The bodies of humans and animals owe their strength especially to a fibrous structural protein called collagen. Collagen is abundant in bones, tendons, ligaments and skin.
Water, a substance that is not often associated with strength, was found out to be an intrinsic component of collagen, as researchers at the Max Planck Institute of Colloids and Interfaces in Potsdam-Golm, together with the scientists from the Massachusetts Institute of Technology in Cambridge USA , have shown. The team, led by Admir Masic and Luca Bertinetti, unveiled that removing water from collagen fibres has dramatic effects on molecular and nanoscopic features.
The fibres contract and generate tensile forces that are times higher than those exerted by human muscles. These findings could help researchers develop novel materials and also suggest that collagen may have more active role in living organisms than previously thought. In fact, it does not act merely as a stabilising framework for the body, but can also generate tensions, for example during the synthesis of bones.
X-ray view of collagen: from the patterns of the two-dimensional X-ray diffraction, information about changes in the molecular and nanoscopic collagen structure can be gained when the protein dries.
The structure of collagen is crucial for power generation. Like a building, collagen has a hierarchical structure consisting of a complex arrangement of individual molecular components. The basic building block is the collagen molecule itself. Its shape reminds of a rope, with three chain-like proteins twisted around each other to form a triple helical motif.
However, being just to nanometres thick, the fibrils are , thinner than actual ropes. Within the fibrils, adjacent collagen molecules are not simply stacked one adjacent to each other but they are laid to form a staggered arrangement. This results in alternating denser and thinner zones along the length of the fibrils. Many fibrils in turn combine to form collagen fibres. Scientists at the Max Planck Institute of Colloids and Interfaces in Potsdam-Golm have now investigated the characteristics of collagen and specifically how its function is influenced by the water content.
The proteoglycan attracts and traps available moisture forming the clear, viscous, colorless matrix you now know as ground substance. Connective tissues perform many functions in the body, but most importantly, they support and connect other tissues; from the connective tissue sheath that surrounds muscle cells, to the tendons that attach muscles to bones, and to the skeleton that supports the positions of the body.
Protection is another major function of connective tissue, in the form of fibrous capsules and bones that protect delicate organs and, of course, the skeletal system. Specialized cells in connective tissue defend the body from microorganisms that enter the body. Transport of fluid, nutrients, waste, and chemical messengers is ensured by specialized fluid connective tissues, such as blood and lymph.
Adipose cells store surplus energy in the form of fat and contribute to the thermal insulation of the body. Loose connective tissue is found between many organs where it acts both to absorb shock and bind tissues together. It allows water, salts, and various nutrients to diffuse through to adjacent or imbedded cells and tissues. Adipose tissue consists mostly of fat storage cells called a dipocytes that store lipids as droplets that fill most of the cytoplasm figure 4.
A large number of capillaries allow rapid storage and mobilization of lipid molecules. Fat contributes mostly to lipid storage, can serve as insulation from cold temperatures and mechanical injuries, and can be found protecting internal organs such as the kidneys and eye. Areolar tissue shows little specialization.
It contains all the cell types and fibers previously described and is distributed in a random, web-like fashion. It fills the spaces between muscle fibers, surrounds blood and lymph vessels, and supports organs in the abdominal cavity. Areolar tissue underlies most epithelia and represents the connective tissue component of epithelial membranes, which are described further in a later section.
Figure 4. Areolar Tissue. This is a loose connective tissue widely spread throughout the body. It contains all three types of fibers collagen, elastin, and reticular with much ground substance and fibroblasts.
Reticular tissue is a mesh-like, supportive framework for soft organs such as lymphatic tissue, the spleen, and the liver Figure 4. Reticular cells produce the reticular fibers that form the network onto which other cells attach.
Dense connective tissue contains more collagen fibers than does loose connective tissue. As a consequence, it displays greater resistance to stretching. There are three major categories of dense connective tissue: regular, irregular, and elastic. Dense regular connective tissue fibers are parallel to each other, enhancing tensile strength and resistance to stretching in the direction of the fiber orientations.
Ligaments and tendons are made of dense regular connective tissue. In dense irregular connective tissue, the direction of fibers is random. However, changes in the fibrocartilaginous structure due to compressive loading vary depending on the adhesion sites of the tendons.
This ensures better protection against compressive forces. The bones of the tendons are composed of four regions within the bone; at the end of the tendon region 1 , collagen fibers enter the fibrocartilage fibrous cartilage—region 2. As the fibrocartilage progresses, it becomes mineral fibrocartilage area 3 and then integrates with cortical bone fourth region. This transformation, which is more bone structure than tendon structure, leads to gradual increase of mechanical properties of the tissue [ 3 , 19 , 20 , 21 ].
In general, they pass through the joints and adhere to their distal. In this way, they increase the effectiveness of the muscles on the joints. At the same time, similar to bones, mechanical properties vary depending on the load carrying place. For this reason, knowing where they are helps us understand the structure. In fact, not every muscle has a tendon. While some tendons are involved in some muscles that play an active role in joint movements, the presence of some tendons is to increase muscle movement distances rather than the movement of the joint.
