A CRYSTALLOID INCLUSION IN THE RABBIT BLASTOCYST

Intracellular crystals or crystalloids have been reported from a wide variety of cells and tissues over the past century, ever since Auerbach (1) in 1855 found crystals in the cytoplasm of an ameba. They represent a heterogeneous group formed by a wide variety of substances from calcium salts to complex viruses, in cell types ranging from bacteria (7) to vertebrate oocytes (3, 10). In many cases crystalloid structures are of sporadic or variable occurrence, as in the digestive glands of certain molluscs (14) or the interstitial cells of the human testis (9). They may also provide an indication of certain pathological conditions, for example hemosiderosis (12). In other cases they may form an integral part of certain biological systems, such as in the yolk platelets of certain snail oocytes (4). Although intranuclear crystalloids have been described in mammalian liver and kidney (2, 16), in most cases they have been found in the cytoplasm. Electron micrograph studies have shown some crystalloids to be apparently unrelated to the membrane systems of the cell (13), but in other cases they are bounded by smooth membranes that suggest an affinity with the Golgi apparatus (14), and certain crystaUoids have been found within the mitochondria (5, 11). Some intracellular crystals are apparently formed from simple inorganic salts such as the calcium phosphate inclusions of Paramecium (8). Crystals of carbonyl diurea are commonly distributed within the cytoplasm of Amoeba (16). Most crystal-like inclusions, however, apparently contain proteins as a major component, and in a number of cases the protein may be conjugated with iron (5, 14). This paper describes a crystalloid inclusion that is widely and abundantly distributed within the early embryonic cells of the domestic rabbit. Although light microscope observations have been made on crystalloids from the rabbit oocyte (3, 10) this is the first description known to us of such inclusions in the early mammal ian embryo.