Journal of Reproduction and Development, Vol. 59, No 2, 2013

—Original Article—

Structure and Steroidogenesis of the Placenta in the Antarctic Minke
Whale (Balaenoptera bonaerensis)
Motoki SASAKI1,2), Yoko AMANO1), Daisuke HAYAKAWA1,2)#, Toshio TSUBOTA3),
Hajime ISHIKAWA4)##, Toshihiro MOGOE4), Seiji OHSUMI4), Masafumi TETSUKA5),
Akio MIYAMOTO6), Yutaka FUKUI5) , Teguh BUDIPITOJO7) and Nobuo KITAMURA1,2)
1)Laboratory of Veterinary Anatomy, Department of Basic Veterinary Medicine, Obihiro University of Agriculture andVeterinary Medicine, Hokkaido 080-8555, Japan2)United Graduate School of Veterinary Medicine, Gifu University, Gifu 501-1193, Japan

3)Laboratory of Wildlife Biology and Medicine, Department of Environmental Veterinary Science, Graduate School of

Veterinary Medicine, Hokkaido University, Hokkaido 060-0818, Japan

4)Institute of Cetacean Research, Tokyo 104-0055, Japan5)Department of Life Science and Agriculture, Obihiro University of Agriculture and Veterinary Medicine,

Hokkaido 080-8555, Japan6)Department of Animal and Food Hygiene, Obihiro University of Agriculture and Veterinary Medicine,

Hokkaido 080-8555, Japan

7)Faculty of Veterinary Medicine, Gadjah Mada University, Yogyakarta 55281, Indonesia#Present: Wanpark Kochi Animal Land, Kochi 780-8010, Japan##Present: Whale Laboratory, Shimonoseki Academy of Marine Science, Yamaguchi 750-0036, Japan

Abstract. There are few reports describing the structure and function of the whale placenta with the advance of pregnancy.
In this study, therefore, the placenta and nonpregnant uterus of the Antarctic minke whale were observed morphologicallyand immunohistochemically. Placentas and nonpregnant uteri were collected from the 15th, 16th and 18th Japanese Whale
Research Programme with Special Permit in the Antarctic (JARPA) and 1st JARPA II organized by the Institute of Cetacean
Research in Tokyo, Japan. In the macro- and microscopic observations, the placenta of the Antarctic minke whale was adiffuse and epitheliochorial placenta. The chorion was interdigitated to the endometrium by primary, secondary and tertiary
villi, which contained no specialized trophoblast cells such as binucleate cells, and the interdigitation became complicatedwith the progress of gestation. Furthermore, fetal and maternal blood vessels indented deeply into the trophoblast cells andendometrial epithelium respectively with fetal growth. The minke whale placenta showed a fold-like shape as opposed to afinger-like shape. In both nonpregnant and pregnant uteri, many uterine glands were distributed. The uterine glands in the
superficial layer of the pregnant endometrium had a wide lumen and large epithelial cells as compared with those in the deep
layer. On the other hand, in the nonpregnant endometrium, the uterine glands had a narrower lumen and smaller epithelial
cells than in the pregnant endometrium. In immunohistochemical detection, immunoreactivity for P450scc was detected inmost trophoblast cells, but not in nonpregnant uteri, suggesting that trophoblast epithelial cells synthesized and secreted thesex steroid hormones and/or their precursors to maintain the pregnancy in the Antarctic minke whale.
Key words: Antarctic minke whale, Balaenoptera bonaerensis, Morphology, Placenta, Steroidogenesis(J. Reprod. Dev. 59: 159–167, 2013)T

he Antarctic minke whale (Balaenoptera bonaerensis) belonging to
the family Balaenopteridae (order Cetacea, suborder Mysticeti) is
one of the baleen whales that lack teeth, inhabits polar to tropical waterin the Southern hemisphere and has been taxonomically distinguishedfrom the common minke whale (Balaenoptera acutorostrata) mainly
distributed in temperate to polar water in the Northern hemisphere[1]. The Antarctic minke whale, which is likely a seasonal breeder
without reproductive activity from early December to March in the

Received: August 17, 2012
Accepted: November 30, 2012
Published online in J-STAGE: December 27, 2012©2013 by the Society for Reproduction and Development
Correspondence: M Sasaki (e-mail: sasakim@obihiro.ac.jp)

southern Antarctic, breeds at low latitude in austral winter, and thepregnant females migrate to Antarctic feeding areas during austral
spring to summer. The gestation period of the Antarctic minke whale
is about 10–11 months, and pregnant females give birth around
September after returning to low latitudes [2].
In female Antarctic minke whales, several studies on the reproductive organs have been reported. Maturation and fertilization in vitroof the follicular oocyte have been investigated [3–6]. Furthermore,
Suzuki et al. [7] measured the concentrations of follicle stimulating
hormone (FSH) and luteinizing hormone (LH) during the feeding
season. Muranishi et al. [8] examined the preantral follicle numberin the fetal ovary and the concentrations of sex hormones in the
fetal heart, umbilical cord and maternal blood and suggested thatsteroidogenesis occurs in the minke whale placenta.

