Impairments of Endocrine Functions: Causes and Consequences - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 189

tissue) by biotests, chemical analyses are then used to detect and quantify the specific con-
taminants responsible for any hormonal disturbance (Amiard et al. 2009; Kinani et al. 2010).
In contrast to aquatic vertebrates for which the impact of endocrine disruptors (ED) on
reproductive function has been widely studied and documented, leading to the development
and validation of several specific biomarkers to predict alterations in reproductive functions
(concentrations in steroid hormones and induction of VTG synthesis in male and juvenile
organisms) (Matthiessen 2003; Langston et al. 2005), few studies have been conducted on
invertebrates, in particular, mollusks and crustaceans, despite their obvious ecological impor-
tance (deFur et al. 1999; deFur 2004; Oetken et al. 2004). This is largely attributed to the lack of
knowledge on the endocrine regulation of the reproductive process in these species, which
are routinely used in ecotoxicology (Porte et al. 2006). In crustaceans, hormonal regulation
is mainly based on ecdysteroid and terpenoid hormones in females and androgenic gland
hormone (AGH) in males (Charniaux-Cotton 1954). Because of this specificity of endocrine
systems, ED biomarkers available in fish cannot be easily used and/or should not be directly
applied to protostome invertebrates (i.e., ecdysozoans and lophotrochozoans). Indeed, as illus-
trated for the steroid hormone signaling pathway (Thornton 2003; Markov et al. 2009), modern
endocrine systems in metazoans result from a complex evolution of molecular players such as
hormone receptors or enzymes implicated in hormone synthesis. This diversifying biological
evolution has given rise to divergent endocrine systems between the main metazoan lineages
with specific modes of hormonal regulation. The impact of endocrine disruptors on repro-
ductive function has been widely studied since the 1990s. Currently, other types of disruption
are studied in order to investigate the general reproduction and development of exposed
species. This is specially the case for thyroid function disruption as THs play a major role in
growth, reproduction, and development, in particular, neurological development.
The ultimate purpose of the evaluation of ecological risk is to protect communities
and ecosystems from the noxious effects caused by chemical pollutants. Disturbance of
sex hormones and other hormones by miscellaneous chemical contaminants can have
cascading effects on the growth and reproduction of individuals, leading to changes at
higher levels of biological organization, populations, and communities (Kidd et al. 2007).
Ecoepidemiological studies show changes at these levels, but any causal link of these
changes to EDCs is usually tenuous.
8.2 Generalities
8.2.1 Normal Mechanisms of Hormonal Action
In nearly all complex multicellular animals, there are two main systems—the nervous
and the endocrine systems—controlling and coordinating processes within the body. The
endocrine system is based on chemical messengers, the hormones, which are secreted into
the blood (or other extracellular fluids), and can reach all parts of the body.
The endocrine system has three main components: the endocrine glands, the hormones,
and the receptors. The endocrine glands are situated at various sites in the body, and the
cells in these glands secrete specific chemicals called hormones. The hormones circulate
in the body via the bloodstream and modulate cellular or organ functions by binding with
receptors in the target cells. The receptors in the target cells, once activated by binding of
the hormone, regulate the functions and processes in the tissue through interactions with
the cell's DNA or other complex intracellular signaling processes.