A 36-year-old primigravida develops peripheral edema late in the second trimester. On physical examination, her blood pressure is 155/95 mm Hg. Urinalysis shows 2+ proteinuria, but no blood, glucose, or ketones. At 36 weeks, she gives birth to a normal viable but low-birth-weight infant. Her blood pressure returns to normal, and she no longer has proteinuria. Which of the following pathologic findings is most likely to be found on examination of the placenta?