Arrangement
The skin is the largest organ in the human body accounting for almost 15% of its weight. It covers the entire external surface of the body and is continuous with the mucosae of the digestive, respiratory, and urogenital tracts. The thickness of the skin varies greatly from 1.5-5 mm, depending on its location (Standring, 2016).
The skin consists of three layers including the superficial epidermis, the dermis in the middle and the deep subcutaneous tissue.
- The epidermis is an epithelial tissue layer that undergoes constant renewal throughout life, mediated by a stem cell population within it. It mainly consists of keratinized stratified squamous epithelium.
- The dermis lies beneath the epidermis. It’s a dense connective tissue that provides strength to the skin through a diffuse interwoven orientation of collagen and elastic fibers throughout it. It’s in the dermis where the skin receives its neurovascular supplies. The dermis has two sublayers, namely a superficial papillary layer and a deeper reticular layer.
- The subcutaneous tissue is the deepest layer and is composed of a loose connective tissue with variable amounts of adipose cells interspersed (Ross and Pawlina, 2006).
There are two main types of skin:
- thick, hairless (glabrous) skin, which is found on the soles of feet, palms of the hands, and flexor surfaces of the digits;
- thin, hairy skin that covers the largest area of the body.
Related parts of the anatomy
Function
The skin functions as a barrier against the external environment, such as harmful microbial organisms and damage from mechanical, chemical, thermal, and UV stimuli. The skin also performs an important endocrine function, such as Vitamin D formation and synthesis of cytokines and growth factors. The skin also plays a role in excretion, through the secretion of sweat and sebum. Also, the skin is important in maintaining homeostasis by controlling the body's temperature and water loss. The various sensory receptors inside the skin make it an important organ for receiving sensory information from the external environment.
Dermatomes
Nerves transmit sensory information from the skin to designated spinal nerves through its anterior and posterior rami. The defined area of skin corresponding to each nerve fiber is known as a dermatome. There are 3 trigeminal, 5 cervical, 12 thoracic, 5 lumbar, and 5 sacral spinal nerves supplying the dermatomes. Various models for the distribution of dermatomes, or dermatome maps, now exist. However, because different experimental procedures were used to establish these maps, significant variation exists throughout the literature (Keegan and Garrett, 1948; Foerster, 1933; Lee, McPhee and Stringer, 2008; Head, Campbell and Kennedy, 1997). Generally, dermatome maps are inconsistent and there is significant overlap between adjacent spinal nerves. The commonly used Keegan and Garrett map is a widely accepted dermatome map and is portrayed in this model (Keegan and Garrett, 1948). Dermatome maps still remain a fundamental concept in human anatomy relevant to clinical practice, e.g., differential diagnosis of referred pain from visceral disease, to determine sensory limits of region anesthesia, determining the level of spinal cord injury, or insult caused by prolapsed intervertebral discs.
The L1 dermatome (from the first lumbar spinal nerve) covers the upper part of the lower limb, around the hip girdle and the groin area. L2 dermatome covers the anterior and medial aspect of the thigh, while L3 dermatome includes the anterolateral part of the thigh, down to the medial part of the knee and covers a portion of the medial leg. L4 covers the anterolateral aspect of the thigh, traveling across the knee joint and the anterior tibial aspect of the leg to the medial part of the ankle, foot, and great toe. The distribution of the L5 dermatome can be traced just inferior to the L4 dermatome on the lateral aspect of the thigh, lateral to the knee joint. It wraps around to the anterolateral surface of the leg and dorsum of the foot. It also includes the plantar aspect of the foot and the second, third, and fourth toes. The S1 dermatome includes the posterolateral thigh, most of the posterolateral leg to lateral part of the ankle and foot. The S2 dermatome can be traced medial to the distribution of S1 on most of the posterior thigh and covers a small area of the posterior leg on its medial aspect. All the above dermatomes have distributions to the skin of the gluteal region.
References
Foerster, O. (1933) 'The Dermatomes in Man', Brain, 56(1), pp. 1-39.
Head, H., Campbell, A. W. and Kennedy, P. G. (1997) 'The pathology of Herpes Zoster and its bearing on sensory localisation', Rev Med Virol, 7(3), pp. 131-143.
Keegan, J. J. and Garrett, F. D. (1948) 'The Segmental Distribution of the Cutaneous Nerves in the Limbs of Man', The Anatomical Record, 102(4), pp. 409-437.
Lee, M. W., McPhee, R. W. and Stringer, M. D. (2008) 'An evidence-based approach to human dermatomes', Clin Anat, 21(5), pp. 363-73.
Ross, M. H. and Pawlina, W. (2006) Histology: A text and atlas. Lippincott Williams & Wilkins.
Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice. Gray's Anatomy Series 41 edn.: Elsevier Limited.
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