2.4: The Uterus and Cervix

2.4.1 The Uterus

The uterus is the muscular organ that nourishes and supports the growing embryo (Figure 2.4.11). Its average size is approximately 5 cm wide by 7 cm long (approximately 2 in by 3 in) when a female is not pregnant. It has three sections. The portion of the uterus superior to the opening of the uterine tubes is called the fundus. The middle section of the uterus is called the body of the uterus (or corpus). The cervix is the narrow inferior portion of the uterus that projects into the vagina. The cervix produces mucus secretions that become thin and stringy under the influence of high systemic plasma estrogen concentrations, and these secretions can facilitate sperm movement through the reproductive tract.

Figure 2.4.11 The drawing illustrates the internal organs of the female reproductive system: paired ovaries, paired oviducts (Fallopian tubes), uterus, and vagina.
Image Source: Illustration used with permission from Digital Histology. Pakurar, Alice S. and John W. Bigbee. Carole W. Christman, Ph.D, Medical Illustrator. Digital Histology, (ND). This work is openly licensed via CC BY SA Creative Commons License. Available from Virginia Commonwealth University.

2.4.1.1 Uterine Wall

The wall of the uterus is made up of three layers (Figure 2.4.12).

Figure 2.4.12 The uterus comprises an endometrium, similar to a mucosa; a myometrium, analogous to muscularis externa; and a perimetrium, which is either a serosa or an adventitia, depending on its location. The endometrium, influenced by ovarian hormones, changes during the menstrual cycle, preparing for implantation of the blastocyst. 10x
Image Source: Illustration used with permission from Digital Histology. Pakurar, Alice S. and John W. Bigbee. Carole W. Christman, Ph.D, Medical Illustrator. Digital Histology, (ND). This work is openly licensed via CC BY SA Creative Commons License. Available from Virginia Commonwealth University.

• Perimetrium

The most superficial layer is the serous membrane, or perimetrium, which consists of epithelial tissue that covers the exterior portion of the uterus.

• Myometrium

The middle layer, or myometrium, is a thick, highly vascularized, smooth muscle layer responsible for uterine contractions. The smooth muscles of the myometrium enlarge in size due to cell growth (hypertrophy) and increase in number due to cell division (hyperplasia) during pregnancy. Most of the uterus is myometrial tissue, and the muscle fibers run horizontally, vertically, and diagonally, allowing the powerful contractions that occur during labor and the less powerful contractions (or cramps) that help to expel menstrual blood during a woman’s period. Anteriorly directed myometrial contractions also occur near the time of ovulation and are thought to possibly facilitate the transport of sperm through the female reproductive tract.

• Endometrium

The innermost layer of the uterus is called the endometrium. The endometrium contains a connective tissue lining, the lamina propria, which is covered by simple columnar epithelial tissue that lines the lumen. Structurally, the endometrium consists of two layers: the stratum basalis and the stratum functionalis (the basal and functional layers) (Figure 2.4.13). The stratum basalis layer is part of the lamina propria and is adjacent to the myometrium; this layer is not shed during menses. In contrast, the thicker stratum functionalis layer contains the glandular portion of the lamina propria and the endothelial tissue that lines the uterine lumen. The stratum functionalis grows and thickens in response to increased levels of estrogen and progesterone. In the luteal phase of the menstrual cycle, special branches of the uterine artery called spiral arteries supply the thickened stratum functionalis. This inner functional layer provides the proper site of implantation for the fertilized egg, and—should fertilization not occur—it is only the stratum functionalis layer of the endometrium that sheds during menstruation.

Recall that during the follicular phase of the ovarian cycle, the tertiary follicles grow and secrete estrogen. At the same time, the stratum functionalis of the endometrium is thickening to prepare for a potential implantation. The post-ovulatory increase in progesterone, which characterizes the luteal phase, is critical for maintaining a thick stratum functionalis. As long as a functional corpus luteum is present in the ovary, the endometrial lining is prepared for implantation. Indeed, if an embryo implants, signals are sent to the corpus luteum to continue secreting progesterone to maintain the endometrium and thus maintain the pregnancy. If an embryo does not implant, no signal is sent to the corpus luteum, and it degrades, ceasing progesterone production and ending the luteal phase. Without progesterone, the endometrium thins, and under the influence of prostaglandins, the spiral arteries of the endometrium constrict and rupture, preventing oxygenated blood from reaching the endometrial tissue. As a result, endometrial tissue dies, and blood, pieces of the endometrial tissue, and white blood cells are shed through the vagina during menstruation or menses. The first menses after puberty, called menarche, can occur either before or after the first ovulation.

Figure 2.4.13 The endometrium, facing the uterine lumen, is composed of a simple columnar epithelium and its underlying lamina propria, called uterine stroma. The basal zone of the endometrium lies adjacent to the myometrium and is retained during menstruation. Following menstruation, cellular proliferation in the basal zone restores the glands, the surface epithelium, and the stroma (connective tissue lamina propria). In response to ovarian hormones, the functional zone of the endometrium is entirely sloughed during menstruation. The appearance of the glands in the functional zone helps to identify each menstrual stage. In this photomicrograph, some glands are wavy. 10x.
Image Source: Illustration used with permission from Digital Histology. Pakurar, Alice S. and John W. Bigbee. Carole W. Christman, Ph.D, Medical Illustrator. Digital Histology, (ND). This work is openly licensed via CC BY SA Creative Commons License. Available from Virginia Commonwealth University.

Glossary Terms

Cervix

Elongate inferior end of the uterus where it connects to the vagina

Embryo

Group of cells of blastocyst that will become an embryo - the embryo is what will develop into a baby

Epithelial tissue

(cuboidal, serosa, squamos) - The term "epithelium" refers to layers of cells that line hollow organs and glands. It is also those cells that make up the outer surface of the body

Fundus

Dome-shaped region of the stomach above and to the left of the cardia

Lamina propria

Areolar connective tissue underlying a mucous membrane

Luteal phase

Period in the menstrual cycle that follows ovulation, characterized by the development of the corpus luteum, increase in progesterone production by the ovary, and secretion by the glandular epithelium of the endometrium. The luteal phase begins with ovulation and ends with the onset of menstruation.

Menarche

First menstruation in a pubertal female

Menses

Shedding of the inner portion of the endometrium out though the vagina; also referred to as menstruation

Stratum basale

Deepest layer of the epidermis, made of epidermal stem cells

Stratum functionalis

A highly vascularized mucosal layer that undergoes monthly cyclical changes and is lost during menstruation

Uterus

Muscular hollow organ in which a fertilized egg develops into a fetus

Image Acknolwedgements

Pakurar, Alice S. and John W. Bigbee. Carole W. Christman, Ph.D, Medical Illustrator. Digital Histology, (ND). This work is openly licensed via CC BY SA Creative Commons License. Available from Virginia Commonwealth University.