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Lecture notes: Histology of the Pituitary Gland and Endocrine System
- Describe the structural organization of endocrine glands
- Define the components of the endocrine system
- Compare and contrast the cellular composition of the adenohypophysis (anterior pituitary) and neurohypophysis (posterior pituitary gland)
- Describe the hypophyseal portal system and its physiological significance
- Compare and contrast the cellular composition of the adrenal cortex and adrenal medulla
- Describe the organization and cellular features of the thyroid gland and parathyroid glands and the hormones their cells produce
- Explain the structural features and functions of the pineal gland
- Bone as an endocrine organ
- Define the diffuse neuro-endocrine system
HISTOLOGY OF ENDOCRINE GLANDS
1. Describe the structural organization of endocrine glands
The endocrine system is a collection of glands that secrete hormones, molecules that transmit chemical messages. Hormones are released to the bloodstream and act on cells that express the appropriate receptor in target organs. Endocrine cells are typically composed of islands of secretory cells of epithelial origin that discharge their products into capillaries, unlike the cells of exocrine glands that release their products into an epithelial duct.
Based on their chemical structure, hormones can be classified as:
- proteins and glycoproteins
- small peptides
- amino-acid derivatives
- steroids
Types of secretion
Endocrine secretion: the cell content is released to the circulation on the basolateral membrane, through a basement membrane that separates the endocrine cells from the blood stream.
Exocrine secretion: the cell is released from the apical membrane, directly to the body surface or cavity. It can be merocrine, apocrine or holocrine, depending on the mechanism that mediates the secretion.
2. Define the components of the endocrine system
- Major endocrine organs: the sole or major function of the organ is the synthesis, storage and secretion of hormones
endocrine gland exocrine gland
Types of secretion
secretory c
capillary
Hypothalamic hormones Hormone Target Functions Growth hormone- releasing hormone Somatotropic cells^
Stimulates synthesis and release of GH Tyrotropin-releasing hormone
Thyrotropic and mammotropic cells
Stimulates synthesis and release of TSH and prolactin Corticotropin-releasing hormone Corticotropic cells
Stimulates synthesis of POMC and release of β-LPH and ACTH Gonadotropin-releasing hormone Gonadotropic cells^
Stimulates the release of FSH and LH Prolactin inhibitory factor (dopamine) Mammotropic cells^ Inhibits release of prolactin Somatostatin Somatotropicthyrotropic cells^ and^ Inhibits the release of GH andTSH Vassopresin (anti-diuretic hormone)
Collecting ducts in the kidney
Stimulates water re- absorption
Oxytocin
Myoepithelial cells (mammary glands) and smooth muscle (uterus)
Stimulates contraction of the target cells
3. Compare and contrast the cellular composition of the adenohypophysis (anterior pituitary) and neurohypophysis (posterior pituitary gland) a. Pituitary gland (hypophysis) The pituitary gland is located below the brain in a cavity of the sphenoid bone, the sella turcica. The pituitary consists of two glands, the anterior adenohypophysis, develops in the embryo from the oral ectoderm, and the posterior neurohypophysis, developed from the brain. The two glands that form the pituitary are united anatomically but have different functions. - Adenohypophysis Formed by 3 structures; the pars distalis or anterior lobe, the pars tuberalis and the pars intermedia. o Pars distalis : its main components are cords of epithelial cells interspersed with fenestrated capillaries. Fibroblasts produce reticular fibers supporting the cords of hormone-secreting cells. Hormones produced in the pars distalis regulate the function of other endocrine glands, milk secretion, melanocyte activity and the metabolism of muscle, bone and adipose tissue. Based on staining affinity, cells can be classified in chromophils, containing
cytoplasmic granules that can be basophilic or acidophilic, and chromophobes. Basophilic chromophils are: Gonadrotropic: produce follicle-stimulating and luteinizing hormones Corticotropic: produce corticotrophin and melanocyte stimulating hormone Thyrotropic: produce thyrotropin Acidophilic chromophils are: Somatotropic: produce growth hormone Mammotropic: produce prolactin Chromophobes are: stem and undifferentiated progenitor cells degranulated chromophilic cells As cells that produce different hormones may stain similarly, specific cell types cannot be identified by light microscopy staining, only by TEM or immunohistochemistry. o Pars tuberalis : funnel-shaped region surrounding the infundibulum of the neurohypophysis. Most of the cells are basophilic gonadrotropic cells. o Pars intermedia : thin zone of basophilic cells between the pars distalis and pars nervosa of the neurohypophysis. It develops from the dorsal wall of the hypophysis and contains colloid-filled cysts that represent remnants of that structure’s lumen.
