felines, and in the following, a review on recent progress will be given for each of these carnivore families.
2. Canines
Canines are monoestrous. Seasonality is not obvious in most breeds of dogs, except for the Basenji, in which females experience estrus during the autumn in the northern
Ž .
Ž .
hemisphere. The blue A. lagopus and red foxes V. Õulpes are seasonal with their
breeding season during January–March. Ovulation occurs 1–2 days after the preovula- tory LH peak at the beginning of estrus in both dogs and foxes. In bitches and vixens,
preovulatory luteinization of follicles occurs, exposing oocytes to increasing concentra- tions of progesterone, as opposed to the situation in many other domestic mammals,
where estrogen dominates the preovulatory follicular environment. In most mammals, ovulation of the oocyte occurs when the oocyte has reached the metaphase of the second
meiotic division. In canines, the oocyte is ovulated at the beginning of the first meiotic division, and the germinal vesicle is broken down shortly after ovulation. Subsequent
Ž stages of oocyte maturation occur in the oviduct Holst and Phemister, 1971; Farstad et
. al., 1989 .
2.1. Oocyte maturation, fertilization and embryonic deÕelopment in Õitro Canine oocytes may resume meiosis spontaneously in vitro using adaptations of
Ž .
bovine and porcine in vitro maturation IVM techniques, although at a much lower rate and efficiency than most other domestic animal oocytes. In canids, IVM has shown
limited success with maturation rates varying from 0 to 58 for oocytes matured to Ž
. Metaphase I, Anaphase I and Metaphase II MIrAIrMII ; maturation is usually around
Ž .
Ž 20 MII , in a variety of different culture systems and media Mahi and Yanagimaci,
1976; Robertson et al., 1992; Nickson et al., 1993; Yamada et al., 1992, 1993; Bolamba et al., 1997; Hewitt and England 1997, 1998a, 1999a; Hewitt et al., 1998; Theiss, 1997;
. Metcalfe, 1999 . Most oocytes used for IVM experiments in the dog have been collected
from random sources, usually from animals undergoing ovariohysterectomy. Hence, oocytes from prepubertal, anestrous, luteal phases of pregnant and non-pregnant animals,
as well as proestrous and estrous females have been used with no apparent effect of the
Ž stage of estrous cycle Nickson et al., 1993; Hewitt and England, 1997; Theiss, 1997;
. Ž
Metcalfe, 1999 , whereas the age of the donor animal Nickson et al., 1993; Hewitt and .
Ž England, 1998a , oocyte size Theiss, 1997; Hewitt and England, 1998a; Srsen et al.,
. Ž
. 1998 and nuclear and cumulus morphology Nickson et al., 1993 all influence IVM
rates. In foxes, the IVM of ovarian oocytes, collected either from preovulatory or Ž
anestrus follicles, have resulted in maturation rates similar to those in the bitch i.e., . Ž
. 80 — 100 GVBD, 25 MII
Krogenæs et al., 1993; Wen et al., 1994 . Recently, maturation rates to MII have been improved for blue fox oocytes collected from
Ž . Ž
. anestrous animals 40 MII
Srsen et al., 1998 . Canine oocytes may undergo IVM in Ž
. intact follicles dissected from the ovary Bolamba et al., 1998 . The production of antral
follicles from small preantral follicles has been attempted using bitch ovarian tissue
grafting to host ovaries of SCID mice. Graft establishment and some follicular recruit- Ž
ment occurred, even though the production of antral follicles was not obtained Met- .
calfe, 1999 . A refinement of this technique may enable the use of ovaries from valuable animals for further production of oocytes posthumously.
Canine embryos have been produced after fertilization in vitro of in vivo matured Ž
. Renton et al., 1991; Farstad et al., 1993a , and from in vitro matured oocytes, but in the
Ž latter development beyond the eight-cell stage has not been reported Yamada et al.,
. 1993; Metcalfe, 1999 . An in vitro ‘‘block’’ to development has been described for
Ž .
Ž many species in which in vitro fertilization IVF has been attempted for a review on
developmental ‘‘block’’ in vitro of mouse, hamster, sheep and cow embryos, see .
