Self Renewal of Spermatogonial Stem Cells : the Most Promising Multipotent Cells-A Review

Germ cells are endowed with unique properties like the ability to undergo meiosis and differentiate into gametes in order to perpetuate the genetic material. Spermatogonial stem cells (SSC) are considered as the best suited undifferentiated germ cells and are being consistently studied to reveal the basic physiology, genomics and transcriptomics of those cells. SSC provide an excellent model system to understand the differentiation, development and functioning of gonads and further use of these cells in transplantation-cell based therapies. In the present article, basic aspects of spermatogonial multiplication, regulation of stem cell renewal and differentiation would be discussed.


Introduction
found in a living organism.These cells have more restricted options to select a differentiation program In the current scenario, stem cells have egressed from a few possible pathways only.Adult and as one of the most fascinating areas of biological embryonic stem cells differ in the number and type of science.Stem cells are subject of increasing interest differentiated cell types they can form.Embryonic because of their biological properties and potential stem cells can form all cell types of body (including medical importance.Unfortunately, it has been germ cells) because they are totipotent (Rosant, 2008).difficult to penetrate much into the field, in part Adult stem cells are generally limited to differenbecause of ambiguity about what exactly constitutes a tiating into different cell types of their tissue of origin stem cell.Stem cells, defined functionally as the only."Although difficult to extract, since they are precursor cells that give rise to other cell types, are also taken from the patient's own body, adult stem cells are called as "progenitor cells".Stem cells have two superior to both umbilical cord and embryonic stem characteristics that distinguish them from other cell cells.There is always an exact DNA match so the types: firstly, they are uncommitted cells capable of body's immune system never rejects them.And as we self renewal for many generations through cell might expect, results have been both profound and division.
promising" (http://www.allaboutpopularissues.org/The second being that under certain pros-and-cons-of-stem-cell-research.htm).physiological and experimental conditions, they can Amongst the adult stem cells the best defined be induced to become cells with specialized functions one is the spermatogenic system which depends on a (Jaenisch and Young, 2008) such as the myocardial small population of spermatogonial stem cells (SSC).cells, insulin-producing cells of the pancreas, SSC can renew them and produce daughter cells that epithelial cells, neurons, macrophages, fat producing are destined to differentiate into spermatozoa (Ogawa, adipose cells etc. Stem cells are characterized 2008); hence they are the hub in the process to according to the tissue from which they are derived.
procreate the male germplasm.Spermatogenic stem Stem cells may be embryonic stem cells or adult stem cells experience around 10 successive mitotic divisions cells.Embryonic stem cells are derived from the inner to yield primary spermatocytes (Tegelenbosch and de cell mass (ICM) of preimplantation embryos (Evans Rooij, 1993) which thereafter undergoes meiosis to and Kaufmann, 1981) embryonic germ cells and fetal beget approximately 2048 to 4096 spermatids (Russell germ cells (Matsui et al., 1992).Adult stem cells et al., 1990).Hence, the regulation of the germ cell (those obtained from the adult tissues) have its primary numbers is accomplished in the spermatogonial role to maintain and repair the tissues in which they are populations to ensure the appropriate ratio of the germ genital ridges.The PGCs proliferate during migration cells to Sertoli cells (de Rooij and Janssen, 1987; de by day 13 of fetal life in the mouse, when these cells Rooij and Lok, 1987).Spermatogonial stem cells can have enriched the genital ridges; their numbers be recognized and studied at the cellular level with increase to about 10,000 in each gonad (Tam and respect to proliferation, differentiation, and the Snow, 1981).Once derived in the genital ridges, the regulation of these activities.
