(C40-48) Slide 102: Human placenta, 3 months (H&E).

Slide 102 shows a human placenta at about 3 months in pregnancy. From fetal side to maternal side, note the amnion (C40 am), chorionic plate (cp), stem villus (sv), branch villus (bv), and anchoring villus (av). All of the white "empty spaces" surrounding the villi is the maternal blood space. The stem villus arises from the chorionic plate while the branch and anchoring villi arise from the stem villus. As the name implies, the anchoring villus anchors the placenta into the decidua basalis (db), which is part of the maternal uterine endometrium. C41 shows a higher magnification view of part of the placenta. Cross-sections of branched villi (bv, singular= branched villus) and terminal villi (tv, singular=terminal villus) are highlighted on C41. Terminal villi arise as branches off the branch villi. Notice that terminal villi have the smallest diameter of the various villi. At this early stage of placental development, most of the villi are of the branch type, with relatively few small, terminal villi. (Most maternal/fetal exchange occurs at the terminal villi.) Identify maternal and fetal blood spaces, and observe the structures that comprise the placental barrier at this stage. Higher magnification views of terminal villi are shown in C42 and C43.

· Q27 Which structures are maternally derived?

· Q28 Which structures are derived from the fetus?

On C43 look at the cell layers in the villi. Working from the maternal blood space inward, the first cell layer is the syntrophoblast layer. This is a multinucleated syncytium which is postmitotic. You will see "tongues" of syntrophoblast (C43 st) protruding from branch and terminal villi. These are a sign that these villi are still growing.

· Q29 What hormone does the syntrophoblast produce in abundance beginning in the earliest stages of pregnancy?

· Q30 What is its role?

Also note the cytotrophoblast layer (ct) which underlies the syntrophoblast layer. The cytotrophoblasts are highly proliferative cells. These cells fuse with the syntrophoblast, enabling the syntrophoblast layer to grow. Inside the cytotrophoblast layer you will see fetal mesenchyme, and fetal blood vessels. Study a well-sectioned terminal villus on C43 and on your glass slide to identify the layers of the early placental barrier.

· Q31 What are the components of the early placental barrier?

Now turning to the maternal side of the placenta (C44 and C45 av), at the base of the anchoring villi (C44, C45 av) there is a concentration of cytotrophoblasts (C46 ct) which are embedded into the decidua basalis. This concentration of cytotrophoblasts is called the cytotrophoblastic shell. Look in the decidua basalis just beneath the cytotrophoblastic shell for very large, polyhedral-shaped cells that stain lightly with eosin. These are called the decidual cells. They differentiated from stromal cells of the stratum compacta in the endometrium when the embryo implanted. Cytotrophoblasts are seen to migrate into the underlying decidua basalis and mingle among the larger, more eosinophilic decidual cells. The migrating cytotrophoblasts can invade maternal arteries, forming a lining on the inside surface (C47 ct). They are even found invading the myometrium. In the myometrium, cytotrophoblasts fuse to form multinucleated trophoblastic giant cells (C48 tgc). The function of this cell is unknown.