/. Embryol. exp. Morph. 98, 269-285 (1986)
269
Printed in Great Britain © The Company of Biologists Limited 1986
Gonadal development of the chick embryo following
microsurgically caused agenesis of the mesonephros
and using interspecific quail—chick chimaeras
EVA S. RODEMER, ALICE IHMER AND H. WARTENBERG
Anatomisches Institut der Universitdt Bonn, Nufiallee 10, D-5300 Bonn 1,
Federal Republic of Germany
SUMMARY
Mesonephric agenesis was achieved by microsurgical excision of the left Wolffian duct and the
underlying intermediate mesoderm of different regions between somites 16 and 23 in chickens
after 50-52 h of incubation (stage 14 HH). Quail-chick chimaeras were produced by transplantation of corresponding quail tissue in the region of somites 18-21.
A morphometrical analysis of the mesonephric and gonadal area in cross sections shows that
the intermediate mesoderm from somites 16 to 23 develops into the mesonephros. A partial
agenesis of the mesonephros brought about by removal of the intermediate mesoderm at the
level of somites 18 to 21 at stage 14 leads to a mean reduction of the gonadal volume of 37-8 %
compared to the volume of the untreated side at stage 30. Transplantation of quail intermediate
mesoderm in this region of the excision results in development of a hybrid mesonephros.
Consequently, the gonads are invaded and colonized by quail cells mobilized from mesonephric
corpuscles examined at stage 30, 35 and 36.
These results are discussed in terms of the origin of the gonadal stroma during this developmental period; they show that in the region from the third to the sixth segment the ventromedial
part of the differentiating mesonephros participates in the contribution of stromal cells to the
gonad.
INTRODUCTION
The development of the gonads of vertebrates starts with the formation of the
germinal epithelium, bilateral thickenings of the coelomic epithelium that appear
ventral to the developing mesonephroi. The proliferation of both germinal and
'nongerminal' cells leads to the formation of distinct gonadal primordia which are
identical in both sexes, the indifferent gonad (see for review Zuckerman & Weir,
1977). The data on the origin of the gonadal stroma in sexually indifferent gonads
are at variance. Considered by Callebaut (1976), Dodd (1977), Kopp & Bertrand
(1978), Fargeix, Didier & Didier (1981), Merchant-Larios & Villalpando (1981)
and Popova & Scheib (1981) to derive from the coelomic epithelium, the stroma of
the gonads was suggested by others to arise either from the mesonephric blastema
(Dang & Fouquet, 1979; Carlon, Pizant & Stahl, 1983) or from the differentiated
Key words: chick embryo, mesonephros, agenesis, quail-chick marker system, gonad,
chimaera.
270
E. S. RODEMER, A. IHMER AND H. WARTENBERG
mesonephros (Witschi, 1914, 1951; Swift, 1916; Upadhyay, Luciani & Zamboni,
1979; Zamboni, Bezard & Mauleon, 1979).
In order to ascertain the dependence of the gonad on the intermediate mesoderm or the mesonephros in chick embryos, we excised the intermediate mesoderm in the presumptive mesonephric area at day two. Following subsequent
incubation, we morphometrically analysed the indifferent chick gonad at day 7
(end of the indifferent stage). To obtain further information about gonad-contributing cells of the mesonephros, we transplanted corresponding quail tissue in
the presumptive gonadal area at day 2 and examined the chick gonads at days 7, 9
and 10.
MATERIALS AND METHODS
Microsurgery to cause mesonephric agenesis
Fertilized eggs of the domestic fowl (Gallus domesticus, strain White Leghorn) were acquired
from a local supplier and incubated at 37-8°C ± 1°C, 70 % to 80 % humidity.
After 50-52 h of incubation the position of the embryo was marked by candling and the
surface of each egg was sterilized by swabbing it with 70 % ethanol. A hole was pricked in the
blunt end of the egg, a procedure that leads to air being forced out of the air space and which
results in the embryo dropping away from the surface.
