94
JAMES CLARKE FENWICK
goes a gradual infolding with the result that the central lumen, although
retaining its connection with the third ventricle, becomes divided into
narrow sinuous spaces. Many of the blood vessels which surround the
pineal body in embryonic and larval stages, are, with the increased
development of the animal, carried into the infoldings of the pineal
epithelium and come to lie in the intralobular septa. This centripetal
invasion by the blood vessels results in the extensive vascularization
characteristic of the adult pineal. Generally, the distal vesicle is closely
apposed to the overlying cranial roof but is not, in teleosts, associated
with a pineal foramen in the skull (U. Holmgren, 1959b; Hafeez and
Ford, 1967). However, as in many deep-sea fishes, the pineal body often
lies beneath a more or less depigmented and translucent area of the
cranial roof (U. Holmgren, 1959b). In other fish, the pineal body is
located in a depression in the roof of the skull or may be associated
with a very complex structure of skull bones which may allow the
passage of light directly to the pineal body (Rivas, 1953). Some authors,
in questioning a photoscnsory role of the pineal body, have attached
special significance to the fact that some fish possess thick skull bones
and have no special structure to allow light to pass though to the pineal
region. It should be noted, however, that measurable quantities of light
have been demonstrated to penetrate the skulls of sheep, dogs, rabbits,
and rats (Ganong et al., 1963).
2. HISTOLOGY
Many of the references on the pineal histology are found in papers
concerned with the secretory activity of this organ and will be discussed
in a later section (Section IV, B, 1 and 2 ) . Histologically, the pineal body
of fishes most closely resembles a sensory structure. The structure of the
vesicular epithelium resembles the retina except that the pineal "retina"
is of a reversed type with the sensory cells in the inner retinal layer;
thus, they are turned in the direction from which the light enters ( Ariens
Kappers, 1965). In cyclostomes the dorsal wall of the pineal forms a
lenslike pellucida, but a lenslike differentiation is never found among
other fishes where the pineal scems to have acquired some characteristics
of a secretory organ (N. Holmgren, 1918a,b; U. Holmgren, 1958b;
Altner, 1965; Hafeez and Ford, 1967). Despite this histological progression from an eyelike structure in some fish to a glandular organ
in others, there is now general agreement that the pineal body of fish
is largely composed of three cell types: sensory cells, supporting cells,
and ganglion or nerve cells (Rasyuin, 1958; U. Holmgren, 1959b;
Riideberg, 1968b).
JAMES CLARKE FENWICK
goes a gradual infolding with the result that the central lumen, although
retaining its connection with the third ventricle, becomes divided into
narrow sinuous spaces. Many of the blood vessels which surround the
pineal body in embryonic and larval stages, are, with the increased
development of the animal, carried into the infoldings of the pineal
epithelium and come to lie in the intralobular septa. This centripetal
invasion by the blood vessels results in the extensive vascularization
characteristic of the adult pineal. Generally, the distal vesicle is closely
apposed to the overlying cranial roof but is not, in teleosts, associated
with a pineal foramen in the skull (U. Holmgren, 1959b; Hafeez and
Ford, 1967). However, as in many deep-sea fishes, the pineal body often
lies beneath a more or less depigmented and translucent area of the
cranial roof (U. Holmgren, 1959b). In other fish, the pineal body is
located in a depression in the roof of the skull or may be associated
with a very complex structure of skull bones which may allow the
passage of light directly to the pineal body (Rivas, 1953). Some authors,
in questioning a photoscnsory role of the pineal body, have attached
special significance to the fact that some fish possess thick skull bones
and have no special structure to allow light to pass though to the pineal
region. It should be noted, however, that measurable quantities of light
have been demonstrated to penetrate the skulls of sheep, dogs, rabbits,
and rats (Ganong et al., 1963).
2. HISTOLOGY
Many of the references on the pineal histology are found in papers
concerned with the secretory activity of this organ and will be discussed
in a later section (Section IV, B, 1 and 2 ) . Histologically, the pineal body
of fishes most closely resembles a sensory structure. The structure of the
vesicular epithelium resembles the retina except that the pineal "retina"
is of a reversed type with the sensory cells in the inner retinal layer;
thus, they are turned in the direction from which the light enters ( Ariens
Kappers, 1965). In cyclostomes the dorsal wall of the pineal forms a
lenslike pellucida, but a lenslike differentiation is never found among
other fishes where the pineal scems to have acquired some characteristics
of a secretory organ (N. Holmgren, 1918a,b; U. Holmgren, 1958b;
Altner, 1965; Hafeez and Ford, 1967). Despite this histological progression from an eyelike structure in some fish to a glandular organ
in others, there is now general agreement that the pineal body of fish
is largely composed of three cell types: sensory cells, supporting cells,
and ganglion or nerve cells (Rasyuin, 1958; U. Holmgren, 1959b;
Riideberg, 1968b).