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160

The placenta of whales has been described as an epitheliochorial
and diffuse placenta [9–14]. This kind of placenta is known as theplacenta of the pig [15] and horse [16], but their detail appearance
varies depending on the species. In the Antarctic minke whale placenta,
the structural development with the progression of pregnancy and theexistence of steroidogenesis have not yet been investigated. In thisstudy, therefore, we examined the placental structure of the Antarctic
minke whale histologically and by scanning electron microscopy
and the immunolocalization of a steroidogenic enzyme, cytochrome
P450 side chain cleavage enzyme (P450scc), to clarify the structuralchanges with the progress of gestation and the existence and site ofsteroidogenesis in the placenta.

Materials and Methods
Animals

Fifty-eight placentas (fetal length of 11.1–187.4 cm; estimatedfetal age of 88–245 days, see below) and 12 nonpregnant uteri (5mature and 7 immature, no corpus luteum and corpus albicans ineach whale ovary) of Antarctic minke whales were collected in the
15th (2001/2002), 16th (2002/2003) and 18th (2004/2005) Japanese
Whale Research Program under Special Permit in the Antarctic(JARPA) and 1st (2005/2006) JARPA II (the Second Phase of
the JARPA) organized by the Institute of Cetacean Research inTokyo, Japan. Special attention to reduction of the time to death wasgiven for all the sampled whales. According to Schedule III of theInternational Convention for the Regulations of Whaling, explosive
harpoons were used for all whales as the primary killing method. The
fetal age was calculated using the equation of Kato and Miyashita(t0=W1/3/0.243+74, t0=estimated age, W=0.059L2.676, W=fetal weight,
L=fetal length) [17], and the resulting number was rounded off andconsidered to be the fetal age. The fetal length at birth was presumed
to be about 290 cm (day 330 of gestation) [2]; thus very early andlate gestational stages were not examined in this study.

Histology and immunohistochemistry

Small pieces of tissue samples were randomly collected fromthe whole placenta and nonpregnant uterus, and immediately fixedin Bouin’s fluid or 10% formalin. After 24 h, the samples weretransferred to 70% ethanol, then dehydrated in graded series of
ethanol, cleared in xylene and embedded in paraffin (Paraplast Plus,
Kendall, Mansfield, MA, USA). Tissue samples were cut serially at a
thickness of 4 μm and placed on aminopropyltriethoxysilane-coatedslides (S8226, Matsunami Glass Ind., Osaka, Japan). Deparaffinizedsections were used for hematoxylin and eosin (HE) and were immunohistochemically stained using the avidin-biotin peroxidase
complex (ABC) method [18]. The sections were treated by microwave
in high pH target retrieval solution (1:10, S3307, DakoCytomation,
Carpinteria, CA, USA) for 20 min to retrieve antigenicity. The
sections were immersed in methanol containing 0.3% H2O2 for 10
min at room temperature (RT) to block the endogenous peroxidase
activity, and then incubated with normal goat serum (1:50, S-1000,
Vector Laboratories, Inc., Burlingame, CA, USA) for 30 min at RT
to prevent nonspecific staining. Then the sections were incubatedovernight with a polyclonal anti-rat P450scc antibody raised in the
rabbit (1:200, AB1244, Chemicon International, Inc. Temecula, CA,

USA) at 4 C in a moisture chamber. After incubation with primary
antibody, biotinylated anti-rabbit IgG raised in the goat (1:200,
BA-1000, Vector Laboratories) was applied for 30 min, and then thesections were incubated with ABC reagent for 30 min (1:2, PK-6100,
Vectastain Elite ABC Kit, Vector Laboratories). The binding sites
were visualized with Tris-HCl buffer (pH 7.4) containing 0.02%
3,3’-diaminobenzidine hydrochloride (DAB) and 0.006% H2O2.
After incubation, the sections were washed with 0.01 M phosphatebuffered saline (PBS, pH 7.4), dehydrated in graded series of ethanol,
cleared in xylene, coverslipped and observed under a conventional
light microscope. The negative control sections were treated withnormal rabbit serum instead of primary antibody and omission of
the primary antibody.