Operations were performed under aseptic conditions with the aid of a stereomicroscope, the
egg being supported horizontally in a cradle of cellulose. After cutting a window (approximately
0-7x0-5 cm) in the shell and its subjacent shell membrane at the marked area, the chick embryo
was floated up to the level of the window by infusion of Locke solution (Hara, 1971). The stages
of development were determined according to the criteria of Hamburger & Hamilton (1951).
We used embryos at stage 14.
For microsurgical procedures, electrolytically pointed tungsten needles were used (Dossel,
1958). After slitting the vitelline membrane, the left intermediate mesoderm and, for technical
reasons, the Wolffian duct were excised over a length of three to four somites at alternating
levels between somites 16 and 23 (Fig. 1). After the microsurgical procedure, 1 to 2 ml albumen
were removed from the egg; it then was sealed with Leukosilk® and subsequently incubated for
5 days.
The operated and normal embryos were killed, fixed in Serra's fluid containing 60%
propanol, 30 % formaldehyde solution and 10 % glacial acetic acid and dehydrated with graded
propanol solutions. The 7 jum paraffin-embedded serial sections were stained with haematoxylin
and eosin. With the aid of a computer-linked planimeter (Interactive Image Analysis System,
IB AS I and IB AS II, Kontron), the cross-sectional areas of the gonads and the mesonephroi
were measured. The levels of the points of measurement were elicited by using the spinal ganglia
as a means of orientation.
In the region of the mesonephros at the beginning and end of each spinal ganglion the
circumference of the mesonephros was marked by a digitizer (Fig. 2). From this, IB AS
calculated the cross-sectional areas in [im2. In the region of the gonad, measurements in the
middle of each spinal ganglion and at the midpoint between two ganglia were added.
In the normal embryos the average of the areas for each point of measurement for the left and
right side were calculated. Then the left and right sides were compared by estimating the
difference as
j
• .•
• m
mean deviation in % =
area left side ^
1nri
, .
,
, ,
. ..
. , . ., x 100 ± standard deviation.
area right side
In embryonal chicken development there are considerable interindividual differences. Therefore we evaluated the effects of our operations by comparing the cross-sectional areas of the
Avian gonadal development following quail-chick grafting
271
Chick embryo
Staee 14
16
Fig. 1. Diagram showing on the right side the unilateral extirpation of the intermediate
mesoderm (im) and the Wolffian duct (Wd) from a chick embryo at stage 14 over a
length of three to four somites in different regions between somites 16 and 23. The left
side shows the substitution by an equal quail graft previously isolated from a quail
embryo of the same stage of development. Transplantation in the region of somites
18 to 21.
mesonephros and the gonad of the operated side with the corresponding areas of the untreated
normal side as
area left side
deviation in % =
xlOO
area right side
in each embryo. The percentage deviation of all points of measurement of one organ was then
used to calculate the arithmetical mean of the difference between the operated and the normal
side.
Grafting procedure
Fertilized eggs of the Japanese quail (Coturnix coturnixjaponica), delivered by a local supplier,
were incubated together with the chick eggs. The chick embryos were operated at the level
of somites 18 to 21 at stage 14HH as described above. For the quail, whose development is
precocious relative to chicks although it does not differ by more than a few hours until the third
day of incubation, the stages were determined by analogy with the chick.
Quail embryos were removed from the egg, placed on agar and covered with Locke solution.
Left intermediate mesoderm and Wolffian duct were cut from the quail donor with fine tungsten
needles at the level of somites 18 to 21 and stained lightly with Nile blue sulphate impregnated
in agar. These were then transferred by means of a Spemann micropipette from donor to host
272
E . S. R O D E M E R , A . I H M E R AND H .
WARTENBERG
embryo and manoeuvred into place with tungsten needles (Fig. 1). In order to ensure a proper
fit and good adhesion between host and graft tissues, excess Locke solution was carefully
withdrawn with a micropipette. After removal of 1 to 2 ml albumen the sealed egg was incubated
for 5, 7 and 8 days. Chimaeras at HH stages 30, 35 and 36 were processed for histological
Fig. 2. Transverse section of a chick embryo in the region of mesonephric agenesis
at stage 30 after excision of the left intermediate mesoderm and Wolffian duct at
stage 14. On the operated side the mesonephros (m) and the Wolffian duct (Wd) are
missing. Both gonads (g) are present.
