J Toxicol Pathol 2006; 19: 169–177

Original Changes in the Reproductive Organs Depending on Phases of Reproductive Cycle and Aging in Female Cynomolgus Monkeys

Dai Watanabe1,2, Toru Hoshiya1, Junko Sato1,#, Yuko Yamaguchi1, Kohsuke Horiguchi1, Yoshikazu Nagashima1, Azusa Okaniwa1, and Hiroyasu Yoshikawa2

1Kannami Laboratory, Bozo Research Center Inc., 1308 Kuwahara-Sanbonmatsu, Kannami-cho, Tagata-gun, Shizuoka 419–0101, Japan 2Department of Veterinary Pathology, School of Veterinary Medicine and Animal Science, Kitasato University, Towada-shi, Aomori-ken 034–8628, Japan #Present: Mitsubishi Chemical Safety Institute Ltd., 14 Sunayama, Kamisu-shi, Ibaraki-ken 314–0255, Japan

Abstract: In order to prepare background data for toxicity studies, reproductive organs of a total of 106 purpose-bred female cynomolgus monkeys 2.0 to 9.0 years of age were surveyed for the sequence of events during reproductive cycles. Based on histological features, these animals were classified into groups according to the six phases of the reproductive cycle in addition to a group at the immature stage. Follicular phases were characterized by development of follicles and luteal phases by development of functional . Proliferation of endometrial glands started at the follicular phase and reached the maximum at the luteal phase. Features in the menstrual phase were almost the same as those in the follicular phases except for the shedding of superficial surface of . The percentage of animals in the phases of follicular development was 51%, and those in luteal development 46%, indicating roughly equal duration of both phases of the reproductive cycle. Absolute and relative weights of the and were lowest in early follicular phase and gradually increased and reached the maximum in the early luteal phase, then decreased to the level of the early follicular phase. The difference in weight between the bigger and smaller ovaries, remarkable between 5.0 to 5.5 years of age, was at the maximum in the early luteal phase. The absolute weight of the ovaries and uterus increased rapidly and reached its peak between 5.0 to 5.5 years of age and remained unchanged thereafter, although the body weight gradually increased up to 9.0 years of age. (J Toxicol Pathol 2006; 19: 169–177) Key words: reproductive cycle-related changes, cynomolgus monkeys, ovaries, uterus

Introduction of each phase of the reproductive cycle by means of various in-life observation and examinations is impractical in routine Considering that few detailed descriptions about toxicity studies. Because the reproductive organs are subject normal cyclic changes of the female reproductive organs are to remarkable changes during the reproductive cycle, available1, the present survey was undertaken to elucidate abundant background data are essential for precise sequential changes in the weight and histology of the major assessment of findings on the reproductive organs. In reproductive organs associated with reproductive cycles and addition, cynomolgus monkeys used in toxicological aging in female cynomolgus monkeys kept in an animal research are frequently at the age of adolescence and laboratory environment at our research facilities. We therefore their reproductive organs are functionally as well concentrated on identification of each phase of the as morphologically in the course of development. reproductive cycle by means of morphology and weight changes in the major reproductive organs, as determination Materials and Methods

Animals and histological specimens Received: 19 May 2006, Accepted: 12 October 2006 The present study examined a total of 106 female Mailing address: Dai Watanabe, Kannami Laboratory, Bozo Research cynomolgus monkeys 2.0 to 9.0 years of age. Seventy-three Center Inc., 1308 Kuwahara-Sanbonmatsu, Kannami-cho, Tagata-gun, per cent of them were between 3 and 5.5 years of age. The Shizuoka 419–0101, Japan TEL: 81-559-78-7851 FAX: 81-559-78-7898 number of animals and body weight for each age-group are E-mail: [email protected] shown in Table 1. These animals were purpose-bred and 170 Reproductive Organs of Female Cynomolgus Monkeys

