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The oestrous cycle in the mare and some associated phenomena.

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THE OESTROUS CYCLE I N T H E M A R E AND SOME
ASSOCIA TED PHENOMENA
EDWIN SEABORN
London, Canada
CONTENTS
..........................
Introduction . . . .
Description of the cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Survey of the oestrous cycle , . .
.......................
A. Uterus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Ovaries . . . . . . . . . . . . . . . . .
................
Fundamental identity of the oestrou
mammals . .
..........................
Fertilieation . . . . . .
Pregnancy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Suprarenals .
.......................................
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bibliography
......................................
277
279
282
282
283
283
284
285
283
285
287
INTRODUCTION
A very favorable opportunity of studying the oestrous
cycle in the mare, together with the gross and microscopic
findings in the ovaries and uteri of these animals at definitely
known periods throughout the cycle, was afforded the writer
while in Paris, France, in the summer of 1923. Inasmuch
as such material is difficult to obtain under ordinary circumstances, especially material from mares where the exact day
in the oestrous cycle is known, it is felt that this study may
be of value to workers in this field, particularly since interest
in the phenomenon of oestrus in the mammals is now undergoing a marked reawakening.
The mares were kept in the stable under good conditions
and were under observation for some time, so that their
sexual behavior could be carefully noted and the exact time
of the oestrous period could be accurately determined f o r each
277
278
EDWIN SEARORN
animal. Willingness to accept the male was taken as an
indication of the oestrous period, and the animals were presented to the male daily, without allowing copulation to occur
except in two cases-one killed immediately and the other on
the second day following. The accurate determination of the
exact stage of the cycle is of the utmost importance, for it
enables us to correlate with the greatest exactness the external objective manifestations of the various stages with
the gross and microscopic anatomical changes of the sexual
and other organs taking place in these stages.
The oestrous cycle in the mare lasts twenty-four days. It
may be divided into a period of rest, lasting eight days; a
prooestrous period of three days, during which the animal
exhibits external evidences of heat, but does not accept the
male ; an oestrous period of three days, characterized by decrease in amount of mucous discharge from the vulva and
willingness to mate, and a metoestrous period of ten days.
Animals whose time of oestrus had been accurately determined were killed at various times throughout the cycle, so
that material from all stages was available. The uteri and
ovaries were removed, their gross appearance noted, and
microscopic preparations made from them. I n the gross examination of the uterus, it was first opened and the condition
of the mucosa carefully noted and recorded. Particular attention was addressed to its looseness and flabbiness, color,
vascularity, and oedematous condition. The ovaries of every
animal were weighed separately, both untouched and after
the follicles had been evacuated. The following report is
based upon the findings in this accurately timed and wellpreserved material. The Fallopian tubes, suprarenals, thyroids, hypophysis, and a portion of the vagina were also removed and examined.
I n all, nineteen mares were so studied. While all of these
could be utilized to show the progression of changes in the
cycle, for the sake of brevity only six will be described in
some detail, since these are all typical cases, illustrating all
the important stages of the oestrous cycle. The stage of the
279
OESTHOUS CYCLE I N THE MARE
cycle a t which the mare was killed and the number of each
specimen are given in the following table :
Fourteenth day following oestrus (rest),
First day of prooestrus,
First day of oestrus,
Third day of oestrus,
Second day following oestrus,
Eighth day following oestrus (metoestrus),
mare
mare
mare
mare
mare
mare
no.
no.
no.
no.
no.
no.
404
398
4\59
51
468
416
DESCRIPTION O F THE CASES
Mare 404-kiJled
on the f o u r t e e f l t h d a y following oestrus
The opened uterus shows a mucosa of a faint bluish color,
containing a minimum of moisturc. The folds of the mucosa
are not voluminous and do not hang down of their own weight
when the uterus is held reversed. When the mucosa is cut,
the tissue does not spread away from the cut edge and extend
itself on the table-a process which we have called ‘flowing.’
The blood vessels of the cut mucosa are not visible with the
naked eye.
Under the microscope, sections show the following characteristics: the superficial cells are cuboidal, with a small
content of cytoplasm; the proportion of stroma is small and
there is an almost total absence of lymphocytes.
