CHAPTER 4 - Tissues

TISSUES

Tissues ‑ 4 types of tissues:

EPITHELIAL TISSUE

general characteristics:

general function:

Types: classification based on shape and arrangement of cells

Based on arrangement:

simple

stratified

Based on shape:

squamous

cuboidal

columnar

pseudostratified

Specific Types of Epithelial Tissue:

1) simple squamous

Simple squamous epithelium, c.s. (400X) thin section
Kidney cortex

The arrows in the image point to the nuclei of simple squamous epithelial cells. This image was made from a thin section of the kidney at the same magnification as the previous image (400X). It is about one-fifth to one-tenth the thickness of the slides used to make the top three images on this page.

Thin sections allow us to see more detail, but they are usually lighter because there is not as much tissue to absorb stain. Another difference between this slide and the previous three is that the glomerular capsule is much harder to see here. In thick sections tissues shrink more during processing, which leaves bigger spaces between the cells. The space in the glomerular capsules that you see in the top three images is not present in living tissue. This thin-section image is more life-like because it didn't shrink as much and there aren't such large spaces between cells. But the one advantage of the shrinkage in the thick sections is that it gives you an easy landmark for finding the simple squamous epithelial tissue. You will usually be using thick section slides in lab.

2) simple cuboidal

Simple cuboidal epithelium (400X)--thick section
Kidney medulla

This image is an enlargement of the one just above it. The color of the stain is different from the top three images, but you can still recognize the pattern formed by the nuclei of the simple cuboidal epithelial cells--just look for the circles. The individual cells can be seen in this image, making the simple cuboidal epithelium even easier to recognize.

3) simple columnar

Simple columnar epithelium (400X)
Primate small intestine

The arrows are pointing to goblet cells that produce and release mucus. They look clear because the molecules in the mucus do not absorb a lot of stain. The entire layer of simple columnar epithelium is indicated by the bar.

4) pseudostratified

Pseudostratified ciliated epithelium (400X)
Human respiratory tract

Now you can see the individual cells. Once again, the bar shows you the thickness of the ciliated pseudostratified epithelium. The cells in this tissue are very tall and thin, and are not all the same shape. All of the cells are in contact with the basement membrane (on the basal surface), but not all of them reach the surface. The cells that do reach the surface are either ciliated or goblet cells (mucus-secreting cells). The cells that do not reach the surface are probably stem cells and will eventually replace the ciliated and goblet cells. The small bar shows the location and height of the cilia.

5) stratified squamous

(a) non-keratinized

Stratified squamous non-keratinized epithelium (100X)

Once again the bar shows you the thickness of the stratified squamous epithelium (sse). Just underneath it you can see a layer of connective tissue (ct). Look at the nuclei of the epithelial cells and notice that there are several layers of them. This is your clue that you are looking at a stratified tissue.

Stratified squamous non-keratinized epithelium (400X), surface

This image shows only the outermost layers of the stratified squamous epithelium. The cells in this tissue are not all squamous (flat). It is named for the shape of the cells on the surface of the tissue. The arrow indicates one of these squamous cells. Notice that two of the cells seem to be separating from the surface of the tissue. This is called sloughing (pronounced "sluffing") and is a normal process in epithelial tissues that form coverings and linings, especially the stratified tissues.

Stratified squamous non-keratinized epithelium (400X), base

This image shows only the lower layers of the stratified squamous epithelium. The dotted line indicates the division between epithelium (above) and connective tissue (below).

The bottom layer is the source of new cells to replace the ones that are sloughing off of the surface. The cells in this layer are usually cuboidal or columnar in shape. As the cells are pushed up towards the surface, their shape changes. You can see that the cells are more flat in the upper part of the image. By the time they reach the surface, they will be squamous.

(b) keratinized

Stratified squamous keratinized epithelium 40X
(Palmar skin)

Although stratified squamous keratinized epithelium covers the entire surface of the body, most of it also includes hair, which makes the basic tissue structure harder to see. If we just want to look at stratified squamous keratinized epithelium, we look at skin from one of the few areas of the body that does not have hair. This tissue is from the palm of the hand (palmar skin). The bar shows the thickness of the stratified squamous keratinized epithelium. In this specimen, the epithelium is stained very dark. The lighter areas underneath are connective tissue (ct).

