Endocrine system

Chapter 23
Endocrine system

Organs located in different parts of the body contain specialised secretory cells which produce hormones. These specialised secretory cells may form defined endocrine organs, the endocrine glands, or they may occur as organised clusters within organs which do not have a solely endocrine function. In addition, endocrine cells may be present as solitary cells distributed in many tissues throughout the body. Collectively the organs, clusters of cells and individual cells with specialised secretory activity constitute the endocrine system. Unlike the products of exocrine glands, which are conveyed through ducts, endocrine secretions diffuse into the bloodstream and are carried to target cells, tissues or organs. Endocrine secretions play a central role in regulating and coordinating the normal physiological activities of the body. The functioning of some endocrine organs may be stimulated or inhibited by hormones secreted by other endocrine organs. Defined endocrine glands include the pituitary gland, the pineal gland, the adrenal glands, the thyroid gland and the parathyroid glands. Organs which contain groups of endocrine cells include the pancreas, the testes and ovaries, and, in pregnant female mammals, the placenta. The endocrine cells of the placenta and gonads and the functional roles of the hormones they produce are briefly reviewed in Chapters 12 and 21 respectively. Because thymic epithelial reticular cells secrete hormones which contribute to the maturation of T lymphocytes, the thymus can be considered as an organ with some endocrine activity. Cells of the diffuse endocrine system are found in gastrointestinal tract epithelium, the conducting airways of the respiratory system, the juxta‐glomerular apparatus of the kidney, atrial myocardium and hepatic tissue. The autonomic nervous system influences the activity of several endocrine organs.

Pituitary gland

Ectoderm of both oral and neural origin contributes to the formation of the pituitary gland (hypophysis cerebri). The portion of the pituitary gland which develops from a midline evagination of oral ectoderm from the roof of the stomodeum, is referred to as the adenohypophysis (Fig 23.1). The primordial structure from which the adenohypophysis develops is known as the adenohypophyseal pouch or Rathke’s pouch. A second component of the pituitary gland, the neurohypophysis, develops from a ventral diverticulum in the floor of the diencephalon known as the infundibulum. In domestic mammals, the two primordial structures meet and fuse, forming the pituitary gland.

Diagram illustrating the sequential stages (A–E) in the formation of the pituitary gland, with parts labeled.

Figure 23.1 Sequential stages in the formation of the pituitary gland (A to E).

The adenohypophyseal pouch grows dorsally towards the infundibulum and gradually loses its connection with the oral ectoderm, forming the adenohypophyseal vesicle. Cells of the rostral wall of the vesicle proliferate at a higher rate than cells of the caudal wall. The space formed following mural proliferation is referred to as the adenohypophyseal cleft. Proliferating cells from the dorsal aspect of the rostral wall surround the stalk of the infundibulum, forming the pars tuberalis. The remaining cells of the rostral wall proliferate, forming aggregations of cells which give rise to the pars distalis. From the infundibulum, the hypophyseal stalk and an enlarged distal area, the pars nervosa of the pituitary, are formed. Cells of the pars distalis differentiate into endocrine cells, which, on the basis of their staining characteristics, can be classified as acidophils, basophils and chromophobes. The acidophils are the source of growth hormone and prolactin, while the basophils give rise to the trophic hormones, adrenocorticotrophic hormone (ACTH), thyroid‐stimulating hormone (TSH), follicle‐stimulating hormone (FSH), and luteinising hormone (LH). Chromophobes are considered to be either stem cells or non‐secreting stages of acidophils or basophils. In the pars distalis, acidophils, basophils and chromophobes are not evenly distributed and show species variation both in their numbers and in their distribution. The cell types in the pars tuberalis are similar to those present in the pars distalis.

The caudal wall of the adenohypophyseal vesicle which undergoes little proliferation and forms the pars intermedia, contacts the infundibulum. The extent of fusion of these two structures accounts for the anatomical relationship of the different regions of the pituitary gland in domestic animals. In humans, following fusion of the pars intermedia with the rostral surface of the neural lobe, continued proliferation of the pars distalis obliterates the adenohypophyseal cleft. Because of limited proliferation of the pars distalis in ruminants, the cleft persists. An unusual feature of the pituitary gland in ruminants is the presence of a small segment of pars distalis‐like tissue attached to the rostral surface of the pars intermedia. In horses, pigs and carnivores, the pars intermedia encloses the infundibulum so that the pars intermedia is in direct contact with the surface of the pars nervosa. The hypophyseal cleft, which persists in carnivores and pigs, is obliterated in horses. The most abundant cell type of the pars intermedia is a large, round, pale‐staining cell which produces melanocyte‐stimulating hormone. These large cells may sometimes form colloid‐filled follicles (Fig 23.2).

Image described by caption.

Figure 23.2 Relationships of the components of the fully‐formed porcine pituitary gland and histological features of the pars distalis, pars intermedia and pars nervosa, A; relationships of the components of the canine, equine, feline and bovine pituitary glands are shown in B, C, D and E respectively.

Processes of neurons from the supraoptic and paraventricular nuclei of the hypothalamus project into the infundibular stalk and extend into the developing pars nervosa. Neurosecretions from the supraoptic and paraventricular nuclei, antidiuretic hormone and oxytocin, are transported along axons to the pars nervosa where they are stored. The majority of glial cells of the pars nervosa are modified astrocytes and are referred to as pituicytes. The functioning of the adenohypophysis is under the control of hypothalamic hormones which either stimulate or inhibit secretions of particular cell types of the pars distalis. These hypothalamic hormones are carried to the pars distalis through the hypothalamic‐hypophyseal portal system. Release of the hypothalamic hormones is modified by feedback mechanisms from target organs influenced by hormones from the pars distalis.

Molecular regulation of pituitary gland development

The earliest transcription factors expressed in the pituitary primordium include Six‐3, Pax‐6 and Rathke’s pouch homeobox (Rpx). Subsequently, Shh, Pitx, Ptx and P‐Otx are expressed continuously throughout the oral ectoderm. Bmp‐4 signals from the ventral diencephalon suppress the expression of Shh, creating a molecular border between oral and pouch ectoderm. Subsequently, expression of Bmp‐2 can be detected at the oral ectoderm–Rathke’s pouch boundary. Concurrently, Fgf‐8 and Wnt‐5a are expressed within the ventral region of the diencephalon. Fgf‐8 is also expressed in the infundibulum. Based on the expression levels of Fgf‐8 and Bmp‐2, gradients of transcription factors Six‐3, Nkx‐3.1 and Prop‐1 are expressed dorsally and Brn‐4, Isl‐1, P‐Frk and GATA‐2 are expressed ventrally. This variable expression of transcription factors along the dorsal–ventral axis not only establishes pituitary commitment but also induces the determination, formation and differentiation of the pituitary gland. Development and differentiation of pituitary gland cells is also specified by the homeodomain transcription factors Rpx, Ptx, Lhx‐3, Prop‐1 and Pit‐1.

Pineal gland

The pineal gland (epiphysis cerebri) develops as a dorsal diverticulum of the caudal part of the roof of the diencephalon (see Fig 16.21

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Sep 27, 2017 | Posted by in GENERAL | Comments Off on Endocrine system
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