Developmental morphology of the turkey pineal organ. Immunocytochemical and ultrastructural studies

Embryonic development of the chick pineal gland throughout the incubation periods

Birds have a very different pineal gland structure morphologically and cytologically. The structure of the organ shows significant changes during the incubation periods. This study, which follows the embryological development of the pineal gland and makes histomorphometric measurements of the cellular elements that make up the gland parenchyma, is a current reference for studies in these areas. These brains were taken from 24 Babcock White Leghorn chick embryos on the 10th, 13th, 16th and 21st days of incubation. At 10th embryonic day, the pineal recess was in the structure of an elongated pineal canal. Solid rosette-shaped cell clusters were transforming into round vesicles with a small lumen. These vesicles had developed into larger, oval-shaped follicles with a well-defined central lumen. On 13th day, it was observed that the number and development of follicles increased considerably. The pineal gland showed a follicular-solid structure in 16th day embryos. While the mean follicle diameter was determined as 123.46 ± 13.28 μm on the 10th embryonic day, the highest value was measured as 187.62 ± 7.37 μm on the hatching day (p < 0.05). While the mean follicle area had the lowest value in the 10th day embryos, it was determined that this value gradually increased compared to the advancing embryonic days (p < 0.05). As conclusion, it is thought that this study provides new data to the literature about pineal gland development by monitoring the histological and histomorphometric developments of chick pineal gland in different incubation periods.

Changes in the Metabolic Profile of Melatonin Synthesis-Related Indoles during Post-Embryonic Development of the Turkey Pineal Organ

Research on age-dependent changes in pineal activity has been limited almost exclusively to melatonin (MLT). This study determined, for the first time, the alterations occurring in the metabolic profile of MLT synthesis-related indoles during the post-embryonic development period in birds. Turkeys reared under a 12 h light/dark cycle were euthanized at 2 h intervals for 24 h at the ages of 2, 7, 14, and 28 days and 10, 20, 30, and 45 weeks. The results showed prominent changes in the metabolic profile of indoles during development and could be distinguished in four stages. The first stage, from hatching to the age of 2 weeks, was characterized by a decrease in the 5-hydroxytryptophan concentration and an increase in the concentrations of serotonin (5-HT), MLT, 5-methoxyindoleacetic acid, and 5-methoxytryptamine (5-MTAM). During the second stage, around the age of 1 month, the concentrations of N-acetylserotonin (NAS) and MLT reached a maximum. The synthesis and degradation of 5-HT were also the highest. The third stage, around the age of 10 weeks, was characterized by decreased levels of 5-HT (approximately 50%) and 5-hydroxyindoleacetic acid and a high level of 5-MTAM. The last stage, covering the age of 20 to 45 weeks, was characterized by a large decrease in the synthesis, content, and degradation of 5-HT. Despite these changes, there were no prominent differences in the nocturnal levels of NAS and MLT between the third and fourth stages. The concentrations of all tryptophan derivatives showed daily fluctuations until the age of 45 weeks.

Developmental morphology of the turkey pineal gland in histological images and 3D models

The histological structure of the avian pineal gland during embryonic life has so far only been studied in chickens. It is known that the pineal organs of hatched chickens and turkeys differ significantly from each other based on their morphology and physiology. The aim of the present study was to investigate the histological structure of the embryonic pineal gland of domestic turkeys. The study was performed on turkey embryos aged 4-28 days. Along with histological analyses, three-dimensional (3D) images of the pineal glands from embryos aged 6-28 days were also obtained. In four-day-old embryos [embryonic day (ED) 4], primary evagination of the pineal gland from the neuroectoderm of the diencephalon was observed. On ED 6, the evagination formed a pineal recess with a thick and folded wall. In the next embryonic stages, the pineal recess was lengthened to the pineal canal, with the lumen opening to the third ventricle. The connection of the pineal lumen with the ventricular lumen was observed in all studied embryos. The first cellular rosettes without the lumen separated from the wall of the pineal recess occurred on ED 6. Several small and round follicles containing their own lumens were visible on ED 8. On ED 10, the pineal parenchyma was composed mainly of small, round follicles. The first oval follicles appeared on ED 12 and branched follicles appeared on ED 16. In some embryos at different stages, follicles formed from secondary evaginations of the diencephalon epithelium were observed. The turkey pineal organ maintained the follicular type of parenchyma without solid cellular aggregates throughout embryonic life. The pineal follicles originated from: 1) rosettes arising from the wall of the pineal canal (from ED 6); 2) an accessory evagination occurring in the neuroectoderm anteriorly and posteriorly to the pineal canal end (from ED 6); 3) direct development in the walls of larger follicles and detaching from them in a manner similar to the budding process (from ED 14); and 4) fusion of smaller follicles into branched ones. The pineal capsule started to develop on ED 6, first as a vascularization and later as a thin mesenchymal outline around the apical part, then at the dorsal and at the end the ventral part of the pineal gland. The pineal stroma was composed of mesenchymal tissue consisting of abundant in cells and blood vessels. The first evagination of the choroid plexus in the diencephalon was observed on ED 8. The attachment of the pineal gland to the dura mater first occurred on ED 16. Finally, the pineal gland of ED 28 embryos consisted of a wide proximal part attached to the dura mater and a narrow distal part that extended into the pineal stalk, which extended to the intercommissural region of the diencephalon. The present study revealed the occurrence of significant morphological differences in the developing embryonic pineal gland of turkeys compared with chickens.