McGinnis and Youngs, 1992; Sparks et al., 1992; for cat: Swanson et al., 1996a . The maternal embryonic transition constitutes a critical phase of embryo development. In
foxes and dogs, structural studies and cultivation with
3
H-uridine, indicate that activa- tion of the embryonic genome occurs at the six- to eight-cell stage in foxes and the
Ž .
eight-cell stage in dog embryos Farstad et al., 1993b; Bysted and Greve, 2000 . The scarcity of reports in the literature of attempts to modify the culture conditions in vitro
for IVM-derived embryos after the eight-cell stage may indicate that some difficulties have been encountered in propagating development past this stage, but too little
information is available to conclude that such an in vitro block exists in dog oocytes. To date, no reports of production of live young after IVF from either in vivo- or in vitro
matured dog or fox oocytes exist in the literature.
2.2. Sperm treatment, cryopreserÕation and assisted fertilization The in vitro capacitation of canine sperm may be achieved in canine capacitation
Ž .
Ž medium Mahi and Yanangimaci, 1976 or in modified Tyrode’s solution Farstad et al.,
. 1993a,b; Hewitt and England, 1999b . Calcium ionophore A23187 can promote capaci-
2q
Ž tation and the acrosome reaction in a similar manner as Ca
in vitro Szasz et al., 1997; .
Hewitt and England, 1998c . The reports on IVF rates in dogs or foxes are few, but cleavage rates of 5–20 and pronuclear formation in 20–37 of oocytes have been
Ž reported Farstad et al., 1993a,b; Nickson et al., 1993; Yamada et al., 1993; Metcalfe,
. Ž
. 1999 . Intracytoplasmic sperm injection ICSI has been attempted with chilled dog
sperm, with the formation of male pronuclei in 8 of oocytes, but no cleavage occurred Ž
. Fulton et al., 1998 .
The cryopreservation of dog and fox semen has been frequently reviewed, and a variety of freezing regimens, extenders and thawing protocols for dog and fox semen
Ž .
have been published England, 1993; Farstad, 1996; Rota, 1998 . Recently, modifica- tions of the commonly used TRIS egg yolk extender by the addition of Equex STM
paste improved post-thaw survival of dog sperm during incubation at 38 8C and produced
Ž .
Ž an overall pregnancy rate after vaginal or intrauterine IU AI of 84 Rota et al., 1997,
. 1999 . Differences between canine species with respect to cooling tolerance may exist,
Ž .
Ž .
since freezing trials with blue foxes A. lagopus and red wolves C. rufus have
resulted in a significant reduction in the percentage of intact acrosomes after freezing Ž
. and thawing Farstad et al., 1992; Goodrowe et al., 1998, 2000 . Dual fluorescent
staining techniques, binding tests and oocyte penetration assays have been developed for
dog sperm, which may also be usable for the assessment of sperm function in vitro for Ž
other canines Hay et al., 1997; Hewitt and England, 1998b,c; Mayenco-Aguirre and .
Peres-Cortez, 1998, Strom Holst, 1999 . Red wolf sperm bind to domestic dog oocytes
¨
Ž .
Goodrowe, 1999, personal communication . 2.3. Artificial insemination and ET
IU artificial insemination may be carried out non-surgically using either endoscopic Ž
visualization or transcervical catheterization, or surgically by laparoscopy for review, .
see Farstad, 2000 . Results in foxes and dogs using IU non-surgical AI are good, and Ž
both birth rates and litter sizes approach those of natural mating see reviews Farstad, .
1996, 1998 . The ET of in vivo-derived embryos has been carried out surgically both in Ž
the silver fox and the dog resulting in live young, but with low success rates Kinney et .
al., 1979; Tsutsui et al., 1989; Jalkanen and Lindeberg, 1998 . Recently, ET has been carried out in the blue fox, using the IU catheter developed for artificial insemination in
Ž .
foxes Lindeberg, 1999, personal communication . To date, the birth of live young from cryopreserved canine embryos has not been reported. However, in the blue fox, freezing
of embryos recently resulted in the observation of implantation sites in naturally Ž
. synchronized recipient females after ET Lindeberg, 1999, personal communication .
The refinement of freezing regimens, improvement of donor-recipient synchronization, in vitro handling of embryos and transfer techniques may soon render both cryobanking
of embryos and ET feasible in foxes.
3. Felines
