Reproduction in Domestic Animals

Reproduction of Domestic Ferret 153over a broad area that eventually becomes a zonaryband of endotheliochorial placenta (Strahl and Ballman1915; cited in Enders and Schlafke 1972). The endotheliochorialplacenta has three foetal (endothelium, connectivetissue, trophoblast) but only two maternal layers(connective tissue, endothelium) because the maternalepithelium is lost. The foetal trophoblast invades theendometrial epithelium, but does not destroy theendothelium of the maternal capillaries (Mossman1987; Valtonen 1992). Between days 12 and 13 aftermating, the embryos are implanted in the endometrium(Enders and Schlafke 1972; Mead et al. 1988b).Prior to implantation, the trophoblast differentiatesrapidly and gives rise to patches of syncytial trophoblast(Gulamhusein and Beck 1973). The first feature ofpenetration is the projection of a thin fold of syncytialtrophoblast between adjacent epithelial cells. As implantationprogresses, more of the blastocyst is involvedin implantation, and by day 14, a continuous sheet,either penetrating or overlying the area of the wall of theuterus, constitutes two-thirds of the circumference of thefuture zonary band placenta (Enders and Schlafke1972). At the cervical and ovarian ends of the chorionicvesicle and in the mesometrial region, the trophoectodermis non-invasive (Gulamhusein and Beck 1975). Inthese non-attached regions, the trophoectodermic cellsabsorb uterine milk (Gulamhusein and Beck 1973). Inthe immediate vicinity of implanting chorionic vesicles, ahighly localized increase in the permeability of uterineblood vessels is associated with the final stage ofattachment to the uterine epithelium, this being firstdetectable in the morning of day 12 after mating (Meadet al. 1988b). Prostaglandins are proposed to play animportant role in the process of implantation, but theprocess is unrelated to decidual formation because theferret is an adeciduate species (Mead et al. 1988b); thatis, ferret endometrium is not known to be capable ofdecidual transformation during implantation (Beck1974) in contrast to the situation of rodents and humansthat have a primary decidualization reaction beforeblastocyst(s) start penetrating the uterine epithelium(Johnson and Everitt 2000). Following implantation, awave of epithelial hypertrophy sweeps progressivelyfrom the uterine lumen towards the bases of the glands,and epithelial cells become extraordinarily enlarged withnuclei as large as 90–100 lm in diameter. Most of theluminal cells lose their integrity and form massescontaining whole or fragmented nuclei. This degeneratetissue is termed a symplasma because, although technicallyit is a syncytium, it is not an active tissue (Amoroso1952; Buchanan 1966). This degenerated tissue probablycontributes to the histiotrophe because it is ingested bythe syncytiotrophoblast (Gulamhusein and Beck 1975).In the ferret, the placental labyrinth is fully developedat day 18, when the greatly hypertrophied maternalcapillaries are completely surrounded by a layer ofsyncytiotrophoblast (Gulamhusein and Beck 1975). Atthe same time, accumulations of maternal blood, whichvary considerably in size and location and constitute the‘haemophagous organ’ (Creed and Biggers 1964),appear in the antimesometrial region between theplacental discs. This organ is fully formed by day 28,and it maintains its size almost to term (Gulamhuseinand Beck 1975). It is thought to act as an alternativesource of iron for the foetuses (Baker and Morgan1973).Gestation and function of corpora luteaGestation length is 41 days (39–42 days) in the domesticferret (Hammond and Marshall 1930; Ternovsky andTernovskaya 1994; Fox and Bell 1998). If fertilizationdoes not occur, pseudopregnancy lasting 40–42 daysensues, the functional life of corpora lutea (CL) beingsimilar to that in normal pregnancy (Hammond andMarshall 1930; Chang and Yanagimachi 1963). FerretCL start secreting progesterone immediately afterovulation, and the progesterone concentrations risecontinuously to peak at approximately days 12–14during the period of implantation (Daniel 1976), thendecrease steadily after approximately day 15 and leveloff by day 24 of pregnancy (Heap and Hammond 1974).Ferret CL consist of small (25 lm) luteal cells, with the smaller cells predominatingon day 6. A shift towards larger sizes is observed bydays 13 and 24 of pregnancy, and concurrently thepercentage of smaller cells declines (Joseph and Mead1988). Progesterone concentrations decline continuouslybetween day 24 and parturition at day 42 (Blatchley andDonovan 1976). Mustelids and other carnivores displaya protracted decline in circulating progesterone, andprogesterone levels reach basal concentrations ‡1 weekafter parturition (Møller 1973). The conceptuses haveno effect on the duration of the luteal phase, becausepregnancy and pseudopregnancy are indistinguishable(Hammond and Marshall 1930; Heap and Hammond1974; Agu et al. 1986). Removal of the uterus during theluteal phase has no effect on the life span of the CL(Deanesly 1967). Factors causing luteal regression in theferret are not known.The functional ferret CL secrete glucose-6-phosphateisomerase on days 6–9 of pregnancy, the time at whichimplantation-promoting activity has been found incorpora lutea. This isomerase has been identified to benecessary for embryo implantation in the domestic ferret(Schulz and Bahr 2003). It seems that oestrogen is notessential for implantation in the ferret (Foresman andMead 1978; Mead and McRae 1982). Experimentallyinduced maternal pregnancy reactions (for instance,traumatization with an intrauterine thread that resultsin the formation and subsequent necrosis of symplasmalnests of endometrial epithelial cells and hypertrophy ofthe maternal capillary endothelium) show that theendometrium is sensitive to implantation between days9 and 14 (Gulamhusein and Beck 1977).Parturition, rebreeding, induction of oestrus andsuperovulationThe domestic ferret gives birth to an average of eightkits (1–18 kits), which weigh 6–12 g at birth (Ternovskyand Ternovskaya 1994; Fox and Bell 1998). Females willreturn to oestrus within 2 weeks after weaning if theyare exposed to a stimulatory photoperiod. If the kits areremoved at birth, the mothers return to oestrus 8 weeksafter mating, as do pseudopregnant animals and femalesÓ 2008 The Author. Journal compilation Ó 2008 Blackwell Verlag