The Endocrine System

Intercellular Communication

Endocrine versus Nervous system

• Nervous system performs short term crisis management

• Endocrine system regulates long term ongoing metabolic

• Endocrine communication is carried out by endocrine cells releasing hormones

• Alter metabolic activities of tissues and organs

• Target cells

• Paracrine communication involves chemical messengers between cells within one tissue

An Overview of the Endocrine System

Endocrine system

• Includes all cells and endocrine tissues that produce hormones or paracrine factors

Hormone structure

• Amino acid derivatives

• Structurally similar to amino acids

• Peptide hormones

• Chains of amino acids

• Lipid derivatives

• Steroid hormones and eicosanoids

Hormones can be

• Freely circulating

• Rapidly removed from bloodstream

• Bound to transport proteins

Mechanisms of hormone action

• Receptors for catecholamines, peptide hormones, eicosanoids are in the cell membranes of target cells

• Thyroid and steroid hormones cross the membrane and bind to receptors in the cytoplasm or nucleus

Control of endocrine activity

• Endocrine reflexes are the counterparts of neural reflexes

• Hypothalamus regulates the activity of the nervous and endocrine systems

• Secreting regulatory hormones that control the anterior pituitary gland

• Releasing hormones at the posterior pituitary gland

• Exerts direct neural control over the endocrine cells of the adrenal medullae

Endocrine System

The Pituitary Gland

Hypophysis

• Releases nine important peptide hormones

• All nine bind to membrane receptors and use cyclic AMP as a second messenger

Gland

The anterior lobe (adenohypophysis)

• Subdivided into the pars distalis, pars intermedia and pars tuberalis

• At the median eminence, neurons release regulatory factors through fenestrated capillaries

• Releasing hormones

• Inhibiting hormones

The Endocrine System

Hypophyseal portal system

• All blood entering the portal system will reach the intended target cells before returning to the general circulation

Hormones of the adenohypophysis

• Thyroid stimulating hormone (TSH)

• Triggers the release of thyroid hormones

• Thyrotropin releasing hormone promotes the release of TSH

• Adrenocorticotropic hormone (ACTH)

• Stimulates the release of glucocorticoids by the adrenal gland

• Corticotrophin releasing hormone causes the secretion of ACTH

Hormones of the adenohypophysis

• Follicle stimulating hormone (FSH)

• Stimulates follicle development and estrogen secretion in females and sperm production in males

• Leutinizing hormone (LH)

• Causes ovulation and progestin production in females and androgen production in males

• Gonadotropin releasing hormone (GNRH) promotes the secretion of FSH and LH

Hormones of the adenohypophysis

• Prolactin (PH)

• Stimulates the development of mammary glands and milk production

• Growth hormone (GH or somatotropin)

• Stimulates cell growth and replication through release of somatomedins or IGF

• Growth-hormone releasing hormone
(GH-RH)

• Growth-hormone inhibiting hormone
(GH-IH)

Melanocyte stimulating hormone (MSH)

• May be secreted by the pars intermedia during fetal development, early childhood, pregnancy or certain diseases

• Stimulates melanocytes to produce melanin

The posterior lobe of the pituitary gland (neurohypophysis)

• Contains axons of hypothalamic nerves

• neurons of the supraoptic nucleus manufacture antidiuretic hormone (ADH)

• Decreases the amount of water lost at the kidneys

• Elevates blood pressure

The posterior lobe of the pituitary gland (neurohypophysis)

• Neurons of the paraventricular nucleus manufacture oxytocin

• Stimulates contractile cells in mammary glands

• Stimulates smooth muscle cells in uterus

The Thyroid Gland

The thyroid

• Lies near the thyroid cartilage of the larynx

• Two lobes connected by an isthmus

Thyroid follicles and thyroid hormones

• Thyroid gland contains numerous follicles

• Release several hormones such as thyroxine (T4) and triiodothyronine (T3)

• Thyroid hormones end up attached to thyroid binding globulins (TBG)

