Stem Cell Regeneration Cycle

PHRTTM: Restoring the Body’s Native Stem Cell Regeneration Cycle

By James Nagel, MD

In recent years, regenerative medicine has shifted the focus of healthcare from simply managing disease to actively restoring health. At the center of this transformation are stem cells, our body’s own built-in repair system. These remarkable cells have the unique ability to develop into many different types of tissues and to help repair damage wherever it occurs.

I like to think of stem cells as our body’s internal maintenance team. When injury or wear and tear occurs, these cells are called into action. The more stem cells circulating in your bloodstream, the greater your body’s ability to repair tissues, maintain function, and support long-term health. This is why increasing the number and activity of these cells, especially through natural, internal (endogenous) mobilization, is a key strategy in promoting longevity.

Endothelial Progenitor Cells and Vascular Renewal

One particularly important group of stem cells is called endothelial progenitor cells (EPCs). These cells play a critical role in maintaining the health of your blood vessels. Endothelial progenitor cells (EPCs) are bone marrow–derived cells that circulate in the bloodstream and play a central role in vascular repair and regeneration³. They contribute to repairing damaged vessel walls, support the formation of new blood vessels, and help maintain healthy circulation.They respond to signals in the body, such as vascular endothelial growth factor (VEGF), to restore and protect the inner lining of blood vessels (the endothelium).

Healthy blood vessels are essential for delivering oxygen and nutrients to every organ in your body,including your heart and brain. When EPC function is strong, your vascular system remains resilient and efficient.

However, as we age, both the number and function of EPCs naturally decline. This reduction contributes to the development of conditions such as atherosclerosis (hardening of the arteries) and cardiovascular disease.

Aging EPCs also show signs of cellular fatigue. Their telomeres (protective caps on DNA) become shorter, and their ability to regenerate declines. Because of this, EPCs are considered a marker of both vascular aging and overall biological aging.

This is where hormone optimization may play a powerful role.

Estrogen as a Driver of Vascular Repair and Regeneration

As you’re aware, estrogen plays a central role in cardiovascular health in women. It supports vascular function by promoting vasodilation, reducing inflammation, preserving endothelial integrity, and helping regulate cholesterol handling within the vessel wall. Estradiol also stimulates endothelial cell growth, migration, and repair, helping maintain healthy blood vessel function over time.

One emerging area of research suggests that estrogen may also support the body’s natural vascular repair system through its effects on circulating progenitor cells. Among these are subsets of CD34+ progenitor cells, which are widely used as markers of circulating progenitors associated with angiogenesis and vascular repair.5,6,7

Experimental and clinical studies suggest that estradiol enhances the proliferation, migration, and mobilization of these vascular repair-associated progenitor cells. Importantly, human studies have shown that circulating CD34+ progenitor cell populations fluctuate across the menstrual cycle, with peak levels occurring during the periovulatory and mid-luteal phases, periods characterized by elevated estradiol levels.⁴ Collectively, these findings suggest that estrogen’s cardiovascular benefits may extend beyond vasodilation alone, potentially helping coordinate endothelial repair, angiogenic signaling, and regenerative vascular maintenance through progenitor cell biology.

Together, these findings support a direct and biologically meaningful relationship between peak estradiol signaling and EPC mobilization, reinforcing the role of estrogen as a regulator of vascular repair dynamics.

This raises an intriguing possibility: the pronounced estradiol surge around ovulation may serve not only a reproductive function, but also act as a systemic regenerative signal.

A New Perspective on the Ovulatory Cycle and Hormone replacement

Modern research increasingly demonstrates that hormone signaling patterns, not simply hormone levels, play a critical role in physiology.

Rather than viewing hormones solely as substances to replace, it may be more accurate to view them as signals that need to be restored to help orchestrate the complex biological programs, including vascular repair, progenitor cell mobilization, and tissue regeneration that the body once performed on its own.

Physiologic Hormone Restoration Therapy® (PHRT™) is built on the concept that hormones function as biological signals that follow natural rhythms, rather than static substances that simply need to be replaced. By seeking to more closely reflect these physiologic signaling patterns, PHRT™ aims to restore not just hormone levels, but the dynamic hormonal communication that supports normal physiology.

Understanding the role of these rhythms may ultimately reshape how we think about hormones, aging, and the restoration of physiologic function after menopause.

References

  1. Gargett CE, Ye L. Endometrial stem/progenitor cells and the regenerative capacity of the endometrium. Hum Reprod Update. 2012.

  2. Critchley HOD, Maybin JA, Armstrong GM, Williams AR. Physiology of the endometrium and regulation of menstruation. Physiol Rev. 2020.

  3. Asahara T, Murohara T, Sullivan A, et al. Isolation of putative progenitor endothelial cells for angiogenesis. Science. 1997.

  4. Sunao Tanaka, Takayuki Ueno, Fumiaki Sato, Yoshitsugu Chigusa, Nobuko Kawaguchi-Sakita, Masahiro Kawashima, Noriyoshi Fujisawa, Kenichi Yoshimura, Satoshi Teramukai, Hiroshi Fujiwara, Masatoshi Fujita, Masakazu Toi, Alterations of Circulating Endothelial Cell and Endothelial Progenitor Cell Counts around the Ovulation, The Journal of Clinical Endocrinology & Metabolism, Volume 97, Issue 11, 1 November 2012, Pages 4182–4192, https://doi.org/10.1210/jc.2012-1736

  5. Nakada D, Oguro H, Levi BP, et al. Oestrogen increases haematopoietic stem-cell self-renewal in females and during pregnancy. Nature. 2014;505(7484):555–558.

  6. Asahara T, Murohara T, Sullivan A, et al. Isolation of putative progenitor endothelial cells for angiogenesis. Science. 1997;275(5302):964–967. .

  7. Fadini GP, Agostini C, Avogaro A. Autologous stem cell therapy for peripheral arterial disease: meta-analysis and systematic review of the literature. Atherosclerosis. 2010;209(1):10–17.

  8. Mendelsohn ME, Karas RH. The protective effects of estrogen on the cardiovascular system. N Engl J Med. 1999.