Regulatory systems in early mammalian development, with especial reference to polarity and totipotency

Increasing evidence is now available to reveal some of the underlying regulatory systems regulating early mammalian embryogenesis. These include timing, integration, multipotency, polarities and other embryological systems. This paper deals mostly with polarities and axes, and their integration with other regulatory phenomena. Various morphogenetic aspects of ovulated oocytes and cleavage planes have long been suspected as displaying gradients, and events such as ooplasmic rotation at sperm entry, the imposition of nucleolar polarity and the highly regulated cleavage planes add to this evidence. Examples of proteins that form gradients according to the embryonic axes are increasing in number, and provide molecular evidence of their nature in establishing a persistent polarity throughout early cleavage stages. The same polarised systems are also involved in the blastocyst and implantation, and their characteristics influence implantation and the post-implantation growth of the embryo. More knowledge on each of these major regulatory systems is needed to clarify the exact nature of early mammalian differentiation and organogenesis.