Author
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RYAN, PETER - Mississippi State University |
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Submitted to: Book Chapter
Publication Type: Book / Chapter Publication Acceptance Date: 5/12/2014 Publication Date: 8/1/2014 Citation: Ryan, P.L. 2014. Endocrine and exocrine function of the bovine testis. Chapter 2. Book Chapter. P. 11-25. Interpretive Summary: The normal bovine male reproductive system consists of paired testes retained within a sack or purse-like structure known as the scrotum which is formed from the outpouching of skin from the abdomen, consisting of complex layers of tissue. The testes are accompanied by a number of supporting structures including spermatic cords, accessory sex glands (prostate, bulbourethral gland, paired vesicular glands), penis, prepuce and the male ductal system. The testicular duct system is extensive and comprises of the vas efferentia found within the testes, the epididymis, vas deferens and the urethra all of which are located external to the testes. The reader is referred to the excellent chapter on the Anatomy of the Reproductive System of the Bull in this volume (Chapter 1). The testes primary functions are to produce male gametes (spermatozoa) and endocrine factors, such as steroid (testosterone) and protein hormones (inhibin, insulin-like peptide 3), that help regulate reproductive function of the bull in concert with hormonal secretions from the hypothalamus (gonadotropin releasing hormone) and pituitary glands (luteinizing hormone, follicle stimulating hormone). The testes consist of parenchymal tissue that supports the interstitial tissue and include the steroid-producing Leydig cells, vascular and lymphatic system and the seminiferous tubules within which the germinal tissue develops with the support of the nurse cells or more commonly known as the Sertoli cells. Chapter 4 of this text (Endocrine control of testicular development and initiation of spermatogenesis in bulls) discusses in detail the endocrine factors responsible for testicular development and initiation of spermatogenesis in the bull, thus this chapter will focus more on the regulation and function of the adult testes. Due to limitations, this chapter will not undertake a treatise of those conditions that disrupt testicular function rather it will focus, as practically as is possible, on what is known of the endocrine and exocrine function of the bovine testes. Much of the endocrine and exocrine function of the testes is similar across mammalian species, and where specific information is absent for the bovine, examples will be given from other domestic species when possible. Technical Abstract: This chapter is devoted to the endocrine and exocrine function of the normal bovine male testes. The discussion begins with a historical review of the literature dating back to Aristotle’s (300 BC) initial description of the anatomy of the mammalian testes. The first microscopic examination of the testes was undertaken by Van Leeuwenhock in 1667 where he described the presence of germinal cells in the seminal fluid. Since that time much progress has been made in understanding the anatomy, physiology, endocrine and exocrine function of the bovine testes. Chapter 4 of this text (Endocrine control of testicular development and initiation of spermatogenesis in bulls) discusses in detail the endocrine factors responsible for testicular development and initiation of spermatogenesis in the bull, thus this chapter will focuses more on the endocrine and exocrine regulation and function of the adult testes that includes the hypothalamic, pituitary and gonadal axis. Much of the endocrine and exocrine function of the testes is similar across mammalian species, and where specific information is absent for the bovine, examples are given from other domestic species when possible. In addition, other regulatory factors are discussed including the important role of relaxin and related peptides involved in testicular decent and spermatozoa motility. The chapter also details the sites of endocrine production and secretion within the testes. Tables and figures are included to assist the reader understand the sites of hormone synthesis and secretion, and the synthetic pathway of the primary steroids from the base structure cholesterol through to the primary androgens that include testosterone and the associated enzyme systems involved in the synthetic process. The latter part of the chapter is devoted to spermatogenesis and the maturation of spermatozoa. The discussion includes spermatocytogenesis, the process of meiosis, spermiogenesis and spermiation. These processes are supported by schematic figures that show representations of the germinal epithelium of the bull testis during specific phases of the seminiferous epithelium cycle and developmental changes of the spermatozoa as they migrate through the rete testis to the epididymis where final maturation takes place. The chapter does not undertake a treatise of those conditions that disrupt testicular function rather it focuses on what is known of the endocrine and exocrine function of the bovine testes. |
