Chapter 18
The Endocrine System
Endocrine system glands
Hormone chemistry and Action
Are chemically composed of either: (p. 516 in Saladin)
– Ring structures = steroids
– Polypeptides = ACTH, TSH, FSH, LH, oxytocin, insulin, etc.
– Monoamines = dopamine, thryoxine (T3/T4)
At their target cell, they may diffuse through the cell
membrane and bind to a receptor site in the cytoplasm
or nucleus (steroid hormones), or they may they may
bind to a receptor site on the cell membrane (watersoluble hormones) and activate a first messenger (e.g.
adenylate cyclase) which, in turn, activates a second
messenger (cyclic AMP).
Endocrine System vs Autonomic
Nervous System
1. The endocrine system releases
chemical messengers (hormones) into the
blood. The autonomic nervous system
communicates by nerve impulses with
effectors.
2. The endocrine system acts relatively
slowly as compared to the autonomic
nervous system.
Endocrine System vs Autonomic
Nervous System
Neurotransmitter
Neuron
Nerve impulse
Endocrine
cells
Hormone in
bloodstream
Target
cells
Comparisons of Nervous and Endocrine Systems
Types of Endocrine Glands
Three types of glands:
1. Pure endocrine glands – thyroid, parathyroid,
adrenal cortex, thymus and pineal.
2. Endocrine/exocrine glands – pancreas,
ovaries and testes
3. Neuroendocrine glands – adrenal medulla
and hypothalamus (supraoptic nuclei and
paraventricular nuclei) to posterior pituitary.
Endocrine Organs
Hypothalamus- neuroendocrine gland
Anterior pituitary gland- endocrine gland
Posterior pituitary gland- neuroendocrine gland
Thyroid gland- endocrine gland
Parathyroid glands- endocrine gland
Adrenal gland (cortex and medulla)- endocrine/neuroendocrine gland
Pancreatic islets- endocrine/exocrine gland
Gonads- Ovaries in females; Testes in males- endocrine/exocrine glands
The Hypothalamus
Location: directly below the thalamus in the
diencephalon of the brain. It lies between the optic
chiasm anteriorly and the mammillary bodies
posteriorly and is inferior to the third ventricle.
Structure: Composed of several groups of nuclei,
the hypothalamus controls the endocrine system
as well as the autonomic nervous system and
produces regulatory hormones that regulate the
release of numerous pituitary hormones. It also
produces the hormones of the posterior pituitary.
The Hypothalamus
The Pituitary Gland or Hypophysis
Location: Sits in the sella turcica of the sphenoid bone
Attached to the hypothalamus by the infundibulum
Consists of two lobes:
1. Adenohypophysis
Releases 7 different hormones
Consists of 3 divisions: pars tuberalis, pars intermedia and
pars distalis (anterior lobe).
2. Neurohypophysis
Releases 2 different hormones
Consists of 3 divisions: median eminence, infundibular
stalk and pars nervosa (posterior lobe)
Pituitary gland
Adenohypophysis
Pars tuberalis
Pars intermedia
Pars distalis
Neurohypophysis
Median eminence
Infindibular stalk
Pars nervosa
Anterior Pituitary Hormones
There are seven anterior pituitary hormones:
– Thyroid stimulating hormone (TSH)*
– Growth hormone (GH)
– Adrenocorticotropic hormone (ACTH)*
– Melanocyte stimulating hormone (MSH)
– Follicle stimulating hormone (FSH)*
– Luteinizing hormone (LH) = ICSH in males*
– Prolactin (PRL)
* indicate trophic hormones
Hypothalamic releasing hormones
Release of anterior pituitary hormones is directed by specific
releasing hormones (factors) from the hypothalamic nuclei.
All of these are polypeptide molecules.
TRH – thyrotropin releasing hormone → (TSH and PRL)
GHRH – growth hormone releasing hormone → (GH)
Somatostatin – inhibits release of growth hormone
CRH – corticotrophin releasing hormone → (ACTH)
MRH- melanocyte releasing hormone → (MSH)
MIF- inhibits release of MSH
GnRH – gonadotropin releasing hormone → (FSH/LH)
PRH – prolactin releasing hormone → (PRL)
PIH – prolactin inhibiting hormone (dopamine)
Anterior/Posterior Pituitary Circulation
Blood flow to pituitary gland is via a portal circulation the
hypophyseal portal. Arterial flow is via superior and inferior
hypophyseal artery into capillary beds in series
Posterior Pituitary Hormones
ADH an Oxytocin are secreted by neurosecretory cells in the
paraventricular and supraoptic nuclei of the hypothalamus and are
transported to posterior pituitary via hypothalamohypophyseal
tract.
Neurohypophyseal Hormones
Antidiuretic hormone (ADH) – produced by supraoptic
nuclei in the hypothalamus.
– Consists of 9 amino acids
– Reduces the excretion of water by kidney collecting
ducts; increases cuddling and grooming behavior.
Oxytocin – produced by the paraventricular nuclei in
the hypothalamus
– Consists of 9 amino acids, but differs from ADH.
– Induces smooth muscle contraction; increases
cuddling and grooming behavior.
Adenohypophyseal cell types
Thyrotropic cells secrete TSH
Somatotropic cells secrete GH
Corticotropic cells secrete ACTH and MSH
Gonadotropic cells secrete FSH and LH
– Tropic hormones regulate the release of other hormones
from the glands that they stimulate (TSH, ACTH, FSH and
LH).
MSH, PRL and GH all act directly on non-endocrine
target tissues.
