Diabetes, Vol 27, Issue 5 530-538, Copyright © 1978 by American Diabetes Association
Nonuniform distribution and grouping of insulin receptors on the surface of human placental syncytial trophoblast
DM Nelson, RM Smith and L Jarett
These studies were designed to investigate the cytologic localization and
topographic distribution of insulin receptors in human placental villi.
Biochemical studies showed placental villi to specifically bind
125I-insulin. Radioautographic studies showed the specific binding to be
localized to the surface of the syncytial trophoblast. Topographic
distribution of insulin binding was determined with ferritin-insulin.
Initial studies using ferritin-insulin containing some oligomers of
ferritin revealed the insulin receptors to be specifically associated with
the glycocalyx region of the surface membranes of microvilli. No insulin
receptors were detectable in association with the intermicrovillous plasma
membrane even though its glycocalyx is in direct continuity with the
glycocalyx of microvilli. Monomeric ferritin-insulin showed the same
nonuniform distribution of the insulin receptor, which suggests that there
is not complete freedom of lateral mobility of the insulin receptors in the
surface membrane of this tissue. The insulin receptors were found to occur
as singletons or in groups of two or more. Incubations with monomeric
ferritin-insulin at 4 degrees or with tissue prefixed with formaldehyde
showed that the groups of insulin receptors were naturally occurring, i.e.,
they are present prior to and independent of insulin binding and thus not
secondary to ligand-induced aggregation. The physiologic meaning of the
nonuniform distribution and the groups of insulin receptors is unclear at
present.