Embryonic Development of Avian Pineal Secretory Activity-A Lesson from the Goose Pineal Organs in Superfusion Culture

The embryonic ontogeny of pineal secretory activity in birds has been investigated almost exclusively in chickens. This study aimed to characterize this process in domestic geese. The pineal organs of embryos aged 18–28 days were incubated in superfusion culture under different light conditions for 4–5 days and treated with norepinephrine (NE). Melatonin (MLT) was measured by radioimmunoassay and other indoles by HPLC with fluorescence detection. Additionally, pineal organs were collected from embryos at 14–28 days of age and used to measure catecholamines by HPLC with electrochemical detection. MLT secretion increased with embryo age, most intensively between the 22nd and 24th days of life. The daily changes in MLT secretion under the 12 L:12D cycle occurred on the first day of culture, starting from an embryonic age of 24 days. MLT secretion was controlled by the light-dark cycle in all age groups studied. However, exposure to light during the scotophase did not alter the secretion of MLT. The endogenous oscillator expressed its activity in regulating MLT secretion in the pineal organs of embryos aged 24 days and older but could not generate a rhythm after one cycle. The rhythm of 5-hydroxytryptophan release during the first day of culture was found in the pineal organs of all embryos, while the rhythmic release of N-acetylserotonin and 5-methoxyindole acetic acid started at the age of 24 days. The proportion of released indoles changed with embryo age. NE caused a decrease in MLT secretion and provoked an increase in serotonin release. Incubation of the pineal organs induced the development of MLT secretory machinery and its diurnal rhythmicity. The pineal content of catecholamines increased prominently at the end of embryonic development.

Monochromatic green light stimulation during incubation shortened the hatching time via pineal function in White Leghorn eggs

Background

Effect of monochromatic green light illumination on embryo development has been reported in chickens. The avian pineal gland is an important photo-endocrine organ formed by a mediodorsal protrusion during embryonic development. However, the involvement of pineal gland in the light transduction process remains to be elucidated. In the present study, we investigated the influence of monochromatic green light on hatching time and explored the possible mechanism via pineal function.

Results

A total of 600 eggs of White Leghorn (Shaver strain) were incubated under photoperiods of either 12 h of light and 12 h of darkness using monochromatic green light (12L:12D group) or 24 h of darkness (0L:24D group) for 18 d. Compared to 0L:24D group, the green light stimulation shortened the hatching time without extending the hatch window or impairing hatchability. The liver of embryos incubated in the 12L:12D light condition was heavier than those of the 0L:24D group on d 21 post incubation which may be linked to the observed increase in the serum concentration of insulin-like growth factor 1 (IGF-1); primary secretion of the liver. Histological structure analysis of pineal gland demonstrated that the light stimulation increased follicle area, wall thickness and lumen area on d 10 and d 12 post incubation. Rhythmic function analysis demonstrated that three clock related genes (brain and muscle ARNT-like-1, BMAL1; circadian locomotor output cycles kaput, CLOCK; and cryptochrome-1, CRY1) and a melatonin rate-limiting enzyme related gene (arylalkylamine N-acetyltransferase, AANAT) were rhythmically expressed in the pineal gland of the 12L:12D group, but not in the 0L:24D group. Simultaneously, the light stimulation also increased the concentration of melatonin (MT), which was linked to hepatocyte proliferation and IGF-1 secretion in previous studies.

Conclusions

The 12L:12D monochromatic green light stimulation during incubation shortened hatching time without impairing hatching performance. Pineal gland’s early histological development and maturation of its rhythmic function were accelerated by the light stimulation. It may be the key organ in the photo-endocrine axis that regulates embryo development, and the potential mechanism could be through enhanced secretion of MT in the 12L:12D group which promotes the secretion of IGF-1.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40104-020-00539-x.