Thyroid gland
Location: largest pure endocrine gland in adults ~
20-25 gms. and located adjacent to trachea
inferior to larynx.
Structure: Butterfly shaped with two lobes joined by
an isthmus. ~ 50% of people have a pyramidal lobe
growing upward off of isthmus.
Gross Anatomy: Bulbous at inferior end and tapers
superiorly.
- Thyroid is highly vascular via thyroidal arteries .
Cellular Anatomy: Composed of sacs of thyroid
follicular cells and lined with simple cuboidal or
simple squamous epithelium that is filled with protein
rich colloid (thyroglobulin).
Thyroid gland
Follicular cells produce tri-iodo thyronine (T3) and
thyroxine (T4) which are stored in thyroglobulin.
– Target cells are every cell and tissue in the body
Parafollicular or “C” cells found between follicular cells
in the thyroid gland produce calcitonin which keeps
blood Ca++ levels within the normal range by
depositing excess Ca++ in the bones and teeth.
– Target cells are osteoblasts in bone
– Has no demonstrable function in adults, most active in fetus,
infants and adolescents.
Thyroid gland
Parathyroid gland
Located on the posterior lateral margins of the thyroid
gland are 4 to 8 small nodules.
Structure is small ovoid nodules ~ 2-5 mm x 3-8 mm.
Produces parathyroid hormone (PTH) which helps
regulate blood Ca++ levels.
Target organs of PTH are bone, kidneys and
intestines.
Histologically it contains numerous small chief cells
and rare large oxyphilic cells.
– Chief cells secrete PTH.
– Oxyphilic cells are probably inactive or immature chief cells.
Parathyroid glands
Adrenal glands
Located in the abdominal cavity attached to superior
pole of each kidney (suprarenal).
Two distinct regions: Cortex and Medulla
Adrenal cortex has 3 layers:
Zona glomerulosa – outer layer → mineralocorticoids.
Zona fasciculata - middle ¾ of cortex →
glucocorticoids.
Zona reticularis – innermost layer → androgens
Adrenal Medulla is neuroendocrine tissue and is part
of sympathetic division (postganglionic) of ANS.
Adrenal glands
Blood supply is via:
Superior suprarenal
from Inferior phrenic
arteries.
Middle suprarenal and
Inferior suprenal off
of aorta
.
Adrenal Cortex
Histologic features of adrenal cortex:
Outer layer is a dense fibrous capsule.
Zona glomerulosa (15% ov) looks like little balls
or knots densely clustered together.
Zona fasiculata (78% ov) looks like cords that
radiate toward the medulla.
Zona reticularis (7% ov) branching network of
pink staining cells between fasciculata and
medulla.
Adrenal medulla is composed of chromafin cells
arranged in spherical clusters.
Adrenal gland histology
Pancreas
Location: Just inferior to the stomach and in the first loop of the
duodenum approximately in the middle of the abdomen.
Structure:- mixed gland (endocrine/exocrine); spongy-like
appearance. Exocrine cells produce digestive enzymes.
Pancreatic “Islet of Langerhans” are endocrine cells.
Hormones produced by 5 classes of islet cells include:
– α-cells → Glucagon- a 29 amino acid molecule which targets
the liver to breakdown glycogen and release glucose.
– β cells → Insulin- a 51 amino acid molecule which targets
the liver and most body cells except the brain to take up
glucose.
– Delta cells → Somatostatin ↓ release of insulin & glucagon.
– “F” cells → Pancreatic polypeptide ↓ gall bladder contraction.
– “G” cells → Gastrin ↑ acid secretion, gastric motility and
stomach emptying.
Pancreas
Ovaries
Primary sex organs of females
Located retroperitoneal in the abdominal cavity lateral
to the uterus and at the proximal end of the uterine
tubes (fallopian tubes).
Pair of almond shaped organs ~ 3 cm x 1.5 cm x 1 cm.
Produce female sex hormones (estrogen and
progesterone) and contain ova.
More about the ovaries in reproduction.
Ovaries
Testes
Primary male sex organs.
Located in the scrotum outside of abdominal
cavity.
Produce sperm and male sex hormones
androgens (testosterone and inhibin).
Size ~ 4 cm ↑ x 3 cm a/p x2.5 cm →.
More about the testes in reproduction
Testes
Thymus
Located in mediastinal space of the thoracic
cavity deep to sternum and supeficial to the
pericardium.
Produces several hormones amongst which are
thymosin, thymopoietin, and IGF-1.
Stimulates the maturation of T- lymphocytes
Largest size occurs at puberty and thereafter
diminishes in size as one gets older. By the age
of 50 it is ~ ¼ its original size.
Thymus
Pineal gland or “epiphysis cerebri”
Part of the epithalamus in the brain
Contains neurons, glial cells and pinealocytes
which produce and secrete melatonin .
Melatonin regulates the circadian cycle as well
as slows the maturation of sperm and ova by
inhibiting FSH and LH release from the
adenohypophysis.
Pineal gland
Other endocrine like organs
Heart secretes atrial naturetic peptide (ANP).
Skin initiates synthesis of calcitrol → vitamin D.
Kidneys secrete renin, erythropoietin, and aids in →
vitamin D synthesis.
Liver secretes erythropoietin and angiotensinogen.
The liver also aids in → vitamin D and insulin-like GF.
Placenta secretes human gonadotropin.
Stomach secretes gastrin and cholecystokinin and
other enteroendocrine hormones that affect digestion.
Hypothalamic releasing and inhibiting
hormones
Pituitary Hormones