Formation of the Decidua
After fertilization the endometrium of the uterus is known as the decidua. Oestrogen increases the size to about 4 times of its pre-gravid thickness, the corpus Luteum, and produce large amount of progesterone which increases the secretion of the endometrial glands and increases the blood vessels. So it makes the endometrium to be softer, spongy and vascular for the fertilized ovum to embed and nourishes itself. The decidua is transformed into 3 layers.
1. The Basal layer (Basement): This lies immediately above the myometrium. It remains unchanged in itself but regenerate the new endometrium after delivery.
2. The functional layer (cavernous layer): it consists of tortuous glands rich in secretions. The stoma cells are enlarged in what is known as the decidua reaction which provides defense against excessive invasion by the syncytiotrophoblast and limit it to this spongy layer. It provides anchor for the placenta and allows it to have access to nutrition and 0xygen. It is the functioning layer.
3. The compact layer it covers the surface of the decidua and composed of closely packed stoma cells, polygonal in shape and it contains necks of glands.
The blastocyst forms a small nodule in the decidua which bulges out into the uterine cavity progressively as it continues to enlarge and divides the decidua into three areas.
1. Decidua Basalis: This is the area of the decidua underneath the developing ovum.
2. Decidua capsularis: the area which covers the ovum.
3. Decidua vera (Perietal) (True Decidua): This lies in the remainder of the uterine cavity.
As development continues the ovum grows and completely fill up the uterine cavity, at about the 12th week the decidua capsularis comes in contact with the decidua vera, it fuses with it and degenerates.