Table 1.

Function and localization of iron transporter proteins in human and mouse placentae^a

Proposed function	Protein	Localization in human placenta	Localization in mouse placenta
Apical iron uptake
	Transferrin	Syncytiotrophoblast: strong, continuous staining on apical membrane23 Other locations: fetal stroma23	No localization data
	TFR1	Syncytiotrophoblast: strong, continuous staining on apical membrane23,47,48 Other locations: cytotrophoblasts, fetal stromal macrophages, fetal endothelium48	Strong, predominant staining on apical membrane of labyrinth trophoblasts; also stains fetal endothelium at E1020
	HFE	Syncytiotrophoblast: staining confined to basal23 or apical membrane49 Other locations: fetal stromal macrophages23	No localization data
	SCARA5	Syncytiotrophoblast: weak cytoplasmic and membranous staining50 Other locations: fetal endothelium50	No localization data
	ZIP8	Syncytiotrophoblast: strong, continuous staining on apical membrane; weak cytoplasmic staining50 Other locations: fetal endothelium50	No localization data
Intracellular iron trafficking
	ZIP14	Syncytiotrophoblast: strong cytoplasmic and membranous staining50	No localization data
	DMT1	Syncytiotrophoblastb: punctuate and diffused staining in cytoplasm and basal membrane23,47; continuous staining in cytoplasm and apical membrane45,46 Other locations: fetal stroma,23,51 fetal stromal macrophages47	No localization data
	Ferritin	Syncytiotrophoblast: very weak/negative staining23,50,51; predominant staining toward apical side52 Other locations: fetal stroma,23,52 fetal endothelium51	No localization data
	STEAP3	Syncytiotrophoblast: weak and diffuse cytoplasmic staining; intense and granular staining along basal membrane50	No localization data
	STEAP4	No localization data	No localization data
Basolateral iron export
	FPN1	Syncytiotrophoblast: strong, continuous staining on basal side23,53 Other locations: fetal stromal macrophages23	Strong, continuous staining in labyrinth trophoblasts at E16.554
Ceruloplasmin	Syncytiotrophoblastb: intense55 or weak diffuse staining56 Other locations: fetal stroma,55 intervillous space, fetal endothelium56	No localization data
	Hephaestin	Syncytiotrophoblast: membranous and cytoplasmic staining 50	No localization data
	Zyklopen	No localization data	Labyrinth trophoblasts and spongiotrophoblasts at E15.557
Heme iron metabolism
	HRG1	No localization data	No localization data
	LRP1	Syncytiotrophoblast: strong cytoplasmic staining, more intense on apical side58 Other locations: fetal stromal macrophages58	Strong staining in ectoplacental cone at E7.559
	PCFT	Syncytiotrophoblast: cytoplasmic and membranous staining, more intense on apical side60 Other locations: fetal endothelium60	Weak staining in labyrinth trophoblasts, spongiotrophoblasts, trophoblast giant cells, and fetal endothelium at E12.5–18.561
	FLVCR1	Syncytiotrophoblast: cytoplasmic and membranous staining, more intense on apical side50 Other locations: fetal stromal macrophages	No protein localization data High mRNA expression in labyrinth trophoblasts, spongiotrophoblasts, and trophoblast giant cells from E9.5 to term62
	FLVCR2	Syncytiotrophoblast: cytoplasmic and membranous staining, very intense on apical side50 Other locations: cytotrophoblasts50	No localization data
	HO1	Syncytiotrophoblastb: intense63,64 or negative/very weak staining65,66 Other locations: fetal endothelium,63,64 cytotrophoblasts63	Labyrinth trophoblasts2: positive staining at E1367; negative staining at E14.568; strong staining in spongiotrophoblasts at E13 and E14.567,68
	HO2	Syncytiotrophoblastb: strong,63 weak,65,66 or negative staining64 Other locations: fetal endothelium63–66	Moderate staining in labyrinth trophoblasts, spongiotrophoblasts, and trophoblast giant cells at E13; weak staining in fetal endothelium67

^aAll human data are from studies in term healthy placentae; mouse studies represent expression after the formation of mature placenta at E10.5, if data are available. Subcellular locations of some proteins are not described because the immunohistochemistry images are at low resolution.

^bDenotes inconsistent findings.

Abbreviations: DMT1, divalent metal transporter 1; E, embryonic day; FLVCR, feline leukemia virus subgroup C cellular receptor; FPN, ferroportin; HO, heme oxygenase; HRG, heme-responsive gene; LRP, low-density lipoprotein receptor-related protein; PCFT, protein-coupled folate transporter; SCARA5, scavenger receptor class A member 5; STEAP, six-transmembrane epithelial antigen of the prostate; TF, transferrin; TFR1, transferrin receptor 1; ZIP, Zrt- and Irt-like protein.