Scanning electron microscopy (SEM)

For scanning electron microscopy, small pieces of the samples
fixed in 10% formalin were washed in PBS, postfixed in 1% osmium
tetroxide in PBS and dehydrated in graded series of ethanol. The
specimens were then freeze-dried with t-butyl alcohol (Freeze-drying
Device F-300, JEOL, Tokyo, Japan). The dried tissues were mounted
on stubs and sputter coated with Pt (Magnetron Sputtering Device
JUC-5000, JEOL). The samples were observed by SEM (JSM-6301F,
JEOL) at an accelerating voltage of 1 or 5 kV.

Results
Morphology

Fetomaternal interface: The placenta of the Antarctic minke
whale was classified as a diffuse placenta according to the distribution of chorionic villi (Fig. 1). Histologically, the placenta was anepitheliochorial placenta characterized by close contact between
the fetal trophoblast (trophectoderm) and maternal endometrial
epithelium (Fig. 2). The chorion was interdigitated to the endometriumby chorionic villi branching complicatedly with the advance ofpregnancy (Figs. 2 and 3).
In early gestation, the interdigitation consisted of simple fetal
primary villi and their corresponding maternal endometrial crypts,
which are ditches among endometrial villi. Both villi branched into
secondary and tertiary villi and continued to become longer and thinnerwith the pregnancy progression. Consequently, the interdigitation
became more complicated during mid gestation with more branchedchorionic villi (Figs. 2–4).
The trophoblast consisted of cuboidal or columnar cells in earlygestation, and their cytoplasm became larger with the progress ofgestation. The trophoblast cells were mostly columnar throughout mid
gestation. On the other hand, the endometrial epithelium consistedof squamous or cuboidal cells in the early stage, and then boththe nuclei and cytoplasm of these cells became flatter as fetal age
progressed (Fig. 4).
Blood capillary indentation was observed on both the fetal trophoblast cells and maternal endometrial epithelium during the mid
stage of pregnancy (Figs. 4 and 5). Specialized trophoblast cells,such as binucleate cells or giant multinucleate cells in ruminants[19] and camels [20], were not observed in the Antarctic minke
whale during the pregnancy period examined in this study (Fig. 4).
The surfaces of the chorion and endometrium were observed

ANTARCTIC MINKE WHALE PLACENTA

Fig. 1.

Placenta of the Antarctic minke whale on day
153 of gestation (fetal length, L79.9 cm). The
fetal side (chorion) of the placenta is pickedup. The upper side is the chorion (c), and thebottom is the maternal side (endometrium)(e). The chorionic villi were interdigitated tothe crypts of the endometrium. Interdigitation
point (arrow).

Fig. 2.

Fig. 4.

161

Blood capillary indentation in the Antarctic minke whale placenta. A: Placenta on
day 100 of gestation (L22.9 cm). Trophoblast cells were cuboidal or columnar.
Endometrial epithelial cells were squamous or cuboidal. B: Placenta on day 219
of gestation (L155.6 cm). Trophoblast cells were mostly columnar. Endometrial
epithelial cells were squamous. Note the deepening indentation of blood capillaries
into both trophoblast cells and the endometrial epithelium and absence of specialized
trophoblast cells. Trophoblast cells (large arrowheads), endometrial epithelial cells(small arrowheads), fetal blood capillaries (large arrows), maternal blood capillaries(small arrows), chorionic villi (c), endometrium (e). Both figures are at the same
magnification. Bar=100 μm.

Histological observation of the Antarctic minke whale placenta. A: Placenta on day 101 of gestation (L23.6 cm). The
chorion was interdigitated to the endometrium by simple primary chorionic villi. B: Placenta on day 122 of gestation(L45.2 cm). Chorionic primary villi began to branch into secondary and tertiary villi. C: Placenta on day 221 of gestation(L158.6 cm). The interdigitation of the chorion and endometrium became more complicated. Chorion (c), endometrium(e), uterine glands (arrows). All figures are at the same magnification. Bar=1 mm.

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162

Fig. 3.

