Autoradiographic Observations on the Silk Glands of Bombix mori

Brachet (1) first called attention to the correlation between tissue RNA content and protein synthesis and to the RNA richness of the silk glands of B. mori. This high RNA content (2), cytological characteristics (3), and synthesis of a specific protein-fibroin-with peculiar properties .and aminoacid composition (4)-suggested an autoradiographic study of the incorporation of radioactive amino acids into the silk glands. Fibroin is synthetized by the posterior part of the silk glands (5). The tubular silk glands are lined by large cells, filled with ergastoplasm erroneously interpreted as secretion by Lesperon (3). In their apices are found hyaline globular masses and complex structures constituted by a fila-mentous grid, stainable by phosphotungstic acid hematoxylin. This apical structure is presumably related to the secretion of fibroin into the lumen of the gland. Cell nuclei are branched and contain numerous nucleoli. Two series of experiments were made. In the first, silkworms of the 5th instar, weighing 1.60 to 2.00 gin. and 3.50 to 4.50 gin., were injected in the body cavity respectively with 0.05 ml. = 0.5 ~zc. and 0.1 ml. = 1 ,uc. of glycine-l-C 14 dissolved in 0.85 per cent NaCl. Groups of silkworms, each composed of 6 worms of each weight, were killed at 1, 2, 6, 12, and 24 hours after the injection. Silk glands were fixed either in 10 per cent formaldehyde, buffered with phosphate to pH 7.1, or Carnoy. In a second series of experiments, silkworms in the 5th instar, weighing 3 to 4 gm. received 0.03 ml. = 3 #c. of C14-glycine, and the glands were collected 15, and 30 minutes, and 1, 2, 6, 12, and 24 hours later, and fixed in 10 per cent formaldehyde. Four larvae were studied for each time period. All glands were paraffin-embedded and cut at 3/z. Slides were deparaffinized, washed for a few hours in tap water, and following that by several changes of a M/100 d-l-glycine solution and distilled water. Some slides were treated with 0.02 per cent crystalline pancreatic ribo-nuclease. Slides were covered with Kodak Ltd. stripping emulsion and developed after a month's exposure (6). The unstained sections were examined by phase contrast microscopy after mounting in water. RESULTS The two fixatives used gave the same autoradio-graphic results, but tissue preservation was better after formaldehyde fixation. As the two experimental series gave similar results, they are jointly described. No clear influence of the larvae weight was found. …