MILNE LECTURE From a Sperm’s Eye View—Revisiting Our Perception of This Intriguing Cell Dickson D. Varner, DVM, MS, Diplomate ACT; and Larry Johnson, MS, PhD Authors’ address: Departments of Large Animal Clinical Sciences and Veterinary Integrative Bio- sciences, College of Veterinary Medicine and Biomedical Sciences, Texas A&M University, College Station, TX 77843-4475; e-mail: [email protected]. © 2007 AAEP. 1. Introduction and the events that accompany a spermatozoon’s The stallion represents one half of the breeding sojourn through the female reproductive tract, fol- equation; however, apportioned time and funding lowed by a discussion of some potential clinical ram- has been considerably less for efforts to advance the ifications. The lecture that accompanies this text discipline of stallion reproduction compared with will be directed, almost exclusively, to practical ap- that of mare reproduction. Modern-day fathers of plications and advancements in the discipline of stallion reproduction, such as Robert M. Kenney, stallion reproduction. Not all published work could Bill W. Pickett, and Marion Tischner, made signifi- be represented in this short review, and we preface cant strides in our understanding of stallion repro- this reading with an apology for unintentional ex- ductive physiology and clinical disease, but this clusion of pertinent data. Although we direct the fascinating area of study begs for individuals with reader to some relevant studies conducted in equids, the aptitude and tenacity required to take the disci- most of the information provided in this manuscript pline to new heights. In recent years, the level of stems from studies conducted in other species, interest in research directed toward the breeding where the process of discovery has been most pro- stallion has increased noticeably, as evidenced by nounced. Although this comparative approach can the 2006 Congress of the International Society of be quite enlightening, a drawback is that the rele- Equine Reproduction (ISER). The stallion was rep- vance to stallions of findings unveiled in other spe- resented in the largest section of papers presented cies will require documentation. and published at this meeting, a first since the in- A manuscript directed exclusively at the sperma- ception of the ISER in 1975. tozoon would seem to represent a miniscule part of This paper will be directed primarily at the male the total reproduction picture. On close scrutiny, gamete, the spermatozoon, and all the fascination however, one becomes enlightened about the level of that it bestows on those of us that have dedicated a scientific interest in this cell. As an example, large portion of our professional lives eavesdropping ϳ50,000 entries are logged in response to a query for on its microcosm. We will begin with a detailed the keywords, spermatozoa or spermatozoon, in overview of spermatozoal structure and function PubMed, a database of indexed citations offered by NOTES 104 2007 ր Vol. 53 ր AAEP PROCEEDINGS MILNE LECTURE the National Institutes of Health. An uninformed tion of the outer acrosomal and overlying individual might describe a spermatozoon as a plasma membranes necessary for fusion and highly specialized, but simple, cell with only one role vesiculation) to fulfill—that of fertilization. Although fertiliza- 11. Penetration of the zona pellucida (release of tion is the endpoint of spermatozoal function, this acrosomal proteins with enzymatic activity cell must be extremely sophisticated and adaptable is required for this event to occur) to achieve this task, and the process involves a se- 12. Binding and fusion with the oolemma (re- ries of highly coordinated cellular and molecular quires specific region-dependent molecular events. The following is a list of some require- interactions) ments ascribed to a mammalian spermatozoon. 13. Dispersion of nuclear contents (requires spe- cific fusogenic alterations of the lipid mem- 1. Loss of most organelles and cytoplasm dur- branes of the spermatozoon and oocyte) ing formation in the testis and maturation in 14. Oocyte activation (a spermatozoon-derived the epididymis (requires a host of intracellu- factor is required for activation of the oocyte lar and intercellular signaling events) and embryonic development) 2. Remodeling of spermatozoal chromatin 15. Pronucleus formation (requires decondensa- within the epididymis as a protective mech- tion of the highly compact spermatozoal anism against environmental injury (re- nucleus) quires repackaging of nuclear DNA into a 16. Organization of the mitotic spindle after pro- highly condensed form through the aid of nuclear formation (requires contribution of specialized proteins termed protamines) proximal centriole from the spermatozoon) 3. Plasma membrane