Figure 2223

Transformation zone of the cervix. The site of the squamocolumnar junction from the upper rectangle in Figure 22.21 is shown here at higher magnification. Note the abrupt change from stratified squamous epithelium to simple columnar epithelium (arrow). Neoplastic changes leading to development of cervical cancer most frequently begin in this transformation zone. Within the connective tissue are the branched, mucus-secreting cervical glands (CG) composed of a simple columnar epithelium that is continuous with the lining epithelium of the cervical canal. xl20.

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 vessels 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

• Primary chorionic villi, which are formed by the rapidly proliferating cytotrophoblast. It sends cords or masses of cells into the blood-filled trophoblastic lacunae in the syncytiotrophoblast (see Fig. 22.19b). The primary villi appear between days 11 and 13 of development.

• Secondary chorionic villi, which 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. 22.24a).

• Tertiary chorionic villi are formed by the end of the third week as the secondary villi become vascularized by

chorionic cavity secondary villus syncytiotrophoblast cytotrophoblast developing vessels in wall of chorionic sac endometrium cytotrophoblastic shell tertiary villus syncytiotrophoblast intervillous space maternal blood maternal sinusoid

0 0

Post a comment