Fully grown oocytes of the frog (Rana pipiens) undergo cytoplasmic and nuclear maturation when treated with progesterone after the follicular envelopes have been removed. The mechanism of this maturation was investigated by injection of cytoplasm from progesterone-treated oocytes at various stages of maturation into fully grown but immature oocytes. The injected cytoplasm becomes effective in inducing maturation by 12 hours after progesterone administration, reaches a maximum effectiveness around 20 hours, and then declines after the donor oocytes complete maturation. However, even cytoplasm from early embryos retains some capacity to induce oocyte maturation. The frequency with which maturation is induced is proportional to the volume of the injected cytoplasm. Progesterone itself is not directly responsible for the maturation-producing effect of injected cytoplasm since injected progesterone does not promote maturation. However, externally applied progesterone does induce the completion of the first meiotic division, presumably by releasing a cytoplasmic "maturation promoting factor." The production of this cytoplasmic factor was not affected by removal of the nucleus.After completion of the first meiotic division, oocytes cease further development a t the metaphase of the second meiotic division, where they remain until fertilized or activated to develop. Cytoplasm from such secondary oocytes when injected into one of the blastomeres at the two-cell stage of development suppresses mitosis as well as cleavage. Mitosis is usually arrested a t metaphase. No such inhibition was brought about by injection of cytoplasm from cleaving blastomeres. Thus, the arrest of mitosis and cleavage can be attributed to a specific "cytostatic factor" i n the cytoplasm of the secondary oocyte. Activation of donor secondary oocytes by insemination or pricking with a glass needle soon destroys the cytostatic factor. Likewise, addition of cortical cytoplasm to endoplasm from the secondary oocyte rapidly destroys the cytostatic capacity. This result implies that cortical material is involved in the process of removing the cytostatic factor at the time of normal activation or fertilization. Enucleation of oocytes demonstrated that production and removal of the cytostatic factor is independent of the nucleus.

No abstract

A cell-free preparation of the cytoplasm from activated eggs of Rana pipiens induces, in demembranated sperm nuclei of Xenopus laevis, formation of a nuclear envelope, chromatin decondensation, initiation of DNA synthesis, and chromosome condensation. Both soluble and particulate cytoplasmic constituents are required to initiate these processes in vitro. The observed changes resemble processes occurring during fertilization and the mitotic cycle in early amphibian embryos. Therefore, this cell-free system may be useful in biochemical analysis of the interactions of nucleus and cytoplasm that control nuclear behavior.

Much interest in vertebrate embryology is now focused on early pattern formation in the frog, Xenopus laevis. In this species, the body plan is specified by a stable positional system set up by a cytoplasmic rotation in the zygote that occurs before first cleavage. Perturbation of this initial cellular event by a variety of means causes permanent distortions of the positional system. Until now it has not been possible to alter the positional system after it has been specified. However, we report here that lithium, when applied after specification of the body plan, can respecify the positional system of the Xenopus embryo such that dorsal, axial structures develop from cells that otherwise contribute to ventral structures. Lithium is usually considered to have negative effects on early embryo development, but our results show that lithium can act in a positive manner to produce structures which represent the uppermost values of the positional system. This discovery introduces a convenient means to study cellular and molecular mechanisms of early vertebrate pattern expression.

No abstract