Maternal part of the placenta

How the maternal part is formed

The formation of the maternal cotyledon of the placenta is a complex process of interaction between maternal and fetal tissues. The basis for all transformations is the basal decidua (decidua basalis). Chorionic villi actively grow into this decidua, destroying some of its structures and establishing complex 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 of the fetal chorionic plate).

Lacunae, septa, and Rohr's fibrinoid

• Lacunae are expansive spaces formed at the site of decidual tissue destroyed by villi. They are filled with maternal blood. It is here that the chorionic villi are immersed: some float freely in the bloodstream, while others (anchoring villi) are firmly fixed to the walls.
• Septa are connective tissue partitions separating the lacunae from one another. Essentially, these are areas of the decidua basalis that survived the invasion of the villi. Inside the septa run maternal blood vessels that ensure the inflow and outflow of blood directly within the lacunae.
• Rohr's fibrinoid is a specific mass located on the surface of the septa bordering the lacunar blood. It is a decay product of the underlying decidua and blood elements (a sort of analogue 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. Two layers are distinguished within it:

1. Compact layer (upper). Facing the intervillous space. Here is a diverse cellular ensemble: extensive clusters of decidual cells, myofibroblasts, and peripheral cytotrophoblast cells that have migrated here along the anchoring villi.
2. Spongy layer (deep). Borders the muscular wall of the uterus, the myometrium. It harbors remnants of uterine glands. Clinically, this is a critical zone: the bases of these glands remain preserved in the myometrium and, after childbirth, become the main source of epithelium for endometrial regeneration.

Decidual cells — the main workers

This is 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 is of bone marrow origin and belongs to the macrophage system.

Morphology: These are large oval cells. Their cytoplasm appears light because it is packed with glycogen granules and lipid droplets. The cells do not lie individually — they form dense clusters, firmly adhering to each other via microvilli.

Functions:

• Synthetic function: actively produce the hormone relaxin (and probably other hormones). Relaxin is vital for preparing the tissues of the birth canal and pelvis for childbirth.
• Lytic (macrophage) function: this activity increases sharply just before labor, facilitating the successful detachment of the placenta from the uterine wall.

Microscopic diagnosis (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 amniotic membrane (epithelium and underlying connective tissue) and the chorionic villi (multiple sections of villi of varying diameters).
• Maternal part: appears as areas of dense decidual tissue (septa and basal plate with large decidual cells). Between the septa, clear spaces—lacunae—are visible, containing massive accumulations of erythrocytes (blood) and anchored and free chorionic villi immersed in them.

Class and exam questions

What else to study on this topic

• Features of the cellular composition of the compact layer of the basal plate.
• Migration of peripheral cytotrophoblast cells from anchoring villi.
• Differences between the macrophage and fibroblastic lineages of decidual cells.
• Topography of the amniotic membrane and chorionic villi on a histological slide.

Other topics in the "Fetal membranes and placenta" section

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