biopsy, all of which point in the same general direction.  Basically, mild inflammation does appear in the biopsy, providing evidence that something is not quite right in her skin.  Without that evidence, the amount and texture of collagen alone would be even less compelling.

If inflammation is present in our patient (which it is), then what is stimulating this?  We know that the patient does not have an infection, and her tissues aren't really damaged, they just have swollen collagen bundles.  What inflammatory trigger is producing the inflammatory response?

Short answer:  I don't know.  Immunity appears to be involved, but science doesn't yet know the target of the immune response.

Something, possibly an autoimmune process, appears to be stimulating fibroblasts to act as if tissue had been damaged.  These cells then go to work "repairing" the tissue by laying down more collagen.  In other words, fibroblasts are converting normal connective tissue into scar tissue, and in so doing interfering with normal tissue functioning.

What is the connection between the autoimmune nature of scleroderma and the inflammation?

Cheap answer:  Inflammation is an immune response.  The one implies the other.  But until we learn what the immune system is attacking, the fact of inflammation is indeed difficult to relate etiologically to the disease of scleroderma (except to note the correlation).

Immunology is a most important topic, one that is deeply relevant to this case (as it is to much of medicine).  But the associated "learning issues" are too vast for one week's accomplishment.  Trust that better understanding will come as the curriculum progresses -- with concentration on immune system function next year.

8.  I originally wondered if perhaps number of infiltrating cells might be related to acute vs. chronic, but now believe this is due to the amount of tissue damage being responded to, so could be large or small infiltrate no matter whether it's acute OR chronic response....? 

Number of infiltrating white blood cells can be an indicator of degree of severity for the immune response, but this degree of severity bears no necessary relationship to degree of tissue damage. 

Also, be cautioned.  The concentrations of lymphocytes vary tremendously among normal tissues.  Thus tonsils have an appearance which superficially resembles severe inflammation.

7.  Some sources say histamine causes vasodilation, others say histamine causes smooth muscle contraction.  Which is right???

Both are right.  It depends on the circumstances.

When released in the inflammatory response, histamine triggers both 1) vasodilation and 2) increased capillary permeability.  However, histamine does not directly cause either event.  Rather, histamine functions as a signalling molecule.  The result of the signal is determined entirely by the receiving cell.  Different populations of smooth muscle cells have different histamine receptors (called H1 and H2) and can respond differently.  Binding of histamine to the H1 receptor causes smooth muscle to contract; binding to the H2 receptor causes smooth muscle to relax.  The usual effect of histamine on smooth muscle of arterioles is relaxation (H2 receptors).  In the lung, histamine can cause bronchospasm (H1 receptors).

6.  Is the difference between acute vs chronic inflammation related to the type of cells infiltrating (e.g., neutrophils are acute, lymphocytes indicate longer term or more chronic condition)???  Should we be able to distinguish these cell types in tissue???  [I find this virtually impossible at this stage...] 

The defining difference between "acute" and "chronic" is how long the process has been going on.  That difference typically correlates with the cell populations which comprise the inflammatory infiltrate, as you have noted. 

Distinguishing cell types in inflammatory infiltrate is a vital skill for pathologists, but it is a bit much for first year students who have not yet become accustomed to tissue microscopy generally. 

But it never hurts to try.  Given a sample of inflamed tissue, one swarming with inflammatory cells, it is usually not too tough to tell whether the vast majority of the infiltrating cells are mononuclear (i.e., each with one ordinary nucleus, as lymphocytes and monocytes, hence chronic) or polymorphonuclear (i.e., with the characteristic pinched and beaded nuclear appearance of neutrophils, hence acute).

5.  Extracellular matrix (ECM) seems to be described as predominantly existing in the extracellular spaces around connective tissue cells.  Does this mean that other tissues do not have ECM, or do they have matrix but not in as large a proportion as in connective tissue?

ECM is indeed a feature of connective tissue.  The concept includes not just intercellular fluid but also fibers and ground substance.  Although the composition of ECM varies markedly from region to region (e.g., in bone it is hardened by mineral; in blood it is fluid and lacks fibers altogether), ECM in some form or other is part of the defining concept of connective tissue.  The ECM concept includes the presence of some stable elements (plasma proteins in blood, fibers and glycosaminoglycans in ordinary connective tissue and cartilage, calcium salts in bone) which form a permanent part of the tissue.  These elements are not present in epithelial and nervous tissue. 

