Growth & Development

Premed I BiologyJanuary 2015

Growth

• All organisms grow, develop and reproduce• Increase in mass or overall size of an organism

is called Growth

• Involves Cell Proliferation or Mitosis

• Development of a Zygote --- > Embryo

Endocrine System

• Endocrine system regulates growth and development by “Chemical Signaling”

• “Hormones” are released by the endocrine system

• Released into body fluids, often blood and then affects tissue or multiple tissue

• Target === > Tissue or Organ

Cell Signaling

• Each hormone binds specific receptors, elicits a response in specific target cells.

• Communication between animal cells via signals such as hormones can be classified into five forms of signaling:

• Paracrine signaling – Target cells lie near the secreting cell

• Autocrine signaling – Secreting cells themselves are target cells

• Eg. Prostaglandins – Physiological processes (during childbirth) and immune system (inflammation), formation of clot by platelets

• Synaptic Signaling – neurons form synapses with target cells, secreting neurotransmitters diffusing over short distance

• Neuroendocrine Signaling – Neurosecretory cells secrete Neurohormone diffusing from nerve cell endings into blood stream– E.g. ADH hormone

• See Image Below

• Endocrine signaling function to maintain homeostasis

• Also mediates response to external stimuli, regulates growth and development, alter metabolic activities, triggers physical and behavioral changes, sexual maturity and reproduction

Classes of Hormones

• Three (3) major chemical classes:

1. Polypeptide (water soluble)2. Steroids (non-polar)3. Amines (most are water soluble)

Eg. Insulin – PolypeptideEg. Cortisol – SteroidEg. Epinephrine (water-soluble), Thyroxine (lipid-soluble)

– Amines

Water-soluble hormones

• Travel freely in blood stream• Cannot diffuse through plasma membranes of target cells

• 1. Bind to cell-surface receptors – e.g Epinephrine binds to G-protein

• 2. Induce changes in cytoplasmic molecules – Signal Transduction – e.g. cyclic AMP synthesis

• 3. Alter gene transcription – eg. Activates enzyme needed for glycogen breakdown and inhibit

glycogen synthesis

Lipid-soluble hormones

• Receptors are located in the cytosol• Hormone binds forming a “Hormone receptor

complex” • HRC moves into the nucleus • Alters transcription of gene by binding to

specific sites in DNA

• Eg. Estrogen, Thyroxine

Multiple Effects of Hormones

• Example – Epinephrine (Adrenaline)

• In liver cells, bind to B-type receptor in plasma membrane result in release of glucose

• In blood vessel smooth muscle – Relaxation of smooth muscle leading to vasodilation (increased blood flow)

• In intestine smooth muscle cells – Constricts blood flow leading to restricted blood flow to intestines

Endocrine vs Exocrine • Exocrine Glands

– Release enzymes to the external environment through ducts – E.g. Sweat glands, Oil glands (sebaceous), mucous, and digestive glands– [ Thyroid and parathyroid glands, testes, ovaries] = Ductless organs, secrete hormones

directly into the surrounding fluid

• Exocrine (Salivary glands) = Ducts that carry secreted substances such as enzymes

• Endocrine Glands– Release hormones directly into body fluids

• Pancreas act as both Exocrine gland and Endocrine gland – Exocrine by releasing digestive enzymes through pancreatic duct– Endocrine by releasing insulin and glucagon into blood

Simple Endocrine Pathway

• Endocrine cells respond directly to an internal or environmental stimulus by secreting hormone

• Eg. pH in duodenum lowers as stomach contents enter it, this stimulates duodenum lining to secrete Secretin hormone into extracellular fluid

• Secretin acts on pancreas, release of bicarbonate through pancreatic duct into duodenum

• See Image Below

Simple Neuroendocrine Pathway

• Stimulus = Neurosecretory cell Neurohormone Bloodstream Target cells

• See Image Below

Endocrine Gland

• Slow, and long lasting effects on target (compared to nervous system)

• All hormones do not move directly to target tissue but released into blood– (E.g. Insulin and glucagon released directly into blood)

• Seconds to days to produce effects

• Hormones act by binding to “Receptors” specific for that particular hormone in the target– Receptors can be on the membrane or inside the cell

Negative Feedback

• “When a change occurs in system, it automatically triggers a corrective mechanism, which reverses the original change and brings back to the set point (normal)”

• Hypothalamus Pituitary Gland Endocrine

• Pathway:• Change is detected by sensor Sends message to integrating

center (hypothalamus) Stimulates an effector

Pituitary Gland

• Hypothalamus controls the release of anterior pituitary hormones with “Releasing” and inhibitory hormones of its own

• Pituitary Gland or master gland: • Adenohypophysis ( Anterior pituitary gland)• Neurohypophysis (Posterior pituitary gland)

• Located beneath the hypothalamus

Hypothalmic Axons reach into the posterior pituitary secreting neurohormones that were synthesized in the hypothalmus

Anterior Pituitary

• Synthesizes and secretes hormones in response to hormones form the hypothalmus– “Hormone cascade pathway”

• Control metabolism, osmoregulation, reproduction

Anterior Pituitary• FSH – Follicle Stimulating Hormone

– Stimulates ovaries to release Estrogen– Reproduction, ovulation, secondary sexual characteristics in females

• LH - Luteinizing Hormone– Female Reproduction, stimulates Progesterone release during second half of menstrual cycle– Male Reproduction, stimulates testes to release Testosterone, secondary sexual characteristics in males