Cross-sectional view of the Antarctic minke whale placenta by SEM. A: Placenta on day 101 of
gestation (L24.0 cm). The interdigitation was simple. Bar=100 μm. B: Placenta on day 173 ofgestation (L102.1 cm). The interdigitation became complicated compared with A. Bar=1 mm. The
border between the fetal and maternal placenta (arrows), uterine glands (arrowheads), chorion (c),endometrium (e).

Fig. 5.

Cross sectional view of the Antarctic minke whale placenta by SEM. A: Trophoblast on day 101 of
gestation (L24.0 cm). Fetal blood capillaries only compressed trophoblast cells. B: Endometrium
on day 101 of gestation. Maternal blood capillaries only compressed endometrial epithelial cells.
C: Trophoblast on day 243 of gestation (L185.3 cm). Fetal blood capillaries invaded trophoblast
cells. D: Endometrium on day 243 of gestation. Maternal blood capillaries invaded endometrial
epithelial cells. Fetal blood capillaries (large arrows), maternal blood capillaries (small arrows),
trophoblast (t), endometrial epithelial cell (e), fetal side (f), maternal side (m). All figures are at
the same magnification. Bar=10 μm.

ANTARCTIC MINKE WHALE PLACENTA

sterically by SEM. As with light microscope observations, there
were chorionic villi and endometrial crypts on the surface of chorion
and endometrium, respectively (Figs. 6 and 7). In early gestation,
the chorionic villi were simple and irregular in shape and showeda fold-like shape (folded placentation) as opposed to the finger-likeshape (villous placentation) in the human [21] and horse [16]. Thenthey became more complicated and compacted as fetal age progressedbut did not branch finger-like villi, keeping the fold-like feature (Fig.
6). On the other hand, the endometrial villi were thick and simple inshape in early stage and then became thinner and more complicatedwith the progress of gestation (Fig. 7).
Endometrium: The endometrium consisted of the epithelium,
uterine glands and connective tissue stroma, and many uterine
glands, which indicate a simple coiled branched tuber gland, were
distributed throughout the endometrial stroma of both the pregnant
and nonpregnant whales. The stroma thickened during gestation,and the density of the uterine glands became lower (Fig. 8). In
nonpregnant whales, the uterine glands in the superficial layer wereslightly larger than those in the deep layer (Fig. 9A and B), and no
significant structural differences were detected between the uterine
glands of the mature and immature nonpregnant whales examinedin this study.
In pregnant whales, the uterine glands were different in appearance
between the superficial endometrial layer positioned near fetal
trophoblast and the deep layer positioned near the myometrium(Fig. 9C and D). In early gestation, the glands in the superficial
layer became much larger and had a wider lumen than those in the
nonpregnant placenta, whose epithelial cells were columnar andcontained rich cytoplasm and a nucleus located in each base (Fig.
9A and C). They did not change with the stage of pregnancy (Fig.
9C and E). The glands in the deep layer were small in size and had
a narrow lumen, whose epithelial cells were cuboidal or columnar
cells and contained scanty cytoplasm in early gestation, although
they were slightly developed as compared with those in nonpregnant
whales (Fig. 9B and D). During mid gestation, they became slightly
larger and had wider lumens than those in the early stage (Fig. 9D
and F). However, the glands in the deep layer were much smaller
than those in the superficial layer (Fig. 9E and F).

Immunohistochemistry

In all samples of placenta, immunoreactivity for P450scc wasdetected only in trophoblast cells; immunoreactivity was not seenin the nonpregnant uterus. Strong P450scc immunoreactivity wasobserved in the supranuclear region of the cells, which was contact
with the maternal endometrial epithelium. The distribution of P450scc
immunoreactive cells and the staining intensity for P450scc did notshow significant changes during the pregnancy period examined inthis study (Fig. 10).

Discussion
This study reports for the first time morphological changes withgestational stages and the localization of steroidogenic enzyme
in the Antarctic minke whale placenta. In this study, it was notedthat the placenta of the Antarctic minke whale was a diffuse andepitheliochorial placenta, corresponding to other cetacean placentas

