Fibers of connective tissues
Recall that CT is composed of cells and extracellular matrix (ECM), with the latter being its major constituent. Two major components of the ECM are protein fibers and ground substance, with the three principal fibers being reticular, collagen, and elastic. Here we will look more closely at collagen and elastic fibers.

Collagen
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.

High magnification TEM shows individual collagen fibrils and their characteristic banding pattern when cut longitudinally (blue bars in image). The alternating light and dark regions represent the overlap and gap regions of the highly organized collagen molecules that self-assemble to form a fibril. The blue arrows indicate cross sections of fibrils, and the individual dots within the circular fibril are individual collagen triple helices. The red line indicates a grouping of fibrils called a fiber.

Elastic Fibers
The following series of TEMs illustrate elastic fiber assembly: (a) Shows microfibrils (fibrillin) which are secreted by fibroblasts (and smooth muscle cells) and forms a scaffold. (b) Next, elastin (dark amorphous material) is secreted and aggregates on the microfibrils. (c) The mature elastic fiber has elastin aggregated in the middle and microfibrils surrounding it.

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 and sample 2). On the connective tissue slide showing small arteries and veins, examine the internal and external elastic laminae on either side of the tunica media.
 

Try this self-assessment quiz.

Next is nervous tissue.