Newport J, Kirschner M
Cell. 1982 Oct;30(3):675-86. doi: 10.1016/0092-8674(82)90272-0.
The Xenopus embryo undergoes 12 rapid synchronous cleavages followed by a period of slower asynchronous divisions more typical of somatic cells. This change in cell cleavage has been termed the midblastula transition (MBT). We show that at the MBT the blastomeres become motile and transcriptionally active for the first time. We have investigated the timing of the MBT and found that it does not depend on cell division, on time since fertilization or on a counting mechanism involving the sequential modification of DNA. Rather, the timing of the MBT depends on reaching a critical ratio of nucleus to cytoplasm. We view the MBT as a consequence of the titration of some substance, originally present in the egg, by the exponentially increasing nuclear material. When this substance is exhausted a new cell program is engaged, leading to the acquisition of several new cell properties.
非洲爪蟾胚胎先经历12次快速同步分裂，随后进入一段较慢的异步分裂期，这更类似于体细胞的分裂方式。这种细胞分裂方式的转变被称为中囊胚转换（MBT）。我们发现，在中囊胚转换时，卵裂球首次变得具有运动性且开始转录活跃。我们研究了中囊胚转换的时间，发现它并不取决于细胞分裂、受精后的时间或涉及DNA顺序修饰的计数机制。相反，中囊胚转换的时间取决于细胞核与细胞质达到临界比例。我们认为中囊胚转换是由于原本存在于卵中的某种物质被呈指数增长的核物质滴定的结果。当这种物质耗尽时，就会启动一个新的细胞程序，导致获得几种新的细胞特性。
