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Subcutaneous tissue
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| Subcutaneous tissue | |
|---|---|
 The hypodermis is the lower layer of skin shown in the diagram above. | |
| Details | |
| System | Integumentary |
| Identifiers | |
| Latin | tela subcutanea[1] |
| MeSH | D040521 |
| TA98 | A16.0.03.001 |
| TA2 | 7083 |
| TH | H3.12.00.2.00001 |
| FMA | 9630 |
| Anatomical terminology | |
The subcutaneous tissue (from Latin subcutaneous 'beneath the skin'), also called the hypodermis, hypoderm (from Greek 'beneath the skin'), subcutis, or superficial fascia,[2] is the lowermost layer of the integumentary system in vertebrates.[3] The types of cells found in the layer are fibroblasts, adipose cells, and macrophages. The subcutaneous tissue is derived from the mesoderm, but unlike the dermis, it is not derived from the mesoderm's dermatome region. It consists primarily of loose connective tissue and contains larger blood vessels and nerves than those found in the dermis. It is a major site of fat storage in the body.
In arthropods, a hypodermis can refer to an epidermal layer of cells that secretes the chitinous cuticle. The term also refers to a layer of cells lying immediately below the epidermis of plants.
Structure
[edit]- Fibrous bands anchoring the skin to the deep fascia[4]
- Collagen and elastin fibers attaching it to the dermis[5]
- Fat is absent from the eyelids, clitoris, penis, much of pinna, and scrotum[1]
- Blood vessels on route to the dermis[6]
- Lymphatic vessels on route from the dermis[7]
- The glandular part of some sweat glands; mammary glands lie entirely within the subcutaneous tissue[8] (which are modified apocrine sweat glands)[9]
- Cutaneous nerves[7] and free endings
- Hair follicle roots
- Ruffini[10]: 478 and Pacinian corpuscles
- Mast cells[11]
- Bursae, in the space overlying joints in order to facilitate smooth passage of overlying skin
- Fine, flat sheets of muscle, in certain locations, including the scalp, face, hand, nipple, and scrotum, called the panniculus carnosus
The hypodermis forms an important insulating layer and/or food store in some animals, such as whales and hibernating mammals.
In some plants, the hypodermis is a layer of cells immediately below the epidermis of leaves. It is often mechanically strengthened, for example, in pine leaves, forming an extra protective layer or a water storage tissue.
Subcutaneous fat
[edit]
Subcutaneous fat is the most widely distributed subcutaneous tissue layer.[1] It is composed of adipocytes, which are grouped together in lobules separated by connective tissue.[5] The number of adipocytes varies among different body areas, while their size varies according to the body's nutritional state.[12] It acts as padding and as an energy reserve, as well as providing some minor thermoregulation via insulation.[10][5] Subcutaneous fat is found just beneath the skin, as opposed to visceral fat, which is found in the peritoneal cavity,[13] and can be measured using body fat calipers to give a rough estimate of total body adiposity.[14]
Clinical significance
[edit]Injection
[edit]Injection into the subcutaneous tissue is a route of administration used for drugs such as insulin: because it possesses few blood vessels, the tissue absorbs drugs slowly.[10]: 135 Subcutaneous injection is believed to be the most effective manner to administer some drugs, such as human growth hormones. Just as the subcutaneous tissue can store fat, it can also provide good storage space for drugs that need to be released gradually because of limited blood flow. "Skin popping" is a slang term that includes this method of administration and is usually used in association with recreational drugs.
Disease
[edit]See also
[edit]References
[edit]- ^ a b c TA A16.0.03.001
- ^ "hypodermis lumenlearning".
- ^ Mosby's Medical, Nursing & Allied Health Dictionary (4th ed.). St. Louis: Mosby. 1994. pp. 998, 774, 1497. ISBN 978-0801672255.
- ^ McMinn, R.M.H. (2003). Lasts Anatomy: Regional and Applied. Churchill Livingstone. p. 3. ISBN 0729537528.
- ^ a b c "The hypodermis". An Organ Revealed. L'Oréal. Retrieved 4 June 2013.
- ^ Tamarkin, Dawn A. "Unit 4: Skin & Bone Structure". Springfield Technical Community College. Retrieved 8 June 2013.
- ^ a b O'Rahilly, Ronan; Müller, Fabiola; Carpenter, Stanley; Swenson, Rand; Dartmouth Medical School. "Chapter 4: The skin, hair and nails". Basic Human Anatomy: A Regional Study of Human Structure. Retrieved 9 June 2013.
