Uterus – Tutorial
Please read Unit 13 – Introduction to Female Reproductive System Tissues prior to completing the activities in this chapter.
Introduction to the Uterus
The uterus is a muscular-walled organ in the lower abdominopelvic cavity adjacent to the posterior-superior surface of the urinary bladder with an inner chamber adapted to protect and nourish a developing embryo/fetus throughout pregnancy. In a nonpregnant woman, the uterus is shaped like an upside-down pear and relatively small measuring approximately 3 inches (7.5 cm) long and 2 inches (5 cm) in diameter. During pregnancy the uterus goes through tremendous changes in both size and shape which allow it to contain a fully developed fetus along with the placenta, amniotic fluid, and associated extraembryonic membranes. The ability of the uterus to greatly stretch is made possible by smooth muscle cells in the uterine wall that are able to stretch to accommodate the increasing size of a developing fetus and yet retain the ability to contract.
The largest region of the uterus is the rounded superior portion known as the uterine body. The lumen of the uterine body is known as the uterine cavity and it is lined with tissue where embryos can implant at a specific phase of the menstrual cycle, develop a placenta, and eventually grow into a fetus. The inferior portion of the uterine body tapers into a narrow region known as the isthmus which is continuous with the most inferior portion of the uterus known as the cervix. The cervix extends approximately 1-2 inches (2.5-5 cm) from the body of the uterus to the vagina. The constricted lumen of the cervix that extends from the inferior uterine cavity to the lumen of the vagina is known as the cervical canal. In a nonpregnant woman, the most distal portion of the cervix slightly bulges into the proximal lumen of the vagina forming a convex surface surrounding the external opening of the cervical canal known as the external os. The external os and the cervical canal act as the passageway for sperm to swim up into the uterine cavity after sexual intercourse and for fluid to exit the body during a woman’s menstrual period. When dilated during labor, the cervical canal and cervical os also act as a passageway for a fetus to leave the body during delivery.
The wall of the uterus is composed of three layers known as the perimetrium, myometrium, and endometrium. The perimetrium is a partial serosa (visceral peritoneum) that covers the body of the uterus. The myometrium is the middle layer and by far the thickest layer of the uterine wall. The myometrium is composed of multiple layers of smooth muscle fibers arranged in a variety of planes (circular, oblique, and longitudinal) which strengthen and reinforce the wall of the uterus. Smooth muscle cells in the myometrium are able to greatly stretch to accommodate the increasing size of a developing fetus and yet retain the ability to contract.
The inner-most layer of the uterine wall is the endometrium. The endometrium is divided into an inner functional layer (functional zone) that lines the uterine cavity and a basal layer (basilar zone) sandwiched between the functional layer and the myometrium. The functional layer of the endometrium goes through cyclical changes in its structure during different phases of the menstrual cycle. After a menstrual period, cells of the functional layer rapidly divide to thicken the endometrial lining. During this time, endometrial glands lined with simple columnar epithelium develop within the functional layer. The endometrial glands eventually secrete nutrient rich fluid creating a hospitable environment for the implantation of an embryo. If an embryo does implant into the functional layer, it will grow a placenta and continue to develop into a fetus. If implantation of an embryo does not occur, the functional layer eventually breaks down and is lost as menstrual flow during the next menstrual period.
The tutorials that follow will focus primarily on the structure and function of the wall of the uterus during the secretory phase of the uterine cycle when the endometrium is fully vascularized and endometrial glands are actively secreting nutrient-rich fluid into the uterine cavity (lumen of the uterus). Keep in mind that the secretory phase of the uterine cycle is often referred to as the progravid phase on microscope slide labels.
Tutorial: Use the image slider below to learn more about the structure and characteristics of the uterus.
Microscopy: Use the image slider below to learn how to use a microscope to study uterine tissue on a microscope slide.
