Anatomy of the Endometrial Microvasculature

Most studies of the endometrial vasculature have focused on the large "spiral" arteries rather than the microvasculature (Figure 2). Reference to the latter is usually limited to the statement that the spiral arteries terminate in a "capillary plexus" in the subepithelial zone. As in most tissues, however, the most fundamental function of the endometrial circulation, i.e., the delivery of nutrients and the removal of wastes, occurs at the level of the microvasculature—in the capillaries. The fact that this capillary plexus is so richly supplied by feed arteries, which penetrate the full width of the functionalis with minimal branching, is in itself an indication of its functional importance. Structurally, the sub-epithelial capillaries have been described as highly variable

Figure 1 Diagram of endometrial development during the human menstrual cycle. F, function-alis layer; B, basalis layer; M, myometrium (modified from Ref. [1]).
Figure 2 Diagram showing the histologic organization and major features of the endometrium and its vasculature. Although this figure shows the subepithelial capillary plexus, it does not show that a plexus also surrounds each gland (from Ref. [2]).

in form, diameter, and length [1], with numerous anastomoses interconnecting the various channels and large-diameter lacunae (Figure 3). This description suggests that flow through the plexus may be relatively slow, which would maximize exchange of bloodborne materials with the subepithelial stroma, the epithelium, and the lumen (analogous to the subcutaneous capillary plexus of the skin that facilitates the exchange of heat with the external environment). The capillaries of the plexus empty via narrow branches to larger collecting venules, which in turn empty into progressively larger veins. These eventually enter a venous plexus between the basalis and the myometrium (the outer muscular layer of the uterus).

During the cycle, epithelial cell-lined invaginations, the endometrial glands, develop and extend from the lumen deep into the functionalis layer, thereby greatly increasing the epithelial surface area of the endometrium. In humans, these glands reach their maximum development during the secretory phase of the cycle under the influence of progesterone. Glands also are present in the endometrium of nonmenstruating species, and their development is also stimulated by progesterone. Although seldom discussed, a distinct capillary plexus also envelops each gland [3]. Such a "basket" of capillaries can clearly be seen in Figure 4, surrounding a gland in the monkey endometrium [4]. Capillary branches penetrate every fold of the gland, bringing them into close proximity to the epithelial cells. This suggests that the capillaries are critical for glandular function, which is to produce and secrete or transport nutrients and other factors into the uterine lumen. Indeed, these secretions play essential roles in embryo survival, development, and pregnancy recognition even before the conceptus invades the endo-metrium to gain more direct access to maternal blood (see later discussion).

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Essentials of Human Physiology

Essentials of Human Physiology

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