ESTROUS CYCLE, ANATOMY AND HISTOLOGY OF THE UTERINE TUBE
OF THE MONGOLIAN GERBIL (Meriones unguiculatus)

CICLO ESTRAL, ANATOMÍA E HISTOLOGÍA  DE LA TUBA UTERINA DEL
GERBO DE MONGOLIA (Meriones unguiculatus)

^*Camila C. D. Almeida; ^*Patrícia F. F. Pinheiro; ^*Tânia Mara Segatelli;
^** Marcelo Martinez; ^***Carlos Roberto Padovani & ^*Francisco E. Martinez

^*       Department of Anatomy, Institute of Biosciences, UNESP, Botucatu, SP, Brazil.
^**     Department of Morphology and Pathology, UFSCar, São Carlos, SP, Brazil.
^***   Department of Bioestatistics, Institute of Biosciences, UNESP, Botucatu, SP, Brazil

SUMMARY: We classified the estrous cycle of the Mongolian gerbil and described the structures and relations of the uterine tube in the pelvic abdominal cavity and the histology of its coating epithelium. From 19 adult female Mongolian gerbils the uterine tube has been investigated by anatomy and histology using intact tissue. A polyestrous and irregular cycle was observed, with five different phases: proestrus, estrus I, estrus II, metaestrus, and diestrus. Estrus I is characterized by a smaller number of dispersed keratinized cells compared to estrus II. The uterine tube is related to the distal extremity of the uterine horn and to the ovary and presents an outer serosal covering (peritoneal visceral lamina), a muscular tunica media consisting of smooth circular fibers and a mucosal layer with simple covering epithelium composed of secretory and ciliated cylindric cells. Histologically, it is divided into four different areas: intramural, isthmus, ampulla and infundibulum. The gerbil is not an ideal model for the study of the estrous cycle due to its irregularity. The anatomy and histology of the uterine tube is similar to that of rodents used as laboratory animals.

            KEY WORDS: 1. Estrous cycle; 2. Anatomy; 3. Histology; 4. Uterine tube; 5. Mongolian gerbil.

   INTRODUCTION

The Mongolian gerbil is a small rodent from the northeast desert areas of China and Mongolia. It belongs to the Cricetidae family, Gerbillinae subfamily, genus Meriones and Meriones  unguiculatus species. The species was recently introduced as a laboratory animal (Kress & Mardi, 1990). Because their docile nature, gerbils have been used in the rehabilitation of children with emotional disturbances, in vascular and renal function studies, laboratory diagnosis, sexual and parental behavior and in hormonal assays. However, few papers are available about the development and the genital system structure of females (Kress & Mardi). The studies conducted by Kress et al. (1989) and Kress & Mardi were the first attempts in this field. Therefore, the objectives of the present study were to classify the estrous cycle in the Mongolian gerbil (Meriones unguiculatus), and to describe the structure and the relations of the uterine tube in the pelvic abdominal cavity and the histology of its coating epithelium.

MATERIAL AND METHOD

Estrous cycle: Nineteen adult female Mongolian gerbils (Meriones unguiculatus) were used. Vaginal smears were collected on 20 consecutive days with sterilized cotton swabs moistened with physiological saline and introduced into the vagina with a slight rotating movement. The material adhering to the swab was then spread on a histological slide, fixed with 1:1 alcohol-ether and stained with hematoxylin-eosin (HE).

Anatomy: Six females in estrus were killed with ethyl ether and a midline sagittal incision made in each from the xiphoid process to the pubic symphysis. Superficially, the skin and the subcutaneous tissue were dissected and deeply, another middle sagittal incision was performed in the anterolateral muscular wall of the abdomen. Intestinal loops were displaced and an `en bloc' dissection of the female reproductive tract was carried out using a Zeiss stereoscopic microscope.

Histology: Five animals were sacrificed in estrus with ethyl ether. The reproductive organs, including uterine tube, ovary and cranial extremity of the uterine horn were collected, fixed in Bouin's solution for 24 h, at room temperature, dehydrated in a graded ethanol series, embedded in paraffin and cut into 7 µm-thick sections. The histological sections were stained with periodic acid Schiff (PAS), toluidine blue (TB) and HE, and examined and photographed with an Olympus BH 12 photomicroscope.

