The fine structure of the lutein cells in the mink (Mustela vison) with special reference to the secretory activity during pregnancy

Summary The ultrastructure of the lutein cells in the mink throughout pregnancy and the regression periodpost partum is described. To correlate the fine structure with the changes in the peripheral plasma progesterone levels, the concentrations of progesterone were measured by a rapid competitive protein-binding assay.Even during the delay period (e.g. as long the plasma progesterone levels remain at the basal level, <8ng/ml), the lutein cells in the mink exhibit structural criteria of functional activity. However, the increase in progesterone secretion is accompanied by some morphological transformations, characterized by the presence of more and more small dense homogenous bodies in the cytoplasm, which become irregular and scalloped during the stage with maximum release of progesterone. At this stage the agranular endoplasmic reticulum is often cisternal or vesicular.During the decline of the progesterone levels, typical and moderate electron-dense lipid droplets are found increasingly more within the lutein cells. The expanded agranular ER is now more sparse, while the granular ER becomes more pronounced, often forming parallel arrays. During this phase the mitochondria become elongated, dumb-bell, or cup shaped. After parturition the corpora lutea consist of cells in various stages of degeneration. At day 14post partum only a few lutein cells are still identifiable.Evidently the observed morphological changes take place in the lutein cells during the life span of corpora lutea. This feature lends further support to the concept that the mink lutein cells are steroid-producing cells and furthermore, that the corpora lutea may be the main sites of gestagen production during pregnancy.     

The development of homeothermy in mink (Mustela vison)

In mink (Mustela vison) kits newborn mortality is very high. One of the major causes of death is hypothermia. The objectives of this study were to observe the development of thermoregulation in mink kits, and their ability to maintain their body temperature during the postnatal period (1-50 days of age). Based on the kit's body weight (BW), and rectal and ambient temperature measurements during cold (+4 degrees C) and warm (+40 degrees C) exposures, a homeothermy index (HI) and cooling and warming rates were calculated. No significant differences in the body temperatures were found between the kits and the dam after 36 days of age. The kits were able to maintain homeothermy by 22 days of age (HI 90%). The body cooling rate was 0.88+/-0.04 degrees C min(-1) on day 1 but only 0.35+/-0.03 degrees C min(-1) at 22 days of age. The body WR was lower: day 1, 0.85+/-0.04 degrees C min(-1) and 0.22+/-0.03 degrees C min(-1) at 22 days of age. All measured and calculated thermophysiological variables were significantly influenced by BW and age of the kit.     

The food of feral mink (Mustela vison) in Scotland

Feral American mink ( Mustela vison ) thrive in Scotland. The potential impact of the mink on the native fauna was investigated through its food. Fifty-five stomachs and 33 scats were analysed by identifying the indigestible remains. Fish formed the main prey (49% occurrence) and birds and mammals 28% and 23% respectively. Most of the fish eaten were Salmonidae (brown trout and salmon). No remains of eels were found.
Mink may compete with otters for food, but the smaller prey taken by the mink and its greater use of land separates it from the otter. Mink seem to be occupying a vacant feeding niche in Scotland.     

Ultrasonographic analysis of gestation in mink (Mustela vison)

Mink are seasonal breeders that display an obligate delay preceding implantation and a post implantation gestation of 31 d. The purpose of this study was to evaluate gestational parameters in mink by ultrasonography. A total of 92 female mink were mated twice during the period from March 2 to 20. The mink were scanned once and allowed to whelp (n=55); or scanned at 3 to 5-d intervals until parturition (n=13); or immediately subjected to autopsy (n=24) after scanning. Embryonic age was calculated from the date of parturition or from crown rump length. Uterine swelling diameter and fetal head size were correlated with embryonic age. The gestational sac grew rapidly once implantation had occurred. Uterine swellings of 4 to 5 mm in diameter were found on Days 2 to 4 post implantation and increased through Days 18 to 20, at which time they began to elongate due to the longitudinal growth of the fetus. Fetal cardiac activity could be detected on Days 10 to 12 post implantation in live embryos. The heart frequency was 198 +/- 3.0 beats per minute and did not vary from Days 12 to 30 post implantation. Fetal head diameter of 5 mm was first detected on Day 19 post implantation and grew gradually to 9 to 10 mm at parturition. It was not possible to accurately assess the number of conceptuses in utero. We conclude that ultrasonography can be employed in mink to diagnose pregnancy, to predict the parturition date and to determine the presence of live fetuses.     

