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Theiler stages (TS) are the most reliable way to describe mouse embryo development. They classify the embryo by visible morphology rather than by calendar age. The embryonic period proper contains 26 Theiler stages, from the one-cell egg (TS01) through the end of embryonic development (TS26). Days post coitum (dpc, often written E-days) are useful orientation, but they are not interchangeable with stage: embryos of the same gestational age can be at different stages.
Use the stage landmarks below to identify what an embryo has actually formed, then report its approximate dpc and strain. The timing examples are reference values for C57BL/6 × CBA F1 embryos, not universal deadlines for every colony or experiment.
What Theiler stages measure
Theiler staging is a morphology-based system. Instead of assigning an embryo to a stage solely because a certain number of hours has passed, you examine anatomical features such as the embryo’s overall form, surface landmarks, somites in younger embryos, and eye, limb, and tail development in older embryos. The e-Mouse Atlas Project (EMAP) stage definitions provide the formal criteria.
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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesEMAP states: “Embryos of the same gestational age may differ in their stage of development.” That difference can reflect normal embryo-to-embryo variation, colony conditions, staging uncertainty, or strain-specific developmental rates. EMAP also notes that “Different mouse strains develop at different rates and, in some cases show differences in the relative rates of development of different organs.” A dpc value should therefore accompany a Theiler stage, not replace it.
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The atlas uses a specific dating convention: the morning after a vaginal plug is found is counted as 0.5 dpc (E0.5). If a laboratory uses a different mating or collection convention, document it rather than silently converting the value.
The 26-stage sequence applies to the embryonic period proper. It is not a complete description of postnatal growth, puberty, or adult development. Keeping that scope explicit prevents TS26 from being interpreted as the end of every developmental process.
Reference map: Theiler stages and approximate dpc
The following map condenses the atlas’ reference series for C57BL/6 × CBA F1 embryos. The ranges are orientation points from the atlas; they are not universal cutoffs.
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| Theiler stage | Approximate dpc | Reference landmark or developmental interval |
|---|---|---|
| TS01 | 0.5 | One-cell egg |
| TS02 | About 1–2 | Cleavage interval before the morula |
| TS03–TS04 | 2–3 | Morula through blastocyst |
| TS05–TS08 | 4–6 | Zona-free blastocyst, attachment, implantation, and egg-cylinder differentiation |
| TS09–TS10 | 6.5–7 | Early- to late-streak development; the allantoic bud begins at TS10 |
| TS11–TS12 | 7.5–8 | Neural-plate progression to the first somites |
| TS13–TS15 | 8.5–9.5 | Head-fold and turning progression; posterior neuropore and forelimb-bud changes by TS15 |
| TS16–TS20 | 10–12 | Organogenesis and continuing external morphological change |
| TS21–TS26 | 13–18 | Later fetal development through the end of the embryonic period |
For exact boundaries and landmark descriptions, consult EMAP’s staging criteria and the individual stage pages. The atlas cautions that typical somite ranges are guides only and that somite number stops being a reliable global indicator after TS12.
A range in this table does not mean that every embryo progresses in a fixed, day-by-day sequence. It means that the listed morphology is commonly encountered during that approximate interval in the specified reference cross. When a specimen falls between two descriptions, the visible landmark should decide the assignment, with the dpc value reported as supporting context.
Stage-by-stage guide to mouse embryo development
TS01: the one-cell egg
TS01 is the starting point of the embryonic series. In the C57BL/6 × CBA F1 reference mapping it is placed at about 0.5 dpc. The defining observation is a single-cell fertilized egg. At this point, a stage assignment should be based on the visible one-cell state rather than on an assumed time since mating.
TS02: cleavage before compaction
Cleavage is the interval in which the early embryo divides without the dramatic increase in overall volume seen later. The atlas’ milestone series places this interval between TS01 and the morula-to-blastocyst stages. Because cleavage embryos can be difficult to distinguish by age alone, record the observed cell-number or cleavage appearance together with the collection time when those observations are available.
TS03–TS04: morula to blastocyst
These stages cover the transition from a compact morula to a blastocyst. The approximate reference timing is 2–3 dpc. The important distinction is the change in overall organization: a compacted preimplantation embryo becomes a blastocyst with a more recognizable cavity and cellular arrangement. The exact stage should come from the morphology described in the EMAP criteria, not from the dpc estimate alone.
