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Metamorphosis of the pubic symphysis III. histological changes in the symphysis of the pregnant guinea pig

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Department of Anatomy, The University of Rochester School of
Medicine and Dentistry
The phenomenon of the relaxation of the pelvic joints, and
the separation of the pubic bones at the symphysis, with
formation of a symphyseal ligament in the pregnant guinea
pig, was observed and described by various authors considerably in advance o€ the actual experimental dcmonstration of the causes of illis metamorphosis. The truth of
Todd’s (’23) statement that “there is probably no single
fact in comparative anatomy which has been more often rediscovered than the extraordinary separation at the symphysis of the guinea pig in late pregnancy and parturition”
is obvious to those who are familiar with the literature on
this subject. It is of historical intercst, however, to note that
Bland-Sutton (’ll),who erroneously believed that he was
the first to dcscribe the pubic ligament of the parturient
female guinea pig, states that “a microscopic examintltion of
the symphyseal ligament shows nothing t o explain its extensibility. ”
I n a recent investigation (Ruth, ’36) it has been shown that
the pubic symphysis of the female guinea pig develops in
the same manner as that of the male, with an uninterrupted
plate of hyaline cartilage between the osseous elements, until
the latter part of the gestation period. At that time a sexual
dimorphism appears, through a cataplasia of the interosseous
plate of cartilage. By this process a fissure appears, and
finally causes the separation of the osseous elements, with a
concomitant elaboration of collagenous fibers in the form of
a connecting ligament. This metamorphosis makes possible
the spectacular separation of the osseous symphyseal elements
(pubic and ischial rami) during parturition, and is accompanied by a hyperplasia of the symphyseal ligament itself,
which allows “the delivery of a fetus having an average head
diameter of 20 mm. through a pelvic canal with an average
diameter of 11mm.” (LeGallois, 1812). The present investigation is concerned with the histological changes observed
at the symphysis of pregnant females during gestation,
parturition and to the fourth week postpartum.
Thirty female guinea pigs (half of which were nulliparous)
were used in this investigation. Daily examinations were
made to detect the occurrence of oestrus in each individual.
Each female was placed in a breeding cage with a male as
soon as the vagina opened. When examination revealed
closure of the vagina, the female was placed back in the cage
with the other females. After an individual had been in the
breeding cage, and then failed to come into oestrus at the
expected time, she was placed in a cage for pregnant females
and considered pregnant unless subsequent examinations
proved otherwise. The daily examination was continued until
fetuses could be palpated and the diagnosis of pregnancy
definitely confirmed, Each female was marked and separate
records were kept for each one. When the diagnosis of
pregnancy was confirmed by the palpation of fetuses, the
number of days of gestation were counted from the date of
closure of the vagina. Pregnant animals were sacrificed at
weekly intervals from the eighteenth day of gestation to and
including the day of parturition. Subsequently, animals
were killed every third day postpartum for 2 weeks, and
again at the end of the fourth week. The symphyses were
fixed in Zenker’s, and embedded in paraffin. Sections were
stained with hematoxylin and eosin, and with hiallory’s triple
connective tissue stain.
During the first 5 weeks of pregnancy little change is to be
noted at the symphysis. The connective tissue forming the
ligament is arranged in bundles of collagenous fibers which
run antero-posteriorly, in the direction of the long axis of
the symphysis. These gradually shift into a transverse direction at the anterior and posterior extremity of the joint, to
become continuous with the anterior and posterior pubic
ligaments. The ligament is practically avascular, and during
this period nothing remarkable is to be noted-the joint appearing much like that of a non-pregnant animal.
After the fifth week of gestation a gradual invasion of the
symphyseal ligament by capillaries can be noted. With the
appearance of capillaries there is a noticeable widening of
the ligament. One or two small islets of hyaline cartilage,
unassociated with other cartilage or bone of the joint, were
found in the ligaments of a few animals at this time. The
increase in vascularity becomes more apparent as gestation
progresses toward term, and the vascular channels become
greatly enlarged. This enlargement of the capillaries is
especially noticeable during the last two weeks of gestation.