, measured circumference of mesonephros,
—, measured circumference of gonad. Scale bar, 0-2 mm; stained with haematoxylin
and eosin.
Avian gonadal development following quail-chick grafting
273
examination: they were fixed in Serra's fluid by transcardiac perfusion and subsequent immersion. After paraffin embedding the specimens were serially sectioned at 7 jum. As the interphase
nuclei of quail cells are characterized by a large mass of nucleolus-associated heterochromatic
DNA, the sections were stained for DNA (Feulgen & Rossenbeck, 1924) and counterstained
with lightgreen, or they were stained with haematoxylin and eosin following acid hydrolysis
(Hutson & Donahoe, 1984).
RESULTS
Morphometry of normal embryos
At stage 30 the mesonephros extends over a region of ten to eleven spinal
ganglia in a craniocaudal direction. It begins at the ganglion corresponding to the
15th spinal nerve and ends at the 25th ganglion. The increase of the cross-sectional
areas from craniad to caudad of the left and right organ proceeds absolutely symmetrically. The difference between left and right mesonephros was calculated as
A • .- -of
area left mesonephros w 1ftft i n n . M , 1O « ~
mean deviation m % =
r-r
*-.
x 100 = 100-4 % ± 12-2 %
area right mesonephros
(Fig. 3).
The gonads start three to four segments caudal of the beginning of the mesonephros and stretch over a region of four to five ganglia. In female birds one gonad
degenerates; in chickens this is invariably the right one. This becomes already
evident at the indifferent stage of the gonads (stage 30): whilst male gonadal
primordia exhibit only a slight difference in size between the right and the left side:
mean deviation in % = a r e a left gonad x m = m ^ % ± n Q %
area right gonad
females show a distinct diminution of the right gonad:
mean deviation in % =
area
left gonad x
area right gonad
m
=
^ ^
% ±
^
%
(Fig. 3).
Therefore we chose the left side for operations.
Morphometry of operated embryos
After excision of the different segments of the intermediate mesoderm and the
Wolffian duct at day 2 (stage 14), a partial dysgenesis or agenesis of the mesonephros was noted at day 7 (stage 30).
In accordance with the different regions where the defects of the mesonephros
appeared, the specimens were divided into three groups (Table 1): operations in
the region of somites 16i to 19 cause defects in the cranial third of the mesonephros
(group I). Defects of the middle third (group II) are evoked by operations in the
region of somites 18 to 21 and exhibit a maximum between somites 18-5 and 21-5
(Fig. 4). Cessation of development of the caudal third of the mesonephros (group
III) results following removal of the intermediate mesoderm and the Wolffian duct
in the zone of somites 20 to 23.
274
E. S. RODEMER, A. IHMER AND H.
0 20 40 60 80 100
0
0
2
4
WARTENBERG
6 8
0
2
4
6
8
Area (,um2)
15
17
0
5
Spinal ganglia
Fig. 3. Diagram shows the mean cross-sectional area of the mesonephroi (n = 7) and
the gonads (d" = 3; $ = 4) in /mi2 of the left ( •
• ) and right (O
O) side in
normal chick embryos at stage 30. In females there is a distinct diminution of the right
gonad already in the indifferent stage.
Reduction of the mesonephros alone does not influence the size of the gonads
in all three groups. But a partial agenesis of the mesonephros in certain regions
causes a reduction of the gonad. Mesonephric tissue from the region of the caudal
third of the mesonephros has no influence on the gonad (Table 1).
Group I
In the region of the upper three segments (ganglia 15-17), an agenesis of the
mesonephros resulted in a reduction of the gonad in only one of the five studied
Avian gonadal development following quail-chick grafting
275
cases. If the agenesis in the two studied cases extends further caudally into the
region of ganglion 18, an effect on the gonads is observed (Table 2), namely a
mean reduction of 17-5 % to an average of 82-5 % as compared to the right side.
