Cellular and molecular drivers of differential organ growth: insights from the limbs of Monodelphis domestica.

A fundamental question in biology is "how is growth differentially regulated during development to produce organs of particular sizes?" We used a new model system for the study of differential organ growth, the limbs of the opossum (Monodelphis domestica), to investigate the cellular and molecular basis of differential organ growth in mammals. Opossum forelimbs grow much faster than hindlimbs, making opossum limbs an exceptional system with which to study differential growth. We first used the great differences in opossum forelimb and hindlimb growth to identify cellular processes and molecular signals that underlie differential limb growth. We then used organ culture and pharmacological addition of FGF ligands and inhibitors to test the role of the Fgf/Mitogen-activated protein kinases (MAPK) signaling pathway in driving these cellular processes. We found that molecular signals from within the limb drive differences in cell proliferation that contribute to the differential growth of the forelimb and hindlimbs of opossums. We also found that alterations in the Fgf/MAPK pathway can generate differences in cell proliferation that mirror those observed between wild-type forelimb and hindlimbs of opossums and that manipulation of Fgf/MAPK signaling affects downstream focal adhesion-extracellular matrix (FA-ECM) and Wnt signaling in opossum limbs. Taken together, these findings suggest that evolutionary changes in the Fgf/MAPK pathway could help drive the observed differences in cell behaviors and growth in opossum forelimb and hindlimbs.

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.