A Tale of Two Gametes -

First stages of division of mammalian embryo. Semidiagrammatic. (From a drawing by Allen Thomson.) z.p. Zona striata. p.gl. Polar bodies. a. Two-cell stage. b. Four-cell stage. c. Eight-cell stage. d, e. Morula stage.

Henry Gray (1825–1861). Anatomy of the Human Body. 1918.

August 2nd, 2013

Our life’s journey is part of a cycle that is a fantastic and fascinating story of complex biology. Perhaps the part of that journey that we are most aware of is the beginning. We all start our journey as an egg (or oocyte [pronounced “oh-oh-sight”]) in our biological mother’s ovary.

The egg is surrounded by many hundreds of smaller support cells called follicular cells in a round structure known as a follicle. As the follicle matures it migrates to the outside surface of the ovary and is greeted by the infundibulum, the trumpet shaped open end of the Fallopian tube. Under the influence of hormonal stimulation the oocyte bursts through the ovarian surface and is entrapped by the infundibulum. Those of us who are the product of in vitro fertilization (test tube babies) had a slightly different beginning, one with the surgical removal of Mom’s egg into a dish filled with Dad’s sperm.

The tube is a transport system for eggs and sperm carrying the one to the other and then both to the uterus. Meanwhile, Dad’s sperm are in several states of readiness in preparation for fertilization. Once mature the sperm can exist in a storage state. They can be in a repressed state or a sort of holding pattern for sperm. They can be fertile and finally they can be inactive.

Mature sperm cells (spermatozoon) are not immediately capable of fertilizing the egg. To have that capacity sperm must first be capacitated. Capacitation is the process by which the mature sperm becomes competent to fertilize the egg. Ejaculated sperm become capacitated in the female genital tract and exist in a fertile state; however, this state is not permanent, and the sperm will eventually become inactivated.

Capacitation results in a series of changes critical to the sperm’s ability to fertilize. First the sperm become hypermotile and just as the name implies this generates force and velocity to the sperm’s swimming ability and allows the sperm to penetrate the cloud of follicular cells that surround the egg after ovulation.

That cloud of cells is arrayed in a complex matrix that includes a common chemical substance found in our bodies called hyaluronic acid, a kind of gluey material sticking cells together. The capacitated sperm releases hyaluronidase from its cap region through fenestrations or holes in its outer member made during the acrosome reaction. Hyaluronidase is a protein enzyme (enzymes are proteins that catalyze specific chemical reactions) that breaks hyaluronic acid down and allows the sperm to get to the egg.

The acrosome reaction leads to vesiculation of the outer membrane of the sperm and release other enzymes as well, like acrosin for example. Importantly though, the membrane is not completely removed and what is left is available to bind the sperm to the outer surface of the egg, the first step in fertilization.

How about the other states of sperm?

Repressed sperm are associated with factors in the seminal fluid that inhibit premature capacitation. Such factors, most likely associated with the glycocalyx, a complex chemical substance on the outer surface of cells include a seminal decapacitating factor, spermine, that reversibly binds to the sperm head.

For more information on fertilization, visit Colorado State University’s Fertilization and Early Embryonic Development site.

And this great review article in the Journal of Clinical Investigation by Masahito Ikawa.

An overview:

• Fertilization is a complex dance of well-timed and complex events played out in two halves.
• The first half is the egg (or oocyte) which gets surrounded by follicular cells.
• Under stimulation by hormones the oocyte bursts through the ovary and into the infundibulum which is the opening of the Fallopian tube.
• The other half of the story is the spermatozoon. The sperm must be activated before they can fertilize the egg.
• To penetrate the cloud of follicular cells around the egg, the sperm begin to secrete enzymes that break down the cloud of cells.
• But sperm do not remain activated forever, and those that are not capacitated remain in a repressed state.

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