Endocrine Department of Histology and Embryology Li jinxin No.0453

Key Points The structure and function of thyroid, parathyroid, adrenal and pituitary gland.

Difficult Points The relationship between hypothalamus （下丘脑） and neurohypophysis （神经垂体）. The component and significance of pituitary portal system （垂体门脉系统）.

Composing endocrine glands and diffuse endocrine cells thyroid and parathyroid adrenal pituitary gland pineal body pancreatic islets interstitial cells corpus luteum thymus

endocrine glands The characteristic of endocrine glands Endocrine glands are ductless. Cells arranged in glomerulus, cords, reticulum or in follicles. Rich in blood and lymphatic capillaries between cells. Release hormones either into the vascular system (endocrine), or to their surroundings (paracrine), and bind to specific receptors of target cells.

Classification of Endocrine Cells Nitrogen-containing hormone-secreting cells （含氮激素分泌细胞） Found in thyroid, parathyroids, adrenal medulla, pituitary and pineal glands, etc. Contain RER, Golgi complex and membrane bound secretory granules. Hormones are secreted by exocytosis （胞外分 泌）.

Steroid hormone-secreting cells （ Steroid hormone-secreting cells （类固醇激素分泌细胞） Found only in adrenal cortex, interstitial cells, corpus luteum. Contain abundant SER, mitochondria with tubular cristae and lipid droplets. Hormones are secreted by diffusion.

1. Thyroid 1.1 General structure Capsule: loose connective tissue sends septa into parenchyma, dividing the gland into lobules. follicles: which are functional units and varied in size. Parafollicular cells: between follicles is rich in fenestrated capillaries and also contains parafollicular cells.

follicle parafollicular cell capillary colloid

1.2 Thyroid follicles a. Composed of a simple follicular epithelium, follicular lumen filled with colloid. b. The follicular epithelium is composed of the follicular epithelial cells and the parafollicular cells. follicular epithelial cell parafollicular cell

follicular epithelial cells Height of cells ranges from squamous to columnar, depending on the functional activity of follicles. The cytoplasm is weakly basophilic or acidophilic.

Ultrastructure Apical surface is covered by numerous microvilli. Cytoplasm rich in RER in basal part, well- developed Golgi complex in supranuclear area, secretory granules, pinocytotic colloid vesicles in apical part and abundant lysosomes. Cells are joined laterally by junctional complexes, preventing thyroglobulin （甲状腺球蛋白） leakage from follicles into surrounding stroma.

Function synthesis and release of thyroid hormones (T3, T4) T3 and T4 increase rate of metabolism and influence development of central nervous system during fetal and neonatal life.

The secretory process of follicular epithelial cell Synthesis of thyroglobulin (TG) in RER and released into lumen of follicles. Iodination of TG. iodide taken up and oxidized. The iodinated TG is stored as colloid in the follicular cavity. Pinocytosis. follicular cells take up iodinated TG, forming pinocytotic vesicles. Release of thyroid hormones. iodinated TG is degraded, and forms thyroxine (T4) and triiodothyronine (T3), released into capillaries.

compensatory thyroid hyperplasia

Low-secretory of thyroid hormones: neonatal→cretinism adult →mucous edema High-secretory of thyroid hormones: hyperthyroidism

1.3 Parafollicular cells or C cells Located between follicular epithelial cells or between follicles as isolated clusters. Large and pale stained with H & E, argyrophilic granules （嗜银颗粒） with silver staining. Under EM, many small secretory granules. Secrete calcitonin （降钙 素）, which lowers blood calcium level.

1.4 Colloid Homogeneous, acidophilic, gelatinous substance. Chemical composition is iodinated thyroglobulin （碘化甲状腺球蛋白）. Vacuoles （空泡） at the periphery may represent pinocytosis of colloid.

2. Parathyroid gland Consist of 2 types of cells: Chief cells are small, pale stained and contain secretory granules; oxyphil cells are large, acidophilic because of many mitochondria. Parathyroid hormone secreted by chief cells elevates blood calcium by increasing activity of osteoclasts.

3. Adrenal gland 3.1 General description Covered by a capsule of dense connective tissue. Parenchyma consists of an outer adrenal cortex, and an inner adrenal medulla. The cortical cells are steroid-secreting, and the medullar cells are nitrogen hormone- secreting.

3.2 Adrenal cortex subdivided into three layers according to cell arrangement Zona glomerulosa ( 球状带） Zona fasciculata ( 束状带 ) Zona reticularis ( 网状带 )

3.2.1 Zona glomerulosa Beneath the capsule, occupy 15% of total volume of the glands. Composed of round clusters of columnar cells surrounded by capillaries. Secrete mineralocorticoids( 盐皮质 激素 ), which maintain electrolyte and water balance by acting on distal renal tubules and stimulating resorption of sodium.

