HYPODERMIS: LAYERS, COMPOSITION, FUNCTIONS, DISEASES - ANATOMY AND PHYSIOLOGY - 2024

The hypodermis, or subcutaneous tissue, is the layer of fibrous connective tissue and fat accumulator that surrounds the body. It is located just below the dermis. It is also known as superficial fascia, fat pad, subcutis, and subcutaneous tissue. There is no consensus on whether the hypodermis should be considered the deepest layer of the skin, or simply subcutaneous tissue.

The only areas of skin with little or no hypodermis are on the eyelids, lips, and outer ear. The hypodermis may contain layers of striated musculature, particularly in the head, nape, areola, anal region (external anal sphincter), and scrotum. On the face, it enables facial expressions, such as a smile.

Source: US-Gov

The hypodermis of women contains more fat than that of men. This difference is the cause of the rounded contours of the female body.

Structure and scale

The skin is composed of an external stratum corneum (8–20 μm thick; up to 1.5 mm on the palms of the hands and soles of the feet), which covers the living epidermis (30–80 μm), which in turn, it covers the dermis (1–2 mm). The hypodermis (0.1 to several centimeters; 4–9 mm on average) is under the skin.

The hypodermis is structurally and functionally integrated with the skin due to the shared possession of vascular and nerve networks, and the continuity of the epidermal appendages, such as hairs and glands. The hypodermis connects the skin with muscles and bones.

Connective tissue is made up of collagen and reticulin fibers, which represent extensions of the dermis. The connective tissue forms septa that, as they are flexible compartments, provide resistance and mechanical mobility to the hypodermis.

The adipose tissue is arranged in olive-shaped lobes (diameter, ~ 1 cm) formed by the aggregation of microlobes (diameter, ~ 1 mm), in turn formed by the aggregation of adipocytes and lipocytes (diameter, 30–100 μm). Each adipocyte is in contact with at least one capillary. The adipose tissue lobes are surrounded by connective tissue septa.

Composition

The hypodermis consists of: 1) fibroblasts; 2) loose connective tissue containing blood and lymphatic vessels, nerve fibers, and Vater-Pacini corpuscles; 3) adipocytes; 4) adipose tissue containing ~ 50% of body fat; 4) leukocytes and macrophages.

Vater-Pacini corpuscles are macroscopic ovoid capsules filled with fluid and penetrated by a myelinated nerve axon. They are important receptors of tactile stimuli, specifically pressure and vibration.

The hypodermis is penetrated by continuations of epidermal appendages, such as hairs, apocrine sweat glands, and mammary glands.

Apocrine sweat is a viscous, milky liquid rich in lipids, nitrogen, lactates, and ions (Na ⁺, K ⁺, Ca ²⁺, Mg ²⁺, Cl ^-, and HCO ^{3 -}) contributed by the dermis and hypodermis.

Apocrine sweat glands flow into hair follicles and are present in the armpits, pubis, anogenital region, foreskin, and around the nipples. The Moll's glands of the eyelid and the ceruminous glands of the ear canal are subtypes of apocrine sweat glands.

The ducts of the mammary glands, which evolve from sweat glands, form a system of branches, terminated in alveoli, which penetrates deep into the hypodermis. These alveoli are surrounded by hypodermic milk-producing cells, which provide fat and other nutrients.

Layers

The hypodermis is continuous with the dermis. The boundary between the two layers is irregular and poorly defined. Some authors consider that the hypodermis does not have sublayers. Others admit the existence of a connective membranous tissue that would divide it into two sub-layers, called superficial adipose tissue (TAS) and deep adipose tissue (TAP).

TAS and TAP have retinaculae that connect the hypodermis to the overlying dermis and the underlying deep fascia. TAS and TAP are the cause that the hypodermis (and therefore the skin) can slide over the deep fascia and then return to its normal position.

The TAS are elastic and strong structures, perpendicular to the surface of the skin, made up of large lobes of fat interspersed (like the tines of a comb and the spaces that separate them) between well-defined fibrous septa (retinaculus cutis superficialis).

The TAS can all be at the same level, or organized in overlapping planes, depending on the local and individual fat content.

The TAPs are not very elastic structures, they have oblique fibrous septa (retinaculus cutis profundus), and small adipose lobes with a tendency to move.

TAS-TAP differentiation is particularly evident in the lower torso, buttocks, and thighs.

