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/. Embryol. exp. Morph. Vol. 25, 1, pp. 141-153, 1971
Printed in Great Britain
In vitro analysis of the
hormonal basis for the sexual dimorphism in
the embryonic development of the
mouse mammary gland
By KLAUS KRATOCHWIL 1
From the Institut fiir Krebsforschung, Universitdt Wien
SUMMARY
Factors underlying the sexual dimorphism in the embryonic development of mouse
mammary glands were analysed in vitro and the following results were obtained:
1. Mammary gland rudiments of 13-day male embryos, explanted immediately before the
onset of their regression, were perfectly capable of developing into female-type glands in vitro.
Even some of the glands of 14-day male embryos, where the regression process had already
begun, recovered after explantation and underwent female-type morphogenesis.
2. Combined explantation of 13-day testes with mammary rudiments of female embryos of
12-14 days gestation resulted in male-type regression of the glands.
3. The addition of testosterone to the culture medium caused a similar regression of
explanted (female) mammary-gland rudiments. The minimal effective concentration of the
hormone was 10~9M, or 000029/*g/ml.
4. Cultured mammary rudiments of 15-day female embryos were no longer responsive to
the presence of testis explants. They failed to undergo regression and continued their development in vitro.
From these results the following conclusions were drawn:
(a) The sexual dimorphism in the embryonic development of mouse mammary glands is
caused by their suppression in males and not by their stimulation in female embryos.
(b) The androgenic hormones in male foetuses are solely responsible for the regression of
the mammary rudiments. They exert their effect directly on the gland without the need for
involvement of other endocrine organs.
(c) The genetic sex of the gland itself has no influence on its developmental capacities as:
(i) glands of male embryos are able to develop in the absence of androgens, and (ii) glands of
female embryos undergo typical male-type regression in vitro when exposed to the presence of
foetal testes or of testosterone.
INTRODUCTION
The development of the mammary glands in mouse embryos starts during the
12th day of gestation. Five pairs of rudiments arise as epidermal thickenings in
all embryos, the sex of which cannot be determined at that early stage. One
day later, morphological differentiation of the gonads makes it possible to
distinguish between male and female litter-mates. Mammary-gland development
proceeds in an identical way in both sexes for one more day, but at the end of
1
Author's address: Institut fur Krebsforschung, Universitat Wien, A-1095 Vienna, Austria.
142
K. KRATOCHWIL
the 14th day a sexual dimorphism becomes apparent. In male embryos a
conspicuous condensation of mesenchymal cells is seen around the epithelial
bud, the stalk of the Anlage becomes constricted and eventually the gland
rudiment is detached from the epidermis (Turner & Gomez, 1933; Raynaud,
1941 b, 1961; Raynaud & Raynaud, 1953a, b; Cowie & Folley, 1961). No nipples
are formed in male embryos. The disconnected distal portion of the epithelial
rudiment either continues its regression and disappears completely, or it may
still undergo some modest development. Considerable variation exists between
different strains of mice, and the degree of development was also found to vary
with the position of the gland (Richardson & Cloudman, 1947). The fifth pair
of glands regresses without prior separation from the epidermis (Raynaud &
Raynaud, 19536).
There is experimental support for the assumption that this inhibition of
mammary-gland development in male embryos is caused by testicular hormones.
Raynaud & Frilley (1947, 1949) succeeded in destroying the gonads of 13-day
embryos in utero by localized X-irradiation. Mammary-gland development then
followed the female-type pattern in all embryos. By contrast, injections of
testosterone-propionate into pregnant mothers resulted in an involution of the
mammary-gland rudiments even in female embryos (Raynaud, 1947a, 1949;
Hoshino, 1965). Finally, the administration of a synthetic steroid with known
antiandrogenic effect to pregnant mothers prevented the regression of the
mammary glands in male embryos (Elger & Neumann, 1966).
So far, all the evidence comes from experiments performed in vivo and the
interpretation of the last two experiments is further complicated by the fact that
the hormone, or its antagonist, respectively, were given to the mother animal.
Possible consequences of castration on other endocrine organs, and the effect
of a hormonal imbalance created in the pregnant mother must be considered.
From these results it cannot be conclusively decided whether at the stage of the
13-day mammary rudiment, the presence or absence of androgens is the only
factor determining the further fate of the gland. In addition, as recently pointed
out by Jean & Delost (1969), evidence for a direct action of testicular hormones
on the mammary gland rudiments is still missing.
