When undifferentiated cell undergoes a process of differentiation, she makes a series of individual steps, showing different features or characteristics, until you reach your final objective (for example, when the cell precursor becomes a red blood cell, a keratinocyte, or a cardiomyocyte). In the developing body signs cells are different not only between different types of cells as they change over time. In the course of differentiation the cell undergoes specialization, acquiring specific properties, but under the influence of environmental factors (signaling molecules, of positional information) can still change their final fate.
These environmental factors occur from adjacent cells by a direct intercellular contact or in the form of the signals received by the surface of cells from solutes including positional information determined by the gradient of different morphogens. Ultimately, the cell or irreversibly acquires specific characteristics, or irreversibly determined by (committeets) to acquire these characteristics (determination) in the future. With the exception of germ and stem, all cells undergo specialization and komitilevai to their final destiny. Specialization and determination include step-by-step acquisition of a stable phenotype of the cells and the gene expression specific for each cell — nerve cells synthesize synaptic proteins, but not hemoglobin, whereas red blood cells do not produce synaptic proteins, but needs to produce hemoglobin.
With the exception of lymphocytic cells-predecessors, subjected to DNA rearrangements in the gene for the receptor of T cells or immunoglobulin, the expression profile of specific genes responsible for the different phenotype cells is not associated with stable changes in the DNA sequence. On the contrary, regulation of gene expression depends on epigenetic changes, for example, stable complexes of transcription, histone modifications in chromatin and DNA methylation. Epigenetic control of gene expression leads to a decrease in evolutionary plasticity, discussed next.
Regulation and mosaic development
Cells of the early embryo theoretically equivalent in the prospective potencies and can specialize in different directions, i.e., they are totipotent, this state and this stage is known as the regulatory development. At the stage of regulatory development the removal or destruction of part of the embryo can be compensated for the remaining cells. At later stages of development (beginning of gastrulation) the cells of different parts of the embryo lose the property of totipotential already have a predefined destiny to evolve in a particular direction, and the embryo is homogenous only in each of these parts. In this situation, known as mosaic development, the loss of the part of the embryo leads to disruption of the development of finite structures, which were komitilevai lost cells. Prospective potency — the potential of early embryo cells to develop in different directions, i.e. the property of totipotency.
Regulatory development and multiple pregnancy
The early stage of development — regulation, fundamental experiments on embryos and confirmed by observations in clinical medicine. Identical twins (monozygotic) — natural experimental evidence that early development — regulation. The most common form of identical twins is formed during the second half of the first week of development, the separation of the inner cell mass into two parts, each develops into a normal embryo. If the embryo at this stage even partially controlled by mosaic development, the twins will be separated only partially and will contain the common parts. It doesn’t happen often, as the twins usually develop normally and eventually reach normal size during prenatal-tion and postnatal growth.
Various forms of monozygotic twins demonstrate regulatory development at other stages. Dioralyte Gemini is associated with the cleavage at the stage of four cells. Monochorial twins come from a split of inner cell mass. Monoamniotic twins occur in case of later separation within the bilayer of the embryo, with the formation of two separate embryos and only one extraembryonal part continuing to form a single amnion. All these events demonstrate that these cell populations can reprogram its development, forming a complete embryo from the cells corresponding to only a part of the embryo.
The successful application of the technique of preimplantation diagnosis also proves the regulatory nature of early development in humans. In this procedure, the parents extracts of male and female gametes and fertilization carried out in vitro. After the embryos reach the eight cell stage (3 day), biopsies microglia remove some cells of the developing blastocyst. Taken the cell with a clearly visible nucleus was subjected to FISH analysis for aneuploidy. In addition, a dedicated genomic DNA can be used for PCR of specific gene sequences to determine whether the inherited embryo pathogenic alleles from the parents. Can then be selected not diseased embryo, consisting of the remaining seven cells is implanted and the mother.