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Serous cysts of the aging guinea pig ovary I. Light microscopy and origin

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Serous Cysts of the Aging Guinea Pig Ovary
Department ofAnatomy, Medical College of Virginia, Virginia Commonwealth University,
Richmond, Virginia 23298
Ovaries from guinea pigs of different ages have been examined
by light microscopy. Serous cysts measuring between 1mm and 2.5 cm are present in all animals over one year of age. The cysts are fewer in number and size a t
six months, are least developed a t ten weeks and are not apparent in the nearterm fetus. The cysts arise from the rete ovarii. They are lined with a simple
cuboidal to columnar epithelium composed of cells bearing solitary cilia or tufts
of cilia. The structure and origin of these cysts is discussed and compared with
what is known of the human serous cystadenoma.
The increased incidence of neoplasms in the
aged ovary (Julian et al., '74) and decline in
fertility in aged mammals (Albrecht et al.,
"75) are two problems which reflect the need
for information on the biology of the ovary in
aging mammals. The rat, one of the most commonly studied aged laboratory animals, is
well known for its disposition toward irregular cycles and polycystic ovaries (Quadri et
al., '73). The cysts of such ovaries are of follicular origin and appear to be structurally
similar to the preovulatory follicles of
younger animals. In humans the most common ovarian neoplasm is the serous cystadenoma which is frequently malignant (Bullock et al., '55; De Santo et al., '59). In a population studied by Bullock et al. ('55),the mean
age of women with benign serous cysts was 41
years with a range of 16 to 84 years. Unlike
the rat these cysts do not resemble ovarian
follicles and several authors (Evans, '68; Hertig and Gore, '61; Novak and Woodruff, '74)
have attributed their origin to the ovarian
The present investigation is concerned with
the appearance of cysts in the ovaries of
guinea pigs. These cysts are more common in
older animals, and are comparable structurally to the serous cystadenoma of the human
rather than the follicular cyst of the rat. The
light microscopic anatomy and origin of these
cysts are the subject of this paper.
Four groups of guinea pigs (Hartley strain)
ANAT. REC., 288: 351-360.
were used in this study: (1) 2-year-old retired
breeders; (2) 1-year-old retired breeders; (3)
6-month-old females; (4) 10-week-old virgin
females. Three females were examined in
each of these groups. In addition, one near
term female fetus was taken from one of the
6-month old females and examined. The animals were killed with sodium pentobarbital
and the ovaries immediately removed and
placed in an aldehyde fixative. This fixative
was composed of 3% glutaraldehyde and 1%
paraformaldehyde in 0.1 M phosphate buffer
(pH 7.4). Portions of some ovaries were postfixed in a 1%solution of OsO, with 0.1 M phosphate buffer (pH 7.41, dehydrated in alcohol,
and embedded in Epon 812. Sections (0.75 p m
thick) of this material were stained with toluidine blue, examined and photographed by
light microscopy. The remaining tissues were
transferred to fresh aldehyde fixative containing 5 ml of saturated picric acid per 100 ml of
solution (It0 and Karnovsky, '68). The tissues
were fixed in this solution for 18 hours a t 4"C,
embedded in paraplast, serially sectioned a t
5 pm and stained with hematoxylin and eosin.
Reconstructions of portions of the serially sectioned ovaries were made using camera lucida
drawings. Photomicrographs of selected sections in these series also were made.
In the present study ovarian cysts measure
Received July 30, '76. Accepted Dec. 10, "76.
' Supported by A. D. Williams Grant 3658 (5301, Medical College of
Virginia, Virginia Commonwealth University.
up to 2.5 cm in diameter. By gross observation
these cysts are unilocular with a smooth inner
surface and are filled with a clear fluid of low
viscosity. The number of cysts, as well as their
diameter varies greatly. When the cysts of a
given ovary are less than 5 mm in diameter,
several are clustered in the cephalic pole near
the hilar region. In the specimens exhibiting
the larger cysts (2 cm or greater) there is
usually only one cyst and no other recognizable ovarian tissue.
While the size and frequency of the cysts
vary among animals of the same age group,
the largest cysts are in the animals of the 1and 2-year-old groups. In each of the animals
examined in these groups, there is a t least one
cyst greater than 1mm in diameter. Three out
of seven animals examined in these groups
have one ovary with a cyst greater than one
centimeter in diameter. In each of the animals of the 1- and 2-year-old age groups in
which the left and right ovaries are identified
(5 of 6 animals), the largest cysts are in the
left ovary. In general cysts in the 10-week-old
and 6-month-old groups are less than 0.1 mm
in diameter. However, there is one cyst with a
diameter of 1 mm in a 6-month-old animal.
