Studies concerning the hormonal control of deciduomata and metrial glands

Morphology and physiology of rat placenta for toxicological evaluation

The placenta plays a pivotal role in fetal growth, and placental dysfunction and injury are associated with embryo/fetal toxicity. Histological examination of the rat placenta for safety evaluation provides valuable clues to the mechanisms of this toxicity. However, the placenta has specific and complex biological features unlike those of other organs, and placental structure dramatically changes depending on the time during the gestation period. Thus, time-dependent histopathological examination of the rat placenta should be performed based on the understanding of normal developmental changes in morphology and function. The placentas of rats and humans are both anatomically classified as discoid and hemochorial types. However, there are differences between rats and humans in terms of placental histological structure, the fetal-maternal interface, and the function of the yolk sac. Therefore, extrapolation of placental toxicity from rats to humans should be done cautiously in the evaluation of risk factors. This review describes the development, morphology, physiology, and toxicological features of the rat placenta and the differences between the rat and human placenta to enable accurate evaluation of reproductive and developmental toxicity in studies.

Effect of estrogen on rat placental development depending on gestation stage

We examined the sequential histopathological changes in the placenta from rats exposed to estrogen. 17 β-estrogiol-3-benzoate was intraperitoneally administered at 100 μg/animal/day during GD 6 to GD 8 (GD6-8 treated group), GD 9 to GD 11 (GD9-11 treated group) and GD 12 to GD 14 (GD12-14 treated group), and the placentas were sampled on GDs 11, 13, 15, 17, and 21. Fetal mortality rates were increased up to approximately 50% in the GD6-8 and 9-11 treated groups, but there was no change of fetal weight on GD 21. An increase in placental weight and a reduction in fetal/placental weight ratio were detected during GD 17 to GD 21 in the GD6-8 treated group. Histopathologically, hypoplasia of metrial gland was detected with defective development of spiral arteries in the GD6-8 and GD9-11 treated groups. A decrease in the thickness of metrial gland was observed from GD 11 onwards in the GD6-8 treated group and from GD 13 onwards in the GD9-11 treated group. The endovascular trophoblasts invaded into the spiral arteries in the deep part of metrial gland in these treated groups. The number of phospho-histone H3 positive cells was decreased on GD 11 or GD 13 in these groups. In the decidua basalis, transitory necrosis was observed with hemorrhage on GD 13 in the GD6-8 and GD9-11 treated groups. In the labyrinth zone, cystic dilatation of the sinusoid was observed with congestion in the GD6-8 treated group, resulting in an increased placental weight. Therefore, we consider that estrogen inhibits the proliferation of decidualized endometrial stromal cells in the metrial gland, and leads to metrial gland hypoplasia with less development of the spiral arteries. The reduced utero-placental blood flow is supposed to be one of the important factors for poor reproductive performance.

Granulated metrial gland cells in 'minor' species

Granulated metrial gland cells, also known as uterine natural killer cells or large granular lymphocytes, are pregnancy associated leucocytes of granular phenotype. They are well characterised in mice and humans in terms of their structure, origin and distribution although the function of these cells has yet to be determined. In this review, granulated metrial gland cells in 'minor' species of rodents, insectivores, primates and species with epitheliochorial placentae are described. Emphasis is given to the comparative structure and distribution of granulated metrial gland cells in these minor species and to their possible functional association with trophoblast. Comparative studies of granulated metrial gland cells in minor species complements other approaches such as can be provided using mutant mice.

Mode of cell death in the rat metrial gland during peripartum regression

In the rodent uterus, the metrial gland develops during midpregnancy and undergoes regression prior to parturation. The involution of the gland is reported to be accompanied by the loss of gland cells due to their death in situ. Cell death has been classified by using morphological criteria into two types: necrosis and apoptosis. To study the mechanism involved in the peripartum regression of the rat metrial gland, we examined the mode of cell death in the gland during the last week of gestation. We identified apoptotic cells in the regressing metrial gland by using DNA fragmentation, in situ DNA 3'-end labeling, and electron microscopy. Expression of progesterone receptor (PR) and estrogen receptor (ER) was also demonstrated by immunohistochemistry in the gland. The mean weight of metrial gland nodes decreased after day 18 of pregnancy. The apoptotic granulated metrial gland (GMG) cells that were detected by using the in situ DNA 3'-end labeling method were observed on day 16 of pregnancy, and they increased in number after day 20 of pregnancy. Intense fragmentation of DNA was also found from day 20 to day 22 of pregnancy. Electron microscopy demonstrated apoptotic GMG cells in the regressing metrial glands, confirming the results of the labeling studies. Immunohistochemical study revealed that expression of PR and ER, which were localized mainly in fibroblast-like stromal cells but not in GMG cells, was almost unchanged during late pregnancy. Apoptotic cell death is the major mode of rat metrial gland cell death in the peripartum loss of metrial gland cells.