Attaches to uterine wall and establishes a connection between fetus and mother
Sources of placenta
Fetal – chorion frondosum
Maternal – decidua basalis
Development of the Placenta
1. Implantation completed on D11, blastocyst surrounded by lacunar spaces around cords of syncytial cells
2. From trabeculae, stem villi develop on Day13 connecting chorionic plate with basal plate
3. Primary, secondary and tertiary villi successively develop from stem villi
4. Arterio-venous system develops in mesenchymal core of each villus completed by D21, connecting with intraembryonic vascular system
Structure of Chorionic Stem Villus
Early
Primary villi
Secondary villi
Tertiary villi
The lacunar spaces become confluent and by 3rd/4th week form a multilocular receptacle lined by syncytium and filled with maternal blood, which is the future intervillous space
There are 4 layers between fetal and maternal blood
Development of the Placenta
1. Decidua capsularis becomes thinner and regresses
2. Exuberant growth of decidua basalis and chorion frondosum
3. Discrete placenta formed from 6/52 and completed by 12/52
4. Placenta grows in thickness and circumference until 16/52
