The Endocrine System

Chapter 25

The Endocrine System

• A system of ductless glands that secrete hormones (‘messenger’) molecules - secrete hormones directly into the blood or lymph

- hormones trigger physiological changes in target cells

• Controls and integrates the functions of other body systems- closely interacts with the nervous system

• Endrocrinology- the study of hormones and endocrine glands

Nervous System• Controls homeostsis

rapidly• Anatomically continuous:

nerve impluse conducted along axons from one neuron to the next

• Neurotransmitters (NTs)• Brief effect (muscle

contraction)

Endocrine System• Controls growth and

metabolism slowly• Scattered:

messenger molecules released into the EC space will immediately enter adjacent capillaries

• Hormones (‘to excite’)• Longer lasting effect with

feedback loops

Together these systems interact to coordinate and integrate activity of our cells

Endocrine Organs

• Series of ductless glands - small and scattered throughout the body

- some may be both endocrine and exocrine

• ‘Pure’ endocrine glands - pituitary, pineal, thyroid, parathyroid, and adrenal

• Organs containing endocrine cells

- pancreas, thymus, gonads, and hypothalamus

• Most endocrine cells are of epithelial origin- others include hormone-secreting neurons, muscle cells, and fibroblast-like cells

• Highly vascularized – blood and lymph vessels

Location of the Major Endocrine Glands

Endocrine cells also

occur in the heart,

alimentary canal,

kidney, skin, placenta,

and elsewhere

Fig 25.1

Endocrine System Overview

• Endocrine glands may be stimulated by the nervous system or chemical changes in the body - respond by secreting hormones into the circulation

• Hormones travel through the bloodstream but affect only specific tissues called target tissues

• Hormones secreted by cells regulate the metabolic function of other cells in the body- effects result from pre-programmed responses of target cells

Endocrine Functions• Regulation of:

- Internal environment (adjust fluid/volume ratio): aldosterone- Metabolism and energy balance: thyroid hormones- Cardiac and smooth muscle contraction: epinephrine and norepinephrine- Immune system: cytokines

- Glandular secretions: hypothalamus and pituitary hormones

• Maintainance and assistance of:- Homeostasis despite disruptions: pancreatic hormones- Growth and development: growth hormones- Reproduction: hormones that influence oogenesis and spermatogenesis

Classes of Hormones

• The body produces many different kinds of hormones with distinct chemical structures

• Two broad molecular categories:1. Amino acid-based hormones - modified amino acids (amines), peptides (short chains of amino acids), and proteins (long chains of amino acids)

2. Steroid hormones - lipid molecules derived from cholesterol

Basic Hormone Action

• Hormones circulate throughout the body in BVs- leave the bloodstream at capillaries encountering all body tissues- influences only specific tissue cells or target cells- same hormone can have different effects on different target cells- similar molecular structures can have very different functions

• Cells have receptors on their surface that bind only specific types of hormones- receptor binding initiates a response/reaction - hormones are just molecular triggers and do not carry any coded information

Control of Hormone Secretion• Secretion is triggered by three major types of

stimuli: humoral, neural, and hormonal stimuli

• Humoral (‘body fluids’) - simplest of endocrine control mechanisms - secretion is in direct response to changing critical ion or nutrient levels in the blood- parathyroid monitors calcium: responds to decline be secreting hormone to reverse decline

• Neural - a few glands secrete their hormones in response to stimuli by the nervous system to induce physiological changes - sympathetic nerve fibers stimulate cells in the adrenal medulla- induces release of epinephrine and norepinephrine

Control of Hormone Secretion

• Hormonal - many endocrine glands secrete their hormones in response to hormonal stimuli received from other endocrine glands

- certain hormones signal secretion of other hormones

- the hypothalamus secretes hormones stimulates the pituitary stimulates other glands (the thyroid, adrenal cortex, and the gonads)

