Posthatching Week Research Articles

Histomorphology and gene expression profiles during early ovarian folliculogenesis in duck and goose

In contrast to the later stages of follicle development, little is known about the characteristics and mechanisms associated with early folliculogenesis in avian species. The objectives of the present study were to examine and compare the histomorphological and molecular changes of primordial, primary, and secondary follicles from duck and goose ovaries during the first 6 post-hatching week. Morphological analysis showed that the length and width of both duck and goose ovaries increased steadily during weeks 1 to 5 but increased acutely at week 6, whereas a greater increment was observed in the ovarian length of ducks than that of geese during weeks 4 to 5. Furthermore, smaller diameters of the 3 categories of follicles were observed in ducks than those in geese at the first appearance, but they reached a similar size at week 6. More importantly, secondary follicles were found in the ovaries of ducks 1 wk earlier than in those of geese. These results indicated a more rapid growth rate for ovarian follicles in ducks than in geese during early post-hatching development. At the molecular level, it was found that the mRNAs encoding follicle stimulating hormone receptor (FSHR), anti-Müllerian hormone (AMH), B-cell leukemia/lymphoma 2, and cysteine-dependent aspartate specific protease 3 (CASPASE3) were ubiquitously expressed in all ovarian follicles of ducks and geese with different expression profiles in each follicular category during the first 6 post-hatching week. Notably, transcript levels of FSHR, AMH, and CASPASE3 changed differently between ducks and geese during weeks 5 to 6, which was postulated to be one of the mechanisms inducing more rapid growth of ovarian follicles in ducks rather than in geese. In conclusion, our results revealed, for the first time, differences in early folliculogenesis, including the rate of growth of each follicular category and the timing of transition of primary to secondary follicles, between ducks and geese, and these differences could result from different expression profiles of FSHR, AMH, and CASPASE3 during early post-hatching development.

Genome-wide transcriptome profiling reveals the mechanisms underlying muscle group–specific phenotypic changes under different raising systems in ducks

Although a number of nongenetic factors have been reported to be able to modulate skeletal muscle phenotypes in meat-type birds, neither the underlying mechanisms nor the muscle group–specific phenotypic and molecular responses have been fully understood. In the present study, a total of 240 broiler ducks were used to compare the effects of floor raising system (FRS) and net raising system (NRS) on the physicochemical properties and global gene expression profiles of both breast and thigh muscles at the posthatching week 4 (W4), W8, and W13. Our results showed that compared with FRS, NRS generally induced higher pH, lower lightness (L∗) and yellowness (b∗), lower drip loss and cooking loss, and lower shear force in either breast or thigh muscles during early posthatching stages but subsequently showed less pronounced or even reverse effects. Meanwhile, it was observed that the raising system differently changed the myofiber characteristics depending on the muscle group and the developmental stage. Genome-wide transcriptome analysis showed that compared with FRS, NRS induced the most extensive gene expression changes in breast muscle (BM) at W4 but in thigh muscle (TM) at W13, suggesting the asynchronous molecular responses of BM and TM to the raising system and period. Most of differentially expressed genes in either BM or TM between NRS and FRS were enriched in the Gene Ontology and Kyoto Encyclopedia of Genes and Genomes terms associated with regulation of muscle cellular functions, metabolic and contractile activities, and tissue remodeling, indicating similar molecular mechanisms principally responsible for the raising system-caused phenotypic changes in both muscle groups. Nevertheless, several crucial pathways (e.g., adipocytokine signaling, AGE-RAGE signaling, and apoptosis) and genes (e.g., ANO6, ACER2, UCP3, DTL, and TMEM120A) were tightly related to the muscle group–specific adaptive remodeling on different raising systems. These data could not only contribute to a better understanding of the molecular mechanisms behind meat quality but also provide novel insights into the molecular causes of the muscle group–specific adaptive remodeling in response to environmental stimuli.

