150
A. M. PEFKS
ser fulvenscens, the nucleus lies in a similar position to that of the teleost
fish ( Sathyanesan and Chavin, 1967). In Acipenser giildenstadt Brandt,
A. stellatus Pallas, and Huso huso L. the nucleus is divided into a ventral
region of small bipolar or unipolar cells, and a dorsal region of large
multipolar cells (Polenov and Barannikova, 1958). In Acipenser fuluescem, a number of the larger type of cells intrude among the smaller
variety, so that the distinction into two zones is not as marked as in the
teleost fish (Sathyanesan and Chavin, 1967). The preoptic cells have
single, eccentric nuclei, which are round or oval in shape. Their cytoplasm contains h e neurofibrils, Nissl substance, and clear vacuoles. More
important, they contain neurosecretory granules, which are variable in
size and less numerous than in the teleosts. According to Polenov and
Barannikova ( 1958), neurosecretion is formed in the perinuclear region
and later spreads throughout the cytoplasm; although granules were
found around the vacuoles, these authors do not consider that they were
formed by the vacuoles themselves (cf. Scyliorhinus caniculus; M a d ,
1952). The preoptic cells extend thick dendrites between the ependymal
cells which line the ventricle of the brain. They give rise also to the preoptico-hypophysial tract. This tract carries neurosecretory granules mainly
in its peripheral fibers and in some specimens granules may be found
throughout its length. At first, in the region of the optic chiasma, the tract
consists of symmetrically arranged bundles, densely packed with secretion, and close to the surface of the infundibulum. Over the adenohypophysis, these groups of fibers merge into a single, thin, broad bundle. In
Acipenser fulvescens, this tract gives off a ventral branch, reminiscent of
that seen in the brachiopterygians. This branch is a compact bundle of
axons, which comes into close contact with the vascular connective tissue
which lies between the infundibular wall and the adenohypophysis. Herring bodies are found in the infundibular floor (Sathyanesan and Chavin,
1967). In contrast, Polenov (1966) does not describe a discrete ventral
branch in Acipenser giildenstadt Brandt or A. stellatus Pallas. However,
he gives a detailed description of a broad “neurosecretory contact area,”
where the loosely arranged, nonmedullated neurosecretory fibers of the
preoptico-hypophysial tract give rise to an outer layer of fine radial axons
(Fig. 7). These axons form club-shaped, ribbonlike, or tapering terminal
bulbs against the vascular connective tissue which separates the brain
from the pituitary. There are no large Herring bodies in the area of contact, but the total amount of neurosecretory material is considerable because of the large size of the area involved. The connective tissue membrane becomes thinner wherever contacts are most intimate. It contains
broad, sinusoidal capillaries, which penetrate the adenohypophysis in its
caudal region. It is reasonable to suggest that this “contact area” is the
homolog of a median eminence ( Polenov, 1966).
A. M. PEFKS
ser fulvenscens, the nucleus lies in a similar position to that of the teleost
fish ( Sathyanesan and Chavin, 1967). In Acipenser giildenstadt Brandt,
A. stellatus Pallas, and Huso huso L. the nucleus is divided into a ventral
region of small bipolar or unipolar cells, and a dorsal region of large
multipolar cells (Polenov and Barannikova, 1958). In Acipenser fuluescem, a number of the larger type of cells intrude among the smaller
variety, so that the distinction into two zones is not as marked as in the
teleost fish (Sathyanesan and Chavin, 1967). The preoptic cells have
single, eccentric nuclei, which are round or oval in shape. Their cytoplasm contains h e neurofibrils, Nissl substance, and clear vacuoles. More
important, they contain neurosecretory granules, which are variable in
size and less numerous than in the teleosts. According to Polenov and
Barannikova ( 1958), neurosecretion is formed in the perinuclear region
and later spreads throughout the cytoplasm; although granules were
found around the vacuoles, these authors do not consider that they were
formed by the vacuoles themselves (cf. Scyliorhinus caniculus; M a d ,
1952). The preoptic cells extend thick dendrites between the ependymal
cells which line the ventricle of the brain. They give rise also to the preoptico-hypophysial tract. This tract carries neurosecretory granules mainly
in its peripheral fibers and in some specimens granules may be found
throughout its length. At first, in the region of the optic chiasma, the tract
consists of symmetrically arranged bundles, densely packed with secretion, and close to the surface of the infundibulum. Over the adenohypophysis, these groups of fibers merge into a single, thin, broad bundle. In
Acipenser fulvescens, this tract gives off a ventral branch, reminiscent of
that seen in the brachiopterygians. This branch is a compact bundle of
axons, which comes into close contact with the vascular connective tissue
which lies between the infundibular wall and the adenohypophysis. Herring bodies are found in the infundibular floor (Sathyanesan and Chavin,
1967). In contrast, Polenov (1966) does not describe a discrete ventral
branch in Acipenser giildenstadt Brandt or A. stellatus Pallas. However,
he gives a detailed description of a broad “neurosecretory contact area,”
where the loosely arranged, nonmedullated neurosecretory fibers of the
preoptico-hypophysial tract give rise to an outer layer of fine radial axons
(Fig. 7). These axons form club-shaped, ribbonlike, or tapering terminal
bulbs against the vascular connective tissue which separates the brain
from the pituitary. There are no large Herring bodies in the area of contact, but the total amount of neurosecretory material is considerable because of the large size of the area involved. The connective tissue membrane becomes thinner wherever contacts are most intimate. It contains
broad, sinusoidal capillaries, which penetrate the adenohypophysis in its
caudal region. It is reasonable to suggest that this “contact area” is the
homolog of a median eminence ( Polenov, 1966).