The pituitary gland produces, accumulates, and stores the gonadotropic hormone(s) which plays a decisive role in ovulation. Insofar as reproduction is concerned, the role of the pituitary gland is that of an intermediary between the central nervous system and the gonads. The gonadotropic hormone(s) is produced by sexually mature fish and the cyclical changes in its concentration in the pituitary gland are correlated with the reproductive cycle of the fish. Its concentration is maximum during the prespawning period, while it is very low or almost nil during and after spawning. The release of gonadotropin(s) by the pituitary gland is “ordered” by the hypothalamus through the secretion of gonadotropin-releasing hormone (GRH). The gonadotropin(s) is also responsible for inducing spawning migration, during which its concentration in the pituitary gland gradually decreases. The gonadal development during spawning migration is most probably directed by the continuously released gonadotropin(s).
The pituitary gland is situated on the ventral side of the brain below the hypothalamus, which is connected to the pituitary gland by a funnel-like structure, the infundibulum. The part of the cranium where the pituitary gland is located is known as the sella turcica. The gland is usually embedded in fatty tissue. When the brain is taken out of the skull, the pituitary gland remains connected to the brain in some fishes, while in most fishes the infundibulum ruptures and the gland is left behind on the base of the skull. In the case of common carp, a small droplet of blood indicates the place where the pituitary gland was connected to the brain. The hypothalamus, below which one has to look for the pituitary, is a ventrally protruding part of the brain and is easy to distinguish.
The collection of pituitary glands on a commercial scale is possible only when fish of suitable size (above 1 kg), maturity, and gonadal development are available alive or shortly after death. Because hormones are known to decompose quickly in a dead body, glands taken from fish that have been dead too long are unreliable.
To gain access to the pituitary gland, the top part of the skull must be removed by a saw or a sharp strong knife, or a cylindrical core of the entire skull in the region of the brain is removed using a cylindrical hole cutter (hole-saw) attached to an electric drill (Figure 73). A plank with a hole drilled in it is kept pressed on the skull to guide the hole-saw, while the fish head is held in position with the help of a wooden frame.
The cylindrical core which is removed from the hole-saw with the help of a screw-driver contains the skull top, the brain, the pituitary gland, and the base of the skull. These are separated with the help of a scissors and the pituitary gland picked out with a forceps and preserved in pure acetone. The drilling technique greatly helps in the quick collection of a large number of pituitary glands. To ensure that the gland comes within the core, larger sized hole-saws are used for larger fishes; viz., 2.5 cm for fish of 1 kg, 4.0 cm for fish up to 3–4 kg weight, and 5.0–6.0 cm for still larger fishes.
After collecting the required number of glands, the acetone in which the glands are kept is drained off and fresh acetone is added. The acetone is changed again after another 8–12 hours. The next day when the glands have been in acetone for a total of about 24 hours, the acetone is drained off and the glands dried on a tissue paper. The acetone dehydrates and defattens the glands. The dried glands are put in a glass phial, pressed down with a ball of fine cotton, corked tightly, and sealed by wax. The phial should be labelled, indicating the date of collection and the origin of the glands (Figure 74).
Acetone-dried glands can be stored for many years if they are kept free from moisture. Even in sub-tropical and tropical regions, they can be preserved for at least 5–8 years. It is not necessary to refrigerate the glands. The sealed phials are kept in a plastic bag with a bag of water absorbent (silica gel) or in a desiccator (Figure 75). When some glands are removed, the remaining glands in the phial must be pressed down with additional cotton and corked and sealed again.
The glands can also be preserved in absolute alcohol, in which case they are immersed in the absolute alcohol immediately after collection. After 24 hours, the glands are washed with absolute alcohol, and put in fresh absolute alcohol and stored in a cool, shady place at room temperature or under refrigeration until needed. To prevent moisture from getting into the phials, they should be sealed with wax or kept in a desiccator.
Another method of preserving glands is to subject them to instant freeze and storing them in a freezer.
Fresh glands can also be used immediately after collection to prepare a hypophysis solution.
Pituitary glands are sometimes available in the market in some countries; e.g., the U.S.A., Hungary, and India. They are marketed in dried unbroken form. Glands are also marketed in powdered form and in the form of extract in glass ampoules ready for injection. But these are less reliable because of the possibility of adulteration.