[Definition] basal cell (CL:0000646) definition conflates position with stemness

[cmungall]

Current definition [GOC:tfm, ISBN:0517223651]:

"Undifferentiated; mitotic stem cell for other epithelial cell types; rounded or elliptical with little cytoplasm and few organelles; contain cytokeratin intermediate filament."

Two problems, independent of the reclassification question in #3694:

1. It is not accurate on its own terms. The basal layer of a stratified epithelium contains both stem cells and transit-amplifying cells. Not every basal cell is a stem cell. CL:4033048 respiratory tract suprabasal cell already acknowledges this landscape in its own definition ("intermediate progenitor cell has limited proliferative capacity").

2. Leading with stemness rather than position is what drives the current placement. Reading "mitotic stem cell for other epithelial cell types" as "precursor to epithelium, therefore not epithelium" is what put basal cell under CL:0000036 epithelial fate stem cell instead of under epithelial cell. It also leaves the door open for a genuinely pre-epithelial progenitor (a nephron-progenitor-like MET case) to be filed here in future.

Position is what the class is actually doing. All eight descendants are defined positionally:

term	differentia
CL:0002187 basal cell of epidermis	part_of some stratum basale of epidermis
CL:0002633 respiratory basal cell	part_of some respiratory tract epithelium
CL:1000447 epithelial cell of stratum germinativum of esophagus	part_of some epithelium of esophagus
CL:1000486 basal cell of urothelium	part_of some urothelium
CL:1000349 basal cell of epithelium of bronchus	part_of some bronchus
CL:2000033 limb basal cell of epidermis	part_of some limb
CL:0020060 basal duct cell of salivary gland	asserted subclass
CL:4033024 airway submucosal gland duct basal cell	asserted subclass

Proposed definition

"An epithelial cell of the basal layer of a stratified or pseudostratified epithelium, resting on the basal lamina and containing cytokeratin intermediate filaments. Basal cells include the stem and transit-amplifying populations that give rise to the overlying layers."

Wording is open to discussion — the point is to lead with position and demote stemness to a typical-but-not-defining property.

Depends on #3694 (same stanza). Part of the epithelial hierarchy cleanup.

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@cmungall drove the agent, checked its reasoning, and is accountable for its output.

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