Nouvelles stratégies alimentaires précoces au service de la production avicole

Le développement de l’embryon d’oiseau dépend de l’environnement dans lequel il baigne et des nutriments disponibles. La période de démarrage (1ère semaine post-éclosion) est souvent délicate avec parfois des mortalités post-natales importantes dues aux délais d’alimentation et aux conditions d’ambiance et de transport sub-optimales. Les liens entre la nutrition parentale, la composition de l’œuf et le comportement ultérieur des animaux, leurs performances, et leur sensibilité aux maladies sont bien établis. Des supplémentations en nutriments de l’œuf via l’alimentation maternelle ou via des injections in ovo (« in ovo feeding ») représentent alors des stratégies innovantes pour optimiser l’apport de nutriments à l’embryon. La finalité de telles approches est d’optimiser les apports nutritionnels des reproductrices ou des embryons en développement dans le but d’obtenir des poussins de meilleure qualité en termes de robustesse, croissance et/ou composition corporelle en tirant parti de la plasticité embryonnaire d’utilisation des nutriments. La manipulation très précoce de l’environnement dans lequel l’embryon se développe est très prometteuse pour analyser l'influence des conditions précoces sur l'élaboration des phénotypes à long terme que ce soit sur des critère de croissance, composition corporelle, caractéristiques tissulaires, robustesse vis-à-vis de challenges ou adaptation à des conditions d’élevage différentes. Outre les bénéfices observés pour les animaux, ces stratégies alimentaires tentent de répondre aussi à des intérêts environnementaux et économiques.

Transcriptional profiling and pathway analysis reveal differences in pituitary gland function, morphology, and vascularization in chickens genetically selected for high or low body weight

BackgroundThough intensive genetic selection has led to extraordinary advances in growth rate and feed efficiency in production of meat-type chickens, endocrine processes controlling these traits are still poorly understood. The anterior pituitary gland is a central component of the neuroendocrine system and plays a key role in regulating important physiological processes that directly impact broiler production efficiency, though how differences in pituitary gland function contribute to various growth and body composition phenotypes is not fully understood.ResultsGlobal anterior pituitary gene expression was evaluated on post-hatch weeks 1, 3, 5, and 7 in male broiler chickens selected for high (HG) or low (LG) growth. Differentially expressed genes (DEGs) were analyzed with gene ontology categorization, self-organizing maps, gene interaction network determination, and upstream regulator identification to uncover novel pituitary genes and pathways contributing to differences in growth and body composition. A total of 263 genes were differentially expressed between HG and LG anterior pituitary glands (P ≤ 0.05 for genetic line-by-age interaction or main effect of line; ≥1.6-fold difference between lines), including genes encoding four anterior pituitary hormones. Genes involved in signal transduction, transcriptional regulation, and vesicle-mediated transport were differentially expressed and are predicted to influence expression and secretion of pituitary hormones. DEGs involved in immune regulation provide evidence that inflammation and response to cellular stressors may compromise pituitary function in LG birds, affecting their ability to adequately produce pituitary hormones. Many DEGs were also predicted to function in processes that regulate organ morphology and angiogenesis, suggesting pituitary gland structure differs between the divergently selected lines.ConclusionsThe large number of DEGs within the anterior pituitary gland of birds selected for high or low body weight highlights the importance of this gland in regulating economically important traits such as growth and body composition in broiler chickens. Intracellular signaling, transcriptional regulation, and membrane trafficking are important cellular processes contributing to proper hormone production and secretion. The data also suggest that pituitary function is intimately tied to structure, and organization of the gland could influence hypothalamic and systemic metabolic inputs and delivery of hormones regulating growth and metabolism into peripheral circulation.

Development of the Cerebellum in the Platypus (Ornithorhynchus anatinus) and Short-Beaked Echidna (Tachyglossus aculeatus)

