Introduction to the Endocrine System

Endocrine System Introduction

• Two systems that control and coordinate the body are the endocrine and nervous systems.
• These systems maintain homeostasis and respond to internal and external changes .
• The endocrine system uses hormones produced by endocrine glands or neurons, often acting at a distance.
• The nervous system uses neurotransmitters for short-distance and immediate changes.
• The endocrine system brings about slow but sustained changes.
• Endocrine glands secrete products into the blood and are located throughout the body.

Hormones

• Endocrine glands secrete chemical signals directly into the blood.
• Hormones travel in the blood and affect cells with specific receptors.
• Hormones bind to receptors to alter cell activity.
• Cells without receptors for a hormone will not be affected.
• Target cells possess receptors for specific hormones.

Feedback Systems

• Negative feedback controls hormone secretion e.g., the thyroid gland produces thyroid hormone in response to TSH from the anterior pituitary gland.
• When thyroid hormone levels drop, the anterior pituitary produces more TSH to stimulate the thyroid gland.
• As thyroid hormone reaches a set point, TSH production decreases.
• Hormone secretion may also be controlled by direct nervous system stimulation e.g., the adrenal medulla secretes hormones during the fight-or-flight response due to sympathetic nervous stimulation.

Hypothalamus

• The hypothalamus is located in the diencephalon (midbrain), caudal to the optic chiasm.
• It is involved in appetite, body temperature, and sleep/wake cycle control.
• The hypothalamus links the conscious mind and the endocrine system.

Hypothalamus-Pituitary Relationship

• The pituitary gland is attached to the hypothalamus by a thin stalk.
• Blood vessels and nerve fibers in the stalk enable hypothalamic control of the pituitary gland.
• The portal system refers to the blood vessels between the hypothalamus and pituitary.
• The hypothalamus secretes releasing or inhibiting hormones into the portal system.
• The portal system connects the hypothalamus to the anterior pituitary.
• Releasing factors cause the anterior pituitary to produce or release hormones, and inhibiting factors block hormone production or release.
• The effect on the posterior pituitary is more direct.
• Two hormones produced in the hypothalamus are stored in the posterior pituitary: antidiuretic hormone (ADH) and oxytocin.

Pituitary Gland

• The pituitary gland is also known as the master gland.
• It is housed in the pituitary fossa, a small pocket within the sphenoid bone, and is divided into two halves.
• The anterior pituitary produces 7 hormones: growth hormone (GH), prolactin, thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle stimulating hormone (FSH), luteinizing hormone (LH), and melanocyte-stimulating hormone (MSH).
• The posterior pituitary stores and releases but does not produce 2 hormones.

Anterior Pituitary

• This section discusses hormones produced by the anterior pituitary, highlighting their functions.

Growth Hormone

• Promotes body growth in young animals and regulates carbohydrate, protein, and lipid metabolism.
• GH has an anabolic effect on protein metabolism, supplying materials for growth.
• GH has a catabolic effect on lipid metabolism, breaking down fat stores for energy.
• GH has a sparing effect on carbohydrates, discouraging cell use and raising blood glucose levels (opposite of insulin's effect).

Prolactin

• Helps initiate and maintain lactation.
• Prolactin is released as long as the teat or nipple is stimulated.
• Milk secretion stops when the stimulus ceases.
• Prolactin has no known male effect.

Thyroid-Stimulating Hormone

• TSH causes thyroid gland growth, development, and hormone production.
• TSH secretion is regulated by negative feedback from the thyroid gland.
• Homeostasis of thyroid hormone is maintained through interaction between the hypothalamus, anterior pituitary, and thyroid gland.

Adrenocorticotropic Hormone

• ACTH stimulates adrenal cortex growth and hormone release.
• ACTH production is regulated by feedback from adrenal cortex hormones.
• ACTH releasing factor from the hypothalamus allows rapid ACTH release from the anterior pituitary during stress.

Follicle-Stimulating Hormone

• FSH stimulates ovarian follicle growth and development.
• Each follicle acts like an incubator for the oocyte as it matures through oogenesis.
• FSH causes the production of estrogens, preparing the female behaviorally and physically for breeding and pregnancy.
• FSH stimulates spermatogenesis in the seminiferous tubules of the testes in males.

