Endocrinology Lecture 1

What makes an endocrine cell?

(Bioregulatory Cells and Tissues) Specialized secretory epithelial cells clumped in groups - Cords - Follicles - Islets

negative feedback

*Any disturbance regardless of the direction of the disturbance (i.e. up or down) causes a homeostatic reflex to be activated to maintain homeostasis* A type of regulation that responds to a change in conditions by initiating responses that will counteract the change. Maintains a steady state. other notes: - Negative feedback reverses changes - In this system, the output shuts off the original effect of the stimulus or reduces its intensity; these mechanisms cause the variable to change in a direction opposite to that of the initial change, returning it to its "ideal" value

positive feedback

*Any disturbance that drives the system even further away from the preprogrammed condition* A type of regulation that responds to a change in conditions by initiating responses that will amplify the change. Takes organism away from a steady state. Other notes: - Positive feedback brings a process to completion - Positive feedback intensifies a change in the body's physiological condition rather than reversing it

feed-forward feedback

*Changes in a regulated variable are anticipated, which accelerates homeostatic responses and minimizes fluctuations in the regulated variable* -Anticipates change (such as energy available) -Anticipates a change in internal environment and meets that need -Usually are learned responses -Gets us ready to respond Example: Salivation before eating, Fear

Placenta

*Human Chorionic Gonadotropin (hCG)* Begins to be secreted by the placenta within a week of implantation

neurohormones

*chemicals released by neurons into the blood* for action at distant targets

What is bioregulation?

*consists of chemical messengers that control bodily functions* Transport system in vertebrates is vascular system (primarily) - Part of the endocrine system includes the neuroendocrine system, which is both neural and endocrine - Some hormones are released into extracellular fluid, not blood secretions of the endocrine system, nervous system, the immune system, and virtually all cells in the body that use chemical messengers to communicate with one another

Allostasis

*explains the complex role that different bioregulators have in integrating the response of many organ systems in response to the environment* - Often applied during times of "stress" takes into account that there is going to be natural variations in organisms and that it doesn't have to be perfect maintaining levels of biological conditions that vary according to an individual's needs and circumstances

Neuromodulator

- Secreted by neurons into synaptic space; modulates sensitivity of postsynaptic cell to other neurotransmitters - ex: Endorphins and various other neuropeptides

Neurohormone

- Secreted by neurons into the blood or CSF; may be stored in neurohemal organ prior to release - ex: TRH, CRH, oxytocin, dopamine

intracrine

- Secretion of hormones inside the same cell - *secretes chemical messenger within itself* - Intracellular chemical messengers that govern intracellular events - a hormone that acts inside a cell, regulating intracellular events.

Hormone-receptor complex

- a hormone bound to a target molecule - causes a measurable change in the target cell

Pheromones

- a semiochemical - Act on members of same species

Allelomones

- a semiochemical - Act on other species

Why do we care about comparative endocrinology?

- approach of evolutionary system is important and provides insight - it works pretty well which means it is being selected - we can use these as models - we can use a lot of these models as framework - informs us how the endocrine system works in the real world - ethically it isn't good to inject thyroid hormone

paracrine

- different cell type - Secreted by cells and affect other cell types - relating to a hormone or to a secretion released by (endocrine) cells into the adjacent cells or surrounding tissue rather than into the bloodstream. - para = alongside of or next to

Receptor

- on or in a cell, a specific protein to whose shape fits that of a specific molecular messenger, such as a hormone - a protein which binds to a specific molecule

cytocrines

- same cell type - "local hormones" secreted by cells into the surrounding extracellular fluid. These local regulators typically travel short distances to nearby target cells - bioregulators that are secreted into extracellular fluid rather than the bloodstream are named "local hormones"

Neurotransmitter

- secreted by neurons into synaptic space - ex: Acetylcholine, dopamine, substance P, GABA

4 types of cells in the islets of Langerhans

Alpha cells - α- cells (glucagon) Beta cells - β- cells (insulin) Delta cells - δ- cells (somatostatin) Polypeptide-secreting cells - aka F cells or PP cells

Adrenal Glands

Bilateral glands adjacent to kidneys Like the pituitary, adrenals are 2 glands with distinct embryological origins. - Adrenal medulla - Adrenal cortex (Zona glomerulosa; Zona fasciculata; Zona reticularis) Both parts of the adrenals work together to regulate metabolism and cope with stress.

example of negative feedback

Body temperature is regulated by negative feedback mechanism Neural control mechanism (like a withdrawal reflex) Hormonal negative feedback mechanism → control of blood sugar (glucose) by insulin

