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Medical Definition of X zone
1. A transient adrenocortical zone present in some rodents at birth, most notably in mice, situated between the zona reticularis and the adrenal medulla; it degenerates in males with the secretion at puberty and in females during their first pregnancy; it slowly enlarges in unmated females after puberty and does not degenerate until middle age; the X zone appears to secrete no hormone. Synonym: androgenic zone. Misnomer for the foetal adrenal cortex of primates. Synonym: foetal reticularis. (05 Mar 2000)