• Some are attached to transthyretin or albumin

Thyroid hormones

• Held in storage

• Bound to mitochondria, thereby increasing ATP production

• Bound to receptors activating genes that control energy utilization

• Exert a calorigenic effect

Cells of the thyroid gland

• C cells produce calcitonin

• Helps regulate calcium concentration in body fluids

The Parathyroid Glands

Four parathyroid glands

• Embedded in the posterior surface of the thyroid gland

• Chief cells produce parathyroid hormone (PTH) in response to lower than normal calcium concentrations

• Parathyroid hormones plus calcitriol are primary regulators of calcium levels in healthy adults

The Adrenal Glands

Adrenal cortex

• Manufactures steroid hormones (corticosteroids)

• Cortex divided into three layers

• Zona glomerulosa (produces mineralocorticoids)

• Zona fasciculate (produces glucocorticoids)

• Zona reticularis (produces androgens)

Adrenal medulla

• Produces epinephrine (~75 - 80%)

• Produces norepinephrine (~25-30%)

The Pineal Gland

Pineal gland

• Contains pinealocytes

• Synthesize melatonin

• Suggested functions include inhibiting reproductive function, protecting against damage by free radicals, setting circadian rhythms

The Pancreas

The pancreatic islets

• Clusters of endocrine cells within the pancreas called Islets of Langerhans or pancreatic islets

• Alpha cells secrete glucagons

• Beta cells secrete insulin

• Delta cells secrete GH-IH

• F cells secrete pancreatic polypeptide

Insulin and glucagon

• Insulin lowers blood glucose by increasing the rate of glucose uptake and utilization

• Glucagon raises blood glucose by increasing the rates of glycogen breakdown and glucose manufacture by the liver

The Endocrine Tissues of Other Systems

The intestines

• Produce hormones important to the coordination of digestive activities

The kidneys

• Produce calcitriol and erythropoietin (EPO) and the enzyme rennin

• Calcitriol = stimulates calcium and phosphate ion absorption along the digestive tract

• EPO stimulates red blood cell production by bone marrow

• Renin converts angiotensinogen to angiotensin I

Angiotensin I converted to angiotensin II in the lungs

• Stimulates adrenal production of aldosterone

• Stimulates pituitary gland release of ADH

• Promotes thirst

• Elevates blood pressure

The heart

• Specialized muscle cells produce natriuretic peptides when blood pressure becomes excessive

• Generally oppose actions of angiotensin II

The thymus

• Produces thymosins

• Help develop and maintain normal immune defenses

The gonads

• Interstitial cells of the testes produce testosterone

• Most important sex hormone in males

• In females, oocytes develop in follicles

• Follicle cells produce estrogens

• After ovulation, the follicle cells form a corpus luteum that releases a mixture of estrogens and progesterone

Adipose tissues secrete

• Leptin, a feedback control for appetite

• Resistin, which reduces insulin sensitivity

Patterns of Hormonal Interaction

Hormones often interact, producing

• Antagonistic (opposing) effects

• Synergistic (additive) effects

• Permissive effects (one hormone is required for the other to produce its effect)

• Integrative effects (hormones produce different but complimentary results)

Hormones and growth

• Normal growth requires the interaction of several endocrine organs

• Six hormones are important

• GH

• Thyroid hormones

• Insulin

• PTH

• Calcitriol

• Reproductive hormones

Hormones and stress

• Stress = any condition that threatens homeostasis

• GAS (General Adaptation Syndrome) is our bodies response to stress-causing factors

• Three phases to GAS

• Alarm phase (immediate, fight or flight, directed by the sympathetic nervous system)

• Resistance phase (dominated by glucocorticoids)

• Exhaustion phase (breakdown of homeostatic regulation and failure of one or more organ systems)

Hormones and behavior

• Many hormones affect the CNS

• Changes in the normal mixture of hormones significantly alters intellectual capabilities, memory, learning and emotional states

Aging and Hormone Production

Endocrine system

• Few functional changes with age

• Chief change is a decline in concentration of reproductive hormones