Role of the testis interstitial compartment in spermatogonial stem cell function

Abstract

Intricate cellular and molecular interactions ensure that spermatogonial stem cells (SSCs) proceed in a step-wise differentiation process through spermatogenesis and spermiogenesis to produce sperm. SSCs lie within the seminiferous tubule compartment, which provides a nurturing environment for the development of sperm. Cells outside of the tubules, such as interstitial and peritubular cells, also help direct SSC activity. This review focuses on interstitial (interstitial macrophages, Leydig cells and vasculature) and peritubular (peritubular macrophages and peritubular myoid cells) cells and their role in regulating the SSC self-renewal and differentiation in mammals. Leydig cells, the major steroidogenic cells in the testis, influence SSCs through secreted factors, such as insulin growth factor 1 (IGF1) and colony-stimulating factor 1 (CSF1). Macrophages interact with SSCs through various potential mechanisms, such as CSF1 and retinoic acid (RA), to induce the proliferation or differentiation of SSCs respectively. Vasculature influences SSC dynamics through CSF1 and vascular endothelial growth factor (VEGF) and by regulating oxygen levels. Lastly, peritubular myoid cells produce one of the most well-known factors that is required for SSC self-renewal, glial cell line-derived neurotrophic factor (GDNF), as well as CSF1. Overall, SSC interactions with interstitial and peritubular cells are critical for SSC function and are an important underlying factor promoting male fertility.

Figure 1. Proposed models of SSC self-renewal

The most accepted model of SSC self-renewal and differentiation (Huckins/Oakberg model) is depicted by black arrows. A subset of A_single cells are proposed to be “true” self-renewing SSCs. These cells divide to give rise to A_paired and A_aligned undifferentiated spermatogonia. The major alternative model (Alt. 1), proposed by Clermont, is that the A₁ cells (A_single, A_paired, and A_aligned) cycle, resulting in long chains of cells that can self-renew (back to A_single) or differentiate into differentiated spermatogonia. Part of this proposed model is that there are quiescent (non-dividing) A₀ cells that, under certain conditions, give rise to A₁ cells. The alternative 2 (Alt. 2) model proposes that, during the progression from A_single to A_paired or A_paired to A_aligned, a few cells can break off to create new A_single or new A_paired cells. A third alternative model (Alt. 3) proposes that A_single cells self-renew or undergo programming to become a “committed progenitor.” In the “committed progenitor” stage, an A_single is committed to differentiation and upon limited divisions gives rise to A_paired undifferentiated spermatogonia.

Figure 2. Overview of mammalian testicular structure

This image represents a high-magnification cross-section of an adult mouse testis. Regions are labeled by white boxes as follows: interstitial; peritubular; and seminiferous tubule regions. The interstitial region contains the following cell types: Leydig cells (peach), macrophages (brown), and vasculature (red). The peritubular region is comprised of myoid cells (blue) and macrophages (brown). The interstitial and peritubular regions are separated from the tubular compartment by a basement membrane (thick black line). The seminiferous tubules contain Sertoli cells (gray), spermatogonial stem cells (SSC, yellow), and later-stage germ cells (different shades of green; not drawn to scale).

Figure 3. Interstitial/peritubular contributions to the SSC niche

The interstitial region contains the following cell types: Leydig cells (peach), macrophages (brown), and vasculature (red). The peritubular region is comprised of myoid cells (blue) and macrophages (brown). For either self-renewal or differentiation, external signals influence SSC fate. Intrinsic germ cell factors promoting self-renewal are found within the yellow (left) side of the SSC, whereas factors which promote differentiation are found on the green (right) side of the SSC, and are placed on the respective sides of the balance within the SSC. Extrinsic cues that regulate the niche are depicted by arrows; their receptors are localized on the yellow side of the SSC if they regulate self-renewal or the green side if they regulate differentiation. IGF1R, CSF1R, GFRA1, and KDR all promote self-renewal (yellow), whereas the RXR/RAR receptor complex promotes the transition to undifferentiated spermatogonia (non-self-renewal capability). The colors of receptors are based on the cell type from which the signal is provided (e.g., IGF1 signals are from Leydig cells to promote SSC proliferation; therefore, the arrow and the receptor are peach-colored). The factors are listed within the cells from which they are secreted.