Trophoblast refers to cells with nourishing function, which comes from the trophoblast outside the embryo. The trophoblast cells grow rapidly, and many hairy processes are formed on the surface of embryo sac, which are called villi. Chorionic trophoblast is mainly cytotrophoblast and syncytial trophoblast. The intermediate trophoblast is an independent type of trophoblast, which covers the morphological and functional characteristics of the above two cells and is the main component of the outer trophoblast. At first, the trophoblast has only one layer of flat cubic cells, and when the villi are formed, this layer of cells gradually differentiates into two layers. The contact between the inner layer and the stroma, formerly called "Langerhans cells", is now called "cytotrophoblast". The outer layer is in contact with the decidua of uterus and is now called syncytial trophoblast. After further understanding, normal trophoblasts have some unique biological characteristics, which are closer to malignant tumors than normal tissues. The trophoblast centrifugally invades the endometrium, myometrium and spiral artery from around the embryo sac, and establishes uteroplacental circulation. During normal pregnancy, the trophoblast spreads widely in the blood, mainly to the lungs, and disappears after delivery due to invasion of blood vessels. The trophoblast covered with chorionic villi is called chorionic trophoblast. The trophoblast in other parts of the uterus is called "extravillous trophoblast". The trophoblast outside the villus forms a trophoblast column, which passes through the villus space from the villus anchor base; Infiltrate decidua around the bottom of embryo sac, form trophoblast shell, and partially evolve into smooth villous epithelial layer; Spiral artery invades placental bed; Infiltrate into the muscle layer under the implantation site. The trophoblast is composed of heterogeneous cell groups, and there are three distinct types in morphology, namely: ① cytotrophoblast (CT); ② syncytiotrophoblast (ST); ③ Intermediate trophoblast. Cytotrophoblast (CT) is composed of uniform, polygonal to oval epithelial cells with single and round nucleus, few cytoplasm, clear cell boundaries and active mitosis. Trophoblastic cells (st) are composed of multinucleated, cytoplasm-rich, double-stained or eosinophilic cells. There are vacuoles of different sizes in the first two weeks of pregnancy, and some vacuoles form lacunae. The trophoblast lacks mitosis because it is the most differentiated trophoblast. Intermediate trophoblast (IT) is mostly composed of monocytes, which is larger than cytotrophoblast. However, it can also be seen that the multinucleated and intermediate trophoblasts are round or polygonal, and they can be spindle-shaped outside villi, with clear and rich cytoplasm, double staining or increased eosinophils, round and lobular nuclei, oval nuclei, irregular chromatin distribution and rare mitosis. The intermediate trophoblast has some common characteristics with cytotrophoblast and syncytiotrophoblast, but it is obviously different from cytotrophoblast and syncytiotrophoblast in light microscope, ultrastructure, biochemistry and function.