TS05–TS08: hatching, attachment, implantation, and the egg cylinder
TS05–TS08 span the end of preimplantation development and the beginning of implantation. The atlas’ landmarks include the zona-free blastocyst, attachment, implantation, and differentiation of the egg cylinder. The reference interval is approximately 4–6 dpc, but implantation timing is especially sensitive to strain and biological variation.
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When examining specimens from this interval, distinguish a free or zona-free blastocyst from one that has attached or implanted. Those are different morphological states even if the nominal dpc values are close. A report should identify the state actually seen and then add the age convention used by the colony.
TS09–TS10: primitive streak and the first allantoic bud
TS09 and TS10 represent early gastrulation and successive primitive-streak subdivisions. The reference mapping places them around 6.5–7 dpc. TS10 is particularly useful because its late-streak early-bud subdivision is defined by the first appearance of the allantoic bud. The EMAP TS10 definition should be used when that transition matters to an experiment.
A visible streak or early bud is stronger evidence for staging than a presumed “day 7” label. If the allantoic bud is not clearly visible, record the uncertainty rather than forcing the embryo into TS10. The primitive streak subdivision and the bud should be considered together with the embryo’s overall configuration, since a single poorly oriented view can hide a small structure.
TS11–TS12: neural plate through the first somites
TS11 leads from neural-plate development toward the first somites. TS12 is the first-somite stage and is listed at about 8 dpc in the atlas, with a reference range of 7.5–8.75 dpc. At TS12 the embryo is unturned, one to seven somite pairs are reported, head folds and neural closure are developing, and the heart rudiment is changing rapidly. The TS12 criteria give the detailed anatomical distinctions.
Somites are useful here because the embryo is young and the criteria explicitly incorporate them. They should still be treated as one part of the assessment: surface form, neural development, and the embryo’s overall configuration also matter. A count at the upper end of the reported range does not by itself establish that the embryo has progressed to a later whole-embryo stage.
TS13–TS15: turning, posterior neuropore, and forelimb-bud condensation
These stages cover the progression from head-fold development through turning. By TS15, the posterior neuropore is present and condensation of the forelimb bud is apparent. The atlas lists TS15 at about 9.5 dpc, with a reference range of 9–10.25 dpc. The TS15 definition is the appropriate reference when distinguishing these landmarks.
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This interval illustrates why a single somite count cannot substitute for a whole-embryo stage. The embryo’s shape, neural closure, turning status, and limb-bud morphology all contribute to the assignment. If turning or the posterior neuropore cannot be seen because of orientation, identify the limitation rather than treating the missing view as evidence that the feature has not formed.
TS16–TS20: organogenesis and external morphological change
TS16–TS20 correspond approximately to 10–12 dpc in the reference series. They cover continuing organogenesis and readily changing external morphology. As embryos progress through this interval, stage assessment should move away from relying on somites alone and toward the full set of visible anatomical landmarks specified in the atlas.
For a reproducible record, note the features that led to the assignment—such as the overall body configuration, eye or limb appearance, and tail development—rather than writing only “day 11.” If a landmark is obscured by orientation or specimen damage, mark the stage as provisional. Comparing several landmarks is particularly important in this interval because different organs can advance at somewhat different relative rates.
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TS21–TS26: later embryonic development
TS21–TS26 span approximately 13–18 dpc in the C57BL/6 × CBA F1 reference series and lead through the end of the embryonic period. These stages capture later fetal development, continued limb and organ maturation, digit separation, and preparation for birth as external morphology becomes more differentiated. The 26-stage count ends with TS26 because it describes the embryonic period proper; it does not silently include postnatal development.
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How to stage an embryo in practice
A consistent workflow prevents age labels from being mistaken for anatomical observations. Use the following sequence for a single specimen or for a batch of embryos.
- Write down the age convention. Record the mating date, plug-finding date, collection time, and whether the laboratory counts the morning after the plug as 0.5 dpc. This makes the age value interpretable.
- Record strain and cross. Include the strain or parental cross because developmental rates can differ between strains and organs may not advance at identical relative rates.
- Start with the whole-embryo form. Decide whether the specimen is preimplantation, implanted or egg-cylinder, streak-stage, unturned with early somites, turning, or in later organogenesis. This narrows the stage range before you inspect individual landmarks.