Group II
A marked influence on the size of the gonads is attributable to the absence of
the mesonephros in the region of ganglia 18 to 20. In nine cases studied, a mean
reduction of 37-8 % to an average of 62-2 % as compared with the opposite side
results (Fig. 4). Agenesis of the mesonephros commencing from the region of
ganglion 20 and further caudad has no more influence on the gonad (Table 2).
The region of the reduction of the gonad is always observed caudally from the
corresponding defect of the mesonephros (Fig. 4). Only in one case did diminution
of the gonad appear at the same level as the defect of the mesonephros, but the
maximum diminution nevertheless occurred further caudad.
Interspecific quail-chick chimaeras
43 out of 175 embryos operated in this way developed after subsequent incubation. Of these 43 specimens, 27 contained quail tissue. 21 of the latter were
fixed on day 7, three on day 9 and three on day 10.
Table 1. Selection of three groups of animals according to the region of operatively
caused defects
Group
Region of operation
(stage 14)
Region of defect of
the mesonephros
(stage 30)
Size of gonads following
agenesis of the mesonephros
(stage 30)
I
II
III
somites 16-7-191
somites 18-4-21-1
somites 20-23
ggl. 15-18
ggl. 18-21
ggl. 21-25
normal/normal
ggl., ganglia; - , reduction.
Table 2. Size of the gonads following agenesis of the mesonephros in group I and II
Region of agenesis
of the mesonephros
Group I
ggl. 15-17 (n = 5)
Size of the gonads
normal (n = 4)
(n = l)
ggl. 15-18 (n = 2)
Group II
ggl. 18-20 (n = 9)
—
ggl. 201-22 (n = 2)
normal
ggl., ganglia; - , reduction.
(8 caudally
1 caudally and also at
the same level)
276
E. S. RODEMER, A. IHMER AND H. WARTENBERG
0 20 40 60 80 100
1
I
1
I
I
1—
0 20 40 60 80 100
-)
1
1
1
1
1
c
Area
0
0
0
0*
0
0
20
21
25
Spinal ganglia
Fig. 4. Diagram demonstrates the mean reduction in % of the cross-sectional areas
of the left side ( •
• ) of the mesonephroi and gonads in relation to the right
(O
O) side (100%) in chick embryos (n = 9) at stage 30. The left intermediate
mesoderm and the Wolffian duct have been excised in the region of somites 18 to 21 at
stage 14. —> indicates the point of maximum reduction.
In all the above experimental cases, healing was very good. Macroscopically
examined, they showed no alteration of form and bilateral symmetry in the
development of the body wall; but 14 of the 43 embryos revealed a dysgenesis of
the left upper limb.
Although the quail grafts were originally of the same size as the dissected
chicken tissue, the expanding lesion in the developing chick mesonephros is never
entirely filled by quail mesonephric tissue. After postoperative incubation for at
least 5 days there is a good connection between host and donor mesonephros at the
Avian gonadal development following quail-chick grafting
277
cranial end of the transplanted tissue, but at the caudal end a distinct gap exists. In
each case, in the region of the graft, mesonephric tubuli consisting of cells with
typical quail nuclei are found. Also hybrid tubuli are demonstrable, one part of
the tubulus wall containing chick cells, the other part formed by quail cells; no
intermingling of the different cells occurs (Fig. 5). If the quail graft develops only
tubuli and no other mesonephric structures, no quail cells are discovered in the
gonad.
The quail tissue is also able to form mesonephric stromal cells and to develop a
Bowman's capsule. Either the Bowman's capsules circumscribe an empty space or
they contain hybrid glomeruli. These glomeruli consist of chick-derived cells with
oblong, flat nuclei which we consider to be endothelial cells of the glomerular
loop; they also contain cells with oval or round nuclei carrying the quail marker,
which we assume to be podocytes (Fig. 6).
