TI - Activin regulates cell division in the involuting mesoderm . AB - Both our microarray experiments and our real-time RT-PCR analyses show that down-regulation of activin B , but not loss of nodal-related activity , causes the mis-regulation of genes involved in cell cycle control . One of the effects of the loss of activin B function is a disruption of gastrulation [3] , and in this connection we note that the mitotic index of involuting dorsal mesoderm is significantly decreased during gastrulation [28] and that arrest of the cell cycle is required for both bottle cell formation [20] and for convergent extension movements [21] , [22] . We therefore asked whether loss of activin B affects cell division during early embryogenesis . Embryos injected with control oligonucleotide mMO1 or specific antisense oligonucleotide MO3 were fixed at the mid gastrula stage and stained using an antibody recognising PHOSphorylated histone H3 , which marks mitotic chromosomes [28] . Inspection of such embryos revealed that the down-regulation of the cell cycle that normally takes place in dorsal axial mesoderm does not occur ( Fig 3 ) . In three control embryos stained as sections the mean mitotic index in dorsal axial mesoderm was 0% ; in six embryos injected with MO3 the mitotic index was 12.7+-2.7% ( mean+-standard deviation ) . Similarly , in a control embryo stained as a whole-mount and then sectioned , the mitotic index was 0% ; in an embryo injected with MO3 it was 20% . This failure of the dorsal axial mesoderm to undergo cell cycle arrest is consistent with the observed mis-regulation of cell cycle genes , and it may explain why embryos lacking activin function fail to gastrulate properly [see refs 20] -[22] .