- ^ Fischer, Josef E.; Bland, Kirby I.; Callery, Mark P. (18 December 2006). Mastery of Surgery. Lippincott Williams & Wilkins. p. 482. ISBN 078177165X.
- ^ Krstic, Radivoj V. (18 March 2004). Human Microscopic Anatomy: An Atlas for Students of Medicine and Biology. Springer. p. 466. ISBN 9783540536666.
- ^ a b c Kenneth, Saladin (2007). Human Anatomy. Rex Bookstore, Inc. pp. 135, 478, 602. ISBN 978-0071259712.
- ^ Goldsmith, Lowell A.; Katz, Stephen I.; Gilchrest, Barbara A.; Paller, Amy S.; Leffell, David J.; Wolff, Klaus (10 April 2012). Fitzpatrick's Dermatology in General Medicine (8 ed.). McGraw-Hill. p. 64. ISBN 978-0071669047.
- ^ "Subcutaneous Tissue". Medical Subject Headings (MeSH). National Library of Medicine. Retrieved 5 June 2013.
- ^ "Abdominal fat and what to do about it". Harvard Health Publications. Harvard Medical School. 2006. Retrieved 5 June 2013.
- ^ Orphanidou, C; McCargar, L; Birmingham, C; Mathieson, J; Goldner, E (August 1994). "Accuracy of subcutaneous fat measurement: comparison of skinfold calipers, ultrasound, and computed tomography". Journal of the American Dietetic Association. 94 (8): 855–858. doi:10.1016/0002-8223(94)92363-9. ISSN 0002-8223. PMID 8046177.
Subcutaneous tissue
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Anatomy and Composition
Macroscopic structure
The subcutaneous tissue, also known as the hypodermis or subcutis, is the deepest layer of the skin, composed primarily of loose connective tissue containing fat deposits that lies directly beneath the dermis and serves to anchor the skin to the underlying deep fascia or muscle.[5][6][7] Superiorly, it attaches to the dermis through fibrous septa that extend from the dermal reticular layer into the hypodermis, while inferiorly it transitions gradually into the deep fascia without a distinct boundary, allowing for mobility between the skin and deeper structures.[8][9] The thickness of the subcutaneous tissue exhibits significant variation across the body and among individuals, typically ranging from less than 1 mm in thin areas such as the eyelids to several centimeters in regions like the abdomen.[10][6] These differences are influenced by factors including age, with thinning observed over time due to fat loss; sex, where females often have greater thickness in areas like the hips and thighs compared to males; and body mass, as higher body mass index correlates with increased subcutaneous fat accumulation.[11][7] In gross appearance, the subcutaneous tissue is organized into lobules of adipose tissue separated by thin fibrous bands or septa, which provide structural support and pathways for blood vessels and nerves; this lobular arrangement is particularly evident during surgical dissection or on cross-sectional imaging modalities such as computed tomography (CT) and magnetic resonance imaging (MRI), where the fat compartments appear as hypodense or hyperintense regions delineated by linear fibrous structures.[8][9][12]Microscopic components
The subcutaneous tissue consists primarily of loose connective tissue interspersed with adipose elements at the histological level. The predominant cell type is the adipocyte, which stores lipids in large droplets; white adipocytes are unilocular with a thin rim of cytoplasm and eccentric nucleus, comprising the majority in adult subcutaneous depots for energy reserve, while brown adipocytes are multilocular with numerous mitochondria and central nuclei, more prevalent in interscapular and perirenal regions during infancy but diminishing with age.[2] Fibroblasts, responsible for synthesizing extracellular matrix components, are scattered throughout, alongside resident immune cells such as macrophages that phagocytose debris and regulate inflammation, and mast cells that release histamine in response to injury; these non-adipocyte cells constitute a smaller proportion, varying regionally, where adipocytes can exceed 90% of cellular volume in the abdomen.[13][14][15] The extracellular matrix forms a supportive scaffold, featuring bundles of type I collagen fibers and scattered elastin fibers organized into fibrous septa that compartmentalize adipocyte lobules, conferring tensile strength and elasticity.[16] Glycosaminoglycans, including hyaluronic acid, are interspersed within this matrix, binding water to maintain hydration and facilitate nutrient diffusion through the avascular adipocyte clusters.[13] Embedded within the matrix are vascular elements, primarily a rich capillary plexus and postcapillary venules arising from deeper arterial branches, which nourish the tissue and enable hormone transport to adipocytes.[2] Neural components include unmyelinated sensory nerve fibers and endings that innervate the region for proprioception and nociception, often branching alongside vessels.[2] In deeper subcutaneous zones, specialized structures integrate with the matrix, such as the dermal roots of hair follicles extending into the tissue for anchorage, excretory ducts of eccrine sweat glands traversing to the surface, and encapsulated mechanoreceptors like Pacinian corpuscles, which consist of concentric lamellae of Schwann cells surrounding a central axon to detect vibratory stimuli.[17]Functions and Physiology