Both these questions can be answered by an in vitro system. Embryonic
mammary glands of the mouse, grown in organ culture, undergo typical
morphogenesis (Kratochwil, 1969), and explanted testes of mouse embryos
secrete androgens in vitro (Weniger, Ehrhardt & Fritig, 1967). If the sexual
dimorphism in the development of the mammary gland is indeed caused by
androgens alone, then rudiments excised from male embryos before the beginning of regression should continue their development after explantation and
yield female-type glands. Secondly, the addition of embryonic testes or of
testosterone to cultures of mammary glands from female embryos should cause
their regression in order to prove a direct effect of the hormones on the gland.
The results reported in this paper are able to confirm both of these assumptions.
Embryonic mammary gland in vitro
143
MATERIALS AND METHODS
Mouse embryos were obtained by mating random bred Swiss (GP) females
with C3H males, the detection of a vaginal plug counting as day zero of
pregnancy. Dissection and explantation of mammary gland rudiments were
performed as previously described in detail (Kratochwil, 1969). Testes of
13-day or 15-day embryos were cut in half before explantation. Both mammary
glands and testes were explanted on to the platform of a filter assembly which
was suspended on the surface of the liquid medium in an organ culture dish
(the method of Grobstein, 1956). The explants—usually two on a filter disc of
3-5 mm diameter—were covered with a plasma clot made up by mixing equal
volumes of rooster plasma and 9-day chick embryo extract. Culturing the
explants on the translucent filter substrate (Millipore THWP, with a thickness
of 25 ft and a pore size of 0-45 /*) allowed continuous observation during their
development in vitro. Photographs of living cultures were made with a Zeiss
Ultraphot II.
The medium consisted of 80 % Eagle's MEM (Grand Island Biological
Comp.), 10 % horse serum (Flow Laboratories) and 10 % chick embryo extract,
prepared from 9-day embryos. Glutamine was added to a final concentration
of 2 mM and 50 units each of penicillin and streptomycin were incorporated in
the medium. Each culture dish contained 1 ml of medium. It was changed daily
except for the experiments with explanted testes where the medium was replaced only every second day to allow greater accumulation of testicular
hormones. When testosterone was used the required amount of a 10"1 M
alcoholic solution (28-8 mg/ml) was added to the medium. In all dilution series
the concentration of ethanol was kept constant throughout the experiment and
only that of testosterone was varied. Control explants were also exposed to the
same concentration of ethanol.
Normal in vivo development of embryonic mammary rudiments in Swiss xC3H
mice. Because of the variation in mammary-gland development between different mouse strains, especially in male foetuses, the normal fate of the glands in
our material had to be checked. In newborn females all five pairs are in a
considerably advanced state of development, the branched ducts extending
very far into the fat pads. In male litter-mates no traces of the third and fifth
pair of glands can be found. The other three glands are detached from the
epidermis—no nipples are present—and they exhibit very rudimentary development with modest growth and little branching. The best structures are formed
by the second pair of glands.
RESULTS
Two types of experiments were performed. In the first, series A, the developmental capacities of mammary-gland explants from male and female embryos
were compared. In series B, gland rudiments of female embryos were cultured
144
K. KRATOCHWIL
in the presence of foetal testes or of testosterone. All experiments were done
with mammary rudiments at various stages of development.
A. In vitro development of mammary-gland rudiments from male and
female embryos
Explantation of 12-day mammary-gland rudiments. At this early stage the sex
of the embryos could not be determined. Mammary-gland rudiments were
taken from all embryos of several litters and the glands of individual embryos
were kept separate throughout the explantation procedure. With this precaution
it would have been possible to detect differences in the development of explants
derived from various embryos. However, virtually all glands from all embryos
developed (Table 1). Differences in the frequency or rate of development of
glands derived from particular embryos were never observed.
Table 1. Development of explanted mammary-gland rudiments
from male or female embryos
Sex of embryo
Age of embryo
(days)
Male
12
13
14
38/40
18/49
Female
65/67
40/42
58/61
Explantation of 13-day rudiments. In this group the sex of the donor embryos
was known at the time of explantation. All five gland pairs were used for cultivation and comparison was made only between litter-mates. No difference in
the behaviour of glands from male or female donors was found, the glands of
male embryos developed with the same frequency and at the same rate (Table 1).
