Changing genetic world of IVF, stem cells and PGD. C. Embryogenesis and the differentiation of the haemopoietic system

From embryonic stem cells to blastema and MRL mice

New scientific knowledge offers fresh opportunities for regenerative medicine and tissue repair. Among various clinical options, multipotent embryonic stem cells (ESC) prepared from inner cell masses of rabbit blastocysts have been tested over many years. More recently, stem cells have been isolated from individual tissues and from umbilical cord blood. These methods seemingly offer similar rates of repair and avoid ethical complexities arising from the need for human embryos to prepare ESC. Different methods of regenerating tissues have now emerged, based on the well-known forms of organ regeneration in urodeles such as salamanders. These methods depend on the formation of a blastema, and recent studies on MRL mice have revealed that they possess similar methods of repair as in salamanders. There is also some evidence showing that this form of repair is also active in human fetuses but not in adults. Detailed knowledge of these various forms of tissue repair is now urgently needed in order to assess the benefits of each form of treatment. These matters are discussed at the end of this review where various investigations clarify the benefits and drawbacks of these varied approaches to tissue repair.

Genetics, epigenetics and gene silencing in differentiating mammalian embryos

A highly complex pattern of differentiation involving maternal and embryonic factors characterizes the early development of mammalian embryos. These complex genetic and proteonomic patterns of early growth also involve various forms of gene silencing and tissue reprogramming. Understanding the nature of fundamental developmental events is hence essential to appreciate the significance of natural and induced forms of remodelling, damaged forms of gene expression and gene silencing during the initial stages of growth. Natural forms of remodelling include subtle genetic events involved in, for example, the changing nature of imprinting from before fertilization or the inactivation of one X chromosome in female blastocysts. Induced forms include the consequences of nuclear transfer and embryo cloning or the immediate effects of placing embryos in culture media. Animal and human studies are described in this paper, relating reprogramming to detailed embryological and clinical knowledge gained through the use of IVF, preimplantation genetic diagnosis and the establishment in vitro of stem cells. Attention concentrates on the consequences of variations in all growth stages from the formation of oocytes, through fertilization, the differentiation of blastocysts and early haemopoietic stages in mammalian species. Unique features of gene expression or gene modification are described for each developmental stage.