Metabolism of Melatonin Synthesis-Related Indoles in the Turkey Pineal Organ and Its Modification by Monochromatic Light

The metabolism of pineal indoles is closely related to alterations in the light and dark phases of a daily cycle. Recent research showed important interspecies differences in the pineal biochemistry, and a strong impact of monochromatic light on many physiological processes in birds. Therefore, the aims of study were to characterize the metabolism of melatonin-synthesis indoles in the pineal organ of the domestic turkey, and to determine the changes occurring in this metabolism under the influence of different wavelengths and intensities of light. For this purpose, 3-week-old turkeys were kept under 16 lx white light, or under blue, green, and red light with intensities of 16, 32, and 64 lx during the photophase, and after 7 d were sacrificed at 4 h intervals. The activities of melatonin-synthesizing enzymes and the contents of indoles were measured in the same pineal organ. The results revealed that the activities of tryptophan hydroxylase and arylalkylamine N-acetyltransferase, and the levels of all tryptophan derivatives had significant daily changes in birds kept under each light condition used. The profile of pineal indole metabolism in 4-week-old turkeys was characterized by high-amplitude rhythms in the activity of arylalkylamine N-acetyltransferase and the contents of N-acetylserotonin and melatonin, equal relative amounts of serotonin and 5-hydroxyindoleacetic acid, and higher content of melatonin than N-acetylserotonin. The monochromatic light significantly modified the pineal indole metabolism, and its effects were dependent on the color and intensity of light. Pronounced changes occurred in the level of serotonin synthesis and the daily rhythm course of melatonin synthesis.

Pinopsin and photoreception in the pineal organ of the domestic turkey during post-hatching development

The avian pineal organ is photosensitive because of the presence of photopigments, of which pinopsin seems to be one of the most important. This organ is subject to far-reaching changes during post-hatching development, but evidence regarding pinopsin presence and direct photoreception during this time is lacking. This study was carried out to demonstrate the following: 1) the structures showing immunoreactivity to pinopsin in the turkey pineal organ, 2) the changes of these structures during development, 3) the pinopsin localization in pinealocytes in monolayer cultures, and 4) the role of direct photoreception in the regulation of melatonin secretion in pineal organs in adult turkeys. Pinopsin immunoreactivity was localized in the apical extensions of columnar cells limiting the follicular lumen, in fiber-like structures located between columnar cells in the inner part of follicle wall, in string-shapes or small spherical structures distributed in the outer part of follicle wall and in amorphous material inside the follicle lumen. In young birds, immunoreactivity was also sporadically noted in cell bodies of rudimentary receptor pinealocytes. The distribution of pinopsin showed prominent age-dependent changes, including a subsequent increase in pinopsin-positive structures in the outer part of the follicle wall and a prominent reduction in the number and size of positive apical extensions in 40- and 56-week-old turkeys. These data demonstrate that the role of secretory pinealocytes in pineal photoreception increases with age. In monolayer cultures, all pinealocytes showed strong reactions in club- or bulbous-shaped prolongations. The pineal organs of adult birds were less sensitive to light exposition at night than those of young turkeys, which points to differences in light sensitivity between rudimentary receptor and secretory pinealocytes. However, direct photoreception could play an important role in the regulation of melatonin secretion in adult turkeys.

Roles of Direct Photoreception and the Internal Circadian Oscillator in the Regulation of Melatonin Secretion in the Pineal Organ of the Domestic Turkey: A Novel In Vitro Clock and Calendar Model

The regulation of melatonin secretion in the avian pineal organ is highly complex and shows prominent interspecies differences. The aim of this study was to determine the roles of direct photoreception and the internal oscillator in the regulation of melatonin secretion in the pineal organ of the domestic turkey. The pineal organs were collected from 12-, 13- and 14-week-old female turkeys reared under a 12 L:12 D cycle with the photophase from 07.00 to 19.00, and were incubated in superfusion culture for 3-6 days. The cultures were subjected to different light conditions including 12 L:12 D cycles with photophases between 07.00 and 19.00, 13.00 and 01.00 or 01.00 and 13.00, a reversed cycle 12 D:12 L, cycles with long (16 L:8 D) and short (8 L:16 D) photophases, and continuous darkness or illumination. The pineal organs were also exposed to light pulses of variable duration during incubation in darkness or to periods of darkness during the photophase. The secretion of melatonin was determined by direct radioimmunoassay. The turkey pineal organs secreted melatonin in a well-entrained diurnal rhythm with a very high amplitude. Direct photoreception as an independently acting mechanism was able to ensure quick and precise adaptation of the melatonin secretion rhythm to changes in light-dark conditions. The pineal organs secreted melatonin in circadian rhythms during incubation in continuous darkness or illumination. The endogenous oscillator of turkey pinealocytes was able to acquire and store information about the light-dark cycle and then to generate the circadian rhythm of melatonin secretion in continuous darkness according to the stored data. The obtained data suggest that the turkey pineal gland is highly autonomous in the generation and regulation of the melatonin secretion rhythm. They also demonstrate that the turkey pineal organ in superfusion culture is a valuable model for chronobiological studies, providing a highly precise clock and calendar. This system has several features which make it an attractive alternative to other avian pineal glands for circadian studies.