The process of decidualization
Endometrial changes begin as early as the first weeks of pregnancy and are termed decidualization. This process specifically affects the propria (connective tissue plate) of the endometrium.
The basis of the process is cellular transformation. Ordinary 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 maintain 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). Represents the superficial section of the decidua. Its main histological feature is a very dense, compact aggregation of large decidual cells.
- Spongy layer. 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.
Topographical division of the decidua
Following successful embryo implantation, the logic of further endometrial development dictates its division into three distinct regions. The classification criterion is the topographical position of a specific area of the decidua 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. Proliferates extensively, clearly differentiating into compact and spongy layers.
- Function: forms the maternal part of the placenta. After childbirth, the fundi of the glands preserved in this layer (and adjacent areas) serve as a source for the regeneration of new endometrium.
- Decidua capsularis
- Localization: acts as a barrier separating the embryo from the free uterine cavity.
- Dynamics: as the fetus grows, the blood supply to this zone gradually deteriorates, and the connection with maternal vessels is lost.
- Development outcome: transforms into the outer fetal membrane.
- Decidua parietalis
- Localization: lines the rest of the uterine cavity, remaining outside the initial embryo implantation zone.
- Function: does not participate in the trophism (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, resulting in the formation of a single continuous outer membrane.
Briefly: answers to topical questions
What functions do decidual cells perform during pregnancy?
During pregnancy, decidual cells are part of the maternal placenta and perform both synthetic and lytic functions.
- Synthetic function — production of the hormone relaxin (ensures preparation of the maternal birth canal and pelvic tissues for labor), as well as likely other hormones.
- Lytic function — macrophage activity, which increases sharply before labor. Possessing high lytic activity, decidual cells facilitate placental detachment during and after childbirth.
Decidua basalis is the BASE for the placenta. Decidua capsularis acts like a BAG (capsule) enclosing the fetus from the cavity. Decidua parietalis simply lies along the uterine WALL far from the fetus.
Class and exam questions
In which layer does placental separation occur during labor?
Detachment of the placenta and fetal membranes during labor occurs at the level of the spongy layer of the decidua.
From which region of the decidua does the maternal part of the placenta form?
The maternal part of the placenta is formed from the decidua basalis, which is located between the embryo and the myometrium.
What happens to the capsular and parietal decidua in late pregnancy?
From the 4th month of pregnancy, the decidua capsularis presses against the decidua parietalis due to fetal growth, after which they fuse into a single outer membrane.
Where does endometrial restoration originate after childbirth?
Endometrial regeneration occurs from the fundi of the uterine glands, which are preserved in the deep sections of the decidua basalis.
What else to study on this topic
- Cellular transformation of fibroblasts into decidual cells
- Histology of the compact layer (stratum compactum)
- Histology of the spongy layer and its vascular network
- Mechanism of endometrial regeneration from the fundi of uterine glands
- Formation of the maternal part of the placenta from the decidua basalis
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