EXPRESSION PATTERNS OF THE PAIRED-RELATED HOMEOBOX GENES MHOX PRX1 AND S8/PRX2 SUGGEST ROLES IN DEVELOPMENT OF THE HEART AND THE FOREBRAIN/

Prx1 and Prx2 (previously called MHox and S8, respectively) are the me mbers of a small subfamily of vertebrate homeobox genes expressed duri ng embryogenesis from gastrulation onwards. We directly compared the e xpression domains of the Prx genes in detail in mouse and in addition some aspects of these patterns in chicken. In addition to the superfic ially similar expression patterns of Prx1 and Prx2 in cranial mesenchy me, limb buds, axial mesoderm, and branchial arches and their derivati ves, we detect major differences at many sites particularly in heart a nd brain. Our analysis indicated in several cases a correlation with r egions developing into connective tissues. From at least day 8.5, Prx- 1 expression was observed in the heart, initially in the endocardial c ushions and later in the developing semilunar and atrioventricular val ves. Prx2 develops early on a diffuse myocardial expression pattern an d is later higher expressed in the ventricular septum and in particula r in the ductus arteriosus. Prx2 is never expressed in the brain, wher eas Prx1 is expressed, from at least day 9.5 onwards, in a unique dist inct domain in the ventral part of the hypothalamus, as well as in a b roader region of the telencephalon.