In nearly all chimaeras the mesonephroi are composed of alternating chick and
quail tissue; in many cases the quail mesonephric tissue occupies the dorsal or
lateral parts of the organs. These dorsolateral sections of the mesonephroi do not
contribute to the gonad, as no quail cells reach the gonad in such situations.
Thus, to form the gonad, the quail graft has to develop mesonephric corpuscles
or stroma, and this in the ventromedial section of the mesonephros lying in
juxtaposition to the gonad.
In 7-day-old embryos, quail cells egressing from the Bowman's capsules of
the quail type had colonized the indifferent gonad in three cases. Outside the
Bowman's capsule the migrating cells form very delicate trabeculae (Fig. 7) which,
in the serial planes of sections, can be followed all the way into the gonad. There,
the colonizing quail cells do not disperse randomly throughout the inner core of
the gonad and do not intermingle with chicken stromal cells. They occupy a
distinct part of the gonadal stroma and in no case do they show contact or
intermingle with the stratified surface epithelium. A band-like space beneath the
latter approximately twice as wide as the epithelium is formed by chick cells
(Fig. 8). Hence, the quail cells establish no association with the primordial germ
cells in the deep layer of the epithelium. Germ cells arranged between the inner
core and the surface epithelium are enclosed by chick and quail stromal cells; the
chick cells form a crescent round the germ cells at the side apposed to the
epithelium; the other side is covered by quail cells (Fig. 9).
In a 9- and 10-day-old ovary, two structural components are demonstrable: an
outer cortex and an inner medulla. The medulla occupying the major portion of
the ovary is partially formed by quail cells. Chick cells constituting the other part
of the medulla hardly intermingle with the quail cells. Most of the germ cells lie in
pairs or small groups in the middle or inner part of the cortex. These germ cells
are closely surrounded by chick cells. Also, the few germ cells lying in the deep
medullary part toward the hilus, constituted of quail tissue, are surrounded by
chick cells (Fig. 10).
In a 10-day-old testis, germ cells show an even distribution throughout the
medulla, indicating the formation of male cords. Although in this case only a few
278
E. S. RODEMER, A. IHMER AND H. WARTENBERG
1?*
-£- >'
Fig. 5. Part of a transverse section through the chimaeric mesonephros of a chick
embryo 8 days after exchange of the left intermediate mesoderm and Wolffian duct by
an equal quail graft. The asterisk marks a hybrid tubulus. The arrows mark the
beginning of the part of the tubulus wall containing quail cells. All cells of the right side
between the arrows were identified as quail cells by varying the focus. The left tubulus
side is formed by chick cells (arrowheads). Scale bar, 10 jwm; stained with haematoxylin
and eosin following acid hydrolysis.
Fig. 6. Hybrid glomerulus 5 days after substitution of the left intermediate mesoderm
and the Wolffian duct by an equal quail graft. The arrowheads mark oblong, flat nuclei
of chick cells. The round nuclei carry the typical quail marker (arrows). Scale bar,
10 jum; stained with haematoxylin and eosin following acid hydrolysis.
Avian gonadal development following quail-chick grafting
* IS
8
Figs 7, 8. For legends see p. 280
279
280
E. S. RODEMER, A. IHMER AND H. WARTENBERG
quail cells colonize the gonad in small groups, they come into contact with the
germ cells. These germ cells are consequently surrounded by cells of the chick as
well as those of the quail type (Fig. 11).
DISCUSSION
The data indicate that the cranial third of the mesonephros develops from the
intermediate mesoderm of the region of somite 16 to the beginning of somite 19.
