Accumulation and localisation of maternal RNA in oocytes of Japanese quail

Accumulation of total RNA and poly(A+)RNA was determined in the oocytes of Japanese quail (Coturnix coturnix japonica) during oogenesis, by a standard spectrophotometric method, after RNA extraction. Intensive RNA accumulation was observed in the oocytes 0.25-2.0 mm in diameter (correlated with the presence of lampbrush chromosomes), followed by a plateau in 2.0-3.0 mm oocytes (correlated with the condensation of lampbrush chromosomes). Resumption of the RNA accumulation in the 3.0-5.0 mm oocytes is interpreted as a result of external RNA delivery by the granulosa cells. Most of the RNA (approximately 3/4, 3-4 micrograms) contained in the ovulated oocyte was found under the vitelline membrane surrounding the yolk. This RNA has been designated as 'extraembryonic RNA', as it is located outside the germinal disc region where the embryo is formed. The extraembryonic RNA is rapidly degraded within 24 h, from ovulation until oviposition, during egg passage through the oviduct, while the RNA present in a germinal disc (approximately 1.1 micrograms) is stable. In bird oocytes the presence of two, territorially separated pools of RNA has been postulated: one contained in a germinal disc and needed for early embryo development, and the second present in the cytoplasmic layer around the yolk supporting oocyte growth and development during oogenesis.