"The blastocyst normally attaches to the uterine mucosa about 6 days after fertilization. The outer layer of the blastocyst proliferates to form the trophoblastic cell mass or chorionic shell. Trophoblastic cells derived from this mass penetrate the columnar epithelial cells of the maternal endometrium. The invasion of the endometrium begins on the 7th day after fertilization and is complete by about the 12th day. The primitive trophoblast rapidly differentiates into two layers. The inner layer is composed of mononuclear cells and is known as the cytotrophoblastic layer, whereas the outer layer consists of multinucleated cells and is known as the syncytiotrophoblast. Intercommunicating fluid-filled spaces (lacune) appear in the rapidly enlarging trophoblastic mass from the 8ht day after fertilization.

1. Blood vessel
2. Endometrial stroma
3. Syncytiotrophoblast
4. Cytotrophoblast
5. Surface epithelium
6. Epiblast
7. Aminotic cavity
8. Hypoblast

1. Syncytiotrophoblast
2. Trophoblastic lacunae
3. Surface epithelium
4. Fibrin coagulum
5. Epiblast
6. Aminotic cavity
7. Hypoblast
8. Cytotrophoblast
9. Exocoelomic cavity

1. Extra-embryonic mesoderm
2. Secondary yolk sac
3. Exocoelomic cyst
4. Aminotic cavity
5. Extra-embryonic coelom
6. Epiblast
7. Connecting stalk
8. Hypoblast
9. Primary villi
10. Trophoblastic lacunae

The end of the second week is characterized by the first appearance of chorionic villi. The layer of the placenta from which the villi project is called the chorionic plate. Primary villous stems have a central core of cells derived by the proliferation of the cytotrophoblast. The primary villi gradually develop mesenchymal cores, which convert them into the secondary villi. The mesenchymal cells within the villi differentiate into blood capillaries, thus forming the tertiary villi. The vessels from the villi soon become connected with the embryonic heart via vessels that differentiate in the mesenchyme of the chorion and in the connecting stalk.

1 . Connective tissue
2 . Placental membrane
3 . Syncytiotrophoblast
4 . Cytotrophoblast
5 . Endothelium
6 . Fetal blood vessels

1. Connective tissue
2. Syncytiotrophoblast
3. Cytotrophoblast
4. Endothelium
5. Fetal blood vessels

By the end of the third week, embryonic blood is circulating through the capillaries of the villi. As the growth continues, the villi on the decidua capsularis (abembryonic) pole degenerate to form the chorion laeve, while the villi adjacent to the decidual plate rapidly grow and expand to form the chorion frondosum. The chorion frondosum forms up the placenta together with the decidual plate.
By the end of the fourth month of the pregnancy, the placenta has attained its definitive form and undergoes no further anatomical modifications. It has two components: the maternal portion, formed by the decidual plate, and a fetal portion, made by the chorion frondosum. Growth continues by further ramification of the stem villi into the surrounding intervillous spaces. On the maternal side decidual septa extend into the intervillous spaces, dividing placenta into 10-38 cotyledons.

1. Decidua capsularis
2. Uterine wall
3. Uterine cavity
4. Placenta
5. Decidua parietalis
6. Decidua basalis
7. Chorion leave
8. Embryo
9. Connecting stalk
10. Yolk sac
11. Chorion frondosum
12. Amnion
13. Chorionic cavity
14. Amniotic cavity

The villous surface area continues to increase until term, although the rate of the increase slows from approximately 34 to 36 weeks of gestation. The placenta has a circular shape and measures about 15 to 20 cm in diameter, weighing 500 to 600 g at full term.
The umbilical cord is a vascular cable (~ 55 cm) that connects the embryo to placenta. The umbilical cord of the fetus is covered by the amniotic epithelium and contains two umbilical arteries and one umbilical vein embedded into the Wharton's jelly.

1. Umbilical arteries
2. Chorionic vessels
3. Umbilical vein
4. Placental villi
5. Chorionic plate
6. Cotyledons
7. Amnion
8. Decidual septum
9. Spiral artery
10. Decidual plate

1. Connecting stalk
2. Allantois
3. Amnion
4. Wharton's jelly
5. Vitelline vessels
6. Umbilical vein
7. Yolk sac
8. Umbilical arteries