Morphometric Analysis: Six females in estrus were sacrificed with ethyl ether. The paraffin blocks containing the uterine tube were cut into 7 µm-thick sections, stained with HE and used to measure the area of the uterine tube lumen, and the area of the epithelium of the isthmus, ampulla and infundibulum regions. The number of mucosal folds was counted in three different regions. An image analyzing program (Optimas 4.10) was used to measure the variables under study.

Statistical Analysis: The data were analyzed by descriptive statistics (Vieira, 1990) and by the repeated measures technique, complemented with the construction of simultaneous confidence intervals (Morrison, 1990), with the level of significance set at p>0.05.

RESULTS

Estrous cycle: The Mongolian gerbil presents a polyestrous and irregular cycle lasting about 4 days with five different phases: Proestrus: predominance of epithelial cells, and a smaller number of clustered keratinized cells. Estrus I: dispersed keratinized cells in smaller numbers compared to estrus II. Estrus II: predominance of keratinized cell clusters and absence of leucocytes and mucus. Metaestrus: predominance of leucocytes, large numbers of keratinized cells and epithelial cell infiltrates; Diestrus: predominance of unstained mucus, leucocytes and epithelial cells (Fig. 1).

Anatomy:

The Mongolian gerbil presents a bicornuate uterus. The ovaries and the uterine tubes are located at the cranial extremities of the uterine horns (Fig. 2a). Between the cranial extremity of the uterine horn and the ovary there is a fibromuscular ligament (a band of connective tissue), the mesosalpinx, a reflection of the peritoneal visceral lamina that provides support to the uterine tube and contains blood and lymph vessels and nerves. The complex formed by the ovary and uterine tube is located in a cavity limited superiorly by the caudal pole of the kidney, inferiorly by the cranial extremity of the uterine horn, medially by the lateral margin of the psoas major muscle, laterally by the muscles of the anterolateral abdominal wall, anteriorly by loops of small intestine, and posteriorly by the quadratus lumborum muscle. The ovaries are kidney shaped and have two distinct poles, a cranial one and a caudal one. The uterine tube is preferentially located on the ventromedial ovarian surface, the region called hilum (Fig. 2b). The uterus, uterine horns, ovaries and uterine tubes are organs in contact with the peritoneal visceral lamina. The arterial vessels of the uterine tube arise from branches of the ovarian and uterine arteries and the veins follow the same pattern as the arteries.

Histology: The uterine tube of the gerbil is a long and convoluted tubular duct organized into three distinct layers: (1) internal mucosal layer with epithelium composed of secretory and ciliated columnar cells and a lamina propria of loose connective tissue, (2) a coat composed of two layers, and (3) external serosa, peritoneal visceral lamina. The uterine tube presents four distinct and consecutive regions: 1st Intramural: the mucosa has longitudinal folds reduced to low ridges in the lumen and the muscle layer is thicker than the other regions, 2nd Isthmus: the folds of the longitudinal mucosa are short, but more salient when compared to the intramural region and the muscle layer is thick, 3rd Ampulla: the mucosa has folds similar to those of the isthmus, but there is a considerable increase of the tubular lumen. The muscle layer is similar to that of the isthmus and of the intramural regions; 4th Infundibulum: the mucosa is composed of long folds that project into the lumen, reducing its diameter, and the muscle layer is thin. The free end of the infundibulum presents extensions called fimbriae, characterized by mucosal folds and by the presence of cilia (Fig. 3).

Morphometric Analysis: The luminal area of the isthmus is smaller than that of the ampulla and infundibulum. The ampulla and infundibulum regions present the same mean luminal area. The isthmus epithelial area is smaller compared with the ampulla and infundibulum regions, which do not differ from one another. The isthmus presents the smallest number of mucosal folds, and the infundibulum the largest (Table I).

