Tissue concept and classification, connective tissue – Klára Dolinová,
Yoni Solomonov
Tissue – definition, quick reminder
Tissues are three-dimensional structures of morphologically and functionally similar cells and
their products.
And what does a cell product mean? For example, proteins or polysaccharides secreted to the
cells environment to support its function and structure of the tissue.
Of course, cells of all of the body tissues have the same genetic material but keep in mind that
each cell type is different in its’ gene expression and therefore its’ function.
That’s why the tissue of the retina differs from skeletal muscle tissue or skin tissue.
Connective tissue
Connective tissue is made up of cells and extracellular matrix. Extracellular matrix (ECM),
according to the general definition consists of fibrous component and amorphous component
(that is ground substance).
Connective tissue is the most abundant tissue of the body. It has a characteristic composition –
it comprises mostly matrix and only few cells (unlike muscle, epithelium or nervous tissue).
What are functions of connective tissue?
We have 4 main functions and these are:
 Structural support – CT provides mechanical support and shape
 Defense – that means mechanical protection (skin, hair, bones) and immunity
(monocytes, macrophages, skin).
 Functional environment – CT provides environment for growth, reproduction or for
storing deposits. These environments are for example reticular fibers in hematopoietic
tissue.
 Repair – CT is very important in injury repair and in the process of scarring because it
comprises the pool of fibroblasts that help to establish new connective tissues.
Ground substance
As mentioned, the ECM includes fibrillar and amorphous component.
Fibrillar component
We distinguish 3 main types of fibers in the ECM. These are:
- Collagen fibers.
- Reticular fibers.
- Elastin fibers.
All of these are produced by the cells of the connective tissue.
Collagen:
There are around 30 types of collagen. Each provides the tissue with different properties. Every
tissue has variable composition of collagen types. I will further discuss 4 main types of collagen:
Collagen I – the most abundant in bone tissue, ligaments and tendons. This type provides strong
mechanical resistance to pull.
Collagen II – very abundant in cartilage, provide strong resistance to pressure.
Collagen III (AKA reticular fibers) – it is high distributed in many tissues, especially in reticular
tissue (also in lymphoid and hematopoietic tissue).
Collagen IV – is found in basement membrane in of epithelium.
The typical staining of collagen fibers HE & saffron – in which collagen appears yellow, or AZAN
where collagen appears blue.
Elastic fibers
Are mostly found in tissues that can expand or constrict – we call them elastic connective
tissues. These are for example lungs or large arteries. The elastic function of those fibers is due
to their unique shape.
Amorphous component
GAG – glycosaminoglycans – are made of long linear chains of polysaccharides. They are
negatively charged. The main function is to attract water to provide hydration of the ECM. As an
example we can name hyaluronic acid, chondroitinsulfate or keratansulfate.
Proteoglycans – are core proteins of the ECM. The special importance is in cartilage where they
bind hyaluronic acid which is important for the function of cartilage. Representative of this
group is aggrecan or syndecan.
Multiadhesive glycoproteins – cells of connective tissue have integrins on the surface. Some of
the multiadhesive glycoproteins can bind those integrins and by that fix cells to their ECM.
Another important glycoprotein is fibronectin.
Classification of connective tissue proper
Firstly, we may classify the cells of connective tissue as resident cells (fibroblast, reticular cells,
adipocytes, stem cells) and wandering/transient cells (plasma cells, mast cells = heparinocytes,
macrophages = histiocytes).
Second type of classification is according to the response to deformation  dense, loose or
elastic connective tissue.
Dense connective tissue
Dense connective tissue is the one that is abundant in collagen type I. Provides mechanical
protection of organs. It is characterized by small amount of cells in contrast with high number of
fibrillar component in ECM. Cells of the connective tissue are called fibroblasts (these are active)
and fibrocytes (these are non-active). We distinguish 2 types of this dense CT:
Regular
Regular means that all fibers are oriented parallel, in the same direction. This organization
provides very strong resistance to pull applied in parallel to fibers. It can be seen in tendons and
ligaments.
Irregular
Here we may again see high content of fibers however, in this case the fibers are oriented in all
directions. They provide mechanical resistance to pull in various directions. This CT can be found
in the sclera, fascia, and fibrous capsules of internal organs.
Loose connective tissue
Loose collagen connective tissue consists of relatively low amount of fibers in contrast with high
amount of cells. It has better blood supply compared to dense C.T. It also houses immune cells.
Found in dermis, in lamina propria and in tunica submucosa of viscera. Fibrous component
consists of collagen and elastic fibers.
Specialized connective tissue
Those connective tissues have some special features allowing the function of a specific body
part or an organ.
Elastic connective tissue
High content of elastic protein (fiber). Relatively low number of cells. Characterized by the
ability of the tissue to return to its original shape after deformation has been applied (= this
phenomenon is called elasticity). Typically seen in the vocal cords or in elastic membranes of the
aorta.
To visualize these tissues we use special staining – orcein – elastic fibers are in brown.
Reticular connective tissue
It comprises reticular cells that produce reticular fibers (type III collagen). This tissue is organized
in a 3D network that houses blood precursor cells and lymphocytes. Therefore, this type of
tissue is seen in lymphoid tissue, and in hematopoietic tissue – namely in bone marrow which is
blood producing.
We stain them by silver impregnation – reticular fibers are in black.
Adipose tissue
Adipose tissue can be subdivided into white and brown adipose tissue.
White adipose tissue
It is specialized in storing triglycerides (= fat storage). This tissue is abundant in cells called
adipocytes. Very important and frequently ignored function is endocrine function – adipose
tissue is pretty active in producing hormones. The most famous is the hormone called leptin that
participates in regulation of food intake.
Typical image of an adipocyte is a cell with a large droplet of fat with its nucleus and rest of
organelles compressed at the periphery by this huge vacuole full of fat. However we can differ
adipocytes with only one vacuole – these are univacuolar. And adipocytes with more vacuoles
that are smaller – these are called multivacuolar.
Brown adipose tissue
This tissue is found mainly in newborns and small children up to first year of life. As mentioned
before, the adipocytes here are multivacuolar.
Brown adipose tissue is specialized in heat production and energy production. Those cells have a
lot of mitochondria which causes the brown color of this tissue.