Changing genetic world of IVF, stem cells and PGD. B. Polarities and gene expression in differentiating embryo cells and stem cells

Derivation of human embryonic stem cells (hESC)

Stem cells are characterized by their absolute or relative lack of specialization their ability for self-renewal, as well as their ability to generate differentiated progeny through cellular lineages with one or more branches. The increased availability of embryonic tissue and greatly improved derivation methods have led to a large increase in the number of hESC lines.

Epidemiology of acephalus/acardius monozygotic twins: new insights into an epigenetic causal hypothesis

Apart from a series of 10 acephalus/acardius (Ac/Ac) cases described from a pathological point of view, and the analysis of a review of published cases, we have been unable to find any epidemiological studies on Ac/Ac. Using data from the Spanish Collaborative Study of Congenital Malformations (ECEMC), we present here what seems to be the first epidemiological analysis of a consecutive series of the Ac/Ac type of monozygotic twins (MZT). Among a total of 2,281,604 consecutive births, 11 cases of Ac/Ac MZT were detected, giving a frequency of 0.48 per 100,000 births. However, we consider the period 1980-1985 as the baseline for our data, as in this period voluntary termination of pregnancy was not possible in Spain, and the frequency of Ac/Ac MZT was 0.49 per 100,000 births. Nonetheless, this frequency should be considered as a minimal estimation. The characteristics of these Ac/Ac cases indicate that they are more frequent in males (sex ratio 2.67). In addition, gestational age in Ac/Ac cases was 2.41 and 3.12 weeks lower than in malformed and control twins, respectively. Similarly, their mothers are 4.54 and 4.68 years younger than mothers of separate malformed and control twins, respectively. To understand the biological basis behind the occurrence of MZT in the context of recent observations, we evaluate the hypothesis that the epigenetic processes involved in the early cleavage of the embryo, and in blastocyst formation during development, may be implicated in twinning.

Translocation of the subplasmalemmal cytoplasm in human blastomeres: possible effects on the distribution and inheritance of regulatory domains

The subplasmalemmal cytoplasm of the mouse and human oocyte contains proteins, high-polarized mitochondria and for the mouse, mRNA that appears to have regulatory roles in early development. For the human, the irreversible loss of certain proteins and high-polarized mitochondria into small fragments that arise spontaneously and incorporate the subplasmalemmal cytoplasm has been suggested to have developmental consequences for the affected cell(s). Time-lapse studies have shown that fragments of this type can be resorbed during cleavage and in certain instances, their disappearance largely restores phenotypic normality to the human embryo. Time-lapse microscopy was used to examine this type of fragmentation and describe behaviours associated with their fusion or resorption, or both, in early human embryos. The transfer of fragments containing high-polarized mitochondria (delta psi m(HIGH)) labelled with a potentiometric fluorescent stain between embryos, and to oocytes and parthenogenetically activated eggs, demonstrates fragment resorption by incorporation of high-polarized mitochondria in the subplasmalemmal cytoplasm. The results indicate the potential for spontaneous transposition of subplasmalemmal regulatory elements within and between cells exists in early human embryos and that loss or gain of function may have different developmental impacts on affected cells. The location of incorporated mitochondria confirms their stability in subplasmalemmal domains, and suggests the possibility that a high polarity may be a factor in spatial specification.

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

In this final review, attention is focused on the formation of several haemopoietic systems and their genetic markers. Very early haemopoietic precursors have been identified in mesoderm and yolk sac, as interactions arise between haemopoietic stem cells (HSC) and mesenchymal stem cells (MSC). The foundation cell for the haemopoietic system has not been identified, although several candidate cells carrying specific markers have been recognized and are highly pluripotent. Haemangioblasts were proposed as the founder haemopoietic stem cell. They may be the source of pluripotent haemopoietic cells formed in blastocyst injection chimaeras, a characteristic typical of ES cells. Their role as the founder cell of haemopoietic and mesenchymal tissues is discussed.