The dynamic structure of rabbit blastocyst coverings

Summary

The extracellular coverings which surround rabbit blastocysts are far more complex structures than the zona pellucida of other species. Since previously published views of their composition, structure and identification of the various layers are highly controversial, a detailed investigation of the stages between morula and implantation has been undertaken using both electron microscopical and histochemical methods.

Rabbit blastocyst coverings undergo considerable structural and chemical transformation from the early until the late blastocyst stages. Morulae are surrounded by zona pellucida and mucoprotein layer (a highly sulfated, sialic acid-free mucosubstance which is derived from the tubal secretion). In the early blastocyst (i.e. from 31/2 d p.c. on), the zona loses its high content of periodate-accessible vicinal hydroxyl groups (PAS reaction) and protein, and there is morphological evidence for an erosion (particularly from the inside), suggesting enzymatic lysis in addition to mechanical stretching due to the expansion of the blastocyst. The zona pellucida disappears completely at 41/2 d p.c. At the same time, deposition of new material begins in its place. This newly formed layer will be called “neozona”. Until implantation, it increases considerably in thickness, finally representing between 2/5 and nearly 1/2 of the total thickness of the blastocyst coverings. Histochemically, the neozona is characterized as a moderately acid mucosubstance, rich in protein and in periodate-accessible vicinal hydroxyl groups, containing sulfate esters as well as sialic acid. Its chemical composition is in many respects comparable to that of the zona pellucida. At least part of the neozona material may be derived from the trophoblast. This new aspect of the physiology of the preimplantation trophoblast, i.e. its secretory activity, is discussed. The possibility that uterine secretion components are also involved in formation of the neozona (as well as in dissolution of the zona pellucida) is envisaged. Observations suggesting a chemical modification of the inner parts of the mucoprotein layer and impregnation of this layer with sialic acid-containing glycoproteins are also discussed.

An additional layer, the gloiolemma, which derives from the uterine secretion, is deposited at the outer surface of the mucoprotein layer after 6 d p.c. At the onset of implantation, i.e. around 7 d p.c., rabbit blastocyst coverings are, therefore, composed of three layers of different origin: neozona, mucoprotein layer and gloiolemma.