The Typical and Atypical Centrioles and Their Potential Roles in the Sperm and Embryo

The embryo of humans and other mammals requires the presence of two subcellular structures, named centrioles. The centrioles act as nucleation points for two massive structures that help build the skeleton of the cell, the centrosome, and the cilium. Centrosomes nucleate and anchor star-shaped arrays of microtubules known as asters that assist in cell division. Cilia (known as the flagella in sperm cells) are hair-like structures that mediate cell movement and signaling. Two centrioles are maintained in each cell of the developing embryo by precisely duplicating and segregating during each cell cycle. It is accepted that the mature oocytes of mammals do not contain centrioles, and therefore do not contribute centrioles to the embryo. Similarly, it is accepted that, in humans and other non-murine mammals, the mature sperm has one centriole with a typical structure, which is contributed to the embryo during fertilization. But, how the embryo acquires its first two centrioles, or what their precise role in the early embryo is, is only now starting to be revealed. Recently, it was discovered that human and bovine sperm also have a second, atypical centriole, and this centriole is functional in the embryo. Therefore, the typical and atypical centrioles of the sperm appear to be the first two centrioles of the embryo. Here, we will focus on sperm centrioles in humans and other mammals, their formation during spermatogenesis, and their role in the embryo.