Author: Dr. Kartini Jasni

Humans have a reproductive cycle, commonly known as the menstrual cycle, which consists of the ovarian and uterine cycle. As for mammals, the reproductive cycle is known as the estrous cycle. The estrous cycle is a cyclical pattern of ovarian activity whereby female animals go from a period of reproductive receptivity to non-receptivity, ultimately allowing the establishment of pregnancy following mating. Walter Heape1 was the first person to use the word “estrus" to describe the reproductive cycle since it is a Latin word that originated from the word “oistros”, which means “sexual season”.  He then further named the 4 phases of the estrous cycle: proestrus, estrus, metestrus and diestrus.

Proestrus is the phase during which the uterus prepares for estrus. Hormone level rises and the ovaries develop eggs. Estrus is the stage when ovulation occurs, and the animal is “in heat”. Metestrus is the phase that occurs post-estrus, or also known as post ovulation. If fertilization occurs, the rats’ body is ready for pregnancy. Diestrus is the resting phase when there is no pregnancy. The metestrus phase of a rat is different from menstruation in humans. In humans, the cells of the uterine lining shed during menstruation due to no implantation or fertilisation. Rats, on the other hand, do not shed off their uterine lining.

The estrous cycle and duration differ among species. For example, the estrous cycle of rats ranges between 4 to 5 days. For rats, proestrus and estrus each last for 1 day and metestrus, which varies between individual rats, lasts either 1 or 2 days. Finally, diestrus often lasts just 1 day. In contrast, the estrous cycle of an elephant lasts for 12 to 18 weeks, while the human menstrual cycle lasts for 28 days. Therefore, rats produce approximately 6 litters per year due to their short estrous cycle and gestation period. 

It is important for researchers, veterinarians and laboratory technicians working in Laboratory Animal Facility to understand the reproductive cycle of the animals. By understanding the estrous cycle, researchers can better comprehend hormonal fluctuations and the fertility health of the animals.

Figure 1: A dam is nursing its pups. She produces approximately 9 pups per litter.

Acknowledgement: 

The author would like to acknowledge Dr Chau De Ming for his useful suggestions.

References:

1. Walter Heape, FRS: a pioneer in reproductive biology. Centenary of his embryo transfer experiments. Journal of Reproductive Fertility (1991) J. D. Biggers. 
2. Staging of the estrous cycle and induction of estrus in experimental rodents: an update. Fertility and Research Practice (2020) Ayodeji Folorunsho Ajayi & Roland Eghoghosoa Akhigbe.
3. A Comparative Study on Staining Techniques for Vaginal Exfoliative Cytology of Rat, Journal of Pharmacology and Clinical Research (2017), MR Srivinasan et.al

Date of Input: 24/12/2024 | Updated: 24/12/2024 | azah