- a transient adrenocortical zone present in some rodents at birth, most notably in mice, situated between the zona reticularis and the suprarenal 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;
- misnomer for the fetal suprarenal cortex of primates.
Synonym(s): androgenic zone1
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