Download Chapter 18 - Endocrine

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, TRH, 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, or they may they may bind to a receptor
site on the cell membrane and activate a first
messenger (e.g. adenylate cyclase).
Types of Endocrine Glands
Endocrine (ductless) glands which release chemicals
(hormones) directly into the bloodstream.
The response time for hormones is relatively slow as
compared to the nervous system.
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.
Comparisons of Nervous and Endocrine Systems
Endocrine vs Nervous system
Neurotransmitter
Neuron
Nerve impulse
Endocrine
cells
Hormone in
bloodstream
Target
cells
Endocrine Organs
Hypothalamus- neuroendocrine organ
Pituitary gland- endocrine 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 is the head gland of the
endocrine 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 “Hypophysis”
Location: Sits in the sella turcica of the sphenoid bone
Attached to the hypothalamus by the infindibular stalk
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
Consits of 3 divisions: Median eminence, Infindibulum
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)
– Parathyroid hormone (PTH)
– Follicle stimulating hormone (FSH)*
– Luteinizing hormone (LH) = ICSH in males*
– Prolactin (PRL)
– * indicate tropic 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)
CRH – corticotrophin releasing hormone → (ACTH)
MRH- melanocyte releasing hormone → (MSH)
GnRH – gonadotropin releasing hormone → (FSH/LH)
PRH – prolactin releasing hormone → (PRL)
PIH – prolactin inhibiting hormone (dopamine)
Somatostatin – inhibits release of growth hormone
MIF- inhibits release of MSH
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.
Parfollicular cells produce calcitonin which ↑ blood
Ca++ levels.
– Target cells are osteoclasts 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 - layer next to adrenal medulla →
sex hormones (androgens and testosterone).
Adrenal Medulla is neuroendocrine tissue and is part
of sympathetic division (postganglionic) of ANS.
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
Adrenal glands
Blood supply is via:
Superior suprarenal
from Inferior phrenic
arteries.
Middle suprarenal and
Inferior suprenal off
of aorta
.
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:
– β cells → Insulin a 51 amino acid molecule which targets the
liver and most body cells except the brain
– α-cells → Glucagon a 29 a.a. molecule produced which
targets the liver to breakdown glycogen → 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 posterior to sternum and superior to the
heart.
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 “epiphysis cerebri”
Part of the epithalamus in the brain
Contains neurons, glial cells and pinealocytes
which produces 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 → Vit. D.
Kidneys secrete renin, erythropoietin, and aids in →
Vit. D synthesis.
Liver secretes erythropoietin and angiotensinogen.
The liver also aids in → Vit. D and insulin-like GF.
Placenta secretes estrogen and progesterone.
Stomach secretes gastrin and cholecystokinin and
other enteroteric hormones that effect digestion.
Hypothalamic releasing and inhibiting
hormones
Pituitary Hormones