Distal cytoplasm

The terminology in the literature is confusing. Terms used for the outer (nucleate) zone include syncytial layer, surface layer, distal cytoplasm, tegument and for the inner (nucleated) zone perikarya, proximal layer, tegumental cells, cytons, perinuclear cytoplasm. Bearing in mind that descriptive terms are more likely to be remembered and used correctly than non-descriptive words, the terms distal cytoplasm and proximal cytoplasm, which are well established in the literature, are used in this text, for these zones. The term tegumental cytons is used for the basic cells which make up the syncytial epithelium (Fig. 2.1). 

The distal cytoplasm contains an abundance of granules, membrane-bound vesicles and mitochondria (Fig. 2.1). The majority of vesicles and granules appear to have their origin in the Golgi apparatus of the tegumental cytons, from where they are passed to the distal cytoplasm. These granules probably play a major role in the formation and replace-... 

The mechanism by which Na" is reabsorbed in coupled exchange with and K+ in the collecting duct has been discussed previously that is, Na+-driven K+ secretion is partially under mineralocorticoid control. Aldosterone and other compounds with mineralocorticoid activity bind to a specific mineralocorticoid receptor in the cytoplasm of late distal tubule cells and of principal cells of the collecting ducts. This hormone-receptor complex is transported to the cell nucleus, where it induces synthesis of multiple proteins that are collectively called aldosterone-induced proteins. The precise mechanisms by which these proteins enhance Na+ transport are incompletely understood. However, the net effect is to increase Na" entry across apical cell membranes and to increase basolateral membrane Na+-K+-ATPase activity and synthesis. 

At the distal tip of the axon are voltage-gated Ca2+ channels. When the wave of depolarization reaches these channels, they open, and Ca2+ enters from the extracellular space. The rise in cytoplasmic [Ca2+] then triggers release of acetylcholine by exocy-tosis into the synaptic cleft (step (3) in Fig. 12-5). Acetylcholine diffuses to the postsynaptic cell (another... 

Mineralocorticoids act by binding to the mineralocorticoid receptor in the cytoplasm of target cells, especially principal cells of the distal convoluted and collecting tubules of the kidney. The drug-receptor complex activates a series of events similar to those described above for the glucocorticoids and illustrated in Figure 39-3. It is of interest that this receptor has the same affinity for cortisol, which is present in much higher concentrations in the extracellular fluid. The specificity for mineralocorticoids at this site appears to be conferred, at least in part, by the presence of the... 

In the 3-hour group, the width of the periodontium at the distal tension side enlarged to a size of 100 pm, resulting in relative mobility of the root and stretching of the periodontal fibers with the formation of interfiber space. The root on the pressure side approached further toward the alveolar bone. On the tension side, cytoplasms of the periodontal fibroblasts were... 

Acentric fragments often are not incorporated into daughter nuclei at anaphase, but rather appear as micronuclei in the cytoplasm separate from the cell nucleus after cell division. The appearance of such micronuclei is a convenient manifestation of earlier chromosomal breakage, with loss of the distal fragment. Micronuclei can thus be used as an index of chromosomal breakage.392 Schmid394 has developed the bone marrow micronucleus test for use in experimental animals. 

Fig. 7.2. The structure of the translocation pathway in mycelial cords. (A) Hyphae fanning out at the distal end of a cord of Phanerochaete velutina (scanning electron microscopy by A. Yarwood) (B) Internal structure of a cord of Serpula lacrymans, showing vessels and cytoplasm-filled hyphae and extracellular matrix material. (C) Diagram of the components of the translocation pathway (adapted from Cairney, 1992) V, vessel hypha f, foraging front a, anastomosis (D) A cord system in beech woodland showing both corded mycelium and diffuse growth in contact with the wood substrate.

---