« 1 is not present in epidermal kératinocytes (Zambruno et al. 1991, Klein et al.
1990). However, «i is present in the outer root sheath of hair follicles (Stamp and Pignatelli 1991). In contrast to previous reports Hertle et al. (1991) described faint expression of «i in neonatal epidermis.
« 2 is strongly expressed pericellularly on basal kératinocytes in skin (Klein et al.
1990, Zutter and Santoro 1990, Zambruno et al. 1991, Watt and Jones 1993).
Zambruno et al. (1991) noted that « 2 expression was stronger along the apical and
lateral cell margins than the basal and that some suprabasal expression was present.
Laijava (1991) using confocal laser scanning demonstrated expression in an intense
dot like manner between basal cells with little expression along the basal surface of
basal kératinocytes. Zutter and Santoro (1990) remarked on the high levels of CK2B1 in
proliferating epithelium and this may suggest that CK2B1 plays a role in orderly regulated
cell proliferation. In culture conditions that allow stratification of human kératinocytes,
(X2R1 polarized to the lateral domains of the plasma membrane both in growing colonies
and reconstituted epithelium (Marchisio et al. 1991).
«3, like « 2 is expressed pericellularly in basal kératinocytes (Watt and Jones
1993). Zambruno et al. (1991) also noted similar « 3 8 1 and 0 1 2 ^ ^ 1 expression in skin but
reported stronger « 3 6 1 expression suprabasally. Both ot2^^ and « 3 6 1 are components of
focal adhesions in kératinocytes (Carter et al. 1990a).
« 4 is not expressed in epidermal kératinocytes (Staquet et al. 1990, Peltonen et
al. 1989, Klein et al. 1990).
« 5 shows faint pericellular expression on basal kératinocytes in neonatal
epidermis (Hertle et al. 1991). Different workers have reported differences in
expression of « 5 in kératinocytes. Peltonen et al. (1989) detected « 3 8 1 in the hair
follicles of foetal skin but not in adult or neonatal epidermis. Laijava et al. (1991)
however reported « 5 8 1 in association with hair follicles whereas Klein et al. (1990)
found no expression of « 5 8 1 on kératinocytes.
« 6 is expressed at the basal pole of basal kératinocytes after stratification (Hertle
Introduction
et al. 1991). A strong continuous line of was demonstrated along the basement
membrane zone (Zambruno et al. 1991). It was also noted that hair follicles in
sebaceous and sweat glands expressed but that expression on the lateral cell surfaces
was less than at the basement membrane zones.
«V is expressed pericellularly but it is weaker and more diffuse than other integrins in skin (Hertle et al. 1991).
Bi is expressed in a similar manner to those oc subunits that it partners.
B4 is present as a continuous line at the basal aspect of the basal cells (Kaiji et
al. 1989, DeLuca et al. 1990).
The expression of the oiy integrins and is discussed in more detail in
Sections 3.1 and 5.1.
1.4.2 Integrin Expression in Oral Epithelium
The overall pattern of Bi integrin expression in the oral mucosa is similar to that
in skin. There is strong «3» «3» «6» and B4 expression and weaker and more variable
expression of «y and « 5 , but there is no expression of « 4 or a, (Jones et al. 1993).
Within non-keratinised buccal mucosa, the subunits are strongly expressed in the basal and immediately suprabasal layers with weaker and patchy expression extending into the
prickle cell layers (Jones et al. 1993, Jones et al. 1995a). Occasional « 3 expression is
seen in the superficial layers, and 8 4 are expressed in a linear pattern at the
basement membrane zone, whereas «3, « 3 and 15i are expressed in a pericellular manner,
tty is expressed in a pericellular manner in the basal layer with occassional expression
in the suprabasal layers (Jones et al. 1993, Jones et al. 1995a). Laijava (1991) also
reported strong pericellular expression of «3, «3, and Bj in the basal layer of keratinised
oral epithelium with occasional weaker expression in the lower prickle layer.
The pattern of integrin expression at other sites in the mouth, such as hard palate, floor of mouth and the lateral border of the tongue was examined by Jones et
al. (1993) and was essentially similar to that seen in buccal mucosa. However,
expression of «3, %, and Bj extended through more layers in the floor of the mouth.
Introduction
strong linear staining of and H4 along the basement membrane zone (Hormia et al.
1990).
1.4.3 Integrin Expression in Cultured Kératinocytes
Epidermal kératinocytes express «2^1» «3^1» «5 ^ 1 » «6 ^ 4 and ayB^and occasionally
(Adams and Watt 1991). «vUg is also present in culture (D. Sheppard-personnal communication).
1.4.4 Integrin Functions in Kératinocytes
1.4.4.1 Adhesion
Integrins play a role as adhesion molecules as discussed in Section 1.2.5
1.4.4.2 Stratification
When basal kératinocytes become committed to terminal differentiation there is a marked reduction in their ability to adhere to extracellular matrix proteins and this ensures their selective migration from the basal layer into the suprabasal layers (Stanley et al. 1980, Watt 1984). Integrin-mediated interactions with the extracellular matrix have a role in linking the initiation of terminal differentiation and migration out of the basal layer (Adams and Watt 1989). The loss of adhesiveness precedes the loss of integrins from the cell surface (Adams and Watt 1990), in particular adhesiveness to fibronectin, laminin and types I and IV collagen is lost (Adams and Watt 1990) as cells differentiate. The loss of integrins on the surface of cells that have left the basal layer reflects both transcriptional and post-translational regulation of integrin expression during terminal differentiation. The decrease is reflected in mRNA levels, as shown by Northern blotting of cultured kératinocytes (Nicholson and Watt 1991).
1.4.4.3 Commitment to Terminal Differentiation
When kératinocytes are plated on an adhesive substrate that restricts spreading, terminal differentiation is stimulated (Watt et al. 1988) and loss of contact with the extracellular matrix may be the primary differentiation stimulus in suspension (Adams
Introduction
and Watt 1989). Inclusion of fibronectin or antibodies to the subunit in methyl
cellulose at the time of plating out cells inhibits suspension-induced terminal differentiation (Adams and Watt 1989, Watt et al. 1993). Laminin and type IV collagen also participate in the inhibition of terminal differentiation, as can a cocktail of function
blocking antibodies to the «2, « 3 and « 5 subunits (Watt et al. 1993). Thus terminal
differentiation may be regulated by the total proportion of Bi heterodimers occupied by ligand, with a decrease acting as a stimulus for differentiation.
1.4.4.4 Stem Cells
It is possible to identify stem cells within the epidermis on the basis of their B^
integrin expression and their rapid adhesion to fibronectin, type IV collagen or
keratinocyte extracellular matrix proteins (Jones and Watt 1993). Transit amplifying cells adhere more slowly to the matrix proteins and express lower levels of Bj integrins. These cells divide 1-5 times and then initiate involucrin expression. One implication of these results is that if a reduction in the number of Bj integrins with bound ligand is a stimulus for terminal differentiation (Adams and Watt 1989), transit cells will be more sensitive to that stimulus than stem cells because they have fewer surface integrins.