AHE II 2.5 - ENDOCRINE SYSTEM: INTORDUCTION, HYPOTHALAMIC CONTROL, AND GLANDS

• Objective: to contextualize the role of the endocrine system by identifying anatomical structures and link their functions to maintenance of homeostasis in the body

Lecture Learning Objectives

• Explain how the endocrine system communicates
• Describe how the hypothalamus controls the endocrine system by describing its influence on the anterior pituitary, posterior pituitary, and adrenal gland
• Describe the anatomy of the pituitary gland, focusing on the structure that comprises it and the differences between the anterior and posterior pituitary in development, blood supply, and hormone synthesis/storage
• Recall the role that the parathyroid and thyroid glands play in the endocrine system, identifying their location, blood supply, and anatomical structures, linking the cells it contains with their primary function
• List the major events in the development of the thyroid gland
• Recall the location of the adrenal gland, and the differences between the adrenal cortex and adrenal medulla
  • Name the hormones produced in the adrenal cortex and medulla, and outline their effects on target cells
  • List the layers of the adrenal cortex and the hormones produced in each
• Recall the anatomy and location of the pancreas, listing the hormones produced by the pancreatic islets
• Describe the anatomy, location, and endocrine function of the pineal gland and thymus, linking the thymus's embryological origins
• Explain how hormones secreted by the kidneys, heart, gastrointestinal tract, and gonads help regulate homeostasis
• Identify the following endocrine glands and their specific structures in the histology slides provided:

Endocrine System Introduction

• It is a regulatory system that produces hormones to control metabolism, growth and development, tissue function, sexual function, sleep, and mood
• Composed of endocrine glands and endocrine cells housed within other organs
• Endocrine glands are ductless organs that secrete their molecular products directly into the bloodstream
• It works with the nervous system to maintain homeostasis in the body
• Both target specific organs
• Communication methods differ, using hormones instead of impulses
• Differ in range and target of effects. The endocrine system is widespread, while the nervous system is specific.
• Response time and duration differ- the nervous system is short, the endocrine system is prolonged
• Recovery time differs, the nervous system is immediate, the endocrine system is not

Endocrine Glands

• Pituitary gland, which contains an anterior and posterior pituitary each
• Thyroid and Parathyroid Glands
• Adrenal glands are regulators of metabolism, immune system, blood preassure, and stress via cortisol and aldosterone
• Pancreas, which is both endocrine and exocrine
• Pineal gland
• Thymus
• The kidneys, heart, GI tract, and gonads also contain endocrine cells

Hormone Overview

• Endocrine glands communicate through the release of hormones into the bloodstream
• Hormones are molecules that affect specific organs by binding to receptors
• Only target cells/organs have specific receptors for specific hormones
• Organs, tissues, or cells that lack receptors for hormones will not be affected

Hypothalamic Control

• The hypothalamus is the master control center of the endocrine system
• It interacts with the pituitary gland
  • The anterior pituitary (adenohypophysis)
  • The posterior pituitary (neurohypophysis)
• Regulation occurs in three ways:
  • Secreting regulatory hormones into blood that influence the secretory activity of the anterior pituitary gland
    • Releasing hormones- stimulate production and secretion
    • Inhibiting hormones- deter production and secretion
  • Produces two hormones that are transported and stored in the posterior pituitary gland
    • Antidiuretic hormone (ADH) - reduces water excretion
    • Oxytocin (OT)- triggers milk letdown and uterine contractions
  • Oversees the stimulation and hormone secretion of the adrenal medulla
    • The adrenal medulla is specialized in that it secretes hormones into the bloodstream in response to the sympathetic nervous system
• Some endocrine cells are not under direct control of the hypothalamus and release hormones without input
  • Ex. The parathyroid gland responds to concentrations of chemical levels in the bloodstream, requiring no input from the hypothalamus

Pituitary Gland

• Also known as the hypophysis
• It is inferior to the hypothalamus and a part of the diencephalon
• A thin stalk called the infundibulum connects the hypothalamus to the pituitary and projects from its median eminence
• It is located within, and protected by, the sella turcica of the sphenoid bone
• Partitioned structurally and functionally into the anterior (AP) and posterior (PP) pituitary, derived embryonically from different structures

Embryology of the Pituitary Gland

• During Week 3 of development:
  • The anterior pituitary's hypophyseal pouch grows superiorly from the roof of the pharynx
  • The posterior pituitary's neurohypophyseal bud forms from the inferior portion of the diencephalon
• During Week 8 of development, the hypophyseal pouch detaches from roof of the pharynx and merges with the neurohypophyseal bud

Anterior Pituitary (Adenohypophysis)

• Produces and secretes hormones in response to the hypothalamus
• Divided into three parts:
  • Pars distalis, which is anterior-most
  • Pars intermedia, which is between pars distalis and posterior pituitary
  • Pars tuberalis, which "hugs" the infundibulum
• Regulatory hormones reach the AP from the hypothalamus through the hypothalamo-hypophyseal portal system
• A portal system consists of two capillary beds that are connected by veins

Posterior Pituitary (Neurohypophysis)

• Stores hormones synthesized by the hypothalamus
• Composed of the pars nervosa and the infundibular stalk
• Derived from nervous tissue at the base of the diencephalon

Hypothalamo-Hypophyseal Portal System

• Blood enters via the internal carotid artery into the superior hypophyseal artery (SHA)
• Primary plexus, capillaries located around the median eminence and infundibulum, drain into hypophyseal portal veins (HPV)
• Hypophyseal portal extends inferiorly carrying hypothalamic hormones to the secondary plexus
• Hormones are released into interstitial space to communicate with anterior pituitary
• Other hormones travel from the anterior pituitary via hypophyseal veins, and eventually to the heart to be pumped throughout the body

