Histology basics – Connective Tissues

Connective Tissues  

(all images can be clicked on for a larger version)

(if you are looking for a specific connective tissue use control f (find) to locate that tissue quickly)

Connective tissue is the most variable of the primary tissue types and it is also probably the most wide spread in the body.  Most connective tissue have two things in common:  1)  cells separated by a lot of extracellular matrix and 2)  All but one contain collagen protein (no collagen in blood tissue, but that’s the only connective tissue without collagen).    I’ll list each of the tissue types with images (where possible) and some details about their functions and where the are found.  This will be a long page because there are at least 10 different types of connective tissue.


02.002 areolar tissue 100X (2)

Areolar Connective Tissue

This is one of the loose connective tissues.  It is found under most epithelial tissues and in many other places in the body (areolar tissue is the most widespread of the connective tissues).  It consists of fibroblast cells, that produce the fibers, and other components of the ECM and also often contains white blood cells.  The fiber types found in the ECM of areolar tissue include collagen and elastic fibers.  The elastic fibers allow the tissue good elasticity (it returns to its original shape if deformed) and the collagen fibers give it strength to resist tearing.  However, compared to other connective tissues areolar tissue has few fibers…  that’s what makes it a loose connective tissue.  The fibers in areolar tissue are arranged with a good amount of space in between, that space contains interstitial fluid that circulates from blood vessels and back to both blood and lymphatic vessels.  Roaming in this interstitial fluid are white blood cells (neutrophils and macrophages) on the lookout for aliens and debris to destroy.  The fluid also delivers nutrients to the cells that live in it as well as adjacent epithelial tissues.  Areolar tissue is soft, flexible, and resilient.  Note on aging:  as we get older the areolar tissues that underlie our skin slowly looses elastic fibers, this causes our skin to sag and wrinkle (UV exposure and smoking speed up this process).


(No picture of reticular tissue right now…  sorry)

Reticular Tissue (I do not have good images of this in my lab)

In reticular tissue the collagen fibers are arranged in what looks like a three dimensional netting.  This gives the tissue a nice framework in which white blood cells can travel easily.  Reticular tissue is seen in organs that contain many white blood cells (spleen, red bone marrow, lymph nodes).


02.13 dense regular connective tissue 450X

Dense Regular Connective Tissue

Dense connective tissues have a lot of collagen, making them very tough.  In dense regular connective tissue all of the collagen fibers are running in the same direction, giving the tissue a very high tensile strength.  We find dense regular connective tissue in tendons and ligaments.  The above image of dense regular connective tissue is from a section of tendon.



02.13.5 thin skin 40X dense irregular connective tissue

Dense irregular connective tissue

Dense irregular connective tissue is mostly made of collagen fibers that are arranged in all directions…  it kind of looks like a mess of collagen.  Because the collagen fibers are running in all directions this tissue is tough, not just in one direction like a tendon is, in all directions.  The image above is from the dermal layer of skin and most of what you see is dense irregular connective tissues (there are also a few glands and a bit of smooth muscle in the view).  The dense irregular connective tissue that you see is important for the skin’s toughness…  this is how skin resists tearing when stretched out.  We also find dense irregular connective tissue in the white of the eye (sclera) and wrapped around many organs.  Our muscles are surrounded by a thin layer of dense irregular connective tissue called fascia.



Images of adipose tissue:   The following images are good for seeing how to recognize the same tissue even from different preparations or magnifications of a tissue.

02.03 adipose tissue 02

The image above contains only adipose tissue (450X magnification).

03.37 Thick skin 100X pacinian corpuscle

The image above is from skin and contains some glands and other structures surrounded by adipose tissue (100X magnification).

02.05 adipose tissue 04 100X

The image above is of adipose tissue (there are also a couple of blood vessels in the image)  (100X magnification).

Adipose tissue and its functions

Adipose tissue (fat tissue) is composed of large cells filled with triglycerides (fat) with the nuclei somewhere on the side of the cell.  The three images above all contain adipose tissue (with differences in staining and magnification).  In each image you can see large white cells, and in two of them you can see the nuclei in those cells.  Adipose tissue serves a few functions…  I’ll list and describe these.

