Ultrastructural and cytochemical studies on nucleolus-like bodies in early postimplantation rat embryos

Fibrillar cytoplasmic bodies, generally referred to as nucleolus-like bodies (NLBs), have been observed in the embryonic ectodermal cells of early postimplantation rat embryos. The NLBs are not delineated by a membrane, and consist of 5-7 nm filaments. The diameter of the NLBs varies widely, but 0.7-1.2 micrometer is common. Small NLBs are frequently present in the vicinity of the nucleus, suggesting that the filamentous elements of the NLBs may originate from there, and also that these filaments may gradually gather and grow into the NLBs in the cytoplasm. Enzymatic treatment of ultrathin sections obtained from glycol methacrylate-embedded material reveal that a large part of NLBs is removed by digestion with pepsin or pronase, but that various fibrous materials remaining are sensitive to RNase digestion and stained with Bernhard's uranyl-EDTA method. These findings indicate that the NLBs consist of ribonucleoprotein.