Note: the hypothalamus is called the master gland

- controls many functions of the endocrine system, through hormonal and other mechanisms

Control of Hormone Release: 3 Mechanisms

Fig 25.2

Control of Hormone Secretion

• Always controlled by feedback loops- ensures that hormone concentrations stay within a narrow ‘desirable’ range in the blood

• Negative feedback loop:- Hormonal blood concentration declines below a minimum set point more hormone is secreted

- Blood concentration exceeds a maximum set point hormone production is halted

• Positive feedback loop:- as blood concentrations of a certain hormone increase the response of the effector organ stimulates further secretion

- progression of labor in childbirth by oxytocin

The Pituitary Gland (Hypophysis)

• The pituitary gland (hypophysis - undergrowth):- secretes 9 major hormone - sits in the hypophyseal fossa of the sella turcica - resembles a golf club: the gland forms the head of the club, and the stalk, called the infundibulum (funnel), forms the shaft- the infundibulum connects superiorly to the hypothalamus

• 2 basic divisions of the pituitary gland:- Anterior adenohypophysis (adeno = glandular) - Posterior neurohypophysis (neuro = neural)

• Blood supply: 2 branches of the internal carotid artery- superior hypophyseal artery supplies the adenohypophysis- inferior hypophyseal artery supplies the pars nervosa

The Pituitary Gland

Figure 25.3a–c

The Adenohypophysis

• Hormone release is controlled by the hypothalamus- stimulated and inhibited by releasing and inhibiting hormones

• 3 subdivisions: pars distalis, pars intermedia, pars tuberalis - pars distalis the largest division has 5 different endocrine cell types- secrete protein hormones, have many secretory granules and a well-developed RER and Golgi apparatus

• Somatotropic cells - secrete growth hormone (GH)

• Mammotropic cells - secrete prolactin (PRL)

Pars Distalis Endocrine Cells• Thyrotropic cells

- secrete thyroid-stimulating hormone (TSH)

• Corticotropic cells - secrete adrenocorticotropic hormone (ACTH) and melanocyte-stimulating hormone (MSH)

• Gonadotropic cells- secrete follicle-stimulating hormone (FSH), luteinizing hormone (LH)

• Tropic (‘changing’) hormones – TSH, ACTH, FSH, LH- regulate secretion of hormones by other endocrine glands- GH, PRL, and MSH act directly on nonendocrine target tissues

• 5 cell types group into 3 categories when stained: acidophils, basophils, chromophobes

• GH or somatotropin - stimulates growth of body tissues, especially in muscle and skeleton

• TSH - stimulates the thyroid to produce and release T4 and T3 (influences metabolism)

• PRL – initiates and maintains milk production (lactation) by mammary glands in the breasts

• ACTH – influences production and secretion of hormones by the adrenal cortex (helps us deal with stress)

• MSH - stimulates melanocytes of the epidermis to produce more melanin, thus darkening the skin

• FSH - stimulates ova maturation and estrogen production in ovaries and sperm production in the testes

• LH or ICSH – stimulates ovulation and progesterone secretion in ovaries and testosterone secretion in the testes

Hypothalamic Control of Hormone Secretion from the Adenohypophysis

• The hypothalamus have neurons that produce and release hormones much like NTs are released- secretes releasing factors to release hormones- secretes inhibiting hormones to turn off hormone secretion- travels through the hypophyseal portal system into the anterior pituitary to stimulate its hormone secretion- the hypophysial portal system involves two beds of capillaries connected by a vein- allows a high level of hormone concentration within a small region- designed so that the hormones released by the hypothalamus travel directly to the anterior pituitary- in turn the anterior pituitary releases hormones into systemic circulation

Fig 25.4

Hypothalamic Control of Hormone Secretion from the Adenohypophysis

The Neurohypophysis• Stores and releases hormones produced by the

hypothalamus - structurally part of the brain

- composed of nervous tissue (unmyelinated axons and neuroglial cells)

