The metrial gland

Early spiral arteriole remodeling in the uterine-placental interface: A rat model

The mammalian placenta's interface with the parent is a richly vascularized tissue whose development relies upon communication between many different cell types within the uterine microenvironment. The uterine blood vessels of the interface are reshaped during pregnancy into wide-bore, flaccid vessels that convey parental blood to the exchange region of the placenta. Invasive trophoblast as well as parental uterine macrophages and Natural Killer cells are involved in the stepwise remodeling of these vessels and their respective contributions to this crucial process are still being delineated. However, the earliest steps in arteriole remodeling are understudied as they are difficult to study in humans, and other species lack the deep trophoblast invasion that is so prominent a feature of placentation in humans. Here, we further characterize the rat, with deep hemochorial placentation akin to humans, as a model system in which to tease apart the earliest, relatively understudied events in spiral arteriole remodeling. We show that the rat uterine-placental interface increases in size and vascularity rapidly, before trophoblast invasion. The remodeling stages in the arterioles of the rat uterine-placental interface follow a sequence of anatomical changes similar to those in humans, and there are changes to the arterioles' muscular tunica media prior to the marked influx of immune cells. The rat is a tractable model in which to better understand the cell/cell interactions occurring in vivo in an intact tissue microenvironment over time.

Sexually dimorphic effects of forkhead box a2 (FOXA2) and uterine glands on decidualization and fetoplacental development

Glands of the uterus are essential for pregnancy establishment. Forkhead box A2 (FOXA2) is expressed specifically in the glands of the uterus and a critical regulator of glandular epithelium (GE) differentiation, development, and function. Mice with a conditional deletion of FOXA2 in the adult uterus, created using the lactotransferrin iCre (Ltf-iCre) model, have a morphologically normal uterus with glands, but lack FOXA2-dependent GE-expressed genes, such as leukemia inhibitory factor (LIF). Adult FOXA2 conditional knockout (cKO; Ltf ^iCre/+ Foxa2 ^f/f ) mice are infertile due to defective embryo implantation arising from a lack of LIF, a critical implantation factor of uterine gland origin. However, intraperitoneal injections of LIF can initiate embryo implantation in the uterus of adult FOXA2 cKO mice with pregnancies maintained to term. Here, we tested the hypothesis that FOXA2-regulated genes in the uterine glands impact development of the decidua, placenta, and fetus. On gestational day 8.5, the antimesometrial and mesometrial decidua transcriptome was noticeably altered in LIF-replaced FOXA2 cKO mice. Viable fetuses were reduced in FOXA2 cKO mice on gestational days 12.5 and 17.5. Sex-dependent differences in fetal weight, placenta histoarchitecture, and the placenta and metrial gland transcriptome were observed between control and FOXA2 cKO mice. The transcriptome of the placenta with a female fetus was considerably more altered than the placenta with a male fetus in FOXA2 cKO dams. These studies reveal previously unrecognized sexually dimorphic effects of FOXA2 and uterine glands on fetoplacental development with potential impacts on offspring health into adulthood.

Mathias Duval on Placental Development in Mice and Rats

Mathias Duval (1844-1907) was one of the pioneers in elucidating the intricate placental histology of different mammalian groups, notably the rodents. Using a well-dated series of mouse conceptuses, he described in detail the successive steps in placental development, and for confirmation he included observations on a (undated) collection of rat specimens. Not only was he able to identify correctly the different extra-embryonic cell layers, but he was also the first to recognize trophoblast invasion in rodents. Not all his interpretations are still valid, however. Re-reading his extensive and detailed work "Le placenta des rongeurs" (1890-1892) confronts us with still existing gaps in our present understanding of placental development, notably the morphogenesis of the different placental layers and the differentiation of invasive trophoblast. His understanding of uteroplacental blood flow was still limited, and he failed to recognize the complexity of the maternal decidua and its vasculature, which is essential for correctly understanding the pathways and extent of trophoblast invasion. Although Duval was active in promoting Darwin's evolutionary ideas, he refrained from extrapolating too quickly his findings in rodents to other mammalian groups including humans. In his view detailed histological studies on complete series of specimens had to come first, and thus provide a firm basis for a proper understanding of placental function and evolution.

