HISTOLOGY AND HISTOPATHOLOGY

Cellular and Molecular Biology

 

Review

Effects of thyroid hormones on Leydig cells in the postnatal testis

S.M.L.C. Mendis-Handagama and H.B.S. Ariyaratne*

Department of Comparative Medicine, The University of Tennessee College of Veterinary Medicine, Knoxville, TN, USA
*Present address: Department of Veterinary Basic Sciences, Faculty of Veterinary Medicine & Animal Science, University of Peradeniya, Sri Lanka

Offprint requests to: Dr. Chamindrani Mendis-Handagama, Department of Comparative Medicine, The College of Veterinary Medicine, The University of Tennessee, 2407 River Drive, Knoxville, TN 37996, USA. Fax: 865-974-5640. e-mail: mendisc@utk.edu

 

Summary. Thyroid hormones (TH) stimulate oxidative metabolism in many tissues in the body, but testis is not one of them. Therefore, in this sense, testis is not considered as a target organ for TH. However, recent findings clearly show that TH have significant functions on the testis in general, and Leydig cells in particular; this begins from the onset of their differentiation through aging. Some of these functions include triggering the Leydig stem cells to differentiate, producing increased numbers of Leydig cells during differentiation by causing proliferation of Leydig stem cells and progenitors, stimulation of the Leydig cell steroidogenic function and cellular maintenance. The mechanism of action of TH on Leydig cell differentiation is still not clear and needs to be determined in future studies. However, some information on the mechanisms of TH action on Leydig cell steroidogenesis is available. TH acutely stimulate testosterone production by the Leydig cells in vitro via stimulating the production of steroidogenic acute regulatory protein (StAR) and StAR mRNA in Leydig cells; StAR is associated with intracellular trafficking of cholesterol into the mitochondria during steroid hormone synthesis. However, the presence and/or the types of TH receptors in Leydig cells and other cell types of the Leydig cell lineage is still to be resolved. Additionally, it has been shown that thyrotropin-releasing hormone (TRH), TRH receptor and TRH mRNA in the testis in many mammalian species are seen exclusively in Leydig cells. Although the significance of the latter observations are yet to be determined, these findings prompt whether hypothalamo-pituitary-thyroid axis and hypothalamo-pituitary-testis axis are short-looped through Leydig cells. Histol. Histopathol. 19, 985-997 (2004)

Key words: Thyroid hormone, Leydig cells, Stem cell differentiation, Leydig progenitor cells, Steroidogenesis, Aging

DOI: 10.14670/HH-19.985