EGG CAPSULE STRUCTURE AND PERMEABILITY IN THE VIVIPAROUS SHARK, MUSTELUS-CANIS

The 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 anal ysis, and through the use of modified Ussing chambers. The egg capsule surrounds the developing embryo during gestation and consists of a fo ur or more orthogonally arranged laminae of fibrous material. SEM and energy dispersive spectroscopy (EDS) of inner and outer capsular surfa ces reveal that the capsule is 4.7 mum in thickness and possesses scat tered calcium deposits along its inner and outer surfaces. Transcapsul ar permeability studies qualitatively demonstrate that capsules are re adily 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 (66,000 Da). Glucose (P = 3.3 x 10(-4) +/- 1.2 x 10 (-4) cm/sec [n = 120]) and urea (P = 4.5 x 10(-4) +/- 2.3 x 10(-4) cm/ sec [n = 139]) readily pass across the egg capsules. It is suggested t hat maternal-embryonic nutrient transfer primarily involves the transc apsular passage of low (<5 kDa) molecular weight components. These dat a are discussed in relation to the mechanisms of placental attachment in matrotrophic sharks. (C) 1993 Wiley-Liss. Inc.