Growth and development require broad changes in many different organ systems. The release of circulating hormones from the endocrine system accomplishes these changes, regulating how animals develop and grow. For example, we discuss in Chapter 42 the importance of the sex hormones estrogen and testosterone in determining female or male sexual characteristics both in the embryo and during puberty.

As another example, let’s consider how the well-studied endocrine system of insects regulates their growth and development. Insects produce a rigid exoskeleton (Chapter 37), which must be periodically shed and replaced by a larger new one to enable the insect to grow (Fig. 38.1). Shedding of the exoskeleton is referred to as molting. Each molting produces a new larval stage. In some insects, such as moths, the animal’s body also changes dramatically, undergoing metamorphosis at a key stage in development (Fig. 38.1a). In other insects, such as grasshoppers and crickets, the animal grows larger after each molting, but with little change in body form (Fig. 38.1b).

Molting and metamorphosis are regulated by hormones released from tissues in the insect’s head. British physiologist Vincent Wigglesworth studied their effects on the blood-sucking insect Rhodnius. Normally, Rhodnius goes through five juvenile stages before developing into its final adult body form. Like grasshoppers and crickets, it does not undergo metamorphosis. Each of the developmental stages is triggered by a blood meal. Wigglesworth showed that he could prevent molting if the head of the insect were removed shortly after a blood meal (Fig. 38.2). Rhodnius can survive for long periods despite having its head removed. If the insect takes a blood meal and the head is removed a week later, the animal’s body undergoes a molt and grows into its adult form. Wigglesworth hypothesized that a diffusing substance from the animal’s head controls its molt. In the first case, this substance does not have time to reach the body to trigger a molt, but in the second it does.

The German endocrinologist Alfred Kuhn found a similar result in experiments performed on moth caterpillars. Tying the caterpillar’s head off from the rest of the body prevented the body from molting. Wigglesworth and Kuhn considered the diffusing substance to be a brain hormone. Subsequent work showed that cells in a specialized region of the insect brain secrete the peptide now known as prothoracicotropic hormone (PTTH) (Table 38.1). In Rhodnius, PTTH is released after a blood meal and acts to trigger the animal’s molt by stimulating the release of a second hormone from the prothoracic gland.

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Two decades later, the German endocrinologist Peter Karlson isolated and purified this second hormone, termed ecdysone. PTTH triggers molting by stimulating the release of ecdysone, a steroid hormone that coordinates the growth and reorganization of body tissues during a molt. The action of ecdysone is a good example of how a hormone can precisely coordinate broad changes in body organization and function, in contrast to the specific regulation that nerves provide. Karlson was able to purify a small amount of ecdysone from large numbers of silk moth Bombyx caterpillars. He ultimately isolated a mere 25-mg sample from 500 kg of moth larvae! This tiny amount highlights the general principle that relatively few hormone molecules can have a large effect on an organism.

In his studies of Rhodnius, Wigglesworth also noted that, whatever the larval stage, the decapitated bug always molted into an adult after its blood meal. By removing just the front brain region of the head, Wigglesworth was able to show that a region in the head close to the brain releases a hormone that normally prevents the earlier larval stages from molting into the adult form (Table 38.1). This hormone, called juvenile hormone, is released in decreasing amounts during each successive larval molt. After the fifth and final stage, the level of juvenile hormone is so low that it no longer blocks maturation, and at the final molt the insect undergoes its final growth into the adult form (Fig. 38.3).

The brains of insects, similar to the nervous systems of other invertebrate and vertebrate animals, contain neurosecretory cells. These cells are neurons that release hormones, which act on endocrine glands or other targets within the insect instead of secreting neurotransmitters that bind to another neuron or to muscle. In the case of insects, peptide hormones released from neurosecretory brain cells act on the corpora allata (paired endocrine glands in insects) and the prothoracic gland (Fig. 38.3).Neurosecretory cells stimulate the corpora allata to produce juvenile hormone so that molting larvae retain juvenile characteristics. At appropriate times, other neurosecretory brain cells secrete the peptide hormone PTTH, which stimulates the release of ecdysone from the prothoracic gland, triggering the transition from larval to pupal and adult forms.

These studies reveal how small amounts of hormones released from key glands within the animal’s body regulate major stages of growth and changes in body form during metamorphosis. When an animal’s body requires coordinated changes in multiple organ systems, hormonal regulation by the endocrine system plays a critical role.

Hormones also regulate growth in humans and other vertebrate animals. Growth hormone controls the growth of the skeleton and many other tissues in the human body. Growth hormone is produced by the pituitary gland, which is located beneath the brain. Tumors of the pituitary that cause an overproduction of growth hormone lead to gigantism, and tumors that result in too little growth hormone cause pituitary dwarfism. The role of the pituitary gland in regulating body function is described later in the chapter.