The TN1 monoclonal antibody recognizes a cell surface epitope that is present on subsets of growing axons in the developing nervous system of moth embryos. This antigen is also found in a variety of other developing tissues: in all cases its expression is cell-specific and transient. Here we show that the first expression of the TN1 epitope in moth embryos occurs specifically on the surfaces of mesodermal cells during gastrulation, and that it is limited to alternate segments. Creation of this pair-rule pattern of expression includes indications of an initial 4-segment periodicity, and transient immunoreactivity in ‘off’ segments. The alternating pattern is most dramatic at the end of gastrulation. It changes rapidly such that, during organogenesis, the TN1 antigen(s) is expressed in many developing tissues of all segments, with little segment-specific variation. Immunolabelling of living embryos under culture conditions demonstrated that the TN1 epitope(s) is associated with cell surfaces, both during neurogenesis and during the earlier period of gastrulation. These observations indicate that pair-rule gene functions operate in insects other than Diptera and suggest that cell surface molecules may be utilized early in insect embryogenesis in the initial establishment of large body regions.

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