Читать книгу Cell Biology - Stephen R. Bolsover - Страница 52

In the days leading up to ovulation, oocytes develop within structures called follicles, in which they are connected to surrounding granulosa cells by gap junctions. During their development oocytes are not yet themselves capable of performing several fundamental homeostatic processes, such as regulating intracellular pH. However, the surrounding granulosa cells have ample ability to regulate pH, and H⁺ ions can pass through the gap junctions, such that the granulosa cells effectively regulate the pH of the oocyte on its behalf. By the time the oocyte is fully grown and ready to be ovulated it can finally regulate its own pH, at which time it jettisons the granulosa cells and becomes ready to be fertilized by a spermatozoon.

Figure 2.7. Not all connexins are compatible. A √ indicates a working gap junction, x indicates that gap junction channels cannot form.

There are over 20 different members of the connexin gene family in humans. Each can, if made in a population of cells, generate complete, working gap junction channels of 12 identical connexins. However, not all connexin isoforms are compatible. A cell with connexin 43 cannot form gap junctions with another cell that makes connexin 50 (Figure 2.7). However, a connexon made from connexin 43 is able to form a good working channel if it meets a connexon made from connexin 46, and a connexon made from connexin 46 is in turn compatible with connexin 50.

Anchoring junctions bind cells tightly together and are found in tissues such as the skin and heart that are subjected to mechanical stress. They are described later (page 229).

Answer to thought question: This can only be speculation, but one obvious effect of cells having incompatible connexin isoforms is that the intracellular route for cell–cell communication is lost. Consider a tissue whose cells make connexin 43 that lies adjacent to another tissue whose cells use connexin 50. The cells of each tissue can coordinate their activity by passing signals via gap junctions, for example by intracellular messengers (page 166) or as voltage changes. However, the signals will remain private to each tissue and will not be shared with the neighboring tissue. The two tissues can still communicate when necessary, for instance by transmitter chemicals that the cells release into the extracellular medium (see Chapters 10 and 11).