The monotremes are a unique group of mammals whose young are incubated in a leathery-shelled egg and fed with milk from teatless areolae after hatching. As soon as they hatch, monotreme young must be able to maneuver around the nest or maternal pouch to locate the areolae and stimulate milk ejection. In the present study, the embryological collections at the Museum für Naturkunde, Berlin, have been used to follow the development of the monotreme cerebellum through incubation and lactational phases, to determine whether cerebellar circuitry is able to contribute to the coordination of locomotion in the monotreme hatchling, and to correlate cerebellar development with behavioral maturation. The structure of the developing monotreme cerebellum and the arrangement of transitory neuronal populations are similar to those reported for fetal and neonatal eutherians, but the time course of the key events of later cerebellar development is spread over a much longer period. Expansion of the rostral rhombic lip and formation of the nuclear and cortical transitory zones occurs by the time of hatching, but it is not until after the end of the first post-hatching week that deep cerebellar neurons begin to settle in their definitive positions and the Purkinje cell layer can be distinguished. Granule cell formation is also prolonged over many post-hatching months and the external granular layer persists for more than 20 weeks after hatching. The findings indicate that cerebellar circuitry is unlikely to contribute to the coordination of movements in the monotreme peri-hatching period. Those activities are most likely controlled by the spinal cord and medullary reticular formation circuitry.

Development of the dorsal and ventral thalamus in platypus (Ornithorhynchus anatinus) and short-beaked echidna (Tachyglossus aculeatus)

The living monotremes (platypus and echidnas) are distinguished from therians as well as each other in part by the unusual structure of the thalamus in each. In particular, the platypus has an enlarged ventral posterior (VP) nucleus reflecting the great behavioural importance of trigeminosensation and electroreception. The embryological collections of the Museum für Naturkunde in Berlin were used to analyse the development of the dorsal thalamus and ventral thalamus (prethalamus) in both species. Prosomeric organization of the forebrain emerged at 6 mm crown-rump length (CRL), but thalamic neurogenesis did not commence until about 8-9 mm CRL. Distinctive features of the dorsal thalamus in the two species began to emerge after hatching (about 14-15 mm CRL). During the first post-hatching week, dense clusters of granular cells aggregated to form the VP of the platypus, whereas the VP complex of the echidna remained smaller and divided into distinct medial and lateral divisions. At the end of the first post-hatching week, the thalamocortical tract was much larger in the platypus than the echidna. The dorsal thalamus of the platypus is essentially adult-like by the sixth week of post-hatching life. The similar appearance of the dorsal thalamus in the two species until the time of hatching, followed by the rapid expansion of the VP in the platypus, is most consistent with ancestral platypuses having undergone changes in the genetic control of thalamic neurogenesis to produce a large VP for trigeminal electroreception after the divergence of the two lineages of monotreme.

Effects of mannanoligosaccharide in broiler chicken diets on growth performance, energy utilisation, nutrient digestibility and intestinal microflora

1. A study was undertaken to evaluate the effects of mannanoligosaccharide (MOS, Bio-MOS, Alltech Inc.) on the growth performance, energy utilisation, nutrient digestibility and intestinal microflora of birds given a sorghum–wheat based diet. Two MOS levels (1 and 2 g/kg) were included in the diet. 2. Inclusion of MOS at both levels in the diet improved the apparent metabolisable energy (AME) values of the diet. However, these effects were not as pronounced as those of zinc bacitracin (ZnB) treatment. Dietary ZnB also significantly improved the net energy value of the diet. No significant differences between the different levels of MOS were noticed in the growth performance, AME and net energy values of the diet. Compared to the negative control, inclusion of 2 g/kg MOS tended to improve feed conversion efficiency (FCE) in the starter phase. 3. Dietary MOS did not affect the apparent total tract digestibility of nutrients compared to the negative control. In contrast, ZnB significantly improved the protein digestibility and tended to increase the starch digestibility. The addition of MOS reduced the concentration of arabinose in the soluble non-starch polysaccharides (NSP) fraction in the excreta of birds; whereas, the concentrations of individual sugars in the insoluble NSP and free sugar fractions were increased by ZnB. 4. A decrease in the populations of lactobacilli and coliforms in the ileal and caecal lumen was observed for MOS and ZnB treatments. Correspondingly, pH and microbial fermentation in the gut was altered. The addition of MOS tended to reduce the coliform load at the gut mucosa. 5. Results from the current study suggest that MOS can improve the apparent energy utilisation of the diet and tend to improve FCE of birds in the first three posthatch weeks, which may be partly related to the modulatory effects of MOS on the gut microflora.

