The Pineal Gland in Newborn Southern Elephant Seals, Mirounga leonina

Abstract

In the newborn southern elephant seal the pineal gland is very large, and both pineal and plasma melatonin concentration is elevated. The pineal gland was investigated during the first 24 h, and up to 20 days of age, in elephant seal pups. A primary aim of this investigation was to determine whether there are obvious ultrastructural characteristics of pinealocytes that are exhibiting extraordinarily high levels of activity. Blood and pineal glands were collected from thirty seven pups of known age which were sampled at random from early September to early November (1985) at Macquarie Island. The pineal gland is large (mean weight, 4.71 +/- 0.35 gm, range 1-9.3 gm) and actively secreting melatonin at birth. Melatonin concentrations were extremely variable, yet very high in pups during the first 24 h post-partum. Mean melatonin plasma concentration for pups 0-24 h was 17632.8 +/- 5723.8 pmol/l (4090.8 +/- 1327.9 pg/ml), ranging from 126 pmol/l (29 pg/ml) to 297000 pmol/l (68904 pg/ml). Electron microscopic examination did not reveal any marked changes in pinealocyte ultrastructure suggestive of increased secretory activity during this period. The large and extremely active pineal gland in newborn southern elephant seal suggests that it is actively involved in thermoregulation.

Quantitative differences in the pineal ultrastructure of perinatal and adult harp (Phoca groenlandica) and hooded seals (Cystophora cristata)

“…This study confirms that the pineal gland in seals is large and active at birth and that both size and activity are reduced in adults [3–6]. In hooded seals the gland is also large and apparently active prior to birth.…”

“…Presumably because of this, plasma melatonin levels are extremely high in newborn seals, even during daytime, compared with other mammals regardless of age [3, 4]. During the first few days of life, however, the plasma levels of melatonin decline rapidly [3–5] and, after a few weeks, become similar to other adult mammals [4]. Eventually, the size of the pineal gland also declines and although it remains large during the first 3 wk of life in southern elephant seals ( Mirounga leonina ) [5], it is considerably smaller in adults compared with newborns [3, 6].…”

“…The finding of a large and apparently very active pineal gland in seals during an ontogenetic stage where all other mammals generally have a small and inactive gland [7, 8], is intriguing and invites questions concerning its development and control as well as its functional significance. The physiological role has been attributed to the need for a precise timing of seasonal events in adult seals [9, 10] or thermoregulation in the newborn seal pup [3, 5]. Recently, we have linked it to the diving lifestyle of seals and, in particular, to the conspicuous cardiovascular changes generally associated with diving [11].…”

“…Typically, the pineal gland of the adult seal has two zones; an inner medulla with mainly blood vessels, nerves and connective tissue (Con) and a cortex with pinealocytes (Pi) [2, 9, 13]. Newborn seals do not show such zonation and Pi are distributed evenly throughout the gland [5, 6]. In the Weddell seal ( Leptonychotes weddellii ), pinealocyte density is similar in pups and adults, but due to a larger gland without zonation, the total number of Pi was estimated to be about 100 times higher in newborns compared with adults [3].…”

“…In the Weddell seal ( Leptonychotes weddellii ), pinealocyte density is similar in pups and adults, but due to a larger gland without zonation, the total number of Pi was estimated to be about 100 times higher in newborns compared with adults [3]. In newborn southern elephant seals the pinealocyte ultrastructure did not indicate increased secretory activity, compared with adults [5].…”