Although somatic stem cells have been reported to exist in various adult organs, there have been few reports concerning stem cells in the heart. We here demonstrate that Sca-1-positive (Sca-1؉) cells in adult hearts have some of the features of stem cells. Sca-1؉ cells were isolated from adult murine hearts by a magnetic cell sorting system and cultured on gelatin-coated dishes. A fraction of Sca-1؉ cells stuck to the culture dish and proliferated slowly. When treated with oxytocin, Sca-1؉ cells expressed genes of cardiac transcription factors and contractile proteins and showed sarcomeric structure and spontaneous beating. Isoproterenol treatment increased the beating rate, which was accompanied by the intracellular Ca 2؉ transients. The cardiac Sca-1؉ cells expressed oxytocin receptor mRNA, and the expression was up-regulated after oxytocin treatment. Some of the Sca-1؉ cells expressed alkaline phosphatase after osteogenic induction and were stained with OilRed O after adipogenic induction. These results suggest that Sca-1؉ cells in the adult murine heart have potential as stem cells and may contribute to the regeneration of injured hearts.

Side population (SP) cells, which can be identified by their ability to exclude Hoechst 33342 dye, are one of the candidates for somatic stem cells. Although bone marrow SP cells are known to be long-term repopulating hematopoietic stem cells, there is little information about the characteristics of cardiac SP cells (CSPs). When cultured CSPs from neonatal rat hearts were treated with oxytocin or trichostatin A, some CSPs expressed cardiac-specific genes and proteins and showed spontaneous beating. When green fluorescent protein–positive CSPs were intravenously infused into adult rats, many more (∼12-fold) CSPs were migrated and homed in injured heart than in normal heart. CSPs in injured heart differentiated into cardiomyocytes, endothelial cells, or smooth muscle cells (4.4%, 6.7%, and 29% of total CSP-derived cells, respectively). These results suggest that CSPs are intrinsic cardiac stem cells and involved in the regeneration of diseased hearts.

Almost the entire sequences of 18S rDNA were determined for two chaetognaths, five echinoderms, a hemichordate, and two urochordates (a larvacean and a salp). Phylogenetic comparisons ofthe sequences, together with those of other deuterostomes (an ascidian, a cephalochordate, and vertebrates) and protostomes (an arthropod and a mollusc), suggest the monophyly ofthe deuterostomes, with the exception of the chaetognaths. Chaetognaths may not be a group of deuterostomes. The deuterostome group closest to vertebrates was the group of cephalochordates. Ascidians, larvaceans, and salps seem to form a discrete group (urochordates), in which the early divergence of larvaceans is evident. These results support the hypothesis that chordates evolved from free-living ancestors.The evolutionary pathway from advanced invertebrates through primitive chordates to vertebrates has been a subject of extensive investigation and vigorous discussion for more than a century (1-5). Chordates are categorized as deuterostomes, which are characterized by several features that include, for example, radial cleavage, the fate of the blastopore that does not form a mouth, an enterocoelic coelom, and a tripartite body plan (4-7). Traditionally, the deuterostomes include pogonophorans, chaetognaths, echinoderms, hemichordates, and chordates (urochordates, cephalochordates, and vertebrates), but recent studies have forced the categorization of pogonophorans near the annelids (7, 8). The chaetognath (arrowworm) remains a mystery in terms of its ancestry.Recent advances in molecular biology have made it possible to answer some of the questions posed by evolutionary biologists. Comparisons based on molecular data, such as the amino acid sequences of certain proteins and the nucleotide sequences of certain RNAs and DNAs, provide powerful tools with which to examine phylogenetic relationships among animal groups since these molecular characteristics can be interpreted more objectively than others. Unfortunately, pioneer studies with sequences of 5S rRNA (9) and partial sequences of 18S rRNA (10) failed to affirm the monophyly ofdeuterostomes, probably because the radiation of bilaterally symmetrical animals occurred over a very short period of time. However, the monophyly of the deuterostomes was suggested by Lake (11) after application of his original method for the construction ofphylogenetic trees. A recent study by Stock and Whitt (12) provided evidence from sequences of 18S rRNAs that lampreys and hagfishes form a natural group, but conclusive support from molecularphylogenetic analysis for the monophyly of the deuterostomes has not yet been obtained.To further our knowledge of the phylogeny of deuterostomes, we determined almost the entire sequences of 18S rDNAs from two chaetognaths, five echinoderms, a hemichordate, and two urochordates (a larvacean and a salp) and, together with the sequences of other deuterostomes (an ascidian, a cephalochordate, and vertebrates) and protostomes (an arthropod and a mollusc) (13), we reexamined the ...

The comparison of Hox genes between vertebrates and their closest invertebrate relatives (amphioxus and ascidia) highlights two derived features of Hox genes in vertebrates: duplication of the Hox gene cluster, and an elaboration of Hox expression patterns and roles compared with non-vertebrate chordates. We have investigated how new expression domains and their associated developmental functions evolved, by testing the cis-regulatory activity of genomic DNA fragments from the cephalochordate amphioxus Hox cluster in transgenic mouse and chick embryos. Here we present evidence for the conservation of cis-regulatory mechanisms controlling gene expression in the neural tube for half a billion years of evolution, including a dependence on retinoic acid signalling. We also identify amphioxus Hox gene regulatory elements that drive spatially localized expression in vertebrate neural crest cells, in derivatives of neurogenic placodes and in branchial arches, despite the fact that cephalochordates lack both neural crest and neurogenic placodes. This implies an elaboration of cis-regulatory elements in the Hox gene cluster of vertebrate ancestors during the evolution of craniofacial patterning.