Connective Tissue I

Embryonic and Ordinary Connective Tissue

Connective tissue comes in many different forms and configurations throughout the body. Connective tissue is made up of certain types of cells and extracellular products secreted by them. This extracellular secretion is composed of fibres and an amorphous ground substance (amorphous means, "shapeless") bathed in tissue fluid. The amorphous ground substance is transparent and homogeneous. It is difficult to appreciate on slides, as it is often removed in the processing of the tissue. Ground substance is composed of glycosaminoglycans (acid mucopolysaccharides), proteoglycans (glycosaminoglycans and protein bound together) and glycoproteins (protein bound with sugars).

The fibres which run through the ground substance are of three types: collagenous, reticular and elastic. Collagenous and reticular fibres are made from a family of protein types called collagen. The different types of collagen are labelled with Roman numerals. Currently, there are 14 different types, with types I, II , III and IV being the most common. The type of collagen determines the particular kind of fibre (there are many kinds). These fibres give strength (especially tensile strength) to the tissues they are in, allowing those tissues to withstand considerably more strain than if the fibres were not present. The different fibre arrangements give rise to many different connective tissue structures, from the thin, sheet-like basal lamina which many cells rest upon, to the large, strong fibre bundles which make up tendons.

Elastic fibres serve a slightly different purpose in that they give resiliency to a tissue. They allow a tissue to be stretched and then return to its original shape (just like an elastic - hence the name). They are composed of proteins know as elastins, whose special bonding to one another allows for stretching without breaking. These fibres are often found in conjunction with collagenous fibres, thereby giving a tissue tensile strength and some resiliency (i.e. tendons).

On this page, we will focus on the embryonic connective tissue and on the ordinary connective tissue types. We will focus on the special connective tissue types (cartilage, adipose, bone and blood) in the next page.

Embryonic Connective Tissue Types
Mesenchyme: Mucoid:
Mucho mesenchyme in the middle, man Mondo mucoid mess, mama

Here we see typical mesenchymal cells. These are pluripotent (undifferentiated) cells which will form, among other tissue types, the cells that will become adult connective tissue cells (which secrete the amorphous ground substance and the protein precursor for fibres, etc.). These cells have a stellate (star-shaped) appearance and have a relatively large amount of extracellular space between them which is filled with ground substance (not seen well here because of the tissue processing, recall). Along the cytoplasmic extensions (the "arms" which give the cells their stellate shape), there are fine reticular fibres which gives the tissue some strength and provides a matrix for the cells. Reticular fibres require a special stain to visualize and are not seen in this section. Mucoid connective tissue (1) is quite similar to mesenchymal connective tissue, but with more space between the cells and fewer reticular fibres. It is found in the umbilical cords of mammals. As a result, there is a large amount of ground substance which contributes to the decreased cohesiveness of this connective tissue.
NOTE: This image is at a lower magnification than the one on the left.

Embryonic Connective Tissue Types
Mesenchyme:	Mucoid:

Here we see typical mesenchymal cells. These are pluripotent (undifferentiated) cells which will form, among other tissue types, the cells that will become adult connective tissue cells (which secrete the amorphous ground substance and the protein precursor for fibres, etc.). These cells have a stellate (star-shaped) appearance and have a relatively large amount of extracellular space between them which is filled with ground substance (not seen well here because of the tissue processing, recall). Along the cytoplasmic extensions (the "arms" which give the cells their stellate shape), there are fine reticular fibres which gives the tissue some strength and provides a matrix for the cells. Reticular fibres require a special stain to visualize and are not seen in this section.	Mucoid connective tissue (1) is quite similar to mesenchymal connective tissue, but with more space between the cells and fewer reticular fibres. It is found in the umbilical cords of mammals. As a result, there is a large amount of ground substance which contributes to the decreased cohesiveness of this connective tissue. NOTE: This image is at a lower magnification than the one on the left.

Ordinary Connective Tissue Types
Loose: Dense:
Loose (areolar) connective tissue Dense irregular connective tissue

Loose connective tissue is found just about everywhere in the body, as it provides support for structures passing through it, such as blood and lymph vessels and nerves. As well, it also serves to bind together other tissues, including organs and their components. Loose connective tissue can also be called areolar connective tissue (from the Latin, areolatus meaning, "with small spaces"). The main cells which make up loose connective tissue are fibroblasts. These cells secrete the components which make up the fibres and ground substance that form the non-cellular component of the loose connective tissue. In the spread film (smear) slide preparation above, both collagen fibres (1 - bundle of fibres circled) and elastic fibres (2 - each a single fibre) can be seen. The darker stained oval shapes are nuclei some of which would be fibroblast nuclei. The above slide shows the skin of a frog. Beneath the epidermis (1) lies the dermis which is composed of the stratum spongiosum (2) and the stratum compactum (3). Both layers of the dermis are composed of dense irregular connective tissue (one layer is just more dense than the other, which accounts for the names). Dense irregular connective tissue features fibres (mainly collagen) that vary in their arrangement. In the stratum spongiosum, collagen fibres form a moderately loose meshwork. In the stratum compactum, collagen fibres are packed closer together and in a more parallel fashion as can be inferred from the position of the nuclei of fibroblasts located among them.

Ordinary Connective Tissue Types
Loose:	Dense:

Loose connective tissue is found just about everywhere in the body, as it provides support for structures passing through it, such as blood and lymph vessels and nerves. As well, it also serves to bind together other tissues, including organs and their components. Loose connective tissue can also be called areolar connective tissue (from the Latin, areolatus meaning, "with small spaces"). The main cells which make up loose connective tissue are fibroblasts. These cells secrete the components which make up the fibres and ground substance that form the non-cellular component of the loose connective tissue. In the spread film (smear) slide preparation above, both collagen fibres (1 - bundle of fibres circled) and elastic fibres (2 - each a single fibre) can be seen. The darker stained oval shapes are nuclei some of which would be fibroblast nuclei.	The above slide shows the skin of a frog. Beneath the epidermis (1) lies the dermis which is composed of the stratum spongiosum (2) and the stratum compactum (3). Both layers of the dermis are composed of dense irregular connective tissue (one layer is just more dense than the other, which accounts for the names). Dense irregular connective tissue features fibres (mainly collagen) that vary in their arrangement. In the stratum spongiosum, collagen fibres form a moderately loose meshwork. In the stratum compactum, collagen fibres are packed closer together and in a more parallel fashion as can be inferred from the position of the nuclei of fibroblasts located among them.

Back to the Histology Index

Go on to Special Connective Tissue Types

Department of Integrative Biology, University of Guelph, Guelph, Ontario, Canada N1G 2W1.

Latest Revision: May 2012