Plasma E2, increased leads to Increased, uterine weight

Estradiol (E2) is a key signalling estrogen hormone in the hypothalamic–pituitary-gonadal (HPG) axis in the estrus cycle of female rodents and some other vertebrates. 

The estrus cycle is a coordinated series of changes that results in fertility in rodents through hormone signaling, including Progesterone, Estradiol, Luteinizing Hormone, and Follicle-Stimulating Hormone, in order to progress through metestrus, diestrus, proestrus, and estrous phases over a period of 4-5 days in rodents, inducing changes in changes to the uterus and vagina (for review see Miller and Takahashi 2014; Swift et al. 2024).  In proestrus, increased estradiol levels occur, and physiological changes include ovarian follicle development and the thickening of the uterine wall in preparation for potential pregnancy.  Thickening of the uterine wall is caused by proliferation of cells, particularly endometrial epithelial (lining) cells (Nicklaus et al. 2007), causing an increase in uterine weight.  In estrus, a surge in luteinizing hormone levels occur, and ovulation of the mature egg.  Metestrus is a short transition between estrus and diestrus, features an increase in progesterone levels, and development of the corpus luteum begins in preparation for pregnancy.  Diestrus includes continued high levels of progesterone and further development of the corpus luteum; if pregnancy does not occur the corpus luteum regresses and resetting of the cycle occurs.

Increased uterine weight shows that reproductive maturity has occurred because of the association with phases of the estrus cycle, and as an indicator of an adverse outcome when puberty occurs at an earlier or later age than in normal development.  Increased estradiol leads to an adverse outcome of puberty occurring at an earlier age than in normal development.  

This Key Event Relationship was part of an Environmental Protection Agency effort to develop AOPs that establish scientifically supported causal linkages between alternative endpoints measured using new approach methodologies (NAMs) and guideline apical endpoints measured in Tier 1 and Tier 2 test guidelines (U.S. EPA, 2024) employed by the Endocrine Disruptor Screening Program (EDSP). A series of key events that represent significant, measurable, milestones connecting molecular initiation to apical endpoints indicative of adversity were identified based on scientific review articles and empirical studies. Additionally, scientific evidence supporting the causal relationships between each pair of key events was assembled and evaluated.   The present effort focused primarily on empirical studies with laboratory rodents and other mammals.  

Empirical studies are focused on increased plasma estradiol and resulting prolonged estrus cycle, in support of development of AOP 637. Authors of KER 3746 did a further evaluation of published peer-reviewed literature to provide additional evidence in support of the key event relationship.  The literature used to support this KER began with the test guidelines and followed to primary, secondary, and/or tertiary works concerning the relevant underlying biology.  In addition, search engines were used to target journal articles with term ‘Estradiol’ and ‘Increased uterine weight’ to locate representative empirical studies that support the key event relationship.  

Increased plasma estradiol leading to increased uterine weight is an established mechanism for onset of estrus cycling from reproductive maturation.  Increased plasma estradiol and the timing of increased uterine weight as an adverse outcome have been studied in laboratory mammals by addition of various forms of estradiol (e.g. 17beta-estradiol, ethynyl estradiol; Laws et al. 2000) and toxicants (Li et al. 2008; Bo et al. 2022; Zhou et al. 2023). Studies involving doses of laboratory mammals with various forms of estradiol (e.g. 17beta-estradiol, ethynyl estradiol) are supportive of the mechanism of increases in exposure to estradiol compounds causing increased uterine weight at an earlier age.

Life Stage: Applies to adult, reproductively mature.

Sex: Applies to females as specific to uterus.

Taxonomic: Primarily studied in laboratory rodents.  Plausible for mammals and marsupials that have uteri.   

Bo T, Liu M, Tang L, Lv J, Wen J, Wang D. 2022.  Effects of High-Fat Diet During Childhood on Precocious Puberty and Gut Microbiota in Mice. Frontiers in Microbiology 13: 930747.

Laws SC, Carey SA, Ferrell JM, Bodman GJ, Cooper RL. 2000.  Estrogenic activity of octylphenol, nonylphenol, bisphenol A and methoxychlor in rats. Toxicological Sciences 54(1):154-167. 

Li Z, Li T, Leng Y, Chen S, Liu Q, Feng J, Chen H, Huang Y, Zhang Q.  2018.  Hormonal changes and folliculogenesis in female offspring of rats exposed to cadmium during gestation and lactation. Environmental Pollution 238: 336-347.

Miller, B.H. and Takahashi, J.S.  2014.  Central circadian control of female reproductive function.  Frontiers in Endocrinology 4(1): 195.

Niklaus AL, Aubuchon M, Zapantis G, Li P, Qian H, Isaac B, Kim MY, Adel G, Pollard JW, Santoro NF. 2007.  Assessment of the proliferative status of epithelial cell types in the endometrium of young and menopausal transition women. Human Reproduction 22(6): 1778-1788.

Swift, K.M., Gary, N.C., and Urbanczyk, P.J.  2024.  On the basis of sex and sleep: the influence of the estrous cycle and sex on sleep-wake behavior.  Frontiers in Neuroscience 18:1426189.

U.S. Environmental Protection Agency.  2004.  EDSP Test Guidelines and Guidance Document. https://www.epa.gov/test-guidelines-pesticides-and-toxic-substances/edsp-test-guidelines-and-guidance-document (retrieved 25 July 2025).

Zhou L, Ren Y, Li D, Zhou W, Li C, Wang Q, Yang X. 2023. Timosaponin AIII attenuates precocious puberty in mice through downregulating the hypothalamic-pituitary-gonadal axis. Acta Biochimica Polonica 70(1): 183-190.

Italics indicate edits from John Frisch March 2026.  A full list of updates can be found in the Change Log on the View History page.