Ch33aP25a Fibrofollicular
Hamartoma (mixed patterns)

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F1Ch33a (N846): One group of follicular hamartomas is characterized by a prominent participation of the perifollicular mesenchyme. Perifollicular mesenchyme was included in the definition of the adventitial dermis; the papillary dermis is the other component of the adventitial dermis but perivascular mesenchyme and the stroma of sweat glands might also be included in the category. In this fibrofollicular hamartoma, the peri-follicular mesenchyme is a tumoral component; its interface with the reticular dermis is well defined - at the right hand edge of the field, a cleft defines this interface. In the plane of this section, the ostium of the parent follicle is represented(top of the field on the left). Thin strands of small squamous cells extend from the parent and, in turn, show branching patterns. Two of the strands terminate in a bulbous expansion in which a portion of the component cells are vacuolated in the pattern of the cells of the outer root sheath. In addition, there is a central component showing the pattern of the inner root sheath. A small hair shaft is cut in cross section. For some of the other branching components, the cells of a bulbous extremity show patterns of hair bulb differentiation with an associated papilla. For some of the extremities, the patterns have a catagen-like quality. It is as if the secondary components (i.e, those terminal branches manifesting patterns of the inconstant portion of a normal follicle) cycle in a manner that is independent of the phenomena of its neighbors.

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F2Ch33a: Each of three secondary follicular units show unique patterns of follicular development. Near the bottom of the field, the patterns are anagen-like with a cross-section represented. Near the top of the field, a lobster claw-like bulb has snared a cellular papilla but the remainder of the branch has catagen-like pattern. The middle strand might be characterized as resting follicle. Each strand is outlined by coarse fibrous lamellae. The lamellae of each strand are confluent with those of its neighbor.

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F3Ch33a: This is a higher magnification of one of the secondary follicular units seen in fig. 2. As a representation in cross-section, the patterns can be compared to a cross-section of an anagen follicle near the upper extremity of the outer root sheath (and the termination of the inner root sheath). Henle’s layer (pale pink keratinized cells, of origin from the column of trichohyalin-rich cells of Huxley’s layer, forms an interface with the cells of the outer root sheath. At this level, the inner row of cells of the outer root sheath undergoes keratinization; in some areas, it is difficult to define the source of the keratinizing cells. Some of the nuclei of the vacuolated cells contain irregular, pink inclusions.

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F4Ch33a (H3400): This is a component of a variant of a fibrofollicular hamartoma. The lesion was polypoid. Multiple fibrofolliculoma-like units were spaced in an expanded, cellular fibrous stroma (as represented along the left side of the field. All the units showed immature anagen patterns (as seen in this field); most of the units were cystic. A cone of trichohyalin-rich cells extends close to the lumen on the right (only a thin layer of flattened squamous cells separate the cone from the lumen).

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F5Ch33a (H3400): A primitive hair bulb and its papilla is represented on the left. A thin basement membrane outlines the wings of the bulb; it is not identifiable along the surface of the fundus of the bulb. The elongated dark cells are interpreted as melanocytic precursors.

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F6Ch33a: The cluster of primitive cells at the level of the fundus are interpreted as trichogenic precursor cells.

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