Functional involvement of P-selectin and MAdCAM-1 in the recruitment of ?4?7-integrin-expressing monocyte-like cells to the pregnant mouse uterus

Leukocyte recruitment to the pregnant mouse uterus has been suggested to be associated with highly regulated expression of distinct patterns of vascular adhesion receptors. One of the most striking observations is the combined expression of mucosal addressin cell adhesion molecule-1 (MAdCAM-1) and P-selectin by maternal vessels of the vascular zone during the critical period of initial placenta development. The predominant cell population within these vessels is of the monocyte/macrophage lineage and expresses the mucosal integrin alpha4beta7, which represents the ligand for MAdCAM-1; neutrophils and lymphocytes are rare. To directly assess the importance of identified adhesion receptors, we undertook long-term in vivo inhibition studies using monoclonal antibodies to inhibit the contribution of MAdCAM-1 in leukocyte trafficking to the decidua or to deplete alpha4beta7(+) leukocytes. In addition, implantation sites of mouse strains genetically deficient in specific adhesion receptors were investigated. Our results underline the importance of predicted adhesion pathways in the recruitment of monocyte-like cells, especially those expressing alpha4beta7. Interestingly, maternal/fetal units with inhibited recruitment of alpha4beta7(+) leukocytes or the absence of these cells are characterized by reduced size and frequency of uterine NK cells.

Thomas Huxley and the rat placenta in the early debates on evolution

The 19th century debates on mammalian classification in the light of the new evolutionary thinking led to controversies between Thomas Huxley and Richard Owen concerning the value of the placenta as a representative key organ. As a main point in his argument, Huxley provided a detailed description of a sectioned rat placenta, highlighting the importance of decidualization of the uterus as an argument supporting an evolutionary relationship between rodents, insectivores and primates, an idea hotly contested by Owen. In addition, he illustrated and correctly interpreted the maternal blood supply from uterus to placenta in striking detail. During the succeeding decades the key role of trophoblast in placenta formation was discovered, and the decidua became neglected in later comparative studies. Nevertheless, at the present time trophoblast-decidual interaction is regarded as an extremely important feature of placental development in both primates and rodents, and Huxley can therefore rightfully be considered as an early pioneer in placental research.

Alterations in the expression of homing-associated molecules at the maternal/fetal interface during the course of pregnancy

One of the most fascinating immunologic questions is how the genetically distinct fetus is able to survive and develop within the mother without provoking an immune rejection response. The pregnant uterus undergoes rapid morphological and functional changes, and these changes may influence the nature of local immune responses at the maternal/fetal interface at different stages of gestation. We hypothesized that specialized mechanisms exist to control access of maternal leukocyte subsets to the decidua and that these mechanisms are modulated during the course of pregnancy. At the critical period of initial placenta development, the maternal/fetal interface displays an unparalleled compartmentalization of microenvironmental domains associated with highly differentiated vessels expressing vascular addressins in nonoverlapping patterns and with recruitment of specialized leukocyte subsets (monocytes, granulated metrial gland cells, and granulocytes) thought to support, modulate, and regulate trophoblast invasion. One of the most striking observations at this time of gestation is the almost complete exclusion of lymphocytes from the maternal/fetal interface. The second half of pregnancy is characterized by a partial loss of microenvironmental specialization and different switches in vascular specificity within the decidua basalis, paralleling dramatic changes in the populations of recruited leukocytes (e.g., a striking influx of lymphocytes, especially T cells). In the term pregnant uterus, the expression of all vascular addressins decreased dramatically; only weakly staining maternal vascular segments remained. These segments may define sites of extremely low residual traffic in the term decidua, which contains remarkably few maternal leukocytes overall. Our results suggest that the maternal/fetal interface represents a situation in which leukocyte trafficking is exquisitely regulated to allow entry of specialized leukocyte subsets that may play a fundamental role in immune regulation during pregnancy.

