Uterus at Time of Implantation
المؤلف:
T.W. Sadler
المصدر:
Langmans Medical Embryology
الجزء والصفحة:
14th E, p46-48
2025-06-22
362
The wall of the uterus consists of three layers:
1. Endometrium or mucosa lining the inside wall
2. Myometrium, a thick layer of smooth muscle
3. Perimetrium, the peritoneal covering lining the outside wall (Fig.1)
Fig1. Events during the first week of human development. 1, oocyte immediately after ovulation; 2, fertilization, approximately 12 to 24 hours after ovulation; 3, stage of the male and female pronuclei; 4, spindle of the first mitotic division; 5, two—cell stage [approximately 30 hours of age]; 6, morula containing 12 to 16 blastomeres [approximately 3 days of age]; 7, advanced morula stage reaching the uterine lumen [approximately 4 days of age]; 8, early blastocyst stage [approximately 4.5 days of age; the zona pellucida has disappeared]; 9, early phase of implantation [blastocyst approximately 6 days of age]. The ovary shows stages of transformation between a primary follicle and a preovulatory follicle as well as a corpus luteum. The uterine endometrium is shown in the progestational stage.
From puberty (11 to 13 years) until menopause (45 to 50 years), the endometrium under goes changes in a cycle of approximately 28 days under hormonal control by the ovaries. During this menstrual cycle, the uterine endometrium passes through three stages:
1. Follicular or proliferative phase
2. Secretory or progestational phase
3. Menstrual phase (Figs. 2 and 3)
Fig2. Changes in the uterine mucosa correlated with those in the ovary. Implantation of the blastocyst has caused development of a large corpus luteum of pregnancy. Secretory activity of the endometrium increases gradually as a result of large amounts of progesterone produced by the corpus luteum of pregnancy.
Fig3. Changes in the uterine mucosa [endometrium] and corresponding changes in the ovary during a regular menstrual cycle without fertilization.
The proliferative phase begins at the end of the menstrual phase, is under the influence of estrogen, and parallels growth of the ovarian follicles. The secretory phase begins approximately 2 to 3 days after ovulation in response to progesterone produced by the corpus luteum.
If fertilization does not occur, shedding of the endometrium (compact and spongy layers) marks the beginning of the menstrual phase. If fertilization does occur, the endometrium assists in implantation and contributes to formation of the placenta. Later in gestation, the placenta assumes the role of hormone production, and the corpus luteum degenerates.
At the time of implantation, the mucosa of the uterus is in the secretory phase (Fig.2), during which time uterine glands and arteries become coiled and the tissue becomes succulent. As a result, three distinct layers can be recognized in the endometrium: a superficial compact layer, an intermediate spongy layer, and a thin basal layer (Fig.2). Normally, the human blastocyst implants in the endometrium along the anterior or posterior wall of the body of the uterus, where it becomes embedded between the openings of the glands (Fig. 2).
If the oocyte is not fertilized, venules and sinusoidal spaces gradually become packed with blood cells, and an extensive diapedesis of blood into the tissue is seen. When the menstrual phase begins, blood escapes from superficial arteries, and small pieces of stroma and glands break away. During the following 3 or 4 days, the compact and spongy layers are expelled from the uterus, and the basal layer is the only part of the endometrium that is retained (Fig. 3). This layer, which is supplied by its own arteries, the basal arteries, functions as the regenerative layer in the rebuilding of glands and arteries in the proliferative phase (Fig. 3).
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