Egg capsule structure and permeability in the viviparous shark, Mustelus canis

Abstract

AbstractThe structure and permeability characteristics of egg capsules of the viviparous matrotrophic elasmobranch Mustelus canis were investigated by scanning electron microscopy (SEM), X‐ray microprobe elemental analysis, and through the use of modified Ussing chambers. The egg capsule surrounds the developing embryo during gestation and consists of a four or more orthogonally arranged laminae of fibrous material. SEM and energy dispersive spectroscopy (EDS) of inner and outer capsular surfaces reveal that the capsule is 4.7 μm in thickness and possesses scattered calcium deposits along its inner and outer surfaces. Transcapsular permeability studies qualitatively demonstrate that capsules are readily permeable to fast green fcf (808 Da) and rose bengal (1,000 Da). Capsules are impermeable to insulin (5, 750 Da), lysozyme (14,300 Da) and bovine albumin (66,000Da). Glucose (P= 3.3 × 10−4 ± 1.2 × 10−4cm/sec[n = 120])and urea(P= 4.5 × 10−4 ± 2.3 × 10−4cm/sec [n = 139]) readily pass across the egg capsules. It is suggested that maternal‐embryonic nutrient transfer primarily involves the transcapsualr passage of low (<5 kDa) molecualr weight components. These data are discussed in relation to the mechanisms of placental attachment in matrotrophic sharks. © 1993 Wiley‐Liss, Inc.