Evidence of specialized leukocyte-vascular homing interactions at the maternal/fetal interface

In normal pregnancy, the maternal immune system fails to reject the fetus or the placenta as an allogeneic graft. We hypothesize that specialized mechanisms of leukocyte recruitment might limit access of circulating maternal immune cells to the maternal/fetal interface. During the critical period of initial trophoblast invasion there is an elegantly orchestrated progression of leukocyte homing events in the decidua basalis, associated with highly regulated expression of vascular addressins and segregation of specialized leukocyte subsets into well-defined decidual microdomains. Neutrophils are limited to the region of necrosis associated with enzymatic digestion at the leading edge of the invading trophoblast, where an almost linear array of maternal blood vessels displays the neutrophil ligand E-selectin. Cells with the phenotype of monocytes but expressing alpha4beta7 integrin are localized in the blood vessels of the specialized "vascular zone", which display the unusual combination of P-selectin (partially associated with platelets) and the alpha4beta7 ligand mucosal vascular addressin-1 (MAdCAM-1). Granulated metrial gland cells (alpha4+beta7-, probably alpha4beta1+) constitute a well-defined cluster positioned in the central decidua basalis around venules prominently expressing the alpha4beta1 ligand VCAM-1 (but not MAdCAM-1). T and B lymphocytes are rare. Our results suggest that selective mechanisms for regulating leukocyte access, associated with microdomain specialization within the decidua basalis, may play a fundamental role in immune regulation during the invasive period of placental development.

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.

Calcyclin in the mouse decidua: expression and effects on placental lactogen secretion

Placental lactogens are protein hormones produced by the placentas of many mammals. These hormones have diverse reproductive functions and are structurally similar to the pituitary hormones growth hormone and prolactin. Isolated decidual cells were previously shown to release a protein that stimulates mouse placental lactogen (mPL)-II release from mouse trophoblast cells in culture. Partial amino acid sequence data suggested that this protein shared sequence identity with mouse calcyclin. In the present study the sequence identity of this protein was determined by sequencing reverse transcription-polymerase chain reaction (RT-PCR) products. The sequence matched that of mouse calcyclin. The distribution of calcyclin message was determined in the conceptus by in situ hybridization, and a gestational profile of calcyclin mRNA was determined by Northern blot analysis. Calcyclin was localized primarily to cells that exhibited a uterine natural killer cell morphology within the decidua and to glycogen cells of the labyrinth and junctional zone. Although calcyclin was detectable by RT-PCR in midterm placentas, isolated trophoblast cells in culture did not contain detectable quantities of calcyclin by RT-PCR. Calcyclin stimulated secretion of mPL-II from isolated trophoblast cells in vitro but did not affect mPL-I secretion.

Detection of metrial gland cells in rat cervix after delivery by smear method

It has been reported that the metrial glands, which form yellowish-white masses on the mesometrial triangle after pregnancy, coincide with the number of implants in the rat. In this study, the migration of the metrial gland cells (MGC) from the metrial glands to the endometrium after placental desquamation was investigated by the smear method. In smeared specimens, MGC appeared as large round cells with an unstained nucleus (a large binucleate cell was frequently seen), surrounded by a pinkish rim of cytoplasm, with PAS-staining; or appeared as a large reddish violet nucleus surrounded by dark violet cytoplasm with Giemsa staining. Numerous MGC were observed on the endometrium on the mesometrial side after artificial desquamation of the placenta on day 20 of pregnancy and day 0 of delivery. Further, MGC were observed on the endometrium on the antimesometrial side or in the cervix (including the internal and external orifice of the uterus) on day 0 of delivery, but no MGC could be detected in the cervix or on the antimesometrial side either during the period of placental signs (days 11-13) or on day 20 of pregnancy. Although MGC disappeared rapidly after delivery, slight traces of MGC remained on the mesometrial side until day 3 after delivery. In the case of abortion, MGC were observed in the cervix on the day of vaginal bleeding. In view of these results, it is considered that confirmation of MGC on the endometrium or the cervix provides evidence of abortion or delivery.