Outgrowth of the glandular ducts occurred in both groups on the 4th day in vitro
after the obligatory resting phase of the gland (Balinsky, 1950; Kratochwil,
1969). Even when at the time of explantation the donor embryos were fairly
advanced in their development, as judged by morphological criteria, the rudiments from male embryos were still able to form typical mammary glands
in vitro.
Explantation of 14-day rudiments. At the time of their explantation the
mammary-gland rudiments of 14-day female embryos did not differ significantly
from those of 13-day embryos except for a very slight increase in size (Fig. 1).
In male 14-day embryos, however, the glands had already started to undergo
visible regression: the epithelial knob was smaller than in female litter-mates,
connected to the epidermis only by a thin stalk and was surrounded by densely
packed mesenchymal cells (Fig. 2). This process appeared to be most advanced
in the second and third pair of glands, the third pair (second thoracic gland)
Embryonic mammary gland in vitro
145
occasionally became hardly distinguishable under the dissecting microscope
from the neighbouring hair rudiments.
Almost all of the glands from female donor embryos developed in vitro
(Table 1). Outgrowth of the primary sprouts occurred after a 'resting period'
of 3 days. In explanted male mammary glands the mesenchymal condensation
persisted for about 2 days in culture and the epithelial rudiments regressed
further. In the majority of the cases the gland epithelium was no longer detectable after 3 days in vitro. Although epidermal cornification and the formation of
hair rudiments proceeded normally, glandular outgrowth failed to occur in
these explants.
In about one-third of the cultures (Table 1), however, the epithelial rudiments,
though reduced in size, were still present after 3 days in vitro. At that time the
mesenchymal condensation around the gland had disappeared—probably due
to outgrowth of the fibroblasts on the substrate. In these cases the epithelia
were found to recover. They started growing again and finally gave rise to
glandular structures identical to those formed by explants of female rudiments.
The only difference was a delay of about 2 days in the development of these
male glands as compared with cultures of mammary glands from female littermates.
Table 2. Development of explanted mammary-gland
rudiments from 14-day male embryos
Developing
explants
Pair of glands
1 (cervical)
2 (1st thoracic)
3 (2nd thoracic)
4 (1st inguinal)
5 (2nd inguinal)
Total
7/11
3/11
2/5
6/11
0/11
18/49
Individual pairs of glands of 14-day male embryos were found to differ in
their capacity to develop in vitro. The cervical (first) and the first inguinal (fourth)
pair of glands appeared to be least affected by the regression process at the time
of their explantation: more than half of them were able to give rise to a
mammary gland in vitro. The results obtained with the thoracic rudiments
(second and third pair) were inferior and not a single gland was formed by
eleven explants of the fifth (second inguinal) pair (Table 2).
B. Exposure of mammary-gland rudiments from female embryos to
the influence of testicular hormones
These experiments were designed to reveal whether testicular androgens act
directly on the mammary gland. The rudiments were exposed to the hormones
by combined explantation with 13-day testes or by the addition of testosterone
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146
K. KRATOCHWIL
Embryonic mammary gland in vitro
147
to the culture medium. Although the previous experiments (section A) had
demonstrated no difference in the in vitro development of rudiments of male or
female embryos up to 13 days of age, the following experiments were done with
glands from female donors only, in order to avoid the possibility that the glands
could have been exposed to testicular androgens before their explantation. The
experiments were done with mammary rudiments at different stages of development between day 12 and day 15 of gestation. Rudiments at the end of the
gland's resting phase (16-day embryos) could no longer be tested as it has
previously been shown that they do not develop in vitro even in the absence of
androgens (Kratochwil, 1969).
Mammary-gland rudiments of 12- to 14-day embryos combined with 13-day
testes. In all experiments the testis explants formed typical and well-developed
testis cords in vitro (Figs. 3-5). The mammary rudiments cultured with these
testes began to show the first morphological signs of regression as early as 18 h
Figs. 1, 2. Histological sections of the first inguinal mammary-gland rudiment
(4th pair of glands) of 14-day embryos, at a time when the sexual dimorphism first
becomes apparent.
Fig. 1. Gland rudiment of a female embryo.
Fig. 2. Gland rudiment of a male litter-mate showing the beginning of the regression
process. The epithelial Anlage, especially its stalk, is surrounded by densely packed
mesenchymal cells. The stalk later ruptures, the lower portion of the gland thus
becoming separated from the epidermis.
