Endocrine System Department of Histology and Embryology Yu hongwei

Introduction I Composition Endocrine glands(thyroid,parathyroid,adrenal gland, hypophysis,and pineal body ) Endocrine cell mass(pancreas islets,etc) Endocrine cells(distributed in the digestive tract)

Introduction II Function Regulate the activities of various cells,tissues,and organs in the body. Maintain homeostasis and coordinate body growth and development. The endocrine system interacts closely with the nervous system and the lymphatic system.

Characteristics of endocrine glands Cells are arranged in cord, follicle, aggregation or net-like structure, Ductless. Ductless. containing rich blood

What is Hormone? Hormone is molecule that function as chemical signals. Most hormones act at a distance from the site of their secretion. Therefore,the endocrine cells are always close to the capillaries,which receive the secrete hormones and distribute them throughout the organism.

paracrine secretion However, many cells produce hormones act at a short distance by diffusing through the extracellular matrix.This is called paracrine secretion.

Classification of the Hormones Steroids(cholesterol-derived compounds) Small peptides,proteins,and glycoproteins Amino acid(catecholamines)

Steroids Are synthesized and secrete by cells of ovaries,testes,and adrenal cortex. These hormones are released into the bloodstream and transported to the target cells with the help of specialized carrier proteins such as androgen –binding protein(ABP)

Small peptides,proteins,and glycoproteins Are synthesized and secrete by cells of hypothalamus,thyroid gland,parathyroid gland,pancreas. Hormones in this group,when release into circulation,dissolve readily in the blood and do not require special carrier proteins.

Amino acid Including the catecholamines(norepinephrine and epinephrine), Are synthesized and secrete by many neurons and the adrenal medulla. Hormones in this group,when release into circulation,dissolve readily in the blood.

How do the Hormones act on the target cell? The target cell have receptors that specifically recognize and response to the hormones. Cell surface receptors(peptide hormones) Intracellular receptors(steroid hormones)

Thyroid gland Locate in the cervical region anterior to the larynx,consists of two lobes united by an isthmus. It originates the endoderm. Its function is to synthesize the hormones thyroxine (T4)and triiadothyroxine(T3).

Thyroid gland It is composed of thyroid follicle.The follicles are lined by a simple epithelium. The central cavity contains a gelatinous substance called colloid, which is composed of a glycoprotein of high molecular mass called thyroglobulin.

Follicular epithelium Follicular cells Parafollicular cells

Follicular cells These cells vary in shape and size according to the functional state of the gland. In routine HE staining, follicular cells exhibit a slight basophilic. They are responsible for production of thyroxine (T4)and triidothyroxine(T3).

Ultrastructure of follicular cells Short microvilli on the apical surface. Golgi complex,mitochodria,lysosome,rough endothelial reticulum(rER) are present in the cytoplasm.

Synthesis of T 3 and T 4 Take place in the thyroid follicle 1 Synthesis of thyroglobulin AA→ RER→ thyroglobulin → Golgi complex (secreting granules) → colloid. 2 Resorption,diffusion, and oxidation of iodide Iodide → (iodide pump) → iodide oxidation in cytoplasm → colloid

Formation of T3 and T4 3 Iodination of thyroglobulin. bound to thyroglobulin → iodated thyroglobulin(MIT,DIT). 4 Formation of T3 and T4 by oxidative coupling reactions The coupling of these molecules (MIT,DIT) Produce T3 and T4.

Formation of T3 and T4 5 Resorption of colloid Stimulated by TSH, Follicular cells take up thyroglobulin by endocytosis. 6 Release of T4 and T3 into circulation T4 and T3 cross the basal membrane and are discharged into the capillaries.

Major function of T4 and T3 regulate growth and development :most cells require T3/T4 for growth. The nervous system is especially sensitive. calorigenic effect: T3/T4 increase basal metabolic rate (BMR) and affect temperature regulation. cardiovascular effects: increase heart rate.

T3 and T4 control almost every metabolic activity in your body so big variations in their levels can cause serious problems.

hypothyroidism hypothyroidism means circulating levels of T3/T4 are below normal. In children this leads to cretinism and in adults myxedema. Adults suffering from hypothyroidism show a very low rate of metabolism and are extremely lethargic. Body temperature and heart rate are all low.

cretinism Congenital absence of T3 and T4, or chronic iodine deficiency in infancy. Cretinism is associated with retarded growth, sluggish movements,and severe mental deficiencies. The growth deficiencies can be corrected by providing thyroid extract or iodine supplementation soon after birth, but mental impairment tends to be irreversible.

Hyperthyroidism In the adult, hyperthyroidism means circulating levels of T3/T4 are above normal. It is associated with a high rate of metabolism, high body temperature, and high heart rates. There are several types of hyperthyroidism including is termed Grave's Disease which is an autoimmune disorder.

Graves' disease is a autoimmune disorder in which the body makes antibodies to the thyroid-stimulating hormone receptor (TSHR), The autoantibodies produced in Graves' disease are not subject to negative feedback, so they continue to be produced and bind to TSHR even when thyroid hormone levels rise too high. A sign often associated with hyperthyroidism is protrusion of the eyeballs.

Parafollicular cells (C cells) Locate in the follicular epithelium or as isolated among thyroid follicles. Larger than follicular cells and less stained. Secrete calcitonin, which can decrease the blood Ca 2+ level by inhibiting the activity of osteoclasts.

Parafollicular cells

Parathyroid Four small glands Locate behind the thyroid gland

Structure Cells are arranged in cords. Chief cells(principal cells) Oxyphil cells

Chief cells More numerous cells of the parathyroid gland. Electron microscope shows granules in the cytoplasm.They are the secretory granules containing parathyroid hormones(PTH),which can increase the blood Ca 2+ level.

PTH is essential for life.Therefore,care must be taken during thyroidectomy.If the glands are totally removed,death will ensue because muscles,including the laryngeal and other respiratory muscles,go into tetanic contraction as the blood calcium level falls. PTH and caltitonin have reciprocal effects in the regulation of blood calcium level.

Oxyphil cells They are larger than chief cells and their cytoplasm contains many acidophilic granules. The TEM reveals these granules are mitochondria. The function and importance of such a large concentration of mitochondria in these cells are unknown.

Homework Describe the structure and function of the thyroid gland and parathyroid gland.