The Hypothalamic-Pituitary-Gonadal Axis

The physiology of the hypothalamic-pituitary-gonadal (HPG) axis plays a critical role in each of the following processes, the last 2 of which are relevant for reproduction:

1. Phenotypic gender development during embryogenesis
2. Sexual maturation during puberty
3. Endocrine function of the testis: testosterone production
4. Exocrine function of the testis: sperm production

Basic Endocrine Concepts

A. Hormone Classes (

Figure 42-1)
Two kinds of hormones classically mediate communication in the reproductive hormone axis: peptide and steroid. Peptide hormones are small secretory proteins that act via receptors on the cell surface membrane. Hormone signals are transduced by one of 3 second-messenger pathways, as outlined in

Figure 42-1. Ultimately, most peptide hormones induce the phosphorylation of various proteins that alter cell function. Examples of peptide hormones are luteinizing hormone (LH) and follicle-stimulating hormone (FSH).

In contrast, steroid hormones are derived from cholesterol and are not stored in secretory granules; consequently, steroid secretion rates directly reflect production rates. In plasma, these hormones are usually bound to carrier proteins. Since they are lipophilic, steroid hormones are generally cell membrane-permeable. After binding to an intracellular receptor, steroids are translocated to DNA recognition sites within the nucleus and regulate the transcription of target genes. Examples of reproductive steroid hormones are testosterone and estradiol.

B. Feedback Loops
Normal reproduction depends on the cooperation of numerous hormones, and thus the signals that hormones generate must be well controlled. Feedback control is the principal mechanism through which this occurs. With feedback, a hormone can regulate the synthesis and action of itself or of another hormone. Further coordination is provided by hormone action at multiple sites and eliciting multiple responses. In the HPG axis, negative feedback activity is responsible for minimizing hormonal perturbations and maintaining homeostasis.