Table 1. Body Weight of Monkeys in Each Age group Table 2. Major Morphological Characteristics of Reproductive Organs in Immature Stage and in Each Phase of Age group No. of Range Mean Median Reproductive Cycle (years) Animals Stage and phases Major characteristics 2.0–3.0 6 2.27–3.11 2.73 2.83 3.0–3.5 16 2.20–3.65 2.77 2.65 Immature Thickness of endometrial layer about 1/2 of that in 3.5–4.0 18 2.13–3.93 2.88 2.78 follicular phase; cornification of vaginal epithelial 4.0–4.5 20 2.12–5.83 3.43 3.37 layer incomplete, primordial follicles conspicuous. 4.5–5.0 12 2.57–5.51 3.33 3.13 Menstrual Hemorrhage and shedding of superficial surface of 5.0–5.5 11 2.51–5.90 3.48 3.35 (Early follicular) endometrial layer, and menstrual corpus luteum 5.5–6.0 9 2.53–4.12 3.16 3.02 with vacuolated luteal cells. 6.0–6.5 9 2.73–5.95 4.22 4.04 Early follicular Endometrial layer: thinner than that in follicular 6.5–9.0 5 3.17–7.62 4.34 3.68 phase and a few endometrial glands running straight. Development of many follicles, some of Unit: kg them are undergoing atresia, and vacuolation of luteal cells is marked in the ovaries. Follicular Thickened endometrial layer, but endometrial glands only slightly coiled. imported from the People’s Republic of China and Pre- Thickened endometrial layer, and a large maturing Nafovanny, Vietnam, subject to quarantine in accordance follicle. with national regulations on import quarantine. The animals Early luteal Corpus luteum with central cavern and hemorrhage. were kept at our facility and were housed in individual cages Beginning of coiling and dilation of endometrial (750 × 750 × 700 mm) under routine controlled conditions: glands prominent. 18 to 28°C; relative humidity, 30 to 80%; lighting, 12 hours Luteal Highly developed functional corpus luteum, and per day. They were provided a pellet diet (restricted feeding markedly coiled endometrial glands. 150 g/animal/day, PS, Oriental Yeast Co. Ltd., Tokyo, Japan), and tap water ad libitum per an automatic supply system. All animals were non-treated animals collected He compared primate menstrual cycles with estrous cycles in from control groups used in toxicological studies during the other species. Our classification of phases of reproductive years 1996 to 2002. All experimental procedures were done cycles, however, is not necessarily identical with current in accordance with the animal welfare guidelines of Bozo functional classification consisting of follicular and luteal Research Center Inc. All the histological specimens of the phases. For example, the phase of menstrual hemorrhage, ovaries, uterus and were hematoxylin eosin stained usually included in the early follicular phase, is dealt with preparations prepared according to routine laboratory separately in the present study because of its morphological procedures. Prior to preparation, the ovaries (each side characteristics. separately) and uterus were weighed. Presentation of data Identification of each phase of the reproductive cycle Morphological characteristics of the major reproductive The sign of the phase of the reproductive cycle organs used as criteria for identification of the immature discernible from routine cage-side observation is menstrual stage and each phase of the reproductive cycle are hemorrhage. Definite periodic menstrual cycles, however, summarized in Table 2. Each individual classified according could be recognized only in 29 out of 106 animals in the to the above criteria was examined for (1) the relationship present study, and they ranged between 26 days and 43 days between each phase and the weights of the ovaries and with 31 days on average with 2 exceptional animals (90 and uterus, (2) the relationship between weights of the ovaries 99 days). In the other 74 animals, no periodic menstrual and uterus and age, and (3) the relationship between the age hemorrhages were recognized. However, these figures do and sexual cycle. All the absolute and relative organ weight not mean that about 70% animals in the present survey data per reproductive phase-group and per age-group are underwent no reproductive cycle. Because of individual expressed as range, group mean and median. The ovaries difference in the amount of menstrual hemorrhage and from each individual were regarded separately according to behavioral nature, routine cage-side observation provides no their weight, larger and smaller. precise results. Some animals licked up the menstrual hemorrhage. In fact, morphological data revealed that all the Results animals were in a certain phase of the reproductive cycle. From these circumstances, data were arranged exclusively Histological characteristics and criteria of the major based on histological pictures of the reproductive organs reproductive organs for identification of the immature representing the six phases of the reproductive cycle, stage and the six phases of the reproductive cycle (Fig. especially that of the ovaries and uterus in the present study. 1) The above description of the six phases of the reproductive Immature stage: In the (Fig. 1A1), primordial cycle depends for the most part on the textbook of Banks2. follicles were conspicuous and first and secondary follicles, Watanabe, Hoshiya, Sato et al. 171