• Hormones made in the neuron cell bodies, transported along the axons, and stored in dilated axon terminals (Herring bodies) – secrete 2 hormones- antidiuretic hormone (ADH) or vasopressin (‘vessel constrictor’): targets the kidney to resorb more water from the urine and return it to the blood

- secretes oxytocin induces contractions of smooth muscle of reproductive organs

The Neurohypophysis

Fig 25.5

When the neurons

fire, they release

stored hormones

into a capillary bed

in the pars nervosa

for distribution

Table 25.1

The Thyroid Gland

• Largest pure endocrine gland - located in the anterior neck

• Internally, is composed of hollow follicles- separated by areolar CT rich in capillaries- walls are formed of cuboidal or squamous epithelial cells (follicular cells)- lying within the epithelium are parafollicular (C) cells- central lumen filled with colloid (‘gluelike’) consisting of thyroglobulin (protein precursor to thyroid hormone)

• Produces 2 hormones: amino-based and protein- Thyroid hormone (TH): thyroxine (T3 ); tri-iodothyronine (T4)- Calcitonin: lowers blood levels of Ca2+, mostly during childhood

The Thyroid Gland

• T4 and T3, consists of 2 amino acids and iodine• Main function is to increase metabolic rate

Fig 25.6

Histology of the Thyroid Gland

Fig 25.6

• Follicle cells continuously synthesize thyroglobulin and secrete it into the follicle lumen for iodination and storage

• TSH (pituitary gland) signals the follicle cells to release TH

The Parathyroid Glands

• Lie on the posterior surface of the thyroid gland surrounded by CT capsules (number varies)

• Contains thick branching cords composed of 2 types of endocrine cells- small abundant chief cells and rare larger oxyphil cells

• Chief cells produce a small protein hormone, PTH- PTH increases calcium levels and is essential to life:

1) stimulates osteoclasts to release calcium from bones

2) decreases secretion of calcium by the kidney

3) activates vit D, which stimulates uptake of Ca by the intestine

The Parathyroid Glands

Posterior view of the pharynx a

and trachea showing the location

of the parathyroid glands on the

posterior aspect of the thyroid

Fig 25.7

Histology of the Parathyroid Gland

• The function of oxyphil (‘acid-loving’) cells is unknown• PTH is essential to life - low Ca2+ levels lead to lethal

neuromuscular disordersWhat is the antagonist of PTH?

Fig 25.7

The Adrenal (Suprarenal) Glands

• Paired pyramidal organs on the superior surface of the kidneys – highly vascularized

• 3 groups of 60 small suprarenal arteries supply each gland

- the superior suprarenal arteries from the inferior phrenic artery;

- middle suprarenal arteries from the aorta;

- inferior suprarenal arteries from the renal artery• Veins

- left suprarenal vein drains into the renal vein and the right suprarenal vein drains into the inferior vena cava

• Nerve supply is almost entirely sympathetic fibers

• 2 endocrine glands in one - internal and external:- Adrenal medulla: a cluster of neurons derived from the neural crest, acts as part of the sympathetic NS

- Adrenal cortex: bulk of the adrenal gland derived from mesoderm

The Adrenal (Suprarenal) Glands

The Adrenal Medulla

• Part of the autonomic nervous system (Ch 15)

• Chromaffin (‘affinity for chromium’) cells- arranged in spherical clusters with some branching cords - modified ganglionic sympathetic neurons- secrete catecholamines: the amine hormones epinephrine and norepinephrine- active in the ‘fight, flight, and fright’ (fight or flight) response- hormones stored in secretory vesicles

The Adrenal Cortex

• Secretes a variety of corticosteroid hormones- all are lipid-based steroids

• Cortex is composed of 3 layers or zones:

