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From the Editor Whatever happened to the “cell-block” during mammalian embryogenesis? Culturing mammalian embryos from the 1-cell to the blastocyst stage in vitro is for most species now a routine procedure. Not only do these embryos develop well, but after transfer to a suitable surrogate they also result in offspring. Research is now focusing on mechanism(s) that regulate the developmental pattern of gene expression that is most compatible with development to term. As we learn more about regulation of transcription by processes like DNA methylation and posttranslational modifi cations to histone proteins, we discover that subtle changes in the culture conditions can have lasting effects on not only the developing embryo and the fetus, but also on the resulting individual and possibly their offspring. Some of this variability could be caused by the source of reagents in the culture medium such as serum (bovine serum albumin or fetal calf serum), with the response specifi c to different lot numbers. Thus there are efforts to develop completely defi ned systems for culturing embryos, e.g. no bovine serum albumin or fetal calf serum. But culturing embryos from the 1-cell to blastocyst stage wasn’t always so easy. There used to be a “block” to in vitro embryo development, i.e. the embryos stopped dividing. This block coincided with the major onset of transcription, e.g. mouse embryos “blocked” at the 2-cell stage; Bovine embryos (orange) in agar chips (blue) after culture pigs and rats at the 4-cell stage; cattle and sheep at the 8- to 16-cell for 6 days in sheep oviducts. stage. In the mouse there was a single strain identifi ed that could be They are in vivo produced cultured through the “2-cell block”, and most research on early embryos recovered as 1 and 2 cells from superovulated development used this C57 BL/6 strain. If it was necessary to work donors. Photo courtesy of with a non-C57 BL/6 mouse and suitable “non-blocking” culture Dr. Willard Eyestone. conditions had not been identifi ed, then embryos had to be cultured in an oviductal or co-culture environment for them to progress through the “block” stage. So, many groups, especially those working with domestic animal embryos, used a surrogate Feature Article rabbit or sheep oviduct to temporarily culture their embryos. Under this scenario in vivo- or in vitro-produced embryos Universal Regulation would be embedded in an agar cylinder prior to transfer to of Calcium? a suitable oviduct. Once the embryos developed beyond the species-specifi c “block” stage, then the agar cylinders could be recovered and the embryos cut from the agar for further Eggs are different: Various culture in vitro prior to transfer to a surrogate that would carry sizes and shapes, different the embryo to term. This was an expensive, time-consuming, and ineffi cient process. types of environmental barriers, and from what little we know, the molecules involved in sperm Fine tuning of the basic culture media ingredients that egg interactions vary all over regulate the osmolarity and ionic content, and purifi ed water for preparation of the media appears to have been the major the map. One constant in fertilization, however, is the release contributors to overcoming the “block” to development of calcium into the cytoplasm from the stores in the smooth (Baltz and Tartia, 2010). Indeed, recent studies revealed endoplasmic reticulum (SER). This wave is seen in the egg that the “2-cell block” in mice is essentially a consequence of of every animal examined at fertilization, and in many plants osmotic stress, and that the strain difference is due to also. Many regulators of this calcium release and re-uptake different threshold sensitivities to hyperosmolarity also appear to be conserved, including the proteins that bind (Hadi et al., 2005). and stabilize calcium in the SER. Zhang et al. report in this issue that the well-known calcium-binding protein calreticulin Though study of the “cell block” may reveal aspects of stress may be a regulator of free intracellular calcium dynamics. responses in early embryos, for most researchers the “cell They fi nd that SER-enriched calreticulin levels are reduced at block” is considered an artifact of culture that can be both the mRNA and protein (yellow) levels during oocyte minimized or avoided. Since we are now beyond the “block”, maturation, and remain low in eggs and early embryos, studies can easily be completed on all stages of preimplanta- perhaps sensitizing the cell to calcium signaling. The tion development to understand the physiological and calcium-reuptake pump – the SERCA pump - is highly cellular processes that guide early development. sensitized over a similar period of development. Taken together, the results suggest that oocytes and early embryos have —Randall S. Prather refi ned their use of and responsiveness to intracellular calcium dynamics, and document a new mechanism as to how calcium Baltz, J.M. and A.P. Tartia. (2010). Human Reprod Update 16:166–176. Hadi, T., M.A et al. (2005). Biol Reprod. 72:179–187. homeostasis is controlled during a crucial time in development.
Molecular Reproduction & Development – Wiley
Published: May 1, 2010
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