DEVELOPMENTAL EXPRESSION OF HOX11 AND SPECIFICATION OF SPLENIC CELL FATE

Hox11 is the first member of a novel class of orphan homeobox genes. W e report that Hox11 is expressed in a discrete temporal and spatially segmented pattern during embryonic development and appears critical fo r the specification of splenic cell fate. Expression is first observed in the developing muscle plates of branchial arches 1, 2, 3, and 4/6, and subsequently within motor neurons of cranial nerves V, VII, IX, a nd X, which innervate these muscles. Hox11 serves as a molecular marke r distinguishing branchial from somatic motor nuclei Additionally, Hox 11 is expressed is the surface ectoderm of the first branchial arch in the region destined to become the tongue and teeth and then in gangli a innervating this area. However Hox11-deficient mice have no apparent morphological or functional defects within these structures.(1) Notab ly the closely related homeobox genes, Hox11L1 and Hox11L2, were not e xpressed in a redundant pattern. Neither Hox11L1 nor Hox11L1 was expre ssed in the branchial arches or their motor nuclei within wild-type or Hox11(-/-) mice. Beginning at E11.5, Hox11 is normally expressed at a single site is the abdomen within splanchnic mesoderm destined to for m the spleen, and Hox11(-/-) mice have no spleen.(1) We noted no incre ase in cell death within the dorsal mesogastrium of Hox11-deficient mi ce. Instead the dorsal mesogastrium fails to separate from the stomach . Hox11(-/-) mice display a larger stomach and possibly pancreas, sugg esting that these mesodermal cells now contribute to other organs.