SUMMARY. Histopathological changes of the expanded human scalp in 10 children aged 2.5 - 12 years old are described. A correlation of the clinical observations during and after the procedure of expansion with the histological changes is reported. These changes were found to be present up to six months after removal of the expanders. Long-term studies, up to two years, are recommended to evaluate the effect of expansion on the histology and functions of the scalp.

Introduction
Factual histological changes and ultrastructural information describing the tissue-expansion phenomenon are still scanty in comparison with the rapid increase in clinical application (Pasyk et al., 1987).
There are sporadic data in the literature connected with the histomorphology of the expanded skin and soft tissue in the human (Argenta et al., 1985; Pasyk et al., 1987; Hegazy et al., 1991).
Pasyk et al. (1987) studied the histopathological changes occurring in expanded human skin at maximum expansion. The expansion process was done at different anatomical sites. There were two cases of scalp expansion. They used light and electron microscopy in their study. The stains used were haematoxylin and eosin and toludine blue stains. With light microscopy, human expanded epidermis showed marked thickening of the stratum spinosum and flattening of the rete ridges. Human expanded dermis showed marked thinning. Skin appendages, hair follicles and sweat and sebaceous glands did not show any histological changes during the process of the expansion.
This study reports a correlation of the histopathological changes and clinical observations during and after the procedure of scalp expansion in children.

Patients and methods
This study included 10 children presenting to our Unit with post-bum cicatricial alopecia at various sites of the scalp. The scalp defect was repaired using Dew Coming Silastic remote valve tissue expanders. One expander and a two-stage procedure were sufficient in seven cases. In two cases the size of the defect was large and required two expanders. In the last case the same expander was removed and re-used in the same case three times to reconstruct an extensive defect on the vertex and sides of the head.
A consent was signed from the parents to take biopsy of normal hair-bearing scalp during placement of the expander from the expanded scalp during removal and three and six months later on. The last two biopsies were taken either combined with other procedures or solely under general anaesthesia.
Every biopsy consisted of 1 x 0.5 cm of skin and subcutaneous tissue down to the pericranium. The biopsy was fixed in formaline 10% processed in paraffin and stained by haematoxylin and eosin (H & E), Van Geisson (VG), periodic acid Schifr (PAS), Alcian blue (AB), Mallory (M) and orcein (0) stains. Examination of these specimens with light microscopy was carried out.

1. The epidermis showed focal thickening of the stratum spinosum and flattening of the rete ridges at maximum expansion (Fig. 1).

These changes were still present 6 months after reconstruction (Fig. 8).

2. The dermis showed marked thinning with maximum expansion. The papillary and specially the reticular dermis were filled with thick bundles of collagen fibres that mostly oriented parallel to the surface of the skin (Fig. 2).

The skin appendages, the sebaceous and sweat glands as well as the hair follicles did not show any manifest histological changes during maximum expansion. However, it was noticed that these structures were situated deeper in the deep dermis or within the subcutaneous fat (Fig. 3).

With maximum expansion there were dilated capillaries in the papillary derinis, and small blood vessels in the deep dermis were also dilated and filled with blood. Oedema was noticed in the papillary dermis pushing the elastic fibres away from the epidermis (Fig. 1). Thinning, shortening and disorientation of the elastic fibres were also observed (Fig. 1). Three months after expansion the oedema was less manifest and the elastic fibres became thicker, longer and more oriented and parallel to the skin surface (Fig. 4).

Fig. 1 At maximum expansion: thickened epidermis, subpapillary oedema, fragmentation of elastic fibres (orcein stain). Fig. 2 At maximum expansion: decreased thickness of dermis, thick collagen bundles; sweat and sebaceous glands normal but with the subcutaneous fat (Mallory stain). Fig. 3 At maximum expansion: effaced rete ridges, subcutaneous fat herniatio ' n and skin appendages within subcutaneous fat (haematoxylin and eosin stain).

Acid mucopolysaccharides were found to be increased in the expanded papillary dermis (Fig. 5, 6) and returned to normal after 6 months. Neutral mucopolysaccharides were not changed during the expansion procedure.

1. The subcutaneous fat subjected to expansion became thinner than normal. The fat cells became flattened and smaller, and in some parts the fibrous tissue surrounded the fat cblls or even replaced them. The subcutaneous fat herniated into the reticular or even the papillary dermis. The skin appendages became present within the subcutaneous fat instead of being normally in the deep dermis or at the dermo-subcutancous junction (Fig. 3).
2. The capsule was found to be formed of four layers:

• the inner layer formed of fibrous tissue and macrophages;
• the second layer: elongated fibroblasts that were pressed within very thick bundles of collagen oriented parallel to the implant surface;
• the third layer: looser collagen fibres and few blood vessels;
• the outer layer: this is a vascular layer, with dilated blood vessels and more loosely dispersed collagen fibres and fibroblasts (Fig. 7).