Distribution of perforin-containing cells in normal and pregnant mice

Cytotoxic T lymphocytes (CTL) and natural killer (NK) cells elaborate a cytolytic protein named perforin or cytolysin. It was widely held that, in vivo, high quantities of perforin are not present in resting lymphocytes and are usually produced only by activated lymphocytes found under pathological conditions. Until now, only one tissue was known to synthesize abundant quantities of perforin under nonpathological conditions, the uterus during pregnancy. To investigate the possibility that perforin might also be synthesized by other tissues, several tissues besides the uterus from pregnant and normal mice were tested by immunofluorescence and immunoperoxidase for the presence of perforin. The tissues studied were the ears, brain, nasal epithelium, tongue, salivary gland, larynx, thymus, stomach, liver, spleen, small intestine, and lymph nodes; two cell populations with different sizes and levels of perforin expression were found. Large cells, displaying the NK cell phenotype and expressing high levels of perforin, were detected not only in the uterus but also in the salivary gland and lungs of pregnant mice. Small cells, expressing low levels of perforin, were detected mainly in the stomach and small intestine, and they were expressed in both pregnant and normal mice. Taken together, these results imply that perforin-containing cells exist in vivo under nonpathological conditions, and that the immune system is endowed with heretofore unknown mechanisms for stimulating the activation of NK cells in a limited number of tissues during pregnancy.

Enzyme histochemistry of the regressing rat decidua and metrial gland

In order to understand more about participation of the basal placental zones in processes of regression and degradation as well as separation on the cellular level, the cell metabolism of the rat decidua and metrial gland was investigated enzyme histochemically in cryosections for activities of oxyradical-forming enzymes and hydrolyzing enzymes. Additionally, plastic sections were studied to facilitate the recognition of cell types. Decidual stromal cells and fibroblasts formed the vast majority amongst many cell types in the decidua and metrial gland. High activities of enzymes involved in purine degradation and oxyradical generation were demonstrated in decidual stromal cells and fibroblasts. Microsomal alanyl aminopeptidase and various acid hydrolases were shown to be extremely active in decidual stromal cells. The abundance of these enzyme activities in the decidua and metrial gland in contrast to other placental areas suggests, that these enzymes may have specialized functions in connection with regression and degradation processes finally contributing to placental separation.

Differentiation inhibiting activity (DIA/LIF) and mouse development

Analysis of the differentiation in culture of murine embryonic stem (ES) cells has resulted in the identification and characterization of the regulatory factor differentiation inhibiting activity (DIA). DIA specifically suppresses differentiation of the pluripotential ES cells without compromise of their developmental potential. DIA is identical to the pleiotropic cytokine leukaemia inhibitory factor (LIF) which has a broad range of biological activities in vitro and in vivo. It is produced in both diffusible and matrix-localised forms whose expression is differentially regulated. The compartmentalization of DIA/LIF and the modulation of its expression during stem cell differentiation and by other cytokines may be significant elements in the control of early embryo development. These features may also indicate general principles of the regulatory networks which govern stem cell renewal and differentiation in later development.

Perforin-expressing granulated metrial gland cells in murine deciduoma

It has previously been shown that granulated metrial gland (GMG) cells of the pregnant uterus express abundant quantities of the lymphocyte pore-forming protein, perforin. No perforin was present before implantation of the embryo, but large numbers of perforin-producing GMG cells were observed after implantation, which coincides with decidualization of the uterus. The possible source of the activation factors responsible for perforin gene induction in GMG cells was studied here with the pseudopregnancy model, in which cervical stimulation of mice during estrus leads to a series of hormonal changes resembling those seen in pregnancy, but in the absence of an embryo. Subsequent stimulation of the uterus of pseudopregnant mice with oil causes the stimulated portion of the endometrium to differentiate into decidual tissue. Perforin-containing GMG cells were in fact present in the deciduomata, but not in adjacent nondecidualized tissues of the same mice. These results suggest that maternal factors associated with decidual tissue are responsible for the local expression of perforin in GMG cells.

Morphologic changes in the rat uterus following natural mating and embryo transfer

In order to gain some insight into the putative immune suppression that may be induced at the placental implantation sites, the morphological changes at these sites following natural matings and following the transfer of embryos fertilized in vivo were studied. The only histologic parameter that showed a significant difference was the number of granulated metrial gland (GMG) cells. More GMG cells were present in allogeneic than in syngeneic pregnancies, and more GMG cells were present following embryo transfer into an allogeneic female than following the comparable natural mating. The role of the GMG cells in pregnancy is, however, still unresolved.

