Age and sex associated organ weight differences in vervets/African green monkeys (Chlorocebus aethiops sabaeus)

Nonhuman primates are crucial biomedical research models, intermediate between rodents and humans. AGMs have been utilized in a variety of research studies for many years.^1-3 These primates have similar behavior and physiology¹ to that of other Old-World primates used in biomedical research and are useful alternatives to Asian species, such as rhesus and cynomolgus macaques. AGMs are phylogenetically similar to other laboratory primates as well as to humans. Furthermore, AGMs are highly adaptable, less expensive than macaques, and are safer to work with for humans since they are not carriers of macacine herpesvirus 1.

AGMs have been used in a wide variety of biomedical studies including infectious disease, diabetes/metabolism, behavior, aging/Alzheimer's disease, toxicity and medical device testing.⁴ Infectious diseases,^5-9 metabolic disorders,³ and aging¹⁰ all influence body^{10, 11} and organ weight variability. Inflammation due to infectious agents, age-related thymic atrophy, hepatic lipidosis in diabetes mellitus, and neoplasms are a few examples. Additionally, AGMs demonstrate sexual dimorphism¹² that results in differences in organ and body weights. Some studies also demonstrate that stress can considerably alter the body and adrenal gland weights.¹³

Among nonhuman primates, organ weight data have been published for baboons (Papio hamadryas),^{14, 15} rhesus macaques (Macaca mulatta),¹⁶ cynomolgus macaques (Macaca fascicularis),¹⁷ and stump-tailed macaques (Macaca arctoides).¹⁸ Normative organ weight data have not been reported for AGMs. Normative organ weights are used as reference points for the study of pathology in various research and diagnostic settings. Establishing a reference for age-associated organ weight changes in both sexes would be an important resource for primate researchers and pathologists. This study was conducted to provide normative body and organ weight data by age category and sex from the breeding colony of AGMs.

The scatterplots of body and organ weights age-by-sex are shown in Figures 1-16. Organ weights as a percentage of body weight age-by-sex are shown in Figures 17-31. Descriptive statistics by age category and sex are shown in Tables 2–6. Percentage organ weights by age category and sex are shown in Tables S1–S5.

AGMs are widely used in biomedical research, but a reference for normal organ weights and organ weights as percentages of body weights has not been previously reported. This retrospective study was designed to generate normal body and organ weights and their relationships to the body weight, providing a reference range for AGMs by sex and age category. This data will be valuable to researchers and pathologists investigating diseases in AGMs. In summary, males have higher body and organ weights than females, but organ weights as percentages of body weights is not significantly different between males and females.

The AGM colony at WFUSM is a breeding colony skewed toward female predominance. The mean weights of the kidneys, liver, lungs, pancreas, pituitary gland, prostate gland, spleen, testes, and uterus were highest in adult animals. The thymus weights were higher in yearlings compared to the infants and gradually declined in older animals. This was similar to that of the cynomolgus macaques.¹⁷ The weights of both adrenal glands, and lungs were higher in elderly females compared to the adults. The heart weight increased with age and was slightly higher in elderly females compared to adults; a similar observation was made in cynomolgus macaques.¹⁷ In contrast, brain weights did not differ significantly between adults and elderly females. The reproductive organs demonstrated a growth pattern compatible with sexual maturity. Male AGMs become sexually mature at about four years.²³ This finding corresponds with our study data.

The organ weights as percentages of the body weights for the kidney and liver tended to plateau after the AGMS reached 1 year of age. This was comparable to the cynomolgus macaques.¹⁷ However, in elderly females both kidney and liver percentage weights tend to increase compared to the adults. The cause of this is uncertain, as tissues were not examined histologically, but, in older animals, interstitial fibrosis and immune cell infiltration are common findings. The testes weights as percentages of the body weights were higher in adults, but there were no differences between the two testes. This contrasted with cynomolgus monkeys, where the left testis has a higher percentage of body weight compared to the right, and also in humans, where the left testis has a 10% lower percentage of body weight compared to the right.^{17, 23} As percentages of body weights, the brain, ovary, spleen, thymus, and thyroid glands declined in older animals. Similar to the observations in cynomolgus¹⁷ and rhesus macaques,¹⁶ except for ovaries. In cynomolgus macaques, the infants and adults had comparable ovarian weights as percentage body weights.

This retrospective study was designed to report normal body and organ weights in AGMs and can be a resource for AGM researchers in their future studies.

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.