Stratified squamous keratinized epithelium 100X
(Palmar skin)

At 100X you can see the distinct cell layers that make this a stratified epithelium. The bar indicates the thickness of the stratified squamous keratinized epithelium (sske). Notice that the nuclei of the cells in the bottom layers tend to have a round shape, but that the nuclei seem to become flatter as you move towards the surface. You have probably noticed that the bottom surface of the epithelium is not flat. The bumpy appearance is caused by connective tissue projections called dermal papillae. On all of the images on this page, the epithelium is stained more darkly than the connective tissue.

Stratified squamous keratinized epithelium 400X
(Palmar skin)

The cells on the surface of stratified squamous keratinized epithelium are very flat. Not only are they flat, but they are no longer alive. They have no nucleus or organelles. They are filled with a protein called keratin, which is what makes our skin waterproof. The arrow at the top of the image is pointing to a keratinized cell that has partially separated from the rest of the skin. These dead cells are continually lost from the surface of the skin, and are replaced by new cells from the layers below. The cells on the basal (bottom) layer are actually cuboidal or columnar in shape. They divide by mitosis to produce a constant supply of new cells that replace the ones that are lost from the surface. As these replacement cells are gradually pushed towards the surface, their shape changes. They start out on the bottom as cuboidal cells, then they become irregular in shape, and finally become very flat as they are transformed in the dead, keratin-filled surface cells.

6) glandular

endocrine

exocrine

CONNECTIVE TISSUE

characteristics:

General Functions:

Specific Types of Connective Tissue:

1) areolar tissue (loose)

Areolar connective tissue 40X

Areolar connective tissue has no obvious structure, like layers or rows of cells. You might think that this would make it harder to identify. But if you realize that the lack of pattern is one of the distinguishing characteristics of
areolar connective tissue, you have learned a cue that will allow you to recognize it.

Areolar connective tissue is made of cells and extracellular matrix ("extra-" means "outside", so the extracellular matrix is material that is outside of the cells). The matrix has two components, fibers and ground substance. In the images on this page, you can see the fibers very easily--they look like threads. The only part of the cells that is visible is the nucleus. The ground substance has no structure, so you can't tell that it is there. The ground substance fills all of the spaces between the cells and fibers.

Areolar connective tissue 100X

The fibers are the dark lines that run through the image. Note that they are not all arranged in the same direction. Of the three types of fibers in areolar connective tissue, only collagen is visible in this image. The other two types of fibers, elastic and reticular, do not show up in this image, even though they are there.

Some of the dark dots in the images are the nuclei of areolar connective tissue cells. The most common cell type is the fibroblast, but areolar connective tissue also contains macrophages, mast cells, and white blood cells.

Areolar connective tissue 400X

This is not a good image because it contains many bubbles in the glue that holds the cover slip in place. One of them is labeled "art" (bubbles are one type of artifact--see main page for explanation). There are many of them, of various sizes, all over the image. At the lower magnifications, these bubbles were not visible, but now they are. They make the tissue components look blurry, and can be very confusing if you don't know what they are. The collagen fibers (cf) have the largest diameter of the three fiber types and stain pink. The reticular fibers (rf) are smaller in diameter and look like thin black lines.

2) adipose tissue

Adipose connective tissue 40X

Adipose tissue is probably the easiest thing to identify. The only other things that looks anything like this is the lung, and there are some obvious differences that are pointed out on the lung page.

Adipose connective tissue 100X

What you are seeing in this image resembles a sponge, with lots of spaces. What look like hundreds of empty spaces on this image are where fat was stored inside the cells. When the tissue is processed, the fat dissolves in the processing chemicals and leaves the space.

Adipose connective tissue 400X

The bar labeled "a" indicates the width of one adipose cell (adipocyte). The light purple dots you see inside the cells are an artifact of process used to make the images, and do not represent real structures. The arrow points to the nucleus of one adipocyte. The nucleus and cytoplasm are pushed to the outside of the cell by the large fat droplet.

3) reticular

Reticular connective tissue 40X

Reticular connective tissue is named for the reticular fibers which are the main structural part of the tissue. The cells that make the reticular fibers are fibroblasts called reticular cells. Reticular connective tissue forms a scaffolding for other cells in several organs, such as lymph nodes and bone marrow. You will never see reticular connective tissue alone--there will always be other cells scattered among the reticular cells and reticular fibers.

