Question:  I'm preparing for an animal experiment but don't know much about lab mice. What should I do?

Answer:  Let's first understand the life cycle of a laboratory mouse.

Laboratory mice play a vital role in areas like drug discovery and disease research. However, their lives are very brief compared to humans.

Question:  I know mice develop very quickly, reaching sexual maturity in just 6-8 weeks.

Answer:  Yes, the mouse lifespan is approximately 18-24 months. Let's look at the key stages of a mouse's growth cycle.

 

Embryonic Stage
 

After the zygote forms in the oviduct ampulla, it develops into a morula in about 3 days, and a blastocyst in 5 days, beginning implantation. The gestation period is 19-21 days.

Postnatal Early Life and Lactation

●Day 0:  Newborn pups are hairless, with pink skin, and eyes tightly closed.

●Day 3:  Umbilical cord detaches; ear pinnae begin to appear.

●Day 7:  Fine hair covers the body; toes are completely separated.

●Days 12-14:  Eyes are fully open; pups begin to consume solid food.

Weaning Period

Pups are weaned at 3 weeks of age, beginning independent life. At this point, male and female pups must be separated. After adapting to the new environment, they gradually exhibit behavioral characteristics of adult mice.

Early Juvenile and Growth Period

●The vaginal opening in females becomes patent by ~4 weeks.

●Testes descend into the scrotum in males by ~5 weeks.

Late Juvenile Period

Occurs 5-8 weeks after birth.

Sexual Maturity Period

Reached at 45-60 days of age. The estrous cycle is 4-5 days.

Physical Maturity / Adulthood

Reached at 60-90 days of age, entering adulthood.

Middle to Old Age

Mice enter middle age around 4 months and old age after 6 months. Generally, healthy mice can live 18-24 months, with a maximum of up to 3 years.

 

A mouse's brief lifespan can be approximately equated to the human average lifespan (~80 years). Theoretically, 1 day in a mouse's life is roughly equivalent to 40 days in a human.

Here is a conversion for different stages:

●Weaning:  Mouse 3-4 weeks ≈ Human 6 months.

●Adolescence:  Mouse 42 days ≈ Human 11.5 years.

●Adulthood:  Mouse 10 weeks (sexual maturity) ≈ Human 20 years.

●Middle Age:  Mouse 15 months ≈ Human 51 years.

●Old Age/Senescence:  Mouse 18 months ≈ Human 70+ years.

 

Question:  What age of mouse is best for experiments?
 

Mice typically have maturing organs by 4-6 weeks. At 6-8 weeks of age, their organs are fully developed, and they are in good health, making them suitable for most experiments.

●Acclimatization Period:  When ordering, it's advisable to select mice 1-2 weeks younger than the required experimental age to allow them to acclimate to the laboratory environment.

●Immunology Studies:  If human cell line xenografts are challenging, 4-6 week-old mice can be chosen as their immune systems are less developed, facilitating tumor engraftment.

●Breeding Experiments:  9-10 week-old mice are recommended as they have stronger reproductive capacity.

●Aging/Geriatric Studies:  Mice 12 months or older are selected to model physiological changes in the elderly.

 

References:

[1] Bilkei-Gorzo, A. et al. (2017). A chronic low dose of Δ9-tetrahydrocannabinol (THC) restores cognitive function in old mice. Nature Medicine.

[2] National Resource Center for Laboratory Animals, China.

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