In Vitro Spermatogenesis
The ability to propagate spermatogonial stem cells indefinitely in culture (2-3) may now result in a comprehensive understanding of how their physiology is controlled by their molecular attributes (Figure 3). This capability provides a key experimental foundation to define the biochemical “building blocks” required for these stem cells to mature into functional spermatozoa.
The ability to conditionally induce mammalian stem cells to progress through spermatogenesis in vitro for the production of haploid male gametes would provide a long-sought-after technology that holds enormous potential to advance a wide scope of endeavors related to human health, animal husbandry and conservation.
Our lab routinely isolates highly enriched fractions of rat spermatogonial stem cells from testes by a simple selection method in culture using collagen and laminin extracellular matricies (4-5). Based on an ability to regenerate spermatogenesis when transplanted into testes of recipient rats, this isolated fraction of highly pure, undifferentiated spermatogonia represents the most enriched source of spermatogonial stem cells published to date. When isolated by this procedure, we term this stem cell fraction, laminin-binding spermatogonia (i.e. LamB spermatogonia), which is based on their high avidity for binding to a laminin matrix (see Figure 3 above). Since establishing this procedure, we have routinely used this population of spermatogonia as an enriched source of stem cells to initiate in vivo and in vitro culture studies on spermatogenesis (1-2, 4-6).
Proliferating spermatogonial lines can be derived from either mouse or rat spermatogonia that bind to laminin (Figure 4 above). These spermatogonial lines maintain their potential to produce functional spermatozoa in recipient testes, even after genetic modification with DNA test constructs designed to express candidate genes of interest.
Importantly, now that we can cultivate fully functional sperm stem cell lines in vitro, one long term goal of our lab is to be able to stimulate their development through the various stages of spermatogenesis in vitro (Figure 5 below).
The developmental process by which spermatogonia differentiate through meiosis to produce haploid gametes for sexual reproduction is termed “Spermatogenesis”. Newly formed haploid gametes are termed “Round Spermatids”. Once produced, rounds spermatids undergo a dramatic metamorphosis into mature spermatozoa by way of a male-specific genetic program termed “Spermiogenesis”.