Posterior Pituitary (Neurohypophysis)

• Hormones released by the PP drain into surrounding blood vessels
• Arterial supply: inferior hypophyseal artery from the internal carotid artery
• Capillary bed surrounds the PP
• Venous drainage is done via the hypophyseal veins

Thyroid Gland

• Large gland located anterior to the trachea directly inferior to the thyroid cartilage and "hugs" the trachea
• Deep to infrahyoid muscles
• Butterfly in shape with left and right loves connected by the isthmus
• Arterial supply via the external carotid to the superior thyroid and subclavian to the thyrocervical trunk, then ultimately the inferior thyroid
• Venous drainage happens via the superior and middle thyroid to the internal jugular, and the inferior thyroid to the brachiocephalic

Parathyroid Glands

• Consist of ~4 small nodules located on the posterior surface of the thyroid gland; number and location may vary
• Derived from pharyngeal pouches and migrate to adult location
• Arterial supply via the inferior thyroid arteries
• Venous drainage is the same as the thyroid gland

Thyroid Development

• Derived from an epithelial outpouching of the endoderm in the floor of the foregut, forming the thyroid diverticulum
• Migrates caudally into position anterior to trachea and a patent thyroglossal duct remains during development
• The thyroglossal duct normally closes, with remains located on posterior tongue; this forms the foramen cecum

Adrenal Glands

• Paired, triangular shaped glands anchored on the superior surface of each kidney
• They are retroperitoneal, embedded in fat and fascia to minimize their movement
• Contain an outer adrenal cortex and inner adrenal medulla that secrete different types of hormones

Adrenal Cortex

• The cortex is separated into three regions, each producing its own hormones, that are derived from the mesoderm
• Zona glomerulosa synthesizes mineralocorticoids to regulate the concentration and composition of electrolytes in body fluids
  • It is the outermost layer
• Zona fasciculata synthesizes glucocorticoids that stimulate the metabolism of lipids and proteins, and regulate glucose levels
  • It is the middle and thyckest layer in the adrenal cortex
• Zona reticularis synthesizes a small amount of gonadocorticoids that exert masculinizing efforts and promote protein anabolism and growth
  • It is the innermost layer

Adrenal Medulla

• Forms inner core of adrenal gland
• Controlled by the sympathetic nervous system
• It releases neurotransmitters directly into the circulatory system for a widespread response
• Composed of chromaffin cells formed from neural crest cells and secrete epinephrine (adrenaline)
• Other cells secrete norepinephrine and dopamine
• Prepares the body for fight-or-flight and prolongs the effects of sympathetic stimulation

Pancreas

• Heterocrine gland that performs both endocrine and exocrine activities
• Exocrine function aids in digestion by secreting pancreatic juices into the duodenum
• Pancreatic acini- groups of cells that produce alkaline pancreati juice
• Endocrine function: clusters of endocrine
• Four types of cells produce four hormones
  • Alpha cells produce glucagon, synthesis stimulated by low levels of blood glucose
  • Beta cells secrete insulin, synthesis stimulated by elevated levels of blood glucose
  • Delta cells secrete somatostatin, synthesis stimulated by high levels of nutrients in the bloodstream
  • F cells secrete pancreatic polypeptide, synthesis stimulated by protein digestion in GI tract

Pineal Gland

• Small structure attached to the posterior region of the epithalamus
• Secretes melatonin from pinealocytes
• The pineal gland helps regulate circadian rhythm by increasing synthesis at night
• It also affects the synthesis of hypothalamic regulatory hormones for FSH and LH, which play a role in sexual maturation
  • Studies have shown that excessive melatonin delays puberty
• Decreases in size with age

Thymus

• Bilobed structure located in the mediastinum superior to the heart
• Large in infants and children and diminish in activity and size with age
• Produces thymopoietin and thymosins
• It works with the lymphatic system to regulate and maintain body immunity
• Stimulates and promotes differentiation, growth, and maturation of T-lymphocytes and the development of a adaptive immune system

Development of the Thymus and Parathyroid Glands

• The third pharyngeal pouch gives rise to the thymus and inferior parathyroid glands
  • Glands separate from pouch, migrate caudally, and inferior parathryoids separate and attach to thymus
• The forth pharyngeal pouch gives rise to superior parathyroid glands, which migrate short distance alone to the thyroid gland

Endocrine Function of Other Organs

• The kidneys help regulate the concentration of electrolytes in body fluids, the rate of red blood cell production, and increase blood volume and pressure by secreting:
  • Calcitriol, which increases uptake of calcium and phosphate
  • Erythropoietin, which increases RBC production and maturation
  • Renin, which is an enzyme released in response to abnormal electrolyte concentrations in urine
• The heart responds to excessive stretching in the wall of the heart due to elevated blood pressure or volume by producing atriopeptin, synthesized modified cardiac muscle cells in the wall of the right atrium to cause water loss and sodium excretion from blood into urine to reduce blood volume
• The gastrointestinal tract produces hormones throughout the tract and accessory organs that help regulate digestive activities
  • This stimulates the liver, gallbladder, and pancreas to release secretions needed for efficient digestion
• The gonads
  • Ovaries produce estrogen and progesterone
  • Testes produce androgens (most converted into testosterone)
  • Also produce inhibin, which inhibits follicle-stimulating hormone secretion

Explore the intricacies of the endocrine system: hormone actions, gland functions, and regulatory mechanisms. Understand hormone effects on target cells and the hypothalamus's role in pituitary control. Delve into gland malfunctions and their physiological impacts.