Interactions between melatonin and prolactin during gestation in mink (Mustela vison)

Immature female mink, 8 weeks of age in July, were treated with implants releasing melatonin. Mating, which induced ovulation, took place during the normal breeding season in the following March. Circulating prolactin and progesterone concentrations did not undergo the expected gestational increases, and no embryos implanted. A similar absence of gestational changes in prolactin and progesterone values ensued in primiparous mink treated with the melatonin implant 2-3 days after the second of 2 matings. Administration of exogenous sheep prolactin (0.5 mg/day) by minipump induced precocious elevation of progesterone concentrations in mated mink. Prolactin administration overcame the effects of melatonin, in that the corpora lutea were activated and embryos implanted, but exogenous prolactin resulted in degeneration of implanted embryos both in the presence and absence of chronic melatonin. The results suggest that melatonin has a single effect in alteration of gestation in mink; i.e. the prevention of prolactin secretion. Hyperprolactinaemia may inhibit embryo development in this species.     

The feeding ecology of mink (Mustela vison) in coastal habitat

The diet of a coast-living population of mink was investigated from the scats collected over a three-year period, and compared with information on the availability of principal prey species. Lagomorphs were the single most important prey, and predation upon them matched the abundance of rabbits as determined by monthly counts. Aquatic foraging was particularly important, with rockpool-inhabiting fish accounting for 29–1% occurrence of food items. Fish predation was more pronounced during winter months when lagomorph prey was less available. Crustacean prey, particularly the shore crab, Carcinus maenas , occurred frequently in the diet. Seabirds figured regularly in the diet; these were either taken as carrion from the strand-line or through predation on breeding colonies during the summer months.     

Three twinning types in mink (Mustela vison) embryogeny

The frequency of emergence of monochorial twins in the postimplantational embryogeny of the American mink and their karyotypes were studied. Monochorial pairs were found in which embryos had different karyotypes: 2n, XX and 2n, XY or 2n and 3n. This fact contradicts the notion that monochorial twins should be monozygotic and genetically identical but confirms our earlier hypothesis that a third twinning type exists in mink: monozygotic but genetically different. The mechanism of the emergence of this twinning type in mammals is discussed. It is suggested that the high (up to 4.5%) frequency of its emergence in the American mink is related to obligate embryonic diapause, causing abnormal fertilization.     

The diet of the feral mink (Mustela vison) in southwest Britain

Mink diet in three study areas in southwest Britain was studied by faeces (scat) analysis. In these areas mink ate a wide range of prey, mainly fish, birds and mammals. Proportions of prey in the diet were determined largely by availability, but were also affected by prey behaviour, the physical nature of the habitat and overall prey abundance. Seasonal variation in predation was recorded on moorhens and fish. Changes in rod and net fishing success on the river Teign could not be correlated with the arrival of the mink. Changes in fish populations in Slapton Ley appeared to have causes other than mink. It is concluded that, in the areas studied, domestic stock make up a very small proportion of the mink's diet, and that the wild prey species are not suffering any obvious population depletion.     

Observations on thermoregulatory ontogeny of mink (Mustela vison)

(1) We measured cooling rate for neonatal mink during a 10min coldroom (3.9 degrees C) exposure and subsequent warming rate during a 20min incubator (37.2 degrees C) exposure, the behaviour of the kits and the changes in their pelage between 1 and 46d of age, in an attempt to monitor the ontogeny of their thermoregulatory capacity. (2) Body weight of the 1d old kits averaged only 12.8+/-2.3g (n=4), but they gained weight rapidly reaching 226.1+/-28.3g (males, n=4) and 207.6+/-16.1g (females, n=4) at 30-31d of age, and 562.3+/-43.2g (males, n=3) and 435.7+/-35.5g (females, n=4) at 45-46d of age. (3) Body cooling rate (C(rate) ( degrees C/min); n=80) was affected by the age (between 1 and 31d), BW, initial rectal temperature (T(r0)), and sex of the kits, in addition to their body posture (P(cold), 1=extended, 2=curled-up) during coldroom exposure. C(rate) ( degrees C/min)=-0.34-0.02age-0.002BW+0.05T(r0)-0.06sex-0.20P(cold) (R(2)=0.75). (4) Body warming rate (W(rate) ( degrees C/min); n=80) was influenced by the age(2) and rectal temperature of the kit after the coldroom exposure (T(r10)). W(rate)( degrees C/min)=1.24+0.0002age(2)-0.04T(r10) (R(2)=0.76). (5) Kit fur fibre length increased from 5.45+/-0.63mm (males, n=2) and 6.20+/-0.20mm (females, n=3) at 22-23d of age to 9.43+/-1.44mm (males, n=4) and 8.70+/-1.89mm (females, n=4) at 30-31d of age, and to 12.93+/-0.47mm (males, n=3) and 11.38+/-0.41mm (females, n=4) at 45-46d of age, the growth averaging about 0.26mm per day. (6) Under normal circumstances newborn mink kits are hypothermic.Their thermoregulation develops only gradually and is dependent on increase in body mass, insulation and behavioural thermoregulation. Their strategy of survival is based on the ability to withstand hypothermia and on the nutrition and warmth provided by the dam.     

