Individual smooth muscle cells or small bundles of smooth muscle cells that originate in this layer form the arrector pili muscles that connect the deep part of hair follicles to the more superficial dermis. Contraction of these muscles in humans produces the erection of hairs and puckering of skin called “goose flesh.” In other animals, the erection of hairs serves in both thermal regulation and fright reactions.
A thin layer of striated muscle, the panniculus carnosus, lies deep to the subcutaneous fascia in many animals. Although largely vestigial in humans, it remains well defined in the skin of the neck, face, and scalp, where it  constitutes the platysma muscle and the other muscles of facial expression.
The cells of the epidermis consist of four different cell types:  Keratinocytes are highly specialized epithelial cells de signed to perform a very specific function: Separation of the organism from its external environment.  Melanocytes are the pigment-producing cells of the epi dermis.
 Langerhans’ cells are involved in signaling in the im mune system. Merkel’s cells are associated with sensory nerve  endings.
The keratinocyte is the predominate cell type of the epidermis. These cells originate in the stratum basale of the epidermis. On leaving this layer, keratinocytes assume two essential  activities:  They produce keratins (cytokeratins), major heteropoly meric structural proteins of the epidermis (see Table 2.3, page 64). Keratins form intermediate filaments; they con stitute almost 85% of fully differentiated keratinocytes.  They participate in the formation of the epidermal water barrier.
The keratinocytes in the basal layer contain numerous free ribosomes, scattered 7to 9-nm intermediate (keratin) filaments, a small Golgi apparatus, mitochondria, and rough-surfaced endoplasmic reticulum (rER). The cytoplasm of immature keratinocytes appears basophilic in histologic sections because of the large number of free ribosomes, most of which are engaged in the synthesis of keratins, which will later be assembled into keratin flaments. These filaments are classified as intermediate filaments, although they are more commonly called tonoflaments.
As the cells enter and are moved through the stratum spinosum, the synthesis of keratin filaments continues, and the filaments become grouped into bundles sufficiently thick to be visualized in the light microscope. These bundles are called tonofbrils. The cytoplasm becomes eosinophilic because of the staining reaction of the tonofibrils that fill more and more of the cytoplasm.
Keratohyalin granules contain intermediate filament– associated proteins that aid in the aggregation of keratin filaments.
In the upper part of the stratum spinosum (Fig. 15.4), the free ribosomes within the keratinocytes begin to synthesize keratohyalin granules that become the distinctive feature of the cells in the stratum granulosum (Plate 42, page 515). Keratohyalin granules contain the two major intermediate filament–associated proteins, filaggrin and trichohyalin. The appearance of the granules and expression of filaggrin in the keratinocytes are often used as a clinical marker for the initiation of the final stage of apoptosis. As the number of granules increases, the contents of the granules are released into the  keratinocyte cytoplasm. Filaggrin and trichohyalin function as promoters in the aggregation of keratin filaments into tonofibrils, thus initiating the conversion of granular cells into cornified cells. This process is called keratinization and occurs in 2 to 6 hours, the time it takes for the cells to leave the stratum granulosum and enter the stratum corneum. The keratin fibril formed in this process is called soft keratin in contrast to the hard keratin of hair and nails (see below).