Introduction

A. General Comments

The endocrine physiology of the male and the interplay of the several hormones associated with male sex determination, fetal development, and, following birth, growth and sexual maturation are still another example of the effectiveness of a highly differentiated endocrine system. Its integrated operation is dependent upon the interaction of signals—both hormonal and neural—between the central nervous system, hypothalamus, pituitary, and testes. The two major functions of the testes are steroid hormone production and gametogenesis.

The hormones that are responsible for the development and maintenance of the male phenotype comprise the gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH) produced by the anterior pituitary, the androgenic steroid hormones, testosterone, androstenedione, dehydroepiandroster-one, and 5a-dihydrotestosterone produced by the gonads, and inhibin, a protein hormone also produced by the gonads. The female steroid hormones, estradiol and estrone, also play an important role in the male in certain select circumstances.

This chapter discusses the biology and biochemistry of the androgens and gonadotropins in the male; the concepts of this chapter should be compared and contrasted with those of Chapter 13, which provides similar information for the female. In addition, a portion of Chapter 14 is devoted to the hormonal aspects of fertilization and sex determination.

B. Characteristics of a Male

As discussed in Chapter 14, the gonads of both males and females in the early embryonic state are morphologically identical. It is only after the onset of sex differentiation (during the fifth and sixth weeks of fetal development) that the inevitable consequences of expression of the genetic information resident in the XY (male) or XX (female) chromosomes normally manifest themselves to convert the "indifferent" gonad into the male testes or female ovaries.

HORMONES, SECOND EDITION

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