SA-1 antigen expression in small intensely fluorescent cells is associated with proliferation

Sympathetic neurons, chromaffin cells, and small, intensely fluorescent (SIF) cells are thought to derive from a common sympathoadrenal precursor cell. Sympathoadrenal precursor cells and adrenal chromaffin cells have been shown to react with an antibody, SA-1; we now report that, like sympathoadrenal precursors, SIF cells during their proliferative phase transiently possess this epitope. Precursors and SIF cells were identified in double-label studies of the superior cervical ganglion (SCG) using SA-1 and an antibody that identifies a noradrenergic trait, tyrosine hydroxylase (TH). At E16 when the earliest SIF precursors were detected and at birth, while postmitotic principal neurons had lost SA-1 reactivity, many SIF cells expressed both TH and SA-1. As development proceeded, the proportion of SIF cells expressing only TH increased. In addition, some small SIF-like cells possessed only SA-1 reactivity at birth. Some SIF cells at P7 possessed SA-1, but it was absent at P10 and in the adult. Bromodeoxyuridine (BrdU) was used to identify proliferating SIF cells, and SA-1+ expression was correlated with the period of SIF cell proliferation. At late embryogenesis, the proportion of SA-1+ SIF cells that possessed BrdU was relatively large (17% at E20), and decreased as SIF cell division ceased (6% at P1). Our results indicate that SA-1 is present on SIF cells when these cells are capable of cell division. In addition, mature SIF cells lack SA-1 and are therefore antigenically distinct from the sympathoadrenal precursor. These data suggest that the expression of SA-1 is correlated with the ability of sympathoadrenal cells to proliferate, in the SCG early during embryogenesis, chromaffin cells both during embryogenesis and in the adult, and SIF cells during their transient period of division in the SCG.