  • Energy storage — triglycerides are packed with energy.  When that energy is needed these cells break the triglycerides into fatty acids that other cells can use for energy (muscles run mostly on free fatty acids, at least until they run low on oxygen).
  • Insulation — adipose tissue is an excellent insulator, helping to keep body temperature stable and keeping the inside of the body warm in cold weather.
  • Protection from trauma — adipose tissue provides some padding to underlying bones and muscles, so if we get hit the adipose tissue absorbs some of the energy helping prevent damage to underlying tissues (men with a “spare tire” belly tend to survive car accidents better).
  • Hold some organs in place while protecting them — your eyes rest in a cup of adipose tissue, this helps hold them in position, but also lets them move a bit if we get hit in the eye (if you do get punched in the eye the eye can move back into its socket a bit instead of being popped like a grape).
  • Contributes to body contours —  one of the ways we recognize different sexes is by body shape, and it is adipose tissue that gives us these different shapes.  Women in general tend to have a higher percentage of body fat (it’s one of the reasons that women are tougher than men and live longer), that fat makes female skin softer, and that fat is more concentrated in the breasts, hips, and buttocks…  giving the female body is beautiful shape.
    • Women are tougher than men!!!!  Women live longer and can survive difficult conditions better than men (starvation, cold, etc).  One reason for this is the increased body fat percentage that women have.  If you look again at the list above you’ll see that this extra bit of fat gives women energy reserves in case the food runs out, it allows women to survive cold better, and it gives women better protection from trauma.




Cartilage comes in a few varieties, each with a different mixture of extracellular matrix (ECM) components.  Here is a list of those components :

  • collagen — as already mentioned this protein gives strength to a tissue.
  • elastic fibers — also mentioned earlier, these proteins give elasticity to a tissue.
  • ground substance — ground substance is basically the rest of what is there in the ECM…  we find tissue fluid here as well as glycoproteins, glycosaminoglycans, and proteoglycans.  One function of these substances is to attract water, making sure that there is enough tissue fluid in the cartilage.

Here is a list of the types of cartilage (followed by a description of each type individually)

  • hyaline cartilage
  • elastic cartilage
  • fibrocartilage

hyaline cartilage:

02.33 hyaline cartilage 05 450X.psd

The above image is hyaline cartilage from human trachea (450X magnification).  Note the features: smooth appearing ECM separating the cells.

02.31 hyaline cartilage 04 100X

The above image is also hyaline cartilage from human trachea, just a lower magnification (100X magnification)

02.29 hyaline cartilage 03 growth plate 450X

The above image is also hyaline cartilage.  This time it’s from a growth plate inside of a growing bone.  Note the stacks of chondrocytes (cartilage cells).  The next image is a lower magnification of the same…

02.27 hyaline cartilage 02 growth plate

In the above image hyaline cartilage is the purple/blueish line in the center flanked on each side by pinkish appearing spongy bone tissue (100X magnification).

Hyaline cartilage contains mostly ground substance with fewer collagen and elastic fibers.  This combination makes hyaline cartilage relatively strong and flexible.  The greater amount of ground substance also makes hyaline cartilage a good pad for joints.  Hyaline cartilage is found at the ends of many bones as articular cartilage.  When compressed the fluid in the hyaline cartilage is squished to the surface helping to lubricate the joint.  Hyaline cartilage is found in your nose, connecting your ribs to your sternum, in your trachea and larynx, and in growth plates of bones.


Elastic cartilage

02.17 elastic cartilage 450X

The above image is an example of elastic cartilage (450X magnification).  Note the irregular shape of the cell spaces and the darker color as compared to hyaline cartilage.  The next image is from the same slide,  just at a lower magnification.

02.15 elastic cartilage 100X

The above image is of elastic cartilage from a human epiglottis (100X magnification).

