ENDOCRINE SYSTEM 1 - Thyroid, Parathyroids and Adrenals
Roger Bick, PhD, MMEd
Learning Objectives: You should know:
Hormones synthesized by thyroid, parathyroid and adrenal glands

Targets of the synthesized hormones

Cell types in the endocrine organs

Synthetic pathway of thyroxine and triiodothyronine

Blood supply of endocrine organs (type and amount)

Aspect of cell(s) through which hormones are secreted (basal v apical)
Key Words: Hormones, vascular, receptors, APUD, follicular, parafollicular, Calcitonin, Thyroxine,
Triiodothyronine, TSH, iodine/iodide (NB), chief cells, oxyphil cells, parathormone. Zona glomerulosa,
fasciculata, reticularis. Aldosterone. Cortisol, dehydroepiandosterone. Catecholamines. Angiotensin
(converting enzyme)
GENERAL FEATURES OF ENDOCRINE GLANDS
Ductless glands, with secretions into copious vascularity of surrounding interstitium. Secretions rapidly
enter bloodstream and travel to distant organs-receptors at target sites. Parenchyma composed of individual
cells, or groups of cells in clumps, cords, arrays or lobules. Secretory products elaborated by endocrine
glands include steroid hormones, peptide hormones and amines.

Discrete endocrine glands - comprised of tissue with solely an endocrine function.

Adrenal Glands

Parathyroids

Thyroid

Pineal

Pituitary
Have abundant parenchyma, sparse connective tissue, many blood vessels of which numerous are
fenestrated capillaries. Often, inclusion granules in the cytoplasm of the cells- either lipid droplets as
precursors to steroid hormone synthesis, or secretory granules for protein elaboration.

