1. Department of Anatomy and Reproductive Biology, John A. Burns School of Medicine, University of Hawaii, Honolulu, Hawaii 96822, USA
2. Department of Veterinary Anatomy, Faculty of Agriculture, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
3. Department of Signalling, Babraham Institute, Cambridge CB2 4AT, UK
4. Dipartimento Biologia Animale, Laboratorio Biologia dello Sviluppo, University of Pavia, Piazza Botta 10, 27100, Pavia, Italy
5. Jackson Laboratory, 600 Main Street, Bar Harbor, Maine 04609, USA

Correspondence to: R. Yanagimachi¹ Correspondence and requests for materials should be addressed to R.Y.

Until recently, fertilization was the only way to produce viable mammalian offspring, a process implicitly involving male and female gametes. However, techniques involving fusion of embryonic or fetal somatic cells with enucleated oocytes have become steadily more successful in generating cloned young^{1, 2, 3}. Dolly the sheep⁴ was produced by electrofusion of sheep mammary-derived cells with enucleated sheep oocytes. Here we investigate the factors governing embryonic development by introducing nuclei from somatic cells (Sertoli, neuronal and cumulus cells) taken from adult mice into enucleated mouse oocytes. We found that some enucleated oocytes receiving Sertoli or neuronal nuclei developed in vitro and implanted following transfer, but none developed beyond 8.5 days post coitum; however, a high percentage of enucleated oocytes receiving cumulus nuclei developed in vitro. Once transferred, many of these embryos implanted and, although most were subsequently resorbed, a significant proportion (2 to 2.8%) developed to term. These experiments show that for mammals, nuclei from terminally differentiated, adult somatic cells of known phenotype introduced into enucleated oocytes are capable of supporting full development.