Uterus

The uterus is lined by a thick mucosa, the endometrium, surrounded by a thick wall of smooth muscle, the myometrium.  

The endometrium consists of a simple columnar epithelium, forming numerous tubular glands, supported by a thick vascular stroma.  Both glands and stroma undergo extensive changes during the menstrual cycle.

The endometrium is functionally subdivided into two layers.

• The stratum functionalis is a thick superficial layer that is sloughed off during menstruation and grows anew during each cycle.  It is therefore a temporary tissue with an "unfinished" appearance -- not quite as tidily organized as mucosal tissues in most other organs.  The stroma more closely resembles embryonic mesenchyme than typical lamina propria.
• The stratum basalis consists of permanent stromal tissue and deep ends of the uterine glands.  These tissues remain through each cycle and serve as sources for cells during regrowth of the superficial stratum functionalis.

When the stratum functionalis is present (i.e, when it has not just been sloughed off), there is no distinct morphological boundary between these two layers.

During the proliferative phase, when the stratum functionalis is growing, the endometrial glands have a relatively smooth contour and mitotic figures are common.

In contrast, when the endometrial glands have matured in the secretory phase, their contour is more tortuous and the epithelium consists of mature secretory cells.

The endometrium is highly vascular, and the blood vessels also participate in the menstrual cycle.  Distal vessels are sloughed off, while the spiral arteries (named for their helical shape) retract into the stratum basalis and constrict to limit blood loss during menstruation.  The spiral arteries then extend again (like springs) as the stratum functionalis regenerates.  

The myometrium consists of smooth muscle, organized into poorly-defined layers with bundles of fibers interwoven in varying directions.  In response to hormonal changes during pregnancy, the myometrium undergoes both hyperplasia (more smooth muscle cells) and hypertrophy (larger smooth muscle cells).

SIUC / School of Medicine / Anatomy / David King

https://histology.siu.edu/erg/uterus.htm
Last updated:  20 May 2022 / dgk