Embryologic, Histologic, and Anatomic Aspects
Capability for accurate diagnosis of inflammatory skin diseases, clinically and histopathologically, requires familiarity thoroughly with embryologic, histologic, and anatomic aspects of the skin. Although examples of this precept are legion, only a few are necessary to illustrate it convincingly. Without knowledge of the course of the vascular plexuses in the skin, reasons for nuances of erythema cannot be conceptualized nor can patterns formed by infiltrates of inflammatory cells be comprehended. Without awareness of variability in epidermal pigmentation among the races, different shades of red exhibited by a single disease as it presents itself in persons of different colors of skin cannot be understood nor can differences histopathologic between postinflammatory hyperpigmentation and hypopigmentation be explained. Without a grasp of the vascular changes that result from the effects of long-standing stasis, a “normal” finding histologic that comes into being as a result of the upright posture of man, expected deviations from conventional appearances of inflammatory skin diseases, clinical and histopathologic, often cannot be recognized when lesions of those diseases are positioned far below the knees, such as in the vicinity of the ankles. Last, without being alert to the distinctiveness of the stratum corneum of palms and soles, namely, thick and composed of corneocytes arranged compactly, it is not possible to comprehend why psoriasis at those sites may appear as spongiotic vesicles.
Knowledge of the function of the skin in general, and of each component of it, is requisite if the effects of inflammatory skin diseases on the well-being of patients are to be recognized and integrated. If, for example, the stratum corneum is lost nearly entirely, as it is, along with viable epidermal cells, as a consequence of an expression of a blistering disease so severe that the changes have been likened clinically to those of a scald, such as occurs at times in erythema multiforme (that expression of it known as “toxic epidermal necrolysis”) and in pemphigus vulgaris, and if that cornified layer is not restored rapidly, a patient may die because the stratum corneum is indispensable for life. If the stratum corneum is shed abnormally and constantly, as it is in conditions in which scaling is universal, as in the erythroderma of psoriasis and of pityriasis rubra pilaris, a patient may suffer major aberrations in control of balance of fluids and electrolytes, the epidermal cornified layer being essential for maintenance of homeostasis. If the stratum corneum on palms becomes thickened strikingly, as it does, for example, in some patients with long-standing allergic contact dermatitis, such as occurs consequent to the effects of an ingredient in cement, a patient may not be able to utilize his or her hands effectively because they are restricted so greatly by hyperkeratosis and by fissures that form within the abnormally cornified epidermis.
In short, without firm grounding in structure and function of skin, a histopathologist lacks a basis for making diagnoses logically of inflammatory skin diseases, and a clinician lacks a fundament for devising therapy for them rationally. This chapter, which seeks to prepare readers for application of a particular method for diagnosis of inflammatory skin diseases, is organized according to individual components of the skin, namely, epidermis, hair follicles, sebaceous units, apocrine units, eccrine units, nail units, melanocytes, Langerhans” cells, Merkel cells, structures of the dermoepithelial interface, blood vessels, lymphatics, collagen, elastic fibers, ground substance, muscles, and nerves. The strategy is to inform about how the skin looks by examination grossly and by inspection using conventional microscopy, and how it works. Although each of the components of the skin is discussed separately, it must be borne in mind constantly that all of the different components are interrelated, structurally and functionally.
At the outset it must be understood clearly that, strictly speaking, the skin consists of but two compartments: (1) epidermis and epithelial structures of adnexa continuous with it, and (2) dermis and nonepithelial structures of adnexa lodged in it. The subcutaneous fat is not a part of the skin, as the word “subcutaneous,” and its synonym, “hypodermis,” denote; it is one of the “soft tissues.” But because of its exceedingly close relationship anatomically to the skin and its tendency to respond together with the skin in many processes pathologic, the subcutaneous fat is given due consideration in this chapter. For these same reasons, “panniculitis” is one of the eight patterns engaged by us in analysis algorithmic for purposes of diagnosis histopathologic of inflammatory diseases.
The epidermis is the thinnest by far of the two essential components of skin, varying in thickness from approximately 0.03 mm on the eyelids to 1.5 mm on the palms of a young adult; the average thickness of epidermis is about 0.4 mm. As is the case for every other part of the skin, as a person ages, the epidermis shrinks, that atrophy being physiologic. The epidermis is a metabolically active, stratified squamous, cornifying epithelium populated by at least four different and distinctive types of cells: keratocytes, melanocytes, Langerhans” cells, and Merkel cells. Within the epidermis, keratocytes, organized cohesively, predominate overwhelmingly, those attributes being typical of an epithelium, in contrast to the dermis, which consists mostly of relatively noncellular connective tissue composed of collagen bundles, elastic fibers, and ground substance, characteristic of a nonepithelium.
The dermis, depending on the anatomic site, is 15 to 40 times thicker than the epidermis, but its requirements metabolic are far less. Cells of various kinds in variable numbers are scattered throughout the mature dermis, those being fibrocytes, dermal dendrocytes, histiocytes, Langerhans” cells, and mast cells. Within the dermis are housed nerves, blood vessels, lymph vessels, smooth muscles, and epithelial structures of adnexa, to wit, the folliculosebaceous-apocrine units and the eccrine units. A fully formed dermis is divisible into two distinct compartments: (1) a thin zone immediately beneath the epidermis (papillary dermis) and around adnexa (periadnexal dermis) and (2) a thick zone (reticular dermis) that extends from the base of the papillary dermis to the surface of the subcutaneous fat (Figs. 1.1 and 1.2). The combination of papillary and periadnexal dermis has been termed the adventitial dermis. It is typified by thin collagen bundles arranged haphazardly, delicate branching elastic fibers, plentiful fibrocytes, abundant ground substance, and a highly developed circulation made up mostly of capillaries. The papillary dermis and the epidermis together form a morphologic and functional unit, just as does adnexal epithelium and adjacent dermis, the interrelatedness being reflected in their alteration, jointly, in various inflammatory processes, for example, interface dermatitides, such as erythema multiforme and lichen planus; spongiotic dermatitides, such as allergic contact dermatitis and pityriasis rosea; ballooning dermatitides, such as farmyard pox (milker’s nodule and orf) and fixed drug eruption; and psoriasiform dermatitides, such as psoriasis and pityriasis rubra pilaris.
Papillary and periadnexal dermis, together, are called the adventitial dermis. They have a similar appearance and function, in contrast to that of the bulk of the dermis, namely, the reticular dermis.
Papillary dermis is composed of thin bundles of collagen arrayed haphazardly, in contrast to the deeper reticular dermis, which is made up of thick bundles of collagen arranged in orthogonal pattern. A capillary can be seen in each papilla.
The reticular dermis is formed mostly of thick bundles of collagen arranged in orthogonal pattern. Elastic fibers course among those bundles. Proportionally fewer fibrocytes and blood vessels, and less ground substance, are present in the thick reticular dermis than in the thin adventitial dermis. Into the reticular dermis in broad vertical columns often extend adipocytes from the subcutaneous fat, these enveloping eccrine units and terminating at the base of hair follicles. Fascicles of striated muscle are numerous in the subcutis of the face and neck.