Development expression of Hox11 and specification of splenic cell fate.

Hox11 is the first member of a novel class of orphan homeobox genes. We report that Hox11 is expressed in a discrete temporal and spatially segmented pattern during embryonic development and appears critical for 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, and X, which innervate these muscles. Hox11 serves as a molecular maker distinguishing branchial from somatic motor nuclei. Additionally, Hox11 is expressed in the surface ectoderm of the first branchial arch in the region destined to become the tongue and teeth and then in ganglia innervating this area. However, Hox11-deficient mice have no apparent morphological of functional defects within these structures. Notably the closely related homeobox genes, Hox11L.1 and Hox1L1.2, were not expressed in a redundant pattern. Neither Hox11L1 nor Hox11L2 was expressed 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 in the abdomen within splanchnic mesoderm destined to form the spleen, and Hox11-/- mice have no spleen. We noted no increase in cell death within the dorsal mesogastrium of Hox11-deficient mice. Instead the dorsal mesogastrium fails to separate from the stomach. Hox11-/- mice display a larger stomach and possibly pancreas, suggesting that these mesodermal cells now contribute to other organs.