Perivitelline threads in cleavage-stage human embryos: observations using time-lapse imaging.

Time-lapse imaging of the human preimplantation embryo in vitro has revealed a transient phenomenon involving the appearance of perivitelline threads, commonly observed at the two-cell stage. These threads span the perivitelline space, arising at the specific area where the cytoplasmic membrane contacts the zona pellucida, before any perivitelline space is formed. The threads persist as the cytoplasmic membrane retracts from the zona pellucida to form the first cleavage furrow.

Genetic analyses reveal a requirement for Dicer1 in the mouse urogenital tract.

Despite the increasing interest in other classes of small RNAs, microRNAs (miRNAs) remain the most widely investigated and have been shown to play a role in a number of different processes in mammals. Many studies investigating miRNA function focus on the processing enzyme Dicer1, which is an RNAseIII protein essential for the biogenesis of active miRNAs through its cleavage of precursor RNA molecules. General deletion of Dicer1 in the mouse confirms that miRNAs are essential for development because embryos lacking Dicer1 fail to reach the end of gastrulation.

Sfrp1 and Sfrp2 are required for normal male sexual development in mice.

Secreted frizzled-related proteins (Sfrps) are antagonists of WNT signalling implicated in a variety of biological processes. However, there are no reports of a direct role for Sfrps in embryonic organogenesis in mammals. Using in vivo loss-of-function studies we report here for the first time a redundant role for Sfrp1 and Sfrp2 in embryonic sexual development of the mouse.