Basal lamina, collagen

Basal laminas are specialized areas of the ECM that surround epithelial and some other cells (eg, muscle cells) here we discuss only the laminas found in the renal glomerulus. In that strucmre, the basal lamina is contributed by two separate sheets of cells (one endothelial and one epithelial), each disposed on opposite sides of the lamina these three layers make up the glomerular membrane. The primary components of the basal lamina are three proteins—laminin, entactin, and type IV collagen—and the GAG heparin or heparan sulfate. These components are synthesized by the underlying cells. 

The cornea is the first structure of the eye to be in contact with incident light. It is composed of five distinct layers lying parallel to its surface the outer epithelium, which is continuous with the epithelial layers of the conjunctiva the epithelial basal lamina the keratocyte-containing stroma, which is a collagen structure arranged so that it is transparent Descemet s membrane and, finally, the endothelium adjacent to the aqueous humour. 

FIGURE 1-19 A myelinated PNS axon (A) is surrounded by a Schwann cell nucleus (N). Note the fuzzy basal lamina around the cell, the rich cytoplasm, the inner and outer mesaxons (arrows), the close proximity of the cell to its myelin sheath and the 1 1 (celhmyelin internode) relationship. A process of an endoneurial cell is seen (lower left), and unstained collagen (c) lies in the endoneurial space (white dots). X20,000. 

The second class of AChEs exists as heteromeric assemblies of catalytic and structural subunits. One form consists of up to 12 catalytic subunits linked by disulfide bonds to filamentous, collagen-containing structural subunits. These forms are often termed asymmetric, since the tail unit imparts substantial dimensional asymmetry to the molecule. The collagenous tail unit links by disulfide bonding at its proline rich N-terminus through a coiled coil arrangement to the C-terminus of two of the catalytic subunits [30]. The tail unit associates with the basal lamina of the synapse rather than the plasma membrane. 

The ECM that surrounds cells also contains variable levels of HA. It is composed predominantly of structural proteins such as collagen and elastin, as well as proteoglycans, and a number of glycoproteins. The HA content is greatest in embryonic ECM, and in tissues undergoing rapid turnover and repair. The basal lamina or basement membrane that separates dermis and epidermis is also considered an ECM structure. The basal lamina contains HA, though the precise structural position is not known. Loss of basement membrane HA in the skin of diabetic patients correlates with skin stiffness.50... 

The vitreous humor is a viscoelastic connective tissue composed of small amounts of glycosaminoglycans, including hyaluronic acid, and of such proteins as collagen.The collagen fibrils are anchored directly to the basal lamina, which forms the boimdaries of the lens, the ciliary body epithelium, and the neuroglial cells of the retina. Although the anterior vitreous is cell free, the posterior vitreous contains a few phagocytic cells, called hyalo-cytes, and is sometimes termed the cortical tissue layer. 

Figure 2 Life cycle of ChEs. AChE is synthesized as a monomer globular form (Gi). Up to 80% is degraded by intracellular processes. Secretory forms are separated from membrane-bound forms, collagen tails are added to asymmetric forms, and the enzyme is glycosylated and becomes enzymatically active. After secretion, globular forms may escape into the body fluids, while asymmetric forms are bound to the synaptic basal lamina. (Modified from Brimijoin WS (1992) US ERA Workshop on Cholinesterase Methodologies.)...

Fig. 3.2 Relationship of the connective tissue stroma to an epithelium. Fibroblasts secrete unique types of laminin proteins that interact with each other and type IV collagen, to form a basal lamina (yellow) that is tightly attached to the connective tissue (stromal) cells and also to the associated muscle or epithelial cell. The kidney glomerulus is a specialized tissue in which the stroma is absent and endothelial cells and epithelial cells are separated by the basal lamina which filters the blood as the first step in urine collection. The attachment of an epithelium to stromal collagen is discussed in detail in Chap. 5 (Fig. 19-55 in The Molecular Biology of the Cell. B. Alberts et al., 4th Ed. 2002. Garland Science, Taylor Francis Group, NY)...

The kidney contains small amounts of fibrillar collagen, but its major collagen is a non-fibrillar network collagen (type IV), a part of basal laminas called the lamina densa (Sect. 5.1.1). Basal laminas are especially well developed in the kidney as exemplified in the glomerulus (Fig. 3.2). The collagen contents of various tissues are indicated in Table 3.2 and their fiber arrangements in Table 3.3. 

Type VII collagen is a non-fibrous, anchoring fibril that binds large type I collagen fibers in the stroma to the lamina densa section of the basal lamina. Type VII procollagen... 

Plakin proteins link intracellular keratin intermediate filaments to hemidesmosomes of basal epidermal cells, or to desmosomes on suprabasal epidermal cells (Sect. 5.2.1), whereas type XVII collagen (the only collagen secreted by epithelial cells ) and integrins attach hemidesmosomes to a basal lamina. Three p 1 integrins (with al, a3 or a5 partners)... 

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