Reproduction in Domestic Animals

152 H LindebergChang 1950). The oocyte is spherical or ovoid; itsnucleus is large, round and eccentric with one or, rarely,two nucleoli (Robinson 1918). It is rich in lipid particles,which in one-half of the oocyte are less numerous thanin the other half, making the oocyte look less dense inthis half (Hamilton 1934) and making it easier tovisualize the nuclear structures on the less dense half ofthe oocyte. This seems to be particular to the ferretoocyte compared with other lipid-rich oocytes observedin other carnivorous species, such as the canine andfeline oocytes.Early embryonic developmentThe first cleavage produces two blastomeres that aresimilar in appearance but unequal in size. The first twoblastomeres divide asynchronously, so that furtherstages of uneven numbers of blastomeres are produced(Hamilton 1934). Asynchronous divisions are a commonfeature for carnivore embryos (Amstislavsky et al. 1993;Lindeberg et al. 1993) but not for embryos of ruminants(Betteridge and Fle´ chon 1988). A rough estimation ofthe intervals between the first and second, and thesecond and third cleavages was 10–16 h for each stage(Chang 1965b). Asynchronous division of the cells anddifferences in sizes are detected up to the 16-cell stage,but no morphological differences can be detectedbetween the central (inner cell mass) cells and thesurrounding (trophoectoderm) cells at the morula stage,in which the cells are grouped closely together and adistinct perivitelline space is present. Fat is equallydistributed among the cells (Hamilton 1934).The degree of development of embryos of the sameage varies considerably (Rider and Heap 1986). At thesame time after mating, embryos from one ferret can beat the blastocyst stage, whereas those of another ferretmay still be at the morula stage (Chang 1969).Blastocysts may be further developed than those inwhich a longer time interval has elapsed after mating.Blastocysts appear to be almost spherical and completelyfill the zona pellucida, which has become thinned bythe presence of the expanding blastocyst. The inner cellmass appears as a dark mass at one pole of theblastocyst. The flattened cells of the trophoectodermand the inner cell mass are dotted with fatty granules(Robinson 1925; Hamilton 1934). Uterine blastocystsexpand from a size of 200 lm in diameter to more than2 mm in diameter during their pre-implantation development(Daniel 1970; Enders and Schlafke 1972).Progesterone has been reported to support blastocystexpansion up to a diameter of 1 mm, but for furtherblastocyst expansion additional ovarian factors arerequired (McRae 1992). Without the presence ofmaternal progesterone, neither cleavage of embryos inthe oviducts or in the uterus (Rider and Heap 1986) norexpansion of blastocysts in the uterus take place(Buchanan 1969; McRae 1992).Oviductal passageFerret embryos enter the uterus over a period of severaldays starting from day 5 after mating. The embryosstart entering the uterus on day 4, if females are treatedwith hCG (Chang 1969), and almost all embryos arefound in the uterus by day 7 as described in studies ofRobinson (1918), Hammond and Walton (1934), whoanalyzed Robinson’s (1918) data and Hamilton’s(1934). Lindeberg and Ja¨ rvinen (2003) reported thatuntil day 5 after the first mating, embryos of farmedEuropean polecats (Mustela putorius) were recoveredonly from the oviducts. Between days 6 and 9, embryoswere recovered both from the oviducts and the uteri.From day 10 onwards, the embryos were recoveredonly from the uteri. Ferret pre-implantation embryos(Fig. 1a and b) experience a prolonged period ofoviductal residence – a phenomenon that has also beendemonstrated in the cat (Swanson et al. 1994), the bluefox (Valtonen et al. 1985; Valtonen and Jalkanen 1993),the silver fox (Jalkanen 1992), the dog (Holst andPhemister 1971; Renton et al. 1991) and the horse(Betteridge et al. 1982). After unilateral or bilateralembryo transfer, ferret pre-implantation embryos havebeen reported to migrate from one horn to another (Liet al. 2006).ImplantationThree types of pregnancy have been identified in theMustelidae (Mead 1989; Ternovsky and Ternovskaya1994). All polecat species (Mustela putorius, Mustelaputorius furo, Mustela eversmanni, Mustela lutreola)have a short period of pregnancy of constant duration(37–44 days). Species such as the stoat (Mustelaerminea) and the sable (Martes zibellina) exhibit anobligatory embryonic diapause at the blastocyst stageand a long gestation period (7–10 months) (Amstislavskyand Ternovskaya 2000). In the American mink(Mustela vison), the gestation period is short butvariable (range 45–61 days), and may or may notinclude implantation delay (Mead 1989).Implantation in the ferret is central with rapidinvasion of the uterine epithelium by the trophoblast(a)(b)Fig. 1. (a) Freshly flushed day-9blastocysts; (b) Frozen-thawedday-9 blastocysts in the farmedEuropean polecatÓ 2008 The Author. Journal compilation Ó 2008 Blackwell Verlag