The left ovary weighs 55 grams and contains four small
Graafian follicle cysts, together with a corpus luteum very
f a r advanced in degeneration. After evacuation of the cysts
the ovary weighs 47 grams.
The right ovary weighs 38 grams and contains six small
Graafian follicle cysts. After emptying the latter the ovary
weighs 31 grams. The uterus and ovaries are considered
to be in that state of low activity characterized as ‘rest.’
Mare 398-killed
041,the
firsl d n y of prooestrus
The uterine mucosa is of faint bluish color and is somewhat
firm in texture. I t is not pendulous and on section does not
‘flow.’ XJnder the microscope, we find great mitotic activity
in the epithelium and beginning degeneration of the superficial stroma. Lymphocytes are frequently seen.
280
EDWIN SEABORN
The left ovary weighs 33.50 grams and, after emptying the
cysts, 29.50 grams. The right ovary weighs 28.50 grams with,
and 25.50 grams without, the fluid of the follicles. Neither
ovary shows any enlarging Graafian follicle, nor any follicle
which is approaching the hilum of the ovary, as occurs in all
cases in which a follicle is about to rupture (Seaborn and
Champy, ’23).
Mare 459-killed
o n thc first d a y of oestrus
The uterine mucosa is covered by a small amount of nearly
colorless fluid, which under the microscope shows no blood
cells o r other formed elements. When the uterus is reversed,
the folds do not hang down of their ow7n weight, but they are
more voluminous than in mares 404 and 398. The mucosa is
rosy in color. The main characteristics, under the microscope, a r e : degeneration in the cells of the superficial epithelium, collagenous degeneration of the stroma, and increase
in the number of lymphocytes.
The left ovary contains four small Graafian follicle cysts
and a very large follicle which is on the point of rupturing.
An exceedingly thin membrane marks the point of imminent
rupture. The ovary weighed 126 grams and, after evacuation
of the cysts, 80 grams. The fluid of the mature Graafian follicle weighed 39 grams. On microscopic examination the walls
of the large follicle showed lutein transformation of the cells
of the theca interna and of the membrana granulosa.
The right ovary weighed 70 grams before, and 65 grams
after sectioning. It contained five small cysts and a degenerated corpus luteum.
,Ware 51-killrd
o n the third day of oestrus
The uterine folds are most vo1umiiious, are semigelatinous,
and show collections of a yellow fluid under the surface.
There is a small amount of fluid free i n the uterus, but this
fluid does not contain blood cells or other formed elements.
On section the mucosa ‘flows,’ and the blood vessels and
OESTROUS CYCLE I N THE MARE
281
ducts of the glands are visible with the naked eye. On reversing the uterus, the folds hang down of their own weight.
The main microscopic characteristics are : the irregularity
of the epithelial cells, increase in degeneration of the stroma,
and great increase in the number of lymphocytes.
The left ovary shows a large Graafian follicle which has
just ruptured and from which fluid can be expressed in a
small stream. The point of rupture is partially occluded by
a minute blood clot. The ovary weighed 132 grams, and
decreased to 75 grams upon evacuation of the cysts.
The right ovary weighed 124 grams and, after evacuation
of the cysts, 72 grams. It contained several large cysts and
a corpus luteum in regression.
illare 468-killed
on the second day following oestrus
The uterus shows less moisture and is not as gelatinous;
when reversed, the folds hang down much less than in no. 53,
of the third day of oestrus. In the cut edges the blood vessels and ducts of the glands can be indistinctly made out.
The main microscopical characteristics are : lessening of
the proportion of stroma to gland tissue, decrease in degeneration of the stroma as shown by diminution of the collagenous tissue, and reduction in the number of lymphocytes.
The left ovary weighed 120 grams, contained eight cysts
each the size of a cherry, and a corpus luteum 2 em. by 3 em.,
which consisted of an outer zone of a mottled dark plum color,
with blood vessels coursing through it, and an inner zone of
a purplish-red color and a consistency of jelly.
The right ovary contained several moderate-sized cysts. It
weighed 101 grams and, after sectioning, 75 grams.