DISCUSSION

The Mongolian gerbil presents a polyestrous cycle in the laboratory, like the albino rat (Rattus norvegicus) and the mouse (Mus musculus) (Fox & Laird, 1970). In the present study it was confirmed that the gerbil's estrous cycle is short (Marston & Chang, 1965) and irregular (Nishino & Totsukawa, 1996; Almeida, 2000). The estrous cycle of the rat and the mouse is short, but differs from that of the gerbil in regularity (Young et al. 1941; Allen, 1922). The rat estrous cycle presents four phases: proestrus, metaestrus, diestrus and estrus with characteristics like estrus II of the gerbil. The mouse cycle is similar to that of the rat, although the mouse presents metaestrus divided into phases I and II. It presents, therefore, five phases like the gerbil (Allen, 1922). Like the albino rat, the Mongolian gerbil presents a bicornuate uterus united in its caudal portion (Hebel & Stromberg, 1976). The gerbil's ovaries are kidney shaped, with two distinct poles, a cranial one and a caudal one. In the rat, the ovary is a follicular mass with an irregular nodular surface (Greene, 1942). The gerbil's uterine tube is convoluted (Williams, 1974). The arterial vessels of the uterine tube arise from ovarian and uterine branches and the veins follow the same pattern. The results show that the intramural portion presents the narrowest lumen and possesses developed muscle layers, like the golden hamster (Abe, 1996). In the rat, the intramural portion is a papilla (Hafez, 1970). The mucosal longitudinal folds in the isthmus region of the gerbil uterine tube are short and the muscle layer is thick, similar to the golden hamster (Abe). The uterine tube muscle tunic of the gerbil consists of two layers without distinct limits, different from the rat and mouse that possess an internal circular tunica and a typical external longitudinal tunica (Hafez). The ampulla region of the gerbil's uterine tube presents a mucosa with longitudinal folds similar to the isthmus, but there is an increase of the lumen and the muscle tunic is thick, differing from the rat and golden hamster which possess a mucosa with countless branching folds and a thin muscle layer. The infundibulum is similar to the one described for the rat and mouse. The gerbil uterine tube is funnel shaped, presenting two distinct extremities that vary in diameter and cell type concentrations. Each region presents a characteristic function-morphology relationship. The increased diameter of the infundibular lumen permits the mucosa to project into the lumen, increasing its contact surface with the ovum. It is suggested that the muscle thickening permits a unidirectional flow to the uterus. Therefore, the gerbil (Meriones unguiculatus) is not an ideal model for the study of the estrous cycle due to its irregularity. Among the uterine tube regions, the ampulla is characteristic of the species, because its structure is like that of the isthmus region, while in the albino rat and in the mouse it is like the infundibular region. The gerbil´s uterine tube anatomy and histology are similar to those of the albino rat and the mouse used in the laboratory.

ACKNOWLEDGMENTS: The authors are grateful to FAPESP (Grants 97/14433-1) for financial support and to Elettra Greene for a careful revision of the English language.

 This paper was supported by FAPESP (Proc. 97/00954-0)

RESUMEN: Fue clasificado el ciclo estral del gerbo de Mongolia y descritas las estructuras y relaciones de la tuba uterina en la cavidad abdomino-pélvica, así como la histología de su epitelio de revestimiento. Las tubas uterinas de 19 hembras adultas del gerbo de Mongolia fueron investigadas a través de técnicas de anatomía e histología. Fue observado el ciclo estral poliéstrico e irregular, con cinco fases: proestro, estro I, estro II, metaestro y diestro. El estro I se caracterizaba por presentar células queratinizadas, esparcidas y en menor número con respecto al estro II. La tuba uterina presentaba relaciones con la extremidad distal del cuerno uterino y con el ovario. La tuba presenta serosa de revestimiento externo (lámina visceral del peritoneo), túnica media muscular de fibras circulares lisas y túnica mucosa con epitelio de revestimiento simple, constituido de células cilíndricas ciliadas y secretoras. Anatómicamente las tubas uterinas, se dividen en cuatro regiones: intramural, istmo, ampolla e infundíbulo. El gerbo no constituye modelo ideal para estudios del ciclo estral debido a su irregularidad. La anatomía e histología de la tuba uterina son semejantes a la presentada por los roedores usados como animales de laboratorio.

PALABRAS CLAVE: 1. Ciclo estral; 2. Anatomía; 3. Histología; 4. Tuba uterina; 5.Gerbo de Mongolia.

Correspondence to Camila:

Prof. Dra. C.D. Almeida
Department of Anatomy, Institute of Biosciences
University of the State of São Paulo (UNESP)
CEP 18618-000
Botucatu / São Paulo
BRAZIL
fax 55-14-68213744

e-mail: contin99@yahoo.com

Recibido : 22-05-2001
Aceptado:  28-06-2001

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