The histological picture at the symphysis varies somewhat
in different females, according to age and to changes brought
about by previous pregnancies. -4t any given stage of pregnancy the symphyseal ligament of the multiparous female
is wider than that of the female that has not borne young
The extent to which the pubic and ischial bones are covered
with cartilage along the syrnphyseal face varies with the age
of the animal. I n the older females the cartilage is almost
wholly absent, and the fibrous tissue of thc ligament is attached directly to the periosteurn.
By the eighth week the vascular channels of the symphyseal
ligament have increased noticeably in size and number.
Actual resorption of the symphyseal faces of the osseons elements has proceeded to the stage where the medullary
cavities of the bones concerned have actually been opened and
invaded by the fibrous tissue of the ligament. The m m e r o u s
67, NO. 4
vascular channels of the ligament anastoniose with, or in
some cases arise from the vascular channels of the bone
(fig. 2). Indeed it seems that the opening of the vascular
channels of the hone is the more important process through
which the spectacular vascularization of the ligament during
late pregnancy and parturition is effected. Bone resorption
in this case contributes only secondarily to the widening of
the birth canal, as the width of the bone destroyed is not at
all remarkable.
The vascularjzation of the ligament and its resultant hpperplasia apparently is concomitant with the appearance of
osteoclasts along the syrnpliyseal faces of the pubis and the
ischiurn, between the bone and the ligament (fig. 1). However, the number of osteoclasts present is not extraordinary,
and would seem wholly inadequate to cause the relatively
cxtensioe resorption of bone by any local osteolytic action
at the points a t which they are found. Rather it would seem
that the resorption of the bone is the stimulus which results
in the formation of these giant multinucleated cells. These
in turn probably assume a phagocytic activity for the removal
of any resulting osseous debris.
The osseous elements of the symphysis by this time show
unmistakable evidence that considerable bone resorption is
occurring. The actual opening up of the marrow cavity in
many places (as mentioned above) is perhaps the most apparent indication that bone is being removed. I n addition to
these actual openings in the symphyseal plates of the
ischium and pubis, the whole plate througliout is noticeably
thinned, and in many places has become so thin that only an
extremely narrow wall of bone intervenes between the marrow
cavity and the ligament. As gestation draws near to term,
even these thin walls of bone are resorbed, and extensive
areas of the medullary space are opened up. The myeloid
elements disappear, and the space is filled with fib~*oiistissue
that is continuous with the symphyseal ligament. In such
areas only the lateral plate of the ischium and pubis remains
(fig. 4). The dissolving away of the bone is shown by the
appearance of canals or cavities in the symphyseal plates of
the bones concerned. These become more numerous and extensive until the walls of bone between them become thin
and finally are removed, permitting adjacent spaces to coalesce, and resulting in a trabeculated appearance of the plates.
I n some areas there is actual necrosis of the bone, and resorption occurs by the phenomenon of ‘creeping replacement ’
(Ham, ’32). By this means new bone forms in minute and
discrete islands around osteocytes bordering the necrosed
area, thereby exerting pressure on the area of dead bone.
Humoral agents, possibly elaborated by the ostcocytes themselves, cause tlie removal of the dead bone, while the chemical
action is accelerated by the pressure exerted by the encroaching islets of new bone (fig. 5 ) .
The greatest change at the ischio-pubic symphysis of the
pregnant female occurs during the ninth and last week of
gestation. This is seen in the marked hyperplasia of the
symphyseal ligament. Up t o this time the lengthening of the
ligament has been relatively gradual, but now vascularization
and proliferation of the connective tissue elements increase
at a tremendous rate, the latter process being dependent upon
the former. The fiber bundles, which run parallel to the
antero-posterior axis of the joint during early pregnancy,
tend to change direction as the ligament lengthens through
the proliferation of the fibrous tissue. At the onset of this
shift in direction of the! fiber bundles they tend to bend away
from their original position toward a more transverse direction, especially those fibers near the middle of the ligament.
