TABLE 1.

Characteristics of critical windows for development.

Characteristics of critical windows	Description	Examples	References
Timing in Development of Critical Windows	Critical developmental windows can occur from shortly after egg fertilization until achievement of sexual maturity.	Human psychiatric illnesses frequently first manifest during critical windows in teenagers, potentially associated with gut microbiome dysfunction.	McVey Neufeld et al., 2016
		Human cardiovascular form and function impacted by environmental toxicants during critical windows starting as early as week 2 after conception	Lage et al., 2012
Duration (‘Width’) of Critical Window	Finite “width” to critical window – i.e., distinct onset and closing of window, but interpretation of critical window “edges” is dependent upon stressor dose	Cardiac development in chicken embryos primarily sensitive during week 2 of 3 weeks incubation	Chan and Burggren, 2005
		Gonad differentiation in zebrafish between 30–44 days post-fertilization	Quintaneiro et al., 2019
		Modeling of critical window as a 3D construct of time, dose and phenotype	Burggren and Mueller, 2015
Duration of Switched Phenotype	Phenotypic switching irreversible, persisting through subsequent life stages	Larval hypoxia has long-term effects on protein digestion and growth in juvenile European sea bass	Zambonino-Infante et al., 2017
		Chicken embryos show aberrant aortic arch morphogenesis when hemodynamic variables are manipulated specifically at Stage 21.	Kowalski et al., 2013
Number of Critical Windows Per Trait	Typically only one, but multiple critical windows can exist for same trait	Lipid and glucose metabolism in adult sheep is similarly affected by undernutrition early in gestation as well as immediately postnatally	Poore et al., 2010
		Correction of structural abnormalities in mouse brain cortex have multiple critical windows	Cox et al., 2018
Stressors Acting During Critical Window	Stressors can be intrinsic or extrinisic (environmental) factors.	Odors (aversive or attractive) in first week post-eclosion fruit fly larvae alter olfactory circuitry	Golovin and Broadie, 2016
		Hypoxia during middle third of avian incubation alters gross morphology and metabolic physiology	Dzialowski et al., 2002
Dose Effects During Critical Window	Phenotypic switching during critical windows is dose-dependent	Body mass changes in Artemia during early development are dependent on strength of environmental salinity	Mueller et al., 2016
		Hypoxia-induced alteractions of morphology and physiology of chicken embryo show differential responses to 13 and 15% oxygen	Zhang and Burggren, 2012
Sex Differences in Critical Window Susceptibility for Same Trait	Phenotypic switching during critical windows is sex-dependent	Prenatal critical window for oranotin toxicant exposure in rats results in greater permanent phenotype switching in males compared to females	Grote et al., 2009
		Prenatal critical window for particulate air pollution exposure causes phenotype switching in human male but not female children	Hsu et al., 2015
Organ System Differences in Critical Windows	Timing of development of window differs between organ systems within an organism	Critical window for hypoxic effects on heart mass and blood pressure are considerably different in timing and width in embryonic alligator hearts	Tate et al., 2015
		Critical windows for sensitivity to environmental toxicant differ in timing and duration for immune and respiratory systems in humans	Dietert et al., 2000
Population Differences in Critical Windows For Same Trait	Timing and width of critical window for a particular phenotypic trait varies between different populations – i.e., “heterokairy”	Human populations differ in critical window for infant weight gain and its effect on adult adiposity	Wells, 2014
		Hypersalinity delays onset of heartbeat and changes timing of foot attachment and eye spot formation in the euryhaline snail Radix balthica	Tills et al., 2010
Species Differences in Critical Windows For Same Trait	Timing of window for a particular phenotypic trait varies between different species – i.e., “heterochrony”	Critical windows for nephrogenesis and morphologica renal development differ between dog, pig, rabbit, monkey, mouse, and rat	Frazier, 2017
		Critical windows for motor activity and motor function performance identified by exposure to environmental neurotoxins differ in rats and mice	Ingber and Pohl, 2016
		Critical window for gut microbiome establishment differs between wood frogs, green frogs and bullfrogs	(Ingber and Pohl, 2016)