For example, Achilles tendon is a very special tendon for the body carrying the loads by centralizing the strength of a few muscles. In contrast, some tendons, such as the posterior tibial tendon, act by distributing the load to several bones. Although it is known that most tendons originate from the muscle and adhere to the bone, some tendons may be the starting point for muscles, or two muscles are connected to each other through a tendon [ 22 , 23 ]. They can be very small and very long, and they can be very large and very short.
Tendons are very variable according to their shape, long, round, rope-shaped such as Achilles tendon , or short; flat tissue adhesion such as bicipital aponeurosis can be seen.
In other words, tendons may change from flat to cylinder, from fan shape to ribbon shape. However, round tendons such as flexor digitorum profundus or flat tendons such as rotator cuff, bicipital aponeurosis are more involved in the body.
In this simple classification, tendons are divided into round and flat and are very different from each other as structural and functional. For example, while round tendons respond equally to tensile loads with parallel collagen patterns, flat tendons such as rotator cuffs can respond microanatomically in the form of compression and shear forces due to longitudinal, oblique, and transverse collagen sequences. However, in round tendons, the section area is proportional to the maximum isometric strength of the muscle.
In other words, due to parallel collagen sequences, flat tendons are resistant to compression and shear forces due to flat, longitudinal, and oblique collagen sequences in comparison to round tendons that respond equally to the tensils [ 3 , 24 ].
Tendons can be classified in many ways according to their location, but the most logical one is the tendon classification in relation to the functions they see as the intraarticular biceps long head and popliteus tendon and the extraarticular Achilles tendon. Most tendons are non-articular, but the intra-articular ones lack the ability to repair after injury as in the same intra-articular ligaments an example of anterior cruciate ligament tear.
At the same time, although most tendons adhere to the bone, some tendons form the origo point for the muscles lumbrical muscles originate from the flexor digitorum profundus or connect two muscles such as omohyoid and digastric muscle. In addition, the large part of the tendon may originate from the muscle itself gastrocnemius and soleus.
For example, in some muscles tendons move into the muscle joint and tendon sticks at an angle. This allows a high proportion of muscle fibers to adhere to the tendon, thereby increasing the strength of the muscle-tendon unit but reducing the range of motion. According to their anatomy, the tendons can also be classified as sheathed or synovial-coated such as the long flexor of the fingers or unsealed or paratenon-coated such as Achilles tendon.
In other words, these tendons, which are separated by intrasynovial and extrasynovial, have a higher slippage resistance compared to the intrasynovial tendon structure, when examined more closely. At the same time, the soft tissue protection and vascularity of these two tendons are different [ 20 ].
According to its functions, tendons can be classified as energy storage or positional tendons Table 1. In general, the muscles tend to tendon to shorten the stress load; the affected tendon is stretched and the muscle can relax again when relaxed.
This makes the tendon a structure that stores elastic voltage energy. The best example of energy storage tendons is Achilles tendon. Tibialis anterior tendons in human are examples of positional tendons, and they can never extend relatively. Positional tendons are rarely injured because they extend less [ 25 , 26 , 27 ]. In conclusion, tendons are composed of multiple bundles, fibroblast, and dense linear collagen fibrils, which form the macroscopic structure of tendons and give the fibrous appearance.
Knowing where tendons are helps us understand the structure. Collagen trimers assemble in ER and each collagen polypeptide has prodomains on N and C termini that prevent trimers from assembling into fibrils within fibroblasts.
After release outside cell, enzymes remove prodomains, allowing trimer to assemble into long fibrils. Lysyl oxidase generates covalent crosslinks between trimers in fibrils. Fibrils can also assemble into large fibers. Elastic fibers consist of elastin and a long, extended protein called fibrillin.
Fibrillin forms a template around which elastin assembles to form a functional elastic fiber. Fibrillin also binds some key signaling molecules that regulate cell growth and function. Mast cells trigger inflammatory response after exposure to allergens and insect bites. Mast cells contain secretory granules filled with heparin, histamine and proteases.
Receptors for IgE antibody on the surface of mast cells trigger fusion of secretory vesicles with plasma membrane. Histamine increases permeability of blood vessels and heparin inhibits blood clotting. Fat cells adipocytes store large amounts of triglycerides in a large lipid droplet. Mesenchymal stem cells are capable of differentiating into all resident cells of connective tissue.
Transient cells of connective tissue enter from the circulatory system and consist primarily of immune cells. Neutrophils are first line of defense against bacterial infections. They detect and track bacterial proteins and then phagocytose captured bacteria. Neutrophils contain vesicles with lysozyme, proteases and free radicals that fuse with phagosome to destroy bacteria.
Macrophages engulf cellular debris and foreign antigens. They have receptors which recognize antigens complexed with antibodies. Macrophages endocytose the antigen complexes and then destroy the engulfed material in their lysosomes.
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