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The proximal tubules make up a significant portion of the kidneys; proximal tubule epithelial cells are the most populous cell type in the kidney, and carry out diverse regulatory and endocrine functions where numerous transporters are located.he proximal tubules make up a significant portion of the kidneys; proximal tubule epithelial cells are the most populous cell type in the kidney, and carry out diverse regulatory and endocrine functions where numerous transporters are located.
The epithelium of the tubule consists of a type of sustentacular cells known as Sertoli cells, which are tall, columnar type cells that line the tubule. In between the Sertoli cells are spermatogenic cells, which differentiate through meiosis to sperm cells. Sertoli cells function to nourish the developing sperm cells. They secrete testis-determining factor, a binding protein which increases the concentration of testosterone inside the seminiferous tubules.