Rozwój trzustki i rola komórek macierzystych dojrzałego narządu w jego regeneracji
The liver and the pancreas are organs that develop from two different groups of endodermal epithelium of the primitive foregut. The liver is formed from the segment of the hepatic recess, and proliferating cells penetrate the mesoderm of the transverse septum. A hepatic bud comprises the head part, which develops into the liver, and the caudal part, responsible for creating the gallbladder and bile ducts. Furthermore, the pancreas develops from two buds, dorsal and ventral, which are bulges of the proximal part of the duodenum. The ventral bud develops into the uncinate process and the inferior part of the pancreatic head, whereas the rest of this organ derives from the dorsal bud endoderm. In humans, the dorsal bud of the pancreas is visible in the fifth week of the development, penetrating into the dorsal mesentery of the stomach. The ventral bud appears slightly later and it is located in the area of the common bile duct. During the turn of the intestines, also of the duodenum, the ventral bud moves in the dorsal direction to finally merge with the dorsal bud and form the organ. In humans, pancreatic islets appear in the third month of the foetal development, whereas insulin secretion begins about the fifth month. The pancreatic and hepatic buds differentiate due to regulatory signals from the surrounding mesodermal cells.
The formation of embryonic stem cell (ES) culture requires their isolation from a blastocyst before implantation. A growth medium for culturing such types of cells should contain, beside standard foetal calf serum, nourishing cells (fibroblasts) and leukemia inhibitory factor (LIF). Under such conditions, ES cells form colonies composed of both undifferentiated and spontaneously differentiating cells. Self-renewable ES require binding LIF to a binary system of receptors, including a receptor specific for LIF and the gp130 receptor. The receptor activation initiates the JAK/STAT3 pathway, that is, one of pathways promoting cell proliferation. The activation of ERK by gp130 and other membranous receptors suppresses the effect of the LIF-STAT3 pathway. Thus, the maintenance of ES ability to divide results from the balance between the actions of different signalling pathways. The expression of the transcription factor Oct-3/4, a member of the POU family identified in embryonic neoplastic cells, and Nanog that plays a key role in blastocyst formation, is necessary for ES to maintain the ability to differentiate. The knowledge of the final formation of B cells from pluripotential precursors is hindered by the fact that they appear at relatively late stages of organogenesis. This brings up a question what molecular events regulate a sequential activation and inhibition of the adequate number of homeotic genes controlling the formation of endodermal cell line, and then, the generation of precursor exocrine cells, endocrine cells and finally, mature B cells (1).
The suppression of mesodermal signalling pathways, Wnt and FGF4, which is predominant in the early phase of the foregut development, enables the induction of morphogenesis of both the liver and the pancreas, whereas the activation of the Wnt pathway in the hindgut prevents the initiation of this process (2). FGF released from the cardiac mesoderm of the ventral part of the foregut and BMP from mesenchymal cells of the transverse septum in a coordinated way, induce the molecular liver formation programme and at the same time block the pancreatic development programme (3, 4). Interestingly enough, the activation of the MAPK pathway as a reaction to FGF occurs earlier in cells located laterally inside the bud than in medially situated cells (5). At the time of foregut closing, ventrolateral endodermal cells, which move in the caudal direction in relation to the cardiac domain, cease to be affected by FGF and initiate the development of the ventral pancreatic bud (6) he dorsal part of the foregut, signals from the dorsal chord, activin and FGF, inhibit the Shh (Sonic hedgehog) pathway in endodermal cells, which activates the pancreatic development programme. It is worth emphasizing that all of the phenomena previously described occur in vertebrate embryos within a few hours only (7, 8).
Bipotential hepatic stem cells called hepatoblasts show the expression of genes characteristic for hepatocytes. They code serum proteins: albumin (alb1) and transthyretin (ttr). They undergo a differentiation into both hepatocytes and cholangiocytes (8). Tbx3 gene promotes the expansion of the hepatoblast population via p19ARFsuppression. At early stages, the pancreatic endoderm is characterised by the expression of genes for transcription factors, Pdx1 and Ptf1a (9). They play a key role in the pancreatic development (10, 11). Following the formation of hepatoblasts and pancreatic progenitor cells, they change their shape from cuboidal into columnar, and then they form a pseudostratified structure. This process is similar to the morphogenesis of the nervous epithelium, and in the foregut it is controlled by the transcription factor Hhex (6). The pancreatic epithelium spreads and penetrates into the stroma, where it forms the pancreatic bud, whereas, in the case of the hepatic epithelium, the basement membrane is disrupted and cells proliferate into the immediate surroundings of the stroma. Later morphological changes are controlled by the genes for transcription factors, Prox1, Hnf6/OC-1, Hnf6 and OC-1 regulating the synthesis of E-cadherin, thrombospondin-4, and Spp1, which participate in the process of the cell adhesion and migration (8). Neighbouring progenitor cells of both organs also receive regulatory signals from surrounding endothelial cells (12, 13). Interestingly enough, endothelial cells may also promote liver regeneration after its parenchymal damage, mainly via the HGF signalling pathway (14). Signalling factors produced by endothelial cells have not been identified yet. However, it has been established that sphingosine phosphate (present in blood) that is a substance of endothelial origin, promotes the development of the dorsal pancreatic bud (15). The cells of neural crests migrating into the developing pancreas are transformed into neurons and thus influence the number of B cells (16). The stimulating role of endothelial cells and neural crest cells illustrates the importance of cooperation of the mesenchymal stroma in the development of progenitor hepatic and pancreatic cells. In the liver and in pancreatic buds, the components of the Notch signalling pathway ensure the balance between hepatocytes and cholangiocytes formed from hepatoblasts as well as endo- and exocrine cells derived from pancreatic progenitor cells. The blockade of the Notch system initiates a preferential development of endocrine cells. The main role in this process is played by the Ngn3 transcription factor which is essential for the development of pancreatic islets and which has the features of a classic marker of pancreatic precursor endocrine cells. Ngn3 expression is considerably reduced at the moment of birth, and in the mature organ it is almost indeterminable. In the nascent endocrine cells, the expression of Notch ligands (delta, serrate, jagged) is considerably increased. These ligands react with the Notch receptor on the neighbouring cells and prevent them from differentiating in a similar direction. Multipotential Cpa-1-positive cells located in distal endings of the branched epithelium give rise to both pancreatic ducts and endocrine cells located along their branches. In mice, this process takes place about the 14th day of the embryonic development. At further stages, cells showing Cpa-1 expression are also differentiated into acinar cells of the exocrine part of the pancreas. This process occurs at the time when mature B cells are formed, which is mediated by the MafaA factor.
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