The first linkage map of the American mink (Mustela vison)

Described herein, the first microsatellite linkage map for the American mink consists of 85 microsatellite markers resolved into 17 linkage groups. The map was constructed using 92 F(1) progeny from five sire families created by crossing mink with different colour types. The linkage groups ranged from 0 to 137 cM. These linkage groups were assigned to 12 of the 14 mink autosomes using a somatic cell hybrid panel. The total map covered 690 sex-averaged Kosambi units with an average marker spacing of 8 cM. This map will facilitate further genetic mapping of monogenic characters and QTL.     

Mapping of 53 loci in American mink (Mustela vison)

Fifty-three genes were mapped in the American mink genome using polymerase chain reaction (PCR)-based analysis of a Chinese hamster-American mink somatic cell hybrid panel. Heterologous primers designed for cat gene mapping were used in this study. Forty-nine of these loci were localized into expected chromosome regions according to Zoo-FISH data, whereas four loci--ALPL, CDC20, ERF-2, and Fc(Mv)23617--were mapped out of expected conserved regions. PCR products amplified with primers corresponding to these four markers were partly sequenced and verified using BLAST. The results showed the homology to be more than 90% between mink and human or cat counterparts. At present, the gene map of American mink has expanded to 127 loci.     

Underwater predatory behaviour of the American mink (Mustela vison)

Fishing behaviour of the American mink ( Mustela vison Schreber) was investigated in the laboratory. Data were recorded using ciné film and tape recorded commentaries. Three species of prey were presented to mink, namely, carp ( Cyprinus curpio ), goldfish ( Carassius auratus ) and minnows ( Phoxinus phoxinus ).
It was found to be necessary to train ranch-bred mink to enter water and catch fish; young mink appeared to be easier to train than adults. Mink spent 5–20 sec under water when fishing; prey had usually previously been located from an aerial vantage point. Predatory behaviour was highly organized sequentially whereas fish were more prone to indulge in unpredictable stratagems; the behaviour of mink and fish were highly correlated.
The mink's efficiency in catching fish was related to prey size (smaller individuals being more vulnerable to capture) and shoaling. Minnows, which form highly organized shoals, were less easily caught when present in large numbers; this was not true of a loosely shoaling species, the carp.
Of the three species of prey presented, vulnerability to capture took the form goldfish> carp> minnow; these differences, however, may have been influenced by the fish's previous experience of underwater predators.     

Patterns of plasma progesterone, androgen and oestrogen concentrations and in-vitro ovarian steroidogenesis during embryonic diapause and implantation in the mink (Mustela vison)

Peripheral plasma progesterone concentrations exhibited an increase 10 days before implantation, coinciding with the resumption of blastocyst growth and with a decrease in plasma androgen values (DHA, androstenedione, testosterone). No definite pattern of oestrone was observed and oestradiol concentrations remained undetectable. The production of steroids by dispersed luteal cells showed that the growth of the corpora lutea paralleled that of blastocysts and resulted in hypertrophy followed by hyperplasia of the luteal cell. The production of progesterone in the medium increased with blastocyst size up to implantation; it was enhanced by mink charcoal-treated serum, but prolactin, LH, FSH or a combination of these hormones did not affect the progesterone production, whatever the stage of diapause. DHA and androstenedione secretion increased in the two last stages of blastocyst growth and was enhanced by LH. The conversion of androstenedione and testosterone into oestrone and oestradiol was observed at all stages of embryonic diapause, indicating that corpora lutea contain aromatase activity even at an early stage. The secretion of oestrone was higher than that of oestradiol. The non-luteal tissue contributed up to 50% of the steroid production; while progesterone and androgen production remained constant, that of oestradiol decreased at the end of the delay period. These results indicated a change in the size and the secretory capacity of the luteal cell related to blastocyst development and implantation. Although progesterone was the main product of the corpora lutea, androgens and oestrogens were also secreted.     

Genetic polymorphism of esterase in plasma of American mink (Mustela vison L.)

Plasma samples of 412 minks, including 20 families and representing 15 lines, have been investigated by isoelectric focusing for the enzyme esterase (ES). The observed variation of the enzyme may be explained as a result of one locus with at least seven codominant alleles. The segregation of six alleles assumed for the locus in 20 families supports this genetic model. Genetic divergence among the lines is observed and may be due to founder effect and/or selection.     

Effect of aleutian gene on mink (Mustela vison) growth

The correlation between the skin size of Mustela vison mink and the combination of the alleles of aleutian colour gene has been shown. The studied relation can be presented as ppAa > ppAA > ppaa.     

