Experiments performed by Wilmut et al. (1997)

   These experiments marked a new technological breakthrough as regards the cloning of mammals.
  Lambs developed following transplantation in oocytes of nuclei from 9 day old embryos, from 26
  day old foetuses, and even from mammary gland tissue from a gestating ewe. This success was
  due to recognition of the fact that the major difficulty in such ex periments is for a somatic cell
  nucleus to adapt to ovocyte cytoplasm and to respond to its signals. Implanted nuclei are
  generally in the S phase (DNA synthesis phase) or G2 (which is the phase immediately after S) of
  the cell cycle. Oocytes in which n uc lei are transferred are generally in the metaphase of the 2nd
  meiotic division (close to cell division). This chronological discrepancy leads to induction of an
  immediate DNA replication in the implanted nucleus followed by premature chromosomal
  condensa tion. These events are generally linked to an abnormal distribution of chromosomes in
  the daughter cells that become aneuploid (i.e. they contain an abnormal number of
  chromosomes). Aneuploidy very frequently leads to arrest of embryonic development.

   Wil mut et al. (1997) took the precaution of cultivating the donor nuclear cells in conditions which
  ensured a state of non-proliferation, i.e. quiescence . This is easily obtained by starving the cells.
  The cells become arrested in the phase of the cell cycle phase known as G0 (i.e. the resting phase
  when the cells are not dividing). This state probably confers on the somatic nucleus the capacity
  to sync hronise DNA replication with the ovocyte programme. It also probably leads to the
  rearrangement of the chr omatin organisation which occurs throughout the period of egg
  cleavage in mammals (Thompson et al., 1995). However data gathered so far does indicate that,
  after nuclear transfer, the structure of the nucleus is not that of the zygotic nucleus. (Christian s
  et al., 1994 ; Chastant et al., 1996).

   Success obtained by Wilmut et al., (1997) with nuclei from the adult mammary gland should not
  obscure the fact that the nature of these donor cells is not precisely known. There could be cells
  which, even in adults , retain some pluripotentiality. There are such cells in almost all tissues.

  Furthermore, it should be noted that the success rate of recently reported experiments is
  extremely low : 1 lamb born out of a series of 277 transplantations of mammary gland nuclei.