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During studies of hypothermic effects upon cyto-plasmic microtubules (1) in chick embryos, a highly ordered aggregate of ribosomes was discovered in the developing fiber cells of the 10-day embryonic lens (Fig. 1). More extensive electron microscope studies revealed greater numbers of these aggregates in the annular and epithelial cells of the lens. In all cases the morphology of this crystal was that of a "sheet," one ribosome thick. Because the concentration of such sheets seemed to be proportional to the native density of ribo-somes in the cell studied, ][ decided to chill and examine the cells of early (1 to 2 day) chick embryos, where ribosomal density is severalfold higher. Here, this hypothermic crystallization was extensive (Fig. 2); all cell types-epidermal and neural ectoderm, mesoderm, and endoderm-possessed crystalline sheets. The procedures were as follows. Fertile eggs of White Rock domestic fowl were incubated to desired stages at 38°C and then transferred to various chambers maintained at 20 °, 15 °, 10 °, or 5°C. After such hypothermic treatment for periods from 1/~ to 8 hr long, some of the embryos were fixed in phosphate-buffered glutaraldehyde (3% at pH 7.2) for electron microscope studies. The fixed tissues were rinsed in buffer, postfixed in osmium tetroxide, dehydrated, and embedded in Epon. Thin sections were stained with solutions of uranyl acetate and lead citrate for viewing in a Philips EM 200. Other cooled embryos were returned to the 38°C incubator for later examination , either for viability or for fine structural changes. Crystalline sheets of ribosomes appeared in all tissues of both the 10-day embryonic lens and the 1 to 2 day embryo only after 3 or more hours of hypothermic treatment at 5°C or 8 or more hours at 10°C. No sheets were found in embryos treated with the higher temperatures (15 ° or 20°C), nor after 5 ° or 10°C treatments for periods shorter than those indicated above. 1 Whereas these ribosomal sheets in lens cells were about 0.25/~ in maximum dimensions, many in ribosome-rich tissues, like the 24 hr embryonic epidermal ectoderm (Fig. 3), exceeded 1.5 # in their greatest dimension. On the other hand, all tissues possessed many aggregates of just four ribosomes (see Figs. 1 and 3) in a square array: an aggregate which might be considered a fundamental unit of the larger sheets. The control experiments demonstrated reversi-bility of these hypothermic effects. First, no such aggregates were …

RIBOSOME CRYSTALLIZATION INDUCED IN CHICK EMBRYO TISSUES BY HYPOTHERMIA