The Pineal

It has been known for almost twenty years that polar animals tend to have a very large pineal gland. Southern elephant seals are no exception. A remarkable feature of this species, however, like the Weddell seal and other polar seal species, is that the pineal gland is particularly large at birth (Bryden et al. 1986; see fig. 22.2).

Histologically, the pineal contains many pinealocytes at birth, indicating that the gland is potentially functional (ibid.). This contrasts with the gonads, whose enlargement at birth results from the great proliferation of interstitial tissue.

Ultrastructurally, the pinealocytes appear relatively immature (Little and Bryden 1990). This suggests that the pineal, although enlarged, may not be very active at birth. However, the density of pinealocytes is similar

Fig. 22.2
Median section of the brain of a dog (top) and a newborn
elephant seal (bottom). The pineal of each is marked (P).

in pups and adults, and the pup pineal contains 50 to 100 times more pinealocytes than the adult (Bryden et al. 1986).

We tested for pineal activity by assay of plasma for the hormone secreted by the pineal gland, melatonin. Details of the assay method are given in D. J. Kennaway, T. A. Gilmore, and R. F. Seamark (1982) and C. R. Earl, M. J. D'Occhio, D. J. Kennaway, and R. F. Seamark (1985).

The assays revealed extremely high concentrations of circulating melatonin in the first days of postnatal life. Concentrations in excess of 60,000 picomoles per liter (pM/l) were measured. (This contrasts with levels in most adult mammals of 100–300 pM/l.) It is possible that at least some of the melatonin may originate from organs other than the pineal, for example, the retina. However, the fact that the pineals of newborn pups also con-

tained high concentrations of melatonin (Little and Bryden 1990) argues for a high level of production of melatonin in the pineal itself.

The pineal and its secretory product, melatonin, have been implicated in the control of several physiological mechanisms, in particular, reproduction, by mediating the influence of the photoperiod on the neuroendocrine-reproductive axis. As currently perceived, the function of the pineal is a rather general one, serving as an intermediary between the external environment (particularly the photoperiod) and the organism as a whole (Reiter 1981). We have observed that it is large and active in the newborn elephant seal and suggest it is involved in thermoregulation at this stage of the seal's life.

The profiles of plasma melatonin and T3 concentration are somewhat similar. They increase in the first hours of postnatal life and decline after about the first week.

Comparison of Southern and Northern Elephant Seals

If our conclusion that the pineal gland in newborn pups is involved in thermoregulation is correct, we should expect to see a lesser role for the pineal in early postnatal life in the northern elephant seal as compared to the subantarctic southern elephant seal. Plasma has been assayed for melatonin concentration in northern elephant seals (fig. 22.3). We see that although the concentration of melatonin in northern elephant seals is very high, and it follows a similar pattern to that in the southern elephant seal, the concentrations for the most part are substantially lower than in the southern elephant seal. This needs to be looked at in more detail, but these results tentatively support the notion that the pineal gland is involved in thermogenesis in elephant seals in early postnatal life.