A monoclonal antibody, 4H12, recognizes a surface antigen found on granulated metrial gland cells in the murine decidua

A monoclonal antibody (MAb), designated 4H12, was selected for reactivity to a surface antigen on PYS-2 teratocarcinoma cells. 4H12 was the product of a fusion of lymphoid cells of a non-immunized pregnant C57BL/6 mouse to NS-1 myeloma cells. Initial studies utilizing immunohistochemistry revealed that MAb 4H12 bound to an antigen found on cells in the decidua basalis of 7-, 8- and 10-day pregnant mice. Antigen-positive cells of 11--19-day pregnant mice were also found predominantly in the decidua. A few antigen-positive cells were found in the labyrinth of the placenta and up against Reichert's membrane. Antigen-positive cells were morphologically and spatially distinct, oval to round with large periodic acid Schiff positive granules. Indirect immunofluorescent (IIF) labeling of decidual cultures showed antigen on the surface of cells that were small, oval to round and adherent. The antigen recognized by MAb 4H12 was removed from tissue sections with trypsin and protease and therefore is suggested to be a protein. We conclude that MAb 4H12 recognizes a surface antigen found on cells historically described as granulated metrial gland (GMG) cells. This MAb should greatly facilitate the further analysis of the life history and function of GMG cells during pregnancy.

Receptors for IgG2b on cells of the mouse metrial gland

Single cells prepared from metrial glands of mice killed at days 10, 13 and 17 of pregnancy were assayed for the expression of Fc gamma receptors in a standard rosetting assay using sheep red blood cells sensitised with a mouse monoclonal IgG2b antibody. Rosettes, indicating Fc gamma receptors, were found on both granulated metrial gland (GMG) cells and non-GMG cells, comprising mainly stromal cells, from each stage of pregnancy. Some animals were given an intravenous injection of horseradish peroxidase 2 h before they were killed in order to identify endocytic cells. No GMG cells were found to have endocytosed the horseradish peroxidase. Non-GMG cells which showed endocytic activity all expressed Fc gamma receptors but these receptors were also found on some of the non-GMG cells which had not exhibited endocytosis. The finding of Fc gamma receptors on GMG cells provides further evidence that these cells may be related to NK cells.

Rat granulated metrial gland cells differentiate in pregnant chimeric mice and may be cytotoxic for mouse trophoblast

Shielding an ovary and part of the uterus of mice during irradiation allowed chimeric mice, reconstituted with rat or mouse bone marrow (respectively, RBM and MBM chimeras), to be studied in a subsequent pregnancy. Some of the granulated metrial gland (GMG) cells which differentiated in the RBM chimeric mice were rat GMG cells, showing the contribution of donor bone marrow precursors to the uterine response to pregnancy. Rat and mouse GMG cells were found in the maternal blood spaces of the placental labyrinth adjacent to the degenerate layer 1 trophoblast. This suggests that both rat and mouse GMG cells exhibit cytotoxic activity against the mouse trophoblast.

Distribution of expression of 2AR (osteopontin) in the embryonic mouse inner ear revealed by in situ hybridisation

Using in situ hybridisation we have determined the distribution of expression of 2ar (also known as osteopontin, bone sialoprotein 1 or 44-kDa bone phosphoprotein) in the developing mouse inner ear. We have identified several discrete sites, both osteogenic and non-osteogenic, that express 2ar from embryonic day 16.5 (E16.5). In addition to the regions of developing bone of the calvaria and temporal bone, we have found 2ar expression in the epithelium of the sensory maculae (but not in the organ of Corti), in the vestibular and auditory ganglia and nerves (but not in the nerves that innervate the whiskers in the snout), in the epithelium that lines the endolymphatic sac (but not in the neighbouring and contiguous endolymphatic duct) and also in the epithelium that lines the semicircular canals. We found also individual cells scattered throughout the brain, loose mesenchyme and blood vessels of the head that were expressing 2ar. Several of the sites in the inner ear, for example the maculae and the endolymphatic sac, are known to be involved in the production of calcified matrix. The results extend the range of tissue types known to express the protein and demonstrate that tissues of histologically similar appearance can nonetheless differ in their gene expression.

