Question 1

What are the four stages of the rodent estrous cycle?

Accepted Answer

The rodent estrous cycle lasts 4-5 days and consists of four stages: metestrus, diestrus, proestrus, and estrus. Estrus is the ovulatory phase when females are ready to conceive. Proestrus immediately precedes estrus and also indicates fertility. These stages can be identified through visual examination of the vaginal opening or more accurately through vaginal cytology analysis.

Question 2

How is vaginal cytology used to determine estrous cycle stage?

Accepted Answer

Vaginal cytology involves collecting cell samples via lavage or swabbing. Lavage uses a sterile pipette tip with water to gently flush the vaginal canal 4-5 times. Swabbing uses a moistened cotton applicator rolled against the vaginal wall. Cells are transferred to a glass slide, dried, stained with Wright-Giemsa stain, and examined microscopically to identify the cycle stage based on cell type and appearance.

Question 3

What is a copulatory plug and why is it important in rodent breeding?

Accepted Answer

A copulatory plug is a whitish mass of vaginal fluid and semen that persists 12-24 hours after mating. It can be detected by visual inspection or by gently inserting a blunt probe into the vaginal opening, where it will impede advancement within 0.5 cm. The plug's presence confirms mating occurred, though it does not guarantee pregnancy. Finding a plug establishes embryonic day zero for pregnancy dating.

Question 4

What are the differences between intensive and non-intensive breeding schemes?

Accepted Answer

In intensive breeding, males remain with females and pups, allowing immediate conception after birth. This maximizes breeding output but causes continuous lactation and gestation stress on females. Non-intensive breeding separates pregnant females from males until pups are weaned, reducing female stress. Weaning timing differs: non-intensive at 21-28 days, intensive at day 20 to prevent older pups from meeting the next litter.

Question 5

How do inbreeding and genetic modifications affect rodent breeding performance?

Accepted Answer

Outbred animals are more hardy and vigorous, producing larger and stronger litters compared to inbred animals. Genetically modified animals tend to be less hardy and fertile, with some mutations causing pup lethality before or shortly after birth. Environmental factors like temperature, humidity, lighting, noise, and vibrations also significantly reduce breeding efficiency in laboratory rodent colonies.

Question 6

What care is needed for newly weaned rodent pups?

Accepted Answer

Newly weaned pups should be checked daily to ensure they are thriving. Male and female pups must be separated at weaning since rodents can mate as young as 8 weeks. Pups should not be housed singly when possible; single pups should be grouped with same-gender pups from other litters. Food should be placed on the cage floor for 7-10 days to aid transition to solid chow, and water bottles should be provided to prevent dehydration.

Question 7

How do researchers use in-house breeding to study genetic and developmental effects?

Accepted Answer

In-house breeding enables researchers to develop homozygous knockout mice by selectively breeding animals with altered genes. It also allows study of prenatal exposure effects; for example, researchers can administer test compounds during specific embryonic days and examine resulting fetal tissues. Additionally, controlled breeding facilitates investigation of postpartum disorders by manipulating maternal stress and observing effects on maternal aggression and care behaviors.

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