Pilosebaceous Unit

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Pilosebaceous units contain sebaceous glands, sensory end organs, arrector pili muscle, hair, and the hair follicle. Thick hair, such as on the scalp, is terminal hair; fine, nearly imperceptible hair, is vellus hair. Depending on the part of the body, the sebaceous unit may be predominant (e.g., nose). The hair follicle has three named portions (from the skin to the base): the infundibulum (to the sebaceous gland duct), the isthmus (from the duct to the arrector pili muscle insertion), and the inferior portion (below the pili muscle insertion). Hair arises from the base of the hair follicle (the hair bulb). In the hair bulb are cells that make the hair along with melanocytes incorporated into the growing hairs. The hair bulb surrounds a dermal structure called the hair papilla. This papilla regulates the hair bulb activity. The mature hair has a ringlike configuration of several distinct layers (from outer to inner): vitreous membrane, outer root sheath, Henle's layer, Huxley's layer, inner root sheath cuticle, hair cuticle, cortex, and medulla (Fig. 14). Hair growth occurs in stages termed anagen, catagen, and telogen. The anagen stage is the growth phase. The catagen phase is an involutional stage in which the inferior portion ascends to the isthmus. The telogen phase is the resting phase during which the inferior portion of the follicle is absent. Gray hair, seen with advancing age, is caused by reduced hair pigmentation with melanocytes containing large cytoplasmic vacuoles (22).

Resting Skin Tension Lines

The resting skin tension lines (RSTLs) are skin furrows formed when the skin is relaxed (23). They radiate circumferentially outside the melolabial folds; inside the

Inside Hair Follicle
Figure 14 Microanatomy of a hair follicle.

melolabial folds, they are vertically oriented to the pogonion where the lines cross at right angles. The RSTLs should be differentiated from wrinkle lines and Langer's lines. Under the general heading ''On the Anatomy and Physiology of the Skin,'' Langer published a series of four articles involving the study of the dynamic properties of incised skin (24). It was from this collection that the term Langer's lines originated. He attempted to explain the properties of skin elasticity and tension. He studied the skin's directional variations by incising circles and squares and observing the resulting deformation. Under a variety of circumstances (cadaveric skin at rest and cadaveric skin placed on tension from movement of an underlying joint), he recorded a series of cleavage lines (Langer's lines). The cleavage lines followed the long axis of the deformed circle and corresponded to the lines of tension. Incising along the cleavage line was considered desirable to minimize wound gaping and scar contracture.

... While the incision is being made parallel to the cleavage lines the knife cuts readily and the skin does not wrinkle and fold under the knife; this, however, occurs when making the cut at right angles to the cleavage lines even with the sharpest well-greased knife and the incision can only be made slowly and with continued pressure. (24-25).

Langer found that the ''the skin of the face has less inbuilt tension than the skin of most other parts of the body.'' The facial cleavage (Langer's) lines correspond poorly to the lines of tension found in living tissue.

The RSTLs represent lines of skin tension in living tissue that are clinically useful in planning incisions that minimize wound separation and scar width. They are not easily visible but are found along the fine skin furrows that form when the skin is relaxed or gathered. The RSTLs are an inherent property of skin unrelated to muscular action. They are formed during embryological development and correspond to the direction of pull in relaxed skin determined by the static elements under the skin including bone, cartilage, and soft tissue bulk, and they are the same in all persons (26).

Langer Lines Borges Lines

Figure 15 The relaxed skin tension lines on the face (Borges). Overlay of Langer's face lines (center) and wrinkle lines. (From Kraissl, C. J. The selection of appropriate lines for elective surgical incisions. Plast. Reconstr. Surg. 8:1, 1951, [right]. and Wilhelmi, BJ, Blackwell, SJ, Phillips, LG. Langer's lines: to use or not to use. Plast. Reconstr. Surg 104(1), 208-214, 1999).

