ST is made up of terminally differentiated cells that cover the chorionic villi and synthesizes and secretes a number of pregnancy-associated hormones including human placental lactogen (hPL), SP-1, and p-hCG (human chorionic gonadotropin). Some of these secretory proteins may also have a paracrine function by regulating the local microenvironment of decidual cells, inflammatory cells, and smooth muscle cells at the placental site. In addition to its role as an endocrine organ, the ST is bathed in maternal blood and is responsible for the exchange of oxygen, nutrients, and a variety of metabolic products between the mother and the fetus.

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