Table 3. Number of Animals in Immature Stage and in Each Phase of Reproductive Cycle in Each Age Group

Age Group* Phase Immature Menstrual Early follicular Follicular Pre-ovulation Early luteal Luteal (3)** (Early follicular) (4) (34) (9) (11) (38) (7)

2 to 3 (6)** 3*** 1 2 3 to 3.5 (16) 2 1 10 3 3.5 to 4 (18) 2 6 2 1 7 4 to 4.5 (20) 6 2 1 11 4.5 to 5 (12) 1 4 1 2 4 5 to 5.5 (11) 5 1 3 2 5.5 to 6 (9) 1 2 1 5 6 to 6.5 (9) 3 1 1 2 2 6.5 to 9 (5) 1 1 1 2

*: Age in years. **: Number of animals. ***: Number of animals in respective groups. some of which were in the atresic process, were developing. formation of typical functional corpus luteum (Fig. 1F1). The endometrium (Fig. 1A2) was still in an underdeveloped Lutein cells in the functional corpus luteum were state with a simple and basophilic glandular structure. The characterized by acidophilic cytoplasm, occasionally faintly vaginal mucosa (Fig. 1A3) was thin exhibiting incomplete stained at the periphery and clear round nucleoli. Abundant cornification. After maturation, the vaginal mucosa was blood capillaries were seen among these lutein cells. thick with distinct cornification and exhibited no apparent Remarkably proliferated endometrial glands exhibited changes which suggested cyclic changes during estrous typical pictures of coiling (Fig. 1F2). cycles. Early follicular and menstrual phases: In the ovaries, many follicles in varying developmental degrees were Frequency of each reproductive phase in each age- recognized mainly on one side while the corpus luteum group contained coarse vacuoles among lutein cells on the other The frequency of each reproductive phase found in each side (Fig. 1B1). Menstruation occurs during the early age-group is shown in Table 3. While the frequency of the follicular phase. In the uterus, hemorrhage following follicular phase was high in the group 3.0 to 3.5 years of age, desquamation of superficial endometrium characterized this the frequency of the luteal phases was somewhat high in the phase (Fig. 1B2). Thereafter, the endometrium (Fig. 1B3) older groups. The frequencies of both the follicular and was thin with a poorly developed simple straight glandular luteal phases were, on the whole, similar, reflecting the structure. Follicular phase: Development of large secondary duration of both phases. Minimum elapsed time from the follicles was characteristic. The corpus luteum in the first day of menstruation to each phase of the reproductive process of regression became smaller with irregular contours cycle was 3, 4, 10, 10 and 15 days for the early follicular, among newly developing follicles and corpus luteum as well follicular, pre-ovulation, early luteal and luteal phases, as stromal tissue adjacent to them (Fig. 1C1). The above respectively. regressive corpus luteum was finally replaced by connective tissue as the corpus atreticum. The endometrium thickened Changes in ovarian and uterine weights during the with swollen and proliferating glandular cells though its reproductive cycle and with aging glandular structure was still mostly running straight (Fig. Changes during the reproductive cycle: The absolute 1C2). Pre-ovulation phase: A large Graafian follicle (Fig. weights of both organs tended to increase from the early 1D1) and thickened endometrium were characteristic. follicular to the early luteal phases (Table 4, Fig. 2A). In Features of the endometrium were almost the same as the primates, usually only one follicle among many developing previous phase (Fig. 1D2). Early luteal phase: This phase at the same time grows finally to the mature follicle and was characterized by so-called “corpus hemorrhagicum” and others undergo atresia during development. Therefore one proliferative endometrium. Swollen granulosa lutein cells ovary bearing the growing follicle is larger and heavier than surrounded residues of hemorrhage from rupture of the the other, especially in the pre-ovulation phase. After the Graafian follicle and were actively proliferating (Fig. 1E1). ovulation, the weight of the larger ovary increased due to Endometrial glands increased in number exhibiting active luteinization leading to the formation of an active beginnings of coiling (Fig. 1E2). Luteal phase: The corpus corpus luteum. The active corpus luteum slowly luteum and endometrium most markedly developed in this degenerated toward follicular development, resulting in stage. Hypertrophied and proliferated granulose lutein cells decreased weight. Consequently, the difference in the occupied young cystic corpus luteum and resulted in weight of the ovaries became larger towards the luteal phase; 172 Reproductive Organs of Female Cynomolgus Monkeys

