Effect of donor age
By using a design similar to the donor cell type comparison, we studied the cloning efficiency of fibroblast cells from donors of different ages. We found that cells from fetuses and newborn animals were more efficient in nuclear transfer. However, when cells from adult animals were used, little changes were observed in the cloning efficiency of cells from cattle varying in age from 2 to16-years-old (Table 4).
Table 4  Cloning competence of cells from donor animals of different ages
Donor age  No. Oocytes used  (%) Development
Cleavage  Blastocyst
Fetus (D57)  630  82  48a
New born  302  76  51a
2 years  158  79  38b
10–12 years  424  73  35b
16 years  269  63  37b
Numbers with different superscripts within columns are significantly different (P < 0.05). Similarly, Renard et al. [31], Hills et al. [29] and Wakayama and Yanagimachi [56] also reported that development rates of somatic cloned embryo remained similar regardless of donor age. However, Kato et al. [36] noted that clones derived from adult cells frequently aborted in the later stages of pregnancy, and calves developing to term showed a higher number of abnormalities than did those derived from newborn or fetal cells. Forsberg et al. [58] transferred a large number of cloned embryos in cattle. They also concluded that, in general, embryos cloned from fetal cells produced higher pregnancy and calving rates than those from adult cells.
In conclusion, it appears that cells from fetuses, as well as aged adults, can lead to comparable blastocyst development of cloned embryos. Nevertheless, fetal cells may be better than adult cells in producing healthy live births. This might be due to the fact that the somatic cells of adult animals have accumulated more genetic mutations/are more terminally differentiated than fetal cells, and are thus more likely to fail at full term development.