EARLY EXPRESSION OF THE KAL GENE DURING EMBRYONIC-DEVELOPMENT OF THE CHICK

The human KAL gene is responsible for the X chromosome-linked Kallmann syndrome, which consists of the association of hypogonadotropic hypog onadism and anosmia. The human and chicken KAL genes have been isolate d. Using in situ hybridization, we studied KAL gene expression during development of the chick. We have previously reported that, from embry onic day 8, the expression is almost restricted to definite neuronal p opulations in the central nervous system, most of which still express the gene after hatching. Here we report that the KAL gene is also expr essed during early embryonic development (days 2-8) in various endoder mal, mesodermal, and neurectodermal derivatives. In most endodermal an d mesodermal derivatives, the expression is transient and precedes cel l differentiation. In contrast, the expression in the nervous system c oncerns postmitotic central neuroblastic populations, most of which st ill express the gene after differentiation. In accordance with such a spatio-temporal pattern of expression, we suggest that the KAL gene is involved both in morphogenetic events and in neuronal late differenti ation. In addition, the absence of detectable expression of the KAL ge ne either in the embryonic olfactory epithelium or in the surrounding nasal mesenchyme reinforces the hypothesis that Kallmann's syndrome re sults from a central olfactory target cell defect.