The ovary is in the late follicular stage just before ovulation, characterized by high levels of estrogen. Estrogen stimulates the growth of the uterine endometrium and induces expression of the progesterone receptor. Furthermore, estrogen triggers the LH surge, which leads to meiotic maturation of the oocyte and the release of the cumulus-oocyte complex during ovulation.

The period between fertilization and implantation lasts approximately 6 days, occurring around day 22 of the menstrual cycle. During this time, the ovary is in the midluteal phase and produces significant amounts of progesterone. Progesterone promotes secretion from the uterine glands, which provide nourishment to the developing embryo through histiotropic nutrition. This nutrition process is crucial for sustaining the embryo.

The events in the menstrual cycle and endometrium are synchronized with fertilization, early embryonic development within the oviduct, and implantation of the embryo (blastocyst) into the uterine endometrium. The graph in Figure 44.30 illustrates the various stages and hormonal fluctuations involved.

Key events and phases include:
- Peak estrogen (E2) levels on day 13
- Onset of LH surge
- Dominant follicle with high estrogen output during the late follicular phase
- Resumption of oocyte meiosis on day 13
- Blastocyst present in the uterus on day 20
- Early luteal phase on day 14-15
- Formation of the first polar body and arrest at metaphase II, 35 hours after the LH surge begins
- Late luteal phase on day 21
- Growth of the functional zone in the uterine endometrium
- Induction of progesterone (P4) receptors
- Progesterone inducing secretion from estrogen-primed glands and opposing estrogen's mitogenic effects
- Receptive period with decreased antiadhesion protein expression and increased adhesion protein expression in the uterine endometrium
- Arrested primary oocyte and the presence of the egg and embryo during the late proliferative phase
- Early secretory phase on day 15
- Late secretory phase on day 28
- Rescue of corpus luteum (CL) by hCG on day 20
- Fertilization and pronuclear stage on day 24
- Blastocyst hatching from the zona pellucida and initiation of implantation on day 22-23 (6-7 days after fertilization)
- 2-cell embryo on day 17
- Completion of interstitial implantation mode on day 26

Progesterone plays additional roles in inhibiting myometrial contraction, preventing the release of paracrine factors that lead to menstruation (e.g., cytokines, prostaglandins, chemokines, and vasoconstrictors). It also induces the "window of receptivity" in the uterine endometrium, which occurs from approximately day 20 to day 24 of the menstrual cycle. This receptive phase is associated with increased adhesivity of the endometrial epithelium. It involves the formation of pinopodes, cellular extensions, on the apical surface of the endometrial epithelia. Additionally, there is increased expression of adhesive proteins (e.g., integrins, cadherins) and decreased expression of antiadhesive proteins (e.g., mucins) in the apical cell membrane.

When a fertilized egg successfully implants in the uterus, the uterine endometrium reaches its full thickness, actively secretes substances, and can firmly adhere to the implanting embryo.
Fig. 44.30 ). Just before ovulation the ovary is in the late follicular stage and produces high levels of estrogen. Estrogen promotes growth of the uterine endometrium and induces expression of the progesterone receptor. Estrogen ultimately induces the LH surge, which in turn induces meiotic maturation of the oocyte and ovulation of the cumulus-oocyte complex.
The events between fertilization and implantation take about 6 days to complete, so implantation occurs at about day 22 of the menstrual cycle. At this time the ovary is in the midluteal phase and secreting large amounts of progesterone. Progesterone stimulates secretion from the uterine glands, which provide nutrients to the embryo. This is referred to as histiotropic nutrition and is an important
CHAPTER 44 The Male and Female Reproductive Systems 817 •Fig. 44.30 Synchronizationofeventsofthemenstrualcycle(ovaryandendometrium)withfertiliza-tion,earlyembryonicdevelopmentwithintheoviduct,andimplantationofembryo(blastocyst)intotheuterineendometrium.E2,estradiol;P4,progesterone.(ModifiedfromWhiteBA,PorterfieldSP.Endocrine Physiology. 4thed.Philadelphia:Mosby;2013.)d13 Peak E2 levels Onset of LH surge Dominant follicle-high E2 output d7 Ovary and pituitary Late follicular phase d13 Resumption of meiosis d20 Blastocyst in uterus d14 d15 Early luteal phase d15 First polar body and arrest at metaphase II 35 hours after onset of LH surge d21 Late luteal phase Growth of functional zone Induction of P4 receptors P4 induces secretion from E2-primed glands, and opposition of E2 mitogenic effects Receptive period Decreased antiadhesion protein expression Increased adhesion protein expression Uterine endometrium Arrested primary oocyte Egg and embryo Late proliferative phase Early secretory phase Late secretory phase d28 Rescue of CL by hCG d20 d24 d16 Fertilization pronuclear stage d23 Blastocyst hatches from zona pellucida and implantation begins on day 22–23 (6–7 days after fertilization) d22d17 2-cell embryo d26 Completion of interstitial implantation mode of maternal-to-fetal transfer of nutrients for about the first trimester of pregnancy, after which it is replaced by hemotropic nutrition (see later). Progesterone inhibits myometrial contraction and prevents release of paracrine factors (e.g., cytokines, prostaglandins, chemokines, and vasoconstrictors) that lead to menstruation. Progesterone induces the “window of receptivity” in the uterine endometrium, which exists from about day 20 to day 24 of the menstrual cycle. This receptive phase is associated with increased adhesivity of the endometrial epithelium and involves formation of cellular extensions called pinopodes on the apical surface of endometrial epithelia, along with increased expression of adhesive proteins (e.g., integrins, cadherins) and decreased expression of antiadhesive proteins (e.g., mucins) in the apical cell membrane.
When a fertilized egg implants in the uterus, the uterine endometrium is at its full thickness, is actively secreting, and is capable of tightly adhering to the implanting embryo.