B) Chorion
(Chorion Frondosum, which ultimately develops into the placenta.)
Answer:
The answer to the question: In terms of what is happening within a resting skeletal muscle cell, why are myosins not able to attach to the thin myofilaments, would be: because there is no presence of calcium ions to enable to fusion of the two.
Explanation:
When muscle cells, and a muscle in general, wish to perform one of their functions, which is movement, there are some factors that need to be present in order for this to work. One vital factor is the presence of calcium ions, because it is calcium which will enable the proper formation of the myosin-actin cross bridges for movement. Basically, calcium allows the removal of the protection on these filaments and makes them active, so that they will immediately seek the formation of the cross-bridges. To stop this from happening, calcium is re-absorbed by the sarcoendoplasmic reticulum.
