Clinical Review

Hair Follicle Bulb Region: A Potential Nidus for the Formation of Osteoma Cutis

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Osteoma cutis (OC) is an extraneous ossification of the skin. Heterotopic ossification can be either primary or secondary, depending on the presence of a preexisting lesion. Little is known about the morphogenesis of OC. During routine dermatopathologic examinations for unrelated conditions, small osteocalcific micronodules were noted in close approximation of the lower aspect of hair follicles. In most instances, osteocalcific lesions were found near a hair bulb and exceptionally within the hair bulb. Small osteocalcific nodules incidentally noted in close proximity to the lower aspect of the hair bulb may be a precursor to OC. They may form near or within the hair bulb, possibly under the influence of bone morphogenetic proteins (BMPs).

Practice Points

  • Understanding the pathogenesis of osteoma cutis (OC) can help physicians devise management of these disfiguring lesions.
  • Small osteocalcific nodules in close proximity to the lower aspect of the hair bulb may be an important precursor to OC.


 

References

The term osteoma cutis (OC) is defined as the ossification or bone formation either in the dermis or hypodermis. 1 It is heterotopic in nature, referring to extraneous bone formation in soft tissue. Osteoma cutis was first described in 1858 2,3 ; in 1868, the multiple miliary form on the face was described. 4 Cutaneous ossification can take many forms, ranging from occurrence in a nevus (nevus of Nanta) to its association with rare genetic disorders, such as fibrodysplasia ossificans progressiva and Albright hereditary osteodystrophy.

Some of these ossifications are classified as primary; others are secondary, depending on the presence of a preexisting lesion (eg, pilomatricoma, basal cell carcinoma). However, certain conditions, such as multiple miliary osteoma of the face, can be difficult to classify due to the presence or absence of a history of acne or dermabrasion, or both. The secondary forms more commonly are encountered due to their incidental association with an excised lesion, such as pilomatricoma.

A precursor of OC has been neglected in the literature despite its common occurrence. It may have been peripherally alluded to in the literature in reference to the miliary form of OC.5,6 The cases reported here demonstrate small round nodules of calcification or ossification, or both, in punch biopsies and excision specimens from hair-bearing areas of skin, especially from the head and neck. These lesions are mainly observed in the peripilar location or more specifically in the approximate location of the hair bulb.

This article reviews a possible mechanism of formation of these osteocalcific micronodules. These often-encountered micronodules are small osteocalcific lesions without typical bone or well-formed OC, such as trabeculae formation or fatty marrow, and may represent earliest stages in the formation of OC.

Clinical Observations

During routine dermatopathologic practice, I observed incidental small osteocalcific micronodules in close proximity to the lower part of the hair follicle in multiple cases. These nodules were not related to the main lesion in the specimen and were not the reason for the biopsy or excision. Most of the time, these micronodules were noted in excision or re-excision specimens or in a punch biopsy.

In my review of multiple unrelated cases over time, incidental osteocalcific micronodules were observed occasionally in punch biopsies and excision specimens during routine practice. These micronodules were mainly located in the vicinity of a hair bulb (Figure 1). If the hair bulb was not present in the sections, these micronodules were noted near or within the fibrous tract (Figure 2) or beneath a sebaceous lobule (Figure 3). In an exceptional case, a small round deposit of osteoid was seen forming just above the dermal papilla of the hair bulb (Figure 4).

Micronodule of osteoid without mineralization next to a hair bulb

Figure 1. Micronodule of osteoid without mineralization next to a hair bulb with an osteoblastic rim (H&E, original magnification ×10).


Osteocalcific micronodule

Figure 2. Osteocalcific micronodule within the fibrous sheath of the hair follicle (H&E, original magnification ×10).

Calcific micronodule

Figure 3. Calcific micronodule beneath a sebaceous gland (H&E, original magnification ×10).

osteoid formation

Figure 4. An exceptional observation demonstrated the beginning of osteoid formation at the junction of matrix epithelium and papilla, where bone morphogenetic protein–assisted cross-talk aimed at regulating the hair cycle transpires (H&E, original magnification ×20).

Multiple osteocalcific micronodules were identified in a case of cicatricial alopecia. These micronodules were observed in sections taken at the levels of hair bulbs, and more or less corresponded to the size of the bulb (Figure 5A). Fortuitously, the patient was dark-skinned; the remnants of melanin within the micronodules provided evidence that the micronodules were formed within hair bulbs. Melanin staining confirmed the presence of melanin within some of the micronodules (Figure 5B).

osteocalcific micronodule

Figure 5. A, A section from subcutaneous tissue revealed an osteocalcific micronodule and an adjacent hair bulb with similar size and shape (H&E, original magnification ×20). B, Melanin stain of the osteocalcific micronodule and adjacent hair bulb (Fontana-Masson, original magnification ×40).

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