Pancreas

Both endocrine and exocrine tissue Endocrine tissues are embedded in exocrine tissues (called Islets of Langerhans) ~~α- cells (glucagon) ~~β- cells (insulin) ~~δ- cells (somatostatin) ~~Polypeptide-secreting cells

Adrenal cortex

Catecholamines - Epinephrine - Norepinephrine outer section of each adrenal gland; secretes cortisol, aldosterone, and sex hormones

C

Controller

E

Effectors

Parathyroid Gland

First discovered in rhinoceros in 1852 Not described in humans until 1880 Both hormones are involved in *Ca+ regulation* *Parathyroid Hormone (PTH)* is produced in the parathyroids and increases blood Ca+ concentrations *Calcitonin (CT)* is produced in the C-cells and decreases blood Ca+ concentrations (opposite of PTH) Conserved across tetrapods Not fish! - Fish have similar hormones produced in ultimobranchial glands. Note: ultimobranchial: relating to or denoting a gland in the neck which in many lower vertebrates regulates the calcium level in the body

Gastrointestinal Endocrine Cells

Gastrin Ghrelin Bombesin Secretin Cholecystokinin (CCK) Galanin Neurotensin Peptide YY Neuropeptide Y Substance P Note: - our guts are endocrine organs - microbiome helps to regulate these organs

Cholecystokinin (CCK)

Gastrointestinal Endocrine Cell Gallbladder secretion; "satiety" hormone

Secretin

Gastrointestinal Endocrine Cell Stimulates digestive "juices" from pancreas

Ghrelin

Gastrointestinal Endocrine Cell Stimulates hunger in brain (↑ before & ↓ after meals)

Gastrin

Gastrointestinal Endocrine Cell Stimulates secretion of gastric acid, H20 and electrolytes by stomach

Bombesin

Gastrointestinal Endocrine Cell Stimulates secretion of gastrin, satiety

Substance P

Gastrointestinal Endocrine Cell also acts as NT; pain perception, vomiting A neurotransmitter that is involved in the transmission of pain messages to the brain.

Galanin

Gastrointestinal Endocrine Cell feeding, mood, eating disorders?

Neuropeptide Y

Gastrointestinal Endocrine Cell initiation of food consumption

Peptide YY

Gastrointestinal Endocrine Cell satiety in response to feeding released by the gastrointestinal tract after a meal in proportion to the number of calories consumed to reduce appetite

Neurotensin

Gastrointestinal Endocrine Cell vasodilation (Vasodilation is the widening of blood vessels as a result of the relaxation of the blood vessel's muscular walls. Vasodilation is a mechanism to enhance blood flow to areas of the body that are lacking oxygen and/or nutrients)

Endocrine glands

Glands of the endocrine system that release hormones into the bloodstream glands of "internal secretion" that secreted their products into the blood

Hypothalamus Hormones

Gonadotropin Releasing Hormone (GnRH) Corticotropin Releasing Hormone (CRH) Thyrotropin Releasing Hormone (TRH) Somatostatin (GHIH) (Inhibitory) Gonadotropin Inhibiting Hormone (GnIH) Somatocrinin (GHRH) Prolactin Inhibitory Hormone (PIF; DA) Prolactin Releasing Hormone (TRH; DA) Substance P

Thyroid Gland

H-shaped structure Consists of many sphere-shaped follicles filled with colloid Triiodothyronine (T3) - more active form; increase metabolic action, important for heat production, bone growth, heart rate Thyroxine (T4) - - Main secreted form Thyroid Parafollicular Cells (C-Cells) - Secrete calcitonin

Reality of Homeostasis

Homeostasis is at best imperfect and applies to only some parameters

Pituitary Gland (Posterior)

Hormones are secreted directly from neurons into the blood system - *Oxytocin* (Mesotocin-herps, Isotocin-fish) - *Arginine Vasopressin* (antidiuretic hormone)

So what do all of these hormones "do"?

Hormones are used as the biochemical messengers to keep the body functioning. They maintain *homeostasis* by acting upon other organs, tissues, and cells.

Pituitary Gland (Anterior)

Hypothalamus and anterior pituitary communicate via a portal blood system - Called the *hypothalamo-hypophysial portal system* Non-neural endocrine tissue Secrete direct acting hormones or *tropic hormones* - Tropic hormones: Act on other endocrine glands

I

Information

δ- cells (somatostatin)

Inhibits gastric secretion

Pineal Gland

Located near or in the brain Works with the suprachiasmatic nucleus (SCN) In mammals and birds: completely enveloped In fishes, reptiles, and amphibians: exposed (3rd eye) Primary hormone is *melatonin* (synthesized from serotonin)

I'

Message

Adrenal medulla

Mineralcorticoids - Aldosterone Glucocorticoids - Cortisol, corticosterone Androgens - Precursor to testosterone

Types of Feedback

Negative Feedback Positive Feedback Feed-forward

Bioregulator Types

Neurons produce neurocrine regulators called *neurotransmitters* or neuromodulators - Typically released into synapses SOME neurons secrete their chemical regulators into blood 🡪 these are the *neurohormones*

Traditional View of Homeostasis

Organisms work tirelessly to maintain all parameters of their internal state within narrowly defined limits and use active regulation to buffer all environmental challenges Note: - strict definition of homeostasis doesn't allow the 10 F degree window of variation - the more flexible an organism is the less energy it can spend

R

Receptor

What are the two important components of homeostasis?