The area in the box has been enlarged in the next image. In the tissue above the box the cells are very dense and it would be hard to see the reticular fibers there. There is no reticular tissue below the box.

Reticular connective tissue 100X

The resolution of this image is so low that you can't see the reticular fibers very clearly. What you can do at this point when looking at a slide in lab is to find a region of the specimen where the cells are not too dense. Then, when you switch to a higher power, the reticular fibers will be easier to see.

Reticular connective tissue 400X

In this image the red arrows point to individual reticular fibers. The dark-colored dots scattered among the fibers are mostly reticular cells and lymphocytes (this image is from a lymph node). On this image, you cannot tell which cells are reticular and which are lymphocytes.

The reticular fibers are attached to the reticular cells, so the two components of reticular connective tissue are "fixed", they cannot move around. The other cells and tissue fluid can move around in the spaces between the reticular fibers.

4) dense fibrous

a. dense regular connective tissue

Dense regular connective tissue 40X
Tendon

In dense regular connective tissue the bundles of collagen are all parallel to each other. The bar in this image shows you the width of this piece of dense regular connective tissue, which comes from a tendon.

Dense regular connective tissue 100X
Tendon

The collagen fibers are parallel to the arrow bar. Some of the dark spots you can see on this image are the nuclei of the fibroblasts that make the collagen fibers.

Dense regular connective tissue 400X
Tendon

In this image a fibroblast nucleus (fb nuc) is labeled, but you can see other nuclei once you know what to look for. You can't see the rest of the fibroblast cell because it stains the same color as the collagen fibers. The collagen fibers (cf) are parallel to the arrow bar. It's hard to see on the image, but the collagen fibers are not really straight. They have a slight "wave".

The "art" label indicates an artifact--a place where the collagen fibers pulled apart slightly during processing. Don't confuse the artifacts with real structures.

b. dense irregular connective tissue

Dense irregular connective tissue 40X
Silver stain

Dense irregular connective tissue (dict) is found in several places in the body. This image is from palmar skin (skin from the palm of the hand), and the dense irregular connective tissue is stained light brown. The very dark tissue on the top of the image is stratified squamous keratinized epithelium.

The reason the colors look so strange in the images on this page is the silver stain that was used to emphasize the location of collagen and reticular fibers. You will also see the more traditional hematoxylin (blue) and eosin (pink) on other skin slides (stratified squamous keratinized epithelium). You have to learn to recognize the tissue from its location relative to other structures and by any patterns that you can see in the cells or extracellular materials. Learning to recognize a tissue by the color it was stained will get you into trouble on a practical exam if the instructor uses a different slide.

Dense irregular connective tissue 100X
Silver stain

Most of the tissue you see in this image is dense irregular connective tissue (dict). There is a small amount of epithelium on the top (black) and some adipose tissue at the bottom and lower left (very light). Most of the rest of the image shows bundles of collagen fibers, which are stained brown.

Dense irregular connective tissue 400X
Silver stain

In this image you can see the collagen fibers (cf) that are the main component of dense irregular connective tissue. The n fibroblasts that make the collagen fibers cannot be seen because they do not pick up as much of the stain as the collagen fibers do.

5) elastic

Elastic connective tissue 40X
Human aorta c.s.

True elastic connective tissue is very rare, and we have no slide specimens that show it. But elastic fibers are present in relatively high concentration in several organs, including the largest arteries in the body. This image shows a portion of the wall of the aorta, the large vessel that carries blood from the heart to the body. Because elastin fibers are so important in the recoil of organs like arteries and lungs, we decided that you should know what they look like.

Elastic connective tissue 100X
Human aorta c.s

At this magnification you can see black wavy lines. Those are the elastin fibers. When an organ containing these fibers is stretched, the elastin fibers recoil (go back to their original length) and pull the organ back into shape.

Elastic connective tissue 400X
Human aorta c.s

The labels indicate individual elastin fibers (ef) in the aorta.
The areas stained pink (between the elastin fibers) contain smooth muscle cells, reticular fibers, and ground substance.

6) cartilage

a. hyaline cartilage

Hyaline cartilage 40X

Cartilage is easy to recognize because it looks so much different from other tissues. This image shows a section of the wall of the trachea. You can feel the hyaline cartilage in your own trachea by pressing you fingers gently against the front of your throat and moving them slightly up and down. The hyaline cartilage in the trachea is in the middle of the tracheal wall. It tends to stain more blue than other kinds of connective tissue (however, remember that color should never be the main cue you use to identify a tissue). The bar shows the position of the hyaline cartilage.

