Localisation of glycans in the placenta: A comparative study of epitheliochorial, endotheliochorial, and haemomonochorial placentation

Specimens of mid-term (horse), near-term (pig, cow, sheep, mink) and term (human) placentae and associated tissues have been examined with a panel of 15 biotinylated lectins combined with an avidin-peroxidase revealing system. The aim of this study has been to analyse the expression of glycans at the materno-fetal interface in order to establish whether the morphological diversity exhibited by these six species is reflected by accompanying biochemical diversity, or whether similar types of glycan are expressed in tissues performing similar functions. Lectin staining intensity was scored in the following elements of the interhaemal placental barrier: maternal capillaries, maternal uterine epithelium, the materno-fetal interdigitating microvillous membrane (brush border in the human), trophoblast, and fetal capillaries. A high degree of biochemical diversity was found in the glycan expression of the various placental components within and among placental types. Each layer showed widely differing patterns of lectin binding between species, with only a few findings in common: 1) the relative lack of simple fucosyl termini, 2) the presence of non-bisected bi/tri-antennary N-glycan in most layers, 3) an abundance of terminal N- acetyl galactosamine, and 4) the restriction of high mannose glycans to intracellular granules. This diversity may be a mechanism to avoid hybridisation, although glycan patterns may change between conception and placental development, or it may have evolved as a consequence of morphological changes. It is possible that it may also be part of the cause, rather than the result, of the structural diversity that is so characteristic of mammalian placentation.