PGC become enclosed by the differentiating Sertoli However, the complexity of the seminiferous cells and the seminiferous cords are formed.The germ epithelium makes it difficult to study the molecular cells present within the seminiferous cords differ morphologically from PGC and are called gonocytes aspects of the regulation of spermatogonial stem cells (Ogawa, 2008, Hersmusa et al., 2008).In rats and behavior.Presence of a variety of cell types mice, the gonocytes are arrested in the G /G phase of complicates the purification of spermatogonial stem 0 1 cells (Brinster and Avarbock, 1994).Moreover, there the cell cycle following a brief proliferation (Franchi and Mandl, 1964;Huckins and Clermont, 1968; is a lot of species wise variation in the pathways Vergouwen et al. 1991).Slowly after birth, the gonocytes regulating pluripotency vis-à-vis the requirement of resume proliferation to give rise to A spermatogonia. growth factor.To the contrary, LIF which is required 0 for LIF\STAT pathway is indispensable for culturing This event marks the start of spermatogenesis.The male germ cell development occurs in successive murine and bovine ESC.mitotic, meiotic and post-meiotic phases with the

Origin of spermatogonial stem cells
germ cells moving from the periphery to the lumen of Spermatogonial stem cells originate from the the seminiferous tubules.The mitotic phase occurs in primordial germ cells (PGC), the progenitor cells for the basal compartment, while the meiotic and postboth the male and female germ line, which in turn meiotic phases occur in the luminal compartment.derive from the epiblast cells (Lawson and Pederson, This takes about 35 days in the mouse, with the 1992).By 7 days post coitum in the mouse embryo, mitotic, meiotic and post-meiotic phases lasting for about 100 alkaline phosphatase positive PGCs can be approximately 11, 10 and 14 days, respectively detected in the extra embryonal mesoderm posterior to (Clermont and Trott, 1969).The process begins every the definitive primitive streak (Ginsburg et al., 1990).8.7-8.9 days; however, the duration of these phases Later in development, the PGCs migrate from the base varies slightly between mouse species.It includes of the allantois, along the hindgut to finally reach the processes unique to germ cells, including meiotic The chains of A spermatogonia can divide al In the testis of adult mammalian species, further into chains of 8, 16 and rarely 32 cells.spermatogonial stem cells maintain their numbers by Spermatogenesis is a cyclic process that in mice can be self-renewal and give rise to differentiating germ cells.divided into 12 stages (I -XII).In stage VIII, the A , A s pr The spermatogonial stem cell system of the rhesus and a few A spermatogonia are present.considered as the male germ-line progenitor i.e. the which brings about a marked change in cell behavior.functional reserve.The combination of a true stem cell These differentiation steps go through a series of 6 with low mitotic activity and a progenitor producing divisions to give rise to A , A , A , Intermediate (I) and high numbers of differentiating daughter cells seems B and finally a primary spermatocyte (de Rooij, 2001; to be the ideal system in the male germ line of de Rooij et al., 1999).To investigate these steps, in primates, where both the maintenance of genomic integrity and output of millions of motile sperms are vitro experiments using isolated germ cells have been equally important to insure perpetuation of the performed (Ogawa, 2008).However, only a limited genome to the next generation (Ramaswamy et al., number of cells can be isolated as germ cells have a 2000).limited viability in culture and it is difficult to Spermatogonial multiplication and stem cell distinguish spermatogonial stem cells from more renewal can be best studied in whole mounts of differentiated spermatogonia in vitro, due to the lack seminiferous tubules, since this is the best technique to of specific stem cell markers.The establishment of preserve the topographical arrangement of the cells stem cell line would overcome these problems (van (Clermont and Bustos-Obregon, 1968).The process of Pelt et al., 2002).stem cell multiplication and renewal has been studied Paucity of research reports on the transcriptoin different mammalian species.The spermatogonial mics of the