Podocalyxin is expressed by all 3 germ layers and boundary elements during development. Frozen sections of embryos recovered at E7.5 (A) and E9.5 (B) were stained with antibodies to podocalyxin (PCLP1) and PECAM-1 (CD31) or control antibody and counterstained with methyl green. Red arrowheads highlight cavities whose luminal/boundary faces are coated with podocalyxin. These include the primitive ectoderm (pe), embryonic endoderm (ee), amnion (am), the branchial pouches (bp), the dorsal aorta (da), aortic sac (as), bulboventricular canal (bvc), surface ectoderm (se), and neuroepithelium of the neural tube (nt). Other labeled structures include the following: ng, neural groove; iem, intraembryonic mesoderm; msba, mandibular component of second branchial arch; mfba, mandibular component of first branchial arch, hg, hindgut. Original magnification × 10 (see “Materials and methods”).