163[9–14]. The interdigitation between the chorion and endometrium
became complex with the progress of gestation in the Antarctic minke
whale, as seen in other animals with an epitheliochorial placenta [15,16, 20, 22–25]. This not only establishes firm fetomaternal attachment
but also induces increase in the surface area of the fetomaternalinterface. Since one of the important functions of the placenta is theexchange of nutrients, oxygen and carbon dioxide between motherand fetus, the complex interdigitation seems to provide an extensive
surface for the exchange, depending on the increase in consumptionof nutrients and oxygen and the emission of waste products andcarbon dioxide with fetal development.
Trophoblast and endometrial epithelial cells changed in shape withthe progress of gestation. The cytoplasm of trophoblast cells became
large, suggesting an increase in trophoblast functions, for example,
synthesis, secretion and absorption of molecules. In the pig, it has
been reported that the trophectoderm cells at the base of chorionicvilli are tall, columnar and specialized for histotroph absorption[26–28]. In contrast, the endometrial epithelial cells became flatter,
which seemed to bring maternal blood capillaries closer to fetal
tissue. Furthermore, maternal and fetal blood vessels indented deeply
into each epithelium, as seen in previous reports [15, 27–30]. Thesemorphological changes probably decrease the distance between each
vessel, and the increasing number of indenting vessels might resultin a much quantity of blood flowing near the fetomaternal interface,supporting fetal growth by efficient molecular exchange.
The appearance of the chorionic surface with fold-like villi in the
Antarctic minke whale was different from those with finger-like villi
in the human [21], horse [16, 31] and cow [32]. This morphologicalcharacteristic in the Antarctic minke whale has also been seen in
the pig [27] and alpaca [33]. Moreover, folded placentation has
been reported also in other cetaceans [14, 28]. In pig and alpaca
placentas, the chorionic villi continue to complexify and retain afold-like shape until term; thus, the minke whale placenta also might
maintain a fold-like shape up to late pregnancy. The placenta fromlate pregnancy will be examined in the future to reveal the structuraldynamics of the placental surface.
In the ruminant and camelid placentas, binucleate (occasionally trinucleate) or giant multinucleate cells were observed in thechorionic villi [19, 20, 28, 33]; however, neither type of cells wasobserved in the chorionic villi of the pig [15, 26–28]. In this study,
these specialized trophoblast cells were not detected in the Antarctic
minke whale placenta. In previous reports, specialized trophoblast
cells were also not observed in other cetacean placentas [9, 10, 12,13]. Therefore, the absence of specialized trophoblast cells might
be characteristic of the cetacean placenta.
Uterine glands synthesize and secrete a complex array of proteins
and related substances termed histotroph [34–37]. Samuel et al. [38]showed that uterine glands produced secretions throughout gestationand suggested that the secretions were very important for the wellbeing of the fetus in the horse. For animals with an epitheliochorial
placenta, uterofferin, an iron-containing glycoprotein, and histotrophin the secretions of uterine glands seem important as resources ofiron and nutrition for the fetus, respectively [25, 39, 40].
At the openings of the uterine glands, areolae, gaps between thechorion and endometrium, are formed that accumulate secretion fromthe uterine glands, which are absorbed by the trophoblast epithelial

SASAKI et al.

164

Fig. 6.

Surface of the chorion of the Antarctic minke whale placenta by SEM. A , B: Placenta on day 98
of gestation (L20.7 cm). Chorionic villi were irregular in shape and simply folded. C, D: Placenta
on day 245 of gestation (L187.4 cm). The fold of chorionic villi was complicated and compacted
as compared with those of the placenta on day 98 of gestation. Bar= 1 mm (A, C), 100 μm (B, D).

Fig. 7.

Surface of the endometrium of the Antarctic minke whale placenta by SEM. A, B: Placenta
on day 101 of gestation (L24.0 cm). Endometrial villi were thick and irregular in shape. C, D:
Placenta on day 245 of gestation (L187.4 cm). Endometrial villi became thin. Bar= 1 mm (A, C),
100 μm (B, D).

ANTARCTIC MINKE WHALE PLACENTA

165

Fig. 8.

Fig. 9.

Endometrium of the Antarctic minke whale.
A: Nonpregnant uterus. The endometrium
was thin, and uterine glands were densely
distributed. B. Placenta on day 157 ofgestation (L83.3 cm). The endometrium
was thickened, and the density of theuterine glands was low. Lumen of uterus(l), myometrium (m), trophoblast (t),endometrium (e). Both figures are at thesame magnification. Bar= 1 mm.

Uterine glands of the Antarctic minke whale. A:
Superficial endometrial layer of the nonpregnant
uterus. B: Deep endometrial layer of thenonpregnant uterus. C: Superficial endometrial
layer of the placenta on day 100 of gestation(L22.9 cm). D: Deep endometrial layer of theplacenta on day 100 of gestation. E: Superficial
endometrial layer of the placenta on day 238 of
gestation (L179.6 cm). F: Deep endometrial layerof the placenta on day 238 of gestation. Note theremarkable difference in appearance between the
superficial and deep layers with gestation. Lumenof uterus (l), interdigitated point by chorionic villi(ic). All figures are at the same magnification.
Bar=100 μm.