3.2.2 Zona fasciculata Most thick layer in middle account for 65% of total volume of glands. Composed of straight cords of large, polyhedral cells with acidophilic, sponge cytoplasm. Cells have typical ultrastructural characteristics of steroid-secreting cells. Synthesize glucocorticoids ( 糖皮质激素 ), which regulate carbohydrate, protein and lipid metabolism, and also suppress immune responses.

3.2.3 Zona reticularis Innermost layer of smaller cells arranged in anastomosing cords, 7% of total volume. Produce sex hormones, mainly androgens （男 性激素）, and small amounts of cortisol.

3.3 Adrenal medulla Composed of polyhedral cells arranged in nests or short cords. Cells contain chromaffin granules ( 嗜铬颗粒 ), so called chromaffin cells. Weekly basophilic cytoplasm.

Function Synthesize and release epinephrine ( 肾上 腺素 ) and norepinephrine ( 去甲肾上腺素 ) by two different groups of cells. Epinephrine increases the heart rate. Norepinephrine increases the blood pressure and blood flow to the heart, brain and skeletal muscle.

4. Pituitary Gland Consists of two parts: adenohypophysis and neurohypophysis. pars distalis and pars tuberalis make up anterior lobe. Pars nervosa and pars intermedia make up posterior lobe. pars distalis pars intermedia pars tuberalis pars nervosa infundibular stalk median eminence

4.1 Pars distalis largest part containing three types of cells interspersed with sinusoids Acidophil cells Basophil cells Chromophobe cells

4.1.1 Acidophil cells 40%. Round or ovoid, and numerous coarse acidophilic granules. Two types of acidophil cells. Somatotrophs synthesize and release growth hormone, which stimulates body growth, particularly growth of long bones. Mammotrophs secrete prolactin （催乳激 素， PRL ）, which initiates and maintains the milk secretion after pregnancy.

Lack of somatotropin →dwarf Plethora of somatotropin infancy→gigantism adult →acromegaly

4.1.2 Basophil cells Thyrotrophic cells secrete thyrotropin or thyroid stimulating hormone (TSH). Corticotrophic cells secrete adrenocorticotrophic hormone (ACTH). Gonadotrophic cells secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH). 10%. Slightly larger than acidophil cells, with basophilic granules. Three types of basophil cells.

4.1.3 Chromophobe cells 50%, Clusters of small cells without distinct boundaries, and pale stained. Few secretory granules, indicating they are undifferentiated precursor cells or degranulated chromophils.

4.2 Pars intermedia 4.2 Pars intermedia Composed of chromophobes and basophil cells arranged either in cords or in colloid- containing follicles. Secretes melanocyte stimulating hormone (MSH).

4.3 Relationship between pars distalis and hypothalamus Activities of the cells in the pars distalis are controlled by hypothalamic neuroendocrine neurons that produce peptide releasing hormones and inhibitory hormones, which are transported to the pars distalis through the hypophyseal portal system. Hypophyseal portal system includes superior hypophyseal artery → the primary plexus → hypophyseal portal veins → the secondary plexus

Hypophyseal portal system superior hypophyseal artery primary plexus hypophyseal portal veins secondary plexus inferior hypophyseal artery

FSH LH GH PRL ACTH MSH TSH

4.4 Neurohypophysis median eminence infundibular stalk pars nervosa

4.4.1 Structure of neurohypophysis Unmyelinated nerve fibers. They originate from neurons of hypothalamic supraoptic and paraventricular nuclei. Herring bodies. Pituicytes are glial cells with irregular shape and short processes contain lipid droplets and pigment granules in their cytoplasm. Sinusoids.

4.4.2 Function Stores and releases vasopressin (antidiuretic hormone, ADH) and oxytocin. Vasopressin stimulates water resorption in kidneys and contraction of vascular smooth muscle. Oxytocin promotes contraction of uterine smooth muscle during parturition and contraction of myoepithelial cells in mammary glands during lactation.

4.5 The Relationship Between Hypothalamus and Neurohypophysis They are the entia of structure and function. Hypothalamus secrete the hormones. Neurohypophysis transport, store and release hormones.

4.6 Diffuse Neuroendocrine System (DNES) APUD ( amine precursor uptake and decarboxylation cell s ，摄取胺前体脱羧细胞 ) and n euroendocrine cells （神经内分泌细胞）. They secrete amine or peptides hormone. It cooperates with APUD and then unify nervous system and endocrine system, called d iffuse neuroendocrine system (DNES).

Endocrine System Exercises: 1.State briefly the structure and function of thyroid gland? 2. State briefly the structural and functional characteristic of adrenal cortex. Pars distalis 3.What are the kind of cells in the Pars distalis of adenohypophysis? Describe their structure and function. 4.What is the functional significance of the hypophyseal portal system?