- Zona glomerulosa (‘ball of yarn’): cell clusters

- Zona fasciculata: cells arranged in bundles

- Zona reticularis (‘network’): cells arranged in a branching network

The Adrenal Gland

Figure 25.8a, b

Adrenal Corticosteroids

• 2 main classes: mineralocorticoid & glucocorticoid

• Main mineralocorticoid is aldosterone

- secreted by the zona glomerulosa

- in response to a decline in blood volume or BP

- prompts kidney to resorb more sodium into the blood; water passively follows, increasing blood volume

• Glucocorticoids: cortisol is the main type- secreted by zona fasciculata and zona reticularis

- helps the body deal with stressful situations by keeping glucose levels high to support the brain

- body cells switch to fats and amino acids as energy sources

- high amounts depress the inflammatory response

Adrenal Corticosteroids

Adrenal Corticosteroids

• Hormonal pathway of stress:- hypothalamus sends corticotropin-releasing hormone (CRH)

to the adenohypophysis, which secretes ACTH

- ATCH travels to the adrenal cortex to signal glucocorticoid secretions

- the sympathetic NS can also stimulate glucocorticoid secretions

• Zona reticularis secretes an androgen hormone, dehydroepiandrosterone (DHEA)- DHEA is converted to testosterone and estrogens in peripheral tissues

- proposed beneficial effects include counteracting stress, boosting immunity, and mood

Adrenal Corticosteroids

Structure of Steroid-Secreting Cells

• Steroid-secreting cells have distinctive features- abundant SER and no secretory granules

- mitochondria have unusual cristae shaped like tubes

- lipid droplets are abundant in cytoplasm (lipids = raw material of steroids)

• Characterize cells of the adrenal cortex - also testicular and ovarian cells which secrete steroid sex hormones: the interstitial cells, theca folliculi cells, and cells of the corpus luteum

Interstitial cell in the testis

Fig 25.9

The Pineal Gland

• Small, pine cone shaped structure at the end of a short stalk on the roof of the diencephalon

• Pinealocytes are arranged in both spherical clusters and branching cords- star-shaped cells with long, branching cell process

- dense particles of calcium lie between the cell clusters, forming the ‘pineal sand’ (which is radiopaque)

- in Xrays used as a landmark to identify brain structures

- secretes melatonin: a hormone that regulates circadian rhythms (hypothalamus responds to a lack of visual input)

The Pineal Gland

The Pancreas

• Located in the posterior abdominal wall

• Contains endocrine and exocrine cells

• Exocrine acinar cells, form most of the gland- secrete digestive enzymes into the small intestine

• Endocrine cells are contained in spherical bodies- pancreatic islets or islets of Langerhans

- about 1 million scattered among the exocrine acini

• Main endocrine cell types:

- Alpha cells (α cells): secrete glucagon signals liver to release glucose from glycogen; raises blood sugar

- Beta cells (β cells): secrete insulin signals most body cells to take up glucose from the blood; promotes glucose storage as glycogen in liver; lowers blood sugar

The Pancreas

Figure 25.10

A Pancreatic Islet

The Thymus

• Located in the lower neck and anterior thorax

• Important immune organ

• Site at which T-lymphocytes arise from lymphocyte-precursor cells - transformation stimulated by thymic hormones, secreted by the thymus epithelial reticular cells

• Thymic hormones – a family of peptide molecules, including thymopoietin and thymosin

The Gonads

• Main sources of sex hormone – testes and ovaries

• Male testes- interstitial cells secrete androgens (primarily testosterone)

- promotes the formation of sperm

- maintains secondary sex characteristics

• Female ovaries- androgens secreted by the theca folliculi directly converted into estrogens by the follicular granulosa cells & progesterone

- estrogens and progesterone secreted by the corpus luteum

- estrogens maintain reproductive organs and secondary sex characteristics

- progesterone signals uterus to prepare for pregnancy

The Gonads

Endocrine cells occur within• The heart - the atria contains atrial natriuretic

peptide (ANP) - hormone that stimulates the kidneys to produce more urine containing salt