- Check the defining landmarks. Look for the features used by the relevant EMAP stage definition: cleavage or blastocyst organization in early embryos; attachment, implantation, and egg-cylinder form; primitive streak and allantoic bud; neural plate, head folds, somites, turning, neuropore, limb buds, eyes, tail, and digits at later stages.
- Use somite number where it helps. In younger embryos, somite counts can supplement the surface assessment. After TS12, do not infer the entire stage from somite number alone.
- Assign the narrowest defensible stage. If two adjacent stages remain plausible, report a range or provisional assignment and state which landmark was ambiguous.
- Attach the approximate dpc as context. Use the atlas’ reference range only as orientation, and label it as strain-specific when presenting it.
- Save the observation. A useful record includes specimen identifier, strain, plug convention, collection age, observed landmarks, assigned TS, confidence, and any imaging or orientation limitation.
For batch work, apply the same order of observations to every embryo. Photograph or sketch the relevant surface when permitted by the protocol, retain the original specimen identifier, and avoid changing a stage simply to make a group look age-matched. A transparent provisional range is more informative than a falsely precise single number.
How to compare two embryos correctly
Comparisons are strongest when both embryos are described on the same four axes: the anatomical landmark present, the assigned Theiler stage, the approximate dpc and strain context, and the confidence or limitation of the observation.
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|---|---|---|
| Morphology | The visible landmark or developmental configuration | It shows what the embryo has actually formed. |
| Theiler stage | TS number, or a two-stage range if uncertain | It provides a standardized morphological label. |
| Age and strain | dpc convention, parental strain or cross, and collection details | It prevents a reference timing from being treated as universal. |
| Confidence | Clear, obscured, damaged, or orientation-limited observation | It distinguishes biological variation from staging uncertainty. |
For example, two embryos collected at nominally 8 dpc should not automatically be treated as identical. One may meet TS11 criteria while another meets TS12 criteria. Report the difference, include the shared age convention, and avoid claiming that either embryo is “wrong” solely because the stages do not match.
The same framework applies when comparing strains or experimental treatments. First state whether the anatomical landmark is present; then give the TS assignment; next provide dpc, cross, and collection details; finally state how clear the observation was. This separates a genuine developmental difference from a difference caused by timing conventions, orientation, or incomplete visualization.
Why dpc and stage do not always agree
dpc is a chronological estimate; TS is a morphological observation. The same dpc can contain embryos at more than one stage, and a stage can occupy a range of dpc values. The atlas’ TS12 reference range of 7.5–8.75 dpc is an example of that overlap, not a promise that every TS12 embryo falls inside exactly those hours.
Strain effects are another source of divergence. The C57BL/6 × CBA F1 series is useful for orientation, but it should not be copied unchanged onto another genetic background. Differences can affect the overall rate of development and the relative timing of organ development. If a study compares strains, preserve the morphological TS assignment and present dpc as a separate contextual variable.
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Even within one colony, embryos from the same plug-positive morning can span adjacent stages. That is why a stage table should be treated as a reference map rather than a deadline chart. If calendar age and morphology disagree, do not “correct” the morphology to fit the expected day; document both observations and investigate collection, mating, and staging conditions.
Common staging errors and how to correct them
Treating a dpc value as a hard cutoff
Problem: An embryo collected at a nominal age is assigned a stage automatically. Correction: inspect the defining morphology first, then use dpc to describe the collection context.
Using somites as the only late-stage clock
Problem: A somite count is used to infer the complete stage after TS12. Correction: combine overall form with neural, eye, limb, tail, and digit landmarks; the atlas specifically warns that somite number is not a reliable global indicator after TS12.
Ignoring the strain or cross
Problem: A reference range from one cross is presented as a universal schedule. Correction: name the strain or cross beside every age-to-stage comparison and describe the table as an orientation reference.
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Confusing a missing landmark with a true earlier stage
Problem: Poor orientation, damage, or imaging quality hides a feature, so the embryo is moved backward in the staging series. Correction: mark the observation as uncertain, inspect another anatomical plane if available, and report a stage range when the defining feature cannot be verified.
Extending the 26-stage count into postnatal life
Problem: TS26 is treated as the end of all mouse development. Correction: state that the 26 stages cover the embryonic period proper; postnatal development requires other age or developmental descriptors.