Protective and supportive roles
The subcutaneous tissue, also known as the hypodermis, plays a critical role in cushioning the body against mechanical impacts through its adipose components, which absorb shock and protect underlying muscles and bones from trauma.[18] This shock-absorbing function is primarily facilitated by the lobular arrangement of fat cells, which distribute forces during physical activities or external pressures.[5] For instance, in weight-bearing areas like the soles of the feet, the adipose tissue in the hypodermis helps mitigate the effects of repetitive stress on deeper structures.[19] In addition to cushioning, the subcutaneous tissue provides anchoring support via fibrous septa, which are bands of connective tissue that tether the dermis to underlying fascia and muscles, thereby preventing excessive skin mobility and enhancing overall structural stability during movement.[20] These septa, composed of collagen and elastic fibers, maintain the skin's position relative to deeper tissues, reducing shear forces and promoting coordinated motion across joints and body surfaces.[8] The layer also serves as a protective reservoir for neurovascular structures, housing major blood vessels, nerves, and lymphatics within its loose connective matrix, which minimizes the risk of injury to these elements by encasing them in a compliant, padded environment.[5] This containment allows for safe passage of these structures between the skin and deeper tissues while providing a buffer against compression or laceration during blunt trauma.[21] Furthermore, the subcutaneous tissue contributes to skin turgor by offering supportive connective tissue that bolsters the skin's elasticity and resilience, helping it return to its original shape after deformation.[6] The interplay between its adipose and fibrous elements ensures that the overlying skin maintains firmness and adaptability, particularly in areas prone to stretching or pressure.[1]Metabolic and thermoregulatory roles
The subcutaneous tissue, primarily composed of adipocytes, serves as the body's principal site for energy storage, where triglycerides are accumulated as neutral lipids within lipid droplets to form a long-term energy reserve. During periods of fasting or increased energy demand such as exercise, these triglycerides undergo lipolysis, a catabolic process mediated by hormone-sensitive lipase and other enzymes, releasing free fatty acids and glycerol for oxidation in peripheral tissues or hepatic gluconeogenesis.[22][23] This mobilization is tightly regulated by hormonal signals like catecholamines and insulin, ensuring metabolic flexibility while preventing excessive lipid accumulation that could impair systemic homeostasis.[24] In thermoregulation, the subcutaneous layer acts as an insulating barrier, with its low thermal conductivity—approximately 0.2 W/m·K—limiting conductive and convective heat loss from deeper tissues to the environment, particularly in cooler conditions.[25] Additionally, subcutaneous adipose tissue contains brown adipocytes specialized for non-shivering thermogenesis; these cells express uncoupling protein 1 (UCP1) in their mitochondrial inner membrane, which dissipates the proton gradient generated by the electron transport chain as heat rather than ATP synthesis, thereby elevating body temperature in response to cold exposure or sympathetic activation.[26] This thermogenic capacity is prominent in infants but persists in adult subcutaneous depots, such as the supraclavicular region, contributing to overall energy expenditure.[27] Adipocytes within the subcutaneous tissue also function as an endocrine organ, secreting hormones that modulate systemic metabolism. Leptin, produced in proportion to adipocyte size and fat mass, signals satiety to the hypothalamus, suppressing appetite and promoting energy expenditure to maintain body weight balance.[28] In contrast, adiponectin enhances insulin sensitivity in skeletal muscle and liver, inhibits gluconeogenesis, and exerts anti-inflammatory effects, with its circulating levels inversely correlated to adiposity.[29] These adipokines thus integrate local lipid storage with broader metabolic regulation, influencing glucose homeostasis and inflammation.[30] The vascular network in subcutaneous tissue, including capillaries and lymphatics, facilitates the absorption of nutrients and therapeutic agents into the systemic circulation, making it an ideal route for subcutaneous drug delivery. Injected substances, such as insulin or monoclonal antibodies, diffuse through the interstitial space and are taken up by blood vessels for rapid bioavailability, bypassing first-pass hepatic metabolism while allowing sustained release due to the tissue's relatively avascular nature compared to intramuscular sites.[31][32] This property supports effective pharmacotherapy for conditions requiring chronic administration, with absorption rates influenced by molecular size and formulation.[33]Development and Variations
