Background: Mesenchymal stem cells (MSC) derived from bone marrow and placentae characteristically differentiate into adipocytes, osteocytes and chondrocytes. In adults, bone marrow MSCs can be stimulated to differentiate into endothelial cells and thus participate in vasculogenesis. Many researchers isolate MSCs from term placentae, but first-trimester placental MSCs (pMSCs) are thought to be phenotypically different from term pMSCs. Due to their vascular differentiation potential and perivascular location in term placenta, we hypothesise that first-trimester pMSCs are involved in early placental vasculogenesis. In order to use pMSCs to study early placental vasculogenesis we have isolated and characterised pMSCs from first-trimester placental explants.

Methods: pMSCs were isolated from 16 placentae (6-12 weeks of gestation) using an adaption of the explant method of Abumaree et al (Stem Cell Rev. 9(1):16-31, 2013). The expression of MSC (CD73, CD90, CD105, HLA-A2) and non-MSC (CD31, CD34, CD45) cell surface markers were evaluated using flow cytometry. Differentiation capacity was determined by incubating MSC in commercial adipocyte or osteocyte differentiation medium (StemPro) for up to 4 weeks and staining with LipidTOX Red or Alizarin Red.

Results: Freshly isolated pMSCs were CD73⁺, CD90⁺, CD105^dim, HLA-A2⁺ and CD31^-, CD34^-, CD45^-, at passage 1-2 (n=5). pMSC expression of CD73, CD105, HLA-A2, CD31 and CD34 did not change over 4-8 serial passages. However, a progressive decrease in CD90 and increase in CD45 expression was observed over this period. When cultured in differentiation media, pMSCs differentiated to adipocytes or osteocytes within 7 or 28 days, respectively. 

Conclusions: We have shown that MSC can be isolated from first-trimester placentae using an explant methodology. Future work will determine whether first-trimester pMSCs are more plastic than term pMSCs, and whether they are involved in placental vasculogenesis in early pregnancy.