Learning Objectives

• Describe the histological features of the different segments of the female genital tract
• Describe the changes in the endometrium in the uterus during the menstrual cycle

Oviduct

After release from the ovary, the ovum enters the oviduct. The oviduct consists of several segments: the infundibulum, which lies closest to the ovary, followed by the ampulla, the isthmus, and the pars interstitialis. The first two of these regions have a characteristic appearance that is dominated by an elaborate mucosa that is thrown into numerous branched folds, surrounded by a relatively thin layer of smooth muscle. As the tube moves away from the ovary and toward the uterus, these folds become smaller and the smooth muscle dominates.

Oviduct Epithelium

The oviduct epithelium consists of two distinct cell types. The ciliated cells dominate and serve to move the ovum away from the ovary and toward the uterus. The non-ciliated secretory cells, also known as peg cells, release a secretion that lubricates the tube and provides nourishment and protection to the traveling ovum.

Uterus

The uterus is divided into several layers that have distinct structural and functional characteristics. The simplest classification of these layers is their division into a mucosal layer, or endometrium, a muscularis layer, or myometrium, and a serosal layer, or perimetrium. The endometrium itself is divided into two layers, the stratum functionalis and stratum basalis. During the menstrual cycle, the stratum functionalis expands and vascularizes and is subsequently sloughed off during the process of menstruation, whereas the stratum basalis remains relatively constant. The myometrium allows for the expansion and contraction of the uterine cavity and is responsive to the hormone oxytocin.

Uterine Cycle

The endometrium undergoes dramatic structural and functional changes during the menstrual cycle. These changes are divided into two phases: proliferative and secretory.

Proliferative Phase

The proliferative phase is characterized by robust growth of the epithelial cells in the stratum functionalis and the formation of coiled and densely packed glands. This changes in this phase are driven by estrogen.

Secretory Phase

The secretory phase of the uterine cycle begins at ovulation. In this phase, the glands become even more complexly coiled and the endometrial lining reaches its maximal thickness, whereas the stratum basalis and myometrium remain relatively unchanged. Note the saw-toothed appearance of the glands. Secretions rich in glycogen and glycoprotein can be observed in the lumina of the glands. This phase is drive by progesterone.

Menstrual Phase

If fertilization does not occur, the placental tissue does not produce hCG and the corpus luteum degenerates. The uterine lining does not receive the progesterone, causing the spiral arteries constrict and the endometrial tissue to become ischemic. This causes cell death and the sloughing of the stratum functionalis.

Cervix

The cervix lies at the base of the uterus and serves to protect it from bacterial infiltration. It is the site of an important epithelial transition. The upper cervix (endocervix) is lined by a simple columnar epithelium that contains mucous-secreting cells. In contrast, the lower cervix (ectocervix) is lined by a stratified squamous epithelium. The transition point between these two epithelia is known as the external os. Note how the underlying layers of the cervix are composed primarily of collagenous and elastic connective tissue rather than smooth muscle fibers.

Vagina

The vagina is lined by a stratified squamous epithelium that features a small degree of keratinization. Below the epithelium is a thick layer of dense connective tissue, like that in the dermis of the skin. A layer of loose connective tissue containing many blood vessels and nerves follows this. The cells of the vaginal wall typically contain a relatively large amount of cytoplasm because they store and release glycogen. Glycogen is metabolized by commensal bacteria to lactic acid that prevents growth of pathogenic microorganisms.