Embryological development
The subcutaneous tissue, also known as the hypodermis, originates from the mesoderm during gastrulation in early embryonic development. Specifically, it derives from the dermatome component of the developing somite, which forms the underlying connective tissue framework of the hypodermis. This mesodermal layer differentiates beneath the developing dermis, initially appearing as loose mesenchyme around the 5th to 8th week of gestation, providing a foundational scaffold for subsequent tissue maturation.[34][35] The formation progresses with the differentiation of mesenchymal cells into fibroblasts and preadipocytes between weeks 8 and 12 of gestation, establishing the connective tissue matrix and early fat cell precursors. Adipocyte differentiation is primarily driven by the transcription factor PPARγ (peroxisome proliferator-activated receptor gamma), which regulates the expression of genes essential for lipid accumulation and mature adipocyte formation. By weeks 14 to 24, visible adipose lobules emerge in the subcutaneous layer, marking the transition from mesenchymal precursors to functional fat-storing tissue.[36][37] Fetal accumulation of subcutaneous fat is influenced by maternal nutritional status during gestation, with adequate caloric and lipid intake promoting greater fat deposition in the fetus. At birth, brown adipose tissue predominates in the hypodermis, particularly in the interscapular region, enabling non-shivering thermogenesis critical for neonatal temperature regulation. Postnatally, the high brown fat content gradually shifts toward white adipose tissue by early infancy, a process modulated by hormonal signals including thyroid hormones that influence adipose remodeling and energy metabolism.[38][39][40]70069-X/abstract)Anatomical variations across body regions
The thickness of subcutaneous tissue exhibits significant regional variations across the body, influenced by its role in cushioning and support in different areas. In adults, it is thickest in the abdomen, buttocks, and thighs, where measurements can reach 3-5 cm, providing substantial padding over bony prominences and organs. Conversely, the tissue is thinnest on the face, hands, and shins, often less than 1 cm, allowing for greater mobility and finer sensory feedback in these regions.[11] These differences arise from the varying proportions of adipose and connective tissue components, with the hypodermis generally bounded by the dermis superiorly and fascia inferiorly.[6] Sex and age play key roles in modulating subcutaneous tissue thickness. Post-puberty, females typically exhibit greater overall thickness due to estrogen-driven fat deposition, particularly in gluteofemoral regions, compared to males who show more centralized patterns.[41] With aging, however, thickness decreases through lipoatrophy, especially in the extremities and face, as adipose cells diminish and fibrous elements may increase, leading to a more rigid structure.[11] This age-related thinning is more pronounced in both sexes after the sixth decade, contributing to altered body contours.[42] Compositional variations further distinguish subcutaneous tissue by region. In the palms and soles, it contains a higher density of fibrous septa and collagen, enhancing durability against mechanical stress, in contrast to the more lipid-dominant composition elsewhere.[7] In infants, the interscapular region features elevated levels of brown adipose tissue, characterized by multilocular adipocytes rich in mitochondria, which diminishes in adulthood.[43] Ethnicity and body mass index (BMI) influence distribution patterns of subcutaneous tissue. For instance, individuals of African descent often display less subcutaneous fat in the trunk relative to BMI compared to those of European descent.[44] These variations manifest in distinct phenotypes, such as gynoid (peripheral, subcutaneous-dominant) patterns more common in females across ethnicities versus android (visceral-dominant) in males, with BMI amplifying overall thickness proportionally.[45]Regional Variations in Subcutaneous Adipose Tissue Thickness
Ultrasound studies provide precise measurements of subcutaneous adipose tissue (SAT) thickness across body segments. A comprehensive mapping study (Störchle et al., 2018) segmented the body into 11 regions and reported mean SAT thicknesses (including fibrous structures):- Hands: lowest at approximately 0.3 mm
- Forearms: low, closer to hands than upper arms
- Upper arms: higher than forearms, typically in the range of 5–11 mm in general populations (with triceps sites around 6–10 mm)
- Buttocks: highest at ~12 mm
- Abdomen: often 13–24 mm or more