The eye is the last to know
Children with CLN3 disease, the most common form of Batten disease, often lose their sight before anything else goes obviously wrong. The retina degenerates where it can be seen: night vision and peripheral vision fail, and an ophthalmologist can find the damage. The seizures, the lost speech, the failing movement come after.
You would think that makes Batten a disease of the eye. It does not. It makes Batten a disease that announces itself through the eye while starting in something else, depending on which species you ask.
In the mouse model Cln3Δex7/8, the cells lost earliest are not the light-sensing photoreceptors but cells of the inner retina, among them rod bipolar cells, the interneurons that relay rod signals (PubMed 32578444). The photoreceptors die later.
In human patient-derived cells the order inside the retina reads differently. Induced retinal pigment epithelium (RPE) from patients loses its microvilli and its ability to take up photoreceptor outer segments; restoring wild-type CLN3 rescues that function. The reading is that primary dysfunction in the RPE, the support layer that feeds and clears the photoreceptors, can be what instigates photoreceptor loss (Communications Biology, 2021, doi:10.1038/s42003-021-01682-5).
Same disease. Two species. Two “first” cells. The shared endpoint is the same: loss of the photoreceptor outer segment, the exact structure the RPE maintains. So “what fails first” has no answer until you say which species you mean, and the earliest measurable change may sit in a support cell nobody thought to inspect.
That is the whole reason this desk exists. The cell that fails first is rarely the cell that shows the wound. Look only at the damaged tissue and you keep finding the damage and missing the cause. If a readout names a marker without naming the model, it has not told you which “first” it means.
Every claim here is tied to a primary source, and the limit is stated: the mouse and human pictures disagree on the order, and that disagreement is part of the finding, not a footnote.