Tympanic membrane retraction over the surface and move out along the ear canal. Even if migration along the ear canal is blocked, new cells continue to grow so the surface of the eardrum becomes larger. This process of proliferation and migration can result in enlargement of a retraction pocket so that the eardrum expands and grows deeper into the ear. The majority of tympanic membrane retractions remain stable for long periods of time, or may even resolve spontaneously so that eardrum becomes normal again. Not all retractions are able to resolve even if middle ear pressure normalizes, as the retracted segment may become
Cell membrane cells to hold them together to form tissues. Fungi, bacteria, most archaea, and plants also have a cell wall, which provides a mechanical support to the cell and precludes the passage of larger molecules. The cell membrane is selectively permeable and able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival. The movement of substances across the membrane can be either "passive", occurring without the input of cellular energy, or "active", requiring the cell to expend energy in transporting it. The membrane also maintains the cell potential. The cell membrane thus works