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The understanding of embryology provides a foundation for the mastery of anatomy. In the treatment of men with infertility, it is only appropriate that one deals with the basics of fetal development of the male reproductive tract. This chapter reviews the germ layers from which all tissues organize themselves and develop, and also reviews the ductal system and its critical role in reproduction. The cloaca, as incorporated yolk sac, is an endoderm-lined cavity, anchored at the caudal end by the cloacal membrane. The mesonephric ducts, which are mesodermal structures, are connected to the urogenital sinus bilaterally. The common excretory ducts eventually move into the prostatic part of the urethra and become known as the verumontanum. Testosterone stimulates numerous changes in the existing ductal system. In the presence of testosterone, the phallus lengthens and enlarges to form the penis.
The neurophysiological control of the erectile process is under the influence of central and peripheral processes. At least three kinds of erection can be distinguished in man: central, reflexogenic, and nocturnal types. Emission, as the first phase of ejaculation, is a sympathetic spinal cord reflex. The spermatozoa undergo final maturation in the epididymis and are stored there prior to ejaculation. The autonomic nervous system plays a key role in the efferent pathway of the ejaculatory reflex. The spinal network plays a significant role in processing and directing afferent and efferent information in the ejaculatory process. The ejaculatory-related cerebral network includes the medial preoptic area (MPOA), the paraventricular nucleus of the hypothalamus (PVN), the nucleus paragigantocellularis (nPGi), the posterodorsal medial amygdaloid nucleus (MeApd), and the parvocellular subparafascicular thalamic nucleus (SPFp). An improved understanding of the complex influences on ejaculation may open new therapeutic strategies for ejaculatory disorders.
Although abnormalities of the Y chromosome have been associated with male infertility since 1976 (Tiepolo and Zuffardi, 1976), it was only in the last decade that the Y chromosome was shown to have regions and genes that govern spermatogenesis. More recently, it has become clear that the X chromosome may be just as important as the Y chromosome in determining male fertility potential. This chapter will review our current understanding of the genotype–phenotype relationships that underlie abnormalities of both the X and Y chromosomes, and discuss recognized syndromes of the gonosomes that are known to cause male infertility.
Over the last 10 years, there has been significant progress both in analyzing the molecular structure of the Y chromosome and understanding the relationship of Y chromosome mutations to infertility phenotypes. Before its firm association with male fertility, the Y chromosome was widely considered a genetic black hole, a chromosome that evolved as a broken remnant of the X chromosome. It was clear that the Y harbored the male sex-determining region (testis-determining region or sex-determining region Y (SRY)), but it was also home to gene regions that govern stature, tooth enamel and hairy ears as well as ‘junk’ gene regions. Now that the genome of the human Y is known, we realize that this chromosome is structurally unique as a fertility chromosome.
The postulation that deletions in the long arm of the Y chromosome cause azoospermia was made 30 years ago (Tiepolo and Zuffardi, 1976).
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