Figs. 3-7. Explan ted mammary-gland rudiments exposed to the presence of foetal testes
or of testosterone, epid., epidermis; hair, hair rudiments; m.ep., mammary-gland
epithelium; mes., condensation of mesenchymal cells.
Fig. 3. Early stage of mammary-gland regression in a 14-day rudiment after 40 h
in combined culture with foetal testes. The mesenchyme immediately adjacent to the
epithelial bud (m.ep.) becomes condensed as seen by the darker zone surrounding
the gland rudiment. Living explant.
Fig. 4. More advanced stage of regression of a 14-day mammary-gland rudiment,
after 3 days in combined culture with foetal testes. The distal part of the gland
epithelium has become separated from the epidermis, the area of the former stalk
of the Anlage is occupied by a streak of condensed mesenchymal cells. Living
explant.
Fig. 5. Explant of a 14-day mammary rudiment after 8 days in vitro in the presence
of foetal testes. The gland epithelium was detached from the epidermis, its distal
portion still undergoing some development. Note the formation of hair rudiments
in the epidermal cyst and the good development of testis cords in the testis explant.
Living explant.
Fig. 6. Two control glands, developed from 14-day mammary rudiments explan ted
without testes. After 7 days in vitro typical glandular structures have been formed
which remained connected to the epidermal cyst (see upper explant). Living explant,
same magnification as Fig. 5.
Fig. 7. Explant of the first inguinal mammary-gland rudiment of a 13-day female
embryo after 42 h in vitro, the culture medium containing 10~ 6 M testosterone
(0-29 /tg/ml). Typical condensation of mesenchymal cells has occurred and the gland
epithelium has become separated from the epidermis. The response of the gland is
the same as in combined culture with foetal testes—cf. Fig. 4. Living explant.
148
K. KRATOCHWIL
after explantation. Typical condensation of mesenchymal cells occurred around
the gland epithelium, particularly around the stalk of the Anlage (Fig. 3). In
almost all explants the mammary rudiment became detached from the epidermis
(Fig. 4). Regression of the gland epithelium proceeded during the second day
in vitro, and between 48 and 60 h in culture the mammary rudiments completely
disappeared in 57 of 65 explants. Although these cultures were maintained for
at least 7 days, no glandular outgrowth was observed. By contrast, the survival
and development of epidermal cysts and the formation of hair rudiments in
these explants were not affected by the presence of testes.
In a few explants the effect appeared to be less severe. Although these glands
became detached from the epidermis, their regression remained incomplete,
sparing a small distal portion of the epithelial rudiment which subsequently
underwent some modest development (Fig. 5).
No obvious difference in the response of 12-day, 13-day, and 14-day mammary
explants was found (Table 3) except for the observation that 14-day gland
rudiments were able to persist for a longer period in culture than the younger
stages. The position of the gland had no influence on its response under these
experimental conditions. In those experiments where the five individual pairs
of glands of 13-day embryos were cultured in separate groups all explants
regressed completely. It was observed that in some, but not all, explants of the
fifth pair, the glandular epithelium became detached from the epidermis.
Although this process is characteristic of the first four glands it does not represent
the normal mode of regression in the fifth pair.
Table 3. Development of mammary-gland rudiments from female
embryos after combined explantation with 13-day testes
Age of mammary
gland at time of
explantation
(days)
12*
13
14
15
15 (with 15-day testis)
Developing explants
With testis
Without (control)
0/20
0/22
0/23
16/20
15/20
20/20
19/23
22/23
7/10
—
* Sex of 12-day embryos could not be determined.
Mammary-gland regression also occurred when the testes were explanted in a
transfilter position. In the control series without testes the mammary rudiments
persisted and glandular outgrowth occurred in all but five explants between the
3rd and 5th day in culture, depending on the age of the gland at the time of its
explantation (Fig. 6). Five of sixty-six control epithelia were lost due to spreading of the filter substrate.
Embryonic mammary gland in vitro
149
Mammary-gland rudiments of 15-day embryos combined with testes. In contrast
to the results obtained with the earlier stages of the gland, the development of
mammary explants from 15-day embryos was not affected by the presence of
testis explants (Table 3). Even the larger and more advanced testes of 15-day
foetuses could not cause their regression. Although all testes were found to
grow as well as in the experiments just described, there was no observable
difference in the frequency and rate of mammary-gland development between
experimental and control groups. The relatively low percentage of rudiments
developing in both groups was due to a greater spreading tendency of the older
epithelium. It was not observed that any of the morphological signs of androgeninduced regression was associated with the loss of these explants.
