Enveloping membrane system

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enveloping membrane system (noun, ascospore term,spore morphology term,no plural; abbreviation: EMS) – a system of two closely appressed membranes that divide the cytoplasm of the ascus (ascoplasm) into separate portions, forming ascus vesicles that will mature into ascospores. During this process of free-cell formation some ascoplasm remains outside the ascus vesicles. This portion called epiplasm, whereas the ascoplasm inside the ascospores becomes the sporoplasm. The outer membrane of the ascus vesicle is called ascospore investing membrane. The inner membrane is called the spore plasma membrane. Both membranes contribute to ascospore wall formation, accumulating cell wall material within the inter-membrane space. Different hypotheses have been proposed to explain the origin of the two membranes. In most ascomycetes both membranes are apparently formed by invagination from the ascus membrane, thus cleaving portions of the ascoplasm apart. This process seems to be characteristic for most ascomycetes. In some ascomycetes, particularly yeasts, it has also been suggested that the EMS forms de novo within the ascoplasm and has no connection to the ascus membrane. This is:part of ascospore.

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