Mare 416, kiLled
0%
the eighth day followirzg oest'rus
The uterus does not show the yellow color of no. 51, of
the third day of oestrus, but is bluish, resembling no. 404,
showing the condition of rest, of the fourteenth day following
oestrus. It contains no free fluid. When the uterus is re-
282
EDWIN SEABORN
versed, the folds hang down of their own weight for a short
distance only.
The main microscopic characteristics are : continued lessening of the proportion of stroma to gland tissue; the more
regular form of the cells of the surface epithelium and of
the glands, and reduction in the lymphocytes.
The left ovary weighed 62 grams and, after sectioning, 55
grams. It contained a few small cysts.
The right ovary weighed 112 grams and contained several
cysts and a corpus luteum showing an outer zone of a dark
plum color, and an inner zone of a light yellow color and the
consistency of jelly. After evacuation of the cysts, the ovary
weighed 80 grams.
S U R V E Y O F THE OESTROUS CYCLE
B. Uterus
Surveying the uterine mucosa of the entire oestrous cycle
of the mare, we find in the rest period a relatively thin bluish
membrane, which upon the onset of prooestrus gradually
changes to rosy color, becomes moister, and shows thickening
and increase in size of the folds. As the three-day period of
oestrus wears on, the much-thickened mucosa becomes yellowish in color, with heavy, wet, and flabby folds. The third day
of oestrus having been left behind, the mucosa returns by
degrees t o normal, there being evident a thinning and dehydration of the mucosa, a shrinking of the folds, a reduction
in size of the blood vessels and gland ducts, with a change in
the general appearance from pink to bluish.
The uterine mucosa shows microscopically, in the prooestrus stage, an awakening growth activity in the epithelial
cells manifesting itself in numerous mitotic figures. This proliferation is followed by degeneration of the epithelial cells
in the oestrous period, with gradual return to normal following this. As prooestrus progresses, there is considerable
collagenous degeneration in the stroma, especially superficially, which is even more pronounced in oestrus, and which
OESTROUS CYCLE I N THE MARE
283
gradually gives way to the normal condition in the metoest ruum.
Lymphocytes, scanty in rest, become progressively more
numerous until the end of oestrus, when their number is gradually reduced following this stage.
B. Ovaries
Briefly considering the changes in the ovaries which are
correlated with the above-described uterine phenomena, we
find during oestrus a considerable increase in the weight of
these organs, this being due to the augmentation of fluid in
the ripening follicle and hypertrophy of ovarian tissue. Rupture of the single ripened follicle occurs during the second
or third day of oestrus; since this is the case, it is evident
that the corpus luteum arising from this follicle cannot have
initiated the prooestrual and oestrnal uterine changes immediately preceding the rupture, nor can it be held accountable for the sexual behavior of the animal during these stages.
All of these phenomena would seem more probably to be related to a hormone of the liquor folliculi or the follicular
epithelium, as found by Allen et al. to be the case in spayed
mice and rats injected with extract from the liquor folliculi
of the pig. Champy and I ('23) brought about the occurrence of rut in unspayed rabbits by injecting into them liquor
folliculi from ripe follicles of the mare. I n this connection
it was noted that in the mare's ripe follicles lutein transformation of the cells of the theca interna and of the membrana
granulosa was found as early as the first day of oestrus.
Lutein cells disappear from the corpus luteum when the
uterus returns to rest.
FUNDAMENTAL IDENTITY O F THE OESTROUS CYCLE I N T H E
VARIOUS MAMMALS
It would appear, from a review of some of the most outstanding contributions to the subject, that the cycle of oestrus
in the mare is essentially the same as that in the other mammals. Stockard and Papanicolaou for the Guinea-pig, Mar-
284
EDWIN SEABORN
shall and Robinson for the ferret, Givkovitch for the sow,
Hill and O’Donoghue for Dasyurus viverrinus, Van der
Stricht, Van Beneden, Courrier, Benecke, and Eimen for the
bat, Heape for the monkey, and Schroeder for man, together
with many others, have recorded their observations, and
although they use different names for the various stages of
the cycle, and although the cycle shows wide individual variations, as in such animals as the bat and the Guinea-pig, yet
the fundamental ground-plan is the same throughout the
mammals, as shown by a careful perusal of their gross and
microscopic findings, coupled with the consideration of the
sexual behavior of the animals studied. Most interesting
of all, perhaps, is the cycle in the human female, which has
been a subject f o r speculation for so many ages and about
which so many erroneous ideas have gained credence.