Elastic cartilage contains more elastic fibers than hyaline cartilage does–  this is why elastic cartilage stains a bit darker and has a more rough appearance than hyaline cartilage.  Elastic cartilage is found in your outer ear (that’s what allows your ears to be so elastic– to bounce back if you bend them over).  Elastic cartilage is also found in your epiglottis (a flap that closes over your windpipe when you swallow, so you don’t swallow into your lungs).



02.23 fibrocartilage 450X

The above image is fibrocartilage from a rat’s intervertebral disk (450X magnification).

02.21 fibrocartilage 100X

Fibrocatilage from a rat’s intervertebral disk (100X magnification)

02.19 fibrocartilage 40X

Fibrocartilage from a rat’s intervertebral disk (40X magnification)

Fibrocartilage contains much more collagen than hyaline or elastic cartilage, this gives fibrocartilage a lot of strength.  We find fibrocatilage in places where that strength is needed, including the vertebral disks, the knees, between the pubic bones as the symphysis pubis, between tendons and the bones they connect to, and when a bone breaks fibrocartilage is formed during the healing process.


Bone tissue

02.11 bone tissue 100X

The above image is of a section of compact bone at high magnification (450X magnification).  Note the tree ring appearance.

04 bone ground 450X.jpg

The image above and below this writing is also compact bone at high magnification.  The image below is simply labeled in case you need to know the structures of compact bone (I usually cover those parts when we cover the skeletal system).

05 bone ground 450X labeled.jpg


02.09 bone tissue 40X

The image above is also compact bone at lower magnification (100X magnification).  Again, note the tree ring like appearance…  It looks as if it is made up of several tree rings (each of those tree ring like structures is called an osteon).

13 bone spongy bone - bone 450X

The above image is of spongy bone that has been decalcified (450X magnification).  The structure going across the image from left to right is one of the pieces of bone that we find in spongy bone.

11 bone spongy bone 100X

The image above is decalcified spongy bone at lower magnification(100X magnification).  The lighter pinkish areas are bone and the darker purple areas are red bone marrow.

Bone tissue is the most brittle of the connective tissues (probably the most brittle of all tissues).  Its functions are support, protection, mineral storage, and blood cell production (all of the cellular components of blood are born in red bone marrow).

Blood Connective Tissue

02.07 blood 450X

The above image is of blood at high magnification (450X magnification).  Blood is pretty easy to recognize because of the many red blood cells (RBCs) that you can see in this image as little pinkish/redish circles.  Here is a closer look…

02.07 blood 450X close in

The above image is a digital zoom in of blood tissue.  In this image you can see the three formed elements of blood.  The red blood cells are the redish circles, the white blood cell (WBC) appears in the upper left (its nucleus stains purplish), and if you look hard you can see a platelet as a small dot between some of the red blood cells.

Blood is the only liquid connective tissue (the only liquid tissue of the body really) and its job is that of a transport medium.  Blood contains gasses like oxygen and carbon dioxide, many nutrients, waste products from tissues, hormones, vitamins, etc.  for transport between organs and tissues.  The blood is circulated, by the heart and blood vessels, to within just a short distance of all living cells in the body in order to exchange nutrients and oxygen for waste products and other substances.


Using blood to look deeper into the interconnectedness of the body

Thinking of blood and the cardiovascular system is a great way to think of our body systems and organs work with each other.  The blood carries nutrients that are absorbed by the digestive system, it also carries oxygen that is absorbed by the respiratory system.  Waste products are taken by the blood to the liver and kidneys for chemical processing (the liver has enzymes for dealing with many toxic wastes) and elimination from the body.  Hormonal signaling works because the blood picks up hormones from the endocrine glands and transports them throughout the body where they eventually make contact with target tissues/cells and have their effects.  All of these systems, the cardiovascular system, the digestive system, the respiratory system, the endocrine system, and all the rest have to work together to keep us alive and healthy.  Your ultimate goal should be to understand the ways in which these tissues, organs, and organ systems all work together, and maybe some of the ways in which this can go wrong and lead to disease and death.



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