The pineal organ and the hypothalamus-pituitary relationship in the minnow (Phoxinus phoxinus L.) with special reference to the control of the reproductive cycle and of colour change

Abstract

The structure, histology and blood supply of the pineal organ of the minnow are described. Its nerve connections are studied and their possible role in the colour response of the fish is suggested. No evidence is found for hormonal activity of the pineal organ in the colour change of the minnow. The structural modifications of the pineal organ in normal and blind fish in conditions of constant light, temperature, darkness and different background tints are described. Constant illumination together with constant temperature in the blind fish result in a very much folded pineal organ. It is suggested that a non-functioning photoreceptive state of the eyes (condition present in these blind fish and the normal and blind fish kept in darkness at a higher temperature) is responsible for the stimulation of the pineal organ at a constant higher temperature. The antagonistic reaction to the pituitary in relation to the ovaries is also described. Changes following hypophysectomy also suggest an antagonistic relation between the pineal organ and the pituitary.The relations of the hypothalamo-hypophyseal system and its vascular supply are discussed with regard to the neurosecretory function of the nucleus preopticus and itspreoptico-hypophyseal tract and the transport and storage of the neurosecretory material. The developmental differentiation of the system is studied. Seasonal changes in the hypothalamo-hypophyseal neurosecretory system, especially the NPO-activity, are correlated with the reproductive cycle and their significance is discussed. An attempt is made to interpret the relation of the optic nerves with the preoptico-hypophyseal tract. The relation of the latter with the commissura transversa is described, especially with regard to the possible relaying of messages from the cerebellum to the pituitary gland. Aspects of the structure and cytology of the pituitary are described. Seasonal changes, especially in the meso-adenohypophysis, can be correlated with the reproductive cycle. Some effects of hypophysectomy on the hypothalamic complex and on the gonads and thyroid gland are described. In this connection some study is made of the morphology and blood supply of the thyroid gland. Possible interrelationships are further investigated by observing the effects of gonadectomy on the pineal organ, nucleus preopticus, pituitary and thyroid glands. The effects upon these structures in normal and blinded fish of illumination, background tint and temperature are also studied.