Cysts are least frequent in the 10-week-old
animal and are absent in the near term fetus.
Microscopicexamination of the ovary of the
near term fetus indicates the presence of numerous follicles which are frequently joined
to one another in linear configurations (fig. 1).
Such configurations are also in continuity
with the ovarian mesothelium. In the hilar
region of this ovary, there are cords of
basophilic cells (fig. 1). These cords extend
into the more cortical regions of the ovary
where their proximity to the follicles gives the
appearance of continuity. A slit-like lumen is
frequently present in these cords but there
are no cystic dilations.
Microscopic examination of the ovary of the
adult guinea pig reveals large numbers of
cystic structures which vary considerably in
diameter but are composed of similar cell
types (fig. 2). Serial reconstruction of ovaries
indicates that the larger cysts are frequently
multilocular (figs. 3, 4) and that the smaller
cysts (less than 500 pm in diameter) form a
network of interconnecting cysts. In figures 510, selected sections from such a series demonstrates the continuity of these cysts in the
ovary of a 2-year-old guinea pig. In this series
profile x of figure 5 is in continuity with profile y in figure 7. Profile y (fig. 10) measures 75
x 112~m
and has been traced through 20-
sections 5 pm in thickness. At each end of the
20-section series the profile ends blindly (fig.
5).The section in figure 5 is the most proximal
of figures 5 through 10 to the hilar region of
this ovary. The lumen of profile x is followed
in sections closer to the hilar region for
300 pm. In this span this lumen becomes continuous with that of another cyst of slightly
larger diameter and finally in the hilar region
is continuous with the lumen of a cyst 200 p m
in diameter.
While such cysts penetrate throughout the
ovary, they are most common in the hilar
region. In this region there is a complex of
branched tubules whose lumen is less obvious
(fig. 2). Cysts are in continuity with these
All cysts are lined with a simple epithelium
which varies from low cuboidal to columnar.
Two cell types can be identified in this epithelium (figs. 11,121. The most abundant of these
cell types is characterized by the presence of a
solitary cilium which projects into the lumen
of the cyst. This cell contains a basophilic cytoplasm but no other cytoplasmic specialization is apparent by light microscopy. The apical surface of the cell is slightly domed with a
few cytoplasmic projections.
A second population of cells is identified by
the presence of a tuft of cilia on the luminal
surface of each cell. In the cytoplasm underlying these cilia is a row of basal bodies. The
cilia of a given cell are all bent in the same direction. The surface of the ciliated cells may
be flat or slightly domed. Cells with the domed
appearance are generally more basophilic than
those with a more flattened surface. Of
the two types of tufted cells the less basophilic is the least numerous.
The nuclei of all cell types are irregularly
shaped and a t times exhibit deep clefts. They
are generally central in location although in
taller columnar cells they are situated more
basally. The nuclei have a single nucleolus
and a peripheral concentration of heterochromatin.
The non-cystic tubular system found in the
hilar region is also composed of a simple
cuboidal to columnar epithelium which is not
ciliated. The dilated or cystic portions of this
system, however, do contain ciliated cells.
No structural evidence of secretory activity
was detected in any of the above cell types.
Descriptions of ovarian development in
rats, humans and monkeys (Gillman, '48;
Merchant, ’75; Quattropani, ’75) indicate that
cords of epithelial and primordial germ cells
develop and grow in the stroma of the ovarian
blastema. These cords are continuous with the
ovarian mesothelium as well as the mesonephric tubules. Portions of these cords containing germ cells later fragment to become
ovarian follicles; while other portions retain
the cord-like appearance. In the hilar region
of the ovary those cords which are derived
from the mesonephric tubules (Merchant, ’75)
are devoid of germ cells and are known as the
rete ovarii. The cords of epithelial cells observed a t each age in the hilar region of the
guinea pig ovary are the remnants of this rete
ovarii. Continuity between the rete ovarii, follicles and ovarian mesothelium is observed in
the guinea pig. The presence of the rete ovarii
is not restricted to the fetus or neonate. It has
been described in a variety of adult mammals
including man (Sauramo, ’54).
In the guinea pig the large serous cysts of
the adult ovary develop from the rete ovarii.