proliferating spermatocytes has stood as an stem cells (SSC) are accordingly termed as A-single impediment towards developing a deeper insight about the proteomics of the spermatogenesis.Some proliferation and differentiation.C-kit is involved in events during spermatogenesis indicate the genetic the differentiation step from A to A .It is spermatocytes and proposed that kit and kit ligand are research has yet to walk a long way before it could find essential for meiosis.and integrins present on the its placement as a reliable tool for medical application 6 1 spermatogonial stem cells is enriched in the basal layer in a multidimensional way.Recent findings have and may help to attach spermatogonial stem cells to stated that the transgenic loss of function and over laminin in the basement membrane (Shinohara et al., expression models reveal that the fate of 1999).Consistent with this idea, transplant assays undifferentiated spermatogonial cells depend on the show that crude cell population can be enriched for dosage of the Sertoli cells produced glial cell linegerm-line stem cells by binding to laminin (the target derived neurotrophic factor (GDNF) (Meng et al., of and integrins) but not to collagen or fibronectin 2000).stimulates the growth and development of early germ cells.Sertoli cells produce another glial cell line The stem cell niche is a specialized microderived neurotrophic factor (GDNF) that affects environment provided by supporting cells, which proliferation of pre-meiotic germ line cells including promotes self-renewal and retention of stem cells in stem cells (Beumer et al., 2000).The GDNF their undifferentiated state.The Sertoli cell is contributes to paracrine regulation of spermatogonial recognized as one of those unique support cells, as it self-renewal and differentiation (Meng et al., 2000).provides extrinsic signals for establishment and The RNA helicase vasa are also expressed in coordination of the complicated steps associated with developing germ cells of many species.In the absence spermatogenesis.As early as in 1865, this tree like cell of vasa, male germ cell proliferation is reduced was identified as "sustentacular" or the mother cell, severely in spermatogonial stages (Spradling et al., because it formed an intimate physical relationship 2001).with germ cells (Yasuzumi et al., 1960).In normal Dazl (deleted in azoospermia-like) RNA seminiferous epithelium, the ratio between selfbinding protein, cyclin D2 and retinoic acid (RA) are renewal and differentiation of spermatogonial stem among the other factors that regulate spermatogenesis cells should be about one (de Rooij, 2001).More self- (Almstrup et al., 2005).RA and retinoid X receptor are renewal than differentiation would reduce the involved in differentiation of spermatogonia.SCF-cseminiferous epithelium to only stem cells and a tumor kit system is involved in A spermatogonial might form (de Rooij, 1998).If incessant 1 differentiation continues, the stem cells would deplete differentiation (De Rooij, 2001).Genetic studies themselves leading to seminiferous tubules with only indicate that the bcl-2 family plays a key role in the supporting Sertoli cells.
integrating the positive and negative signals on The complexity of the seminiferous epithelium spermatogonial survival with certain members makes it difficult to study the spermatogonial multipli-promoting cell survival (viz.Bcl-2, Bcl-xl, Bcl-w and cation and stem cells renewal at the molecular level.A1/Bfl-1) and others promoting cell death (viz.Bax, Prior to 1990's, there was a desperate need for suitable Bak, Bad and Bim) (Adams and Cory, 1998).biochemical, intracellular and surface marker to Transgenic loss of the Sertoli cell produced glial cell identify the stem cells from the aggrandized spurious line derived neurotrophic factor (GDNF) function cell types in a culture.Discovery of c-kit receptors as results in depletion of stem cell reserves, whereas in spermatogonial stem cell markers has opened new mice over expressed GDNF accumulate undifferenavenue in stem cell research (Shinohara et al. 1999; tiated interactions between hormones and cytokines is Albanesi et al. 1996).C-kit receptor ligand system most likely required to establish and sustain optimal plays an important role in spermatogonial spermatogonial development (McIntyre et al., 2008).
