From the Sertolian syncytium to Sertoli cells in anamniotes, especially Gymnophionan (Caecilian) Amphibians J.-M. Exbrayat Université de Lyon, UMRS 449, Laboratoire de Biologie Générale, Université Catholique de Lyon; Laboratoire de Reproduction et Développement Comparé, EPHE/PSL; 10, place des Archives, 69288 Lyon Cedex 02, France In Vertebrates, seminiferous tubes are bordered with an epithelium producing gametes included between Sertoli cells. These cells were first described like a syncytium. With electron microscopy, the observations showed Sertoli cells contained a single nucleus and were limited by a classical plasma membrane. Some studies showed the Sertoli cells of anamniotes originated from follicle cells surrounding the cysts of germ cells. At the end of 1960’s the paradigm of Sertolian syncytium was out. But Sertolian syncytium remained described during a long time. In anamniotes, Sertoli cells constitute the wall of cysts grouping germ cells. In burrowing or aquatic lengthened gymnophionan (caecilian) amphibians, the testes are constituted with several lobes containing locules in which spermatogenesis occurs. Light microscopy indicated gametes were grouped in cysts included into a filamentous matrix containing fat globules. Some nuclei were observed in a peripheral position. For a long time it was considered the Sertoli cells of these animals were limited to those nuclei with a very small cytoplasm. The use of SEM and TEM revealed the matrix belonged to the cytoplasm of Sertoli cells. The fibrous aspect corresponded to an artefact due to the fixative. The study of the testis development showed Sertoli cells were giant cells delimited with a plasma membrane. Finally, in these amphibians, the structure of Sertoli cells is reminiscent of the Sertoli cells belonging to other anamniotes. Keywords: Sertoli cell, testis, germ cell, anamniote, amphibian, gymnophionan 1. Introduction In Vertebrates, testes are constituted with seminiferous tubes or ampullae bordered with a germinal epithelium producing gametes. This epithelium is put on a connective wall by a basal lamina. Endocrine Leydig cells are included into connective tissue. Germinal epithelium is constituted with cells of spermatic lineage: spermatogonia, spermatocytes I and II, spermatids and spermatozoa. This epithelium also contains somatic Sertoli cells between which are included germ cells. This pattern is found in amniotes and anamniotes, with some differences in their organization. Firstly studied in man and mammals, Sertoli cells were considered as merged, giving a syncytium. The use of TEM allowed one to find Sertoli cells were well individualized, but a long time has been necessary to accept this new view of Sertoli cells. 2. A brief story of the discovery of Sertoli cell 2.1 The discovery of Sertoli cells: syncytium or single cells? Genital tracts were subject of interrogation since a long time. After several centuries of macroscopical observations and deductions often imprinted with philosophy, the invention of microscope by Hooke and Van Leeuwenhoek allowed to increase the knowledge of these organs. Van Leeuwenhoek (1632-1723) was one of the first to describe spermatozoa (animalcule spermatici) in 1677. Later, Spallanzani in 1780, performing a famous experiment in frog, showed the seminal fluid was at the onset of fertilization of female oocytes. XIXth was to be expected to meet several researchers working on male gonads. Among these researchers, Sertoli published a first description of “branched cells” in human testes [1]. These branched cells constituted the Sertolian syncytium. He also published fundamental observations on spermatogenesis, resuming Von Ebner’s hypothesis [2]: the spermatozoa were issued from the branched cells. Sertoli thought these cells performed a nutritional or mechanical role in spermatogenesis. He also showed spermatozoa were coming from rounded spermatids (called nematoblasts), he described spermiogenesis dividing the development of spermatocytes into three steps: leptotene/zygotene, pachytene, and diplotene. Spermatogonia were divided into two stages. Sertoli observed some waves of spermatogenesis in which germ cells evolved grouped into isogenic series, and linked each other with cytoplasm bridges. 2.2 The persistence of the “syncytium paradigm” Sertoli cells were described in 1865 such as branched, i.e. like a syncytium, a cell mass with a lot of nuclei. But Sertoli using only light microscopy, it was not possible for him to visualize the plasma membrane of these cells with this technique. So, all nuclei appeared to be included into the same cytoplasm. Even Sertoli already suggested the branched cells were a mass of single cells from which the limits could not be observed [3], the syncytium concept was integrated into the scientific community, exceeding Sertoli himself. Regaud [4,5] was opposed to Sertoli’s deduction of cells with Microscopy and imaging science: practical approaches to applied research and education (A. Méndez-Vilas, Ed.) 167 ___________________________________________________________________________________________