Nuclear Transfer and Activation
All micromanipulations are carried out in Hepes buffered
media. In our laboratory we use a SOF Hepes-based formulation
(Gardner et al., 1994) supplemented with 6 mg/ml of
BSA and/or 10% FCS for zona-free work. The zona pellucida
of in vitro matured oocytes is very thick and heterogeneous,
and due to hardening it is difficult to penetrate with a
bevelled standard pipette such as those used for enucleating
in vitro matured oocytes of other species. As a result, the
zona-enclosed conventional method for enucleation is more
difficult and less efficient. Moreover, the transfer of the
donor cell into the perivitelline space results in poor contact
between the oocyte and somatic cell membranes, thus
reducing the chance of successful electrofusion (Lagutina
et al., 2005; Li et al., 2002) even at high voltage. Further,
the presence of the zona pellucida acts as an electrical
shield that requires an increase in the voltage of fusion to
2–2.5 kV/cm, and this high voltage reduces subsequent
cleavage of the reconstructed embryos. Therefore, two
methods have been used to overcome this problem: the use
of piezo-electric manipulation (Kimura and Yanagimachi
1995), or pronase removal (the zona-free method) (Oback
et al., 2003). The piezo-driven micropipette is used to
penetrate the zona, to remove the first polar body and the
surrounding cytoplasm containing the metaphase plate
(visualized by Hoechst stain and UV light, in presence of
cytocalasin B), and also to microinject a somatic cell with
a broken cell membrane (Choi et al., 2002b) to obtain the
transfer of the donor nucleus into the enucleated oocyte.
For the zona-free method, the zona pellucida is
digested with 0.5% pronase in PBS. Zona-free oocytes
are enucleated under UV light with a blunt micropipette.
Subsequently, zona-free cytoplasts are individually washed
for a few seconds in 300 μg/ml phytohemagglutinin P in
PBS and then quickly dropped over a single donor cell
(Vajta et al., 2003) settled at the bottom of a drop of SOF–
Hepes. Formed cell couplets are washed in 0.3 M mannitol
(50 μM Ca and 100 μM Mg) solution and fused once
or twice at 15- to 30-minute intervals by two DC pulses
of 1.2 KV/cm applied for 30 μs at 26–27 h after the beginning
of maturation. With such an approach the fusion rate
approximates 100% and, because of the lower electric field
required, the rates of cleavage and of post-cleavage development
to the blastocyst stage are higher (Lagutina et al.,
2005, 2007).
Activation is usually performed 2–4 h after fusion. In
our experience horse oocytes are difficult to activate, and
the standard protocols used in ruminants do not yield a
high rate of cleavage. In our laboratory, we found that the
use of 5 μM ionomycin (a calcium ionophore) followed by
the synergistic effect of 6-DMAP (1 mM) and cycloheximide
(5 μg/ml) resulted in over 90% parthenogenetic
activation (Galli et al., 2007; Lazzari et al., 2002) and
cleavage of cloned embryos (Lagutina et al., 2005). Woods
et al. (2003) increased the calcium level during activation
of in vivo matured oocytes. Hinrichs et al. (2006,
2007) compared different activation treatments, including
the injection of sperm extract, which, in combination
with ionomycin, while not statistically different, provided
the highest blastocyst development rate and development
to term. However injection of murine PLC zeta cRNA
TABLE 22.5 Development of Horse NT Embryos Obtained with Adult Fibroblast Cells Treated with Trichostatin A
Horse TSA Treatment No. of Embryos Cleaved No. (%) MCD5 No. (%) MCD6 No. (%) BLD7 No. (%) BLD8 No. (%)
C Control 56 52 (92.9) 15 (26.8) 18 (32.1) 23 (41.1) 24 (42.9)
TSA 69 66 (95.7) 16 (23.2) 23 (33.3) 29 (42) 30 (43.5)
D Control 79 66 (83.5) 6 (7.6) 10 (12.7) 10 (12.7) 10 (12.7)
TSA 92 84 (91.3) 5 (5.4) 12 (13) 13 (14.1) 12 (13)
Number of replicates >> 4. No statistical difference between control and TSA treatment. Chi square test, P<0.05.MCD5, compacted morula day 5; MCD6,
total number of compacted morula on day 6; BlD7, blastocysts on day 7; BlD8, total number of blastocysts on day 8.
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