Case Study: Spermatozoa And Its Arrival To The Egg

DISCUSSION
Spermatozoa and its Arrival to the Egg
Before embarking in the complexity of CatSper channels, the particular characteristics of sperm and its movement towards the egg in female that allows for fertilization should be addressed. Spermatozoa are differentiated motile cells that have two main structures: the head, which houses packed DNA, and the flagellum, the motility portion that transports the sperm genetic information to the egg (1). Within the spermatozoa head both a vesicle which covers the head, known as the acrosome, and the nucleus reside; whereas the flagellum (tail) is divided by the principal and end piece (4). It is in this principal flagellum piece that we can localize the CatSper channels (2,4,5). Moreover, between
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As previously mentioned, CatSper channels are activated by Ca2+ concentrations; thus, it is crucial to understand the structure in which this ion makes its way to the sperm cytoplasm. The CatSper channel is composed of a transmembrane protein that contains 4 non-identical alpha (α) subunits and thus form a heterotetrameric Ca2+ channel and its pore (4). The first discovered subunit was termed the CatSper1 channel which was found exclusively in sperm cells and thus linked its function to sperm’ unique purpose: fertilization (2). Later on, CatSper 2-4 and their "helper" subunits known as CatSper β (beta), CatSper γ (gamma), and CatSper δ (delta), all known as auxiliary channels, have been identified (1, 3, 4, 9, 10). Within each CatSper channel (1 through 4), there are six transmembrane sections divided into a sensory domain and a pore-creator domain (4). It is also important to identify the structure of the auxiliary channels. The CatSper β channel was first discovered by databases and reverse transcriptase methods (4). The CatSper β channel has two transmembrane sections along with a cytoplasmic small domain and a extracellular large domain (9). On the other hand, both the CatSper γ channel(10) and the CatSper δ channel(3) are composed of one transmembrane section each that …show more content…
One of the most important signaling messengers is Ca2+ (13). These ions come to regulate and influence variations in localization, relationships, and distinct cell roles (13). Therefore, it is essential for the purposes of the CatSper channel investigation to understand why and how intracellular concentration of Ca2+ have an effect in sperm. Ion pumps usually regulate intracellular concentrations of calcium and protons in the sperm cell (14). Intracellular resting concentration of calcium in the sperm is lower (100-200 nM) than in its surrounding extracellular medium (1-2 mM) (15, 16). Ca2+ concentrations are regulated to be lower inside the sperm (14). Within the sperm functionality in the female reproductive tract environment, it appears that acidity and changes in pH are also regulated by H+ channels and pumps (4). Hydrogen ions (H+) concentration is constantly greater in the cytoplasm of the sperm cell with a pH of 0.4-1.0 while in the testes (15,17). When ejaculation occurs, the spermatozoon is present to an alkaline pH in the female reproductive tract (17) compared to that when it was stored in the epididymis of the testes (18) and thus has a lower relative intracellular pH. During the movement through the female reproductive tract, the intracellular pH in the sperm increases as a consequence of sperm capacitation where there is a release of acidic contents (19). However,

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