Stereological analysis of daily variation in the ultrastructure of the pars intermedia of the pituitary gland of the Djungarian hamster (Phodopus sungorus).

A stereological analysis has been made of the daily changes occurring in the ultrastructure of the melanotrophs of the pars intermedia of the pituitary gland of the Djungarian or Siberian hamster, Phodopus sungorus, maintained in long day photoperiods. The rough endoplasmic reticulum, Golgi apparatus and lysosomes all declined in fractional volume throughout the photophase reaching minima at mid-scotophase and rising to reach their maxima at about the time of onset of the photophase. The mitochondria reached their peak fractional volume just before the cessation of the photophase but then also declined to a minimum at mid-scotophase. No significant changes were found to occur in the fractional volumes of the nucleus or the secretory granules. These morphological findings are compared with the changes in plasma and pituitary alpha-melanocyte-stimulating hormone levels found in the rat.