Functional delay of myelination of auditory delay lines in the nucleus laminaris of the barn owl

In the barn owl, maps of interaural time difference (ITD) are created in the nucleus laminaris (NL) by interdigitating axons that act as delay lines. Adult delay line axons are myelinated, and this myelination is timely, coinciding with the attainment of adult head size, and stable ITD cues. The proximal portions of the axons become myelinated in late embryonic life, but the delay line portions of the axon in NL remain unmyelinated until the first postnatal week. Myelination of the delay lines peaks at the third week posthatch, and myelinating oligodendrocyte density approaches adult levels by one month, when the head reaches its adult width. Migration of oligodendrocyte progenitors into NL and the subsequent onset of myelination may be restricted by a glial barrier in late embryonic stages and the first posthatch week, since the loss of tenascin-C immunoreactivity in NL is correlated with oligodendrocyte progenitor migration into NL.

Development of Defense Mechanisms in the Digestive Tract of the Chick

The intestinal lumen contains food and bacteria in close contact with the mucus layer, which lines the absorptive surface. This matrix contains IgA and other antibacterial substances and acts as a barrier to harmful components and organisms. The underlying epithelial layer is protected by intraepithelial lymphocytes at the junctions between enterocytes and by the action of IgA within the enterocytes. The luminal contents are continuously sampled by M cells and dendritic cells in lymphoid tissue, which then present antigens to the complete array of immune cells present in the lamina propria, which can then mount appropriate defenses.This system is immature at hatch with the intestinal lymphoid tissues developing in the first posthatch days, mainly in the distal intestines. Lymphocyte colonization begins in the first posthatch week, whereas functional T lymphocytes and the presence of IgA secreting cells are only observed in the second posthatch week, and maturation occurs later. Thus, the hatchling appears to be susceptible to pathogens in the immediate posthatch period.

Relationship between intraocular pressure and eye growth in chick.

In infants, abnormally high intraocular pressure (IOP) results in excessive eye enlargement. In the study reported here, we investigated whether IOP might be a determinant of ocular elongation using the chick as an animal model. Specifically, we examined IOP changes in (I). normally developing eyes, and (II). eyes undergoing altered growth. In the first case (I). developmental changes in IOP, axial length and refractive error were assessed at approximately daily intervals during early development (days 2-11 post-hatch, n = 8), and at weekly intervals from weeks 1 to 6 (n = 8). In the second case (II). opposite ocular growth responses were elicited using -15 D and +15 D defocusing spectacle lenses fitted monocularly to 8-day-old chicks (n = 8 and 7, respectively). Treated eyes were measured 3 and 7 h after lens application (between 9 and 10 am), as well as 1, 2 and 4 days later, around the same time as the initial lens application. In normal development (I). IOP increased over the first post-hatch week, peaking at 18.0 +/- 5.1 mmHg, and declined slowly thereafter to be back to near 'hatching' values at 6 weeks (12.7 +/- 5.3 mmHg at day 2 cf. 13.4 +/- 1.9 mmHg). Eyes elongated at an approximately linear rate over the entire monitoring period. The -15 D and +15 D lenses (II). produced opposing effects on eye growth, as indicated by axial length changes of + 0.67 +/- 0.25 mm cf. -0.33 +/- 0.17 mm over the 4-day treatment period. Both groups showed decreases in IOP over the first 20 h of lens wear. The faster growing eyes of the -15 D group exhibited a later relative increase in IOP. The described changes in IOP are compatible with, but do not prove, a modulatory influence of IOP on early eye growth.

Distribution and developmental change of lymphoid tissues in the chicken proventriculus.

