Question

1.  Successful reproduction requires the precise coordination multiple, diverse processes in time and space. Similar to other species, these diverse processes are often regulated by a single factor.

A. Describe the multiple purposes and processes LH are involved in within the reproductive tract that permits the successful meeting of sperm and newly ovulated oocyte though the time the sperm is in proximity to the cumulus oophorus.

B. Describe the when, where and how of the multiple critical processes calcium induces or is involved in (i) within the reproductive tract and (ii) after the sperm attaches to the oocyte and (iii) after it enters the cytoplasm.

Answer 1

Over the nucleus of each mammalian sperm is a membranous sac known as the acrosome, which is filled with many kinds of hydrolytic enzymes. In the female reproductive tract or in an IVF medium, sperm undergo capacitation, which permits the acrosome reaction. Near the eggs, probably stimulated by the cumulus cells and the ZP, sperm release their acrosomal contents by exocytosis and penetrate the ZP. Only acrosome-reacted sperm fuse with eggs, but their competency for fusion does not last long. Cumulus cells are packed together by hyaluronic acid at ovulation and become diffuse during fertilization. PVS, perivitelline space.

Mammalian sperm have to travel a long distance through the female reproductive tract to the oviduct, in which fertilization takes place. To accomplish the journey to the egg, sperm are equipped to overcome various obstacles that lie ahead, such as navigating the uterotubal junction (UTJ) and penetrating the egg extracellular matrices. After passing through the UTJ, sperm are held on the surface of mucosal folds in the isthmus and remain there until the time of ovulation draws near. During sperm storage, the isthmic epithelium creates a microenvironment that delays capacitation and stabilizes sperm for a period of approximately 24 hours, at least in humans. When ovulation draws near, unknown female factors trigger the sperm to leave the reservoir and move up to the ampulla. Release of sperm from the isthmic epithelium is reported to depend mainly on sperm changes that are associated with capacitation.

After leaving the storage reservoir, sperm move into the ampulla and locate the cumulus-cell oocyte complex (COC). The COC is comprised of ovulated eggs covered by the ZP and a multicellular cumulus oophorus. Sperm chemotaxis is implicated in locating the COC. Soon after interacting with COCs, sperm penetrate the matrix of the cumulus oophorus, which is rich in proteins and carbohydrates such as hyaluronan, an unsulfated glycosaminoglycan. In marsupials, eggs shed their cumulus layers just before ovulation, and the cumulus-free eggs are fertilized in vivo.

The acrosome is a Golgi-derived exocytotic organelle that covers the tip of the sperm head. Acrosomal exocytosis, the so-called acrosome reaction, happens only in capacitated sperm and is a prerequisite for a sperm to fuse with an egg. The mechanism of the acrosome reaction itself has been well characterized. In brief, transient calcium influx leads to activation of phospholipase C (PLC), and activated PLC generates IP3 and diacylglycerol (DAG) from PIP2. IP3 releases calcium from intracellular stores, and DAG mediates PKC activation and phosphorylation of substrate proteins. These early events promote a subsequent calcium influx via transient receptor potential cation channels (TRPCs), which induces the complete acrosome reaction. Disruption of Plcd4 impairs the in vitro ZP-induced acrosome reaction, while the A23187 ionophore-induced acrosome reaction occurs normally.

After passing through the cumulus oophorus, sperm encounter the ZP, their last hurdle before meeting the egg. After penetration of the ZP, sperm immediately meet and fuse with the egg plasma membrane.  During the acrosome reaction, Izumo1 relocates from the anterior head of the sperm to the site(s) in which fusion will take place.