The Foundation of Human Anatomy: Tissues Part: 2

Hey! Welcome Back! It's the second part of the blog. After waiting for 1 long weekend, it's finally here. Today we are going to explore the 4th family of Tissues, Connective Tissue! And after reading this blog, you will finally get to know why I decided to divide it into 2 parts. So let's begin. 

 Connective Tissue

You can see connective tissue pretty much everywhere in the body. 

It is the most diverse type of tissue. The tendons to ligaments to bones to cartilage to blood, pretty much everything is connective tissue and this tissue is the most abundant as it is found everywhere. It connects everything in the body, and performs certain functions which we are going to learn about now. 

There are 4 types of Connective tissue according to current medical classification:

1. Proper Connective Tissue

2. Bone

3. Blood

4. Cartilage

It might be weird to think about but our bones and blood are just connective tissues. When we think about tissues, the image of bones or blood might not spring to mind, but they are just connective tissues. 

Common Things in every Connective Tissue

Connective tissues are of so many different types, and boast so much diversity. So, how do we know that anything is a connective tissue? There are 3 things that every connective tissue has in common:

1. All the Connective tissues in the body are formed from Mesenchyme, a loose and fluid type of Embryonic tissue.

2. All the Connective tissues in the body are mostly composed of non-living material, called the Extracellular Matrix. The space between the cells of the connective tissue is actually more important than what is inside the cells. The extracellular matrix is the connective tissue itself.

3. All the Connective tissues in the body have a vast degree of vascularity. Ex- Cartilage is Avascular, while the Dense Irregular Connective tissue in our skin is brimming with blood vessels.

Extracellular Matrix

It is composed of 2 main substances- The Ground Substance and Fibers.

The Ground Substance- It is a watery fluid which fills the spaces in between cells and protects them from their surroundings. It is a flexible fluid and is composed mainly of Proteoglycans called Glycosaminoglycans (GAG's) or Mucopolysaccharides, which are basically long sugar molecule chains. The ground substance is very sticky as it contains Starch and holds everything together.

Fibers- These provide a structure to the Extracellular Matrix and they have 3 types:

1. Collagen Fibers- These fibers are the strongest and the most abundant type. They are essentially made up of a lot of proteins.

2. Elastic Fibers- They are longer and thinner, making them very stretchy. They are made out of a protein called elastin, which make them flexible. They form a branching framework in the Extracellular Matrix. Found in the skin, blood vessel walls and lungs.

3. Reticular Fibers- These are shorter and finer collagen fibers. They have an extra coating of glycoproteins on their surface and form a framework to support organs.

So, there are fibers and Ground Substance in every Extracellular Matrix, which is in every Connective tissue.

Cells of the Connective Tissue

 The connective tissue is so diverse, so it also has a lot of different types of cells. 2 types of cells in Connective Tissue are Mature and Immature cells. 

Immature Cells

Every immature cell's name ends with the suffix-blast. Here, blast might sound destructive but it actually means "forming". Immature cells are stem cells which are still growing and dividing rapidly and choosing what type of cell to become. All blast cells have a specialized function, a common one is to secrete the ground substance and make the fibers which in turn make up the extracellular matrix. 

Ex- Chondroblasts are the blast cells of cartilage and when they secrete the ground substance, they make the matrix spongy. Ex- Cartilage of Nose, Ear etc.

 Osteoblasts are the blast cells of the bone and help in remodelling the bone by secreting the ground substance and laying the ground to form the bone.

 See the catch here! All blast cells' first function is to secrete the ground substance, which is their common function and specialized function is their 2nd function, after secreting the ground substance. 

Mature Cells

Every mature cell's name ends with the suffix-cyte. When the blast cells are done forming the matrix, they transition into a less active phase, where they become a cyte cell. Ex- A chondroblast becomes a Chondrocyte and an osteoblast becomes an osteocyte.

These cells mantain the matrix built by the blast cells and these cells are also the regulators of the connective tissue. These cells can transition back into blast cells if they need to do some repair work, for example- if you break a bone, some osteocytes will transition back into osteoblasts and secrete the ground substance and make the fibers to lay the foundation of a new extracellular matrix to form a bone. 

Not all cells share the suffix, as for example- Macrophages are the guard cells of the body. They devour harmful bacteria and protect the body against infections.

Proper Connective Tissue

Now, that we have covered the similarities and basic concepts in Connective tissues. We can talk about their types and subtypes. Proper Connective tissue has 2 types which further have 3 subtypes-

1. Loose Connective Tissue- Areolar, Adipose and Reticular

2. Dense Connective Tissue- Regular, Irregular and Elastic

Loose Connective tissues have a lot more Elastic Fibers, so they are more stretchy. There is more ground substance than fibers in these tissues. Ex- Skin.

Dense Connective tissues have more Collagen Fibers, so they are not really that stretchy and possess great force. There are more fibers than ground substance in these tissues. Ex- Tendons.

Areolar Connective Tissue (Loose Type I)

It is found everywhere in our body, wrapped around our organs, just under our epithelial tissue. They have a random arrangement of fibers, with more ground substance and a few fibroblast cells which make those fibers. Other tissues borrow some ground substance from this tissue, as it contains a lot of it. Areola means "open space" as this tissue has a lot of open space in it.

Adipose Connective Tissue (Loose Type II)

This tissue is found in a lot of the body's empty spaces. Another name for it is fat tissue. This one has mostly cells called Adipocytes, which swell up when they store fat to give us energy when we haven't eaten anything in a few days. This tissue also stores lipids (lipids store energy and are structural components of cell membrane) and insulates the body against heat loss.

