Somites are spherical aggregations of mesodermal cells located on either sides of neural tube and are differentiated into sclerotome and dermomyotome. Notochord as an axial mesoderm has a major role in somitic cell survival and differentiation in vivo. Despite secreting the survival factors, how to notochord inhibits somitic cells apoptosis remains to be elusive. So, this study was aimed to investigate downregulation of caspase-2 expression in somitic cells upon coculturing with notochord. By using alginate system to encapsulate the isolated notochord in Somite + Notochord group, the embryonic somites were cocultured with the notochord on different days. Concurrently in somite group, the somites were cultured alone. Survival assay with MTT showed that the rate of viability in somitic cells cocultured with notochord increased from 59% on day 2 to 89.7% on day 6 but decreased to 38.5% on day 10 after coculturing. Reverse transcriptase-polymerase chain reaction and spectrophotometry analysis also confirmed these findings and showed low caspase-2 and high Bcl-2 expressions and low caspase-2 enzyme activity in somitic cells cocultured with notochord, respectively. These results clearly show that the notochord enhances survival of somitic cells in vitro through downregulating of caspase-2 expression along with triggering differentiation of somitic cells to Pax-1 expressing mesenchymal cells. 1. Introduction The body axis of early embryo is made up of repetitive series of segments called somites that develop from paraxial mesoderm flanking neural tube and notochord. They form epithelial blocks of mesodermal cells budding from the rostral end of bilaterally presomitic mesoderm which originates from primitive streak [1–3]. Upon formation, signals emanating from neighboring tissues promote the newly formed somitic cells differentiation into dorsal dermomyotome and ventral sclerotome. Then, the sclerotomal cells migrate toward the notochord and surround it to form axial skeletons from which the vertebrae and ribs will be shaped. The dermomyotomal cells are also differentiated to dorsomedial epaxial muscle precursor cells forming axial skeleton associated muscles and dorsolateral hypaxial muscle cells which give rise to body wall and limb muscles. The central dermal portion of dermomyotome also develops to the dermis [2, 4]. During gastrulation, the notochord, as an axial structure arising from primitive node, functions as an axis to support the early embryo and finally gives rise to intervertebral disc. Several studies indicate that the notochord also stimulates
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