alterations within the ep- ididymis to yield proteins important to fertil- After viewing this lengthy list of functions, one ization (requires various enzymatic-linked becomes quite appreciative of the highly complex alterations of existing proteins and uptake of and specialized features of a spermatozoon. In fact, proteins from epididymal fluid or from the the biochemical and biophysical features are so so- epididymal epithelium) phisticated that many of the cellular and molecular 4. Passage through the uterus and uterotubal mechanisms remain unresolved to this day. Fur- junction of the female at the time of insemi- thermore, spermatozoa (and oocytes) represent nation (requires activated flagellar move- some of the most highly differentiated cells in the ments and protection against immunologic mammalian body; yet, when sperm and oocyte are attack) combined, they retain their potential for totipotency 5. Binding to oviductal epithelial cells to form a (ability to divide and produce all cell types of the spermatozoal reservoir (requires specific cell- body) through creation of a zygote. cell attachment, possibly mediated through spermatozoal surface carbohydrate-binding 2. Origin of the Spermatozoon proteins, termed lectins) The life of a spermatozoon begins within the testes, 6. Acquisition of additional maturational unless, of course, one wishes to consider the embry- changes, collectively termed capacitation, onic origin of the primordial germ cells. The testis, that permit a spermatozoon to fertilize an an elaborately designed organ, is classically consid- oocyte (requires an assortment of signal ered to possess two functions: (1) exocrine—sper- transduction cascades) matogenesis, and (2) endocrine—production of 7. Release from oviductal epithelial cells and hormones important to spermatogenesis, sexual dif- passage to the vicinity of the oocyte at the ferentiation, development of secondary sex charac- isthmic-ampullar junction of the oviduct (re- teristics, and libido. Although this simplistic quires a coordinated spermatozoal-release description provides one with the general concept of mechanism, hyperactivated motility, and testicular function, it does not portray the extremely probably chemotaxis) complex nature and elegant interplay of these two 8. Penetration through the extracellular matrix processes. Conventional descriptions convey the of the oocyte cumulus (possibly mediated by role of hypothalamic- and pituitary-derived hor- hyperactivated motility and redistribution/ mones on regulation of testicular function, as well as unmasking of surface-associated hyaluroni- feedback mechanisms required for homeostasis. dase, because the cumulus matrix is rich in Although such pathways are undoubtedly the key hyaluronic acid) orchestrators of testicular function, emerging infor- 9. Binding to the zona pellucida, a highly gly- mation is revealing a multitude of subcellular, mo- cosylated protein matrix surrounding the oo- lecular-mediated events that “cloud” our cyte (seems to involve specific affinity understanding of the events that actually occur between spermatozoal surface molecules and within the testes. As with any area of study, the the zona pellucida components) more learned we become about a topic, the more 10. Acquisition of the acrosome reaction, a regu- queries surface that require additional clarification. lated form of exocytosis (requires reorganiza- Such is the case with testicular function. Without AAEP PROCEEDINGS ր Vol. 53 ր 2007 105 MILNE LECTURE question, a thorough understanding of testicular function will require a keen appreciation of the mechanisms by which genes and gene products are expressed and repressed.2–8 As an example of the genetic complexity surrounding control of testicular function, new information has revealed that single nucleotide polymorphisms (SNPs) have been identi- fied in the follicle-stimulating hormone (FSH) recep- tor gene of humans, resulting in a mutation of the FSH receptor (as is found on the surface of the Sertoli cell) that can influence its activity.9 As an- other example, use of transgenic mice deficient in estrogen receptor genes has shown that estrogens are likely to play a more important role in testicular function than was once thought to be the case.10 Similarly, use of mice with a selective androgen receptor knockout in Sertoli cells revealed that the androgen receptor in the Sertoli cell is an abso- lute requirement for normal spermatogenesis.11,12 Furthermore, experimentation with germ cell–spe- cific androgen receptor knockout mice revealed nor- mal spermatogenesis, suggesting that germ cell androgen receptors may play different roles as the germ cells progress through spermatogenesis.13 As seen here, to more fully understand what makes the testes