There always seems to be a big hubbub and much focus on egg quality. However, there’s another part to this story; sperm. Sperm is a funny word and it is not often heard in public as opposed to the word egg. That would change if sperm appeared on menus like eggs: sunny side up or over easy? Nevertheless, sperm is a vital part of the fertilization question – a very import half.

Germinal tissue – that tissue that gives rise to gametes – simply divides and reduces cell volume to make sperm. It is not all that easy, but in comparison to eggs where yolk must be made, incorporated and packaged, it is a whole lot simpler. There are three stages to growing sperm cells: spermatocytes (no tails), spermatids (tails, but no action) and spermatozoa (ready to be mature sperm). All are made in a continuous stream within the testis. There are some structural modifications that occur along the line, but essentially there are three parts to a sperm: the capsule, the engine and the propeller.

The capsule carries the goods: one-half the genetic component of the nucleus of the original cell that if all goes well, will marry up with the egg and compliment it’s genetic material. The engine for the journey to the center of the egg is composed of little furnaces called mitochondria that churn out ATP – the gasoline of a cell. The mitochondria are packed in at the bottom of the capsule and physically join the propeller to the capsule. The propeller’s proper name is flagellum and it is essential a whip that oscillates back a forth to propel the sperm forward to conjugal bliss.

When sperm are undergoing construction and maturation, it’s a little like making hand-loaded shotgun shells in the basement: the propellant must remain stable and quiet or the goods are delivered prematurely and in vain. To keep the sperm in a stable and quiet state, the testis provides conditions that block activation and energization. These conditions are: osmotic pressure, ion concentration and pH.

Osmotic pressure can be envisioned as the ability of a salt solution to draw or pull water. Classically it’s the force a dissolved substance puts on a semi permeable membrane to draw in a solvent. Think of it as those little bags of silica that absorb moisture. Ion concentration is the amount of salt present, and is closely related here to the osmotic pressure. The pH is again ions, so the picture here is that the sperm are inactivated within the testis by local conditions that are high in salts.

Once the goods have been prepared and are looking for a place to party, seminal fluid has to be made as part of the final prep. Seminal fluid contains a large amount of dissolved salts, particularly sodium and potassium. There are some organics in the mix such as sugars, proteins and amino acids. This is sort of like a bagged lunch for the journey.

The production of sperm into milt is done in the collecting ducts and lumen of the testis. The same DOHP that causes eggs to mature causes spermatozoa to be released into the collecting ducts and lumen. Here, fluids are added, but the sperm is not activated. This process is referred to as hydration.

Salmon sperm are immobilized by high potassium ions: K⁺. The drop in K⁺ during dilution (activation) sets the action going. It’s thought that the change in osmotic pressure changes permeability in the membranes of the mitochondrial section of the sperm and the engine fires up. Pores called calcium channels open up, sugar is converted to ATP and the flagellum starts whipping back and forth.

Nothing lasts forever and so too with a sperm’s active state. There are a few things that effect the duration and intensity of sperm activation: temperature, sugars and ions. All of these affect the sperm’s energy supply. As temperature decreases the sperm lives longer. That’s because the machinery in the sperm uses energy more efficiently at a lower temp and heat degrades the process. Sugars supply the sperm with a source of energy, but there is little time to metabolize carbohydrates, so the role of sugars may also be to assist in the constancy of osmotic pressures of the microenvironment. Ions are important because that’s what keeps membranes stable and prevents the sperm from taking on too much water before getting the job done.

In addition to sugars, the mitochondria also need oxygen to keep going. That’s part of the fuel. This is why oxygen is added as an overlay during shipping. When a high K⁺ milt extender or diluents are added to the milt for shipping the oxygen preserves the sperm by keeping the cells alive. The seminal fluid is not enough to preserve milt for long periods. Parts of the proteins in seminal fluid are proteases - enzymes that break down protein. If shipping is to take more than a couple of hours from squeeze to eggs, an extender and oxygen should be added to the mix.

Like eggs, how much one fish can produce depends on size. Rainbow trout produce about 60 billion sperm per gram of testis per year. However, only about 50-60% of the total amount produced is released.

In sex-reversed males, the gonads are not connected to the outside. This makes checking difficult and the selection of males can be risky: the males have to be killed to checked. This can reflect on milt quality because, like eggs, milt can be past its prime, but it is not such a big deal because unused sperm are recycled. Induction of these males really helps eliminate the guesswork of ripeness. As with normal males, induction with a slow-release implant will increase volume and help to assure yield.