The skin covers the body with the superficial epidermis, made of epithelial tissue, and the deeper dermis, containing much connective tissue. Under the dermis is the superficial fascia (also called the hypodermis despite NOT being part of the skin). The most superficial layer of the epidermis is the waterproof stratum corneum. It is not comprised of living cells, hence affords the body great protection against dehydration and from bacteria in the same way that the peel of (say) an apple protects its contents. The deepest layer of the epidermis is the stratum germinativum. Here keratinocytes are actively dividing (and producing keratin and previtamin D3). Melanocytes also inhabit this layer and produce melanin. Dendrocytes migrate to the skin from the bone marrow and become macrophages there. Merkel cells are one of the many sensory receptors found in the skin. The dermis contains blood capillaries, hair roots, and several types of sensory receptors which may be free nerve endings or encapsulated. The dermis also contains sudoriferous (sweat) and sebaceous (oil) glands – these are “exocrine” glands because they secrete their product through a duct. The papillary dermal layer overlies the deeper reticular dermal layer. The former rises up to push papillae into the epidermis and forms troughs to receive epidermal ridges and hence has a wavy boundary with the epidermis. Both layers of the dermis are connective tissue, the deeper layer containing collagen and elastic fibres.