The primitive uteroplacental circulation is established during which period of fetal development?

The primitive uteroplacental circulation is established during which period of fetal development?

  1. First week
  2. Second week
  3. Third week
  4. End of first month
  5. Second trimester

View Explanation

During the second week of fetal development, lacunar spaces develop between cells of the syncytiotrophoblast, particularly in the region of the embryonic pole as the conceptus invades the endometrium. Endometrial capillaries in this region become dilated and engorged with blood to form sinusoids. The syncytial cells direct erosion of the endothelium of the maternal capillaries, allowing maternal blood to enter the lacunae and bathe the syncytial cells. This establishes the uteroplacental circulation. During the second week, primary villi consist of projections of syncytial cells surrounding a core of cytotrophoblast cells. During the third week, the villus core is invaded by mesodermal cells to form a secondary villus. Cells of the mesodermal core will then differentiate to form capillaries and blood cells by the end of the third week (tertiary villus). These vessels become connected to the fetal circulation early in the fourth week.