The middle third is formed by material belonging to the region extending from
somite 18 to the beginning of somite 21, and the caudal third of the mesonephros
originates in the intermediate mesoderm of the region comprising somites 21 to 23
(Fig. 12). As shown in Fig. 12, mesonephric tissue of the region extending from
the end of ganglion 17 to the beginning of ganglion 20 (black area) contributes to
the gonad in each case. Mesonephric material of two small regions (hatched areas)
adjacent to the cranial and caudal end of this region has influence on the gonad
only in certain cases. In contradiction to authors who consider the presumptive
gonadal area to be at the level of somites 9 to 14 (Romanoff, 1960) or at the level of
somites 20 to 26 (Willier, 1933; Griinwald, 1937) or at the level of somites 24 to 29
(Dantschakoff, 1931) or spread over a more extended region from somites 13 to 22
(Didier, Fargeix & Bergeaud, 1980), our results of the extirpation and transplantation experiments suggest a presumptive gonadal area of the intermediate mesoderm extending from the beginning of somite 17 to the beginning of somite 21.
The diminution of the gonadal volume was caused by excision of the intermediate mesoderm; thus on the basis of our experiments alone it cannot be
Fig. 7. Section through parts of the chimaeric mesonephros and gonad 5 days after
substitution of the left intermediate mesoderm and the Wolffian duct by an equal quail
graft. (i) empty Bowman's capsule of quail cells, (2) gonad. The arrows demonstrate
delicate trabeculae formed by quail cells migrating from the Bowman's capsule into
the gonad. Scale bar, 10 jinn; stained with haematoxylin and eosin following acid
hydrolysis.
Fig. 8. Section through a part of the chimaeric mesonephros (1) and the gonad (2)
5 days after exchange of the left intermediate mesoderm and the Wolffian duct by an
equal quail graft. The arrows show the distinct part of the gonadal stroma formed by
quail cells which do not reach the surface epithelium. Scale bar, 40jum; stained with
haematoxylin and eosin following acid hydrolysis.
Fig. 9. Section through a part of the chimaeric gonad 5 days after substitution of the
left intermediate mesoderm and the Wolffian duct by an equal quail graft. The asterisk
marks two germ cells lying between the inner core (1) and the surface epithelium (2) of
the gonad. These germ cells are covered by chick cells (arrowheads) at the side
apposed to the surface epithelium; the other side is covered by quail cells (arrows).
Scale bar, 10 /-tm; stained with haematoxylin and eosin following acid hydrolysis.
Fig. 10. Section through a 10-day-old ovary 8 days after the exchange of the left
intermediate mesoderm and the Wolffian duct of an equal quail graft. Deep medullary
part constituted by quail cells (arrows). Germ cells (*) surrounded by chick cells
(arrowheads). Scale bar, lOjum; stained with haematoxylin and eosin following acid
hydrolysis.
Fig. 11. Section through the medulla of a 10-day-old testis 8 days after transplantation.
Germ cells (*) surrounded simultaneously by quail (arrows) and chick (arrowheads)
cells. Scale bar, lOjum; stained with haematoxylin and eosin following acid hydrolysis.
Avian gonadal development following quail-chick grafting
281
decided whether this reduction of the chicken gonad is due to loss of the
mesonephric blastema or to loss of the differentiated mesonephros. However,
Bishop-Calame (1966), Popova & Scheib (1981) and Merchant-Larios, Popova &
282
E. S. RODEMER, A. IHMER AND H. WARTENBERG
Somites
| 1 6 | | 1 7 | | l 8 | | l 9 | |20| | 2 l | |22| | 2 3 |
Intermediate
mesoderm
Mesonephros
ganglia
—
Fig. 12. Diagram summarizing the results of extirpation of the intermediate mesoderm
at stage 14 (above) in three overlapping regions. At stage 30, segments of the mesonephros (below) are missing in three corresponding regions. • = segments of the
mesonephros which definitely contribute to the gonad. H = region of the mesonephros
which possibly contributes to the gonad.
Reyss-Brion (1984) observed a reduction of the gonad following interruption
of the Wolfflan duct. Since they did not excise the mesonephric blastema, but
prevented mesonephric development, we attribute the diminution of the gonadal
volume in their and our experiments to the missing contribution of a differentiated
mesonephros. This is confirmed by the transplantation experiments. As these
show, the gonads are colonized by cells originating in mesonephric corpuscles.
