Apoptosis, a physiological cell death, has been shown to be involved i
n tissue homeostasis as well as in tissue regression due to hormone de
privation. The thyroid cell population slowly turns over, with cell lo
ss compensating mitogenicity. In the absence of thyrotropin, the cell
population decreases. The possible involvement of apoptosis in this lo
ss has been studied. We show in this report that deprivation of serum,
epidermal growth factor and thyrotropin triggers internucleosomal DNA
fragmentation and morphological modifications characteristic of apopt
otic cell death in dog thyroid cells in primary culture; cycloheximide
treatment has the same effect. This indicates that thyrocytes are end
owed with a constitutive apoptosis ''program'' and that the latter mig
ht be involved in the thyroid regression observed in vivo in the absen
ce of full growth stimulation of the gland. (C) 1994 Academic Press, I
nc.