Increased acid mucopolysaccharides were noticed in the fourth layer, which also contained few elastic fibres (Fig. 7).
All these changes were still present three and six months after expansion (Fig. 8).

Discussion
The histopathological examination of the expanded human epidermis showed focal thickening and absence of the rete ridges. These changes were still present six months after removal of the expander. These changes were focal in our study as the specimens were taken from the periphery of the expanded flaps during removal of the expander. Pasyk et al. (1987) stated that these changes may be due to a reaction around the expander. The effacement of the rete ridges may be due to increased pressure of the inflated expander.
Epidermal thickening was reported in guinea-pigs by Austad et al. (1982, 1986), Brobman and Huber (1985), Johnson et al. (1988) and Vander Kolk et al.

Fig. 4 Three months after removal of expander: rete ridges still effaced but elastic fibres become more oriented (orcein stain). Fig. 5 Increased acid mucopolysaccharides (Alcian blue stain). Fig. 7 The four dayers of the capsule (haematoxylin and eosin stain). Fig. 8 Six months after removal of expander: rete ridges still effaced, remnants of capsule and fat herniation (haematoxylin and eosin stain).

(1988). Austad et al. (1986), reported increased mitotic activity of expanded guinea-pig skin and they suggested it was due to surgical manipulation or stretching. Francis and Marks (1977) maintained that skin stretching stimulates epidermal proliferation only sufficiently to relieve tension or perhaps relieves crowding in the basal layer which normally inhibits mitosis.
The expanded human dermis, in this study, showed thinning at maximum expansion. It was filled with thick bundles of collagen fibres. These changes may be due to compression of collagen fibres as a result of serial expander inflation that decreased the spaces between the collagen bundles, making them closer and morphologically thickened (Pasyk et al., 1987). Thinning of the expanded human dermis was reported by Pasyk et al. (1987) and in the guinea-pig by Cherry et al. (1983). However, Vander Kolk et al. (1988) reported thickening of guinea-pig dermis with expansion.
In this study, the elastic fibres were found to be thinner, shorter, and more fragmented and disoriented than normal. These changes improved three months after expansion. The difference of response between elastic and collagen fibres may be due to the difference in their nature and structure. The elastic fibres are thin in comparison with collagen bundles and are wavy (Breathnach et al., 1983). They are therefore the first to be fragmented and broken with expansion. These are similar to the physiological changes in elastic fibres with aging, as shown by Lever and Lever (1990). This results in decreased skin elasticity and may affect the rate of stretching and inflation. It may also play a role in expander exposure.
The increased acid mucopolysaccharides noticed in our cases that returned to normal six months after expansion may be due to increased activity of the fibroblasts in the expanded scalp. Johnson and Helwing (1963) found increased acid mucopolysaccharides in dermatofibrome and connective tissue around the tumour islands of basal cell carcinoma.
The dilated capillaries in the papillary dermis and small blood vessels in the deep dermis shown in this study and reported by Cherry et al. (1983) may explain the increased vascularity of expanded flaps.
Cherry et al. (1983) stated that there may be angiogenesis as a result of stretching and tissue reaction. These changes may explain the papillary oedema, which may also be due to compression of the lymphatics by the inflated expander.
The skin appendages did not show any manifest histological changes during the process of expansion. However, we observed clinically dryness and thinning of the scalp hair during maximum expansion.
The secretory function of the sebaceous and sweat glands and the hair follicles may be affected by compression. This may predispose to infection and expander esposure, as we have previously stated (Hegazy et al., 199 1). Herniation of the subcutaneous fat and the presence of the skin appendages within this -fat may be a predisposing factor for skin disruption and exposure of the expander. The function of the skin appendages may also be affected. Thinning of subcutaneous fat was reported by Pasyk et al. (1987) and its herniation through the dermis was demonstrated in our study in 199 1.
The four layers of the capsule were previously reported by Pasyk et al. (1988) and the same was found in this study.
However, the vascularity of the outer layer of the capsule was better clarified in this study. The presence of elastic fibres in this layer was also evident. The preservation of the capsule with the advanced or rotated flap is important, especially in extended flaps.
The capsule remnants were present in the specimens examined three and six months after expansion. Pasyk et al. (1988) reported absence of the capsule two years after expansion of the cheek in one case. Does the capsule disappear before two years in humans? In the guinea-pig Pasyk et al. (1988) found that the capsule was absent in the specimens examined one year after expansion. Does this apply to humans? And is this important clinically?

RÉSUMÉ. Aprés avoir décrit les changements histopathologiques du cuir chevelu humain expansé, chez 10 enfants ágés de 2,5 jusqu'á 12 ans, les auteurs distinguent une corrélation entre les observations cliniques pendant et aprés la procédure d'expansion et les changements histologiques. Ces changements étaient présents jusqu'á six mois aprés I'enlévement des extenseurs. On recommande d'effectuer des études á long terme Ousqu'á deux ans) pour évaluer les effects de 1'expansion sur Phistologie et les fonctions du cuir chevelu.

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