This conclusion is based on four factors: (1)
the cysts are most concentrated in the hilar
region of the ovary; (2) the smaller cysts are
in continuity with one another; (3) the rete
ovarii of young animals (and old animals
when not obscured by large cysts) exhibits
dilated or cystic regions and the epithelium of
these cystic regions is similar in appearance
to that of the larger cysts; (4) there is dissimilarity between the epithelium of the
atretic follicles and the cysts. The cystic
nature of the rete ovarii in the guinea pig was
first noted by Mossman (‘73). However, the
guinea pig is not unique in this property; the
heifer (Archbald et al., ’71),pig (Wilkerson,
’23)and man (Sauramo, ’54) have also been
reported as being cystic. Preliminary observations in this laboratory of a population of
prairie deermice also indicates the presence of
cysts similar to those seen in the guinea pig.
In man it is commonly accepted that the
serous cyst develops from the ovarian mesothelium (Novak and Woodruff, ’74). According
to Hertig and Gore (‘61), this conclusion is
based on the presence of germinal inclusion
cysts and infoldings of the ovarian mesothelium. These investigators have listed the rete
ovarii as a n “unlikely” origin. However, the
similarities in ovarian organogenesis in mammals and the reports of cystic enlargement of
the rete ovarii in humans (Sauramo, ’541, suggests that a re-evaluation of the origin of the
serous cysts in man might be appropriate.
Such a study is underway in this laboratory.
The mechanism of growth of these cysts has
not been resolved. The expansion or dilation of
the rete ovarii, an increase in the number of
epithelial cells, or a combination of these two
mechanisms are possible explanations. In the
present study mitotic figures were not observed. Whether this is due to the fact that
samples were taken only a t one time of the
day or that the dividing cells are few and the
division time is short has not yet been established. However, the presence of light and
dark cells of the tufted type may be indicative
of a cell cycle.
There are several other questions pertaining to the growth of these cysts which remain
to be resolved. No signs of secretory activity
were observed in the epithelial cells of these
cysts. This would suggest that the cyst fluid is
a plasma filtrate. We are unable a t this time
to state whether or not cystogenesis in the
guinea pig is hormonally influenced. It has
been reported, however, that in the heifer
cysts vary with the estrus cycle (Archbald e t
al., ’71).
The cystadenoma of the guinea pig described herein bears striking resemblance to
the benign serous cyst of the human (Gondos,
’71; Hertig and Gore, ’61). Both are composed
of a simple epithelium with ciliated and nonciliated cells. The benign cystoma of humans
also exhibits a smooth surface and may be
unilocular or multilocular. In the human,
however, there is a high frequency of malignancy among the serous cystomas (Hertig and
Gore, ’61). Malignant forms exhibit papillary
modification of the surface, stromal invasion,
a decrease in the number of ciliated cells and
irregularly shaped nuclei (Gondos, ”71).In the
ages examined to date, no signs of malignant
transformation have been observed in the
ovarian cysts of the guinea pig.
The increased size and number of cysts in
older specimens suggest that cyst formation is
a normal function of aging in the guinea pig.
In humans (Sauramo (‘54) reports that the
cystic appearance of the rete ovarii is more
prominant in older women. The fact that
small cysts are found in young animals suggests that cyst development is not incompatible with fertility. One 6-month-old female
which was pregnant when killed did exhibit
one serous cyst l m m in diameter. In this
female each horn of the uterus contained a
fetus; and the cystic ovary contained what appeared to be a functional corpus luteum.
Cystic development in the guinea pig is, therefore, unlike the microcystic syndrome of rats
in which there is ovulatory failure (Quadri et
al., '73). The high incidence of cysts does not
appear to be peculiar to the population of
guinea pigs used in this study. Preliminary
observations of ovaries from Hartley strain
guinea pigs from a second supplier indicates
similar cystogenesis.
Several studies of the cystic ovary of the
guinea pig are currently underway in this laboratory to determine the hormonal dependency, growth pattern, and potential for malignancy of these cysts. We believe that these
studies will enable the establishment of a
model for the study of the serous cystadenoma
in humans.
Albrecht, E. D., R. D. Kws and W. B. Wehrenberg 1975
Ovarian A5-3P-hydroxysteroid dehydrogenase and cholesterol in the aged mouse during pregnancy. Biol. Reprod., 13: 158-162.
