Anatomy of the skin

对于皮肤解剖结构、血供以及其生物学及生物力学特性等的了解是提高局部皮瓣成活率以及提高整复效果的基础。这一章是对皮肤解剖的回顾，后面相继的两章为对其生物学及生物力学性能的介绍，尤其是生物力学特性的介绍在国内相关书籍中较少，阅读后发现深受启发。

General characeristics

• 皮肤在厚度以及附件和弹性纤维含量方面的区别具有性别、年龄和解剖区域特异性，也正是这些区别构成了美学区域的划分和区域之间的分界和邻接特点
• 全厚皮瓣对于供区的选择需要从厚度、颜色、质地以及毛发汗腺单位含量多方面考虑
• 皮肤及人种特点与瘢痕特点之间的关系：Individuals with fair skin, light hair, and blue eyes may develop postoperative scars that remain pink for an extended period. Persons with dark skin, hair, and eyes may develop scars that remain pigmented for a prolonged period after surgery. An assessment of previous scars and keloids should be made. Individuals with hyperelastic skin features are characterized by hyperextensibility of the joints (elbows, wrists, and knees), anterior hooding of the navel, and lax skin. These individuals are at higher risk for development of wide scars, permanent railroad tracking suture marks, hypertrophic scars, and prolonged erythema of the scars lasting up to 1 year, eventually resulting in a porcelain-colored white scar.

Epidermis

• The stratum corneum is thickest on the palms and soles and thinnest on the eyelids and genitalia.
• Total epidermal turnover time from the basal layer to the stratum corneum is approximately 30 days. The thickness of the epidermis is generally about 0.075 to 0.15 mm. The epidermis is thin at birth, becomes thicker during puberty and early adulthood, and thins in the fifth and sixth decades of life.
• melenocytes are confined to the basal layer. The number of melanocytes does not differ between races. The number and size of melanosomes is greater in pigmented skin and accounts for the darker skin color seen in pigmented persons. Tyrosinase activity and melanocyte density decrease with age.

• like melanocytes, Langerhans cells are characterized by dendritic processes. langerhans cells are not unique to the epidermis and are found in other squamous epithelia and in the normal dermis. Langerhans cells are responsible for recognizing and presenting antigens to lymphocytes in the skin and are implicated in the pathologic mechanism underlying allergic contact dermatitis and skin allograft reactions. The number of Langerhans cells decreases after ultraviolet irradiation. This results in a diminished capacity for immune surveillance,which may play a role in cutaneous carcinogenesis. The number of Langerhans cells also decreases with age.
• Merkel cells are neuroendocrine cells of epidermal origin that function as slow-adapting mechanoreceptors primarily concerned with touch sensation. They are primarily found among basal keratinocytes in areas of high tactile sensitivity, such as the lips, digits, oral cavity, and hair follicles. At these sites, Merkel cells often aggregate in specialized structures, called tactile disks or touch domes, in close association with peripheral nerve endings to form the Merkel cell–neurite complex. Merkel cell carcinoma, or cutaneous neuroendocrine carcinoma, most like arises from epidermal Merkel cells.
• The follicular epithelium in the dermis provides an additional source of germinative cells for reepithelialization of partial-thickness wounds. The follicular dermal extension of the epidermis also allows epidermal diseases, such as Bowen’s disease (squamous cell carcinoma in situ), to extend into the dermis. This may result in a higher recurrence rate if superficial treatment methods (such as CO2 laser) do not destroy the follicular downward extension of the disease process.
• Sebaceous glands may enlarge considerably in middle-aged and elderly persons, resulting in benign yellow papular lesions known as sebaceous hyperplasia. In certain locations, sebaceous glands arise independently and are not associated with a hair follicle, such as the vermilion border of the lip (Fordyce spots) and the eyelids (meibomian glands).

Dermis

• Ultraviolet irradiation induces matrix metalloproteinases in the epidermis and dermis, leading to dermal collagen degradation. This is manifested histologically as the disorganization of collagen fibrils and clinically as skin wrinkling in photoaging. Tretinoin inhibits the induction of matrix metalloproteinases and improves the appearance of photoaged skin by reducing fine lines and wrinkles. It is believed that matrix metalloproteinases are induced by CO2 laser treatment to degrade photodamaged collagen. This degradation is followed by the formation and deposition of new collagen.
• In the papillary dermis, the fibers are thin and run perpendicular to the skin surface, whereas those in the reticular dermis are thicker and run parallel to the skin surface.
• Ground substance plays a role in skin hydration and helps preserve the tensile elasticity of compressed skin by redistributing the pressure forces. Relative dehydration in the skin due to displacement of ground substance is partly responsible for the phenomenon termed mechanical creep.
• Fibroblasts are abundant in the papillary dermis and scant in the reticular dermis.

Vasculature

• Most of the blood flow in the skin is directed toward the more metabolically active components, namely, the epidermis, the follicular papillae, and the epidermal appendages. Two vascular plexuses connected by communicating vessels arepresent in the dermis (Fig. 1-11).21 At the junction of the dermis and subcutaneous adipose tissue lays the deep vascular plexus, which receives its vascular supply from musculocutaneous arteries perforating the subcutaneous adipose tissue. Arterioles from the deep vascular plexus supply the epidermal appendages and the superficial vascular plexus. The superficial vascular plexus lies in the superficial aspect of the reticular dermis and gives rise to a rich capillary loop system in the papillary dermis. This capillary loop system abuts the epidermis and provides it with nutrients by diffusion. The dermis also contains a lymphatic system that resembles the vascular plexuses.

《皮瓣外科学》第二版中对于皮瓣血管分布与构筑的描述：

• 深筋膜血管网：肌间隔皮动脉、肌间隙皮动脉以及肌皮动脉穿支等等在穿过深筋膜前、后均发出许多细小的分支，在深筋膜浅面和深面分别吻合形成深筋膜血管网
• 皮下血管
  • 干线型
  • 分散型
• 真皮下血管网：位于真皮与皮下组织交界处，由皮下动脉发出上行穿支进入真皮而形成
• 真皮血管网：位于真皮与乳头层交界处
• 乳头血管网：位于真皮乳头层内，随乳头与表皮基底膜的形状呈波浪形起伏，每个乳头由一直乳头动脉供应，分支后形成细小稠密的乳头血管网。
  

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上述皮肤血管的解剖层次和构筑特点是局部皮瓣成型的血供基础，比如真皮下血管网是皮瓣修薄的基础。