A New Ray of Hope to Augment Fertility directly or are intermediates in the gonadotrophins response remains unclear.Several pertinent queries with Spermatogonial Stem Cells await valid explanations on the process of SSC SSC could herald a new era in medical science if differentiation and self-renewal.At present the we could apply it for gene therapy and controlling primary challenges are to develop methods for obtaining reproduction failure in males, both in human medicine pure populations of stem cells, for integrating stable and animal sciences.The spermatogonial stem cells exogenous DNA into these populations.The stem cell (SSC) are the only adult stem cells that donate genetic research is virtually in its infancy.A number of queries material to the progeny.Researchers are harnessing to are yet to be answered, like, identification of utilize the harvested SSC to culture and thereafter constituents of media to culture the stem cells that they transplanting them into the recipient animals, with an could be maintained in the uncommitted state.The aim to boost the sperm production of prized bulls.The pathways regulating pluripotency vis-à-vis the technology would help to produce semen from exotic required growth factor vary considerably with species.bulls having considerably higher genetic worth from For example, human and mouse ESC merely require indigenous scrub bull.In veterinary practice, many LIF but bFGF.To the contrary, LIF which is required prized bulls often fail to reproduce due to certain for LIF\STAT pathway is indispensable for culturing andrological problems like poor libido or compromurine and bovine ESC.In conclusion, despite a lot of mised spermatogenesis.Valuable germplasm of such controversies over the hype and hope of stem cell males may be produced and utilized for artificial research, it can be expected that the field holds great insemination by SSC culture and in vitro maturation of promise in treating many diseases that were virtually sperm cells.Moreover, the time lapsed during thought impossible once upon a time.estimating the sire summary could be drastically

Figure- 1 .
Figure-1.The phases of germ cell development.Stem cells give rise to spermatogonia that undergo several mitotic divisions over the next 10 days.More than half of the spermatogonia undergo apoptosis during the latter part of the mitotic phase.During much of the meiotic phase, the spermatocytes are in the G phase of the cell 2 cycle.After the two meiotic divisions at the end of this phase, germ cells enter the post-meiotic phase.Transcription occurs throughout the germ cell development until the midpoint of the post-meiotic phase.During the latter half of this phase, proteins are synthesized from transcripts that have been stored since the early part of the post-meiotic phase.
al are commonly referred to as spermatogonial stem during the period of active proliferation differentiate cells, their biological functions are very different.The into A spermatogonia, without division.1 A cells show typical characteristics of a progenitor pale The A spermatogonia enter S-phase and in 1 and proliferate at defined periods during each cycle of stage IX divide into A spermatogonia.There are 5 the seminiferous epithelium to produce both A and 2 pale subsequent divisions into A , A , intermediate (I ) and there after B spermatogonia .The A cells also show and primary spermatocytes, high proliferative activity during pre-pubertal respectively.In total, there are 9-11 mitotic divisions testicular development when the pool of both types of during spermatogonial development (van Alphen and A spermatogonia is expanding (Simorangkir et al., de Rooij, 1986).Two differentiation steps take place 2000).Therefore, the A spermatogonium has been dark in the developmental path of spermatogonia; namely, recognized as the 'true' testicular stem cell i.e. the from the A spermatogonia to the A spermatogonia s pr regenerative reserve (Amann, 2008) while the A is pale and the second one is from A to A spermatogonia al 1 spermatogonial stem cell proliferation, predominantly expressed in germ cells of testis and differentiation and stem cell renewal.For instance, the also expressed in Leydig cells (Oatley and Brinster, spermatogonial stem cell renewal is highly preferred 2008).to A formation at the time of dearth of spermatocytes, The presence of functional c-kit receptor has PR similarly, the proliferation of A , A and A depends been implicated in spermatogonial proliferation, S PR AL survival and adhesion to Sertoli cells.(Robinson et al., on the proportion of the A to B type cells towards the 4 2001) showed the expression of c-kit by pachytene lumen (de Rooij, 2001).The spermatogonial stem cell

(
Spradling et al., 2001).Bone morphogenetic protein Regulation of self-renewal and differen-(BMP)-4 produced by extra embryonic ectoderm tiation of spermatogonial stem cells From stage X al monkey is well studied (Clermont and Leblond, 1959; onwards, these cells start to proliferate in such a way de Rooij et al., 1986; de Rooij et al., 2002; van Alphen that the number of A and A spermatogonia remain s pr and de Rooij, 1986; Marshall et al., 1995; relatively constant and more and more A spermatoal Ramaswamy et al., 2000).In these macaques as well gonia are formed.At about stages II-III (stage XII