Table 1. Factors/receptors produced or present in testes regulate steroidogenesis and spermatogenesis in stallions

Factor/receptor	Identified in stallion	Localization within testes	Effects	Reference
IGF-1	+++	Leydig cells	Proliferation of Leydig cells and germ cells	[12]
Inhibin	+++	Leydig cells, Sertoli cells	Suppression of FSH for maintaining spermatogenesis	[21,22]
Activin	---	Germ cells, Leydig cells, Sertoli cells	Modulate the release of FSH from the pituitary	[29]
INSL3	+++	Leydig cells	Development of the gubernaculum testis	[39]
AR	+++	Germ cells, Leydig cells, Sertoli cells, PM cells	Sexual differentiation and behavior. Regulate steroidogenesis and spermatogenesis	[43,44]
GSTA3	+++	Leydig cells	Testosterone production via 3-ketosteroid isomerase activity	[54]
DHCR24	+++	Leydig cells	Cholesterol synthesis and regulate steroidogenesis	[54]
SQLE	+++	Leydig cells	Regulate steroidogenesis	[54]
Oxytocin	+++	Germ cells, Leydig cells	Regulate 5α-reductase synthesis activity	[58]
Aromatase P450	+++	Leydig cells, Sertoli cells	Convert testosterone into 17β-estradiol	[64]
LH receptors	+++	Leydig cells	Production of testosterone and estrogen	[66]
ESR-α	+++	Germ cells, Leydig cells, Sertoli cells, PM cells	Regulate steroidogenesis	[42]
ESR- β	+++	Germ cells, Leydig cells, Sertoli cells	Regulate steroidogenesis	[42]
KIT	+++	Germ cells	Differentiation or proliferation of SSCs by PI3K/Akt pathways	[84]
BMP4	---	Germ cells, Sertoli cells	differentiation of SSCs	[82]
VEGF	+++	Germ cells, Leydig cells, Sertoli cells	Proliferation of endothelial cells	[91]
LIF	---	PM cells	Regulate proliferation and survival of SSCs	[111]
GDNF	---	Sertoli cells, PM cells	self-renewal of SSCs	[88]
GFRA1	+++	Germ cells	SSCs self-renewal	[90]
Transferrin	---	Sertoli cells	Iron supplementation to seminiferous tubules	[93]
FSHR	+++	Germ cells, Sertoli cells	Regulate steroidogenesis	[65]

+++, identified in stallions; ---, not identified in stallions.
IGF-1, insulin-like growth factor-1; FSH, follicle stimulating hormone; INSL3, insulin like peptide 3; AR, androgen receptor; PM, peritubular myoid; GSTA3, glutathione S-transferase A3; DHCR24, dehydrocholesterol reductase; SQLE, squalene epoxidase; LH, luteinizing hormone; ESR, estrogen receptor; KIT, receptor tyrosine kinase; SSCs, spermatogonial stem cells; PI3K, phosphoinositide 3-kinases; Akt, Ak strain transforming; BMP4, bone morphogenetic protein 4; VEGF, vascular endothelial growth factor; LIF, leukemia inhibitory factor; GDNF, glial cell line-derived neurotrophic factor; GFRA1, GDNF family receptor alpha-1; FSHR, follicle stimulating hormone receptor.