Endocrine System: Hormones and Glands

• The nervous system facilitates immediate responses via action potentials.
• The endocrine system maintains homeostasis and long-term control through chemical signals, paralleling the nervous system in regulating growth, maturation, and homeostasis.
• The vertebrate endocrine system includes glands like the pituitary, thyroid, and adrenal glands, alongside scattered cell groups in epithelial tissues.
• More than 50 hormones are secreted by the endocrine system.
• Endocrine glands originate from all three embryologic tissue layers: endoderm, mesoderm, and ectoderm.
• The tissue layer of origin determines the type of endocrine product: Ectodermal and endodermal glands produce peptide and amine hormones, while mesodermal glands secrete lipid-based hormones.

Glands

• The endocrine system comprises glands that secrete hormones, which are chemical messengers.
• Endocrine glands secrete hormones into the bloodstream for transport to target organs containing cells with specific receptors.
• Exocrine glands are not part of the endocrine system and release products onto body surfaces, either internal (like the intestine) or external (like the skin).
• Examples of exocrine glands include sweat, salivary, and digestive glands.

Hormones

• Hormones are chemical messengers released in one tissue and transported via the bloodstream to target cells in other tissues.
• Hormones are classified into four groups based on their structure: steroids, fatty acid derivatives, peptides, and amines (amino acid derivatives).

Steroids

• Steroid hormones are derived from cholesterol through biochemical reactions.
• Defects in steroid synthesis pathways lead to hormonal imbalances.
• Steroid hormones are synthesized and then released into the bloodstream, with synthesis rate controlling their levels.
• Testosterone is the primary male sex hormone.
• Estradiol's structure is similar to testosterone and is a key female sex hormone.
• Steroid hormones are secreted by the gonads, adrenal cortex, and placenta.

Fatty Acid Derivatives

• These hormones are derived from arachidonic acid, a 20-carbon fatty acid produced from phospholipids by phospholipase.
• Prostaglandins are fatty acid-based hormones produced in various tissues, coordinating local cellular activities and enzymatic processes like blood clotting in extracellular fluids.

Peptides

• Peptide hormones consist of amino acid chains, ranging from short chains like antidiuretic hormone (ADH) to small proteins like growth hormone.
• The hypothalamus, pituitary gland, heart, stomach, liver, pancreas, and kidneys secrete peptide hormones, making it the largest class of hormones.
• Peptide hormones are synthesized from precursor molecules, processed by the endoplasmic reticulum and Golgi apparatus, stored in secretory vesicles, and released via exocytosis into the bloodstream.
• Different hormones can be derived from the same precursor molecule through enzymatic cutting.

Amines

• Amines are derived from the amino acid tyrosine and secreted by the thyroid gland, adrenal medulla, and pineal gland.
• This group includes epinephrine (adrenaline), norepinephrine (noradrenaline), thyroid hormones, and melatonin.

Mechanisms of Hormone Action

• The endocrine system triggers actions in specific target cells by releasing hormones.
• Receptors on target cell membranes bind selectively to one type of hormone.
• A hormone influences cell activity by fitting a specific receptor, similar to a lock and key.
• There are over 50 identified human hormones, all of which act by binding to receptor molecules, causing a change in the receptor's shape and initiating a cellular response.
• Two mechanisms of hormone action exist on all target cells.
• Peptide hormones, being water-soluble (like insulin) or too large for membrane channels (like growth hormone), bind to surface receptors on the cell membrane.
• Binding generates a chemical signal (second messenger) inside the target cell.
• Five second messenger chemicals identified include cAMP, IP3, and Ca2+ ions.
• Second messengers then activate intracellular chemicals to produce the target cell response.
• Steroid and thyroid hormones pass through the plasma membrane and act in these two steps because they are small lipid-soluble molecules.
• Steroid hormones bind to receptors on the nuclear membrane to produce an activated hormone-receptor complex once inside the cell.
• The activated complex enters the nucleus, binds to DNA, and directly activates specific genes, increasing protein production.
• Thyroid hormones diffuse across the cell membrane and bind to intracellular receptors in the cytoplasm.
• The resulting hormone-receptor complexes activate specific genes in the nucleus, which increases metabolic activity through changes in the nature and number of enzymes.