Hyaline cartilage 100X

You can begin to see the details in hyaline cartilage (hc) structure in this image. The bar shows you the extent of the cartilage in the tracheal wall. At the very top of this image is a layer of pseudostratified ciliated epithelium. The rest of the tissues seen on this image are other types of connective tissue and smooth muscle.

Hyaline cartilage 400X

Cartilage consists of cells embedded in a matrix (mat) of fibers and ground substance. The cells are called chondrocytes (ch) and the spaces in the cartilage in which they are found are called lacunae. Hyaline cartilage has very few fibers in its matrix, so the matrix usually looks smooth. The cells you see in the upper left corner of this image are part of the perichondrium, which consists of dense connective tissue.

b. fibrocartilage

Fibrocartilage 40X

Fibrocartilage is harder to recognize than hyaline cartilage because there are so many collagen fibers embedded in the matrix. The chondrocytes are not obvious in this image.

Fibrocartilage 100X

Now you can see the chondrocytes (ch) more clearly. Their nuclei look like little dots inside the lacunae. The black lines are the collagen fibers in the matrix.

Fibrocartilage 400X

The chondrocytes (ch) are located in lacunae (cavities in the maxtrix). Their nuclei (nuc) look like dark spots in the lacunae. Although the ground substance in fibrocartilage and hyaline cartilage is made of the same materials (hyaluronic acid and complex organic molecules), fibrocartilage looks a lot different because of the number of collagen fibers embedded in the matrix. If you think you are looking at cartilage because you see chondrocytes, the only thing you have to see to know whether it is fibrocartilage or hyaline cartilage are the cartilage fibers.

c. elastic cartilage

7) bone

(a) compact bone

Bone, compact, ground c.s.
100X

On this image you can see several of the structural units of bone tissue (osteons or Haversian systems). Each osteon looks like a ring with a light spot in the center. The light spot is a canal that carries a blood vessel and a nerve fiber. The darker ring consists of layers of bone matrix made by cells called osteoblasts (check your textbook for an explanation of the difference between osteoblasts and osteocytes). Between the osteons are layers of bone matrix that don't have the circular shape. They may be the remnants of osteons that are being remodeled into new bone tissue. See how many osteons you can pick out in this image. There is a good one in the lower center of the image.

Compact bone is very different from the other tissues you have seen. This image was made from a cross-section specimen of bone that has been ground to a very thin plate. Slides have to be made this way because the matrix of bone is too hard to be cut with a knife as the other tissues are. Another way of preparing bone slides is to remove the calcium salts from the matrix and then make sections by cutting off thin slices with a knife. Ground bone preparations are still very thick, and not much light can pass through them. They are excellent for showing the laminar (layered) structure of compact bone matrix, and the canals that link the osteocytes.

Bone, compact, ground c.s.
400X

This image is from a different slide than the other two images on this page. That's why the color looks different. We have added a dotted line around the outside of the osteon in case you had trouble picking them out on the previous image. Notice the layered effect in the matrix. The layers of matrix (lamellae) are added to the outside of the osteon as it grows larger. The amount of matrix that can be added to an osteon is limited by the distance that nutrients can diffuse to the osteoblasts from the blood vessel in the central canal.

Bone, compact, ground c.s.
400X

In this image, the canal is the light spot in the upper right corner. The dark spots are the lacunae where osteocytes would normally be found. The osteocytes themselves are not preserved in this type of preparation. Look at the lacunae in the lower left corner of the image. The small wavy lines that connect them are the canaliculi that connect osteocytes to each other and to the central canal in living bone tissue.

(b) cancellous (spongy) bone

Bone, cancellous, decalcified 40X

Cancellous or spongy bone has a much simpler structure than compact bone. The structural units are called trabeculae (tr). They are small, irregularly shaped slivers of bone that are attached to each other at their ends. This leaves spaces in the tissue that give the bone its name of "spongy". Spongy bone is in the upper left portion of the image, and compact bone (cb) is in the lower right. It would be difficult to confuse spongy bone with other tissues because of the distinctive and irregular shape of the trabeculae.