Fig. 10. Immunostaining of P450scc in the Antarctic
minke whale placenta. A: Placenta on day 100 ofgestation (L22.9 cm). B: Placenta on day 219 of
gestation (L155.6 cm). P450scc immunoreactivity
was detected in the trophoblast epithelium(arrows). Conspicuous changes with the stagesof pregnancy were not observed. Chorion (c),endometrium (e). Both figures are at the same
magnification. Bar=100 μm.

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SASAKI et al.

cells [38, 41]. In the Antarctic minke whales examined in this study,
however, areolae were not noted in any of the samples.
Whale uterine glands may have similar functions to those in
the animals with an epitheliochorial placenta. The glands in the
superficial endometrial layer in pregnant whales are different fromthose in nonpregnant whales; they contained epithelial cells withrich cytoplasm and a quite wide lumen throughout the gestation
period examined. These suggest that the uterine gland activity might
already be intensified in the superficial layer in early gestation to
supply uteroferrin and histotroph to the growing fetus. In sheep,
cattle and pigs, the uterine glands are dramatically changed byextensive hyperplasia and subsequent hypertrophy during pregnancy,
increasing the secretions for fetal development [37, 39, 42]. In
alpacas, on the other hand, they apparently compensate for this byintense biosynthetic activity, although the density of the glands at
the final stages of pregnancy was not high [33]. In pregnant minke
whales, the density of the uterine glands became lower than that of
the nonpregnant uterus. However, as seen in alpacas, the activityof the uterine glands might be intensified instead of the number ofglands increasing.
Progesterone and estrogen are essential to maintenance of pregnancy
in most mammals. The presence of P450scc in the trophoblast cellsof the minke whale shows that sex steroid hormones are synthesized
there. However, it remains to be seen whether progesterone or
estrogen is secreted by the trophoblast cells because P450scc only
converts cholesterol into pregnenolone. It is thought that estrogenis predominantly secreted in the placenta of mammals with a longgestation period (human, monkey, horse, cow, sheep and goat) [43].
Therefore, the trophoblast epithelial cell of the minke whale very
likely synthesizes and secretes a large amount of estrogen, and highconcentration of estradiol-17β in the fetal heart and umbilical cord
blood might coincide with the secretion from epithelial cells [8]. In
the humpback whale (Megaptera novaeangliae), estrone sulfate andprogesterone were detected in homogenized placenta [44]. Moreover,it has been reported that progesterone and relaxin are also detected inplacental extracts of the bottlenose dolphin (Tursiops truncatus) [45].
The cow and camel have specialized trophoblast epithelial cells[19, 20]. In the cow, binucleate cells produce estrogens at the fetal
cotyledons in the placenta [19, 46, 47]. Furthermore, it has beenreported that syncytiotrophoblasts in the Shiba goat [48] and giant
cells in the camel [20] also have an ability to synthesize estrogen. In
the cetacean placenta, no specialized trophoblast epithelial cells havebeen reported in previous studies [9, 10, 12, 13]. In this study, suchspecialized cells were also not observed, and most mononucleate
trophoblast cells showed positive immunoreactivity for P450scc,
suggesting that the mononucleate trophoblast epithelial cells synthesize
sex steroid hormones and/or their precursors in the Antarctic minke
whale and possibly also in other cetaceans. In further studies of the
Antarctic minke whale placenta, investigation of the localization ofother steroidogenic enzymes including cytochrome P450 aromatase,which converts testosterone to estradiol, is needed to identify thesex steroid hormones secreted from the placenta.
In conclusion, the results of the present observations demonstratethat the Antarctic minke whale placenta has a diffuse and epitheliochorial placenta with the chorion interdigitated to the endometrium
by primary, secondary and tertiary villi, which have no specialized

trophoblast cells and blood vessels deeply indented into the trophoblast
cells and endometrial epithelium. Moreover, we could confirm
steroidogenesis in the trophoblast cells of the minke whale placenta.

Acknowledgments
This study was financially supported in part by a Grant-in-Aid
for Scientific Research (No.13575026, No. 23570103) from the
Ministry of Education, Culture, Sports, Science and Technologyof Japan.

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