ABSTRACT
The formation of connective tissue structures begins with forming condensations in embryonal mesenchyme; this process is referred to as condensation (Hall & Miyake, 2000). Originating essentially from mesoderm, the middle germ layer, and comprising the basic embryo body mass until a certain period (in a human, until the end of the eighth week), embryonal mesenchyme contains pluripotent mesenchymal progenitor cells, which can serve as precursors of various connective tissue cellular differons (fibroblastic, chondroblastic, osteoblastic). Mesenchymal condensations are localized clusters of a progenitor cells, which constitute the first stage in the formation of all dense connective tissue structures, such as tendons and ligaments, aponeuroses and fascia; the derma undergoes a condensation stage in its evolution as well (Michon et al., 2007). Other mesenchymal conden-sations serve as source material for the formation of cartilaginous and bone structures. Becoming regulating signaling systems beforehand is a necessary precondition for condensation formation, the most important stage of embryonal morphogenesis. These systems consist of signaling molecules, components (extra-and intracellular) of the signal transduction pathways, and cell (cytoplasmic) membrane receptors. The so-called maternal effect produces the earliest regulating influence on morphogenesis. This effect is due to the presence of protein molecules in a zygote (fertilized ovum) and, possibly, related transfer RNAs, which originate in a mother’s body and remain unchanged in an oocyte (Elis et al., 2008). These molecules serve as primary signals for the expression of earliest activated genes’ (genes of the maternal effect) encoding of a number of signaling molecules – morphogens, determining the onset of gastrulation and the formation of a body segmented pattern. A set, called a morphome (by analogy with genome) of the intrinsic genes of the zygote, including the gap, pairrule, bicoid and other families, has been described in detail in the drosophila (Surkova et al., 2007). A homologous mechanism for activiting zygote genes by maternal effect factors functions at even significantly higher steps of phylogenesis, including in mammals (Minami et al., 2007). In vertebrates’ zygote, “zinc fingers” containing
transcription factor XDFL156, functions. In inhibiting the activity of the p53 gene, this factor controls the formation of germ layers and mesoderm differentiation (Sasai et al., 2008). With the onset of gastrulation, a number of regulation signaling systems, establishing the bonds between the gastrula proliferating cells, join the regulation factors encoded by the zygote genes. The main families of signaling molecules among these systems operating on a genetic level are: a) morphogenes of the Hh and Wnt families; b) members of the transforming growth factors-β (TGF-β)
superfamily, primarily bone morphogenetic proteins (BMPs);
c) members of the fibroblast growth factors (FGF) family; d) a signaling system associated with Notch trans-mem-
brane receptors; and e) a the retinoic acid system. The leading role in controlling cell differentiation, proliferation and functions is transferred to these signaling systems comprising a comprehensive regulatory network. These systems control gastrula morphogenesis, particularly the formation of the primitive streak, a transient structure wherein mesoderm and endoderm differentiation begins. Later, the same systems control the detailed patterning of an organism, namely the formation of cell condensations, including those that determine segmentation (assembling an organism from uniform modules – segments) inherent to the majority of multicellular organisms. The effictiveness of signaling molecules actions is provided by the early active expression of related transmembrane receptors by the cell. The receptors are a starting point for intracellular signal transduction pathways, eventually achieving the cell nucleus and regulating gene expression. Signaling molecules (messengers) affect the cells by means of serpentine receptors (molecules with convoluted conformation, crossing the cytoplasmic membrane seven times, i.e. they contain seven transmembrane domains). The majority of such receptors, called GPCR, are coupled with G proteins (guanine nucleotide binding proteins), located within the cytoplasm immediately under the membrane. G proteins are latent guanosine triphosphatases (GTPases), whose enzymatic activity appears in the interaction of GPCRs with ligands (Neves et al., 2002). GTP hydrolysis, the initial link of the intracellular transduction of a signal arising from a ligand, is controlled by proteins – regulators of G-protein signaling (RGS). RGS-5, RGS-7 and RGS-10 stimulate condensation
(and the subsequent chondroblastic differen-tiation) of progenitor mesenchymal cells, with RGS-4 acting in the opposite direction (Appleton et al., 2006). Among messengers, morphogens of the Hh and Wnt1 families play a leading role in regulating morphogenetic processes (see section 4.2). Hh (Hedgehog) family consist of three homologous proteins of the morphogen family, among which Shh (Sonic hedgehog, the alternative name of which is HHG-1) plays a more significant generalized role in regulating the development of mammals. The involvement of other Hh, Ihh and Dhh. in morphogenetic processes is restrained by certain localizations; thus, the role of Ihh is especially important in enchondral ossification (Varjusalo & Taipale, 2008). Produced by cells as a preprotein consisting of 424 amino acid residues (in mice), Shh is exposed to proteolytic processing (autoproteolysis) within the endoplasmic reticulum, where is deprived of the signal peptide and the large C-terminal region. The N-terminal peptide (Shh-N), consisting of 173 amino acid residues (residues 25 to 198), possess the specific morphogenetic effect. Another peculiar effect of processing is the coupling of lipid components to Shh, the residue of a palmitic acid (palmitate) to the Nterminal, and of cholesterol molecules to the C-terminal (Nusse, 2003). Shh-N affects cells as a ligand via the Patch receptor. The Patch bound to its ligand derepresses the coreceptor, Smo, which serves as a source for signals coming into the cell that influence the processing of transcription factors with zinc fingers from the Gli protein family. Impairments in processing inhibit their function as transcription repressors (Ehlen et al., 2006). Many molecular details of this process are not clarified (Jia & Liang, 2006). Shh, by affecting cell proliferation and differentiation, plays a key role in many aspects of patterning an organism’s development, with the direction of its activity altered at various embryogenesis stages. The expression of the Shh gene in mouse embryos becomes distinctly apparent on the midline by the eighth day. In this early stage it controls the left-right and dorsal-ventral axis specification of an embryo. Later, the Shh expression is of critical importance in forming the limb distal parts and in the development of structures of ectodermal origin. The switching off (or deletion) of Shh results in the malformation of somites, a lack of vertebrae and ribs, limb and lung hypoplasia (Varjusalo & Taipale, 2008). Shh is also essential in an adult, where it maintains tissue homeostasis and participates in the regulation of stem cell differentiation. According to some data (Dorus et al., 2006), molecular evolution of the Shh gene occurred in primates in an accelerated manner. Herewith, intensive accumulation of serine and threonin residues, which are potential substrates of posttranslational modification, occurred. These gene altera-tions were especially pronounced in the central nervous system of anthropoidea (prehominids) – and could be one of the factors promoting the origin of the human being.
