The divergent development of the apical ectodermal ridge in the marsupial Monodelphis domestica

Marsupials give birth after short gestation times to neonates that have an intriguing combination of precocial and altricial features, based on their functional necessity after birth. Perhaps most noticeably, marsupial newborns have highly developed forelimbs, which provide the propulsion necessary for the newborn's crawl to the teat. To achieve their advanced state at birth, the development of marsupial forelimbs is accelerated. The development of the newborn's hind limb, which plays no part in the crawl, is not accelerated, and is likely even delayed. Given the large differences in the rate of limb outgrowth among marsupials and placentals, we hypothesize that the pathways underlying the early development and outgrowth of marsupial limbs, especially that of their forelimbs, will also be divergent. As a first step toward testing this, we examine the development of one of the two major signaling centers of the developing limb, the apical ectodermal ridge (AER), in a marsupial, Monodelphis domestica. We found that, while both opossum limbs have reduced physical AER's, in the opossum forelimb this reduction has been taken to the extreme. Where the M. domestica forelimb should have an AER, it instead has only a few patches of disorganized cells. These results make the marsupial, M. domestica, the only known amniote (without reduced limbs) to exhibit no morphological AER. However, both M. domestica limbs normally express Fgf8, a molecular marker of the AER.