- getting rid of the excess fluid and salt reduces excess blood volume and salt levels; reduces blood pressure

• The GI tract has scattered enteroendocrine cells

- release amino acid/peptide hormones chemically similar to neurotransmitters

- affect functions related to regulating digestion, blood chemistry, and blood flow

Other Endocrine Structures

Other Endocrine Structures

• The placenta is produced when conception occurs

- secretes hormones that prevent the uterus from getting rid of the nutrient layer to which it is attached

- produces other steroid protein hormones: estrogen, progesterone, corticotropin-releasing hormone, and human chorionic gonadotropin

• The kidneys

- cells of the juxtaglomerular apparatus (JGA) secrete rennin which regulates blood pressure

- endothelial cells and interstitial CT secrete erythropoietin which stimulates erythrocyte production

• The skin

- when exposed to UV rays produces a steroid hormone precursor to vitamin D essential for calcium metabolism

Other Endocrine Structures

Pituitary Disorders

• Gigantism

- hypersecretion of GH in children

• Acromegaly

- hypersecretion in adults causes

• Pituitary dwarfism

- hyposecretion of GH

• Diabetes insipidus

- pars nervosa does not make enough ADH

A Closer Look Potential Uses for Growth Hormone

Disorders of the Pancreas

• Diabetes Mellitus

- caused by insufficient secretion of insulin

- or resistance of body cells to the effects of insulin

• Type 1 diabetes

- develop suddenly, usually before age 15

- T cell-mediated autoimmune response destroys β cells

Diabetes Mellitus

• Type 2 diabetes

- Adult onset

- Usually occurs after age 40

- Cells have lowered sensitivity to insulin

- Controlled by dietary changes and regular exercise

Disorders of the Thyroid Gland

• Grave’s Disease- most common type of hyperthyroidism

- immune system makes abnormal antibodies

- stimulates the oversecretion of TH by follicle cells

- leads to nervousness, weight loss, sweating, and rapid heart rate

• Myxedema

- adult hypothyroidism

- antibodies attack and destroy thyroid tissue

- common symptoms include low metabolic rate and weight gain

Disorders of the Thyroid Gland

• Endemic goiter- due to lack of iodine in the diet

• Cretinism- hypothyroidism in children

- short, disproportionate body, thick tongue and mental retardation

Figure 25.11

Thyroid Disorders

Disorders of the Adrenal Cortex

• Cushing’s syndrome- caused by hypersecretion of glucocorticoid hormones usually due to a pituitary tumor

• Addison’s disease- hyposecretory disorder of the adrenal cortex

- deficiencies of both mineralocorticoids and glucocorticoids

Figure 25.12

Thyroid Disorders

The Endocrine System Throughout Life

• Endocrine organs operate effectively until old age

• Adenohypophysis- increase in CT and lipofuscin

- decrease in vascularization and number of hormone-secreting cells

• Adrenal cortex- normal rates of glucocorticoid secretion continue

• Adrenal medulla- no age-related changes in catecholamines

• Pituitary gland – dual origin- adenohypophysis originates from the roof of the mouth

- neurohypophysis grows inferiorly from the floor of the brain

• Thyroid hormones- decrease slightly with age

• Parathyroid glands- little change with aging

• GH, DHEA, and the sex hormones- marked drops in secretion with age

The Endocrine System Throughout Life

Embryological Origin of Endocrine Organs

• Thyroid gland- forms from a thickening of endoderm on the floor of the pharynx

• Parathyroids and the thymus gland- from endoderm lining the pharyngeal pouches

• Pineal gland- originates from ependymal cells

• Adrenal gland – dual origin gland- adrenal medulla from neural crest cells of nearby sympathetic trunk ganglis

- adrenal cortex from mesoderm lining the coelom

Figure 25.13

Embryonic Development

Figure 25.b–d