In this chapter, we discuss various aspects of classic growth factors and their relevance to endocrinology. Although "growth factors" have traditionally been considered to be represented by the family of peptide growth factors, this definition is too restricted given that nonpeptide hormones, e.g., steroid hormones such as estrogen, also stimulate cell growth. Similarly, growth factors have traditionally been considered as tissue factors, functioning locally as autocrine or paracrine factors, as compared to hormones that function in a classic endocrine fashion. We focus here on insulin-like growth factors (IGFs), which represent a paradigm that has both endocrine and autocrine/paracrine modalities. We then discuss other members of classic growth factor families, allowing the reader to compare and contrast them to the IGFs. We also briefly address the numerous cell-surface receptors and the cross talk between receptors. Because we cannot describe here all aspects of the growth factors, their receptors, and interacting proteins, we refer the reader to various other excellent reviews in the Selected Reading section.
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