Relative stability or constancy Active regulation (we are always working to have a certain set point)

Exocrine Glands

Secrete products into ducts, which are conveyed to sites of action on the surface of the epithelium - Digestive tract or skin These are externally excreted bioregulators called *semiochemicals* (Pheromones and Allelomones)

Gonads (Male = Testes)

Steroid Hormones - *Androgens* (leydig) - *Estrogens* (leydig) Peptide Hormones - *Inhibin* (sertoli) - *Mullerian Inhibitory Hormone (sertoli)* (during development) ~~Activin (sertoli)

Gonads (Female = Ovaries)

Steroid Hormones - *Androgens/Estrogens* (theca/granulosa) - *Progestins* (endometrium/ corpus luteum) Peptide Hormones - *Inhibin* (granulosa/corpus luteum) - *Activin* (granulosa/corpus luteum)

T4 have to be converted to?

T3

Which is more active, T3 or T4?

T3 is more active

Homeostasis vs. Allostasis vs. XYZ?

There is no perfect way to accurately describe the way that our body is regulated. Organisms make "decisions" to: - Avoid changes - Tolerate changes - Actively regulate to conform to changes All of these are controlled by the endocrine system!

Homeostatic Reflex Model

To explain how the endocrine system controls the internal environment relative to the external environment, we use the Homeostatic Reflex Model I = Information R = Receptor I' = Message C = Controller E = Effectors

What are the adrenal glands?

a pair of endocrine glands that sit just above the kidneys and secrete hormones (epinephrine and norepinephrine) that help arouse the body in times of stress. - Adrenal medulla - Adrenal cortex

Apocrine secretion

apical portion of cell is sloughed along with stored secretions - Mammary gland milk

Follicles

cells form a spherical mass of one cell layer surrounding a fluid-filled space or lumen (C) - Thyroid

Hormones

chemical messengers that are manufactured by the endocrine glands, travel through the bloodstream, and affect other tissues Each hormone binds to a specific receptor molecule located in or on a target cell Secreted by specialized nonneural cells into the blood hormon = to stimulate or excite

example of positive feedback

childbirth oxytocin - you have baby pushing on structures - more oxytocin --> the more contraction is released positive feedback mechanism regulates formation of a platelet plug - Blood clotting Enhancement of labor contractions ....very rare positive feedback examples

Walter B. Cannon

coined the term "homeostasis" in 1929 Physiological systems operate to maintain a condition that are "relatively constant"

What is comparative endocrinology?

comparing the endocrine system across species

Endocrine glands are very ______ in animals!

diverse

Claude Bernard

formulated the concept of homeostasis in the 19th century

"Constancy of the internal environment is the condition for a ____ life."

free

tropic hormones

hormones that stimulate other glands to release their hormones acts on another organ to release another hormone

Holocrine secretion

lysis and death of secretory cell - Sebaceous gland secretion

Hypothalamus

major control system Group of nuclei located below the thalamus All vertebrates have a hypothalamus *Portal between nervous and endocrine system* Produces inhibitory and releasing hormones

dotted line

negative feedback

Endocrine Organs

pineal gland hypothalamus pituitary thyroid parathyroids thymus adrenals pancreas ovary testes

bolded line

positive feedback

Cytogenous secretion

release of entire cells - Spermatozoa

Cords

secretory cells forming folded sheets (A) - Pituitary gland and adrenal cortex

Merocrine secretion

secretory products released by exocytosis with no damage to the cell - Thyroxine

semiochemicals

semio = signal externally excreted bioregulators Secreted into environment

Islets

separated clumps of a few cells (B and D) - Islets of Langerhans in the pancreas

autocrine

term for hormones that act on same cells that secrete them the production and secretion of an extracellular mediator by a cell followed by the binding of that mediator to receptors on the same cell to initiate signal transduction. Secreted by cells and affect emitting cell/type

What is homeostasis?

the ability to maintain a relatively stable internal environment in an ever-changing outside world regulating internal processes even though the outside changes endotherms maintain an internal temperature even though the outside is changing

Target cells

the cells on which a hormone acts