The endocrine system originally appeared to be a relatively simple system of discrete glands (Fig. 3.1) that secreted chemical messengers, or hormones, into the blood where they would be carried to specific target cells at a distant site, inducing a reaction. However, it is now clear that the endocrine system is more complex. Some hormones are secreted into ducts and not into blood; for instance, androgens are secreted into the seminiferous tubules. Some organs that have other functions also produce hormones. For instance, the atrium of the heart produces atrial natriuretic peptide (ANP), which inhibits reabsorption of sodium chloride in the kidneys and hence affects blood pressure. Some hormones are produced by several different glands, for instance somatostatin, which is produced by the hypothalamus, pancreas, stomach and intestine. Although the trophoblast is the prime site of human chorionic gonadotrophin (hCG) production, it can also be produced by other tissues, albeit in very low concentrations (Iles and Chard, 1991). The placenta appears to be capable of synthesizing a very broad range of hormones and releasing factors that interact with both maternal and fetal physiology. Some substances such as noradrenaline can act as both hormone and neurotransmitter depending on their mode of delivery and whether they are released from a gland or from a nerve. The hypothalamus produces neurohormones that are important in the interaction between the endocrine and nervous systems.

Overall, the endocrine system (in partnership with the neural system) has the following functions:

The evolution of the endocrine system has its rudiments within the activity of single-cell (unicellular) organisms. The unicellular organisms developed the ability to be attracted to chemicals, described as a chemotactic response, or to chemicals that were vital for the functioning of the organism, described as a chemotrophic response. Equally, these organisms developed the ability to recognize noxious chemicals (toxins) and were thus able to avoid them. The cell reacting to chemical signals interacting with receptor sites upon the cell membrane and within the cytoplasm led to the development of active mobility.

As multicellular organisms developed, the group of unicellular organisms that were the prototypes of multicellular organisms evolved chemical communication as an extension of the chemotrophic response. As multicellular evolution progressed further, the cells became more differentiated and specialized. Regulation therefore became the function of more specialized types of cells. This is reflected in the developmental sequence of a fetus, beginning with the division of a single cell (see Chapter 7). With each successive division, the resulting cells are slightly different from the original zygote cell (although differentiation during the initial divisions may be induced by the presence of maternally derived factors within the cytoplasm of the zygote). Although this differentiation is primarily under genetic control, it is achieved through a process of induction from chemical signals produced by one cell type that influence the division of other neighbouring cells. The altered gene expression of the dividing cells results in a changed morphology and developmental pathway.

As organisms became larger and more complex, cell-to-cell communication became more complicated. It evolved in two ways: the endocrine system (of chemical transmission via the circulating blood system) and the neural system (via transmission of an action potential; see Chapter 1). Under the traditional approach to biological science, the endocrine and neural systems were always considered in isolation; however, they are now considered to be extensions of the same system that are highly interactive. Many endocrine responses are initiated by a neuronal influence. Many neurotransmitters and neuromodulators have also been found to be endocrine hormones.

Hormones regulate metabolism, activate or inhibit the immune system, stimulate or inhibit growth, induce or suppress apoptosis (see below) and prepare the body to respond (such as fleeing or fighting) or undergo transition to a new stage of life such as puberty, pregnancy or the menopause. Hormones are produced by almost every organ and type of tissue; they function as cellular messengers. The action of hormones depends on the responses of the target cells and the pattern of hormonal secretion. Endocrine means ‘secreted inwards’ and is applied to hormones that fit the classical description of being secreted into the bloodstream and having an effect at a distant target. There are also exocrine hormones, which are ‘secreted outwards’ into ducts. These include hormones that are secreted into the vas deferens and uterine tubes.

A number of hormones have a local or paracrine effect, diffusing short distances to act on neighbouring cells or cells separated only by an intracellular space. Examples of a paracrine response are the effects of testosterone and anti-Müllerian hormone (AMH; also known as Müllerian-inhibiting hormone or substance, MIH or MIS) on sexual differentiation (see Chapter 5). If the hormone produced acts upon the same cell that produced it, it is described as autocrine. For example, an autocrine hormone may induce cellular division or signal the programmed death of the cell (apoptosis). If it affects adjacent cells and has a very localized action, it is described as a juxtacrine hormone. Therefore, the effect of a hormone depends on how and where it is secreted, the mode of transport (e.g. whether it is soluble or carried by a binding protein) and how quickly it is metabolized or inactivated.

Neuroendocrine hormones are synthesized in specialized neurons, and their effects can also be paracrine in nature (these are usually described as neurotransmitters and neuromodulators). Oxytocin is an example of a neuroendocrine hormone. It is released from the posterior lobe of the pituitary gland and influences the contractility of the myometrium (see Chapter 13) and myoepithelial cells in the breast (see Chapter 16). In these respects, oxytocin has an endocrine effect, but in many mammals it also modifies female behaviour by inducing parental behaviour in the presence of the sex steroids (Insel, 1992). Oxytocin is thought to influence the successful transition to parenthood in different ways; maternal behavioural changes tending to influence affectionate behaviour and emotional bonding and paternal parenting behaviour tending to affect play and social interaction with their infants (Gordon et al., 2010).

A pheromone is a hormone produced by an individual that induces a response, usually social, within another member of the same species. Releaser pheromones stimulate rapid behavioural responses such as attracting potential mates. Primer pheromones act via the olfactory and neuroendocrine system to produce delayed responses which are usually developmental. Receptors for pheromones are found on the vomeronasal organ close to the nasal cavity of mammals, which use pheromones to indicate identity of kin or family territory. It is controversial as to whether the human vomeronasal organ retains a function; some scientists think it is involved in social behaviour such as pair bonding, parental attachment, sexual attraction and synchrony of menstrual cycles (Halpern and Martinez-Marcos, 2003). The synchronization of menstrual cycles within a group of women, responses between lactating women and their offspring and female responses to non-odorants in male perspiration are suggested to be examples of pheromone effects in humans (Bhutta, 2007).

Secretion of hormones is influenced by a number of factors, including the nervous system, hormone-binding proteins, plasma concentrations of nutrients and ions, environmental changes and other hormones, such as stimulating and releasing hormones.