= Rat dorsal air sac assay
The \b[<rat dorsal air sac assay>] is an experimental <bioassay> of <angiogenesis> in which locally presented material is evaluated by the vascular response of <tissue> bordering a dorsal air space. It was used to test whether material derived from <tumours> could promote new <blood vessels>, connecting biochemical separation with a visible biological effect.
In the 1971 study by <Judah Folkman>, <Ezio Merler>, <Charles Abernathy> and <Gretchen Williams>, early experiments used membrane chambers containing live <tumour> <cells>. For testing separated material, the apparatus used silicone tubing with side openings covered by a short membrane sleeve. That sleeve acted as a diffusing wick, not as a filter. After 48 hours the vascular response was inspected with a <stereomicroscope> at $20\times$ magnification, followed by collection of sections for <histology>.<Isolation of a Tumor Factor Responsible for Angiogenesis by Judah Folkman and others>[Printed p.279, Biological Assay of TAF and Reading the Air Sac Assay; Figure1]{ref}
The investigators graded vascular proliferation from 0 to 5+. Their illustrations show a wick without the active preparation as a <negative control> and progressively stronger vascular responses to active material.<Isolation of a Tumor Factor Responsible for Angiogenesis by Judah Folkman and others>[Figures2–4 and captions]{ref} The scientific question is whether the preparation alters the vascular response relative to an appropriate control. A visible response is not automatically a count of <endothelial cells>, a measured <cell proliferation> rate or evidence of functional <perfusion>.
An original scoring example shows why numerical coding must follow the measurement. Suppose one group receives grades 1, 1 and 4, and another receives 2, 2 and 2. Both arithmetic means are 2, although the response patterns differ. Moreover, the ordered relabeling $1\mapsto1$, $2\mapsto2$, $4\mapsto10$ preserves the ranking but changes those means to 4 and 2. An ordered grade alone therefore does not define an interval scale on which differences have a fixed physical meaning. This example is not a reconstruction of the historical observations.
A useful contemporary comparison would specify the test material, solvent control, observation time and scoring rule, and keep the observer unaware of group assignment. <Histology> and explicit vessel measurements can help distinguish new vascular growth from dilation or local tissue disturbance. Such refinements answer additional measurement questions; this assay does not by itself identify a single angiogenic molecule or demonstrate a treatment effect in human <cancer>.
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