Embryology

The epidermis develops from the ectoderm. The dermis and hypodermis make it from the mesoderm. Fat cells and fibroblasts come from mesenchymal cells. During the first two months of fetal life, the dermis and hypodermis are highly cellular and cannot be distinguished from one another.

From the second month of embryonic development, the interstitial fibrillar substance appears. Elastic fibers emerge from it. This is followed by the differentiation of the mesenchyme into a compact and dense peripheral layer (dermis) and a deeper and laxer layer (hypodermis).

Subcutaneous fatty tissue appears for the first time (15–16 weeks of embryonic development) in the hypodermis of the thorax. Next (17 weeks) it appears on the cheeks, around the mouth and on the chin.

The dermatocranial bones that surround the neurocranium are formed from the embryonic dermis and hypodermis. These bones arise from mesenchymal cells. The process is called intramembranous bone formation.

Features

Allows the skin to slide over muscles and bones. Its mobility dissipates mechanical stress from the skin. Its adipose tissue cushions harmful impacts to internal organs, muscles and bones.

Your adipose tissue stores and generates energy. It also provides thermal insulation, thus facilitating thermoregulation. This fabric maintains the smoothness of the skin and the contour of the body, promoting sexual attraction and creating padded areas on which you can rest when sitting or lying down.

Your adipocytes are a component of innate immunity. In the presence of bacteria or bacterial products, there is proliferation of preadipocytes and expansion of adipose tissue, which act as a protective barrier. The production of adenosine 5'-monophosphate (AMP) by adipocytes is also increased.

It has sensory and circulatory (blood and lymph) and endocrine functions. Stores steroids and produces estrogen. Inside, androstenedione is transformed into estrone. Leptin, a hormone produced by lipocytes, regulates body mass via the hypothalamus.

Provides nutrients for healing. Houses and nourishes skin structures, such as the mammary and apocrine glands, and hair follicles. The production of milk and the protective lipids of apocrine sweat, as well as hair regeneration, are linked to adipogenesis.

Diseases

Obesity, injuries, infections

Obesity, or excessive accumulation of body fat, is the most common disease linked to the hypodermis. It affects half the population of Western developed countries. Life expectancy varies between 13% and 42% lower if the overweight is, respectively, 10% to 30%.

In burn scars in which the hypodermis has disappeared, the skin loses its mobility. It suffers more damage than normal in case of stretching or rubbing.

The close relationship between the hypodermis and the skin explains why inflammation of the latter can secondarily affect adipose tissue, producing subcutaneous annular granuloma, lipoid necrobiosis, rheumatoid nodules, septal panniculitis, or necrobiotic xanthogranuloma.

As we age, the thickness of the subcutaneous fatty tissue is reduced in many parts of the body. This reduces the protective capacity of the hypodermis, increasing the skin's susceptibility to mechanical damage and sunburn, as well as making muscles and bones more vulnerable to impact damage.

Skin infections that often involve the hypodermis include: 1) eripysellas, caused by streptococci; 2) cellulitis, caused by Staphylococcus aureus and streptococci; 3) abscesses of one (furunculosis) or multiple (carbunculosis) hair follicles, caused by S. aureus. Carbunculosis can cause fever and turn into cellulite.

Tumors

Lipomas and hibernomas are benign tumors formed, respectively, by the adipocytes of white and brown body fat.

Fibrohistocystic tumors (= macrophage-like cells) are a heterogeneous group of neoplasms that often present side-by-side histocystic, fibroblastic, and myofibroblastic differentiations. Fibrohistocystic tumors involving the hypodermis include fibrous histocytoma and atypical fibroxanthoma.

Fibrous histocytoma, also called dermatofibroma, is the most common fibrohistocystic tumor. It is benign. It is most common in middle-aged people and women and usually develops on the trunk or extremities. Often deeply penetrates the hypodermis. Its trigger is a traumatic injury, or an insect bite.

Atypical fibroxanthoma is a fast-growing, dome-shaped, ulcerated tumor. It appears almost exclusively on skin that has been damaged by sunlight. It typically affects the skin.

However, a variety of atypical fibroxanthoma, called pleomorphic skin sarcoma, penetrates deeply into the hypodermis. This variety is a malignant tumor, with metastatic potential. Even if it is removed, it tends to recur.

References

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