Mixed Exocrine and Endocrine Glands - components of other tissues - both endocrine and exocrine in
function. Examples of mixed glands:
Organ
Exocrine Function
Endocrine Function
Endocrine Component
Pancreas
Digestive enzymes
Glucagon (A)
Insulin (B)
Somatostatin (d)
Islets of Langerhans
Testis
Sperm production
Testosterone
Leydig cells
Liver
Bile production
Albumin
Clotting factors
Liver
Parenchyma
Kidney
Urine production
Renin
Juxtaglomerular cells
APUD Cells are unicellular endocrine glands; secrete low molecular weight peptides and proteins with
hormone-like activity called candidate or putative hormones. APUD = Amine Precursor Uptake and
Decarboxylation; cells store and concentrate both biogenic amines and precursors in the cytoplasm - They
DO NOT stain with haematoxylin and eosin dyes, but DO with silver stains. Therefore they are sometimes
referred to as Argentaffin cells. APUD cells found wedged or interspersed among other cells comprising
the epithelial lining of organs and systems. These cells have a vast array of physiological functions:
Distribution and Secretory Products of APUD cells.
Cell name
Source
Polypeptide product
A
Pancreas
Glucagon
B
Pancreas
Insulin
C
Thyroid
Calcitonin
D
Stomach, Small intestine, Pancreas
Somatostatin
D1
Stomach, Small and Large intestine, Pancreas
Vasoactive Intestinal Polypeptide*
EC
Stomach, Small and Large intestine, Pancreas
Motilin*, Substance P
G
Stomach, Small intestine, Pancreas
Gastrin
I
Small intestine
Cholecystokinin
K
Small intestine
Gastric Inhibitory peptide
N
Small intestine
Neurotensin
S
Small intestine
* denotes a candidate or putative hormone
Secretin
THE PARATHYROIDS (Para = around)
Embryology - Arise from brachial pouches III and IV. Parathyroids III migrate with the thymus to
inferior poles of the thyroid gland. Parathyroids IV do not migrate as far. Become superior pair near the
superior poles of the thyroid.
Anatomy - Two pairs of glands, each weighing 30-50m; smooth, yellow-brown, flattened ovoid bodies.
Function - Secrete parathormone, and are essential for the physiologic maintenance of circulating
calcium concentrations. Decreased circulating calcium stimulates PTH secretion; increases the number
of osteoclasts, therefore greater calcium absorption from bone matrix; Renal excretion of calcium is
prevented; enhanced absorption of calcium from the GI tract; overall increase in serum calcium
HISTOLOGY OF THE PARATHYROIDS:- Stroma: connective tissue capsule envelopes glands;
sends delicate septa into the anterior portion, dividing glands into course clumps of cells; Stroma contains
abundant vasculature, nerves, lymphatics and unilocular fat, the fat content increasing with age
(diagnostic tool for accurate identification).
Parenchyma: two cell types, 1) Chief (principal) Cells and 2) Oxyphil Cells.
1) Chief cells - bulk of the parathyroid; small and polygonal; clear or lightly basophilic staining
cytoplasm; responsible for the synthesis and release of parathormone (PTH); PTH increases ionized
calcium in the blood; cells therefore contain abundant rough ER, glycogen, secretory granules and a
prominent golgi apparatus.
2) Oxyphil cells - round and larger than Chief cells; Stain intensely eosinophilic due to large
numbers of mitochondria; Function uncertain (variant form of Chief cells?).
Other cells; water-clear cells (Wasserhelle); extremely clear, apparent at times of high secretion;
Transitional cells - between water-clear and Chief cells (evidence of one cell type derived from another?)
Parathyroid cells are arranged in cords and nests, separated by scanty fibrous stroma; abundant
capillaries. Occasional follicles containing protein and lined with columnar/cuboidal epithelia are present.
THE THYROID (Greek meaning shield)
Embryology - Arises as a midline diverticulum of the floor of the pharynx. Develops into bilobed organ.
Site of origin marked by a permanent pit in the tongue, called the foramen caecum. During development,
the thyroid receives a small, significant contribution from the ultimo-branchial pharyngeal pouches, which
give rise to the parafollicular cells.
Anatomy - Bilobed, bilaterally symmetrical, firm, smooth and red-tan in color. Located in anterior neck.
An isthmus connects two lobes. Surface may be faintly lobulated. Follicles may be visible in cut sections,
as they can be up to 500 m in diameter.
HISTOLOGY OF THE THYROID
Stroma (Connective Tissue) - covered by loose connective tissue which invaginates via finger-like
protrusions called septa - gland divided into ill-defined lobes and lobules; septa become more delicate,
eventually consisting of only reticular fibers. Within stroma are numerous nerves, lymphatics and blood
vessels.
Parenchyma (Functional) - Most of the gland comprised of several million follicles - single follicle is a ball
of cells, lumen filled with amorphous, eosinophilic colloid thyroglobulin, stored as non-active form of
thyroid hormone. N.B. (nota bene) The thyroid is the only endocrine organ that stores its secretory
product in an extracellular environment. Epithelial cells lining the follicle are a single layer, resting on a
basement membrane. Two cell types, with dissimilar function and ultrastructure, comprise the epithelial
layer.
1. Follicular cells: present in an overwhelming majority of follicle lining cells - responsible for the
synthesis of thyroid hormone. Active follicular cells contain all the expected organelles and markers for
cells involved in protein synthesis (Know these markers):- Euchromatic nuclei with nucleolus or nucleoli;
lots of rough endoplasmic reticulum in basal aspect of the cell; well developed golgi apparatus for
protein trafficking in supra-aspect of the cell; many secretory vesicles in apical cytoplasm; abundance of
lysosomes. Apex of the cells is covered with short microvilli facing the lumen of the follicle. Remember,
shape and size of cells and follicles varies depending upon "active" state (See Function). Thyroid made up
of follicles - sacs filled with colloid - lined with epithelium which secrete colloid. Surrounded by a rich
capillary network. Iodide in blood is captured by the epithelium, bound to protein, and stored in the
follicles as thyroglobulin.
2. Parafollicular cells: Calcitonin secreting; individually or clustered within follicular epithelium or
scattered in the stroma; difficult to distinguish these cells with haematoxylin and eosin stain (Silver
stains and/or electron microscopy); Characteristic feature is presence of numerous membrane-bound
secretory granule, containing calcitonin which reduces blood calcium levels. As the name implies, to tone
down.
Situated singly and in clusters between the follicles, and intercalated between follicular cells, are
parafollicular cells (light cells or C cells) - source of calcitonin; argyrophilic. Thyroidectomy does not
result in complete calcitonin deficiency, as the parathyroid and thymus contribute to calcitonin production.
Functional state of the thyroid determined by: a) size and shape of the epithelium, b) size of the follicles
and c) amount of colloid present. Height of the cells is most important criteria. Columnar = hyperactive.
Flat = inactive. Height of cells is an index of TSH (thyroid-stimulating or thyrotrophic hormone)
activity.
Follicular size also indicates the level of thyroid activity; large follicles = inactive and colloid/hormone
storage; small follicles and scanty colloid = increased activity.
Regulation, Synthesis, Storage and Release of Thyroid Hormones
 Follicular cells synthesize and secrete thyroglobulin into follicular lumen. Thyroglobulin taken
back up and modified for synthesis and production of the active forms of thyroid hormone - a bidirectional process.
 Thyroglobulin Synthesis. Synthesized in rough endoplasmic reticulum, modified in the golgi
apparatus, then secreted into follicular lumen as non-iodinated glycoprotein (colloid). Tyrosine
residues on thyroglobulin will become iodinated in the active thyroid hormone.

