The endocrine system comprises cells which synthesise particular molecules and secrete them into blood vessels. This contrasts with exocrine secretions in which secreted molecules pass along a duct system to their site of action, e.g. from salivary glands to the mouth. The particular molecules secreted by endocrine cells are known as hormones and the vascular circulation carries them around the body where they interact with various cells described as target cells. The principal means by which hormones achieve their specific action is by interacting with receptor molecules expressed by the target cells in various tissues and organs. Once stimulated, target cells respond in a variety of ways, e.g. by increasing synthesis of certain molecules. Hormones may be steroids, peptides or proteins, or other molecules derived from amino acids. In general, hormones are involved in regulating metabolic activities in cells in many organs and tissues of the body, many of which are important in controlling homeostasis.
The endocrine organs comprise the pituitary gland, thyroid gland, parathyroid glands, adrenal (suprarenal) glands and the pineal gland. In addition, the pancreas contains clusters of endocrine cells known as islets of Langerhans amongst the pancreatic exocrine cells (Chapter 12), the gonads contain cells secreting reproductive hormones (Chapters 15 and 16) and secretions from endocrine cells in the hypothalamus affect the secretory activities of the pituitary gland (see below). There are also small groups of neuroendocrine cells in many other regions, e.g. in the epithelial linings of the gastrointestinal tract. Some are known as paracrine cells and their secretions act in surrounding areas.
All endocrine glands and endocrine cells are well supplied by blood providing the metabolites needed to synthesise hormones. The capillaries in endocrine glands are fenestrated and most hormones are secreted directly into them. A fine meshwork of reticulin fibres supports most endocrine cells and capillaries in endocrine glands. This arrangement facilitates the movement of molecules from and to the capillaries.
Hypothalamus and pituitary gland (hypophysis)
The hypothalamus and pituitary gland function together (Fig. 14.1). The hypothalamus is the part of the brain connected by a stalk to the pituitary gland. The hormones produced by the hypothalamus and the pituitary affect the function of many cells in different parts of the body.
The pituitary gland is situated in the cranial cavity in a depression in the sphenoid bone of the skull known as the pituitary fossa. The pituitary has a secretory (glandular) and a neural component called, respectively, the adeno-hypophysis and neurohypophysis. Connective tissue surrounds these two components as a capsule. The components differ in embryological origin, structure and function. The adenohypophysis develops from oral ectoderm and it has clusters or strands of secretory (glandular) cells. The neurohypophysis develops from the part of the brain which forms the hypothalamus. Nerve axons are a prominent structural component of the neurohypophysis and hormones are released from the axons.
The hormones secreted by the pituitary gland are dependent in two distinct ways on hormones produced by cells in the hypothalamus (Fig. 14.1).
■ Adenohypophysis and its vascular connections (Fig. 14.1). A portal system of capillaries connects the hypothalamus, via the pituitary stalk, with the adenohypophysis. (The definition of a portal system is two capillary beds connected by a set of veins; see hepatic portal system Chapters 10 and 12.) Some hormones produced by neurons in the hypothalamus are secreted into the first capillary bed in the hypothalamus. These capillaries drain into portal veins which carry the hormones along the pituitary stalk to the second capillary bed in the adenohypophysis. By this route hormones from the hypothalamus interact specifically with endocrine cells in the adenohypophysis and control their secretion of a variety of other hormones.
The hypothalamic hormones which pass in the portal system to the adenohypophysis are peptides and most stimulate specific cells in the adenohypophysis to release their specific hormone. These hypothalamic peptides are known as releasing hormones (rH). For example, the rH from the hypothalamus that stimulates cells in the adenohypophysis to release their hormone which stimulates the thyroid is known as thyroid-releasing hormone (TrH). The pituitary hormone is known as thyroid-stimulating hormone (TSH) and it stimulates the thyroid gland to secrete thyroid hormones, e.g. thyroxine (Fig. 14.1). (In contrast, a few peptide hormones from the hypothalamus inhibit the release of some hormones from the adenohypophysis.)
