The uteroplacental circulatory system begins to develop around day 9, with development of vascular spaces called trophoblastic lacunae within the syncytiotrophoblast. Maternal sinusoids, which develop from capillaries of the maternal side, anastomose with the trophoblastic lacunae (Fig. 23.23). The differential pressure between the arterial and venous channels that communicate with the lacunae establishes directional flow from the arteries into the veins, thereby establishing a primitive uteroplacental circulation. Numerous pinocytotic vesicles present in the syncytiotrophoblast indicate the transfer of nutrients from the maternal vessels to the embryo.
Proliferation of the cytotrophoblast, growth of chorionic mesoderm, and blood vessel development successively give rise to the following.
 Primary chorionic villi are formed by the rapidly proliferating cytotrophoblast. They send cords or masses of cells into the blood-filled trophoblastic lacunae in the syncytiotrophoblast (see Fig. 23.17b). The primary villi appear between days 11 and 13 of development.
 Secondary chorionic villi are composed of a central core of mesenchyme surrounded by an inner layer of cytotrophoblast and an outer layer of syncytiotrophoblast. They develop at about day 16 when the primary chorionic villi become invaded by loose connective tissue from chorionic mesenchyme. The secondary villi cover the entire surface of the chorionic sac (Fig. 23.23a).
 Tertiary chorionic villi are formed by the end of the third week as the secondary villi become vascularized by blood vessels that have developed in their connective tissue cores (Fig. 23.23b and Plate 100, page 888).
As the tertiary villi are forming, cytotrophoblastic cells in the villi continue to grow out through the syncytiotrophoblast.
FIGURE 23.19 • Photomicrograph of a human cervix. This H&E–stained specimen is from a postmenopausal woman. Its lower portion projects into the upper vagina where an opening, the external os, leads to the uterus through the cervical canal. The surface of the cervix is covered by stratified squamous epithelium (SSE) that is continuous with the epithelial lining of the vagina. An abrupt transition from stratified squamous epithelium to simple columnar epithelium (SCE) occurs at the entry to the cervical canal. In this specimen, the stratified epithelium has extended into the canal, an event that occurs with aging. Mucus-secreting cervical glands are seen along the cervical canal. These are simple branched tubular glands that arise as invaginations of the epithelium lining the canal. Frequently, the glands develop into nabothian cysts as a result of retention of mucous secretion by blockage of the gland opening. The material marked by the X is mucus secreted from the cervical glands. 10.
When they meet the maternal endometrium, they grow laterally and meet similar processes growing from neighboring villi. Thus, a thin layer of cytotrophoblastic cells called the trophoblastic shell is formed around the syncytiotrophoblast. The trophoblastic shell is interrupted only at sites where maternal vessels communicate with the intervillous spaces. Future growth of the placenta is accomplished by interstitial growth of the trophoblastic shell.