16 Fibrous connective tissues

16.1 Introduction

Here is an image a fibrous connective tissue.

It has three essential features: (1) cells (stained with methylene blue), (2) fibres (stained pink with eosin), and (3) matrix (in the spaces between the cells and the fibres).

The fibrous connective tissues in meat form a continuous mesh, as shown in the image to the left, from the microscopic strands of endomysium around individual myofibres, to the thick layers of perimysium around bundles of myofibres (fasciculi), all being gathered and connected to the very thick epimysium on the surfaces of individual muscles.

 The image below shows a thick layer of perimysium.

 The endomysium, perimysium and epimysium contain two types of protein fibres, collagen and elastin, which now we will consider in detail.

16.2 Collagen fibers

Tropocollagen is a high molecular weight protein (300,000 Daltons) formed from three polypeptide strands twisted into a triple helix. Each strand is a left-handed helix twisted on itself, but the three strands are twisted into a larger right-handed triple helix. The triple helix is responsible for the stability of the molecule and for the property of self-assembly of molecules into microfibrils. The flexible parts of each strand projecting beyond the triple helix (telopeptides) are responsible for the bonding between adjacent molecules. In other words, the cross links binding tropocollagen molecules together laterally are made between the helical shaft of one molecule and the non-helical extension of an adjacent molecule.

In the polypeptide strands, the small amino acid glycine occurs at every third position, and proline and hydroxyproline account for 23% of the total residues. The regular distribution of glycine is required for the packing of tropocollagen molecules and has been claimed as evidence all animals are derived by evolution from a single ancestral stock, since the chance development of this unique regularity in unrelated animals is thought unlikely. Hydroxyproline is quite rare in other proteins of the body, and an assay for this imino acid (an imino acid is chemically similar, but not the same as an amino acid) provides a measure of the collagen or connective tissue content in a meat sample. Tropocollagen also contains a fairly high proportion of glutamic acid and alanine as well as some hydroxylysine.

16.4 Biochemical types of collagen

The various types of collagen of interest in understanding the structure of meat are as follows.    
Tendons often extend into the belly of a muscle or along its surface before they merge with its connective tissue framework, and Types I and III collagen both may be extracted from meat. Even within tendons, there may be some Type III collagen forming the endotendineum or fine sheath around bundles of collagen fibrils. In fibres composed of collagen Types I and II, fibrils have a straight arrangement whereas, in fibres of Type III collagen, the fibrils have a helicoidal arrangement.
Small diameter Type III collagen fibres are called reticular fibers since, when stained with silver for light microscopy, they often appear as a network or reticulum of fine fibres. The larger diameter collagen fibres formed from Type I collagen are not blackened by silver.

Collagen fibres shrink when they are placed in hot water, and ultimately they may be converted to gelatin. Around 65ºC, the triple helix is disrupted and the alpha chains fall into a random arrangement. The importance of this change?  It tenderizes meat with a high connective tissue content.

Tropocollagen molecules from older animals are more resistant to heat disruption than those from younger animals.

16.4 Collagen biosynthesis

The synthesis of the different polypeptide strands combining to make different types of tropocollagen is genetically regulated by the production of messenger RNA. The synthesis of polypeptide strands occurs on membrane-bounded polysomes, but the hydroxylation of lysine and proline occurs after the strands are assembled. Ascorbic acid is required for the hydroxylation of lysine and proline. Polypeptide strands enter the cisternae of the endoplasmic reticulum (a membranous assembly labyrinth within the cell), the terminal extensions of the strands are aligned, and then the strands spiral around each other. Procollagen or immature tropocollagen has long terminal extensions protruding from each end of the newly formed triple helix. Procollagen moves to the golgi apparatus and is packaged into vesicles moved to the cell surface, probably by microtubules. Except for some Type III procollagen molecules, the long terminal extensions are then enzymatically reduced in length.

Outside the cell, tropocollagen molecules become aligned in parallel formations, and then they link up laterally to form fibrils. It is likely that tropocollagen monomers are partially assembled together in groups before they are added to an existing collagen fibril. Firstly, vacuoles containing procollagen fuse to form a fibril-containing compartment. Then the cytoplasmic extensions withdraw from between several fibril-forming compartments to create a bundle-forming compartment. 

16.5 Crosslinking of tropocollagen molecules

16.6 Elastic fibres & elastin

 Individual collagen fibres only lengthen by about 5% when stretched and little elasticity is possible where collagen is formed into cable-like tendons. However, much of the collagen present in meat forms a meshwork and stretching of the whole meshwork is possible because its configuration changes. Fibres with truly elastic properties, however, are necessary in structures such as the ligamentum nuchae of the neck and in the abdominal wall. And all arteries, from the aorta down to the finest microscopic arterioles, rely on elastic fibres to accommodate the surge of blood from contraction of the heart. Elastic fibres may be stretched to several times their original length but rapidly resume their original length once released. Elastic fibres are composed of the protein, elastin. Elastin is found in all vertebrates except primitive jawless fish, and in evolution it appeared first in cartilaginous fish. The elastic fibers in the image below (from loose connective tissue around the intestine) are the thin black ones. The much thicker, red-brown fibres are collagen.

Elastin resists severe chemical conditions, such as the extremes of alkalinity, acidity and heat destroying collagen.

16.7 Why meat must be a natural food for us

During the digestion of meat in the human gut, elastic fibres are broken down by elastase, an enzyme from the pancreas.  We would not have this enzyme if our evolutionary ancestors had not been at least partly carnivorous. In other words, I have never read of the occurrence of elastin in any human food except meat. So if we have evolved a highly specific enzyme, elastase, to deal with elastin in our food, this can only mean that we are the descendants of meat eaters.

Further information

Structure and Development of Meat Animals and Poultry. Pages 82-95.