The endometrium is the specialized tissue lining the inside of the uterus. This lining undergoes changes each month as part of the menstrual cycle in preparation for a potential pregnancy. The secretory endometrium refers to the matured state of this lining that occurs during the second half of the cycle, transforming the uterine environment into one receptive to a fertilized egg.
Timing and Characteristics of the Secretory Phase
This phase begins immediately following ovulation, marking the transition from the proliferative phase. For a typical 28-day cycle, the secretory phase generally corresponds to days 15 through 28. It is also referred to as the luteal phase, referencing the corresponding events in the ovary where the egg was released. The phase is named “secretory” because the glands within the lining begin to actively produce and release various substances.
During this time, the endometrial lining becomes thicker, transforming into a dense, cushion-like tissue. It develops a spongy texture and becomes richly supplied with blood vessels to support a developing embryo. A healthy secretory endometrium typically measures between 7 and 14 millimeters in thickness, which is considered optimal for implantation.
The Role of Progesterone and Structural Changes
The transformation into the secretory state is driven primarily by the hormone progesterone, which is produced by the corpus luteum, a temporary structure formed from the follicle that released the egg during ovulation. Progesterone acts on the endometrial tissue, causing changes that define the secretory phase.
Under the influence of this hormone, the glands that line the endometrium change from straight tubes to highly coiled, tortuous structures, often described as “saw-toothed.” These glands become engorged with a glycogen-rich fluid. Simultaneously, the spiral arteries, which are the main blood supply to the functional layer, lengthen rapidly and become distinctly coiled.
The supportive tissue, known as the stroma, also undergoes predecidualization. This involves the stromal cells surrounding the spiral arteries swelling and becoming plump, creating a softer, more edematous environment. This restructuring increases the surface area for nutrient exchange and prepares the tissue to become the maternal part of the placenta, called the decidua, if pregnancy occurs.
Preparing the Uterus for Implantation
The secretory phase establishes a receptive environment for a fertilized egg, which reaches the uterus as a blastocyst. The nutrients secreted by the endometrial glands, including glycogen, proteins, and lipids, provide immediate sustenance to the blastocyst. This nourishment is required before the embryo can fully implant and establish a direct connection to the maternal blood supply.
This period of maximum receptivity is called the “window of implantation,” which typically occurs about six to ten days after ovulation. The secretory endometrium coordinates its molecular signaling and structural readiness with the arrival of the embryo. Specific molecules and receptors are expressed on the surface of the endometrial cells to mediate the firm attachment and adhesion of the blastocyst.
The highly vascularized and nutrient-rich lining ensures the embryo can successfully burrow into the uterine wall. Without this precise, hormone-driven preparation, successful implantation cannot occur. The synchronized timing between the embryo’s development and the endometrial transformation is paramount for fertility.
The Two Possible Outcomes
The fate of the secretory endometrium depends on whether a fertilized egg implants. If a pregnancy is established, the developing embryo begins to release the hormone human chorionic gonadotropin (hCG). This hormone acts to sustain the corpus luteum in the ovary.
The continued presence of the corpus luteum ensures that progesterone levels remain high. These sustained high levels prevent the breakdown of the secretory endometrium, keeping the lining thick, nourished, and receptive to support the growing pregnancy.
If fertilization does not occur, the corpus luteum begins to degrade approximately 14 days after ovulation. This leads to a decline in the production of progesterone and estrogen. The loss of hormonal support causes the spiral arteries to constrict and the functional layer of the secretory endometrium to break down and shed, resulting in menstruation.