In the chicken proventricular mucosa, aggregations of lymphocytes were localized in three different sites of the lamina propria, namely, underneath the surface epithelium, near the duct orifice of the deep proventricular gland, and in the gland tissue itself. In the lymphoid masses underneath the surface epithelium and in those near the duct orifice, CD4+ T lymphocytes and TCR2+ T lymphocytes occupied their central part, and B lymphocytes were localized in the periphery. CD8+ T lymphocytes and TCR1+ lymphocytes were evenly distributed in the masses. Infiltration of lymphocytes into these sites was first observed on the 20th embryonic day. At 1 week after hatching, CD3+ lymphocytes began to occupy the central area of the masses and His-C1+ B lymphocytes tended to be located in the periphery. Ultrastructurally, M cells were found neither in the epithelium of the mucosa nor in that of the excretory duct close to the lymphoid masses. In the deep proventricular gland, the lymphoid masses had a germinal center consisting of B lymphocytes, surrounded by the T lymphocyte-rich periphery. These masses were first recognized at the 3rd post-hatching week, presumably being formed against possible antigens invading into the lumen of the proventricular gland. On the other hand, the lymphoid masses beneath the surface epithelium and those near the duct orifice existing before the hatching period were considered to be prepared to establish the local mucosal immune barriers against the expectant antigenic invasion.

Developmental changes in 7-ethoxyresorufin- O-deethylase (EROD) and δ-aminolevulinic acid dehydratase (ALA-D) activities in three passerines

In this study, we report changes in 7-ethoxyresorufin- O-deethylase (EROD) and δ-aminolevulinic acid dehydratase (ALA-D) enzyme activities during development of three wild passerine bird species: pied flycather ( Ficedula hypoleuca), great tit ( Parus major) and blue tit ( P. caeruleus). Activities were determined from late embryos, newly hatched, 1-week-old and 15-day-old nestlings and adult birds. EROD activity from hepatic microsomes and ALA-D activity from liver tissue and erythrocytes were measured. In all species investigated EROD and ALA-D activities were increased during the first posthatching week and decreased thereafter. Also, differences between activities in late-term nestlings and adults were seen. Based on these results, we suggest that the developmental status of an animal must always be taken into account when evaluating the results obtained in the biomarker studies.

Distribution of glial fibrillary acidic protein and vimentin-immunopositive elements in the developing chicken brain from hatch to adulthood.

The present study describes the distribution of glial fibrillary acidic protein (GFAP) and vimentin-immunopositive structures in the brain of the domestic chicken (Gallus domesticus) from hatching to maturity. The telencephalon is penetrated by a vimentin-immunopositive radial fibre system, representing a modified form of radial glia, in day-old chicks. Numerous fibres of this system persist until adulthood, mainly in the lobus parolfactorius, lamina medullaris dorsalis and lamina frontalis superior. GFAP immunoreactivity also appears in the course of development in these fibres. The distribution of GFAP-immunopositive astrocytes in the post-hatch telencephalon is like that found in adult chicken, except for the ectostriatum, in which an adult-like GFAP-immunostaining only develops during week three. This delay may be associated with a relatively slow maturation of this visual centre. In the diencephalon and in the mesencephalic tegmentum of day-old chicks GFAP-immunopositive astrocytes are confined to the border zone of several nuclei. In these areas as well as in the pons most GFAP positive astrocytes only appear gradually during the first two post-hatch weeks, although radial fibres occur only sparsely at hatch. Summarizing these results, a gradual replacement of radial fibres by astrocytes, typical of mammals, cannot be found in chicken. In the nucleus laminaris we observed a characteristic palisade of non-ependymal glia, reactive to GFAP but not to vimentin, which almost completely disappears by adulthood. We suggest that this glial system is instrumental in the development of the dendritic organisation of this nucleus. The optic tectum displays a dense array of GFAP-immunopositive radial glia at hatching, similar in this to the situation found in reptiles. However, in the tectum of reptiles this radial glia persists for the lifetime, whereas in the chick it disappears from the superficial tectal layers. This phenomenon may reflect the fact that there is no replacement of tectal cells or regeneration of retinotectal pathways in the chicken. In the early stage, the large cerebral tracts were found to contain dense accumulations of GFAP-positive cells, with peculiarly long outgrowths accompanying nerve fibres. No vimentin-immunopositivity was found in these glial elements; however vimentin was present in the glia situated at the optic chiasm, the anterior commissure and at other decussations. These structures, as well as the raphe, displayed the most intense vimentin-immunopositivity in the post-hatch chicken. This characteristic glial population may represent glial elements that have been reported to regulate fibre-crossing at the midline.

The timing of food availability affects growth in chicks.