The fibers immediately adjacent t o the bone, on the other
hand, retain their antero-posterior direction throughout
pregnancy. As the fiber bundles toward the middle of the
ligament progressively change direction they appear in the
Iiistological sections to run in many different directions until
the last week of gestation, when (with the great acceleration
of proliferation of fibrous tissue) these middle bundles assume
a definite direction at riglit angles t o those fibers immediately
adjacent to the syinphyseal faces of the bones. The rapidly
proliferating fibroblasts have the typical appearance of such
cells in multiplication-the young cytoplasm taking a light
basic stain, and the nucleus being relatively large, distinctly
vesicular, and varying from spherical to oval in shape.
The e~t~raordinary
vascularization of the symphyseal ligament is developed to the highest degree at parturition (fig. 3).
Arterioles and capillaries permeate the ligament in all directions, but show a general tendency to run parallel with the
long axis. The vessels vary in size from capillaries 7 ~1 in
diameter to the larger arterioles which average 0.1 mm. in
diameter. I n many places there is considerable hemorrhage
into the ligament-more o r less extensive aggregations of
erythrocytes being found free among the bundles of fibers. I n
other areas relatively great extravasations of blood are found
in large sinusoidal spaces measuring as much as 1 mm. in
diameter and 1.5 mm. to 2.0 mm. in length. Some of these
blood-filled sinuses have a definite endothelial lining, although
it may not be continuous, while others apparently lack a
definite endothelium. I n several of the microscopic sections
arterioles and capillaries can be seen actually emptying into
these blood spaces.
After parturition the regressive changes in the symphyseal
ligament are marked and rapid. By the third day postpartum
the ligament has shortened approximately by half. The
numerous blood channels and spaces have been reduced in
size and number, and most of those that remain are empty,
except those vessels immediately adjacent to the periosteum.
Already the fiber bundles show a tendency to move into the
antero-posterior direction characteristic of the non-pregnant
or early pregnant animal, and the beginning of the repair of
the symphyseal faces of the pubis and ischiurn is apparent.
Bone repair is accelerated to such an extent that the
symphyseal plates of the ischiurn and pubis have been regenerated by the end of the second week. The fibrous tissue
has withdrawn from the medullary space, and the new cavity
contains the usual myeloid elements. The method by which
the bone is regenerated suggests the development of membrane bone, inasmuch as there is no new formation of cartilage. The abundant active young fibroblasts are metamorphosed into osteogenic cells along the line of bone formation, and osteoid tissue is deposited by their activity. At this
time the ischio-pubic joint has returned to the condition observed in the non-pregnant multiparous female. The bundles
of fibers again are passing in an antero-posterior direction,
and the ligament is practically avascular. The cells (fibroblasts) have returned to a resting condition with the
elongated form of cell and the dense elongated nucleus that
is typical of dense fibrous connective tissue. The histological
picture as described 2 weeks postpartum remains unchanged
at the fourth week, and it seems reasonable to assume that
the symphyseal response to pregnancy has been fully repaired.
The phenomenal hyperplasia and accompanying vascularization of the symphyseal ligament of the guinea pig during
the last week of gestation may be readily compared with those
changes accompanying connective tissue repair. Indeed, the
whole course of the progressive and regressive metamorphosis
in the ligament during the last week of pregnancy and for
2 weeks following parturition is almost identical histologically with the phenomenon of repair of connective tissue. A
spectacular invasion of the fiber bundles by arterioles and
capillaries, accompanied by an extremely active proliferation
of fibrous tissue elements is characteristic of both cases. The
final withdrawal of vascular channels and the consolidation
of the cicatrix in the case of connective tissue repair is likewise paralleled by the similar vascular withdrawal and the
concomitant atrophy of the connective tissue which results in
a shortening of the symphyseal ligament. Concerning cicatrization Karsner ( ’26) states that “the purpose of the rich
vascularization in organization is to provide nutrition for the
rapidly multiplying cells. When multiplication stops, the need
for capillaries no longer exists, and their atrophy may be a
manifestation of adapiation to the changed conditions . . . .
