The stratum spinosum is at least several cells thick. Keratinocytes in this layer are larger than those of the stratum basale. They exhibit numerous cytoplasmic processes or spines, which gives this layer its name (Fig. 15.3 and Plate 42, page 515). The processes are attached to similar processes of adjacent cells by desmosomes. In the light microscope, the site of the desmosome appears as a slight thickening called the node of Bizzozero. The processes are usually conspicuous, in part because the cells shrink during preparation and a resultant expanded intercellular space develops between the spines. Because of their appearance, the cells that constitute this layer are often referred to as prickle cells. As the cells mature and move to the surface, they  increase in size and  become flattened in a plane parallel to the surface. This  arrangement is particularly notable in the most superficial spinous cells, where the nuclei also become elongate instead of ovoid, matching the acquired squamous shape of the cells.
The cells of the stratum granulosum contain conspicuous keratohyalin granules.
The stratum granulosum is the most superficial layer of the nonkeratinized portion of the epidermis. This layer varies from one to three cells thick. Keratinocytes in this layer contain numerous keratohyalin granules, hence the name of the layer. These granules contain cystine-rich and histidine-rich proteins, which are the precursors of the protein  flaggrin, which aggregates the keratin flaments present within the cornified cells of the stratum corneum. Keratohyalin granules are irregular in shape and variable in size.  Because of their  intense basophilic staining, they are readily seen in routine  histologic sections.
The stratum corneum consists of anucleate squamous cells largely filled with keratin filaments.
Usually, an abrupt transition occurs between the nucleated cells of the stratum granulosum and the flattened, desiccated, anucleate cells of the stratum corneum. The cells in the stratum corneum are the most differentiated cells in the skin. They lose their nucleus and cytoplasmic organelles and become filled almost entirely with keratin filaments. The thick plasma membrane of these cornified, keratinized cells is coated from the outside, in the deeper portion of this layer, with an extracellular layer of lipids that form the major constituent of the water barrier in the epidermis.
The stratum corneum is the layer that varies most in thickness, being thickest in thick skin. The thickness of this layer constitutes the principal difference between the epidermis of thick and thin skin. This cornified layer will become even thicker at sites subjected to unusual amounts of friction, as in the formation of calluses on the palms of the hand and on the fingertips.
The stratum lucidum, considered a subdivision of the stratum corneum by some histologists, is normally only well seen in thick skin. In the light microscope, it often has a refractile appearance and may stain poorly. This highly refractile layer contains eosinophilic cells in which the process of keratinization is well advanced. The nucleus and cytoplasmic organelles become disrupted and disappear as the cell gradually fills with keratin.
Attachment of epidermis to dermis is enhanced by an increased interface between the two tissues.