Three experiments challenged the ability of domestic chicks to grow normally by differentially restricting when and for how long food was available. In Experiments 1 and 2, food was available for six 1-hr, three 2-hr, two 3-hr, or one 6-hr (a.m., p.m.) periods/day over the first 3 posthatch weeks. Control groups received continuous access to food. In Experiment 3, different amounts of light surrounded the 6-hr feeding period. In Experiments 2 and 3, chicks composed their own diets from separate sources high in protein or carbohydrate. Except for the single 6-hr meal preceding dark, large meals at other times of day impaired growth--primarily because chicks consumed insufficient dietary protein and ate less earlier in the light phase. We conclude that both the amount eaten and the proportion of the diet consumed as protein at given times of the day are phylogenetically acquired strategies that fit the omnivorous, diurnal chicken to its niche, independent of its momentary requirements, and appear early in development.

Development of dynorphin-like immunoreactive auditory nerve terminals in the chick

The novel discovery that auditory nerve terminals in the chick cochlear nucleus magnocellularis (NM) are immunoreactive for the opioid peptide dynorphin (DYN) was recently reported [3]. The present study examines the development of DYN-immunoreactivity (DYN-I) in auditory nerve terminals in NM from embryos through young post-hatch chicks. No DYN-I was observed in NM at embryonic day 13 (E13). DYN-I first appeared at E16 as short flat structures partially surrounding NM cell bodies. Around post-hatch day 1 (P1), these structures had a more rounded, chalice-type of morphology reminiscent of the specialized auditory nerve terminals found in birds, the end-bulbs of Held. At P6, most NM neurons were circumscribed by a prominent DYN-I calyceal-type of ending. By P13, fewer NM cells were ringed by this DYN-I and by the third post-hatch week, there was very little DYN-I in NM. There were no obvious differences in the density of DYN-I terminals across either the rostrocaudal length or the mediolateral width of NM at any age examined. These results suggest that during a restricted time of development, end-bulbs of Held in the chick NM contain DYN.

Microglia development in the quail cerebellum

We used the QH1 antibody to study changes in the morphological features and distribution of microglial cells throughout development in the quail cerebellum. Few microglial precursors were present in the cerebellar anlage before the ninth incubation day (E9), whereas many precursors apparently entered the cerebellum from the meninges in the basal region of the cerebellar peduncles between E9 and E16. From this point of entry into the nervous parenchyma, they spread through the cerebellar white matter, forming a ‘stream’ of labeled cells that could be seen until hatching (E16). The number of microglial cells in the cerebellar cortex increased during the last days of embryonic life and first posthatching week, whereas microglial density within the white matter decreased after hatching. As a consequence, the differences in microglial cell density observed in the cerebellar cortex and the white matter during embryonic life diminished after hatching, and microglia showed a nearly homogeneous pattern of distribution in adult cerebella. Ameboid and poorly ramified microglial cells were found in developing stages, whereas only mature microglia appeared in adult cerebella. Our observations suggest that microglial precursors enter the cerebellar anlage mainly by traversing the pial surface at the basal region of the peduncles, then migrate along the white matter, and finally move radially to the different cortical layers. Differentiation occurs after the microglial cells have reached their final position. In other brain regions the development of microglia follows similar stages, suggesting that these steps are general rules of microglial development in the central nervous system. J. Comp. Neurol. 389:390–401, 1997. © 1997 Wiley-Liss, Inc.

Microglia development in the quail cerebellum

We used the QH1 antibody to study changes in the morphological features and distribution of microglial cells throughout development in the quail cerebellum. Few microglial precursors were present in the cerebellar anlage before the ninth incubation day (E9), whereas many precursors apparently entered the cerebellum from the meninges in the basal region of the cerebellar peduncles between E9 and E16. From this point of entry into the nervous parenchyma, they spread through the cerebellar white matter, forming a 'stream' of labeled cells that could be seen until hatching (E16). The number of microglial cells in the cerebellar cortex increased during the last days of embryonic life and first posthatching week, whereas microglial density within the white matter decreased after hatching. As a consequence, the differences in microglial cell density observed in the cerebellar cortex and the white matter during embryonic life diminished after hatching, and microglia showed a nearly homogeneous pattern of distribution in adult cerebella. Ameboid and poorly ramified microglial cells were found in developing stages, whereas only mature microglia appeared in adult cerebella. Our observations suggest that microglial precursors enter the cerebellar anlage mainly by traversing the pial surface at the basal region of the peduncles, then migrate along the white matter, and finally move radially to the different cortical layers. Differentiation occurs after the microglial cells have reached their final position. In other brain regions the development of microglia follows similar stages, suggesting that these steps are general rules of microglial development in the central nervous system.

