Day 16 endometrium exhibits proliferative phase endometrium with the added feature of developing subnuclear vacuoles. Proliferative endometirum is characterized by tortuous glands, pseudostratified epithelium and mitotic activity.

The subnuclear vacuoles are partially developed. The stroma is edematous and contains thin-walled blood vessels. Note that the stroma in proliferative phase can be variable, ranging from spindled and cellular to edematous. Mitotic figures are common in the proliferative stroma.

Another image demonstrating partially formed subnuclear vacuoles and tubular glands of proliferative phase endometrium. The glands are evenly spaced, although this may not be evident due to tangential sections of tortuous glands.

Although the subnuclear vacuoles are more fully developed and line up at the basement membrane, less than 50% of glands exhibit subnuclear vacuoles and thus, the criteria (50% rule) for day 17 endometrium confirming ovulation has not been met.

A closer view demonstrates developing subnuclear vacuoles, which are in the process of forming a neat row flush against the basement membrane.

Mitotic figures are frequent findings in Day 16 endometrium, although the mitotic activity is not as robust as in the proliferative phase (day 4-14).

Subnuclear vacuoles are at various stages of development in this image. Some subnuclear vacuoles have lined up at the basement membrane (center), while other glands have not yet formed vacuoles (right).

In this image, the subnuclear vacuoles are well-developed and form an even row at the basement membrane, resembling piano keys. If over 50% of the glands exhibit these changes and no mitotic activity is seen, a day 17 endometrium can be assigned and one can reasonably assume that ovulation has occurred.

Day 16 through Day 28 is considered the secretory phase of the endometrium. In early secretory phase (day 16 through day 20), glandular changes predominate. In mid to late secretory phase (day 21 through day 27), stromal changes take precedence.

Some authors consider day 16 as part of the "interval phase", which occurs 36-48 hours after ovulation (day 14). On day 16, the endometrium exhibits changes attributed to ovulation, however, do not fully meet the criteria to confidently confirm ovulation. The glands are tortuous and mitotically active (similar to late proliferative endometrium). Subnuclear vacuoles are present, but discontinuous and only partially developed.

Note that estrogen alone can produce subnuclear vacuoles, therefore, their presence do NOT confirm that ovulation has occurred. Only when the "50% rule" has been met, can you confirm ovulation. The rule dictates that at least 50% of the glands contain subnuclear vacuoles AND at least 50% of the cells in each gland should exhibit the subnuclear vacuoles. Furthermore, mitotic figures are extremely rare in day 17 endometrium.