A1 A2 A3

B1B2 B3

C1 C2 Watanabe, Hoshiya, Sato et al. 173

D1 D2 E1

E2 F1 F2

Fig. 1. Typical pictures of ovary (A1), uterus (A2) and vagina (A3) of animals in the immature stage. (A1) Many primordial follicles are seen in the cortical area. Secondary follicles in varying degrees of development and undergoing atresia are seen, × 40. (A2) Development of uterine glands is lacking, × 40. (A3) Epithelial layer is thin and there is almost no sign of cornification, × 40. (B1) Ovary of animals in the early follicular phase, many luteal cells contain coarse vacuoles, × 100. Uterus (B2, B3) of animals in the menstrual and early follicular phases. (B2) Shedding of the endometrium leaving the basal layer with dense infiltration of small round cells, × 40. (B3) Uterine glands exhibit a short and still underdeveloped profile, × 40. Ovary (C1) and uterus (C2) of animals in the follicular phase. (C1) Many follicles in varying degrees of development are seen, × 40. (C2) Thickened endometrium and many profiles of uterine glands are seen, × 40. Ovary (D1) and uterus (D2) of animals in the pre-ovulation phase. (D1) A portion of Graafian follicle is seen together with atretic follicles, × 40. (D2) Thickened endometrium with some signs of coiling of endometrial glands, × 40. Ovary (E1) and uterus (E2) of animals in the early luteal phase. (E1) Portion of corpus hemorrhagicum exhibiting active proliferation of granulosa lutein cells, × 100. (E2) Actively proliferating endometrial glands meander among abundant stromal cells, × 40. Ovary (F1) and uterus (F2) of animals in the luteal phase. (F1) Typical picture of functional corpus luteum indicating mass of hypertrophic granulosa lutein cells, × 100. (F2) Conspicuous coiling of endometrial glands, × 40. HE stain. 174 Reproductive Organs of Female Cynomolgus Monkeys

Table 4. Absolute Organ Weights in Immature Stage and in Each Phase of Reproductive Cycle

Organs Phase Immature Menstrual Early follicular Follicular Pre-ovulation Early luteal Luteal (3)* (Early follicular) (4) (34) (9) (11) (38) (7)

Ovaries (unit: mg) Smaller Range 67–78 108–219 68–153 47–282 56–165 101–224 70–186 Mean 72 161 101 121 121 134 122 Median 70 160 70 108 125 140 168 Larger Range 77–93 130–304 86–238 56–290 157–349 211–350 127–339 Mean 87 205 148 150 225 266 223 Median 90 192 153 137 196 264 220 Both (Total) Range 144–171 238–523 159–349 103–572 216–455 335–520 204–519 Mean 159 366 254 271 346 420 345 Median 160 332 252 242 296 412 341

Uterus (unit: g) Range 1.67–2.63 3.65–8.71 3.20–6.86 1.34–11.79 4.59–11.08 4.93–14.51 4.84–12.12 Mean 2.27 6.03 5.07 5.33 7.04 8.99 8.04 Median 2.51 5.18 3.93 4.80 6.83 8.98 7.38

*: Number of animals.

