These two layers are differing in their function, histological appearance and their embryological origin. The outer layer or epidermis is formed by an epithelium and is of ectodermal origin. The underlying thicker layer, the dermis, consists of connective tissue and develops from the mesoderm. Beneath the two layers, a subcutaneous layer of loose connective tissue or hypodermis is found which binds the skin to underlying structures. Hair nails and sweat, & sebaceous glands are of epithelial origin and collectively called the appendages of the skin.
It is the upper layer formed of keratinized stratified squamous epithelium. It has branches of sensory nerves forming free nerve endings which are tactile. It has no blood vessels i.e.
vascular; hence substances pass to and from it by diffusion. The main function of the epidermis is to protect the body from harmful influences from the environment and against fluid loss. It consists of several layers of cells. The lowermost of which is known as stratum germinativum or stratum Malpighi or stratum basale made of columnar cells which keep on dividing actively and rest on a thin basement membrane. Basal cells are the stem cells of the epidermis.
As new cells are formed, the older cells are pushed up, at the same time, their protoplasm becomes granular and horny due to the formation of a hard protein called keratin; the cells gradually become flattened. Above stratum germinativum there are six or seven layers of polygonal cells that constitute the stratum spinosum. Due to tightly hold together by finger like cytoplasmic processes the cells of this layer appear to have a number of spines. Since stratum basale and stratum spinosum undergo mitosis, hence both are sometimes collectively referred to as the stratum germinativum. Above the stratum spinosum is a layer of granular cells known as stratum granulosum. This layer contains basophilic and refractile keratohyaline granules.
The keratohyalin is not located in membrane bound organelles but forms “free” accumulations in the cytoplasm of the cells. The cells begin to release the contents of the lamellar granules. The lipids contained in the granules come to fill the entire interstitial space, which is important for the function of the epidermis as a barrier towards the external environment. Outside the stratum granulosum may be a layer known as stratum lucidum. In stratum lucidum, keratohyaline granules are dissolved and transformed into eleidin which makes cells semitransparent, shiny and waterproof. It is found in places of friction, such as soles and palms. The outermost cells of stratum lucidum from a thick layer of hard scale-like, fully keratinized, flattened cells called stratum comeum.
It contains hard keratin filaments (horny cells) which prevents the passage of water and solutes. In places of friction this layer becomes very thick. Its cells have no nuclei but they are not dead as was believed they secrete hormones, one of which reduces the rate of cell division of the Malpighian layer. To restore- normal rate of cell division the stratum corneum must be cast off in pieces periodically. Pigment granules are present in the lower layers of the epidermis, but there are no chromatophores. The melanin producing melanophores or pigmentary dendritic cells are located in the basal layers of the epidermis; these are ectodermal branched cells with long entwining processes. Melanophores form a second barrier and protect the skin from damaging effects of ultraviolet radiation.
Synthesis of melanin in the melanocytes is dominant genetic trait. Hence persons inheriting only its recessive genes have no melanin. This condition is called albinism. That’s why albino quickly gets skin burn in the sun as they do not have melanin pigment. In sunburn the cells of the germinative layer of epideimis are injured causing blisters.
It is a thick layer below the epidermis and forms a tough, flexible and somewhat elastic covering over the body. The dermis is formed of connective tissue, elastic and collagn fibres, un-striped muscles, blood vessels, nerves, fat cells, some glands, and tactile corpuscles, all arranged intricately. Nerve-endings, receptors and blood capillaries are distributed throughout the dermis.
Some of the glands derived from the epidermis are lodged in the dermis. Functions of the dermis are to hold together, protect and support the body and to carry blood to the surface. The dermis consists of two layers: The Papillary layer is the outer layer closest to the epidermis. It is composed of areolar loose connective tissue proper.
Its superior region contains fingerlike projections called dermal papillae that indent the epidermis. Dermal papillae contain capillaries, bare nerve endings (pain receptors), Meissner corpuscles (touch receptors: make us feel light touching) and the disc-like sensory nerve endings of the Merkel discs. The Reticular layer is the deeper layer and the thickest. It is made of dense irregular connective tissue proper containing thick bundles of interlacing collagen fibres and some coarse elastic fibers that run in several directions.
The collagen fibers in the reticular region provide the skin with strength and extensibility (= ability to stretch) and elastic fibers provide its elasticity (= ability to return to the original shape after stretching). The gradual stretching of collagen fibres causes wrinkles in a later life. The reticular layer is richly supplied with blood vessels and nerves, and contains sensory endings for touch (Pacinian corpuscle for sensing deep pressure such as bumps), pain, heat, cold, etc.
The epidermis projects down into the dermis to form sweat glands, sebaceous glands and hair follicles.