 Functions of the Skin
The skin protects the body. The skin protects the body from water loss. The skin is involved in the production of vitamin D from precursors with the aid of sunlight. There are many sensory receptors in the skin: pain, pressure, fine touch. The skin is also involved in heat regulation.
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Types of Skin
Thick skin is found on the palms of the hand and the sole of the feet. Thin skin is found everywhere else.
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Layers of the Skin
The skin is composed of two layers: the epidermis and the dermis. Underneath these layers lies the
hypodermis (subcutaneous tissue). The hypodermis is a layer of loose connective tissue.
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Histology of the Epidermis
The epidermis is formed by stratified squamous epithelium. Keratinization is seen in the epidermis. Keratinocytes, melanocytes Merkel cells and Langerhans cells are all found in the epidermis. The keratinocyte is the most abundant cell in the epidermis. The melanocyte produces melanin, which is responsible for skin pigmentation. The Merkel cell is a mechanoreceptor. The
Langerhans cell is a phagocyte. Langerhans cells are macrophages seen in the skin.
The epidermis is divided into five layers: stratum basale, stratum spinosum, stratum granulosum, stratum
lucidum, and stratum corneum.
Stratum Basale
The stratum basale contains the dividing cells. This layer is also called the stratum germinativum.
Stratum Spinosum
The stratum spinosum consists of a layer several cells deep. The cells have pointy or spiny processes on them.
Stratum Granulosum
The cells in the stratum granulosum contain keratohyaline granules.
Stratum Lucidum
The stratum lucidum is present only in thick skin.
Stratum Corneum
The stratum corneum is the outermost layer. It is also called the horny layer. The cells in this layer are
essentially bags of keratin. They contain no nuclei or organelles.
Histology hint from Sarah Bellham: The epithelium is classified as "squamous" based on the cells of the surface layer.
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Histology of the Dermis
Beneath the epidermis is the dermis. The
dermis is composed of a papillary layer and a reticular layer. The reticular layer of the dermis is made up of
dense irregular connective tissue. Fibroblasts are found in the dermis. Fibroblasts produces collagen.
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Histology of the Subcutaneous Layer
The subcutaneous tissue layer (hypodermis) is the loose connective tissue layer underneath the dermis.
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Receptors in the Skin
There are several different sensory receptors in the skin.
Ruffini endings, pacinian corpuscles, meissner's corpuscles, and merkel cells are all encapsulated sensory
receptors. Free nerve endings are not encapsulated.
The most abundant sensory receptor are the free nerve endings. Free nerve endings respond to pain and temperature. Ruffini's corpuscles respond to continuous pressure. Pacinian corpuscles respond to vibration and rapidly changing pressure. Krause's end bulbs are a receptor for fine touch which are located in mucous membranes and the tongue. Meissner's corpuscles are also a receptor for fine touch but they are located in the dermis. Pacinian corpuscles are pressure receptors in the skin.
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Skin Appendages
Sweat glands, hair, nails and sebaceous glands are all considered epidermal appendages.
 Nails
The lunula is the half moon shaped white area on a nail. The anatomical term for the cuticle is the
eponychium. The matrix is the region of the nails where there are dividing cells and nail growth. The nail
plate rests on the nail bed. The nail root is the proximal portion of the nail that is underneath skin.
Ceruminous Glands
The ceruminous glands of the ear are apocrine sweat glands.
Glands of Moll
The glands of Moll in the eyelid are apocrine sweat glands.
Histology of Sweat Glands
Sweat gland are also called sudoriferous glands.
Classification of Sweat Glands
Sweat glands are divided into apocrine and eccrine. Apocrine sweat glands are found on the areola, external genitalia, axilla, and curcumanal region. Eccrine sweat glands are distributed over most of the body.
Innervation of Sweat Glands
Eccrine sweat glands are innervated by the sympathetic nervous system. The neurotransmitter for the eccrine
sweat glands is acetylcholine. Thus, it is cholinergic.
Histology hint from Sarah Bellham: For most postganglionic sympathetic neurons, the neurotransmitter is
norepinephrine. Eccrine sweat glands are an exception to this generalization, as the innervation for eccrine
sweat glands is cholinergic sympathetic.
Apocrine sweat glands are innervated by the sympathetic nervous system. The neurotransmitter for the
apocrine sweat glands is norepinephrine. Thus, it is adrenergic.
 Hair
Hair is present over most of the body. It is not found on the palms of the hand, soles of the feet, urogenital
openings, and lips. Huxley's layer is a layer in the hair follicle.
Henle's layer is a layer in the hair follicle.
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Sources:
Histology: A Text and Atlas
Michael H. Ross/Edward J. Reith
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