Uptake and Oxidation of Iodide. Iodide taken up into follicular epithelium by active transport;
oxidized by thyroperoxidase to iodine at the apex of the cell then released into the follicular
lumen; thyroglobulin iodination at the aforementioned tyrosines. Process occurs rapidly at the
interface between the follicle and the microvillus border of the follicular cells. Iodinated
thyroglobulin remains in the lumen, until stimulated by TSH (Thyroid Stimulating Hormone).
Thyroid hormones, both T3 (Triiodothyronine) and T4 (Thyroxine), are synthesized and released.

Liberation of T3 and T4.
Luminal colloid taken up by pinocytosis - vesicles fuse with lysosomes. Proteolytic cleavage of
iodinated thyroglobulin yields Triiodothyronine and Thyroxine; ratio of T 3 to T4 is 1:20.
Hormones enter interstitial capillaries for transport. Although T3 is less abundant than T4, it is the
more potent of the thyroid hormones. Both T3 and T4 influence basal metabolic rates in the
adult, as well as influencing growth and neurological development in the fetus. (See p150 High
Yield Histology)

Thyroid Hormone Regulation.
Thyroid Stimulating Hormone (TSH) from anterior pituitary increases thyroid hormone (TH)
production by stimulating all phases of TH synthesis. Control via a simple negative feedback
loop - high levels of TH suppress TSH release and low levels of TH increase TSH release.
Calcitonin.
Secreted by parafollicular cells in response to high circulating levels of ionized calcium - low
levels of Ca2+ suppress calcitonin release. Calcitonin reduces blood calcium levels by inhibiting
bone resorption by osteoclasts and promoting adsorption of calcium by the bones.
THE ADRENALS
Embryology - Arise from 2 separate primordia, one ectodermal (medulla) and one mesodermal (cortex).
Cortex develops from mesenchymal cells on posterior body wall between gonad and mesentery (urogenital
ridge). Medulla formed by ectodermal cells that migrate from neural crest of cortical region, via celiac
ganglion.
Anatomy - Located in retroperitoneum, just above kidneys (suprarenal glands); embedded in unilocular fat;
4-6 grams/adrenal but varies widely; Triangular in cross-section; Outer cortex yellow, inner cortex brown,
medulla gray; Each gland has multiple arterial branches (superior, middle and inferior suprarenal arteries),
but only one major vein (adrenal or suprarenal vein) - right enters vena cava directly, left drains into left
renal vein; arteries branch to form a subcapsular plexus from which the following arise - a) capsular
arteries, b) arteries for cortex which arborize extensively to form a fenestrated capillary bed between
parenchymal cells and drain into medullary arteries and c) branches that pass directly through the cortex to
form capillary plexus of the medulla.
Histology and Function Cortex has three zones:
1) Zona glomerulosa
2) Zona Fasciculata
3) Zona reticularis
When layers are sharply defined, the ratio is approximately 1:3:2 - good guideline for determining where
you are in the cortex. The Zona glomerulosa and Zona Fasciculata often have abundant lipid.
Cortical Zones
 Zona glomerulosa. Cells form small nest or glomerules, one to three cells across; Foamy
appearance due to lipid droplets; responsible for the production of mineralocorticoid
ALDOSTERONE, which modulates electrolytes in blood. Aldosterone acts on salivary and sweat
glands, and the kidneys, to reabsorb sodium at the expense of potassium (i.e. sodium retained,
potassium excreted). N.B. This zone IS NOT dependent on the pituitary whereas Fasciculata and
reticularis ARE; Control of Aldosterone secretion is conditioned by RENIN-ANGIOTENSIN
system. REMEMBER SALT!!!

Zona Fasciculata. Cells arranged in long, straight columns separated by capillaries; have finely
vacuolated eosinophilic cytoplasm; Contain abundant lipid droplets hence oft-used term,
Spongiocytes; Dependent upon ACTH (adrenocorticotrophic hormone) for maintenance of
structure and function; zone secretes CLUCOCORTICOIDS following ACTH stimulation, 95%
of which is Cortisol. Glucocorticoids: enhance glucose synthesis by the liver to elevate blood
sugar levels, suppress inflammatory and immune responses and cause catabolism (break
down) of proteins, which may lead to muscle wasting. Fatty Acid Storage Increased, But
REMEMBER SUGAR!!!

