The Collaborative International Dictionary of English v.0.48:

Mesoblast \Mes"o*blast\, n. [Meso- + -blast.] (Biol.)
   (a) The mesoderm.
   (b) The cell nucleus; mesoplast.
       [1913 Webster]

WordNet (r) 3.0 (2006):

mesoblast
    n 1: the middle germ layer that develops into muscle and bone
         and cartilage and blood and connective tissue [syn:
         mesoderm, mesoblast]