EXPRESSION AND DISTRIBUTION OF FETUIN IN THE DEVELOPING SHEEP FETUS

Tissue distribution and developmental expression of fetuin were studie d in the sheep fetus from em bryonic day (E) 30 to adult (gestational period is 150 days). The presence of fetuin was demonstrated immunocyt ochemically 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 fetui n, and reverse transcription-polymerase chain reaction (RT PCR) was us ed to estimate the level of fetuin mRNA in selected tissues. Tissue di stribution of fetuin was widespread in the younger fetuses (E30 to E40 ). The most prominent presence due to in situ synthesis was demonstrat ed in the liver, central nervous system (CNS) including anterior horn cells, dorsal root ganglia and in skeletal muscle cells. Other develop ing tissues and organs that showed evidence of fetuin synthesis and pr esence 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 hae mopoietic cells; the proportion of these latter cells that was positiv e for fetuin mRNA increased between E30 and E40. Only some hepatocytes and a proportion of the haemopoietic stem cells were immunoreactive f or fetuin itself at E30-40; immunoreactive hepatocytes were more frequ ently observed in the more mature outer regions of the developing live r. Lung and gut contained scattered fetuin-positive epithelial cells, especially at E30; a weak fetuin mRNA signal could be detected above b ackground 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 m uscle contained cells that were positive for fetuin mRNA. Mesenchyme a t these ages was also very strongly stained for fetuin protein, much o f which may reflect fetuin in tissue extracellular spaces and be deriv ed from the high concentration in plasma. By E80 fetuin mRNA was mainl y 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) fet uses; there was also fetuin protein staining of chondrocytes at the gr owing surfaces of bones and in bone marrow at this age. In the adult, weak immunocytochemical staining for fetuin itself was present in hepa tocytes, but the mRNA signal was barely above the threshold limit of d etection. 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 ti ssues and in their interstitial spaces. The highest levels of fetuin m RNA, as demonstrated by RT-PCR, were detected in E40 and E60 liver fol lowed 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 f etuin could be demonstrated immunocytochemically, its presence is like ly to be due to synthesis in situ. However in some instances (e.g. gut and mesenchymal tissue) fetuin probably originates predominantly by u ptake from plasma or extracellular fluid. The functional significance of the presence of fetuin in different tissues during their developmen t is considered.