The ultrastructure of the cortical granules and their products in the sea urchin egg as studied with the electron microscope

Abstract

The ultrastructure of the cortical granules, of the vitelline membrane and of their products in sea urchin eggs has been studied by means of electron microscopy in ultra-thin sections prepared according to the technique of Sjöstrand. 1. 1. The cortical granules exhibit clear-cut species-specific differences in the five sea urchin species examined. In four of the species the cortical granules have been followed from a rather regular “immature” state in the endoplasm, where the interior of the granule appears as lamellae in a spiral or concentrical arrangement. During egg maturation the cortical granules move to the cortex and the spiral order may be more or less lost, and peripheral spaces appear. 2. 2. Beside the normal cortical granules there are also dwarf cortical granules of a submicroscopical size. These have the same ultrastructure as the larger ones except that no peripheral spaces appear. 3. 3. To test the reliability of the osmium tetroxide fixation the following fixatives have been employed, all of which gave approximately the same results with regard to the cortical granule inner structure: uranyl nitrate, methyl mercury chloride, formaldehyde, Aoyama's fluid, osmium tetroxide + potassium dichromate, formaldehyde + p-chloro-mercury-benzonate and the freeze-dry method followed by osmium tetroxide treatment. 4. 4. There is a continuous limiting membrane of the unfertilized egg with a thickness of about 100 A. This is probably the vitelline membrane or part of it. 5. 5. The outer layer of the cortex and the cortical granules are rich in sulfhydryl groups. 6. 6. The fertilization membrane is shown to be composed of two different layers, the thin outer part is probably the vitelline membrane, the main part of it is derived from the cortical granules. Thickness and fine structure of the fertilization membrane are described. 7. 7. There is strong evidence that the hyaline layer is formed from the cortical granules that do not merge with the fertilization membrane. The fine structure usually is amorphous or fibrous. Treatment with surface-active agents may give the hyaline layer a lamellar character. 8. 8. Cortical granules have also been found in embryos and these could be supposed to form cell cementing substances. 9. 9. Most of the new cell surface of the fertilized egg is derived from the cortical granule outer membrane. 10. 10. The granular fertilization membrane obtained after a lysozyme treatment is described. 11. 11. Cortical rods, here obtained with tannic acid treatment, are shown to have the same fine structure as the main part of the fertilization membrane and are probably the cortical granule lamellae unrolled.