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Microfibril

From Wikipedia, the free encyclopedia

A microfibril is a very fine fibril, or fiber-like strand, consisting of glycoproteins and cellulose. It is usually, but not always, used as a general term in describing the structure of protein fiber, e.g. hair and sperm tail. Its most frequently observed structural pattern is the 9+2 pattern in which two central protofibrils are surrounded by nine other pairs. Cellulose inside plants is one of the examples of non-protein compounds that are using this term with the same purpose. Cellulose microfibrils are laid down in the inner surface of the primary cell wall. As the cell absorbs water, its volume increases and the existing microfibrils separate and new ones are formed to help increase cell strength. As the cell absorbs water, microfibrils separate and new ones are forming.

Contents

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• 1Synthesis and function
• 2Marfan syndrome
• 3See also
• 4References

Synthesis and Function

Cellulose is synthesized by cellulose synthase or Rosette terminal complexes which reside on a cells membrane. As cellulose fibrils are synthesized and grow extracellularly they push up against neighboring cells. Since the neighboring cell can not move easily the Rosette complex is instead pushed around the cell through the fluid phospholipid membrane. Eventually this results in the cell becoming wrapped in a microfibril layer. This layer becomes the cell wall. The organization of microfibrils forming the primary cell wall is rather disorganized. However, another mechanism is used in secondary cell walls leading to its organization. Essentially, lanes on the secondary cell wall are built with microtubules. These lanes force microfibrils to remain in a certain area while they wrap. During this process microtubules can spontaneously depolymerize and repolymerize in a different orientation. This leads to a different direction in which the cell continues getting wrapped.

Microfibrils in The Plant Cell Wall

The plant cell wall is strengthened by networks of microfibrils. Cellulose is assembled into long microfibrils which are a few nanometers in diameter. Cellulose Microfibrils are responsible for the tensile strength in the plant cell wall as each microfibril is comparable to steel in regards to strength. The microfibrils then form a network of cross linking glycans with the use of hydrogen bonds. These cross linking chains will then form into groups called bundles, which consist of about 40-50 chains. These bundles are the structural units in the primary cell wall.

Cellulose Microfibrils are synthesized such that newly synthesized microfibrils form below the pre-existing microfibrils. This means the oldest microfibrils are pushed to the outer of the plant and the newest are more interior. It is important to note that microfibrils which are newly synthesized in the interior play a larger role in expansion due to turgor pressure.

Expansion in plants occurs in response to turgor pressure, and involves widening of the space in between cellulose microfibrils. The direction and rate of a plants expansion is determined but cellulose microfibrils. They allow for plant movement as a response to their environment. The layers of cellulose microfibrils can expand and move to allow for expansion or compression response. Orientation of these microfibrils in the cell wall are often related to the direction of plant growth. Microfibrils reorientate themselves in the direction of plant growth. The stiffness of the cell wall increases in the direction of cellulose microfibrils.

Alignment of Microfibrils

In the plant cell wall, microfibrils layer and form in random directions to create an alignment that strengthens the cell wall. Cellulose microfibril are aligned by cortical microtubules according to the alignment hypothesis. However, they can also be aligned without the use of microtubules, resulting in a random assembly. However, looking across multiple plants it is observed that the random assembly results in the same final pattern, suggesting that there is something responsible for this orientation.

Marfan syndrome

Marfan syndrome is a heritable disorder of the connective tissue with an estimated prevalence of 1 in 5000 humans.  The disease is often inherited as an autosomal dominant trait with complete penetrance and often arises because of a defect in fibrillin, the glycoprotein component of microfibrils.^[1] Marfan syndrome occurs as the result of structural weakness in connective tissue which is attributed to abnormal microfibril formation, specifically within fibrillin-1. Fibrillin‐1, and the closely related fibrillin‐2 protein are major components of the 10 nm microfibrils of the extracellular matrix. These fibrillins are extracellular glycoproteins, and they both contribute to specific physical properties of elastic and non‐elastic tissues. Fibrillin-1, when mutated, can alter microfibril assembly causing a wide range of detrimental effects. In Marfan syndrome a mutation to fibrillins leads to increased elasticity in critical tissues, which leads to abnormal development. Individuals who are affected by this syndrome tend to be characterized by varying patterns of organ development, which include the cardiovascular, ocular, skeletal, and pulmonary systems, as well as the skin and dura matter.

Marfan syndrome patients have many complications and symptoms that are present during puberty or later, however severe complications rarely develop prior to adulthood. Recent findings have challenged many pathogenetic concepts of Marfan syndrome and may impact future treatment strategies specifically focusing on treatment of microfibrils.