Invertebrate Hormones
Invertebrate
Hormones
In many metazoan phyla, the principal source of hormones is neurosecretory cells, specialized nerve cells capable of synthesizing and secreting hormones. Their products, called neurosecretions or neurosecretory hormones, are discharged directly into the circulation, and serve as a crucial link between the nervous and endocrine systems.
Neurosecretory hormones occur in all metazoan groups. An extensively studied neurosecretory process in invertebrates is control of development and metamorphosis of insects. In insects, as in other arthropods, growth is a series of steps in which the rigid, nonexpansible exoskeleton is periodically discarded and replaced with a new, larger one. Most insects undergo a process of metamorphosis, in which a series of juvenile stages, each requiring formation of a new exoskeleton, end with a molt.
Insect physiologists have discovered that molting and metamorphosis are primarily controlled by interaction of two hormones, one favoring growth and differentiation of adult structures and another favoring retention of juvenile structures. These two hormones are molting hormone or ecdysone, produced by the prothoracic gland, and juvenile hormone, produced by the corpora allata (Figure 36-4). The structures of both hormones have been determined. Extraction from 1000 kg (about 1 ton) of silkworm pupae was required to show that ecdysone is a steroid.
Ecdysone is controlled by prothoracicotropic hormone or PTTH. This hormone is a polypeptide (molecular weight about 5000) produced by neurosecretory cells of the brain, and transported by axons to the corpora allata where it is stored. At intervals during juvenile growth, release of PTTH into the blood stimulates the prothoracic gland to secrete ecdysone. Ecdysone appears to act directly on the chromosomes to set in motion changes resulting in a molt, by favoring development of adult structures. It is held in check, however, by juvenile hormone, which favors development of juvenile characteristics. During juvenile life, juvenile hormone predominates and each molt yields another larger juvenile (Figure 36-4). Finally output of juvenile hormone decreases, allowing final metamorphosis to the adult stage.
The precise location of brain hormone in the brain of pupal tobacco hornworms was revealed by N. Agui by delicate microdissection. Using a human eyebrow hair, he was able to isolate the single cell in each brain hemisphere that contained brain hormone activity. Thus only two cells, each about 20 µm in diameter,produce this insect’s total supply of PTTH. In an age when sophisticated instrumentation has removed much of the tedium (and some creativity) from research, it is refreshing to learn that certain biological mysteries succumb only to skillful use of the human hand.
Chemists have synthesized several potent analogs of juvenile hormone, which hold great promise as insecticides. Minute quantities of these synthetic analogs induce abnormal final molts or prolong or block development. Unlike chemical insecticides, they are highly specific and ecologically benign.
In many metazoan phyla, the principal source of hormones is neurosecretory cells, specialized nerve cells capable of synthesizing and secreting hormones. Their products, called neurosecretions or neurosecretory hormones, are discharged directly into the circulation, and serve as a crucial link between the nervous and endocrine systems.
Neurosecretory hormones occur in all metazoan groups. An extensively studied neurosecretory process in invertebrates is control of development and metamorphosis of insects. In insects, as in other arthropods, growth is a series of steps in which the rigid, nonexpansible exoskeleton is periodically discarded and replaced with a new, larger one. Most insects undergo a process of metamorphosis, in which a series of juvenile stages, each requiring formation of a new exoskeleton, end with a molt.
Insect physiologists have discovered that molting and metamorphosis are primarily controlled by interaction of two hormones, one favoring growth and differentiation of adult structures and another favoring retention of juvenile structures. These two hormones are molting hormone or ecdysone, produced by the prothoracic gland, and juvenile hormone, produced by the corpora allata (Figure 36-4). The structures of both hormones have been determined. Extraction from 1000 kg (about 1 ton) of silkworm pupae was required to show that ecdysone is a steroid.
Ecdysone is controlled by prothoracicotropic hormone or PTTH. This hormone is a polypeptide (molecular weight about 5000) produced by neurosecretory cells of the brain, and transported by axons to the corpora allata where it is stored. At intervals during juvenile growth, release of PTTH into the blood stimulates the prothoracic gland to secrete ecdysone. Ecdysone appears to act directly on the chromosomes to set in motion changes resulting in a molt, by favoring development of adult structures. It is held in check, however, by juvenile hormone, which favors development of juvenile characteristics. During juvenile life, juvenile hormone predominates and each molt yields another larger juvenile (Figure 36-4). Finally output of juvenile hormone decreases, allowing final metamorphosis to the adult stage.
The precise location of brain hormone in the brain of pupal tobacco hornworms was revealed by N. Agui by delicate microdissection. Using a human eyebrow hair, he was able to isolate the single cell in each brain hemisphere that contained brain hormone activity. Thus only two cells, each about 20 µm in diameter,produce this insect’s total supply of PTTH. In an age when sophisticated instrumentation has removed much of the tedium (and some creativity) from research, it is refreshing to learn that certain biological mysteries succumb only to skillful use of the human hand.
Chemists have synthesized several potent analogs of juvenile hormone, which hold great promise as insecticides. Minute quantities of these synthetic analogs induce abnormal final molts or prolong or block development. Unlike chemical insecticides, they are highly specific and ecologically benign.
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