J Pathol Transl Med.  2016 Sep;50(5):361-368. 10.4132/jptm.2016.06.19.

CD99 Is Strongly Expressed in Basal Cells of the Normal Adult Epidermis and Some Subpopulations of Appendages: Comparison with Developing Fetal Skin

• Choi G
• Roh J
• Park CS

• KMID: 2353598
• DOI: http://doi.org/10.4132/jptm.2016.06.19

Abstract

BACKGROUND
CD99 is a cell surface transmembrane glycoprotein expressed in various tissues. CD99 is differentially expressed between subpopulations of each tissue and is highly expressed in certain hematopoietic and precursor cells. However, there has been no comprehensive study of CD99 expression in normal skin. We evaluated CD99 expression in normal human skin and developing fetal skin.
METHODS
Seventy-five adult skin samples containing normal skin and eight fetal skin samples of different gestational ages were collected. CD99 immunohistochemical staining was performed to evaluate expression pattern in adult and fetal skin samples. CD99 and CD34 expression were compared by double immunofluorescence.
RESULTS
In normal adult skin, CD99 was strongly expressed in the membrane of epidermal basal keratinocytes, hair follicle bulges and outer root sheaths, and inner secretory cells of eccrine sweat glands. In fetal skin, CD99 was not expressed on the periderm at 16 weeks of gestation but was expressed in basal cells of fetal skin at around 19 weeks of gestation. CD99 expression became comparable to that of the adult skin after 20 weeks of gestation. CD99 and CD34 were co-expressed in hair follicle outer root sheaths, as seen by double immunofluorescence study.
CONCLUSIONS
This is the first study examining CD99 expression pattern in normal adult and fetal skin. CD99 tends to be expressed in the basal/precursor cells of epidermis and in hair follicles. These results provide a basis for future investigation on functions of CD99 in the skin and provide a novel potential target for the treatment of dermatologic lesions.

Keyword

CD99 protein; Skin; Immunohistochemistry

MeSH Terms

Adult*
Epidermis*
Fluorescent Antibody Technique
Gestational Age
Glycoproteins
Hair Follicle
Humans
Immunohistochemistry
Keratinocytes
Membranes
Pregnancy
Skin*
Sweat Glands
Glycoproteins

Figure

• 

  Fig. 1. CD99 expression in normal adult epidermis and hair follicles. (A) Overall CD99 expression pattern in normal adult skin. (B) Basal cells of the epidermis show strong immunopositivity for CD99. Epidermal Langerhans cells are also strongly immunopositive for CD99 (arrow). (C) Basaloid cells in the bulge, the insertion of the arrector pili muscle, show high CD99 expression. (D) In a terminal anagen hair follicle, the outer root sheath cells are strongly positive for CD99 stain. The inner root sheath cells are weakly positive for CD99 stain. O, outer root sheath; I, inner root sheath.

• 

  Fig. 2. CD99 expression in the normal adult sweat glands and sebaceous glands. (A) In apocrine glands, flattened lining cells are immunonegative for CD99 (left). Meanwhile, tall cells in apocrine glands express CD99 (right). (B) Inner small cells of eccrine glands express CD99 in opposition to outer large cells (arrow). Apicoluminal surface of the eccrine glands are immunonegative for CD99. (C) Two layers of excretory ductal cells are immunopositive for CD99. (D) In sebaceous glands, outer basal cells express CD99. However, mature sebocytes do not express CD99.

• 

  Fig. 3. CD99 expression pattern during normal development of the hair follicles. (A) At 16 weeks of gestation, only mesenchymal cells around the hair germ express CD99 (arrow). (B) CD99 positive epidermal basal cells appear at 19 weeks of gestation. Basaloid cells in the bulge and basal cells of the outer root sheath and lower inner root sheath also express CD99. (C, D) CD99 expression pattern is identical to that of adult skins at 20 and 25 weeks of gestation.

• 

  Fig. 4. CD99 and CD34 expression pattern by double-immunofluorescence staining in normal adult epidermis and hair follicles. (A) Overall expression pattern of CD99 (red) and CD34 (green). Epidermal basal cells (arrowhead) (B) and basaloid cells (C) in the bulge only express CD99 but not CD34. (D) Some cells in the lower outer root sheath show co-expression of CD99 and CD34 (arrow) (horizontal section).

• 

  Fig. 5. CD99 and CD34 expression pattern by double-immunofluorescence staining in the fetal epidermis and hair follicles at 20 weeks of gestation. (A) Overall expression pattern of CD99 (red) and CD34 (green). Dermal papilla expresses both CD99 (red) and CD34 (green) (arrow). (B, C) Epidermal basal cells and basaloid cells in the bulge (arrowhead) express only CD99.

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