Hemopoietic origin of certain decidual cell precursors in murine pregnancy

The possible hemopoietic origin of certain precursors of uterine decidual cells appearing during normal murine pregnancy was investigated in semiallogeneic hemopoietic chimeras with retained or regained fertility. Chimeras were produced by three different methods in two donor-host combinations: F1 [BALB/c female x C3H/HeJ male] cells introduced into the parental strain BALB/c female hosts or F1 [CBA/J female x C57BL/6 male] cells introduced into CBA/J female hosts. Prenatal chimeras (PN) were made by reconstituting mouse fetuses (day 13-17) with 10(6)-10(7) adult bone marrow or fetal liver cells through the yolk sac and they were allowed to be delivered naturally. Neonatal chimeras (NN) were made by injecting 1-2 x 10(7) adult bone marrow cells into the anterior facial vein of neonatal mice (less than 24 hr old). In both cases, experimental animals were raised to maturity. Ovary-transplanted chimeras (OT) were made by injecting 10(7) bone marrow cells into lethally irradiated (9.5 Gy) young adult female mice, followed 6 weeks later with bilateral orthotopic transplants of syngeneic ovary grafts to restore fertility. All female chimeras produced by the three different methods were mated with syngeneic male partners to produce normal pregnancy. The extent of chimerism at the cellular level was determined in all cases by a radioautographic identification of the H-2 phenotype of splenic lymphocytes and decidual cells and macrophages in the collagenase-dispersed decidua at day 11-16 of normal pregnancy, following a sandwich labelling with monospecific anti-H-2 antibodies and 125I-protein A. Morphological discrimination of typical decidual cells from macrophages in the collagenase-dispersed decidua was carried out on the basis of several distinctive markers: presence of surface Dec-1 and Thy-1 and absence of surface F4/80 or latex phagocytosis for decidual cells, in contrast to macrophages which were phagocytic and expressed F4/80 but not Dec-1 or Thy-1. While the degree of hemopoietic chimerism (judged by the incidence of donor-derived lymphocytes in the spleen) varied from animal to animal, in all three groups (PN, NN, and OT) comprising a total of 26 chimeras, the percentage of typical decidual cells expressing donor H-2 phenotype showed an excellent correlation with that for small lymphocytes in the spleen. These results reveal that at least a subpopulation of typical decidual cells of the pregnant uterus has a hemopoietic genealogy. A possible familial relationship of these cells to granulated metrial gland cells remains unclear.

Developmental expression of 2ar (osteopontin) and SPARC (osteonectin) RNA as revealed by in situ hybridization

2ar has been identified as a gene inducible by tumor promoters and growth factors in a variety of cultured mouse cell lines (Smith, J. H., and D. T. Denhardt. 1987. J. Cell. Biochem. 34:13-22). Sequence analysis shows that it codes for mouse osteopontin, an RGDS-containing, phosphorylated, sialic acid-rich Ca++-binding protein originally isolated from bone (Oldberg, A., A. Franzen, and D. Heinegard. 1986. Proc. Natl. Acad. Sci. USA. 83:8819-8823; Prince, C. W., T. Oosawa, W. T. Butler, M. Tomana, A. S. Brown, and R. E. Schrohenloer. 1987. J. Biol. Chem. 262:2900-3907.). In this paper we use Northern blot analysis and in situ hybridization to localize expression of 2ar during mouse embryogenesis. 2ar RNA is first detected in developing limb bones and calvaria at 14.5 d p.c., in a population of cells distinct from those expressing SPARC (osteonectin). High levels of 2ar expression are also seen in the bone marrow-derived granulated metrial gland cells of the deciduum and placenta, and in a number of epithelial tissues, including embryonic and postnatal kidney tubules, uterine epithelium and sensory epithelium of the embryonic ear. The temporal and spatial pattern of 2ar expression seen in vivo suggests that the protein plays a wider role than previously realized, in processes which are not confined to bone development.