• hGH – human Growth hormone– Stimulates growth in cells of body, anabolic or synthesis of proteins and glucose– Decrease breakdown of proteins and use of glucose

• ACTH – Adrenocorticotropic Hormone– Stimulates adrenal cortex in release of stress hormones – Cortisol– Aldoesterone (Sodium reabsorption and potassium secretion in distal tubule & collecting duct)

• TSH – Thyroid stimulating hormone– Stimulates Thyroid to release T4 and T3

• Prolactin– Stimulates milk production

Posterior Pituitary

• Oxytocin– Uterine contraction– Milk ejection during pregnancy (NOT production)

• ADH or Vasopressin– Collecting ducts of kidney to become permeable

to water– Increases blood pressure

Adrenal Glands• Located sitting on top of the kidneys

• Two portions:• Adrenal Cortex— Secretes only steroid hormones (Cortisol and Aldosterone),

Family of Corticosteroids– Glucocorticoids - Cortisol (promote glucose synthesis) by pancreas, in skeletal muscle,

breakdown of protein to glucose, supress body’s immune system– Mineralocorticoids - Aldosterone, maintaining salt and water balance

• Adrenal Medulla --- Secretes Epinephrine (adrenalin) and Norepinephrine– Both are Catecholamines, derivative of amino acid Tyrosine

– “Fight or Flight” Response– Both promote release of glucose in blood– Vasoconstrictors of blood vessels– Vasodilators of skeletal muscles (increase blood flow)

Thyroid Regulation

• Regulates blood pressure, heart rate, and muscle tone, digestive and reproductive functions

• Hypothalmus secretes Thyrotropin-releasing hormone (TRH)

• This leads to Anterior Pituitary to secrete Thyroid-stimulating hormone (TSH)

• TSH stimulates release of thyroid hormone by the thyroid gland

• As Thyroid hormone accumulates, it increases metabolic rate, while also initiating negative feedback that prevents overproduction

Thyroid Gland

• Located just in front of larynx and along trachea

• Increase metabolic rate & initiates negative feedback that prevents its overproduction

• Thyroid hormones are derived from amino acid Tyrosine : T3 (triiodothyronine) and T4 (Thyroxine)

• Also Calcitonin

*Iodine needed for synthesis of thyroid hormones

• Low blood levels of thyroid hormone in iodine deficiency is not able to provide the usual negative feedback on the hypothalmus and anterior pituitary

• As a result, the pituitary continues to secrete TSH causing enlargement of thyroid gland, swelling in neck called “GOITER”

Hyperthyroidism• Increased circulating levels of thyroid hormones• Thyroid gland is overactive• Seen usually in women• Enlarged goitre (swelling of thyroid gland)

• Symptoms:– Elevated metabolic rate– Profuse Sweating– Heat intolerance– Nervousness, irritability– Pounding heart– Weight loss– High blood pressure– Fatigue– Abnormal menses or menstruation

Hyperthyroidism

• GRAVE’S DISEASE (protruding eyes due to fluid accumulation)

– Autoimmune antibodies bind and activate receptor for TSH causing continuouos thyroid production

Hypothyroidism

• Decreased production of thyroid hormones• Iodine deficiency (endemic or in geographical locations)

• Children --- Cretinism ( growth & mental retardation)• Adults ---- Lower metabolic rate

– Cold intolerance– Constipation– Slow mental function– Weight gain– Abnormal menses– Dry / think skin, hair loss, and hoarse voice– Heart rate decreased

Growth Hormone

• Stimulates growth through tropic and non-tropic effects

• Targets liver in release of Insulin-like Growth Factors (IGFs) Circulate in blood and stimulate bone and cartilage growth

• GH deficiency Abrupts growth

• Metabolic effects of GH Raise blood glucose levels, opposing effects of Insulin

• Excess GH in childhood Gigantism• Excess GH in adults Acromegaly

• Little GH Retards long bone growth Pituitary Dwarfism

GIGANTISIM (Tallest Man in history 8 feet 11 inches)

Parathyroid Hormone

• Parathyroid Hormone & Vitamin D– Control blood calcium

• Parathyroid Gland – Set of four small glands embedded in the posterior surface of thyroid gland

• Raises blood calcium level by direct effects in bones and kidneys & indirect effect on intestines

• Bone matrix breakdown to release calcium• In kidneys, PTH stimulates uptake of calcium in renal tubules and

conversion of precursor to active Vitamin D• In intestine, uptake of calcium in food

Pancreas• Islets of langherans of pancreas

• Two important peptide hormones: Insulin & Glucagon

• Insulin – Released by beta (b) cells of the pancreas– Lower blood glucose level

• Glucagon – Released by alpha (a) cells of the pancreas – Raise blood glucose level– Stimulates breakdown of glycogen to glucose– Glycogenolysis and gluconeogenesis in liver

*Another hormone produced by pancreas is Somatostatin ( Inhibits insulin and glucagon)

Diabetes• Blood sugar or glucose levels are high

• Type 1 Diabetes --- Body does not make enough insulin• Type 2 Diabetes --- Body does not respond to insulin well

Symptoms: • Polyuria (Frequent urination)• Polydipsia ( Increased thirst)• Polyphagia ( Increased hunger)• Weight loss• Fatigue• Slow healing of infections (high sugar)• Tingling hands and feet (nerve damage)

• Fasting glucose level greater than 126mg/dl (normal is less than 100mg/dl)