Bone, cancellous, decalcified 100X

The trabeculae (tr) consist of matrix that is made by osteoblasts. The bone cells build a trabecula by adding layers of matrix on the outside of the trabecula. This continues until it runs into another growing trabecula, and they fuse together. You can see pale lines in the trabecula in the center of this image. The lines are a result of building up the trabecula layer by layer. The spaces between the trabecula are filled with active or inactive bone marrow. Since this bone has adipose tissue in the spaces, the bone marrow in this area was inactive.

Bone, cancellous, decalcified 400X

The osteocytes (o) of cancellous or spongy bone are also found in spaces called lacunae. The layers of matrix are very clear on this image.

7) blood

Blood, 40X

Blood is an unusual connective tissue because it is normally in liquid form. It consists of a fluid called plasma and cells (formed elements) that are suspended in the plasma. The slide from which this image was prepared was a blood smear--it was made by putting a drop of blood on one end of a slide, and using a second slide to spread the blood into a thin, uniform layer over the slide. Some smears are better than others, meaning that the cells are more evenly spread out. Never use the part of a blood smear slide where cells are piled up on top of each other. Look for part of the slide where the cells are in a single layer. You can do that while you are using the 4X objective lens because you can see a larger area of the slide that way.

Blood, 100X

Using the 10X objective lens you can see individual cells and tell the difference between red and white blood cells. You can even see platelets if you know what to look for. The platelets on this image are very faint, but you can see them in the image below.

Most of the cells you see here are erythrocytes or red blood cells. They are small and don't have a nucleus. They are thin in the middle, and look like red doughnuts in this image. The leukocytes (white blood cells) are larger than red blood cells and they have nuclei that stain dark purple. Many of the white blood cells have segmented nuclei, meaning that the nucleus is pinched into two or more smaller parts that are still connected to each other (sort of like when you twist one of those long balloons to make a sculpture). Can you find the white blood cell in this image? Its nucleus has two segments.

Blood, 400X

The red blood cells in this image are stacked up on top of each other. We included it to show you what an unacceptable smear looks like! But it does have the advantage of including two kinds of white blood cell that are different from the one seen in the image above. The leukocyte on the left has many very dark granules in its cytoplasm. The granules are so dark that you can't see the nucleus. The leukocyte on the right has a two-lobed nucleus and reddish-orange granules in its cytoplasm. Consult your textbook to find out what they are.

The thrombocytes, or platelets, do how show very well in these images. You can see them if you look very carefully between the other cells. They will look like small purple dots.

BODY MEMBRANES

A. Cutaneous

B. Mucous

C. Serous

layers:

parietal

visceral

peritoneum

pleura

pericardium

MUSCLE TISSUE

Types:

1) skeletal

Skeletal muscle 40X

Skeletal muscle can be confused with dense regular connective tissue at low magnification (especially 40X). They stain the same color, and the skeletal muscle cell nuclei are flattened just like the fibroblast nuclei in dense regular connective tissue.

In this image you are looking at three bundles of skeletal muscle cells (fascicles). The bars show you the location of the connective tissue (perimysium) that separates the bundles.

Some of the purple dots you see in the image are the nuclei of the skeletal muscle cells, but some of the purple dots are artifacts of the digitizing procedure.

Skeletal muscle 100X

Although the resolution of this image does not reveal the edges of individual muscle cells, you can tell from the position of the nuclei where the cells are located. The nuclei are pushed to the edge of the cell by the proteins that allow the cell to contract. Where you see lines of flattened purple dots, you are looking at one side of a muscle cell.

There is connective tissue (endomysium) between the muscle cells. The nuclei of the connective tissue cells (fibroblasts) in the connective tissue (ct) may be smaller and rounder than the nuclei of the skeletal muscle cells.

Skeletal muscle 400X

The bar shows the width of one skeletal muscle cell. Most of the muscle cell nuclei you see will be along the sides of the cells. The nucleus identified in the image (nuc) is just inside the cell membrane, but the top the cell was caught by chance in this section. When you look at skeletal muscle cells sectioned longitudinally the nuclei will look long and flat or oval. When you look at cells that were sectioned transversely (cross section) the nuclei will look like round dots. The images on this page only show cells that are sectioned longitudinally.

The faint lines that run across the cells are called striations. They are not actual structures inside the cell, but are caused by the way the light from the microscope shines through the proteins inside the cell. Because the proteins are lined up precisely, they scatter the light as it passes through the specimen and makes a striped or banded pattern. If you cannot see the striations in lab, try closing the iris diaphragm a little to increase contrast, and then use the fine focus knob to focus up and down until the striations appear.