It derives from a downgrowth of the nervous tissue from the hypothalamus. Contains nerve axons and glial cells (called pituicytes); it is comprised of two parts: o Pars nervosa : expanded distal portion containing the neurosececretory axons of the hypothalamo-hypophyseal tract. The cell bodies of these fibers lie in the supraoptic and paraventricular nuclei of the hypothalamus. The nerve terminals of these neurons are atypical in that they are non-synaptic and secrete hormones rather than neurotransmitters. The hormones are stored in granules at the nerve terminal and are visible in the light microscope as Herring bodies (also called neurosecretory bodies). The pars nervosa also contains fibroblasts, mast cells and pituicytes. o Infundibular stack : It also contains neurosecretory nerve endings from hypothalamic neurons (other than supraoptic and paraventricular nuclei). The hormones secreted in these regions enter the hypothalamo-
o Zona reticularis : the innermost part of the cortex, in contact with the adrenal medulla. The cells in this region produce mainly dehydroepiandrosterone, a weak androgen that can be converted into testosterone in several tissues.
Adrenal gland
- Adrenal medulla : it derives from the neural crest. It is composed of chromaffin cells that can be considered modified postganglionic neurons. Chromaffin cells contain electron-dense granules filled with either epinephrine or norepinephrine. Epinephrine-producing cells locate near cortical sinusoids, whereas norepinephrine-producing cells locate near medullary arterioles. 6. Describe the organization and cellular features of the thyroid gland and parathyroid glands and the hormones their cells produce
a. Thyroid gland The thyroid gland is located in the cervical region anterior to the larynx. It originates from the foregut endoderm near the base of the future tongue. The parenchyma is formed by rounded epithelial structures called thyroid follicles. Each follicle consists of a simple epithelium surrounding a central lumen filled with colloid, which contains tyroglobulin, the precursor for the active thyroid hormones, thyroxine (or tetra- iodotyronine, T 4 ) and tri-iodotyronine, T 3. Parafollicular cells (C cells), which produce calcitonin, can be found between follicles. Synthesis of thyroid hormones
The production, storage and release of thyroid hormones involve a multistage process with both exocrine and endocrine:
- Thyroglobulin (hormonally inactive) is synthesized in the RER and glycosylated in the RER and Golgi
- Circulating iodine is incorporated through the basolateral cell membrane of the follicular cells
- Iodine is transferred to the follicular lumen where it undergoes oxidation to active iodine
- Tyrosine residues of tyroglobulin are iodinated in the lumen
- Follicular cells take up colloid by endocytosis or pinocytosis. The endocytic vesicles fuse with lysosomes and move to the basolateral cell membrane while the thyroglobulin is degraded to render T 3 and T 4 , which are then release to the circulation.
b. Parathyroid gland The parathyroids are four small glands located on the back of the thyroid gland. They derive from the pharyngeal pouches in the embryo. Each parathyroid gland is contained within a capsule that sends septa into the gland. The glands are composed of two types of cells, the chief cells that contain granules with parathyroid hormone; and the oxyphil cells, larger than chief cells, with acidophilic cytoplasm and abnormally shaped mitochondria. Parathyroid hormone (PTH) is produced by the chief cells. Lowered extracellular Ca 2+^ removes the calcium sensing receptor-dependent repression of PTH production. PTH is then free to mobilize Ca 2+^ from stores in bone. PTH production may also be regulated by phosphate.
7. Explain the structural features and functions of the pineal gland
The pineal gland is a small, pine cone-shaped organ in the brain. It develops with the brain from neuroectoderm in the roof of the diencephalon. It is covered by connective tissue. The most abundant secretory cells are the pinealocytes, which produce melatonin. It also contains interstitial glial cells, similar to astrocytes.
8. Bone as an endocrine organ
In addition to its function providing protection and support to the soft organs and as a source of calcium, bone cells can produce molecules that are released to the circulation and affect other cells and tissues. Examples of hormone-like molecules produced by bone cells are fibroblast growth factor-23 (FGF-23) and osteocalcin.
