Technical Modifications in Maraglas Embedding

Marglas 655 is proving to be a versatile embedding medium for biological electron microscopy (1-5) in that it provides favorable cutting qualities and excellent cellular preservation (Figs. 1, 2, 4). Since the publication of the original technic (1), we have found several useful technical modifications and developed methods for staining sections for light microscopy. Two technical difficulties were encountered in the routine use of Maraglas-embedding medium. The first and most important, noted in an occasional block, was soft tissue surrounded by a hard matrix. Originally, we suggested double embedding to rectify this problem (1). It was apparent that soft tissue in a hard matrix was due to incomplete impregnation of the tissue by the Maraglas mixture (Maraglas, plasticizer, flexi-bilizer, and hardener). The embedding schedule has been modified to allow the tissue a longer period of impregnation in the Maraglas mixture prior to the final embedding and hardening. Complete dehydration has been achieved consistently by removing the tissue from absolute alcohol to pure propylene oxide and then to a 1:1 propylene oxide-Maraglas mixture. The tissue is then placed in pure Maraglas mixture in a stoppered vial and kept at 10°C in a refrigerator for 8 to 12 hours. The vial containing the Mara-glas mixture and tissue is then warmed to room temperature before removing the stopper. Other than warming before exposure to room air, no precautions need be taken to avoid water condensation. In our experience, water condensation has not been apparent to the naked eye. Desiccated gelatin capsules are filled with fresh Mara-glas mixture, the tissue is placed in the capsule, and the specimen is cured by dry heat in an oven at 60°C for 24 to 48 hours. The second technical difficulty was bubble formation between the gelatin capsule and plastic. This probably would not be considered serious for the investigator using specimens sufficiently large to trim away the distorted area; however, for an investigator dealing with very small blocks or single cells, this distortion could be a serious deterrent. The formation of bubbles occurs after the capsules are removed from the oven and allowed to remain at room temperature for prolonged periods of time; therefore, it is desirable to remove the gelatin capsule from the plastic shortly after its removal from the oven. This is best accomplished by cooling to room temperature for 15 minutes, and then removing the gelatin capsule by soaking in cold tap water. We …