2) cardiac

Cardiac muscle 40X

The individual cardiac muscle cells are arranged in bundles that form a spiral pattern in the wall of the heart. On any slide of cardiac muscle you will see cells that have been sectioned in every possible direction, from transverse to oblique to longitudinal. The cells and their detailed structure is best seen on cells that are sectioned longitudinally. While you are on low power, scan the slide for an area where the cardiac muscle cells seem to be the longest.

The area in the box is enlarged in the next image.

Cardiac muscle 100X

In this image, you can see cells sectioned longitudinally (ls) and transversely (cs). In the lower part of the image you can see a coronary blood vessel (cv).

Cardiac muscle 400X

Cardiac muscle cells branch and attach to each other. The circle indicates a place where two cardiac cells are branched and connected to each other. Some of the nuclei (nuc) will look round and others will look flat. It all depends on how the cell was cut.

The striations in cardiac muscle are not as obvious as those of skeletal muscle. But you can see faint lines running across the cells in this image. If you have trouble seeing these on your slide in lab, close the iris diaphragm a little to increase the contrast.

Cardiac muscle 400X
showing intercalated disks

Cardiac muscle cells are joined end to end at special junctions called intercalated discs (id). These appear as dark lines that are perpendicular to the axis of the cell (they run across the cell). If you have trouble finding them in lab, first increase the contrast by closing the iris diaphragm a little, then use the fine focus knob to focus up and down. As you do this, structures that are thinner than the tissue section will come into and go out of focus. You should be able to see hundreds of intercalated discs on each slide.

The circle and arrow indicate another point where two cells are branched and interconnected.

3) smooth

Smooth muscle 40X

Smooth muscle can be confused with cardiac muscle because the cells are often running in different directions, just as they are in cardiac muscle. Smooth muscle cells are a lot smaller than cardiac muscle cells, and they do not branch or connect end to end the way cardiac cells do.

The area inside the box is enlarged in the next image.

Smooth muscle 100X

To get an idea of the arrangement of the individual cells, look at the nuclei, which look like purple spots in this image. If the nuclei look long and thin, the cells have been sectioned longitudinally (ls). If the cells look round, the cells have been sectioned transversely.

Smooth muscle 400X

This image provides an even better comparison of smooth muscle cells that have been sectioned in different planes (ls and cs). The nuclei (nuc) of smooth muscle cells are located in the center of the cell. Even though you can't see the cell membranes or the edges of the cells, you can visualize their arrangement just by looking at the nuclei.

NERVOUS TISSUE

cell types:

neurons

Nervous tissue 40X
Motor Neuron smear

There are many different kinds of cells in the nervous system, but they can be organized into two major categories: neurons and supporting cells. Neurons (n) are the ones that generate and conduct nerve impulses. Supporting cells do not conduct nerve impulses, but they perform many other functions for the nerve tissue.

The images on this page were made from a slide called a motor neuron smear. Motor neurons are large and easy to see, so they are usually used as examples. A smear means that a small chunk of nerve tissue from the spinal cord or brain was literally squashed and spread out on a slide. That's the only way to see neurons, because they have many extensions that would be cut off in a typical section.

Nervous tissue 100X
Motor Neuron smear

Each neuron (n) has extensions called processes (axons and dendrites) that allow it to communicate with other neurons. The pink lines that are attached to these neurons are their processes. About 90% of the cells in the central nervous system--brain and spinal cord--are supporting cells (sc). You can see that they are much smaller than neurons.

Nervous tissue 400X
Motor Neuron smear

Do you recognize this image? Tilt your head to the right and look again. We used this neuron as our mascot on the main page. The small dark dots are probably all nuclei of supporting cells that are either on top of or underneath the neuron. The large dark spot in the neuron is where its nucleus is located. Several cell processes (cp) extend outwards from the main body of the neuron. If you look at the processes where they are attached to the neuron's body, you can see neurofilaments (refer to your text book for an explanation of neurofilaments).

support cells (neuroglial cells or glial cells)

Wound Healing ‑ repair of tissues

Regeneration ‑ destroyed tissues replaced with tissues of same kind

Fibrosis ‑ destroyed tissues replaced by fibrous connective tissue