Developmental expression of Ki‐67 antigen and proliferating cell nuclear antigen in macaque placentas

Abstract

Placental growth is largely determined by the proliferation of cytotrophoblast cells. However, the distribution of cytotrophoblast cells engaged in the cell cycle during placental development is poorly understood. Recently, antibodies have been developed that identify two proteins directly involved with DNA synthesis: Ki-67 protein and proliferating cell nuclear antigen (PCNA). Immunolocalization of Ki-67 and PCNA provides a measure of the proliferating cells in tissues. We examined, in macaque placentas, the spatiotemporal pattern of expression of these proteins during gestation. Tissues from 24 macaque placentas collected from 22-153 days of pregnancy were prepared for paraffin sections. Standard immunoperoxidase techniques were used to identify Ki-67 and PCNA. The proteins generally co-localized, although PCNA was usually represented in more cells than Ki-67. Early in gestation the cell columns contained many labeled cells. The cytotrophoblastic shell was occupied by numerous cells with PCNA positive nuclei, but few were reactive for Ki-67. By 45 days of pregnancy the immunolabeled cells in the cell columns were concentrated in the proximal regions, adjacent to the anchoring villus tips. The number of positive cells decreased by 100 days when the cell columns were diminished, leaving the anchoring villus tips buried in the shell. Labeled cells were rarely present in the shell at late pregnancy. The single layer of cytotrophoblast cells in the chorionic plate contained numerous reactive cells throughout early and mid-gestation. After approximately 100 days the cytotrophoblast layer of the chorionic plate was stratified over large areas. Soon thereafter few cells of the chorionic plate were labeled. The chorionic villi contained reactive cytotrophoblastic cells throughout gestation. Extravillous cytotrophoblast cells invading spiral arteries were sometimes labeled for PCNA but not Ki-67. We conclude that compartments of the placenta are distinguished by specific patterns of cytotrophoblast cell proliferation. Moreover, these patterns correspond to macroscopic growth parameters of the placenta. Evidence suggests that the macaque placenta slows its rate of diametrical growth at approximately 100 days of gestation. It is at about this time that the cell columns are absorbed into the trophoblastic shell and this pool ofproliferating cells is diminished. The growth in diameter of the chorionic plate matches that of the shell. In this compartment also the architecture changes at about 100 days as the cytotrophoblast layer stratifies. This stratification may result from continued proliferation of cytotrophoblast cells when the diametrical rate of growth is decreasing. Soon thereafter, proliferation decreases in this compartment also. By contrast, labeled cells were found in chorionic villi throughout gestation. Continued villous growth and branching probably accounts for most of the increases in placental thickness and weight that occurs during late gestation. 0 1994 Wiley-Liss, Inc.

Recent applications of the new stereology have thrown fresh light on how the human placenta grows and develops its form

“…This supports immunochemical and autoradiographic labelling studies which have shown that there are centres of proliferation for epithelium, stroma and endothelium (Kaufmann et al, 1991;Blankenship & King, 1994). Allometric studies demonstrate that all nuclear compartments expand as the placenta grows but suggest that the relative growth of stromal nuclei lags behind that of endothelial nuclei.…”

“…Attenuation and proliferation could result in permanent enlargement of sinusoids and, thereby, help to match vascular and diffusive conductances to fetal growth (Burton & Feneley, 1992;Mayhew et al, 1993). The recruitment of new TPU throughout gestation is compatible with recent evidence from immunocytochemistry of human and Macaque placentas which shows that Langhans cells are still mitotically active near term (Arnholdt et al, 1991;Blankenship & King, 1994). It is also consistent with discontinuity of the Langhans cell layer but incompatible with loss of Langhans cells (Boyd & Hamilton, 1970;Dearden & Ockleford, 1983;Arnholdt et al, 1991).…”

Differential Evolutionary Fate of an Ancestral Primate Endogenous Retrovirus Envelope Gene, the EnvV Syncytin, Captured for a Function in Placentation

“…In the macaque placenta, envV2 expression is localized at the level of the syncytiotrophoblast, at the materno-fetal interface, as expected for a gene involved in syncytiotrophoblast morphogenesis. Of note, envV2 expression is not detected at the level of the cytotrophoblasts which can be distinguished by their mitotic activity using an anti-Ki67 antibody [40] (Figure 7B, right panels). Finally, the figure also shows a similar localization of envV2 transcripts in the human placenta.…”

Preference of Invasive Cytotrophoblast for Maternal Vessels in Early Implantation in the Macaque

“…Evidence to date suggests that extravillous tropho blast, including endovascular trophoblast. has lost its capac ity for cell division [Bulmer et al, 1988;Blankenship and King. 1994a], Consequently hyperplasia of cytotrophoblast within arterioles cannot be the basis for its deeper migration.…”