It (the scar) gradually becomes paler . . . . at the same
time decreasing in size and contracting until it is likely t o be
smaller than the original injured area. With the shrinkage
comes increased density.”
Kawata (’24) reports an investigation of the histological
changes at the ischio-pubic symphysis of the pregnant guinea
pig, in which he states that “the most interesting histological
finding is the remarkable proliferation of the osteoclasts along
the symphyseal surface of the pubis and iscliium during pregnancy, especially near the time of parturition. Some parts
of the bones are even perforated to the marrow.’’ €Ie explains the widening of the space between the osseous elements
of the symphysis as being due t o destruction of the margins of
the bones (inferring that this destruction is due to the
activity of the osteoclasts), mueous degeneration of the
symphyseal connective tissue, and fibrosis of “both fibrocartilage and hyaline cartilage over the bones. ”
As has been noted on page 412, the author was unable to find
ostcoelasts in any unusual number in the specimens studied,
nor was there evidence of mucous degeneration. Fibrosis of
the cartilage covering the symphyseal surfaces of the bones
(in a female young enough to have such cartilage) would contribute relatively little to the widening of the ligament. On
the other hand, it has been dcmonstratcd that the outstanding
phenomenon in the separation of the osseous elements of the
symphysis at parturition is the proiiounced hyperplasia of the
fibrous tissue through a tremendous proliferation of the connective tissue cells of the ligament.
The ligament itself is made up of dense bundles of collagenous fibers, with a few elastic fibers. It is not yet possible
to explain fully the mechanism controlling the lengthening of
the symphyscal ligament during gestation. It seems probable,
however, that the stimulus resulting in the formation of the
rapidly multiplying fibroblasts by an active proliferation of
the original connective tissue, is of hormonal origin (IIisaw,
The actual mechanism of the bone resorption is not known.
No one of the several theories of€ered to explain ~ h and
how bone is resorbed adequately covers all of the phenomena
observed in the various normal and pathological conditions
where resorption occurs. Besorption by the specific osteolytic
activity of the multinacleated gliant cells, known as osteoclasts,
is a concept that is no longer universally accepted. Two
outstanding objections to this theory might he mentioned :
1) When these cells are present in an area in which resorption is occiirring, they are seldom in sufficient concentration
to impress tlie observer that it is their osteolytic action that
is dissolving away the bone; and 2 ) in those pathological
cases wherein resorption progresses in the complete absence
of osteoclasts the postulation of a specific localized osteoelastic activity would be un-lenable.
Iii the present instance the extensive bone resorption that
is seen in the ischio-pubic s p p h y s i s cannot be accounted
for by the local osteoclastic activity of the relatively few
osteoclasts that are found in the histological sections. No
osteoclasts at all are found in intimate relation to the areas
of necrotic bone. For this reason the author adopts the view
that resorption is brought about by ‘creeping replacement’
(as already explained) in the areas where bone necrosis is
observed. Resorption in other areas is under humoral control,
with the osteoclasts acting a s phagocytes for the removal
of osteoid debris after the inorganic salts are dissolved away.
The notable histological changes at the ischio-pubic symphysis of the pregnant female guinea pig appear after the
fifth week of pregnancy. These changes are slow at first, and
consist of a progressive vascularization of the symphyseal
ligament, accompanied by a proliferation of the connective
tissue cells. The resultant lengthening of the ligament is
gradual and at first almost imperceptible. Boiie resorption is
noted toward the end of gestation but probably begins much
earlier. It is so gradual that it would be difficult to say when
it first begins, but the end result is the complete resorption
of relatively extensive areas of the medial tables of the pubis
and ischium, with the resultant opening up of the respective
medullary spaces.
