How the Maternal Part Forms
The formation of the maternal placenta is a complex process of interaction between maternal and fetal tissues. The foundation for all transformations is the basal decidua (decidua basalis). Chorionic villi actively grow into this decidua, destroying some of its structures and forming an intimate contact. What remains of the maternal tissues and undergoes transformation forms the three main components of the maternal part: lacunae, septa, and the basal plate (which is the maternal counterpart to the fetal chorionic plate).
Lacunae, Septa, and Rohr's Fibrinoid
- Lacunae are extensive spaces formed where decidual tissue was destroyed by the villi. They are filled with maternal blood. This is where the chorionic villi are immersed: some float freely in the bloodstream, while others (anchoring villi) firmly attach to the walls.
- Septa are connective tissue partitions dividing the lacunae from one another. Essentially, these are areas of decidua basalis that survived the invasion of the villi. Within the septa run maternal blood vessels that supply and drain blood directly into the lacunae.
- Rohr's fibrinoid is a specific matrix located on the surface of the septa bordering the lacunar blood. It is a product of the breakdown of the underlying decidua and blood elements (a structural counterpart to Langhans' fibrinoid).
Structure of the Basal Plate
The basal plate represents the preserved foundation of the decidua basalis. It lies beneath the chorionic villi and serves as a support for the septa extending from it. It consists of two layers:
- Compact layer (superficial). Facing the intervillous space. It contains a diverse cellular ensemble: large clusters of decidual cells, myofibroblasts, and peripheral cytotrophoblast cells that migrated here via anchoring villi.
- Spongy layer (deep). Bordering the muscular wall of the uterus, the myometrium. It contains remnants of uterine glands. Clinically, this is a vital zone: the fundi of these glands persist in the myometrium and after birth serve as the main source of epithelium for endometrial regeneration.
Decidual Cells: The Main Functional Units
These are the key and most numerous cell population in the maternal part of the placenta.
Origin: The population is heterogeneous. One part differentiates from endometrial connective tissue fibroblasts. Another part is bone marrow-derived and belongs to the macrophage system.
Morphology: These are large, oval cells. Their cytoplasm appears pale because it is packed with glycogen granules and lipid droplets. The cells do not lie individually; they form dense clusters, tightly adhering to one another via microvilli.
Functions:
- Synthetic: Actively produce the hormone relaxin (and likely other hormones). Relaxin is vital for preparing the tissues of the birth canal and pelvis for labor.
- Lytic (macrophage-like): This activity increases sharply right before labor, facilitating the successful detachment of the placenta from the uterine wall.
Microscopic Examination (Slide)
On a histological section of the placenta stained with hematoxylin and eosin, one must be able to clearly distinguish the fetal and maternal parts:
- Fetal part: Represented by the amnion (epithelium and underlying connective tissue) and the chorion frondosum (multiple cross-sections of villi of varying diameters).
- Maternal part: Appears as areas of dense decidual tissue (septa and the basal plate with large decidual cells). Between the septa, pale spaces—the lacunae—are visible, containing massive accumulations of erythrocytes (blood) along with anchored and free chorionic villi.