FIG. 4.3 Collagen
(a) EM ×32 000 (b) SEM ×32 000, teased preparation
The typical appearance of type I collagen is shown here. The fibres are seen in transverse T and longitudinal L sections. A characteristic feature is the cross-banding, with a periodicity of about 64 nm which results from the polymerisation of the tropocollagen molecules (300 nm long) each overlapping the next by about a quarter of their length.

FIG. 4.4 Collagen
(a) H&E (HP) (b-d) H&E (MP) (e) Trichrome (HP)
These micrographs show variations in the size and packing of collagen fibres (type I collagen) and demonstrates fibroblasts F. Micrographs (a) and (b) are from a fascia in the hand, with (a) longitudinal and (b) transverse; in these the collagen fibres are large, tightly packed and oriented in one direction for maximal tensile strength. Micrograph (c) is from the dermis of the skin, with less tightly packed collagen fibres running perpendicular to each other (longitudinal and transverse) to give strength in both directions. Image (d) is fibroadipose tissue from a finger, with fine collagen fibres coursing between adipocytes Ap and blood vessels BV. Image (e) is a trichrome stain of skin; collagen stains blue, smooth muscle SM red, and elastin fibres E red.

FIG. 4.5 Reticulin fibres (type III collagen)
Silver impregnation method/neutral red (HP)
Reticulin fibres form a delicate supporting framework for many cellular organs such as endocrine glands, lymph nodes, bone marrow and liver. A fine network of branching fibres ramifies throughout the parenchyma, often anchored to collagenous septa which traverse the tissue. Reticulin is a non-banded form of collagen, type III collagen.
Reticulin fibres stain poorly in H&E preparations but are able to absorb metallic silver, staining them black. Reticulin is the earliest type of collagen fibre to be produced during the development of all supporting tissues. It is found in varying quantities in most mature supporting tissues.
This micrograph shows the fine reticulin scaffolding of the liver; the framework supports the hepatocytes (the purple-stained plates of cells) and the sinusoids through which blood flows.

FIG. 4.6 Elastin fibres
(a) H&E (HP) (b) Elastin stain (HP)
Micrograph (a) shows the wall of an elastic artery, made up mainly of alternating smooth muscle cells and thick sheets of elastin admixed with collagen (see Ch. 8 for blood vessels). Like collagen and smooth muscle cytoplasm, elastin E is eosinophilic; it is recognisable here because the elastin sheets are thick and slightly refractile, slightly more eosinophilic than the other components and have a wave-like conformation due to relaxation of the vessel wall.
Micrograph (b) shows a histological section of an elastic artery stained specifically for elastin; with this method, elastin is stained black and collagen red. The functional properties of large arteries are mainly determined by the amount of elastin in their walls, which allows stretching and recoil with the pulse pressure generated by the heart.

FIG. 4.7 Elastin fibres
(a) Elastin stain (MP) (b) Elastin stain (HP)
In micrograph (a) of skin, the pink-stained, coarse, closely packed bundles of collagen in the dermis are interwoven by elastic fibres, stained black. Elastic fibres in the dermis allow the skin to stretch and recoil, keeping it wrinkle-free. The epidermis Ep is just visible.
Micrograph (b) shows pleura (see Ch. 12) where a layer of elastic fibres, stained black, is woven into the collagen supporting tissues (stained red). The lung contains abundant elastic fibres which help to expel air in expiration.

FIG. 4.8 Elastin fibres (spread preparation)
Elastin H&E (HP)
In most tissues, elastin occurs as short, branching fibres which form an irregular network throughout the tissue. This is not easily seen in tissue sections. It can be better demonstrated in spread preparations such as in this micrograph in which elastin fibres E are stained dark purple, collagen fibres L are stained pink and nuclei are stained blue. A branched capillary Cap crosses the field and two densely stained mast cells Ma are also seen (see Fig. 4.20).

FIG. 4.9 Elastin
EM ×50 000
This micrograph shows elastin E in the delicate supporting tissue underlying the epithelium of mouse trachea. The field also contains collagen fibrils T (cut in transverse section) and the fine cytoplasmic extensions of fibroblasts F, responsible for elaboration of the extracellular constituents.
The elastin is mostly an amorphous mass of polymerised tropoelastin. Microfibrils M of the structural glycoprotein fibrillin, which is involved in the process of elastin deposition, can just be discerned at this magnification as dots (in transverse section) lying within and around the elastin. Microfibrils can also be seen in the lower part of the field cut in longitudinal section ML in association with small amounts of elastin protein.