Connective Tissue: General Considerations

Connective tissue provides a matrix that connects and binds the cells and organs and ultimately gives support to the body. (Hence called support tissue also.)
The main constituent of connective tissue is the extracellular matrix. Due to this abundance of extracellular matrix, the cells in connective tissues are widely placed. Other tissues (like epithelium, muscular and nervous) are formed mainly by cells.

The function of connective tissues:

Mechanical function i.e. supporting function as described below:

  1. The Loose areolar tissue holds together structures like skin, muscles, blood vessels etc. and binds together the various layers of hollow viscera (stomach, intestine, urinary bladder, uterus).
  2. Reticular tissue forms a framework that supports the cellular elements of various organs like spleen, lymph nodes and glands.
  3. Enables the movement of skin over underlying structure.
  4. Allows mobility and stretching in hollow organs.
  5. Hold the bone at joints (in the form of ligament).
  6. Provide attachment for origin and insertions of many muscles (in the form of deep fascia, intermuscular septa, aponurosis and tendon). Tendon also transmits the pull of muscles to their insertion.
  7. Hold the tendons of muscles at wrist and ankle (in the form of retinacula- thickened deep fascia).
  8. Provide planes along which blood vessels, lymphatics and nerves travel (through areolar tissue and fascial membrane).
  9. Provides support and protection to the brain and spinal cord (in the form of dura mater).

Other functions:

  1. The matrix serves as medium through which nutrients and metabolic wastes are exchanged between cells and their blood supply.
  2. Provides immunity: due to presence of cells of immune system - macrophages and plasma cells.
  3. Wound repair: Fibroblast produces the collagen fibres necessary for wound repair.
  4. Adipose tissue stores nutrition as well as provides insulation.
  5. Regeneration of tissues (like cartilage and bone) due to the presence of undifferentiated mesenchymal cells. (Undifferentiated mesenchymal cells are capable of differentiating into specialized cells like chondroblast-cartilage forming cells and osteoblast- bone forming cells)

Connective Tissues are broadly classified into:
General Connective tissue
Specialized connective tissue (Bone, Blood, cartilage)

General Connective Tissue
The constituents for general connective tissues are as follow:
Ground substance


Fibroblast- these are the most numerous cells of connective tissue. They are so named because they produce collagen fibers (also reticular and elastic fibers).In tissue sections, these cells appear to be spindle shaped and nucleus appears to be flattened. Fibroblast become very active when there is need to lay down the collagen fibres e. g. during wound repair. Inactive forms are known as fibrocytes.

Mesenchymal cell- Embryonic connective tissue is known as mesenchyme which is made up of small cells with slender branching processes that join to form a network. Various components of mature connective tissue are derived from mesenchyme. Mesenchymal cells are capable of differentiating into any specialized cells. It is believed that some undifferentiated mesenchymal cells persist as such and these are the cells from which other types can be formed when required.

Pigment cells- they are easily distinguished as they contain brown pigment (melanin) in their cytoplasm. They are most abundant in the connective tissue of skin, of choroid and iris of eyeball. Of the many cells that contain pigment in their cytoplasm only a few are actually capable of synthesizing melanin. Such cells are called melanocytes, remaining cells are those that engulf pigment released by other cells. Such cells are called chromatophores or melanophore and are probably modified fibroblasts.

Fat cells or adipocytes- some cells store fat in large amounts and become distended with it. These are called fat cells, adipocytes or lipocytes.

Mast cells- these are small round or oval cells. The nucleus is small and centrally placed. The distinguish feature of these cells is the presence of numerous granules in the cytoplasm. They release various substances when appropriately stimulated e.g. release of histamine is associated with allergic reaction when a tissue is exposed to an antigen to which it is sensitive. They are most frequently seen around blood vessels and nerves.

