Wall structure of a bitunicate ascus

The bitunicate ascus develops in two stages prior to ascospore formation: 1) initial growth and expansion of the ascus mother-cell, and 2) deposition of a secondary wall layer, the endotunica, within the outer primary wall layer, the ectotunica. The layers of the bitunicate ascus are composed of microfibrils embedded in an amorphous matrix. The ectotunica and the endotunica differonly in the arrangement of the microfibrils. The primary wall layer is deposited during growth and expansion of the ascus mother-cell; the microfibrils are parallel to the ascus-protoplast surface. The secondary wall layer is deposited after the ascus mother-cell has fully expanded and before ascospore formation; the microfibrils are arranged in a banded pattern. Expansion of the ascus wall during ascospore expulsion is accomplished by a rapid reorientation of the microfibrils of the secondary layer.