Genetic factors set the underlying limitations and potentialities for growth and development, but the life experiences and environment of the organism determine how the body will grow within those parameters. In fact, there is increasing evidence that environmental factors can change the ways in which genes are expressed without having an effect on the genes themselves, and that people with identical genotypes (i.e., identical twins) could have very different phenotypes. In other words, identical twins aren’t really identical, and they become even more different as they age (Fraga et al., 2005; Gluckman et al., 2009). Phenotypic differences emerge in identical twins because of the “software” that provides instructions to the unfolding genotype. These instructions are known as the epigenome; they are responsible for turning some genes on and some genes off. All of the cells in our body except the sex cells have the same genes, but they do different things because of the epigenome. Indifferent individuals, the epigenome may turn off some genes or turn on others, resulting in different phenotypes. This is one of the main ways in which the environment interacts with genes, and it helps to explain why one member of a pair of identical twins may suffer from a genetically based cancer while the other is disease-free. Lifestyle factors are particularly important influences on the epigenome, especially diet and smoking.
The ongoing “nature‒nurture debate” has pitted genetic factors against environmental factors in determining how an individual grows, develops, and behaves. The field of epigenetics helps to resolve this conflict by revealing that structural changes to DNA and associated proteins (without causing changes in the nucleotide sequence) can underlie gene expression. The changes are transmitted through mitosis, so that when they are established during development, they persist with further cell division. In this way, environmental factors (such as smoke or air pollution) can bring about changes during development that affect a person in adulthood, partially explaining differences in disease risk (Fig.16-7). Although these changes in gene expression are not usually passed on to offspring, there is increasing evidence of epigenetic inheritance that transcends generations (Whitelaw and Whitelaw, 2006). Certainly, research in epigenetics calls into question the whole idea of genetic determinism for many traits.
One of the primary ways in which genes have an effect on growth and development is through their effects on hormones. Hormones are substances produced in one cell that have an effect on another cell. Most hormones are produced by endocrine glands and are transported to other cells in the bloodstream; virtually all have an effect on growth. Just above the roof of your mouth are two of the most important organs related to hormone action: the hypothalamus and the pituitary gland. They are connected to each other and are in almost constant communication. The hypothalamus has been described as the central command center or relay station for all kinds of actions going on in the body. The pituitary, on the other hand, is the primary regulator of hormonal interactions related to reproduction, growth, and development. The hypothalamus “tells” the pituitary what to do based on input it receives from throughout the body and brain. There are two parts of the pituitary, the anterior and the posterior. The anterior pituitary produces hormones that regulate reproduction (follicle- stimulating hormone [FSH] and luteinizing hormone [LH]), milk production (prolactin), metabolism (adrenocorticotropic hormone [ACTH]), and growth (growth hormone [GH]). The posterior pituitary secretes several hormones that, in turn, act on the gonads (ovaries and testes) and the thyroid, adrenal, and mammary glands.
We can use the hormone thyroxine, produced by the thyroid gland in the neck, to illustrate the action of hormones and the communication system among the endocrine glands (Fig. 16-8). Thyroxine regulates metabolism and aids in body heat production. When thyroxine levels fall too low for normal metabolism, the brain senses this and sends a message to the hypothalamus. The hypothalamus reacts by releasing thyrotropin-releasing hormone (TRH), which goes to the anterior pituitary, where it stimulates the release of thyroid- stimulating hormone (TSH). TSH then goes to the thyroid gland, stimulating it to release thyroxine. When the brain senses that the levels of thyroxine are adequate, it sends signals that inhibit the further release of TRH and TSH. In many ways, this process is similar to what your household thermostat does: When it senses that the temperature has dropped too low for comfort, it sends a message to the heating system to begin producing more heat; when the temperature reaches or exceeds the preset level, the thermostat sends another message to turn the heat off.
Two other hormones that are important in growth include insulin and GH, already mentioned. Insulin, produced by the pancreas, regulates the use of glucose in the body, as noted in our discussion of diabetes. GH, secreted by the anterior pituitary, promotes growth and has an effect on just about every cell in the body. Tumors and other disorders can result in excessive or insufficient amounts of growth hormone secretion, which in turn can result in gigantism or dwarfism. Short stature is not always due to pathology, however. One group of people who have notably short stature are African Efe pygmies (Fig. 16-9). There is evidence that altered levels of growth hormone and its controlling factors interact with nutritional factors and infectious diseases to produce the relatively short adult stature of these people (Shea and Bailey, 1996), providing another example of epigenetics and the interaction of biological and cultural forces. More recent research suggests that their short stature may be due to decreased expression of the receptors for growth hormone and that epigenetic factors such as diet could play a role in modifying how genes are expressed in both African and Philippine pygmies (Dávila et al., 2002; Bozzola et al., 2009). Another hormone that influences growth and development is cortisol, which is elevated during stress. Up to a point, cortisol elevation is adaptive, but if the response is prolonged or severe, there appear to be negative effects on health and behavior (Flinn, 1999; Flinn and England, 2003). Under conditions of chronic emotional and psychosocial stress, cortisol levels may remain high and suppress normal immune function. This means that a child living in a stressful situation is more vulnerable to infectious diseases and may experience periods of slowed growth if the stress is prolonged. A reasonable argument could be made that people today of all ages and in all parts of the world experience significantly higher levels of stress than our ancestors did, suggesting that effects on growth may also be different. This is not to imply that our ancestors didn’t experience stress (few things are more stressful than a lion chasing you and your children), but the sources and duration of stressors were probably very different from those we experience today.
The levels of reproductive hormones in women from health-rich nations appear to be elevated over what is reported for women in traditional societies and what was probably the ancestral profile (Vitzthum, 2009) (Fig. 16-10). Furthermore, women who use contraception, have few or no children, and breast-feed for only a few months have repeated menstrual cycles and the associated high levels of estrogen for the majority of their reproductive lives. This is a very different hormonal profile from ancestral women, who spent most of their adult years either pregnant or nursing infants, yielding very few menstrual cycles. This disconnect between today’s hormonal profiles and those of our ancestors may result in a higher incidence of reproductive cancers, especially when coupled with high-fat diets and low levels of exercise (Trevathan, 2010).