Cell differentiation is a primary growth process in developing limbs of Artemia

The limb of the brine shrimp Artemia develops over a four-instar period when the protopod, endite, exopod, endopod, and epipod are defined and cell differentiation (change in cell shape) occurs. To understand the importance of cell differentiation in limb growth, development of the epidermis was studied in the first thoracopod of instar V-VIII larvae. Each region was established by instar V, and the larval epidermal cells developed into general epidermal (GEC), tendinal, setal, or transport cells by instar VI. Basal extensions of the GECs formed pillar structures. The epidermal cells decreased in height from 10 to 4 microm by instar VI. Increase in length and width resulted from both cell replication and expansion of the apical cell surface in differentiating cells. Growth occurred mainly by cell replication in instar V, whereas expansion of the cell surface in GEC and setae was the major growth process in instar VII. Increase in apical cell surface area occurred primarily by change in cell shape from columnar to squamous during instar V and by increase in total cell surface in subsequent instars. The results demonstrated that cell differentiation is a significant component of growth during limb development.