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Describe the anatomical organisation of the pituitary gland. Explain the control and actions of ACTH and the consequences if control is dysregulated.
organisation of the pituitary gland The pituitary gland is a pea sized endocrine gland that sits in the pocket of the sphenoid bone and is attached inferiorly to the median eminence of the hypothalamus of the brain via a pituitary stalk, also known as the ifundibulum. It consists of an anterior lobe (adenohypophysis) which consititutes for 2/3 rd of the volume and a posterior lobe (neurohypophysis) which forms the remaining 1/3rd. The formation of the pituitary gland begins in the 5 th week of embryonic development. The adenohypophysis is formed by an upward invagination of the oropharyngeal ectoderm which forms the Rathke's pouch and this later constricts and is pinched of from the ectoderm. Whereas the pituitary stalk (ifundibulum) and the neurohypophysis are formed from the downward extension of the neural ectoderm in the diencephalon region. The site where the ectoderm of both the anterior and posterior lobe meet forms the intermediate lobe but is anatomically considered to be part of the anterior lobe. The posterior lobe of the pituitary gland is supplied by the internal carotid artery and its branches form a capillary plexus at the base of the hypothalamus. This plexus is continuous with the hypothalamo-hypophysial portal veins which travels down the pituitary stalk into the anterior lobe. This portal vein is a vital connection and allows the neuroendocrine cells of the hypothalamus to control secretions of the hypothalamus. The posterior lobe is continuous with the hypothalamus and is formed by nerve terminals of hypothalamic magnocellular neurosecretory neurons, which are large neuroendocrine cells that secrete oxytyocin and vasopressin on electrical excitation. The hormones secreted by the posterior lobe are synthesised in the hypothalamus which are then packaged into vesicles and are transported down the axons. On nervous stimulation the hormones are released into the systemic veins by excocytosis. The terminals are surrounded by pituicytes.
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