Uterine Glands: Function, Anatomy, and Role in Pregnancy

Uterine glands are microscopic structures within the endometrium, the lining of the uterus. These glands are central to the reproductive process, undergoing dynamic changes throughout the menstrual cycle in response to hormonal signals from the ovaries. Their primary purpose is to create a nourishing environment for a potential pregnancy by secreting a complex fluid that supports an embryo in its earliest stages.

Anatomy and Location of Uterine Glands

Uterine glands are simple tubular structures originating from the surface of the endometrium. They are formed by an infolding of this surface layer, extending down into the supportive tissue beneath, called the endometrial stroma. This placement allows them to deliver secretions directly into the uterine cavity.

These glands are primarily located in the endometrium’s functional layer, the portion that is shed during menstruation if pregnancy does not occur. Their structure consists of a simple columnar epithelium, a single layer of column-shaped cells. Among these are secretory cells that produce glandular fluids and ciliated cells that help move secretions into the uterine lumen.

The glands are not static; their shape changes in response to hormonal cues. Initially, they are small, straight tubes. As the reproductive cycle progresses, they become elongated and coiled, which increases their surface area for secretion. This structural transformation is tied to their function of preparing the uterus for pregnancy.

Hormonal Influence and Cyclical Activity

The structure and function of uterine glands are directly controlled by the ovarian hormones estrogen and progesterone. Their activity follows a distinct pattern aligned with the menstrual cycle, ensuring the endometrium is prepared for embryo implantation at the correct time.

During the proliferative phase, the first half of the cycle, rising estrogen levels stimulate the growth of the endometrium. The uterine glands elongate and their cells multiply, causing them to appear as long, straight tubes. This phase rebuilds the functional layer of the endometrium.

Following ovulation, the secretory phase begins, driven by progesterone. Progesterone causes the glands to mature, becoming coiled structures with wider inner spaces, or lumens. This coiling maximizes their secretory capacity, and they begin to produce a nutrient-rich fluid. If fertilization does not occur, progesterone levels fall, and the functional layer, including the glands, is shed during menstruation.

Functions in Early Pregnancy

If fertilization and implantation occur, uterine glands support the early embryo. Their secretions, often called histotroph or “uterine milk,” provide nutritional support for the conceptus before the placenta is fully functional. This histotrophic nutrition sustains the embryo before a direct maternal blood supply is established.

Glandular secretions create a receptive environment for the attachment of the blastocyst, or early-stage embryo, to the uterine wall. Components in the fluid modulate the local maternal immune system, preventing rejection of the embryo. This modulation ensures the embryo’s survival without compromising the mother’s overall immune defenses.

Studies on animal models without uterine glands show that pregnancy cannot be established. This indicates the glands are active participants in signaling events, not just passive providers of nutrition. They are involved in the dialogue between the embryo and endometrium, coordinating implantation timing, stromal cell changes, and early placental development.

Composition and Importance of Uterine Secretions

The fluid secreted by uterine glands is a complex mixture. This histotroph includes nutrients like glucose, lipids, and amino acids, which are building blocks for the embryo’s dividing cells. It also contains glycogen, a stored form of glucose, that provides a ready energy source for the conceptus.

Beyond nutrients, the secretions are rich in bioactive molecules that regulate embryonic development, including various growth factors. For example, Leukemia Inhibitory Factor (LIF) helps make the uterus receptive to the embryo and initiates the implantation process.

The fluid also contains specific proteins with multiple roles. Glycodelin-A has immunomodulatory effects, helping create a tolerant environment for the embryo. Osteopontin is an adhesive protein that helps the embryo attach to the endometrial surface. This combination of molecules supports the conceptus’s survival and development.