This early implantation site demonstrates the invading front of trophoblasts. Note the difference between the uniform decidual cells versus the more irregular and hyperchromatic look of cytotrophoblasts and intermediate trophoblasts.
The placental implantation site consists of invading intermediate trophoblasts (IT) that initially invades the endometrium (early implantation) and eventually permeates the decidua and superficial myometrium (late implantation). Note that although there may be some associated inflammation cells and the terms 'infiltrate' and 'invasion' are employed, this is a physiologic process.
Microscopically, it may be difficult to distinguish between the decidual cells of the fertile endometrium and the invading intermediate trophoblasts. Decidual stromal cells have abundant pink cytoplasm with uniform, round or oval nuclei with finely distributed chromatin. In contrast, IT cells are variably sized and shaped, ranging from polygonal to spindled to round in shape. The nuclear features are the most helpful in separating them from decidua. IT nuclei are hyperchromatic, irregularly contoured and sometimes lobulated, bi- or multi-nucleated.1,2
Vascular remodeling is also an important part of implantation. IT cells invade the muscular wall of uterine arteries and gradually replace them. This leads to a fixed and open arterial lumen which facilitates circulation and protects against vasospasm.2
IT cells are immunoreactive for keratin and human placental lactogen, but decidual cells in the endometrium and myometrium are not reactive. Note that endometrial glands will stain positively with keratin, therefore, one must examine the pattern of staining. Look for keratin-positive cells in the decidua or myometrium.1
1 Mazur MT, Kurman RJ. Diagnosis of Endometrial Biopsies and Curettings. New York, NY: Springer; 2005: 48-52.
2 Kraus TK, Redline RW, Gersell DJ. Placental Pathology: Atlas of Nontumor Pathology.First series, Fascicle 3. Washington DC: AFIP; 2004: 14