Although there are some features of it which are obscure when
interpreted with reference t o the cycle in other mammals,
yet it seems evident that the stages which have been described in the mare are all represented in the woman and in
the same order. Thus the oestrous cycle of the lower animals
is equivalent to the menstrual cycle in women. This view is
in line with the statement of Corner in his recent excellent
review of oestrus, ovulation, and menstruation (p. 479) : “it
may fairly be stated that the trend of recent work is to suggest the basic similarity of the primate reproductive cycle t o
that of the other mammalia.” It seems probable that oestrus
of the mare includes more than three periods of the cycle in
of
woman, according to Schroeder ’s terminology-‘middle
interval, ’ ‘end of interval, ’ ‘beginning of pre-menstruum,’
and part of ‘end of pre-menstruum.’ This would push menstruation proper into the metoestruum of the mare.
FERTILIZATION
It has been noted that the signs of ‘heat’ last some six days,
but that during the first three days, the prooestrous stage, the
mare does not accept the male willingly. Ordinarily, if a
male be available, the mare is covered as soon as oestrus
OESTROUS CYCLE I N T H E MARE
285
proper begins. Rupture of the Graafian follicle, as noted,
occurs during the second o r third day of oestrus properthus it is a day or so after the usual time of copulation. The
distance from the external 0s uteri to the ampulla of the
Fallopian tube is some 48 em., and it seems probable that
the spermatozoa would take one o r two days to travel this
distance. Thus, in the ordinary course of events, the spermatozoa and ovum would arrive in the ampulla at about
the same time.
PREGNANCY
If the ovum lives, i.e., if it is impregnated, the mucoitl
degeneration and swelling of the stroma continue and the
proportion of stroma to gland tissue becomes very much
greater. This increase of degeneration was shown in mare
no. 32, killed on the fourth day following her oestrus. She
had been covered twice during the oestrus. If the ovum dies,
however, the degenerated stroma is gradually absorbed, the
lymphocytes disappear, and the uterus in time returns to a
condition of comparative rest.
8 IJPRAKENALS
As to the changes in other organs of the mare during the
oestrous cycle, nothing will be said here except to remark
that the suprarenal glands in some cases increase in weight
during the oestrous period by a s much as 100 per cent. Macrnscopically, the glands show an increase of the yellow material
which forms the peculiar rays of the cortex. Microscopically,
the principal changes consist in an increase of the spongiocytic tissue of the cortex. This tissue is probably formed by
the dissolving of the labile fats by the reagents employed in
the preparation of the specimen.
CONCLUSION8
1. The oestrous cycle in the mare lasts twenty-four days
and can be divided into four periods: rest-about eight days;
days, and metoesprooestrus-three
days ; oestrus-three
trus-about ten days.
286
EDWIX SEABORN
2. ‘Heat’ lasts six days, and in its first half, the prooestrous
stage, although the signs are well marked, there is an unwillingness to accept the male; in the second half, or oestrus
proper, the mucous discharge from the vulva has diminished
and the mare is willing t o accept the male.
3. I n heat there are well-marked uterine changes. In the
prooestruum the mucosa becomes hypertrophied, there being
mucoid degeneration of the stroma, multiplication of the
epithelial cells, and augmentation of the lymphocyte content.
In the oestruum these changes are very outspoken, and on
the second and third days, synchronous with the rupture of
the Graafian follicle, the mucosa is very thick, wet, and yellowish, with heavy, flabby, and loose folds. Blood vessels
and gland ducts are seen with the naked eye and there is much
degeneration of the epithelial structure and of the stroma.
These changes appear to be in the nature of a preparation
of the uterus for the reception of the fertilized ovum.
4. Correlated with these uterine phenomena we find an
increase in weight of the ovaries in heat, the ripening of a
follicle, and, at the middle o r end of oestrus, ovulation. The
suprarenals may double their weight in some cases.
