CHORDATE TESTIS - THE EXTRACELLULAR-MATRIX HYPOTHESIS

Testis structure in the chordates has been evaluated based upon relationships between primary spermatogonia, Sertoli cells, and the changes in the chronological secretion of the extracellular basement membrane. Prespermatocystic, polyspermatocystic, lobular, and tubular testicular types are described. In all chordates, germinal and interstitial compartments are separated by a basement membrane. The timing of the formation of this separation varies with the chordate class. Although primary spermatogonia are not initially separated from the interstitial tissue in agnathans and elasmobranchs, a basement membrane forms around secondary spermatogonia and associated Sertoli cells before meiosis. The spermatocyst so formed is a unit of testicular evolution. In fishes and amphibians, Sertoli cells, primary spermatogonia, and the subsequent stages of germ cell development reside in spermatocysts, They are always sequestered from interstitial tissues by a basement membrane. Sertoli cells rest upon the basement membrane and their processes form the borders of spermatocysts. I propose the Extracellular Matrix Hypothesis to explain how changes in the chronological formation of basement membranes resulted in the evolution of lobular and tubular testes from spermatocystic testes.