Macrophages- these cells are part of mononuclear phagocyte system. Macrophage cells of connective tissue are also called histiocytes or clasmatocytes. They have ability to phagocytose unwanted material like bacteria invading the tissue and damaged tissues. Fixed macrophages resemble fibroblast but free or motile macrophages are round. The nuclei of macrophages are small and stain intensely than those off fibroblasts.

Lyhmphocytes-lymphocytes represent one variety of leukocytes and are in aggregation in lymphoid tissues. They reach connective tissue from these sources and are numerous when tissue undergoes inflammation. They have the ability to recognize the substances that are foreign to host body and destroy them by producing antibodies against them. They are of two types B lymphocytes and T lymphocytes.

Plasma cells- very few plasma cells can be seen in normal connective tissue. Their number increases in the presence of certain types of inflammation. These are mature B lymphocyte that have lost their power of further division. Plasma cells is seen to be small and rounded with nucleus having car wheel resemblance. The cytoplasm is basophilic.


Collagen –Collagen fibers are most abundant. With light microscopy they are seen in bundles. the bundles are made up of collections of individual collagen fibers which are 1-12 micrometer in diameter. In turn collagen fibers are made up of fibrils which are 20-200 nm in diameter. Each fibril consists of a number of microfibrils, 3.5 nm in diameter.

Bundle of collagen fibers appear white with naked eye. With H & E, they are stained light pink.
Collagen fibers can resist considerable tensile forces without significance increase in their length. They are also pliable and can bend easily.

Chemically collagen fibers are made up of protein called collagen.

Types of Collagen fibers

Type I- they are found in connective tissue, tendons, ligaments, fasciae, aponeurosis, bone, dermis, meninges etc.
Type II- found in hyaline cartilage, vitreous body.
Type III is the reticular fibres
Type IV- in the basal laminae of basement membrane.
Various other types are also recognized.

Elastic fiber- Elastic fibers are made up of a protein called elastin. They run singly (not in bundles), branch and anastomose with other fibers. Elastic fibers are thinner than those of collagen (0.1-0.2 micrometer). In some situation they are thick e.g. ligamentum flava and in other they are fenestrated as in walls of arteries.

Elastic fibers can be stretched (like a rubber band) and returns to their original length when tension is released. They are seen as shining line in unstained preparations.

Reticular fiber- these fibers are variety of collagen fiber (Type III). They are much finer and uneven in thickness. They form a network (or reticulum) by branching and anastomosing with each other. They do not run in bundles. Reticular fibers provide a supporting network in many situations e.g. spleen, lymph nodes and bone marrow; most glands including liver; and the kidneys. They are the essential component of all basement membrane.

Ground substance
The intercellular ground substance is a highly hydrated, complex mixture of glycosaminoglycans, proteoglycans and multiadhesive glycoproteins. The complex molecular mixture of the ground substance is colorless and transparent, it fills the space between cells and fibers of the connective tissue and because it is viscous acts as both a lubricant and a barrier to the invaders.

Glycosaminoglycans (GAGs) are linear polysaccharides formed by repeating disaccharide units usually composed of uronic acid and a hexosamine. The hexosamine can be glucuronic or iduronic acid. With exception of hyaluronic acid, these linear chains are bounded covalently to a protein core, forming a proteoglycan molecule. Because of abundance of hydroxyl, carboxyl and sulfate groups in the carbohydrate moiety of most GAGs, the GAGs are intensely hydrophilic and acts as polyanions. With the exception of hyaluronic acid, all other GAGs are sulfated to some degree in adult state. The carbohydtrate portion of proteoglycans constitutes 80-90% of this molecule.

Types of Connective tissue
Loose areolar- e.g. superficial fascia
Dense irregular – e.g. dermis of the skin
Dense regular- e.g. tendon
Elastic Tissue- e.g.  ligamentum flava
Adipose Tissue

Following resources are used while preparing this post (readers are strongly recommended to go through them for more details):

Wheater's Functional Histology: A Text and Color Atlas
Junqueira's Basic Histology: Text and Atlas