Microscopic observations of the development of the human placenta

Abstract

The placenta in the first trimester of pregnancy is composed of separate and independent trunks and branching villi resembling a miniature thicket of trees. Later, and pronounced in the last half of pregnancy, newly formed placental tissue results from the organization of fibrin masses, the remains of multiple fetal hemorrhages from villi and trunks into regional intervillous sinuses. Many of the villi and trunks in the last half of pregnancy, consequently, are firmly adherent to one another and the organ resembles a sponge.