Epithelium is an entirely cellular tissue.  Every epithelial cell is attached to neighboring epithelial cells, with very little space in between.  In that space, there may be some extracellular fluid but it doesn't have any special components of its own.  The basement membrane, which separates epithelium from underlying connective tissue, may be produced in part by epithelial cells; it is by definition part of the ECM.

Nervous tissue is also cellular, with a small but very important volume of extracellular fluid.  Again, this fluid does not contain any special components of its own.

Muscle is intimately embedded in connective tissue matrix.   Each muscle cell/fiber is surrounded by connective tissue. 

The physiological concept of "extracellular tissue water" or "interstitial fluid" is related to, but not synonymous with, ECM.  It includes the water component of ECM (which is substantial) and also the much smaller amount of water found between individual cells.  The interstitial fluid of the central nervous system forms a distinct fluid compartment, separated from the fluid compartment of blood by the blood-brain barrier.

Blood plasma is a special case.  In physiology, it is treated as distinct from interstitial fluid.  But in most parts of the body there is free exchange between blood and interstitial fluid.  Whether or not blood plasma should be included in the class of ECM is a matter of definition; usually it is treated separately but I have described it here as a special case of ECM.

4.  RE: Nutrient and excretory diffusion from blood vessels in dermis up thru the epidermis - do these nutrients have to pass thru each individual cell, with overflow going on "up-line" to other cells? or do they somehow diffuse between the gap junctions (which I thought were to prevent this sort of diffusion)?????  Several sources each just refer to diffusion to upper level cells.... 

Short answer:  Nutrients reach keratinocytes by diffusion through extracellular channels between the cells.  This diffusion is NOT blocked by intercellular junctions.  (The question confuses function of different junctional types; more below.)

There are several types of cell junctions.

Tight (occluding) junctions do block diffusion; they typically form a seal or gasket around the apical end of cells comprising simple epithelia (i.e., epithelia comprising only a single layer of cells).  This junction helps assure adequate separation between different fluid compartments (i.e., between the contents of the intestine and the interstitial fluid of the body). 

Gap junctions provide direct intercytoplasmic communication between joined cells, such as cardiac muscle cells.  That is, ions or small molecules can pass through gap junctions directly from the cytoplasm of one cell into the cytoplasm of an adjacent cell, without passing into intercellular space.  Gap junctions are usually localized patches where adjacent cells touch; they do not significantly affect extracellular diffusion within intercellular spaces.  (If there were gap junctions between epidermal cells, then nutrients could be passed directly from cell to cell.  However, epidermal cells do not appear to work this way.  The situation in bone is different.  Here, where the extracellular space is occupied by impermeable mineral, osteocytes do make gap-junctional contact to share nutrients.)

Adhering junctions provide mechanical attachment.  Their shape may be that of bands (zonulae adherens) or spots (maculae adherens, or desmosomes).

The junctions between keratinocytes are of the adhering type.  You can actually see the between-cell gaps with your microscope, within the prickle-cell-layer, or stratum spinosum.  The "prickles" which attach adjacent cells do not occlude this space, they just reach across it.  The diffusion-barrier role of epidermis is served not by occluding junctions but by keratinization of the outer epidermal cells together with secretion of intercellular "filler" (complex glycophospholipids) at the level of stratum granulosum.

The situation may be different for other organs. 


3.  In organization of cells vs. tissue vs. organs - I understand that an organ is a compilation of various tissue types to perform a specific function.  Do all cells belong to one tissue type or another?  Or are there some cells types that do not fit the "tissue" context.  Example - a keratinocyte is an epithelial tissue type.  A lymphocyte would then be a connective tissue type?  A hepatocyte is.....????   

With the usual caveat that all rules have exceptions -- Yes, all cells do belong to one of the four basic tissue types (epithelium, CT, nerve, muscle).  For more, see Basic Tissue Types.  [The exceptions include germ cells (i.e., eggs and sperm), which don't fit the scheme at all, and several specialized variaties of connective tissue which masquerade as epithelium, such as synovial membranes of joint capsules and stria vascularis of the inner ear.  Endothelium and mesothelium are special cases, usually classified as epithelial even though they derive from mesenchyme.]

Keratinocytes are indeed epithelial tissue, each one attached to its neighbors to form a continuous protective surface of cells.

Lymphocytes belong to connective tissue, along with other blood cells.  The logic of this involves the unity of cell populations (including their stem cells) which inhabit and wander through connective tissue.  (The Stevens & Lowe textbook replaces the classic unitary "connective tissue" category with the functional categories of "support tissue" and "immune cells," in which case lymphocytes belong to the immune-cells category.)