Table 5. Relative Organ Weights in Immature Stage and in Each Phase of Reproductive Cycle

Organs Phase Immature Menstrual Early follicular Follicular Pre-ovulation Early luteal Luteal (3)* (Early follicular) (4) (34) (9) (11) (38) (7)

Ovaries (unit: mg%) Smaller Range 2.37–3.36 3.60–7.87 2.64–5.60 2.07–7.24 1.82–5.21 1.70–6.18 1.45–6.22 Mean 2.71 4.79 3.50 3.94 3.62 4.17 3.79 Median 2.41 4.40 2.82 3.68 3.69 4.51 3.72 Larger Range 2.72–4.01 4.58–9.37 3.25–6.89 2.47–8.06 4.82–8.84 4.12–9.76 2.72–11.69 Mean 3.28 6.07 5.15 4.92 6.62 7.12 6.95 Median 3.10 5.27 4.41 4.92 6.96 6.82 6.63 Both (Total) Range 5.09–7.37 8.37–17.25 5.89–12.49 4.54–14.56 7.60–13.80 6.08–14.61 4.18–16.41 Mean 5.99 10.87 8.66 8.87 10.43 11.29 10.72 Median 5.52 9.12 7.38 8.95 9.39 11.83 10.48

Uterus (unit: g%) Range 0.06–0.11 0.10–0.34 0.10–0.28 0.05–0.35 0.13–0.48 0.08–0.39 0.13–0.38 Mean 0.09 0.19 0.19 0.18 0.22 0.24 0.24 Median 0.09 0.17 0.12 0.16 0.20 0.26 0.22

*: Number of animals. the group-mean difference between the larger and smaller Changes during aging: The absolute weights of both ovaries in the immature stage and the menstrual, early smaller and larger ovaries tended to increase rapidly follicular, follicular, pre-ovulation, early luteal, and luteal reaching their peak between 5.0 and 5.5 years, then phases were 15, 44, 47, 29, 104, 132 and 101 mg, remaining unchanged thereafter, irrespective of the phase of respectively. The differences in the weights of the ovaries the estrous cycle (Table 6, Fig. 3A). The absolute weight of were small during follicular development, and then became the uterus also tended to increase rapidly up to around 5.0 to larger in the later phases. The weight of the uterus increased, 5.5 years of age, but gradually decreased thereafter. Because almost parallel to that of the ovaries, from the follicular body weight gradually increases up to 9.0 years of age, the phases to the luteal ones, and reverted again to the level at above shift in absolute weight of the ovaries and uterus, the follicular phase. Features of the above shift in relative especially after 5.5 to 6.0 years of age, became marked when weight of the ovaries and uterus were almost the same as these values were expressed as relative weights (Table 7, those in the absolute weight (Table 5, Fig. 2B). Fig. 3B). Watanabe, Hoshiya, Sato et al. 175

Fig. 2. Shift in absolute (2A) and relative (2B) weights of the ovaries Fig. 3. Shift in absolute (3A) and relative (3B) weights of the ovaries and uterus in each phase group. Sample numbers of each and uterus in each age group. Age in years. Sample numbers phase group are shown in Table 3. of each age group are shown in Table 1.