■ Neurohypophysis and its neural connections (Fig. 14.1). Axonal processes of some nerve cell bodies in the hypothalamus extend along the pituitary stalk and into the part of the pituitary gland formed by the neurohypophysis. These neurons synthesise peptide hormones which pass along their axons. These hormones are stored in the end regions of the axons in the neurohypophysis before they are released into the vascular system.
Adenohypophysis
The adenohypophysis consists of three parts, the pars distalis (anterior lobe), the pars intermedia and the pars tuberalis (Fig. 14.1). The cells of the pars distalis form several specific hormones (see below). In humans, the pars intermedia and pars tuberalis are relatively small regions and their significance is not clear.
The cells of the pars distalis have been categorised by the reaction of their cytoplasm to various dyes. Cells that bind dyes are termed chromophils and those that do not are categorised as chromophobes. In routine H&E preparations some chromophils bind eosin and are known as acidophils; others bind haematoxylin and are known as basophils (Fig. 14.2). The differences are due to variations in the pH of the hormones stored as cytoplasmic granules in the endocrine cells. The cytoplasm of chromophobes is virtually unstained with most dyes, and in some chromophobes the cytoplasm is sparse (Fig. 14.3). Other methods of staining cells of the pars distalis emphasise the differences between the chemical composition of their cells by specifically staining the glycoproteins in basophils (Fig. 14.3). Immunohistochemical methods using light and electron microscopy have now determined precisely which cells in the pars distalis produce the various anterior lobe hormones.
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| Fig. 14.2 |
Hormones produced by the adenohypophysis are collectively termed trophic hormones in that they stimulate the release of other hormones from endocrine cells in endocrine glands or in other regions. In general, each endocrine cell in the pars distalis produces only one type of trophic hormone. Some acidophils produce the protein somatotrophin, which is also known as growth hormone (GH) as it is important in stimulating growth, particularly in bones. Other acidophils produce the protein prolactin (PrL), a hormone which has a major effect on milk production by the mammary glands and is also known as mammotrophin. All basophils produce glycoproteins; some produce TSH and others produce adrenocorticotrophic hormone (ACTH), which stimulate, respectively, the thyroid gland and the cells in the cortex of the adrenal glands (see below). Yet other basophils produce two trophic hormones per cell, both categorised as gonadotrophins as they affect endocrine secretion by cells in the gonads (ovaries and testes). The gonadotrophins are follicle-stimulating hormone (FSH) and luteinising hormone (LH). Further details of the function of FSH and LH are described in Chapters 15 and 16.
The pituitary gland is sometimes referred to as the ‘conductor of the endocrine orchestra’ and this term has also been applied to the hypothalamus. They both have coordinating roles in determining the function of many and varied endocrine-secreting cells in the body. In turn, however, many hormones produced by endocrine cells in endocrine glands and in other organs in the body are able to regulate the secretory activity of cells in the hypothalamus and/or the hypophysis that are stimulating their own levels of hormone production. This type of regulatory mechanism is known as negative feedback (Fig. 14.1).
Neurohypophysis
The neurohypophysis is described in three parts: the median eminence, the infundibulum and the pars nervosa (posterior lobe of the pituitary) (Fig. 14.1).
■ Median eminence. This part of the hypothalamus contains:
■ neurons that secrete the hypothalamic releasing hormones which regulate the secretory activity of cells in the pars distalis of the pituitary
■ the primary capillary bed of the portal system which carries these hypothalamic hormones to the pars distalis
■ other hormone-producing neuronal cell bodies which have axons that extend along the infundibulum to the pars nervosa.
■ Infundibulum. This is a continuation of the hypothalamus along the pituitary stalk.
■ Pars nervosa. This is the region where axons from neurons in the median eminence terminate (Fig. 14.4
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