Epidermal cross /3-forms

Fig. 2. A high-power view of the basement membrane zone. The lamina densa (D) is composed of poorly delineated cord structures that vary from 3 to 8 nm in diameter. Strands from the lamina densa (arrow) are seen to cross the lamine lucida (L) to the surface of the epidermal cells (Ep). Careful examination of the lamina densa has shown the presence of 8-nm hollow tubes (see circled structure in the lower right quadrant) the basotubules. These are formed of pentagonal units stacked one over the other and are believed to contain amyloid P. The insets at the lower left of the figure show double peg structures (arrows) that occur throughout basement membranes. Courtesy of Dr. S. Inoue and Dr. C. P. Leblond, McGill University.

Intrinsic tyrosine kinases are covalently incorporated in the intracellular domains of some receptors, such as epidermal growdh factor receptor and the insulin receptor. When such receptor tyrosine kinases dimerize, cross-phosphorylation occurs. The phosphorylated tyrosines in activated receptor tyrosine kinases serve as docking sites for SH2 domains present in numerous signaling proteins and permit further propagation of the signal. A prominent component of such pathways is the small GTPase Ras. The Ras protein, like the G subunit, cycles between an inactive form bound to GDP... 

Pig. 46. —I, Epidermis of oak leaf 2, epidermis of Iris leaf, both viewed from the surface 3, group of cells from petal of Viola tricolor 4, two epidermal cells in cross-section showing thickened outer wall differentiated into three layers, namely, an outer cuticle, cutinized layer (shaded), and an inner cellulose layer 5 and 6. epidermal outgrowths in the form of scales and haif.s. (i, 2, 6 after Stevens, 3 after Strasbur ger, 4 after Sachs, and S after de Bary.)... 

Fig. 95.—Cross-section of a mature Uly anther. The pairs of pollen chambers unite to form two pollen sacs, filled with pollen grains s, mbdified epidermal cells at line of splitting. From a Text-book of Botany by Coulter, Barnes, and Cowles. CopyrightJby the American Book Company, Publishers.)...

The epidermis Is a tough outer layer of tissue, which acts as a water-tight barrier to prevent desiccation and serves as a protection against abrasion. In epidermal cells, bundles of keratin filaments are cross-linked by filaggrin, an IFAP, and are anchored at their ends to desmosomes. As epidermal cells differentiate, the cells condense and die, but the keratin filaments remain intact, forming the structural core of the dead, keratinized layer of skin. The structural Integrity of keratin Is essential in order for this layer to withstand abrasion. 

Figure 6.2 Discinisca cf. tenuis, same stage as in Fig. 6.1 A. (A) Reconstruction of a larval seta from serial cross-sections. Only two cells accompany the larval seta (Is). The chaetoblast (ch) secretes most of the setal material, but the adjacent epidermal cell (Epi) is involved in spine formation (arrow) (B-E) cross-sections of the seta (TEM-micrographs), corresponding to the planes B-E In Fig. 6.2A. Notice the basal structures of the rudimentary clllum (cl) In B (not shown In the drawing), the spine formation (arrow) in C, and the narrow bridge of cytoplasm (arrowheads), connecting the seta-forming part of the chaetoblast (ch) with the rest of the cell in E. ECM = extracellular matrix, za = zonula adhaerens. Scale bars A = 2 gm, B-E= I gm.

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