Hepatocytes are epithelial cells.  In addition to derivation from embryonic endoderm, hepatocytes also look and act like epithelium.  Every hepatocyte is attached to several neighbors, and each one forms part of the "outside" surface of the body.  That is, each hepatocyte has a surface facing a space that leads along a duct tree into the intestine.  In other words, each hepatocyte lies on the "outside" of the body -- if a bug were small enough, it could crawl on a free surface all the way from your skin into your mouth, down your esophagus and stomach, up your bile duct, and eventually reach any hepatocyte.  That said, the arrangement of hepatocytes has several features which are unique to the liver and not typical of other glands.  (For example, the concepts of "apical," "basal," and "lateral" surfaces, which apply quite intuitively to most epithelial cells, need some topological stretching before they make sense with hepatocytes.) 

2.  How does one calculate the size of microscopic structures?

Example:  A red blood cell appears to be 3 mm in diameter when magnified 500x.  What is the cell's actual diameter?

Answer:  3 mm / 500 = 0.006 mm = 6 µm (6 micrometers)

Explanation:  Divide the dimension of interest in the magnified image (e.g., the 3 mm diameter of a magnified RBC) by the magnification (in this case, 500x) to get actual size.

If this does not seem obvious, think of it in reverse.  If you were to magnify a 6 micrometer object (such as a red blood cell) by 500 times, how large would the image be?

6 µm x 500 = 3000 µm      [  3000 µm x (1 mm / 1000 µm) = 3 mm ]

Another example:   The image of a mitochondrion in an electron micrograph, printed at a magnification of 100,000x, is 5 cm across.  What is the actual breadth of the mitochondrion?

Answer:  5 cm / 100,000 = 0.00005 cm = 0.0005 mm = 0.5 µm

Measuring by comparison:  In a slide of connective tissue, the clear space representing the fat droplet in an adipocyte has a diameter about 10 times the size of a red blood cell seen in a nearby venule.  How big is the adipocyte?

Answer:  [approximately] 60 µm =  6 µm x [approximately] 10

Explanation:  Red blood cells (RBCs) can be found somewhere in most tissue specimens.  Normal RBCs have a remarkably uniform diameter of 6 - 8 µm, which can be used as a reference standard to estimate the size of other structures.  Thus something whose dimension is about 10 times the diameter of an RBC must be about 60 - 80 µm.

"Standard" objects, which are fairly easy to find and have fairly consistent sizes include:

For light microscopy...
(6 - 8 µm),
(variable, commonly about 50 µm),
skeletal muscle fibers (variable, commonly about 50 µm), and
terminal arterioles
(about 30 - 50 µm);

For electron microscopy...
mitochondria (variable, but commonly about 0.5 µm across, similar to many bacteria, and ribosomes (about 18 nm; note the units - nanometers - puts the ribosome in the range of macromolecular structures).

Measuring by sequential estimation:  Even if you can't find (or don't remember) any "standard" objects, and don't know the magnification of your microscope, you can always estimate size by beginning with an ordinary ruler to measure a conspicuous naked-eye feature on the specimen.  Using that measurement, you can then calibrate the diameter of your microscope's field of view at low power.  Using that figure, you can then use the size of a feature as estimated within the low power view to calibrate a higher magnification view, etc.

Example:  You measure the visible basophilic margin of a particular specimen of thin skin (the epidermis) at about 1/4 mm in thickness.  You place the specimen under the microscope, and notice that the thickness of the epidermis extends across about 1/10 of the field of view.  Therefore, your field of view is approximately 10 x 0.25mm = 2.5mm or 2500 µm.  If the epidermis comprises about 10 cell layers, then the diameter of each cell must be about 0.25mm / 10 = 0.025mm or 25 µm.

1.  How much histology should we be trying to learn in this unit?

As much as you can, and no more. 
But note that there are really two distinct jobs here:

  • First is becoming familiar with the basic tissue composition of the human body -- understanding the various characteristics, components, functions, and appearances of the four basic tissue types.  You might consider this website to be your tour-guide for a guided tour of the human body, as it is revealed by microscopy. 
  • Second is learning specific details of particular organs (trachea, lung, heart, kidney, etc.), especially as these details relate to physiology and pathology.  You should always try to integrate your study of histology with your study of other disciplies.

There are also two distinct motivations:

  • First is learning enough to pass Year One.  For this, enough is enough.
  • Second is learning enough to best serve your future patients.  For this, too much may not be enough.  Let your conscience (and your workload) be your guide. 

For more, see "How to study histology" and "Why study histology?."

Comments and questions:

SIUC / School of Medicine / Anatomy / David King
Last updated:  7 June 2022 / dgk