Hormones play major roles in development. Because they circulate throughout the body, they can simultaneously affect many cells and tissues, but in different ways depending on the receptors and intracellular pathways in those cells. Thus hormones can coordinate the complex sets of responses involved in development. Arthropod development involves complete or incomplete metamorphosis controlled by ecdysone, juvenile hormone, and prothoracicotropic hormone (PTTH). Human gonadal and phenotypic sex development is controlled by steroid hormones.
learning outcomes
You should be able to:
Analyze observations made during experiments of arthropod development.
Explain how an individual can have a male genotype but female body characteristics and genitalia.
Explain the role of negative feedback in the timing of puberty in humans.
Why did decapitation of Rhodnius prevent molting when done 1 hour after feeding but not when done 1 week after feeding?
Decapitation of Rhodnius prevented molting when done 1 hour after feeding but not when done 1 week after feeding because of the time necessary for PTTH to be released and diffuse to the prothoracic gland in sufficient amounts to stimulate the release of ecdysone.
How can a person with an XY genotype develop with the phenotype of a female?
The female external phenotype is the default pathway in humans. That pathway is altered in normal XY individuals because of genes on the Y chromosome that stimulate the development of testes rather than ovaries. The testes produce the male steroid sex hormone that stimulates development of the male external phenotype. However, if an individual lacks receptors for the male steroid sex hormone, then it has no effect and the female phenotype develops.
What is tthe role of negative feedback in the timing of puberty?
In the prepubertal individual, sex steroids are produced in low amounts, but these low amounts provide sufficient negative feedback to the hypothalamus to inhibit the production of GnRH. As a result, the release of LH and FSH from the anterior pituitary is low. However, at the time of puberty, sensitivity of the hypothalamus to the circulating sex steroids is reduced, releasing the negative feedback control. Therefore production and release of GnRH as well as LH and FSH go up, stimulating the changes associated with puberty.
A common feature of the roles of hormones in development is that they coordinate the execution of events programmed into the genome. These functions have evolved to be rather stable in the face of changes in the internal and external environments. In contrast, hormones involved in maintaining homeostasis of the internal environment have evolved to be very sensitive to changes in the internal or external environment that could compromise homeostasis. We will look at some of these hormonal regulatory mechanisms in the next section.