Proliferation and differentiation of thyrotrophs in the pars distalis of the rat pituitary gland during the fetal and postnatal period

Pituitary glands from rat fetuses (gestational age 17.5–21.5 days) and rat pups (3, 7, 10, 14, 28 days old) were labeled with bromodeoxyuridine (BrdU) 2 h prior to sacrifice and embedded in paraffin. Sections were consecutively immunostained with anti-BrdU and anti-rat TSH. The number of cells stained with anti-BrdU, anti-rTSH, or both of them were counted. The area of the section and the volume of the pituitary were measured and the number of immunostained cells per mm3 or per pituitary was calculated. Thyrotrophs were few in 17.5 day-fetuses but increased thereafter, with a rapid increase during the 2nd week after birth. The number of cells labeled with both BrdU and TSH peaked at 7 days after birth. It was estimated that about 1/5 of the thyrotrophs increased during this period was derived from the mitosis of existing thyrotrophs.