Overview of the endocrine system, anatomy and microanatomy of the thymus, pituitary gland, hypothalamus, thyroid gland, parathyroid glands, adrenal glands, gonades, pancreas. Hormones secretions and main pathways.
The Endocrine System
The endocrine system is a chemical messenger system consisting of a group of glands that secretes
hormones directly into the circulatory system to regulate the function of distant targets, and the feedback
loops which modulate hormone release so that homeostasis is maintained. Special features of endocrine
glands are, in general, their ductless nature, their vascularity, and the presence of intracellular granules
that store hormones. A number of glands that signal each other in sequence are usually referred to as an
axis, for example, the hypothalamic-pituitary-adrenal axis. Hormones are normally secreted in the
bloodstream, however sometimes they might be released in the connective tissue to perform paracrine
signalling or autocrine control.
The major glands of the endocrine system include the pineal gland, pituitary gland, pancreas, ovaries,
testes, thyroid gland, parathyroid gland, thymus, hypothalamus and adrenal glands. The
hypothalamus and pituitary gland are neuroendocrine organs. In addition, endocrine secretions are
produced by bone tissue, digestive tract, heart, adipose tissue and kidneys.
Although there are many different hormones in the human body, they can be divided into three classes
based on their chemical structure:
1. Lipid-Derived Hormones derive from cholesterol and thus are structurally similar to it. The primary
class of lipid hormones in humans is the steroid hormones. They are secreted by the gonads, as as
oestrogen and testosterone, and by adrenal glands, as aldosterone and cortisol. Steroid hormones are
insoluble in water, and they are transported by transport proteins in blood. As a result, they remain in
circulation longer than peptide hormones.
2. Amino acid-Derived Hormones are relatively small molecules derived from tyrosine and tryptophan.
Their chemical name ends in “-ine”. They include catecholamines, and arachidonic acid derivatives
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as prostaglandins, prostacyclins and leukotrienes. They readily solubilise in the bloodstream. Also in
this group are thyroid hormones, which need to bind thyroxine binding proteins in the bloodstream.
3. Peptide Hormones have a polypeptide chain structure. They include molecules that are short
polypeptide chains, such as antidiuretic hormone and oxytocin. This class also includes small proteins,
like growth hormones, and large glycoproteins such as FSH. Amino acid-derived and polypeptide
hormones are water-soluble and insoluble in lipids. These hormones cannot pass through plasma
membranes of cells; therefore, their receptors are found on the surface of the target cells. This class of
hormones is produced by the hypothalamus, pituitary gland, thyroid, parathyroid, pancreas and
scattered enteroendocrine cells in the GI tract and respiratory system. Small peptides readily dissolve
in circulation, while polypeptides and proteins might need protein carriers.
PITUITARY GLAND
It is a pea-sized gland, centrally located at the base of the brain into the cella turcica. A short stalk, the
infundibulum, and a vascular network, connect it to the hypothalamus. Together they regulate the other
endocrine glands. The pituitary gland has two functional components, of different embryologic origin, the
anterior lobe (adeno-hypophysis) is composed of glandular epithelial tissue, while the posterior lobe
(neurohypophysis) of neural secretory tissue. The anterior lobe derives from ectoderm of the
oropharynx, also known as Rathke’s pouch, while the posterior lobe from neuroectoderm.
The anterior part of the pituitary gland is subdivided into three. The pars distalis is the bulk of the
anterior lobe, the pars intermedia is a remnant of the posterior wall of the Rathke’s pouch, and the pars
tuberalis forms a collar around the infundibulum. In the anterior lobe cells are typically organised in
clumps and cords separated by fenestrated capillaries. These cells respond to signals from the
hypothalamus, and start synthesising and secreting various hormones:
• The tropic hormones regulate the activity of other endocrine glands. These include the
adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), follicle-stimulating
hormone (FSH) and luteinising hormone (LH).
• The non tropic hormones regulate the activity of exocrine glands or other tissues. These include the
growth hormone (GH) and prolactin (PRL).
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