Figure 15 The relaxed skin tension lines on the face (Borges). Overlay of Langer's face lines (center) and wrinkle lines. (From Kraissl, C. J. The selection of appropriate lines for elective surgical incisions. Plast. Reconstr. Surg. 8:1, 1951, [right]. and Wilhelmi, BJ, Blackwell, SJ, Phillips, LG. Langer's lines: to use or not to use. Plast. Reconstr. Surg 104(1), 208-214, 1999).

Wrinkles may cross the RSTLs, especially in the glabellar region and chin. Over the glabella, wrinkles are vertical, yet the RSTLs are horizontal, and (over the chin) the melolabial sulcus is perpendicular to the RSTLs. Langer's lines represent the skin tension in rigor mortis and have only a loose correlation with the RSTLs (24-25). An incision or scar exactly on an RSTL is ideal, and within 30 degrees is acceptable (Fig. 15).

Facial Topographical Units

Replacing soft tissue defects of the face with identically sized flaps or grafts often produces an obviously patched area. Gonzalez-Ulloa et al. (27) realized that, in the face, there are definite regions in which skin differs in color, texture, mobility, and thickness. The boundaries of these regions represent natural transition zones where incisions and flap or graft borders can escape notice. Therefore, more satisfactory results can be obtained through regional restoration than with local repairs. Gonzalez-Ulloa et al. outlined the patterns of the shape of seven facial regions on cadavers as templates for flaps and grafts.

The borders of the regional facial units present favorable sites of scar placement, while incisions and scars through the borders tend to be unsightly. Reconstructing an entire unit allows the placement of incisions at the unit borders, and scar contracture and depressions become less noticeable. Even trapdoor scarring and bunching of the flap are less noticeable and may result in a net aesthetic improvement in areas that are naturally bulbous (e.g., the nasal alae). Millard's teachings include conforming reconstructive plans to the unit principle. If a large part of a unit is missing, then it is appropriate to extend the defect to include the entire unit and reconstruct it as a whole. Millard's seagull midline forehead flap represents the application of this principle to nasal reconstruction.

The nose, considered a single topographical (aesthetic) unit by Gonzalez-Ulloa et al., can be further divided into aesthetic subunits with borders that fall into natural visual rifts created by subtle shadows, concavities, and convexities (28). They include the dorsum, tip, alae, sidewalls, and soft triangles (Fig. 16). Although the subunits are not exactly the same nose to nose, the general shapes are consistent. The supratip depression divides the dorsum from the tip; this division may be somewhat variable depending upon ethnicity (29). The soft triangle represents a shadowy depression between the start of the cephalic bend on the caudal lower lateral cartilage and the nasal rim. Notching in this area is a problem after reconstructive surgery if the flap tissue extends to the vestibular skin. However, if one respects the superior margin of the soft triangle and sutures flap tissue to this border, the problems of notching give way to depressions in the soft triangle that provide good camouflage. Loss of greater than 50% of a subunit justifies its total replacement. However, |

removing normal structures to attain a more aesthetic result carries additional risk and requires a motivated and informed patient. c

The facial regions (topographical units) and subunits do not necessarily correo spond to the RSTLs. If incisions are within a topographical unit, attention to the

RSTLs is important to maximize camouflage and to attain the thinnest scar possible by following the natural skin tension lines. An area of obvious disagreement between o the unit concept and the RSTLs is around the chin. The RSTLs of the chin are vertical and perpendicular to the melolabial sulcus, while the chin unit follows the

Figure 16 The topographical units of the nose.

melolabial sulcus extending circumferentially around the mentum. Another area of disagreement is along the nasal sidewalls where the RSTLs and the subunit borders are again perpendicular. In most cases, the unit principle will take precedence, especially along the nasal sidewall. Following the chin's unit border may result in a significant widened trapdoor scar and irregular bulging of the chin. However, vertical incisions are obvious due to scar depressions and contour irregularities. A possible solution applicable to similar dilemmas may be to follow the unit borders by placing incisions using techniques that allow the majority of the incision to be within 30 degrees of the RSTLs (e.g., W-plasty).

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  • gorbadoc
    What is pilosebaceous unit?
    1 year ago

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