Pituitary gland adenohypophysis

The adenohypophysis is derived embryonically from glandular tissue, specifically, Rathke s pouch. This tissue originates from the oropharynx, or the roof of the mouth. It then migrates toward the embryonic nervous tissue destined to form the neurohypophysis. When these two tissues come into contact, Rathke s pouch loses its connection with the roof of the mouth and the pituitary gland is formed. Unlike the neurohypophysis, which releases hormones originally synthesized in the hypothalamus, the adenohypophysis synthesizes its own hormones in specialized groups of cells. Similar to the neurohypophysis, however, the release of these hormones into the blood is regulated by the hypothalamus. 

The pituitary gland is situated in sella turcica or hypophyseal fossa of the sphenoid bone attached to the brain by a stalk which is continuous with the part of brain i.e. hypothalamus and there is a communication between the hypothalamus and the pituitary gland by means of nerve fibres and a complex of blood vessels. Pituitary gland consists of three parts - anterior lobe or adenohypophysis, posterior lobe or neurohypophysis and middle lobe or pars intermedia. 

The control of metabolism, growth, and reproduction is mediated by a combination of neural and endocrine systems located in the hypothalamus and pituitary gland. The pituitary weighs about 0.6 g and rests at the base of the brain in the bony sella turcica near the optic chiasm and the cavernous sinuses. The pituitary consists of an anterior lobe (adenohypophysis) and a posterior lobe (neurohypophysis) (Figure 37-1). It is connected to the overlying hypothalamus... 

Connected to the brain by a stalk (Fig. 30-1), the pituitary gland releases at least ten peptide or protein hormones that regulate the activity of other endocrine (hormone-producing) glands in distant parts of the body. The pituitary is composed of several distinct parts the anterior lobe (adenohypophysis), a thin intermediate portion (pars intermedia), and a posterior lobe (neurohypophysis). Each has its own characteristic endocrine functions. 

Pituitary Hormones. The hormones of the hypophysis (pituitary gland) are quite numerous, being secreted variously in three parts of the gland — the ncurohypophysis (posterior lobel. the adenohypophysis (anterior lithe), and the pars intermedia, which connects the other two. 

Administration of estrogens is well known to cause an increase in plasma prolactin (PRL) levels in man (43, 44) as well as in the rat (45-48). This stimulatory effect of estrogens is also observed in vitro in anterior pituitary gland explants (49), tumoral adenohypophysial cells (50) and normal rat anterior pituitary cells in primary culture (39, 40, 42, 51). Seventeen-g-estradiol (E2) does not only stimulate basal and TRH-induced PRL secretion in rat anterior pituitary cells in culture but it can also reverse almost completely the inhibitory effect of dopamine (DA) agonists on PRL release (40). 

The pituitary gland (hypophysis) is located at the base of the skull (Figure 50-1) in a bone cavity called the sella turcica (Turkish saddle). The gland is small—1 cm or less in height and width and weighs approximately 500 mg. As discussed previously, the gland is anatomically divided into the anterior (adenohypophysis) and the posterior (neurohypoph-ysis) lobes. A third lobe (the intermediate lobe) is present in most vertebrates and in the human fetus this lobe is rudimentary in the adult human. 

Arterial blood reaches the pituitary gland via the superior hypophyseal artery, a branch of the internal carotid artery. Venous blood is supplied through a venous portal system that originates in the median eminence of the hypothalamus and ends in sinusoidal capillaries of the pituitary gland. This venous system is known as the hypothalamic-hypophyseal portal system. This system carries neurosecretory hormones from the hypothalamus to the adenohypophysis. These hypothalamic factors stimulate or inhibit the release of hormones from the adenohypophysis. Retrograde flow from the adenohypophysis to the median eminence of the hypothalamus is also believed to occur. With upstream flow, pituitary hormones can reach the hypothalamus and influence hypothalamic function through a short feedback loop. 

The pituitary gland, also referred to as the hypophysis, is located at the base of the brain in a cavity of the sphenoid bone known as the sella turcica. The pituitary is separated from the brain by an extension of the dura mater known as the diaphragma sella. The pituitary is a very small gland, weighing between 0.4 and 1 g in adults. It is divided into two distinct regions, the anterior lobe, or adenohypophysis, and the posterior lobe, or the neurohypophysis (see Fig. 75-1). 

It is now firmly established that the synthesis and release of the hormones of the adenohypophysis are controlled by substances released from nerve endings in the hypothalamus and conveyed to the anterior pituitary gland via the portal vessels. The evidence which demonstrated the fundamental importance of the hypothalamus and the hypothalamo-hypophysial portal vessels came mainly from experiments which involved transection of the pituitary stalk, transplantation of the pituitary gland to a site remote from the sella turcica or electrical stimulation of the hypothalamus. 

As early as 1936, it was realised that diffuse electrical stimuli applied to the head or lumbar spinal cord of rabbits [6] or rats [7] enhance adenohypophysial activity. In an attempt to delimit the neural structures involved, closely localised electrical stimuli were applied directly to regions of the hypothalamus and anterior pituitary gland of anaesthetised animals. Electrical stimulation of the pituitary gland was ineffective in causing gonadotrophin release but such stimulation was fully effective when applied to discreet areas of the hypothalamus... 

The realisation that the hypothalamus contains substances capable of affecting pituitary activity naturally led to attempts to isolate and identify these neurohormones. It seemed unlikely that the traditional chemical transmitters of the parasympathetic and sympathetic nervous system, acetylcholine and noradrenaline, were responsible since neither influenced adenohypophysial activity when injected directly into the pituitary gland. Histamine and adrenaline were also ineffective in this respect despite their being present in considerable quantities in the hypothalamus. Attempts to separate the hypothalamic hormones, or Veleasing hormones as they are known, has therefore primarily involved chemical extractions of the hypothalamus. This laborarious work, which has been... 

Since prolactin has no target organ-inhibitory feedback system similar to those of TSH, ACTH, LH or FSH, it was proposed that high circulating levels of prolactin may inhibit the release of the hormone by the pituitary gland [251]. Ample experimental confirmation of this hypothesis has appeared in recent years. Either systemic injection or implantation of minute amounts of prolactin into the median eminence significantly reduces prolactin in the adenohypophysis and blood and inhibits mammary development and lactation in the rat. The inhibitory action of prolactin is believed to be exerted at the hypothalamic level since increases in hypothalamic PIF activity [252,253] and in the activity of the... 

Combined immunohistochemical, in situ hybridisation, and biochemical studies provide evidence for nitric oxide synthase synthesis in two cell populations of the anterior pituitary gland, gonadotrophs and folliculo-stellate cells, and for an inhibitory effect of NO on stimulated release of LH (Cec-CATELLi et al. 1993). The folhculo-stellate cells do not produce any known hormones (Horvath and Kovacs 1988) but may indirectly influence growth-hormone secretion. Thioredoxin was prominently localised in the folliculo-stellate cells of the mammalian adenohypophysis while only a minor proportion of the glandular cells were positive (Padilla et al. 1992). Glutaredoxin localisation in the adenohypophysis resembled that of thioredoxin. 

The gland consists of the posterior pituitary (neurohypophysis) and the anterior pituitary (adenohypophysis). The functions of the two are not related but both are controlled by the hypothalamus. (See Figure 6.2.)... 

Kidneys, pineal gland, and Ihymus may also be considered by some as endocrine glands, and during pr nancy, Ihe placenta has some endocrine functions. The anterior pituitary is also called the adenohypophysis while the posterior ptuitary may he called the neurohypophysis. 

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