Discussion based on histological features of the ovaries and uterus. In addition, transitions of the ovarian and uterine weights Menstrual hemorrhage was recognized in less than 30% associated with the course of the reproductive cycle and of animals in in-life observation in the present study. Lack aging were studied. No similar attempt has been made up to of recognition of menstrual bleeding and/or prolonged the present time. Histological features of the ovaries and periods of amenorrhea were attributed to abnormally long uterus in various stages of the reproductive cycle were reproductive cycles3. In the present study, therefore, the mostly the same as those described in previous textbooks1,2 authors identified the six phases of the reproductive cycle and the literature4. 176 Reproductive Organs of Female Cynomolgus Monkeys

Table 6. Absolute Organ Weights in Each Age group

Organs Age group in years 2.0–3.0 3.0–3.5 3.5–4.0 4.0–4.5 4.5–5.0 5.0–5.5 5.5–6.0 6.0–6.5 6.5–9.0 (6)* (16) (18) (20) (12) (11) (9) (9) (5)

Ovaries (unit: mg) Smaller Range 67–128 61–257 68–167 77–220 78–211 77–282 89–219 101–185 76–224 Mean 78 114 109 125 135 150 134 145 142 Median 70 99 107 118 134 140 141 142 134 Larger Range 56–197 81–260 81–262 116–349 82–339 103–350 157–304 88–337 123–247 Mean 117 147 179 145 209 250 233 242 198 Median 90 131 170 177 196 276 242 213 207 Both (Total) Range 103–315 144–517 157–421 194–455 164–476 180–572 246–523 327–519 199–471 Mean 195 261 288 328 344 400 368 387 340 Median 160 255 266 306 346 412 364 365 341

Uterus (unit: g) Range 1.67–10.43 1.62–8.71 1.34–11.08 3.41–10.91 3.45–10.80 4.32–12.20 5.00–8.61 4.93–12.12 5.17–14.51 Mean 4.52 4.71 6.72 6.97 6.91 8.22 6.68 7.72 7.75 Median 2.63 4.28 6.38 6.70 6.61 7.97 6.84 6.86 7.74

*: Number of animals.

Table 7. Relative Organ Weights in Each Age group

Organs Age group in years 2.0–3.0 3.0–3.5 3.5–4.0 4.0–4.5 4.5–5.0 5.0–5.5 5.5–6.0 6.0–6.5 6.5–9.0 (6)* (16) (18) (20) (12) (11) (9) (9) (5)

Ovaries (unit: mg%) Smaller Range 2.07–3.36 2.55–7.87 2.43–5.68 1.45–6.29 1.96–5.42 2.52–5.00 2.56–6.22 1.70–5.60 1.76–6.09 Mean 2.71 4.07 3.81 3.83 4.13 4.26 4.24 3.72 3.57 Median 2.41 4.02 3.71 3.61 4.42 4.49 3.92 3.60 3.69 Larger Range 2.47–6.63 3.25–9.37 3.19–10.33 2.73–8.43 3.19–11.69 3.37–9.72 4.82–9.87 4.26–9.28 2.72–6.71 Mean 4.16 5.29 6.21 5.97 6.39 7.22 7.47 5.95 4.93 Median 3.10 4.58 6.06 6.07 5.42 7.95 7.38 5.27 5.02 Both (Total) Range 4.54–10.13 5.89–17.25 6.18–16.01 4.18–14.61 6.08–16.41 5.88–14.56 7.60–15.25 6.13–14.30 4.48–12.80 Mean 6.99 9.34 9.99 9.86 10.52 11.48 11.70 9.68 8.49 Median 5.52 8.73 9.40 9.63 10.84 12.43 12.70 9.12 6.28

Uterus (unit: g%) Range 0.06–0.34 0.07–0.34 0.05–0.48 0.13–0.35 0.11–0.31 0.07–0.33 0.15–0.34 0.08–0.33 0.13–0.39 Mean 0.16 0.17 0.24 0.21 0.21 0.25 0.22 0.19 0.21 Median 0.11 0.14 0.25 0.20 0.21 0.25 0.21 0.15 0.16

*: Number of animals.