Endocrine Systems and Feedback Cycles

• The endocrine system regulates physiological functions via cycles and negative feedback.
• Negative feedback regulates the secretion of most hormones, maintaining physiological and homeostatic control through cycles lasting from hours to months.
• In the thyroxine release reflex, thyroxine increases the metabolic rate and body temperature.
• Cold exposure triggers a two-step stimulation process for thyroid gland cells to produce more thyroxine.
• Elevated thyroxine levels inhibit hormone production in the hypothalamus and anterior pituitary gland, which reduces stimulation of thyroid gland cells.

Main Endocrine Glands

• The endocrine system comprises endocrine glands and specialized cells within various tissues, with the main glands including:
• Hypothalamus
• Pituitary gland (hypophysis)
• Thyroid gland
• Parathyroid gland
• Adrenal glands
• Gonads
• Pineal gland
• Thymus
• Endocrine part of the pancreas
• Placenta (in pregnant females).

Hypothalamus

• The hypothalamus, located below the thalamus, functions in both the nervous and endocrine systems.
• It contains centers that control adrenal medulla endocrine cells through sympathetic innervation and responds by producing epinephrine (adrenaline) and norepinephrine (noradrenaline) to initiate the "fight or flight" response.
• As an endocrine gland, it secretes antidiuretic hormone (ADH) and oxytocin, which are released to the posterior pituitary gland.
• The hypothalamus secretes regulatory hormones that either stimulate or inhibit hormone production in the anterior pituitary gland.
• These hormones reach the anterior hypophysis via the hypothalamo-hypophyseal portal system.

Stimulating Hormones (Hypothalamus)

• Growth hormone-releasing hormone (GHRH) stimulates growth hormone production in the anterior pituitary.
• Thyrotropin-releasing hormone (TRH) stimulates TSH production in the anterior pituitary.
• Corticotropin-releasing hormone (CRH) stimulates ACTH production in the anterior pituitary gland.
• Gonadotropin-releasing hormone (GNRH) stimulates the production of gonadotroph hormones in the anterior pituitary.

Inhibiting Hormones (Hypothalamus)

• Growth hormone-inhibiting hormone (GHIH) inhibits growth hormone production in anterior pituitary cells.
• Prolactin-inhibiting hormone (PIH) inhibits prolactin production in adenohypophysis.

Pituitary Gland

• The pituitary gland is located in the sella turcica, a small bony cavity at the brain's base.
• A stalk links the pituitary to the hypothalamus, which controls pituitary hormone release.
• Anterior and posterior lobes comprise the pituitary gland.
• The anterior pituitary has endocrine cells surrounded by an extensive capillary network, part of the hypothalamo-hypophyseal portal system.
• Hypothalamic regulatory hormones control the anterior pituitary gland's activity.

Hormones of Anterior Pituitary Gland

• Growth hormone (GH), also called somatotropin, is a peptide hormone essential for growth and stimulates protein synthesis and cell division.
• GH-releasing hormone (GHRH) stimulates GH release, while GH-inhibiting hormone suppresses it.
• GH increases cell size (hypertrophy) and number (hyperplasia).
• It increases bone length and thickness via cartilage deposition.
• GH stimulates liver cells to produce somatomedins, hormone-like chemicals that act on target cells by binding to receptors to increase protein synthesis.
• Sex hormones in adolescence halt bone growth by replacing cartilage with bone.
• Too little or too much GH causes dwarfism or gigantism, respectively.
• Thyroid-stimulating hormone (TSH) release from the anterior pituitary because of TRH.
• TSH promotes thyroid hormone production in the thyroid gland, which regulates metabolic rates and body temperatures.
• ACTH stimulates the adrenal cortex to produce steroid hormones.
• Gonadotropins, like FSH and LH, affect the gonads by stimulating gamete formation and sex hormone production.
• Prolactin prepares the breasts for milk production near the end of pregnancy.

