The fibers of Connective Tissue

Connective tissue is largely composed of extracellular matrix. Extracellular matrix is composed of fibrillar components and ground substance. The main fibers are elastic and collagens. Eosin effectively stains type I collagen and this accounts for the bundle staining observed in most H&E sections. However, fibrillar collagens have a unique structure that can be observed in the EM.

Fibrillar collagens

The high magnification TEM shows collagen. The short blue lines indicate collagen fibrils cut longitudinally showing the alternating light and dark regions, representing the overlap and gap regions respectively. The blue arrows are at cross sections of fibrils, and the individual dots within the circle are individual triple helices. The red line indicates a grouping of fibrils called a fiber. (Image from Junqueira’s Basic Histology 14th edition)

Elastic Fibers

The series of TEMs illustrate elastic fiber assembly. a) Shows the microfibers (fibrillin) which are secreted first by fibroblasts. Next, elastin (dark amorphous material) is secreted and aggregates on the microfibrils (b). (c) The mature elastic fiber has elastin aggregated in the middle and microfibrils surrounding it. (Image Junqueira’s Basic Histology 14th edition)

Elastic fibers have a high concentration of glycosylated amino acid residues and generally don't stain well with H&E in the light microscope. Typically elastic fibers are visualized with specialized stains such as aldehyde fuchsin. Now look for elastic fibers in the tunica media (middle layer) of these two slides (sample 1, sample 2). On the connective tissue slide showing small arteries and vein, examine the internal and external elastic laminae on either side of the tunica media.
 

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