Ultrastructure and development of a thick basement membrane-like layer in the anchoring villi of macaque placentas.

BACKGROUND

Anchoring villi and cytotrophoblastic cell columns are important structural components involved in placental morphogenesis. We have previously described the presence of an unusual basement membrane-like layer (BMLL) that separates these placental compartments. The purpose of the present study was to identify developmental changes in the ultrastructure of the BMLL and to assess its changes in extracellular matrix composition over the course of gestation.

METHODS

Conventional techniques were used to examine macaque placental tissue by transmission electron microscopy. Standard immunoperoxidase methods were used to identify type IV collagen, laminin and fibronectin in paraffin sections.

RESULTS

Until day 35 of gestation the BMLL was 70-100 nm thick and appeared similar to basement membranes seen in other regions of the villus, although it usually lacked a lamina lucida along the surface adjacent to the cytotrophoblast cells. Immunohistochemistry revealed the presence of laminin and type IV collagen in the BMLL. By 53 days of gestation the BMLL had hypertrophied at the junction of the anchoring villus and cell column, measuring 2,000-5,000 nm in thickness. The BMLL retained immunoreactivity for laminin and type IV collagen. Ultrastructural examination revealed the presence of a new component in the form of 10 nm microfibrils. By 89 days of gestation the BMLL was not reactive for laminin or type IV collagen but otherwise maintained the structural organization seen at 53 days. No additional changes were observed in the BMLL during late pregnancy.

CONCLUSIONS

The BMLL is a distinct extracellular matrix region that separates the distal aspect of the anchoring villus from the proximal portion of the cell columns. Evidence indicates that adjacent cytotrophoblast plays a prominent role in the production of the BMLL. The BMLL may serve to organize this complex tissue by separating fetal mesenchyme from cytotrophoblast cells that are proliferating, differentiating, and migrating. Modifications to the composition of the BMLL may indicate changes in the role this matrix plays in the development of the placenta.