Bone resorption, vasculariza tion of the symphyseal ligament, and proliferation of the connective tissue cells show a
great increase in activity after the eighth week of gestation,
and move rapidly toward the climax of these processes at
parturition. At parturition the ligament is found to be
lengthened greatly through a hyperplasia of the connective
tissue elements. Vascular channels appear in large numbers,
and many of them open into blood spaces where considerable
amounts of blood have collected. The medullary spaces
opened by bone resorption are invaded by fibrous tissue, and
all myeloid elements are lost.
After parturition regressive chanyes comparable t o cicatrization are seen in the symphyseal ligament, while bone regeneration with the reappearance of myeloid elements in the
mediillary spaces is relatively rapid. Complete repair is
observed as early as the end of the second week.
J. 1911 The symphyseal ligament of the parturient guinea pig.
Brit. Med. J., vol. 2, p. 976.
F. H. 1911 The symphyseal ligament of the parturient guinea
pig. Brit. Med. J., vol. 2, p. 1136.
DUNCAN,MATHEWS 1854 On the pelvic articulations in parturition. Dublin
Quart. J. Med. Sei. Quoted by F. H. Champneys, 1911. Brit. Med. J.,
vol. 2, p. 1136.
HAM, ARTHUR 1933 Cartilage and bone. Section XXV in Cowdry’s Special
Cytology, vol. 11, p. 1041. Paul B. Hoeber, he., New York.
L. 1926 Experimental relaxation of the pubic ligament of
the guinea pig. Proc. SOC.Exp. Biol. and Med., vol. 23, p. 661.
-1929 The corpus luteum hormone. I. Experimental relaxatim of
vol. 2, p. 59.
the pelvic ligaments of the guinea pig. Physiol. ZOO~.,
T. 1926 Human pathology, pp. 190-194. J. B. Lippincott
Co., Philadelphia.
8. 1824 On the morphological changes of the symphysis puboischiadica
in the guinea pig during pregnancy. Folia Anat. Japon., vol. 2, p. 370.
LEGALLOIS, M. J. 1812 Experiences sur la Vie. Paris.
RUTH, ELBERTB. 1932 A study of the development of the mammalian pelvis.
Anat. Rec., vol. 53, p. 207.
1936 Metamorphosis of the pubic symphgsis. 11. The guinea pig.
Anat. Rec., vol. 67, p . 69.
W. 1902 Parturition in the guinea pig. Brit. Mad. J., vol. 2, p. 777.
TODD,T. WINGATE 1923 The pubic symphpsis of the guinea pig in relation to
pregnancy and parturition. Am. J. Anat., vol. 31, p. 345.
1 Section of symphysis of a female 66 days pregnant along the symphyseal
f a r e of the ischial ranius, showing three osteoclasts in a space between the
ligament and bone. Note the numerous capillaries. Note small area of necrotic
bone above and below osteoclasts. X 210.
2 Section of the same symphysis as above, showing the resorption of bone and
invasion of the marrow cavity by fibrous tissue. Section shows a capillary communication between the vessels of the marrow cavity and thosc of the ligament.
x 210.
3 Small area of the pubic ligament of a female 3 hours postpartum, showing
extreme vascularity. Note the sections of the large blood sinuses above and
below the bundles of collagenous fibers, and the capillaries ramifying througb
the fibers. This is a typical section-the whole ligament being highly vascuIar.
x 210.
4 Section of the symphysis of a female 3 days postpartum, illustrating the
extreme degree of bone resorption. The marrow cavity is completely filled with
fibrous tissue, whirh is continuous with the bundles of fibers of the ligament,
and the medial table of bone is removed from relatively extensive areas. X100.
5 Small area of bone and adjacent ligament of the symphysis of a female
56 days pregnant, showing the process of resorption of bone by creeping replacement. Unstained area is dead bone, in which small invading islands of newly
developed bone, surrounding single osteocytes, can be seen. X 430.
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guinea, metamorphosis, change, pig, pregnant, iii, pubic, histological, symphysis
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