Rathjen J, Lake J A, Bettess M D, Washington J M, Chapman G, Rathjen P D
Department of Biochemistry, University of Adelaide, North Terrace, Adelaide 5005, South Australia.
J Cell Sci. 1999 Mar;112 ( Pt 5):601-12. doi: 10.1242/jcs.112.5.601.
The primitive ectoderm of the mouse embryo arises from the inner cell mass between 4.75 and 5.25 days post coitum, around the time of implantation. Positioned at a pivotal time in development, just prior to formation of the three germ layers of the embryo proper, the primitive ectoderm responds directly to the signals generated during gastrulation. We have identified a conditioned medium, MEDII, which caused the homogeneous conversion of ES cells to a morphologically distinct cell population, termed early primitive ectoderm-like (EPL) cells. EPL cells expressed the pluripotent cell markers Oct4, SSEA1 and alkaline phosphatase. However, the formation of EPL cells was accompanied by alterations in Fgf5, Gbx2 and Rex1 expression, a loss in chimaera forming ability, changes in factor responsiveness and modified differentiation capabilities, all consistent with the identification of EPL cells as equivalent to the primitive ectoderm population of the 5.5 to 6.0 days post coitum embryo. EPL cell formation could be reversed in the presence of LIF and withdrawal of MEDII, which suggested that EPL cell formation was not a terminal differentiation event but reflected the ability of pluripotent cells to adopt distinct cell states in response to specific factors. Partial purification of MEDII revealed the presence of two separable biological activities, both of which were required for the induction and maintenance of EPL cells. We show here the first demonstration of uniform differentiation of ES cells in response to biological factors. The formation of primitive ectoderm, both in vivo and in vitro, appears to be an obligatory step in the differentiation of the inner cell mass or ES cells into cell lineages of the embryonic germ layers. EPL cells potentially represent a model for the development of lineage specific differentiation protocols and analysis of gastrulation at a molecular level. An understanding of the active components of MEDII may provide a route for the identification of factors which induce primitive ectoderm formation in vivo.
小鼠胚胎的原始外胚层在交配后4.75至5.25天，即着床前后，由内细胞团产生。原始外胚层处于发育的关键时期，恰好在胚胎本身的三个胚层形成之前，它直接对原肠胚形成过程中产生的信号作出反应。我们已经鉴定出一种条件培养基MEDII，它能使胚胎干细胞均匀转化为形态上不同的细胞群体，称为早期原始外胚层样（EPL）细胞。EPL细胞表达多能细胞标记物Oct4、SSEA1和碱性磷酸酶。然而，EPL细胞的形成伴随着Fgf5、Gbx2和Rex1表达的改变、嵌合体形成能力的丧失、因子反应性的变化以及分化能力的改变，所有这些都与将EPL细胞鉴定为等同于交配后5.5至6.0天胚胎的原始外胚层群体一致。在存在白血病抑制因子（LIF）并撤除MEDII的情况下，EPL细胞的形成可以逆转，这表明EPL细胞的形成不是终末分化事件，而是反映了多能细胞响应特定因子而采用不同细胞状态的能力。MEDII的部分纯化显示存在两种可分离的生物活性，这两种活性都是诱导和维持EPL细胞所必需的。我们在此首次证明了胚胎干细胞对生物因子的均匀分化。体内和体外原始外胚层的形成似乎是内细胞团或胚胎干细胞分化为胚胎胚层细胞谱系的一个必经步骤。EPL细胞可能代表了一种用于开发谱系特异性分化方案和在分子水平分析原肠胚形成的模型。了解MEDII的活性成分可能为鉴定体内诱导原始外胚层形成的因子提供一条途径。