The effect of testosterone on explanted 13-day mammary rudiments. Testosterone was incorporated in the medium at concentrations ranging from 10~5 M
(2-9 /tg/ml) to 10"11 M, each group consisting of twenty explants. The alcohol
concentration in the medium was 0-01 % for the experiments with 10~5-10~7 M
testosterone (including controls), and 0-0001 % for the experiments with < 10~7 M
testosterone.
The response of mammary-gland explants to testosterone was the same as
observed in the presence of testes. Condensation of mesenchymal cells around
the gland epithelia was seen 30 h after explantation; all glands became separated
from the epidermis and, with very few exceptions again, eventually disappeared
completely. The formation of epidermal cysts with hair rudiments proceeded
normally. The lowest concentration of testosterone still causing regression of all
twenty mammary-gland explants was 10~9 M (0-00029 /tg/ml). At the next lower
concentrations (10~10 and 1 0 - U M ) testosterone was completely ineffective: all
glands developed as in the controls.
DISCUSSION
For the study of the effect of hormones on organ cultures a completely
synthetic medium would be desirable. The use of a semi-defined culture medium
in these experiments does not allow an answer to the question whether the
embryonic development of the mouse mammary gland is completely independent
of hormones. Lasfargues & Murray (1959) reported the formation of mammary
buds from explanted skin in medium 199 but they did not comment on the
morphology of the gland. Mitoses were not observed in these cultures and the
connective tissue showed areas of necrosis.
Despite the reservations due to the use of serum and of embryo extract in our
medium the experiments allow some conclusions to be drawn concerning the
role of hormones in the sexual dimorphism of the mammary gland. From the
survival and development in vitro of explanted mammary rudiments from male
embryos it appears that the gland—irrespective of its genetic sex type—follows
the female-type pattern of development in the absence of androgenic hormones.
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K. KRATOCHWIL
This result fully agrees with the female-type development of mammary rudiments in male foetuses after X-ray destruction of their testes (Raynaud &
Frilley, 1947, 1949) and it lends further support to the assumption that this
female-type development of the mammary gland does not require a particular
hormonal environment, present only in female embryos, but is the one which is
expressed in the absence of any sex steroids (Raynaud, 1961). Furthermore, in
the experiments with simultaneously explanted testes or with testosterone in the
medium, the hormonal 'background' due to the natural medium ingredients
was identical in the experimental and control groups. It was shown in these
experiments that the regression of the gland was caused solely by the addition of
a testis or of testosterone. This strongly suggests that the same mechanism
prevails in vivo and that the only cause of the sexual dimorphism in mammarygland development is the presence of androgenic hormones in male foetuses.
Continued presence of androgens seems to be required at least during the
initial phase of mammary-gland regression, as revealed by the in vitro development of some gland rudiments taken from 14-day male foetuses. Withdrawal of the hormone(s)—by explanation—allowed recovery and subsequent
female-type development even of glands already showing the characteristic
mesenchymal condensation. It appears, therefore, that the process of tissue
destruction in the male mammary gland of the mouse follows rules different
from those governing the regression of Miillerian ducts in the mouse (Brewer,
1962), rat (Price & Pannabecker, 1956), and chick embryo (Hamilton & Teng,
1965). In the Miillerian ducts an apparently irreversible pattern of cell death is
imposed by the testis during a rather brief period early in development. No
survival or even recovery in vitro was observed when explantation occurred
after that event.
The regression of explanted (female) glands in the presence of foetal testes
or of testosterone demonstrates that androgens act directly on the mammary
gland without necessary involvement of other endocrine organs. This result
supports the assumption of Raynaud & Raynaud (1953 a) that the destruction
of mammary rudiments in female foetuses, whose mothers had received
injections of testosterone propionate, was caused by a direct action of the
hormone on the gland. The sensitivity to testosterone of the 13-day mammary
rudiment is several orders of magnitude greater than in the adult gland where
it has been found that a concentration of 10"4 M is required to stop all epithelial
DNA synthesis (Turkington & Topper, 1967). This unusually high sensitivity of the embryonic rudiment (to 10~9 M testosterone) is an additional
argument for the assumption that in normal development also androgens act
directly on the mammary rudiment and not via the possible action of the
pituitary gland, as was considered by Jean & Delost (1969). However, this
response of the mammary gland to androgens is limited to a relatively short
time in its development: explanted 15-day rudiments were no longer affected by
the presence of testes. Hoshino (1965) applied testosterone propionate in vivo on
Embryonic
mammary gland in vitro
151
different days of pregnancy and obtained most extensive inhibition of mammary
development when the injection was done on day 12 of gestation. Testosterone
given on day 15 produced much less effect and none at all on day 16. In view of
our in vitro findings it appears that Hoshino's results indeed reflect a changing
sensitivity of the mammary-gland rudiments and not alterations in the steroid
metabolism of the mother and/or foetus.
