Endocrine System Department of Histology and Embryology Zhang Xi-mei

Introduction Composition: Endocrine glands (thyroid, parathyroid, adrenal gland, hypophysis ,and pineal body) Endocrine cell mass Endocrine cells Function: Regulate the activities of various cells, tissues, and organs in the body. Maintain homeostasis and coordinate body growth and development.

Characteristics of endocrine glands Cells are arranged in cord, follicle, aggregation or net-like structure. Ductless. Contain rich blood vessels. The endocrine cells are always close to the capillaries, which receive the secreted hormones.

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

Thyroid It is composed of thyroid follicles. The follicles are lined by a simple cuboidal epithelium. The central cavity contains a gelatinous substance called colloid, which is composed of thyroglobulin.

Follicular epithelium Follicular cells Parafollicular cells

Follicular cells They are responsible for production of thyroxine (T4)and triiodothyronine(T3). 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.

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 Ca2+ level by inhibiting the activity of osteoclasts.

Parathyroid Four small glands. Located behind the thyroid gland.

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

Chief cells More numerous. Secretory granules in the cytoplasm (contain parathyroid hormones, PTH), which can increase the blood Ca2+ level. PTH and calcitonin have reciprocal effects in the regulation of blood calcium level .

Oxyphil cells Larger than chief cells, contains many acidophilic granules (mitochondria). The function is unknown.

They are paired organs lie near the superior poles of the kidneys. They are flattened structures with a half-moon shape.

Adrenal gland Cortex: located in the periphery. It is the steroid-secreting portion. Medulla: located in the center. It is the catecholamine-secreting portion.

Adrenal gland Cortex: it is divided into 3 zones : Zona glomerulosa (15%) Zona fasciculata (80%) Zona reticularis (5%)

Zona glomerulosa It is the narrow outer zone. The cells are arranged in closely packed, rounded, or arched cords. The cells secrete mineralocorticoid (aldosterone). Aldosterone acts on the distal tubules of the kidney,and the gastric mucosa to stimulate the resorption of sodium by the epithelial cells.

Zona fasciculata Arranged in one-or two- cell thick straight cords. A great number of lipid droplets in the cytoplasm. Secrete glucocorticoid (cortisol). Promote normal metabolism, particularly carbohydrate metabolism, which increases blood glucose levels. Suppress the immune response by destroying circulating lymphocytes.

Zona reticularis It lies between the zona fasciculata and the medulla. It contains cells disposed in irregular cords that form network. It secrete gonadocorticoids or sex hormones, influences the sexual development.

Adrenal medulla Chromaffin cells (medullary cells). They have abundant granules in the cytoplasm. Epinephrine-secreting cells and norepinephrine-secreting cells secrete epinephrine and norepinephrine, respectively. These two hormones are secreted in response to stimulation by sympathetic nerve, particularly during stressful situations.

Hypophysis Weight 0.5g. Most important endocrine gland. Lies in a cavity of the sphenoid bone—the sella turcica.

Hypophysis Adenohypophysis pars distalis pars tuberalis pars intermedia Neurohypophysis pars nervosa infundibulum median eminence neural stalk (Anterior lobe) (Posterior lobe)

Adenohypophysis Acidophils Somatotropic cell Mammotropic cell Chromophils Thyrotropic cell Basophils Gonadotropic cell Chromophobe Corticotropic cell

Cells of Adenohypophysis Somatotropic cell They produce growth hormone (GH, somatotropin) which can stimulates growth, stimulates growth of long bone, muscular and skeletal development. Mammotropic cell They produce prolactin (PRL) which promotes mammary gland development, initiates milk formation.

Thyrotropic cell They produce Thyroid-Stimulating Hormone (TSH), or thyrotropin. It stimulates thyroid hormone synthesis, storage and liberation. Gonadotropic cell They secrete Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH). In females, FSH promotes follicle development and secretion of estrogens. In males, FSH stimulates the maturation of developing sperm. In females, LH promotes ovarian follicle maturation and progesterone secretion. In males, LH is essential for maintenance of androgen secretion. Corticotropic cell They produce adrenocorticotropic hormone (ACTH), or corticotropin, which stimulates the release of the adrenal cortex.

Pars tuberalis Pars intermedia It is a funnel-shaped region surrounding the infundibulum of the neurohypophysis. Most of the cells in the Pars tuberalis secrete gonadotropins. Pars intermedia Contains follicles. Functions unknown.

Neurohypophysis pars nervosa is composed of unmyelinated axons of neurosecretory neurons, whose cell bodies locate in paraventricular and supraoptic nuclei in hypothalamus Infundibulum (Composed of median eminence and neural stalk). Herring bodies: the axon dilations in pars nervosa come from neurosecretory neurons, are light acidophilic and contain many secretory granules (antidiuretic hormone, ADH and oxytocin, OCT).

Major function of ADH The antidiuretic hormone (ADH, vasopressin), which is produced by supraoptic nuclei, promote water reabsorption in kidney. In large dose, ADH also stimulates the contractions of smooth muscle of small arteries and arterioles. Major function of Oxytocin Oxytocin is synthesized by paraventricular nuclei in the hypothalamus and is stored in the neurohypophysis. In the uterus, oxytocin stimulates the contraction of the uterus in order to deliver the fetus. In the mammary glands, oxytocin also stimulates the ejection of milk when infants suckle.

Homework Describe the secretory cells of the pars distalis. Herring bodies. Parafollicular cells. Describe the structure and function of adrenal gland.