The pars intermedia of the mink,Mustela vison

Summary The pars intermedia of intact and experimental female minks has been studied by light, electron and fluorescence microscopy. The general structure of the mink intermediate lobe is described. Two main cell types are recognized. One, termed glandular cell, predominates in number and is characterized by the presence of electron-dense granules about 200 nm in diameter and numerous large vesicles up to 300 nm in diameter. The other, termed stellate cell, is devoid of cytoplasmic vesicles and granules and possesses microfilaments, junctional complexes and elongated processes inserted between the glandular cells. Different treatments (photostimulation and administration of hypertonic saline and metopirone) induced morphological reactions in the glandular cells. The significance of these changes and the possibility of a functional relation between the pars intermedia and ACTH secretion are discussed.Numerous nerve fibres and axon terminals containing electron-dense granules (60–120 nm) and electron-lucent vesicles (30–40 nm) are observed throughout the pars intermedia.With the histochemical fluorescence method of Falck-Hillarp a rich system of delicate fluorescent varicose fibres, sometimes provided with irregular swellings or droplets, is observed in the pars intermedia and also in the pars nervosa. Microspectrofluorometrically these fibres exhibit the spectral characteristics of catecholamines. All the cells of the pars intermedia and a large number of cells in the pars distalis show a yellowish weak fluorescence, which becomes much stronger after combined formaldehyde-ozone treatment. This cellular fluorophore shows the same microspectrofluorometric characteristics as does the fluorophores of tryptamine and of certain peptides with NH₂-terminal tryptophan.Supported by the Swedish Fur Breeders' Association and the Swedish Natural Science Research Council (grant No. 2124). Thanks are due to Miss W. Carlsson and Miss Y. Lilliemarck for their helpful technical assistance.Supported by the Harald and Greta Jeanssons Stiftelse and by the Ford Foundation. The skilful technical assistance of Mrs. Eva Svensson and Miss Annika Borgelin is greatfully acknowledged.     

Selective fatty acid mobilization in the American mink (Mustela vison) during food deprivation

The mobilization of fatty acids (FAs) during food deprivation is a selective process in laboratory rodents and humans. The site-specific differences in adipose tissue functions - e.g. energy storage versus insulation - should also affect the use of different FAs. To study this, 16 female minks were randomly assigned into the control group or fasted for 5 days. Preferential mobilization of n-3 polyunsaturated FAs (PUFAs) during fasting caused a decrease in the n-3/n-6 PUFA ratio in fat and liver. In addition, the minks utilized short-chain FAs efficiently in all fat depots, but long-chain FAs - 20:0, 20:1n-11, 20:1n-9, 22:1n-11 and 24:1n-9 - were preserved. The number of double bonds in the FA chain correlated positively with mobilization rate in the retroperitoneal fat. The observed negative correlation between mobilization rate and the location of the first double bond from the methyl end may be due to peroxisomal chain-shortening of long-chain FAs and not the double bond position per se. As a result, minks are able to preserve a low melting point and fluidity of the subcutaneous fat depots, which would be essential to a Northern semi-aquatic mammal.     

Cloning of leukemia inhibitory factor (LIF) and its expression in the uterus during embryonic diapause and implantation in the mink (Mustela vison)

Leukemia inhibitory factor (LIF) is essential for embryo implantation in mice. Whether LIF plays a role in termination of embryonic diapause and initiation of implantation in carnivores, especially in species with obligate delayed implantation such as the mink, is not known. The objectives of this study were to clone the LIF coding sequence in the mink and determine its mRNA abundance in the uterus through embryonic diapause, implantation, and early postimplantation. We show that the mink LIF cDNA contains 609 nt encoding a deduced protein of 203 amino acids. The homologies are 80.6, 90, 88.2, 87.6, and 86.8% in coding sequence and 79.2, 90.1, 91, 90.1 and 85.4% in amino acid sequence with mouse, human, pig, cow, and sheep respectively. Glycosylation sites and disulfide bonds present in other species are generally conserved in the mink LIF sequence. Quantitation by polymerase chain reaction amplification indicates that LIF mRNA is expressed in mink uterus just prior to implantation and during the first two days after implantation, but not during diapause or later after implantation pregnancy. The abundance of LIF mRNA was significantly higher in the uterus at the embryo expansion stage (P < 0.05) than at days 1–2 of postimplantation. By immunohistochemical localization it was shown that LIF is expressed in the uterine epithelial glands at time of embryonic expansion and in early postimplantation. The coincidence of LIF expression with implantation in this species suggests that LIF is involved in the implantation process, and may be a maternal signal which terminates obligate embryonic diapause. Mol. Reprod. Dev. 51:13–21, 1998. © 1998 Wiley-Liss, Inc.