Expression and distribution of fetuin in the developing sheep fetus

Abstract

Tissue distribution and developmental expression of fetuin were studied in the sheep fetus from embryonic day (E) 30 to adult (gestational period is 150 days). The presence of fetuin was demonstrated immunocytochemically using anti-fetuin antibodies; in situ hybridisation using short anti-sense oligonucleotide probes labelled with digoxigenin was used to study the ability of the developing tissue to synthesise fetuin, and reverse transcription-polymerase chain reaction (RT-PCR) was used to estimate the level of fetuin mRNA in selected tissues. Tissue distribution of fetuin was widespread in the younger fetuses (E30 to E40). The most prominent presence due to in situ synthesis was demonstrated in the liver, central nervous system (CNS) including anterior horn cells, dorsal root ganglia and in skeletal muscle cells. Other developing tissues and organs that showed evidence of fetuin synthesis and presence of the protein included mesenchyme, kidney, adrenal, developing bone, gut, lung and heart. In the immature liver (E30–40) there was a strong signal for fetuin mRNA in hepatocytes and also in numerous haemopoietic cells; the proportion of these latter cells that was positive for fetuin mRNA increased between E30 and E40. Only some hepatocytes and a proportion of the haemopoietic stem cells were immunoreactive for fetuin itself at E30–40; immunoreactive hepatocytes were more frequently observed in the more mature outer regions of the developing liver. Lung and gut contained scattered fetuin-positive epithelial cells, especially at E30; a weak fetuin mRNA signal could be detected above background in many of these cells up to E40, but not at E60–E115 or in the adult. Particularly at E30 to E40, mesenchymal tissue both within organs such as the gut and lung and around forming bone and skeletal muscle contained cells that were positive for fetuin mRNA. Mesenchyme at these ages was also very strongly stained for fetuin protein, much of which may reflect fetuin in tissue extracellular spaces and be derived from the high concentration in plasma. By E80 fetuin mRNA was mainly present in the liver and the CNS; staining of the muscle tissue was becoming less pronounced. However in developing bone tissue, staining of chondrocytes for fetuin mRNA was still prominent in older (E80) fetuses; there was also fetuin protein staining of chondrocytes at the growing surfaces of bones and in bone marrow at this age. In the adult, weak immunocytochemical staining for fetuin itself was present in hepatocytes, but the mRNA signal was barely above the threshold limit of detection. Other tissues in the adult were generally negative for both fetuin mRNA and fetuin, except that fetuin could generally be detected immunocytochemically in precipitated plasma within vessels in many tissues and in their interstitial spaces. The highest levels of fetuin mRNA, as demonstrated by RT-PCR, were detected in E40 and E60 liver followed by E40 muscle. The very low level of fetuin mRNA in adult liver, evident from in situ hybridisation, was confirmed by RT-PCR (about 0.1% of that at E60). These results show that in many tissues in which fetuin could be demonstrated immunocytochemically, its presence is likely to be due to synthesis in situ. However in some instances (e.g. gut and mesenchymal tissue) fetuin probably originates predominantly by uptake from plasma or extracellular fluid. The functional significance of the presence of fetuin in different tissues during their development is considered.