The pluripotency factor LIN28 marks undifferentiated spermatogonia in mouse

In the present study, we show that LIN28, a pluripotency factor, is specifically expressed in undifferentiated spermatogonia (As, Apr, and Aal) in mouse. Ngn3 also specifically labels undifferentiated spermatogonia. We used Ngn3-GFP knockin mice, in which GFP expression is under the control of all Ngn3 transcription regulatory elements. Remarkably, Ngn3-GFP is only expressed in ~40% of LIN28-positive As (single) cells. The percentage of Ngn3-GFP-positive clusters increases dramatically with the chain length of interconnected spermatogonia.Our study demonstrates that LIN28 specifically marks undifferentiated spermatogonia in mice. These data, together with previous studies, suggest that the LIN28-expressing undifferentiated spermatogonia exist as two subpopulations: Ngn3-GFP-negative (high stem cell potential) and Ngn3-GFP-positive (high differentiation commitment). Furthermore, Ngn3-GFP-negative cells are found in chains of Ngn3-GFP-positive spermatogonia, suggesting that cells in the Aal spermatogonia could revert to a more primitive state.Spermatogenesis is a productive self-renewing system of adult stem cells that continuously generates spermatozoa through life. At the foundation of this system is the spermatogonial stem cells (SSCs) [1-4]. In mouse testis, isolated A (single) spermatogonia (As) are believed to be the most primitive cells and contain the stem cells. In normal situations, while half of As cells divide and give rise to Apr (paired) spermatogonia that are interconnected by cytoplasmic bridges due to incomplete cytokinesis, the remaining half of As cells undergo self-renewal divisions. The Apr spermatogonia further divide to become chains of 4, 8, 16, or 32 Aal (aligned) spermatogonia. The As, Apr, and Aal spermatogonia can only be identified by their topographical configurations on the basement membrane of the seminiferous tubules and are collectively referred to as "undifferentiated" spermatogonia, although this nomenclature causes confusion becaus