Fun Fact: 18% of a Human's weight is Adipose tissue.

Reticular Connective Tissue (Loose Type III)

This type of tissue is like areolar tissue but instead it contains a lot of reticular fibers (hence the name) instead of collagen or elastic fibers. Instead of ground substance in the place of areolar tissue, this tissue holds the blood in place for many organs. It also supports a lot of developing blood cells. It forms the internal framework or stroma of the spleen, lymph nodes and bone marrow.

Now, lets talk about Dense Connective Tissue.

Dense Regular Connective Tissue 

This tissue is made up of mostly collagen fibers running parallel and has very little ground substance. It is found where force is exerted in one direction, ex- Tendons, Ligaments.

Dense Irregular Connective Tissue

This tissue is also made up of mostly collagen fibers, but they are stacked irregularly, hence the name. Having very little ground substance, the fibers in this tissue are thicker and it is found in places where tension might be exerted in different directions like the Dermis of the Skin.

Dense Elastic Connective Tissue

This tissue is made up of mostly Elastic fibers and is found in places where more elasticity is needed. Ex-In joints. Some of the largest arteries, for ex-Aorta are made of this tissue as it helps them dilate or constrict but also provides great support. 

You can't have reticular or areolar connective tissue in arteries just because you want elasticity, those arteries can get torn apart pretty easily and that is why you need dense connective tissue to be there.

Cartilage

Our body needs something to provide support to all the bones, joints, tendons and ligaments and that is where Cartilage comes in. It stands up well against both tension and compression pretty well. Cartilage is Avascular too, just like epithelial tissue. There are 3 types of Cartilage:

1. Hyaline Cartilage- It's ground substance is rich with Proteoglycans providing that sticky environment and it looks like glass. "Hyal" means glassy or transparent. It contains more Collagen fibers than other types of fibers. You can't really see the Collagen Fibers it contains under a microscope. 

Found at the tip of the nose, and it attached our ribs to the sternum.

2. Elastic Cartilage- It has more elastic fibers as the name suggests and it is present in places where strength and stretchability both are needed. Found in the pinna of the ear.

3. Fibro Cartilage- This type of cartilage is also known as a "Shock absorber". Its composed of thick collagen fibers and is great at withstanding pressure. It makes up the discs of the vertebrae and the space between our joints, where it helps the bones so that they don't grind together.

With that being said, lets move on to the next type of connective tissue.

Bone

Bone or Osseous tissue makes up the 3rd type of Connective tissue. Bones provide the body with a great framework, and help it move and support it. Without bones, we would be just blobbing around like an Amoeba. Bones contain bone marrow which is the house of the production of RBC's or Red Blood Cells.

A bone consists of 2 parts:

1. Compact Bone (Outer part)

2. Spongy Bone (Inner part)

Spongy Bone Tissue

It is found in the inner part of the bone, typically in flat bones of the sternum and the heads of long bones. This tissue is porous and contains a lot of space, which it uses to store bone marrow. Mostly containing fibers and ground substance with little to no cells.

Compact Bone Tissue

Forms the external layer of Bones and is more compact as the name suggests. It has no visible spaces, and contains a lot of bone cells with very little ground substance and fibers. It also stores calcium, within its spaces known as lacunae, for the cells to make more bone tissue.

Now, onto the last one!

Blood

Blood doesn't really seem to be a connective tissue but it connects distant parts of the body and provides them with nutrients, hormones, oxygen etc. It's ground substance is its plasma in which all of its cells or blood corpuscles float. Plasma is a yellow fluid which has protein fibers floating around in it. Blood does not really have those fibers we talked about, but it has a bunch of proteins dissolved in the plasma, which can form fibers like heparin when blood needs to clot. 

The cells of blood are as follows:

1. Erythrocytes (RBC's)- They contain haemoglobin, which binds with oxygen. They give the blood its red colour because they are present in abundance inside blood. They transport oxygen and carbon dioxide to different parts of the body.

2. Leukocytes (WBC's)- They are part of the immune system and help the body fight off infections. Macrophage which I talked about earlier is a type of WBC.

3. Thrombocytes (Platelet)- These cells help the blood clot whenever we get a cut. They take the help from the proteins dissolved in plasma to form fibers like heparin to clot blood.

As you can notice, all of these share the suffix-cyte as they are all mature cells. There are also a type of cells, which share the suffix-clast at the end of their names. These cells are responsible for breaking down tissues to form them again. Ex- Osteoclasts break down osseous tissue and Chondroclasts break down cartilage.

Lymph

Although, not a traditional type of connective tissue. Lymph has been started to being considered as a Connective tissue. It is formed from interstitial fluid (fluid between spaces of cells) and floats in the lymph vessels.

 Its part of the immune system and contains WBC's in abundance. It has an extracellular matrix, where the fluid lymph floats, which is composed of water, sugar and some other proteins. It also contains reticular fibers. It helps in transporting nutrients from the small intestine to blood. It also keeps the organs moist. 

Phew!There you have it! That was a big blog! Pat yourself on the back as you just finished all of it. It was a great experience exploring the tissues of the human body and I know that even you at one point thought that Connective tissue was too much to take in. But reading about it a bit more and trying to understand will definitely clarify things. I have wanted to write this blog for 5-6 months and I am proud that I was finally able to do it. See you next time!

Regards,

Deeparsh Bhanot