What is the decidua?

Definition of Decidua

The decidua is the specialized endometrial tissue that forms during pregnancy, serving as the maternal component of the placenta that embraces and supports the developing embryo. 1

Structure and Classification

The decidua consists of terminally differentiated uterine stromal cells, blood vessels, and a repertoire of uterine local immune cells. It can be anatomically classified into three distinct regions:

1. Decidua basalis - Located at the site of embryo implantation, it has rough surfaces, a dense texture, and high blood content. This region contains extravillous trophoblast cells and is where the placenta develops. 2

2. Decidua capsularis - A thin membrane tissue that surrounds the developing embryo, with or without blood clots. 2

3. Decidua parietalis - A thick tissue with less blood content that lines the remainder of the uterine cavity, with one side being intact epidermis and the other side being rough tissue. 2

Functions

The decidua performs several critical functions essential for successful pregnancy:

• Immunological protection - Creates an immunologically privileged environment that prevents maternal immune rejection of the embryo while maintaining protection against infections 1

• Nutritional support - Provides nutrition to the developing embryo before placental vascular connections are established 1

• Physical scaffolding - Offers structural support for the growing embryo 1

• Regulation of trophoblast invasion - Controls the extent of extravillous trophoblast invasion through the secretion of regulatory factors 3

• Vascular remodeling - Facilitates the remodeling of spiral arteries to establish adequate blood flow to the placenta 1, 4

Cellular Components

The decidua contains various cell types that contribute to its function:

• Decidualized stromal cells - Terminally differentiated endometrial stromal cells that undergo dramatic morphological and functional changes 5

• Uterine natural killer (uNK) cells - The dominant leukocyte population with a unique phenotype that regulates trophoblast invasion and vascular remodeling 4

• Innate lymphoid cells (ILCs) - Recently discovered immune cells that contribute to the local immune environment 1

• Decidual macrophages and dendritic cells - Modulate adaptive immunity by balancing cytokines and promoting regulatory T cell generation 1

Clinical Significance

The decidua has important clinical implications:

• Placenta accreta spectrum disorders - Abnormal placental attachment occurs when there is deficient decidua, allowing direct contact between chorionic villi and myometrium 6

• Pregnancy complications - Defects in decidual development during early pregnancy can result in pregnancy loss or complications in later gestational stages 1

• Retained products of conception - After delivery or pregnancy termination, retained decidual tissue can cause bleeding, infection, and future fertility complications 7

• Placental insufficiency - Inadequate decidualization may contribute to placental insufficiency and related obstetric complications 3

Understanding the decidua's structure and function is essential for diagnosing and managing various pregnancy-related conditions and complications.