Luteinizing Hormone

• LH completes follicular development in females.

• Growing follicles increase estrogen production, which reduces FSH and increases LH production.

• LH stimulates the remaining cells to form a corpus luteum (CL) once the follicle ruptures.

• The corpus luteum produces progesterone to maintain pregnancy.

• LH stimulates the interstitial cells of the testes to produce testosterone in males.

• FSH and LH are sometimes called gonadotropins due to their effect on the gonads.

Melanocyte-Stimulating Hormone

• MSH relates to color changes in reptiles, fish, and amphibians.
• It has no purpose in higher mammals.

Posterior Pituitary

• This section describes the function of hormones released via the posterior pituitary.

Antidiuretic Hormone

• ADH prevents diuresis (urine loss).
• ADH causes the kidneys to resorb water, concentrating urine.
• ADH is released when the hypothalamus detects dehydration via receptors that detect hemoconcentration.
• ADH is inhibited by alcohol and caffeine.
• ADH deficiency causes diabetes insipidus, producing polyuria and polydipsia.

Oxytocin

• The uterus and mammary glands are target tissues.
• Oxytocin causes contractions in the myometrium during breeding and parturition, injectable oxytocin helps to deliver fetus or placenta during parturition
• Oxytocin stimulates milk letdown in the mammary glands, is released with teat stimulation.
• Oxytocin causes myoepithelial cell contraction around alveoli, squeezing milk into the lower gland parts.

The Thyroid Gland

• The thyroid gland has two lobes on either side of the caudal larynx and trachea, sometimes connected by the isthmus (tissue band).

• The precursor of thyroid hormone is stored in the follicles.

• The thyroid gland produces thyroid hormone, which regulates metabolic rate and calcitonin to regulate blood calcium levels.

Thyroid Hormone

• Thyroid hormone is released from the thyroid gland when stimulated by TSH from the anterior pituitary.
• It consists of two hormones: T3 (triiodothyronine) and T4 (thyroxine).
• T3 is the active form, and T4 is mostly converted to T3 before exerting its effect.
• Thyroid hormone has a calorigenic effect, heating the body, and influencing energy usage.
• Hyperthyroid animals tend to lose weight, while hypothyroid animals tend to gain weight.
• Thyroid hormone production is inhibited by physical or emotional stresses, which can make animals more vulnerable to disease.
• Thyroid hormone encourages protein anabolism if the diet is adequate.
• Thyroid controls blood glucose.
• Encourages lipid breakdown.

Calcitonin

• Calcitonin is the other hormone produced by the thyroid gland.
• Maintains blood calcium homeostasis through muscle contraction, blood clotting, milk secretion and skeleton maintenance.
• Calcitonin prevents hypercalcemia by decreasing calcium levels.

Parathyroid Glands

• These are small pale nodules closely related to the thyroid gland.
• Parathyroid hormone (PTH) maintains blood calcium levels counteracting calcitonin effects and preventing hypocalcemia.
• PTH promotes calcium retention by the kidneys, calcium absorption by the intestines, and calcium release from the bones.

Adrenal Glands

• Adrenal glands are located near the kidneys.
• The adrenals consist of two glands.
  • Adrenal cortex (outer)
  • Adrenal medulla (inner)

Adrenal Cortex

• The adrenal cortex produces hormones under the direction of ACTH from the anterior pituitary.
  • Glucocorticoids
  • Mineralocorticoids
  • Sex hormones

Glucocorticoids

• The name comes from effect they have on blood glucose.
• Cortisol, cortisone, and corticosterone all have a general hyperglycemic effect.
• They also maintain blood pressure and help the body resist stress.

Mineralocorticoids

• Help regulate electrolytes.
• The main mineralocorticoid, aldosterone, affects sodium, potassium, and hydrogen ion levels.
• Aldosterone affects hydration due to water accompanying sodium.