Reporting an age without the dating convention
Problem: “E8” or “8 dpc” is listed without saying when the plug was found or when collection occurred. Correction: state the plug convention and collection timing, especially when comparing laboratories.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What to include in a methods or figure legend
A concise staging description should identify the strain or cross, the dpc convention, the collection time, and the morphological criteria used. Give the Theiler stage for each embryo or group, not merely the nominal embryonic day. If the stage is provisional, say why—for example, an obscured posterior neuropore, uncertain turning, or incomplete visualization of a limb or digit landmark.
For a figure, pair the image with a short landmark description such as “unturned embryo with early somites and developing head folds” or “posterior neuropore present with forelimb-bud condensation.” Such wording lets another reader understand the assignment without treating the image’s calendar label as proof. When two adjacent stages cannot be separated confidently, a TS range plus the decisive and missing landmarks is preferable to unsupported precision.
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Authoritative references and further reading
EMAP’s Theiler Stage Definition and Staging Notes and Staging Criteria provide the core definitions and cautions. The stage-specific pages for TS10, TS12, and TS15 are useful when an experiment depends on the allantoic bud, first somites, posterior neuropore, or forelimb-bud morphology.
The EMAP FAQ on Theiler stages and dpc explains the reference age series. EMAP also provides an electronic atlas resource and background on what EMAP is. For a detailed print reference, Springer lists Karl Theiler’s The House Mouse: Atlas of Embryonic Development at the publisher’s catalog page; the softcover ISBN given there is 978-3-642-88420-7.
The online atlas supplies stage criteria and anatomical models for readers who need a free starting point. The printed atlas is useful as a detailed anatomical reference, but edition details and availability should be checked with the publisher or current seller.
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What is a Theiler stage, and how does it differ from dpc?
A Theiler stage is assigned from visible morphology, such as implantation state, primitive streak, somites, neural closure, limb buds, eyes, tail, and digits. dpc is a chronological estimate based on mating and plug timing. They should be reported together because embryos of the same dpc can occupy different stages and one stage can span a dpc range.
How many Theiler stages are there?
The embryonic period proper is divided into 26 stages, TS01 through TS26. TS01 is the one-cell egg, while TS26 marks the end of the embryonic sequence. The count does not include postnatal development.
Which stage is the first-somite stage?
TS12 is the first-somite stage. It is an unturned embryo with one to seven reported somite pairs, developing head folds and neural closure, and a rapidly changing heart rudiment. The atlas places it at about 8 dpc in the C57BL/6 × CBA F1 reference series, with a range of 7.5–8.75 dpc.
What appears at TS10?
TS10 covers late-streak to early-bud development. In its late-streak early-bud subdivision, the allantoic bud first appears. Because orientation or image quality can hide a small bud, record uncertainty rather than assigning TS10 from a nominal day alone.
Can two embryos collected on the same day have different stages?
Yes. EMAP explicitly cautions that embryos of the same gestational age may differ in developmental stage. Normal embryo variation, strain effects, collection timing, and staging limitations can all contribute. Compare the visible landmarks and report each embryo’s stage with the shared age convention.
How should older embryos be staged after TS12?
Do not use somite number as the sole global indicator after TS12. Use the whole-embryo configuration and the atlas’ external criteria, including neural and turning features, eyes, limbs, tail, and digits. If a feature is hidden or damaged, provide a provisional assignment or adjacent-stage range.
Is the EMAP atlas available online?
EMAP hosts electronic Theiler-stage definitions, staging criteria, anatomical models, and an electronic atlas resource. These pages are a practical starting point for stage-specific landmarks. Karl Theiler’s printed The House Mouse: Atlas of Embryonic Development is a more extensive reference; Springer lists the softcover ISBN as 978-3-642-88420-7.
What information should accompany a stage assignment in a paper or dataset?
Include the specimen identifier, strain or cross, mating and plug convention, collection age and time, observed landmarks, assigned TS, confidence or stage range, and any orientation, imaging, or damage limitation. This allows readers to distinguish biological variation from uncertainty in observation.
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Use morphology to assign the Theiler stage and dpc to provide context. The 26-stage EMAP sequence gives a common language from the one-cell egg through the end of the embryonic period, but its timing examples are reference values for a particular cross, not universal deadlines. Record strain, dating convention, landmarks, and confidence—especially after TS12, when somite number alone is no longer a reliable whole-embryo clock.
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