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Microfibril

A microfibril is a small fiber-like unit found in structural biological materials of organisms. It consists of a number of smaller fibril units interacting with each other to form highly ordered structures with diameters in the range of 10-20 nm. The term is commonly used to describe cellulose microfibrils, but it could also refer to the structure found in fiber-like proteins such as fibrillin, collagen, and ellastin. Microfibril structures are very strong thanks to interactions between its fibers and can also provide elastic properties. For these reasons, they are found primarily in connective and structural tissue such as skin, lungs,hair, and plant cell walls.

Cellulose

The image illustrates three strands of cellulose connected through hydrogen boding to produce a fiber. White balls are hydrogen atoms, black balls are carbon atoms, red balls are oxygen atoms, and turquoise lines are electrostatic hydrogen-bonds.

Synthesis and Function

Microfibrils form the major structural elements in the cell wall, and they are composed of (1-4)-β-d-glucan (cellulose) chains hydrogen bonded to one another. This leads to the formation of very ordered and packed layers which then form macrofibrils. Glucan chains are synthesized by the cellulose synthase complex from UDP-glucose monomers. Cellulose synthase is a very organized complex composed of six catalytic units bound to the plasma membrane. These complexes are organized to form a macromolecular unit known as a rosette which contains six complexes. It is the close association of these biosynthetic units that allows the 36 strands of glucan chains to coalesce and assemble into microfibrils at the cell surface. The rosettes are mobile in the fluid plasma membrane and travel around the cell as the microfibrils are deposited. The orientation of microfibrils is regulated by microtubules. Microtubules can form lanes forcing microfibrils to remain in a certain  area. Even further, microtubules can de-polymerize and re-polymerize in different orientations to control the waymicrofibrils are wrapped.

Structure

Glucan chains found in microfibrils vary in length depending on their position within the cell, and they can reach several hundred micrometers in length depending on their start and end positions. The degree of crystallinity within fibrils reflects the degree of hydrogen bonding between the glucan chains. If all sites for hydrogen bonds are occupied, then very highly organized structures will form; otherwise, glucan chains are arranged in a paracrystalline assembly and this allows cross linking glycans to hydrogen bond to the surface of the microfibril. Cellulose microfibrils are examples of a highly organized biochemical material. A typical plant microfibril contains 36 glucan chains in cross-section.

Cell Wall

Microfibrils are not completely bound to one another and spaces between them, intermicrofibrillar spaces, exist with non cellulose materials like lignin or pectin substance. Microfibrils are not homogeneously arranged in cell walls, and differences in their arrangement distinguish the primary and secondary wall of the cell. The primary wall is very thin and is composed of loose weaving microfibrils. The secondary wall, on the other hand, contains microfibrils closely packed and it is divided in S1 (outer), S2 (middle), and S3 (inner) layer. The S1 layer is thin and has a crossed microfibrillar texture. It exhibits an alternating left-hand and right-hand helical arrangement. In each lamella, the helical angle is about 50 – 90º. The S2 layer is thick, and is composed of 30 to 150 lamellae. This layers are parallel to one another and to the cell axis as well, usually have an angle below 30º. The S3 layer is usually thinner than the S1 and it is sometimes missing. The angle of microfibrils likewise varies from about 50 to 90º.

Growth in plants is controlled by expansion of the walls of individual cells. This expansion requires either increasing slippage in microfibrils or widening the space between them. Cross bridges between microfibrils resist deformations being responsibles to control the rate of cell growth.

Fibrillin Microfibrills

Fibrillin microfibrils are widely distributed extracellular matrix assemblies that connect elastic and nonelastic connective tissues. They are very conserve proteins among different species, which shows its importance in structural and biomechanical properties . Fibrillin assembles into microfibrillar arrays that form parallel bundles and crosslinked regions in which different types of molecules can be deposited (such as tropoelastin) to enhance elastic or tensile strength properties. For example, in the case of elastic fibers, structures comprising an outer fibrillin microfibrillar mantle and an inner core of ellastin are formed allowing the structure to extend and recoil .

Microfibril arrays are also abundant in dynamic tissues that do not express elastin, such as the ciliary zonules of the eye. Mutations in fibrillin-1 has been found to cause Marfan Syndrome, a disease associated with structural defects (skeletal and ocular) due to defective elastic fibers.

Microfibril assembly is, at least in part, a cell-regulated process . Fibrillin-1 limited assembly may start in its secretory pathway, just like other major extracellular matrix (ECM) macromolecules such as collagens, laminins, and proteoglycans. Chaperone associations probably play a key role in molecular folding and N-glycosylation, and in preventing inappropriate intracellular assembly. Also, microscopy studies have shown that assembly occurs in association with cell surfaces and predict a key role for receptors in this process.

Collagen

Collagen triple helix. Rearrangement of triple helices in parallel staggered conformation give rise to collagen microfibrils.

Collagen is the most abundant protein building block in vertebrates and provides mechanical integrity to connective tissues including bone, tendon, cornea, cardiac tissue, and extra- cellular matrix. Malfunctions involving this protein could cause several diseases including osteoporosis.

The collagen molecule is such that packing neighbors are arranged to form a right-handed microfibril that interdigitates with neighboring microfibrils. The mechanical properties of collagen are related to its hierarchical conformation, the lowest level consisting of triple helical collagen molecules, followed by collagen molecules arranging in a parallel staggered conformation along the axial direction to form microfibrils, which then combine and interact with each other to form fibrils. Collagen molecules within the micro-fibrils are stabilized by electrostatic interactions and covalent cross-links. They show a periodicity of 67 nm with respect to the neighboring molecules, and form a periodic gap region of ∼0.54D and overlap region of ∼0.46D.

Elastic Fibers

Elastic fibers are insoluble extracellular matrix assemblies that contribute to connective tissue's resilience, deformation properties, and recoil abilities without the need for energy input. The characteristics of elastic fibers are essential in biological structures that undergo constant extension and recoil, such as blood vessels, lungs, and skin. Elastic fiber assembly is composed of two components, elastin and microfibrils. Elastin is the product of crosslink between tropoelastin proteins and lysyl oxidase. Once the crosslink between tropoelastin protiens and lysyl oxadase occurs, the elastin complex associates with microfibrils to form elastic fibers. In conjunction with collagen bundles, elastic fibers help provide strength and elasticity to skin and other deformation structures. Tropoelastin proteins are the building blocks of elastin and is the product of tropoelastin gene expression (ELN). It is a ~70kDa precursor molecule that consist of both hydrophobic and crosslink domains which interact with other hydrophobic domains and lysyl oxadase respectively. The interaction between the hydrophobic regions of tropoelastin proteins are important in assembly and elasticity, and the crosslink of these proteins with lysyl oxidase allows for polymerization of the insoluble elastin product.