Archbald, L. F., R. H. Schultz, M. L. Fahning, H. J. Kurtz
and R. Zemjanis 1971 Rete ovarii in heifers: A preliminary study. J. Reprod. Fert.. 26: 413-414.
Bullock, W. K., R. E. Houts and J. J. Gikane 1955 Ovarian
tumors. AMA arch. Surg., 71:153-166.
DeSanto, D. A., W. K. Bullock and F. J. Moore 1959
Ovarian cystomas. Arch. Surg., 78: 98-107.
Gillman, J. 1948 The development of the gonads in man,
with a consideration of the role of fetal endocrines and
the histogenesis of ovarian tumors. Carnegie Inst. Contrib. Embryol., 32: 81-131.
Gondos, B. 1971 Electron microscopic study of papillary
serous tumors of the ovary. Cancer, 27: 1455-1464.
Hertig, A. T., and H. Gore 1961 Tumors of the female sex
organs. Part 3. Tumors of the ovary and fallopian tube.
In: Atlas of Tumor Pathology. Sect. IX, Fasc. 33, AFIP,
Washington, D.C.
Ito, S., and M. J. Karnovsky 1968 Formaldehyde-glutaraldehyde fixatives containing trinitro compounda. J. Cell
Biol., 39: 168a.
Julian, C. G., J. Goss, K. Blanchard and J. D. Woodruff
1974 Biologic behavior of primary ovarian malignancy.
Obstet. Gynec.. 44:873-884.
Merchant, H. 1975 Rat gonadal and ovarian organogenesis with and without germ cells. An ultrastructural
study. Dev. Biol., 44: 1-21.
Mossman, H. W., and K. L. Duke 1973 Comparative Morphology
-. of the Mammalian Ovary. Univ. Wisconsin
Press, Madison, 461 pp.
Novak, E. R., and J. D. Woodruff 1974 Novak's Gynecologic
and Obstetric Pathology. Saunders, New York.
Quadri, S. K., G. S. Kledzik and J. Meites 1973 Reinitiation of estrous cycles in old constant estrus rats by central-acting drugs. Neuroendocrinology, 11: 248-255.
Quattropani, S. L. 1975 Microscopy of ovarian cords and
mesothelium in the fetal monkey (Macaca arctiods) and
fetal human. Anat. Rec., 181: 454 (Abstract).
Sauramo, H. 1954 Development, occurrence, function
and pathology of the rete ovarii. Acta Obstet. Gynec.
Scand. (Suppl. 2), 33: 29-46.
Wilkerson, W.V. 1923 The rete ovarii a s a normal structure of the adult mammalian ovary. Anat. Rec., 26: 75-78.
1 Mid-sagittal section of ovary of a near-term fetus showing a densely stained rete ova.
rii (R) cords of follicles (F)continuity of follicles with mesothelium (black arrowhead)
and with rete ovarii (white arrowhead). Paraplast, Hematoxylineosin. X 77.
2 Cysts of varying size tC) from the hilar reaon of the left ovary of a 1-year-oldguinea
pig. A dilated portion of the rete ovarii (or branched tubules, is seen a t D. Paraplast.
Hematoxylin-eosin. X 170.
3 This section is from the same specimen as figure 2, but is five sections removed and
shows the smooth surface of two cysts in the hilar region. X 70.
4 This is from the same section a s is seen in figure 2 and shows continuity
the two cysts shown in figure 3.
Steven L. Quattropani
Figures 5-10 are sections from a serially sectioned ovary from a 2-year-old animal. The
lumen of the cyst in figure 5 has been labeled X in figures 5-8. The lumen of the cyst in
figure 10 has been labeled Y in figures 6-10. Paraplast, Hematoxylin-eosin. X 166.
5 This is the first of the series or section 1.
6 Section 3.
7 Section 8
8 Section 10.
9 Section 12.
10 Section 14.
Steven L. Quattropani
11 This Epon section is from the ovary of a 1-year-old guinea pig. The different cell
types which comprise the cystic epithelium are seen: cell with solitary cilium (S),
hasophilic cell with tuft of cilia (TD), less basophilic cell with tuft of cilia (TL).
X 1,800.
12 This micrograph was taken with Nomarski optics. The specimen was fixed in the
aldehyde solution and a portion of a cyst was sliced and photographed in buffer. Note
the tufted cell (TI and the cell with the solitary cilium (S). X 1,800.
Steven L. Quattropani
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guinea, ovary, microscopy, pig, serous, light, origin, aging, cysts
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