Several studies concerning gonadal development in mammals also postulate or
show a mesonephric origin of the somatic cells of the gonad (Witschi, 1951;
Wartenberg, 1978; Fraedrich, 1979; Kinsky, 1979; Upadhyay etal. 1979; Zamboni
et al. 1979). These cells originate in developing as well as in regressing or disintegrating structures of the mesonephros. In accordance with the assumption that
mesonephric degeneration in chickens does not begin earlier than the 11th day of
incubation (Romanoff, 1960), in our examination (days 7, 9 and 10) the chickens
exhibited no signs of degeneration of the mesonephros. Thus in the chicken the
segregation of cells of the mesonephros takes place during the differentiating
phase at these stages of development.
In rabbit (Kinsky, 1979), sheep, Macaca mulatto, (Zamboni etal. 1980) and man
(Wartenberg, 1978) the gonad is colonized by mesonephric cells of the cranial third
of the organ. This seems to be different in mammals from in chickens. Neither the
cranial fifth of the mesonephros nor the caudal half is observed to contribute to the
gonad. Only a complete agenesis of the mesonephros over a region of more than
three segments successive to the cranial fifth causes a reduction of the gonad
(Fig. 12). Likewise, the quail mesonephric tissue has to appear in the latter region
Avian gonadal development following quail-chick grafting
283
of the mesonephros in order to form gonadal stroma. Quail structures further
craniad or caudad have no connection with the gonad.
During the operations, we took pains to avoid injuring both the dorsal aorta
lying directly ventral to the intermediate mesoderm and the coelomic epithelium
medial of the latter. For this reason we could not always be sure that this
ventromedial part of the intermediate mesoderm was excised in toto. We suppose
this incomplete removal to be the reason why in many extirpation experiments no
partial agenesis but a partially reduced mesonephros resulted in the operated
region. If only scant mesonephric tissue appeared in the gonad-contributing area
of the mesonephros, no reduction of the gonad was observed. We consider that
this ventromedial part of the intermediate mesoderm contributes to the formation
of the ventromedial section of the mesonephros and, as the transplantation
experiments show, it is only this ventromedial section of the mesonephros that
contributes to the gonadal stroma.
An agenesis of the mesonephros restricted to two or three segments never
resulted in agenesis of the gonad. In the same way, there was no case in which
quail cells originating from quail mesonephric corpuscles formed the entire
gonadal stroma at any given level. On the one hand it is possible that the persisting
cranial and caudal parts of the chicken mesonephros are extensive enough to
restore the gonadal stroma and act as a regulator; for technical reasons we are
not able to excise more extended parts of the intermediate mesoderm to eliminate
such a regulative mechanism.
On the other hand there still remains the question of a contribution of the
coelomic epithelium to the gonadal stroma. With this method we cannot exclude
it, because we are not able to decide whether the chick cells in the gonad derive
from persisting parts of the chick mesonephros or from the chick coelomic
epithelium. This method only proves the mesonephros to participate, at least, in
the gonadal stroma.
The reduction of the gonad was found almost exclusively caudal to the corresponding agenesis of the mesonephros (Fig. 4). According to observations in the
rabbit (Kinsky, 1979), in sheep (Zamboni et al. 1979), in the Macaca fascicularis
(Dang & Fouquet, 1979) and in man (Wartenberg, 1978), confirmed by the transplantation experiments, the contribution of the mesonephros to the gonads would
seem to proceed in a craniocaudal direction.
The relationship of the gonadal stromal cells of mesonephric origin to the germ
cells seems to differ in male and female gonads. In contradiction to findings in
sheep (Zamboni et al. 1979), they do not come into contact with the germ cells in
chicken ovaries at this early stage of development. Male germ cells are surrounded
by mesonephric quail cells and chicken cells at the same time. This result may
correspond with the postulated dual Sertoli cell system in man and mammals
(Wartenberg, 1978,1979); the possibility needs to be explored in further studies.
This work was supported by Deutsche Forschungsgemeinschaft.
284
E. S. RODEMER, A. IHMER AND H. WARTENBERG
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(Accepted 12 August 1986)