Fine structure of the stratum intermedium, stellate reticulum, and outer enamel epithelium in the enamel organ of the kitten.

Stratum intermedium, stellate reticulum and outer enamel epithelium at the secretion stage in lower second molars of 1 week old kittens were studied with the electron microscope after perfusion fixation. All cell types had a well-developed Golgi apparatus, free ribosomes and little RER. In the stratum intermedium, cytoplasmic processes occasionally contained many vesicles of different types and were connected to neighbouring cells by gap junctions. The number of gap junctions in the stratum intermedium increased greatly with advanced secretion. The cells of the stellate reticulum had large sheet-like cell extensions and surrounded large extracellular spaces. Often, two cell extensions ran parallel to each other, with a narrow extracellular space between them. The narrow spaces were filled with a fluffy material. The outer enamel epithelium showed a smooth basal surface when close to a blood vessel. Facing a larger expanse of connective tissue, the basal surface became folded, the basal lamina formed extended loops into the connective tissue and showed areas of increased density, and cell processes extended through the lamina into the connective tissue. The blood vessels associated with the outer enamel epithelium had many pericytes and resembled post-capillary venules. Macrophages showing vacuoles, aggregations of small vesicles, and peripheral flaps of cytoplasm were present, mainly in the stellate reticulum. These observations are compared with the structure of the human enamel organ, as reported in the literature, and their possible functional significance is briefly discussed.