The experiments done in vivo as well as ours in vitro show that the sex type of
the embryo has no influence on the response of the gland. This indifference is
one more reason for caution in comparing mammary-gland regression with the
involution of the Miillerian ducts. Exposure of the Miillerian ducts from female
mouse (Brewer, 1962) and rat (Price, 1957; Price & Pannabecker, 1959) embryos
to the presence of testes or of steroidal androgens not only failed to cause their
regression but was even found to stimulate their growth in vitro. Controversial
results were reported for chick Miillerian ducts exposed to androgens: regression
(Wolff, Lutz-Ostertag & Haffen, 1952) and stimulation (Hamilton & Teng, 1965).
The fact that mammary-gland regression is directly and solely caused by
testicular androgens, without complication by other hormones or by the sex
type of the donor embryo, appears to offer experimental advantages not found
in other systems. The reliable reproduction of this regression in vitro—with all
the morphological features observed in the embryo—could make it a valuable
system for the study of hormone-initiated tissue destruction.
ZUSAMMENFASSUNG
In-vitro Analyse der hormonellen Grundlage des Sexualdimorphismus in der
Embryonalentwicklung der Milchdruse der Maus
Die Ursachen fur den bei Mausen gefundenen Sexualdimorphismus in der Embryonalentwicklung der Milchdruse wurden mit Hilfe der Organkultur untersucht. Dabei wurden
folgende Ergebnisse erzielt:
1. Milchdriisenanlagen, die 13-tagigen mannlichen Embryonen und somit noch vor dem
Einsetzen ihrer Regression (13 1/2 Tage) entnommen wurden, entwickelten sich in vitro wie
Driisen weiblicher Tiere ohne Anzeichen einer Regression. Selbst ein Drittel aller Driisenanlagen von 14-tagigen mannlichen Embryonen war noch in der Lage, sich in Kultur zu entwickeln, obwohl sie zum Zeitpunkt ihrer Explantation bereits deutlich in Regression begriffen
waren.
2. Wurden Milchdriisenanlagen 12- bis 14-tagiger weiblicher Embryonen zusammen mit
embryonalem Hoden explantiert, so zeigten alle Driisen dieselben Regressionserscheinungen
wie sie in mannlichen Embryonen beobachtet werden.
3. Testosteron im Kulturmedium verursachte ebenfalls Regression explantierter Milchdriisenanlagen weiblicher Embryonen. Die niedrigste noch wirksame Konzentration des
Hormons war 10~9 M, oder 0,00029/*g/ml.
4. Milchdriisenanlagen von 15-tagigen weiblichen Embryonen reagierten nicht mehr auf
die Anwesenheit gleichzeitig explantierter Hoden. Diese Driisen entwickelten sich in vitro
ohne Anzeichen einer Regression.
Aus diesen Ergebnissen wurden folgende Schliisse gezogen:
(a) Der Sexualdimorphismus der Milchdruse kommt allein dadurch zustande, daB die
Entwicklung des Organs in mannlichen Embryonen unterdriickt wird. Die unterschiedliche
Entwicklung wird nicht etwa durch eine Wachstumsstimulation in weiblichen Embryonen
bedingt.
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K. KRATOCHWIL
(b) Die Regression der Milchdriisen wird durch die Anwesenheit androgener Hormone in
mannlichen Embryonen hervorgerufen. Diese Hormone iiben ihre Wirkung unmittelbar auf
die Druse aus, ohne Beteiligung anderer endocriner Organe.
(c) Das genetische Geschlecht der Druse hat keinen EinfluB auf ihre Entwicklungsmoglichkeiten, da: (i) Driisen mannlicher Embryonen sich bei Abwesenheit androgener
Hormone normal entwickeln konnen, und (ii) Driisen aus weiblichen Embryonen auf die
Gegenwart von Androgenen mit der fur mannliche Foeten charakteristischen Regression
reagieren.
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(Manuscript received 29 June 1970)