Safety and evidence boundary
Use prepared images, paper models, supplied datasets, or teacher-approved classroom microscopy. Do not culture unknown cells or microbes, collect human tissue, handle stains or preservatives without qualified supervision, or use personal medical records. This course does not diagnose growth disorders or disease.
Explicit science instruction
The mitotic index is the fraction or percentage of observed cells that are in mitosis at a given sampling time. A common calculation is: cells in mitosis ÷ total cells counted × 100%. It summarizes a population snapshot.
A higher mitotic index can be consistent with more cells occupying mitosis, but the interpretation depends on both entry rate and how long mitosis lasts. Sampling location, field selection, tissue type, image quality, and stage-classification rules also affect the estimate.
Vocabulary: mitotic index; sample; field of view; numerator; denominator; sampling bias; classification error; confidence; snapshot.
Worked example with reasoning
In a prepared image, 36 of 240 counted cells are classified as mitotic. Mitotic index = 36 ÷ 240 × 100 = 15%. A second region has 18 mitotic cells among 90 total: 18 ÷ 90 × 100 = 20%. The second sample has a higher observed index, but the result alone does not identify the molecular cause.
Guided question
If two tissues have different mitotic indices, what additional information would you need before concluding that one tissue's cells divide faster?
Independent work
- Calculate mitotic index for 27 mitotic cells among 180 total.
- Calculate it for 42 among 350 total.
- Compare the percentages.
- Name two sampling biases.
- Write one mechanism claim these counts cannot establish by themselves.
Misconception check
A high mitotic index is not automatically evidence of disease. Normal growth regions, developmental stages, tissue renewal, sampling choices, and differences in mitosis duration can all affect the percentage.
Formative understanding check
A sample has a mitotic index of 12%. Does that mean each cell spends exactly 12% of its life in mitosis?
Show the worked response
Not necessarily. That inference requires assumptions about a stable population, representative random sampling, similar cell-cycle behavior, and the relationship between stage duration and snapshot frequency.
Open-book formative practice only; not an official mastery attempt or grade.
Mentor evidence conference
Ask Archaemenes to test your chromosome-counting logic, cell-cycle model, evidence classification, and stated limitations. The mentor may challenge reasoning but does not provide protected assessment answers.
Open Archaemenes for this lessonTeacher guidance
Foundation: use pre-counted cells and a calculator. Core: calculate and compare indices while stating sampling limitations. Extended: evaluate whether repeated fields, larger samples, or blinded classification would improve the estimate and why.
Preserve learning evidence
Keep the model, calculation or image analysis, correction note, and explanation in the matching Week 15 evidence field.
Return to wednesday Evidence Workspace