A homeobox-containing clone has been isolated from an adult mouse kidney cDNA library and shown by DNA sequence analysis to be a new isolate, Hox-6.1. A genomic clone containing Hox-6.1 has been isolated and found to contain another putative homeobox sequence (Hox-6.2), within 7 kb of Hox-6.1. In situ hybridization of mouse metaphase chromosomes shows this Hox-6 locus to be located on chromosome 14 (14E2). Hox-6.1 has been studied in detail and the predicted protein sequence of the homeobox is 100% homologous to the Xenopus Xeb1 (formally AC1) homeobox and the human c8 homeobox (Carrasco et al. 1984; Boncinelli et al. 1985; Simeone et al. 1987). Southern blotting shows that the DNA sequence encoding Hox-6.1 is single copy. Expression of Hox-6.1 has been studied in adult tissues and embryos by RNase protection assays, Northern blotting analysis and in situ hybridization. RNase protection assays show that Hox-6.1 transcripts are present in embryos between days 9 1/2 and 13 1/2 of gestation and in extraembryonic tissues at day 9 1/2. Adult expression is detectable in kidney and testis but not in liver, spleen and brain. One major transcript is detectable on Northern blots of kidney and day-13 1/2 embryo RNA. In kidney, this transcript is 2.7 kb whereas in embryos the major transcript is smaller at 1.9 kb, a much fainter band being visible at 2.7 kb. Localized expression of Hox-6.1 is observed in the spinal cord and prevertebral column of day-12 1/2 embryos, and in the posterior mesoderm and ectoderm of day-8 1/4 embryos. An anterior boundary of expression is located just behind the hindbrain whereas the boundary in the mesoderm is located at the level of the 7th prevertebra.
Cloning of cells from peri-implantation embryos by blastocyst injection was used to investigate the time of X-chromosome inactivation in that part of the ectoderm lineage giving rise to foetal tissues of the mouse. Matings were arranged so that the two X-chromosomes of female donor cells controlled two distinct coat colours and host blastocysts were of a third colour genotype. No coat chimaeras were obtained in experiments using donor cells from the primitive ectoderm of 6th or 7th day embryos or from lactationally delayed implanting or reactivated blastocysts. In contrast, a minimum of 80 unequivocal coat chimaeras were obtained in experiments in which primitive ectoderm cells from 5th day implanting blastocysts were used for injection. The majority of these chimaeras that had received a female cell exhibited both donor colours in addition to host colour in their coats, suggesting that the donor cell had not undergone X-inactivation until one or more cycles after transplantation. The remainder of such chimaeras exhibited only one or other donor coat colour. Determination of the parental origin of the allocyclic X-chromosome in donor metaphase preparations in internal tissues of several chimaeras revealed that the coat pattern did not always reflect the X-activity status of the donor cell clone as a whole. Nevertheless, the findings suggest that X-inactivation takes place shortly after implantation in the primitive ectoderm cell population from which the foetus is derived. Of the 68 chimaeras in which the sex of both the donor and host component was established 62 proved to be fertile. Furthermore, 21 of the 37 fertile chimaeras whose sex corresponded with that of the donor cell yielded functional gametes of donor origin. Injection of cells from a single donor blastocyst into a series of host blastocysts established that at least 2 cells in 5th day primitive ectoderm can give rise to both somatic cells and functional germ cells among their mitotic descendants.