Decidualization Process
Endometrial changes begin as early as the first weeks of pregnancy, a process known as decidualization. This process specifically affects the lamina propria of the endometrium.
The core of the process is cellular transformation. Standard fibroblasts, as well as bone marrow-derived cells, undergo morphological changes and transform into large, specialized decidual cells. As a result of these changes, the functional layer of the endometrium thickens significantly and acquires new properties necessary to support pregnancy.
Histological Structure (Layers)
The subepithelial part of the decidua has a heterogeneous structure and is histologically divided into two distinct layers:
- Compact layer (stratum compactum). This is the superficial region of the decidua. Its main histological feature is a very dense, compact aggregation of large decidual cells.
- Spongy layer (stratum spongiosum). Located deeper than the compact layer. It retains remnants of uterine glands and contains an abundant, well-developed vascular network.
Clinical significance: The spongy layer plays a key role in the final stage of pregnancy, as it is at this level that the natural detachment of the placenta and fetal membranes occurs during labor.
Topographic Divisions
Following successful embryo implantation, the continued development of the endometrium leads to its division into three distinct regions based on the topographic position relative to the developing fetus.
- Decidua basalis
- Localization: Located directly between the embryo and the muscular layer of the uterus (myometrium).
- Features: Has the most active blood supply among all regions. It proliferates massively, clearly differentiating into compact and spongy layers.
- Function: Forms the maternal part of the placenta. After birth, the fundi of the glands preserved in this layer (and adjacent areas) serve as a source for the regeneration of the new endometrium.
- Decidua capsularis
- Localization: Acts as a barrier separating the embryo from the main uterine cavity.
- Dynamics: As the fetus grows, the blood supply to this zone progressively deteriorates, and its connection with maternal vessels is lost.
- Outcome: Transforms into the outer fetal membrane.
- Decidua parietalis
- Localization: Lines the rest of the uterine cavity, outside the initial site of embryo implantation.
- Function: Does not participate in the trophic support (nutrition) of the developing embryo.
- Dynamics: Starting from the 4th month of pregnancy, the actively growing fetus forces the decidua capsularis to press tightly against the decidua parietalis. They fuse together, forming a single continuous outer membrane.