Zona reticularis. Innermost layer; Cells form short, intertwining cords that abut the medulla, cells
are "lipid poor", with pink cytoplasm containing abundant mitochondria, but rich in RNA;
Dependent upon ACTH for maintenance of structure and function; zone secretes ANDROGENS,
primarily dehydroepiandrosterone (less potent than testosterone), but also progesterone and
estrogen, following ACTH stimulation. REMEMBER SEX!!!
Medulla
Composed of pleomorphic cells (i.e. different shapes and sizes), from polyhedral to circular, grouped in
clumps and cords around sinusoidal vessel; cells secrete and contain epinephrine (80% of the cells) and
norepinephrine (20% of the cells, which are strong reducing agents that can be oxidized with quinones and
dichromates (chromaffins) to yield brown pigments, a very useful histochemical test; sometimes referred to
as Chromaffin cells; stain more basophilic than cortical cells with H & E. Circulating catecholamines
have similar effects on different organs to catecholamines released by direct neural stimulation. However,
response can last 10 times as long because hormones are slowly removed from blood; Effects of these
hormones include: elevating heart rate, elevating blood glucose, splanchnic vasoconstriction, etc, i.e.
Flight/Fight functions. REMEMBER SYMPATHETIC!!!
Regulation of Adrenal gland secretions
Cortex
 Zona Glomerulosa - Aldosterone secreted in response to circulating ANGIOTENSIN levels and
high serum potassium levels:- See flow chart that follows
 Zona Fasciculata - Secretion of glucocorticoids entirely dependent on ACTH from anterior
pituitary; negative feedback loop - low concentrations of Cortisol cause increased ACTH
production and vice versa.
 Zona Reticularis - Small, basal secretion of sex hormones affected by ACTH levels; Secretion of
male sex hormones, androgens, has little effect other than development of secondary sex
characteristics and in the development of sex organs in early life.
Medulla
 Catecholamines are "fight-or-flight" hormones released by the adrenal glands in response to
stress. They are part of the sympathetic nervous system.
ENDOCRINE I LABORATORY
Thyroid, Parathyroid and Adrenals
Slide #56 Human Thyroid - Identify the following in this section:
 Capsule and connective tissue stroma
 Follicles of various sizes filled with the eosinophilic staining colloid (thyroglobulin)
 Follicular epithelium of varying heights - Relate this to the active state of the thyroid from which
your section was taken
 Parafollicular cells require special stains - Which stains? However, it may be possible to identify
clumps of cells in the interstitium or cells in the follicular epithelium that are slightly raised
above the other cells.
 Note the abundant vasculature of the thyroid (endocrine!!!)
Slide #15 Parathyroid - Identify the following in this section:
 Capsule and connective tissue stroma
 Unilocular fat (What is this fat type used for, and what happens to the fat with increasing age?)
 Chief Cells - most populous cells in the gland. They are small, round cells that stain faintly with
H & E, and therefore have a scant, basophilic cytoplasm.
 Oxyphil cells - Easy to identify. Larger than the Chief cells and present as islands amongst the
rest of the parenchyma. These cells stain intensely eosinophilic due to what?
Note! In some of your sections, there will be a preponderance of Oxyphil cells. However, under normal
circumstances, the Chief cell is the main cell type of the parathyroid.
Slide #57 Human Adrenal Hold the slide up to the light, or against a white sheet of paper, and identify
the cortex and medulla. Some sections, if you are lucky, contain a medullary vein. After orienting yourself,
identify the following:
 Cortex - Capsule of thick connective tissue containing numerous blood vessels and nerves. Try to
identify connective tissue trabeculae.
 Zona Glomerulosa - A thin layer of cells that stain slightly basophilic. These cells may contain a
few lipid droplets. This layer is just beneath the connective tissue capsule. The cells are found in
ball-like clusters, or loose round-oval groups.
 Zona Fasciculata - Biggest zone. Contains large cells with a "foamy" appearance due to the
inclusion of what? These cells are sometimes referred to as spongiocytes because of this staining
quality. Cells are arranged in long radial arrays or columns.
 Zona Reticularis - A thin layer of cells on top of the medulla, comprised of small cells with
darkly staining nuclei and an abundance of Lipofuscin in the cytoplasm. These cells are also
arranged in radial arrays or cords.
 Medulla - The medulla contains Chromaffin cells that tend to be larger than other cells. These
cells are round to polyhedral in shape. Cells of the medulla produce....................?
It is possible that some slides will NOT contain a medulla. Please share if that situation arises.
Slide # 58 - Adrenal - Methacrylate and Toluidine Blue stain. Identify similar features as you did in slide
#56. This animal was perfused at death, so red blood cells in blood vessels may not be seen. Again, not all
slides will contain a medulla.