Oriented clonal cell growth in the developing mouse myocardium underlies cardiac morphogenesis.

During heart morphogenesis, cardiac chambers arise by differential expansion of regions of the primitive cardiac tube. This process is under the control of specific transcription factors such as Tbx5 and dHAND. To gain insight into the cellular mechanisms that underlie cardiogenesis, we have used a retrospective clonal approach based on the spontaneous recombination of an nlaacZ reporter gene targeted to the murine alpha-cardiac actin locus. We show that clonal growth of myocardial cells is oriented. At embryonic day (E) 10.5, the shape of clones is characteristic of a given cardiac region and reflects its morphology. This is already detectable in the primitive cardiac tube at E8.5, and is maintained after septation at E14.5 with additional modulations. The clonal analysis reveals new subdivisions of the myocardium, including an interventricular boundary region. Our results show that the myocardium, from the time of its formation, is a polarized and regionalized tissue and point to the role of oriented clonal cell growth in cardiac chamber morphogenesis.

Genetic background influences embryonic lethality and the occurrence of neural tube defects in Men1 null mice: relevance to genetic modifiers.

Germline mutations of the multiple endocrine neoplasia type 1 (MEN1) gene cause parathyroid, pancreatic and pituitary tumours in man. MEN1 mutations also cause familial isolated primary hyperparathyroidism (FIHP) and the same MEN1 mutations, in different families, can cause either FIHP or MEN1. This suggests a role for genetic background and modifier genes in altering the expression of a mutation. We investigated the effects of genetic background on the phenotype of embryonic lethality that occurs in a mouse model for MEN1. Men1(+/-) mice were backcrossed to generate C57BL/6 and 129S6/SvEv incipient congenic strains, and used to obtain homozygous Men1(-/-) mice. No viable Men1(-/-) mice were obtained. The analysis of 411 live embryos obtained at 9.5-16.5 days post-coitum (dpc) revealed that significant deviations from the expected Mendelian 1:2:1 genotype ratio were first observed at 12.5 and 14.5 dpc in the 129S6/SvEv and C57BL/6 strains respectively (P<0.05). Moreover, live Men1(-/-) embryos were absent by 13.5 and 15.5 dpc in the 129S6/SvEv and C57BL/6 strains respectively thereby indicating an earlier lethality by 2 days in the 129S6/SvEv strain (P<0.01). Men1(-/-) embryos had macroscopic haemorrhages, and histology and optical projection tomography revealed them to have internal haemorrhages, myocardial hypotrophy, pericardial effusion, hepatic abnormalities and neural tube defects. The neural tube defects occurred exclusively in 129S6/SvEv embryos (21 vs 0%, P<0.01). Thus, our findings demonstrate the importance of genetic background in influencing the phenotypes of embryonic lethality and neural tube defects in Men1(-/-) mice, and implicate a role for genetic modifiers.