1. Introduction

Stem cells are undifferentiated cells capable of simultaneously self-renewing and differentiating into multiple tissue-specific cell types under appropriate stimuli [1,2]. Traditionally, stem cells can be classified according to their location and differentiation potency. The most potent stem cells are zygotes, classified as totipotent stem cells, which have the ability to generate a whole embryo and extra-embryonic structures. Then, pluripotent stem cells such as embryonic stem cells give rise to the three primary germ layers, namely endoderm, ectoderm, and mesoderm. Induced pluripotent stem cells, generated by the reprogramming of somatic cells, are included in this group. Unipotent stem cells have the narrowest differentiation capability and divide themselves repeatedly into a single cell type. Finally, multipotent cells give rise to specific lineages [3]. Adult stem cells (ASCs), also referred to as somatic stem cells, are a genre of multipotent stem cells located in specific differentiated organs and can differentiate into a limited type of mature cell to maintain tissue homeostasis [2]. The necessary conditions are provided by the specific anatomical location surrounding the ASCs. This microenvironment, called the stem cell niche, gives rise to autocrine, paracrine, and systemic signals that enable stem cell maintenance and differentiation into specific cell types that participate in tissue repair or regeneration [4,5]. Most of the described ASCs reside in the bone marrow, but they are also detected in several organs and play a crucial role in tissue homeostasis, renewal, and repair [6]. Consequently, this transdifferentiation capacity enables research into therapeutic approaches in tissues such as the blood [7], intestine [8], skin [9], muscle [10], brain [11], and endometrium [12].

The endometrium is the innermost lining of the uterus and its main function is preparing for implantation and attracting the blastocyst towards the uterus. The human endometrium is divided into two different layers with different properties. First, the functionalis, which is the upper layer, is formed by luminal epithelium and subjacent stroma as well as microvasculature. Second, the basalis is constituted by glands and stroma that are preserved throughout the female’s life [13]. During menstruation, the functionalis layer is removed from the body through the menstrual blood, while the basalis remains as an endometrial supply for regeneration of a new functional layer in the next cycle. The human endometrium has some features including menstruation that make it physiologically unique from the murine models. The human menstrual cycle consists of three stages: growth, differentiation, and shedding, which occur around 400 times until menopause [14]. In the mouse, the estrous cycle is divided into four stages (proestrus, estrus, metestrus, and diestrus) occurring every 4 to 5 days, but does not involve menstruation and regeneration of the functionalis layer [15]. Mice undergo up to 80 estrous cycles and can give rise to 8-10 litters during their reproductive life, so active repair and regeneration mechanisms in the endometrial mucosa are critically important [16]. This pronounced remodeling ability and the later proliferative changes seen in adult mammals have driven the hypothesis that there is a niche of ASCs in the endometrium that is activated in every cycle [17,18]. Accordingly, alterations in this endogenous niche could be responsible for endometrial pathology, causing fertility problems [19]. Endometrial pathologies such as Asherman syndrome (AS), caused by the presence of intrauterine adhesions [20], or endometrial atrophy (EA), characterized by an atrophic and usually thin endometrium [21], could originate from insufficient production of endogenous cells and/or a non-functional ASC niche. Other gynecological pathologies such as endometriosis could also be caused by variations in endogenous endometrial ASC activity [19].

Even though endometrial ASCs have been described in different mammals, such as cows [22], pigs [23], sheep [24], horses [25], and non-human primates [26], in this review we focus on the discovery of stem cells in mice and humans, pointing out how these ASCs were identified and describing the different techniques employed (Figure 1). In addition, we highlight the value of pre-clinical models of stem cell therapy to treat AS and EA.

Figure 1. Milestones in the identification, isolation, and characterization of endometrial stem cells. Timeline of the principal findings concerning endometrial stem cells and specific isolation techniques in murine and human models. BM: bone marrow; BrdU: bromodeoxyuridine; CD: cluster of differentiation; endoSSCs: endometrial stromal stem cells; endoEPCs: endometrial epithelial progenitor cells; GFP: green fluorescent protein; H2B: histone 2B; LRCs: label-retaining cells; LGR: leucine-rich repeat containing G-protein-coupled receptor; MenSCs: menstrual blood stem cells; NTPDase2: ectonucleoside triphosphate diphosphohydrolase-2; SSEA-1: stage-specific embryonic antigen-1; SP: side population; SUSD2: sushi domain-containing-2.

2. Endometrial Stem Cells and Specific Niches

In 1978, Schofield proposed the concept of a stem cell niche for the first time, referring to an anatomically defined compartment in which stem cells reside and are able to renew and/or remain undifferentiated [27]. We know now how the niche is specialized and the dynamic microenvironment in which stem cells interact with differentiated cells, secreted factors, and/or components of the extracellular matrix (ECM) [28]. These stimuli determine the behavior (rate and pattern of division) of the stem cells in each specific tissue [29]. Prianishnikov was the first to propose the existence of ASCs in the endometrium [30] as an immature hormone-independent population, residing in the deepest basalis layer with the capability of differentiating towards hormone-responsible endometrial cells. The constant maintenance of this layer along the menstrual (or estrous) cycle makes it a reasonable candidate to be a reservoir of ASCs, even if some stem cells are also found in the functionalis [31]. In 1991, Padykula claimed that the population of stem-like cells migrates and produces different progenitor cells that differentiate specifically into vascular, epithelial, or stromal compartments, thanks to their specific niches [32]. Over the years, these endogenous endometrial ASCs have been proposed not only to be responsible for the cyclic endometrial growth and, consequently, for certain gynecological disorders, but also as useful in therapeutic approaches. According to the origin of the stem cells, endometrial stem cell niches include epithelial, stromal, and endothelial cells, and are likely to contribute to endometrial regeneration [33].