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Muscular dysgenesis or dysgeny is a lethal, recessive, autosomal genetic disease in mice that is caused by a mutation in the mdg gene. Due to this genetic abnormality, skeletal muscles in dysgenic mice are paralyzed and the animals die shortly after birth. This study is made to find the causes of the cellular defect associated with this genetic disease and to discuss the results found in comparison with other investigations done on this genetic defect.

AIM: To study the abnormalities caused by the genetic disease at a cellular level.

METHOD

MATERIALS: Experiment Chamber, muscle fiber extracted from a dysgenic mouse fetus.

PROCEDURE: After surgically removing muscle fiber from a dysgenic mouse fetus, it was placed in an experimental chamber in order to study some of the abnormalities in control of the skeletal muscle activity during the muscle dysgenesis.

A high concentration of Acetylcholine (ACh) was applied to the motor end plate directly.

Observation: It was found that, there was no contraction of the fiber.

Inference:  Since the muscular dysgenesis affects only one type of cell in the body, would it be possible, that the motor neurons of the dysgenic mice were working properly? ( )

A high concentration of Ach (Acetylcholine) was applied directly to the muscle fiber.

Observation: An action potential was generated in the sarcolemma.

Inference:  This finding demonstrated that certain events or conditions took place within the neuromuscular junctions.

Calcium concentration in the sarcoplasm was raised artificially in the muscle fiber.

Observation: It was found that the cell began to contract normally, on increased calcium concentrations.

RESULT:

Since the application of higher concentrations of calcium led to the contraction of the cell, it would seem that the defect of muscular dysgenesis occurs during the calcium ion storage and transport in the muscle cells.

DISCUSSION:

Before discussing the above procedures, observations and results one would need to understand the exact nature of muscular dysgeny and in order to understand the harmful effects of this genetic disease at a cellular level, one needs to have a brief idea about what actually happens during the normal working of a muscular contraction.

Muscle fibers are excitable, normally, a nerve impulse arriving at a neuromuscular junction initiates the contractile response and the impulse spreads rapidly due to the depolarization of the surface of sarcomeres. A neurotransmitter released at the neuromuscular junction, enters into the sarcomere through its membrane channel, resulting in the generation of an action potential, which travels all along the muscle fiber.

The sarcoplasmic reticulum then releases its stored calcium ions (Ca+ +) which moves on to bind with the specific sites on the troponin molecules of the thin filament. This leads to the contraction of the cell (Whereas, during relaxation, the Ca+ + is pumped back into the sarcoplasmic reticulum.).

Now, we compare the results obtained from our experiment with other articles published related to this genetic disease after taking a look at the experimental procedures followed and observations made above and how they are important to this study:

As a high concentration of ACh was applied to the muscle fiber and an action potential was observed in the sarcolemma, it would mean that, certain events were taking place in the neuromuscular junction, trying to initiate a contractile response. On the basis of this finding, a high concentration of calcium was raised artificially in the sarcoplasm leading to the contraction of the cell. This points that, though the action potential was generated, as in a normal cell, it took forced increase in the concentrations of calcium in the sarcoplasmic reticulum to see the cell undergo contraction normally, which clearly suggested an abnormality in the structure of the sarcoplasmic reticulum.

In an influential 1971 article, “Fine structure of mutant (muscular dysgenesis) embryonic mouse muscle,”Ann C. Platzer and Salome Gluecksohn-Waelsch suggested that, “The earliest abnormalities are detected at a gestational age of 14 days when the sarcoplasmic reticulum of mutant cells begins to appear dilated. In later stages swollen sarcoplasmic reticulum is seen in all muscle cells as well as nuclear abnormalities and cell death. The correlation between the ultrastructural abnormalities of the sarcoplasmic reticulum and the other cellular abnormalities is discussed, as well as the possible relevance of the structural abnormalities to the total absence of muscle function in mutant homozygotes.”

This supports the observation made in this study about the abnormality of the structure of the sarcoplasm. Their article was concerned with the ultrastructure of the muscle cells in the embryo of mice affected with muscular dysgenesis. This might suggest that the protein by a normal mdg gene in normal mice might be working near the sacroplasmic reticulum region aiding in the transport of the calcium ions.

Although studies have shown that the extracellular calcium is transported into the cell due to the differentiation of the myoblast , there exists a controversy in how the myoblasts induce extracellular calcium entry into the cell. This is suggested in a 2004 article, “Insulin-Like Growth Factor-Induced Transcriptional Activity of the Skeletal alpha-actin Gene Is Regulated by Signaling Mechanisms Linked to Voltage-Gated Calcium Channels during Myoblast Differentiation” written by Espen E.Spangenburg, Douglas K. Bowles and Frank W. Booth. They further suggest that the differentiation in the skeletal muscle myoblasts is dependent on a number of growth factors, ions and transcription factors and it is still not clear how these growth factors interact at the cell membrane to bring changes in the genetic expression which happens during the differentiation and also in the intracellular calcium movement.

But these problems aside, several studies have been made to learn more about this genetic defect and how it affects different anatomical and physiological aspects of the mice’s bodies.For instance, a 1984 article, “Effects of the muscular dysgenesis gene on developmental stability in the mouse mandible,” Atchley WR, Herring SW, Riska B, Plummer AA, showed how the muscular dysgenesis alters the mandible of the affected mice. They suggested, “The more unstable traits include height at the mandibular notch, height at the incisive process, condyloid width, height and area of the coronoid process, and size of the tooth-bearing region. All of these latter traits are closely associated with areas of muscle attachment and/or the muscular dysgenesis phenotype, suggesting that the presence of a single mdg allele is sufficient to alter developmental pathways. Traits not showing significantly increased instability in +/mdg mice bear no clear relationship to either muscle attachment areas or to the mdg/mdg phenotype.”

What else is  known about this genetic disease:

Studies have found that the muscular dysgenesis occurring in mice is hereditary and it is a recessive, autosomal genetic defect, which is expressed primarily as a disruption in the development of the skeletal muscle, in contrast with muscular dystrophies in animals including mice in which only the mature skeletal muscles are affected.

This genetic disease is shown to be homozygous recessive and most seen abnormalities in the newborns include frozen fetal position, small malfunctioning jaw and severe reduction of the skeletal muscles all over the body with often indistinguishable muscle groups. Skeletal muscle contraction is completely absent and newborns die due to respiratory failure. The mutant homozygous embryos can be positively identified within 13 days after fertilization.

The abnormalities were first seen in the embryos which were homozygous to the mdg mutation during the differentiation of the myoblasts into striated myotubes, with no relation to the embryonic age.

A single mdg allele is necessary to alter the skeletal muscles resulting causing muscular dysgenesis.

Although this genetic defect is shown to affect the skeletal muscles, the smooth and cardiac (striated) muscles were seen to be normal in the dysgenic embryos.

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