Primate reproductive cycles have been described as 3.5 years of age, and many young animals were in the consisting of follicular and luteal phases, though various follicular phases. These findings suggest that their terminologies have been employed1,2,4. The short ovulatory reproductive cycles are still unstable and that juvenile phase occurs between the above 2 phases, and menstruation animals require a long time to develop follicles. occurs in the early follicular phase2. The percentage of Both the absolute and relative weights of the ovaries animals judged to be in the menstrual, early follicular, and uterus were lowest in the immature stage. They follicular and pre-ovulation phases was 51%, and those in gradually increased from the early follicular phase and the early luteal and luteal phases were 46% in the present reached the maximum in the early luteal phase, then study. These figures probably indicate, consistent with decreased to the level of the early follicular phase. The descriptions in previous textbooks1,2, that the phases of above transition in the weights of ovaries and uterus are follicular development and of luteal development are of ascribed to the development of follicle(s) resulting in roughly equal duration during the reproductive cycle. The ovulation and successive formation and regression of active proportion of animals, however, in the follicular phases to corpus luteum as well as corresponding changes in the those in the luteal phases was 14: 5 in younger animals up to uterus, mainly those in the endometrium. In the present Watanabe, Hoshiya, Sato et al. 177 study, both organs were remarkably heavier in the menstrual of the ovaries and uterus reached the maximum at 5.0 to 5.5 phase than in the late follicular phases. This difference in years of age, although the body weight increased up to 9.0 organ weight by phase of reproductive cycle probably years of age. From these results we confirmed that not only depends on the age of animals in both phase-groups. The histological features, but also weight data are essential for explanation is that 4 out of 7 or 3 out of 7 monkeys in the toxicological evaluation of the genital organs of female menstrual phase were more than 5.5 or 6.0 years of age, cynomolgus monkeys. Comparison of both ovarian weights respectively, whereas 23 out of 34 monkeys in the follicular provides useful information in assessing the phase of phase were less than 4.5 years of age. As will be mentioned follicular and luteal development. Younger animals less later, organ weights increased up to around 5.5 years of age. than 3.5 years of age are not suitable for assessment of toxic Both ovaries of each animal were dealt with separately insults to the genital organs because their reproductive by their weight, smaller and larger. The difference in weight cycles are still unstable. between the smaller and larger was at the maximum in the early luteal phase-group. Larger ovaries were probably References ascribable to the presence of a dominant follicle6. In dominance of a selected (dominant) follicle or its successor, 1. Yuan Y-A and Forey GL. Female reproductive system. In: the corpus luteum was reported to maintain its eminence Handbook of Toxicologic Pathology, 2nd ed. WM Haschek, bilaterally for the duration of the same menstrual cycle6. Our CG Rousseaux, and MA Wallig (eds). Academic Press, San present results on the shift of ovarian weight during the Diego, Calif. Vol. 2. 847–865. 2002. reproductive cycle roughly corresponds to the above theory 2. Banks WJ. Applied Veterinary Histology, 3rd ed. Mosby Year Book, St. Louis, Missouri. 456–460. 1993. proposed after extensive experimental studies. 3. Nawar MM and Hafez ESE. The reproductive cycle of the The age of puberty in cynomolgus monkeys was crab-eating macaque (Macaca fascicularis). Primates. 13: 5 reported to be 4.0 to 6.0 years . In the present study, the 43–56. 1972. absolute weights of the ovaries and uterus increased 4. Attia MA. Cyclic changes in genital organs and vaginal gradually up to 5.0 to 5.5 years of age, and then remained cytology in cynomolgus monkeys (Macaca fascicularis). unchanged or slightly decreased. Increase of both organ Dtsch. Tieraerztl. Wschr. 105: 399–404. 1998. weights was ascribed to increased remnants of atretic 5. Einspanier A and Gore MA. Reproduction: Definition of a follicles and corpus luteum in the ovaries and development primate model of female fertility. In: The Laboratory of smooth muscle cells and interstitial tissue in the uterus. Primate, S Wolfe-Coote (ed). Elsevier Academic Press, San In conclusion, the present survey revealed cyclic Diego, Calif. 105–117. 2005. changes in the weights of the ovaries and uterus which 6. Hodgen GD. The dominant . Fertility and Sterility. 38: 282–300. 1982. reflect cyclic follicular and luteal development. The weights