H19, which is one of the most abundantly expressed imprinted genes during 
mammalian embryonic and foetal development, has been cloned from a ruminant. The 
sheep (Ovis aries) gene contains five exons interspersed by four exceptionally 
small introns; only short stretches of the nucleotide sequence, particularly in 
exon 1, show good homology with the human gene. The size of the exons and 
introns and the sequences around the splice junctions however, are well 
conserved between the species. The gene encodes a approximately 2.6 kb 
transcript which contains several potential short open reading frames, none of 
which is conserved between the ovine and human or murine transcripts, supporting 
a previous hypothesis that the gene product is the untranslated RNA itself. H19 
mRNA is highly abundant in most ovine embryonic and foetal tissues of mesodermal 
and endodermal origins but was not detected in tissues of ectodermal origin such 
as the trophectoderm and the foetal brain. Expression of H19 in the 
extraembryonic membranes was detected only after the ovine conceptus began 
attachment to the endometrium and the embryo itself had undergone early 
organogenesis. This may be regarded as the first step in implantation; thus, in 
comparison with the mouse, the initiation of H19 expression appears to be 
determined by the timing of implantation rather than by the stage of development 
of the embryo itself. In most tissues, H19 expression is temporally linked to 
IGF2, a major foetal growth factor. The exceptions were the elongated 
blastocyst, the trophectoderm and brain, where low levels of IGF2 were observed 
in the absence of detectable H19. The abundance of H19 mRNA was in general, 
directly correlated with IGF2 mRNA abundance in mesodermal and endodermal 
tissues, suggesting that the two ovine genes share common regulatory elements 
that co-ordinately regulate their expression. Though both are generally regarded 
as embryonic and foetal genes, their expression was still maintained at a fairly 
high level in the adult sheep liver, lung, skeletal muscle, adrenal gland and 
kidney, suggesting that these organs are significant sources of IGF II in the 
adult.