Neuroendocrine Systems and Avian Sexual Differentiation

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Abstract
Sexual differentiation of the endocrine and behavioral components of the avian reproductive system occurs throughout embryonic development, initiating some of the early cellular events that form the central nervous system (CNS) and culminating in the organization of male or female neuroendocrine responses. Early cellular events have been studied intensively in recent years and these developmental processes appear to involve specific growth factors in the development of certain tissues. Subsequent to cellular differentiation, primordial germ cells migrate to the appropriate anatomical location and contribute to the development of the single ovary or testes; steroidogenesis begins soon thereafter. Other portions of the hypothalamo-pituitary-gonad (HPG) axis also appear during early embryonic development with migration of the gonadotropin-releasing hormone (GnRH) neurons from the olfactory region of the CNS to the midbrain and separate formation of the pituitary gland. The gonadal steroid hormones affect development of accessory sex structures as well as the later organization of neuroendocrine regulatory systems and secondary sex characteristics. Manipulation of steroids during embryonic and early posthatch periods results in altered endocrine and behavioral responses in adult birds. There are marked species differences in the timing of these events, especially when precocial and altricial species are compared. Altricial species hatch in a less developed state and as such are more dependent on parental care. Of necessity, the precocial species must be capable of feeding and other motor capabilities at hatch and coincidentally their other physiological systems also appear to be more mature at this time. Finally, there is the separate issue of song birds versus those avian species that do not have elaborate learned songs. It appears that most of the species with elaborate neural circuitry responsible for song are altricial. This may benefit them from the standpoint of gaining more time and contact with the parents to enable them to learn the appropriate song. There are also hormonal and neuroendocrine components critical in the process of song development. Finally, the effects of environmental factors, such as endocrine disrupting chemicals take on added significance when viewed in the context of exerting permanent organizational effects which are likely to alter endocrine and behavioral components of reproduction in the adult

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