Epidermal Cell Proliferation During Thoracic Development in Larvae of Artemia

ABSTRACT Growth of the thorax in Artemia occurred primarily in the longitudinal direction with little increase in width. The epidermal cells were arranged in longitudinally directed rows throughout most of the ventral thorax. In segment-forming (anterior) regions, the cells changed from longitudinally to transversely directed rows. Epidermal cell density increased throughout the first instar. The greatest increase occurred in the anterior region. After a drop in cell density at ecdysis the density increased again during the second instar. Cells in the anterior region underwent a more rapid increase in density than did cells in the middle or posterior regions. Mitotic figures were observed in all stages of the molt cycle. Most of the mitotic figures were oriented in a longitudinal direction. Mitotic figures oriented in transverse or oblique directions were always located in the anterior region and increased in number during the second instar. The initial events of segmentation in the anterior epidermis included the decrease in epidermal cell density in the intersegmental borders and ventrolateral evagination of the epidermal cells in the first segment. These findings indicate that segment morphogenesis in the thorax involves a spatiotemporal pattern of cell replication in the ventral epidermis.

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