The pineal gland is an endocrine organ synthesizing and secreting substances of well-defined hormonal nature. The pinealocytes represent the major cellular component of its parenchyma (Vollrath, 1981). The term pinealocytes is used as an inclusive term i.e. all pineal cells excluding neurons and glial elements. Several attempts have been made to classify the pinealocytes into structurally and functionally distinct groups of cells.
Blumfield and Tapp (1970) studying the rat pineal have distinguished three types of pinealocytes according to the nuclear size and suggested a correlation with pinealocyte function. Differences in mitochondria have also been recorded in hamsters where the pinealocytes were classified into P1 pinealocytes with small cristiforrn mitochondria and P2 pinealocytes with mitochondria exhibiting a plexiform array of crests in a dense matrix (Clabough, 1971). Pevet and Collin (1976) studying the mole pineals have introduced another classification according to the number and form of
pinealocyte processes. Using the terminology employed to nerve cells, they have distinguished three types of pinealocytes. Type I pinealocytes with two processes have been considered bipolar cells, type II pinealocytes with
one process have been classified as unipolar elements and type III pinealocytes have been regarded as multipolar cells due to the presence of multiple processes. Light and dark pinealocytes have also been distinguished in many species, e.g. mouse (Upson et aI., 1976), rat (Johnson, 1980; MiJin et aI., 1990; Humbert and Pevet, 1995), horse (Cozzi, 1986), rabbit (Krakowski and Cieciura 1992), gerbil (Redecker, 1993) and pig (Przybylska, 1989; Lewczuk et al., 1994). However, the nature of the two forms of cells was controversial. Some investigators have considered them as two distinct cell populations (Upson et al., 1976; MiJin et aI., 1990). Others have regarded them as different functional states of one and the same cell type (Przybylska, 1989; Redecker, 1993).