Sex Hormones

• The adrenal cortex produces small amounts of sex hormones.
  • Androgens in the male
  • Estrogens in the female

Adrenal Medulla

• The cells are modified neurons that secrete directly into the bloodstream.
• Produces two similar hormones: epinephrine and norepinephrine.
• Their secretion is controlled by the sympathetic nervous system.
• The sympathetic system directs the fight-or-flight response, which include: increasing HR and CO, increasing BP, dialating airways, and decreasing GI functions.

The Pancreas

• The pancreas is adjacent to the duodenum and has both exocrine and endocrine functions.
• Pancreatic islets are the endocrine portion of the pancreas, also known as the islets of Langerhans.
• Alpha cells produce glucagon.
• Beta cells produce insulin.
• Delta cells produce somatostatin.
• Glucagon and insulin control metabolism and the maintenance of blood glucose levels.
• Somatostatin inhibits insulin, glucagon, and GH secretion.

Insulin

• Essential for life.
• Causes glucose, amino acids, and fatty acids to be absorbed through the cell for energy.
• Insulin lowers glucose levels in the blood, preventing hyperglycemia.

Glucagon

• Has the opposite effect of insulin and raises blood glucose levels, preventing hypoglycemia.
• Stimulates the liver to convert glycogen to glucose.
• Stimulates gluconeogenesis, the conversion of fat and protein breakdown products into glucose.
• Growth hormone and glucocorticoids have similar hyperglycemic effects.

The Testes

• The majority of each testicle is made up of seminiferous tubules, where spermatozoa are produced.
• Among the seminiferous tubules are the interstitial cells, which are the endocrine cells.
• Interstitial cells produce androgens when stimulated by LH from the anterior pituitary.
• The main androgen produced is testosterone, which include:
  • Produces secondary sexual characteristics
  • Stimulates the development of accessory sex glands
  • Activates sperm production
  • Stimulates growth of the penis, and an anabolic effect by stimulating the buildup of protein in muscle and bone

Ovaries

• The ovaries produce female reproductive cells and hormones in cycles.
• Ovarian cycles are controlled by two hormones from the anterior pituitary:
  • Follicle stimulating hormone (FSH)
  • Luteinizing hormone (LH)
• The ovaries produce estrogen and progestins.

Estrogens

• Estradiol and estrone are the female sex hormones.
• Produced when FSH stimulates follicles to develop:
  • Follicles are fluid filled structures in which the ova develop
  • The cells that make up the follicles produce and release estrogens
• Estrogens are responsible for the physical and behavioral changes that prepare the female for breeding and pregnancy.
• Indicate to males when a female is ready.
• As estrogen production increases, there is negative feedback to the anterior pituitary that reduces FSH and increases LH production.
• Ovulation occurs when the follicle is completely mature and LH peaks.
• The blister-like follicle ruptures and releases the ovum into the oviduct.

Progestins

• After ovulation, high LH levels cause the cells remaining in the empty follicle to develop into the corpus luteum (CL), which produces progestins.
• Progestins help prepare the uterus to receive a fertilized ovum and are required for maintenance of pregnancy.
• If the animal becomes pregnant, the uterus signals the ovary to maintain the CL; otherwise, the CL regresses.
• Progestin drugs are used therapeutically for breeding in horses.

Kidneys

• The kidneys produce erythropoietin, which stimulates red bone marrow to produce red blood cells.
• Hypoxia stimulates the production of erythropoietin.
• Increased oxygen levels in the blood trigger negative feedback, reducing erythropoietin production.
• The relationship between hypoxia and stimulus to produce red blood cells is the reason many athletes train at high altitude

Placenta

• The placenta is the interface between the fetus and the maternal circulation.
• Hormones, estrogen and progesterone, are produced to support and maintain pregnancy.
• Large amounts of chorionic gonadotropin are produced in some species (horses and humans especially).
• Chorionic gonadotropin is the basis of OTC pregnancy tests in people.

Pineal Body

• Located in the brain caudal to the thalamus.
• It influences cyclic activities of the body, acting as the body's biological clock.
• The pineal body produces melatonin, which affects moods and wake-sleep cycles, and seasonal estrous cycles in some species.