The Posterior Pituitary

• The posterior pituitary stores and releases hormones into the blood.
• ADH and oxytocin are produced in the hypothalamus and transported to the posterior pituitary via axons and control water balance and blood pressure by increasing water reabsorption in the kidneys, respectively.
• Oxytocin is a small peptide hormone that stimulates uterine contractions during childbirth and milk ejection.

The Adrenal Glands

• The adrenal gland is located above each kidney and divided into a medulla and cortex.
• The medulla synthesizes amine hormones, while the cortex secretes steroid hormones.
• The adrenal medulla consists of modified neurons that secrete epinephrine and norepinephrine and the sympathetic nervous system stimulates the cortex, releasing hormones into the blood in response to initiate the "fight or flight".
• The adrenal cortex produces three classes of steroid hormones: mineralocorticoids, glucocorticoids, and sex hormones.
• Mineralocorticoids, like aldosterone, maintain electrolyte balance by increasing Na+ reabsorption in the kidney.
• Glucocorticoids, like cortisol, increase blood glucose levels through the breakdown of fats and proteins while suppressing the immune and inflammatory responses.
• The adrenal cortex also produces small amounts of male sex steroids, like testosterone.

The Thyroid Gland

• The thyroid gland is located in the neck and consists of follicles that secrete thyroglobulin, a storage form of thyroid hormone.
• TSH causes the conversion of thyroglobulin into triiodothyronine (T3) and thyroxine (T4).
• Almost all body cells are targets of thyroid hormones, which increase the overall metabolic rate, regulate growth and development, and influence sexual maturity.
• C cells of the thyroid gland produce calcitonin, which regulates blood calcium levels by stimulating calcium excretion in the kidney and calcium incorporation into bone tissue.

The Parathyroid Gland

• Located on the posterior thyroid gland, secrete parathyroid hormone, which regulates blood calcium levels.
• Calcitonin and parathyroid hormone are antagonistic, having opposing effects that increase blood calcium levels by increasing calcium reabsorption in the kidney and stimulating calcium release from bones.

The Pancreas

• The pancreas contains exocrine cells that secrete digestive enzymes into the small intestine and endocrine cells (pancreatic islets).
• Endocrine cells secrete insulin (by beta cells) and glucagon (by alpha cells), which regulate blood glucose levels.
• After a meal, blood glucose rises, prompting insulin release, which causes cells to take up glucose and form glycogen.
• Further insulin production inhibits as glucose levels fall.
• Glucagon causes the breakdown of glycogen into glucose, maintaining glucose levels within a homeostatic range that is stimulated when blood glucose levels fall and inhibited when they rise.
• Type I diabetes mellitus is characterized by inadequate secretion of insulin often caused by genetic or autoimmune reasons.
• Type II diabetes mellitus usually develops in adults caused by genetic or environmental reasons that result in loss of response to insulin rather than a lack of insulin.
• Diabetes can cause impairment in the eyes, circulatory system, nervous system, and kidneys, which is the second leading cause of blindness in the US.
• Treatments involve daily insulin injections, monitoring blood glucose levels, and a controlled diet.

Pineal gland

• The pineal gland is a small endocrine gland in the brain that secretes melatonin.
• Melatonin influences daily rhythms, like sleep/wake patterns.

Thymus

• The thymus, located under the sternum, reaches its largest size in childhood.
• Thymus secretes thymosins, which stimulate lymphocyte differentiation and maturation.

Other Chemical Messengers

• Interferons are proteins released when a cell is attacked by a virus, causing neighboring cells to produce antiviral proteins and slow the spread of viruses.
• Pheromones are chemical signals that travel between organisms and are used to signal prospective mates, mark territory, and communicate.
• The presence of a human sex attractant/pheromone has not been established.