Early segregation of the adrenal cortex and gonad in chicken embryos

The adrenal gland is an endocrine organ that plays essential roles in stress responses. This organ consists of two types of tissues, adrenomedulla and adrenocortex, deriving from different embryonic origins. Whereas it is well accepted that the adrenomedulla derives from neural crest cells, the origin of the adrenocortex remains elusive. In addition, the adrenocortex and gonads, two major steroid hormone-producing tissues, have been thought to share the same origin, although the experimental evidence is lacking. In this study, to identify the origin of adrenocortex and to compare it to that of gonads, we scrutinized the medial portion of the coelomic epithelium (CE) after the lateral plate mesoderm has split into two CE components with a concomitant opening of the coelomic cavity in between them. We found that early medial CE consists of a two-cell layer-thick band of epithelial-like cells, the outer and inner CEs. The outer CE faces the coelomic cavity, whereas the inner CE is juxtaposed to nascent blood vessels. Combining direct cell labeling with early molecular markers, we found that outer CE was the origin of the gonad but not the adrenocortex. The adrenocortex, instead, appears to derive from inner CE. Thus, the adrenocortical and gonadal progenitors are already segregated from each other when the coelomic cavity has opened. This study provides a new basis for understanding how the adrenal gland forms and how steroid hormone-producing tissues arise during development.