Testosterone elevates expression of tenascin-R and oligomannosidic carbohydrates in developing male zebra finches.

The song system of zebra finches is a model for studying the influence of steroids on neural connectivity and behavior during development. To investigate the molecular mechanisms underlying the song-related and gonadal hormone-regulated development of neural activity, we have chosen to investigate the expression of recognition molecules in the brain nuclei associated with motor control of song production. Here we show that testosterone accelerates expression of the predominantly oligodendroglia-, but also neuron-associated extracellular matrix glycoprotein tenascin-R and the oligomannosidic carbohydrate L3 during the third and seventh posthatching week in the higher vocal center (HVC) and robust nucleus of the archistriatum (RA), but not in other brain regions. The results suggest that recognition molecules and associated carbohydrate structures can be regulated by testosterone and that an increased expression of these molecules correlates with testosterone-induced modifications of song behavior.

Estrogen promotes neuron addition to an avian song-control nucleus by regulating post-mitotic events

Only male zebra finches sing and several telencephalic song control regions exhibit sex differences in neuron number that presumably reflect effects of estrogen (E2) exerted during the first few posthatch weeks. That is, implanting females with E2 during this time masculinizes neuron number and instills the capacity for vocal behavior. In certain song regions, E2 masculinizes neuron number by preventing the naturally-occurring death of neurons, long after their production, migration and process outgrowth are complete. However, in the Higher Vocal Center (HVC), the cellular mechanisms by which E2 establishes sex differences in neuron number are poorly understood. In contrast with other song regions, HVC neurogenesis overlaps with sexual differentiation and the incorporation of new neurons is greater in young males and E2-treated females, than in normal females. However, it is not known whether E2 promotes the addition of HVC neurons by stimulating their production, specification, and/or survival. To address this issue we injected males and females with [ 3H]thymidine on days 15 and 16 to label a small group of sexually dimorphic HVC neuronal cohorts born during sexual differentiation. Afterwards, on day 17, females were implanted with Silastic pellets filled with estradiol benzoate (EB) or left empty. We report here that EB exposure on day 17 masculinized (increased) the number of neurons in the HVC at day 35 that were labeled by [ 3H]thymidine injections on days 15/16. Thus, EB was able to increase cell number among at least some HVC neuronal cohorts after their final division, implying estrogenic regulation of post-mitotic events. When viewed with other studies that have examined the development of sex differences in the HVC, our results suggest that during sexual differentiation E2 promotes the specification and/or early survival of HVC neurons.

Sexual differentiation of brain and behavior in the zebra finch: Critical periods for effects of early estrogen treatment

In order to determine the critical period(s) during which estrogen alters sexually dimorphic behavior and neuroanatomy in zebra finches (Poephila guttata), nestlings were injected daily with 20 micrograms estradiol benzoate (EB) during posthatching week 1, week 2, week 3, or weeks 1, 2, and 3. At 7 months of age, birds were implanted with testosterone propionate and tested with female partners for singing, dancing, and copulatory mounting. Brains were subsequently processed for morphometry, and the volumes of the song system nuclei HVC, area X, and RA and the soma sizes and densities of neurons in RA were determined. Males given EB during week 1 failed to mount. Females given EB during week 1 were fully masculinized with respect to dancing and RA neuron soma size and density, and were partially masculinized with respect to song nuclei volumes and singing. Treatment beginning after week 1 was ineffective or less effective for all measures. Only for RA neuron measures was treatment for all three weeks more effective than week 1 treatment. Thus the first post-hatching week is the most influential period of those tested for effects of exogenous estrogen on sexual differentiation in this species, and is a period during which both masculinization of females and demasculinization of males is possible.