5. Since the corpus luteum is not found until after oestrus
is well advanced and degenerates before the next oestrus
begins, it is probably not concerned with the initiation of
oestrus. The uterine changes and sexual behavior assocciated with heat would seem more probably to be an effect of
the influence of the maturing Graafian follicle.
6. Since ovulation doesnot occur until the middle or end
of oestrus proper, the probability of fecundation is increased,
doubtless, if the mare is covered during the first day that
she is willing to accept the male, for this allows time for the
sperm to make their way up the uterus and tube.
7. The oestrous cycle in the mare is fundamentally identical with that in other mammals and also with the menstrual
cycle of woman.
I n conclusion, I wish to extend thanks to Institut Pasteur,
University of Paris, St. George’s Hospital of London, and
rniversity of Western Ontario.
OESTROUS CYCLE I N T H E MARE
287
BIBLIOGH APHY
BYRONF. FRANCIS,
LEROYL. ROBERTSON,
CLEON E. COLGATE,
ALLEN, EDGAR,
CHARLESG . JOHNSTON,
EDWARD
A. DOISY, WILLIAMB. KOUNTZ,AND
HARRYV. GIBSON 1924 The hormone of the ovarian follicle; its
localization and action in test animals and additional points bearing
upon the internal secretion of the ovary. Am. Jour. Anat., vol. 34,
no. 1, pp. 133-181.
BENECKE1879 Ueber Reifung uiid Befruchtung des Eies bei den Fledermausen.
Zool. Anz., Bd. 2.
CORNER, GEO. W. 1923 Oestrus, ovulation and menstruation. Physiological
Reviews, vol. 3, no. 4, pp. 457-482.
COURRIER,R. 1921 S u r le r81e physiologique des s6crBtions ut6rine et tubaire
chez la chauve-sonris hibernante. Comp. rend. SOC.de Biol., T. 84,
pp. 572-574.
EIMER 1879 Ueber die Fortpflanzung rler Fledermsuse. 2001. Anz., Bd. 2.
HEAPE, W. 1894 The menstruation of Semnopithecus entellus. Phil. Trans.
Royal Soc. B., vol. 185.
__-_
1897 The menstruation and ovulation of Macacus rhesus. Phil.
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1900 The sexual season of mammals. Quar. Jour. Mic. Sci., vol. 44.
HIILL,J. P., AND c. H. O’DONOGIIUE 1913 The reproductive cycle in the marsupial Daryurus viverrinus.
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pp. 133-174.
GIVKOVITCH 1914 Nidation in women a n d mammals. These, Nancy, 1913 and
1914, no. 8.
MARSHALL,F. H. A. 1904 The oestrus cycle in the common ferret. Quar.
Jour. Mic. Sciences, rol. 48, pp. 323-345.
ROBINSON,ARTHUR 1918 The formation, rupture and closure of ovarian follicles in ferrets a n d ferret-polecat hybrids, and some associated phenomena. Trans. of the Royal Society of Edinburgh, vol. 52, p a r t 2
(no. 13).
SEABORN,
E., ET CHAMPY,CH. 1023 Structure de l’ovaire de la Jument et
son cycle Bvolutif en dehors de la gestation. Comptes Rendus des
SQances de la SociCtB de Biologie. T. 89, no. 34, Dee. 1st.
SCHROEDER,
R. 1914 Revue de Gynecologie et de Chirurgie abdominal. Feb.,
1914.
STOCKARD,
C. R., AND PAFANICOLAOU,
G. N. 1917 The existence of a typical
oestrous cycle in the guinea-pig, with a study of its histological and
physiological changes. Am. Jour. Anat., vol. 22, no. 2, pp. 225-285.
VAN BENEDENET JULIN
1880 Observations sur la maturation la fecundation,
et la segmentatipn de l’oeuf chez les Cheiroptsres. Arch. de Biol., T. 1.
VAN DER STRICHT,
0. 1906 Les mitoses de maturation de l’oeuf de chauvesouris. M6moire prBsentC au V I I I e Congres de l’Assoc. des Anatomistes, Nancy, 1906.
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