Histology of Endocrine SystemMK

7/30/2019 Histology of Endocrine SystemMK

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Lecture by Prof. Marina Kapitonova

for the year 2 UiTM Dental Students


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OBJECTIVES:

1. Define and provide classification of theendocrine glands.

2. Describe microscopic anatomy of the pituitary as

the central endocrine gland.

3. To describe microscopic anatomy of the adrenal

glands, thyroid and parathyroid glands as

peripheral endocrine glands.


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General provisions.

1. Endocrine system in collaboration with the nervous system, orchestrates

homeostasis by influencing, coordinating, and integrating the

physiological functions of the body.2. Whereas the nervous system responds rapidly with a response of short

duration, the endocrine system responds slowly, and the response is of

longer duration.

3. Endocrine glands are responsible for the synthesis and secretion of

chemical messengers known as hormones which are disseminatedthroughout the body by bloodstream where they act on specific target

organs. Therefore endocrine glands possess an extensive vascular

supply that is particularly rich in fenestrated capillaries. They have no

excretory ducts as they discharge their product directly in blood.

4. Endocrine glands function by a regulatory system known as negative

feedback in which production of a hormone affects a target organ to

initiate a response that eventually reduces secretion of that hormone.

5. The endocrine system consists of several glands, composed of islands

of secretory cells of epithelial origin,as well as ofisolated groups of cells

within certain organs, and individual cells scattered among parenchymal

cells of the body.


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Classification of the Endocrine System1.Endocrine glands (pituitary, epiphysis, thyroid gland,

parathyroid glands, adrenal glands).

2. Mixed glands (testes, ovaries, pancreas).3. Paracrine system (APUD).

Endocrine glands:

1. Central (pituitary hypothalamo-hypophyseal system;pineal gland epithalamo-epiphyseal system).

2. Peripheral (thyroid, parathyroid and adrenal glands, sex

glands, pancreas).

Peripheral endocrine glands:1.Pituitary-dependent (thyroid gland, adrenal cortex, sex

glands).

2.Pituitary independent (adrenal medulla, parathyroid

glands, pancreas).


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Pituitary Glandand its hormones

The hypophysis is a

master endocrine gland

connected to the hypothalamus at

the base of the brain, with which is

has important anatomic andfunctional relationship and formes

hypothalamo-hypophyseal system

providing the highest level of control

of the endocrine functions.

Similar relations exist

between pineal gland andepithalamus forming epithalamo-

epiphyseal system.

Other endocrine glands

(thyroid, parathyroid, adrenal

glands) are considered to beperipheral endocrine glands (purely

endocrine ones).

Pancreas, ovaries, testes

combine functions of the endocrine

and exocrine glands.


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cavity of the

3rd ventricle

pituitary

stalk

median

eminence

mammil

bodyoptic chiasm

pars tuberalis

diaphragm

sella

anterior pituitary

pars

intermediafibrocollagenous

setum with cysts

pituitary

fossa

sella

turcica

dura

mater

sphenoid bone

post.

pitui-

tary

neural

lobe

Scheme of pituitary

The pituitary or

hypophysis is dividedinto adenohypophysis

and neurohypophysis

having different

embryonic origin.

Adenohypophysis

includes pars distalis (orpars anterior), pars

tuberalis and pars

intermedia.

Neurohypophysis is

formed by the posteriorlobe of the gland.


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The pituitary is suspended from the hypothalamus (H) by the

infundibulum which is composed of ther neural portion (infundibular stem,

IS) and the surrounding pars tuberalis (PT). The 3rd

ventricle of the brain iscontinuous with the infundibular recess (IR). Pars anterior (PA) is the

largest portion of the pituitary which is glandular and secretes numerous

hormones. Pars nervosa (PN) does not manufacture hormones but stores

and releases them. Pars intermedia (PI) is located between pars anterior

and pars nervosa, it frequently presents intraglandular cleft (colloid-filled

cyst) which is a remnant ofRathkes pouch.

Pituitary gland,H & E


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Anterior lobe of the hypophysis, toluidine blue staining

Endocrine cells (E)

of the

adenohypophysis

are arranged in

groups andsurrounded by

capillaries (C). The

capillaries are wide,

endothelially linedvessels known as

sinusoids.

E

E

E

C

C


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Lobus anterior (or pars distalis) comprises 80% of the pituitary,while pars intermedia in humans is less developed.

Pars distalis is composed of large cords of cells that branch and

anastomose with each other. Traditionally cells of the anterior lobe of the

pituitary have been classified into three types: acidophils (A), (cytoplasm

staining by acidic dyes), basophils (B) (cytoplasm staining by basic dyesand PAS-method) and chromophobes (CO) which do not stain.

Pars distalis of the

pituitary, H & E


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The chromophobes do not take up the stain well and only their

nuclei are demonstrable. They make about 50% of cells of pars distalis.

These cells are small, therefore chromophobes are easily recognizable since

their nuclei appear to be clumped together. They represent either non-

specific stem cells or partially degranulated chromophils.

Pars Distalis of the Pituitary, H & E.


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Adenohypophysis,

PAS-Orage G.

h

On the picture acidophils (A) are stained

bright, basophils (B) are stained dark

while chromophobes (C) are not

stained.

It is now customary to classify cells according to their hormone content which

is demonstrable by modern immunohistochemocal methods of staing usingantibodies to each hormone type. It was shown that acidophils are cells secreteing

growth hormone and prolactine (i.e. somatotrophs and lactotrophs), and basophils

(PAS+) are gonadotrophs, thyrotrophs or corticotrophs. Basophils stain well with

hematoxylin and PAS which detect glycosil groups, as luteinizing hormone (LH),

follicle stimulating hormone (FSG) and thyrocyte stimulating hormone (TSH) are

glycoproteins, and the ACTH precursor protein is glycosylated.


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Most of the chromophil cells in the pituitary (40%) are

acydophils, and among them the majority are somatotrophs

(arrows). Besides the granules, there are mant parallelcysterns of the RER in the cytoplasm. In the malignant cells

thick bundles of the intermediate filaments may be discovered.

Most of the granules measure 350-450 nm in diameter, though

the range is between 300 and 600 nm.

Anterior lobe of the

pituitary,

immunohistoche-mical staining for

somatotropin.


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Lactotrophs make up to 25% of the anterior pituitary. While some are

rounded and polygonal (arrows), most are compressed by adjacent cells intonarrow angular profiles). They increase in size and number during pregnancy and

lactation. Ultrastructurally they have a prominent Golgi compared to all other

anterior pituitary cells and their granules measure 200-350 nm in diameter.

Interesingly, exocytosis may be seen at their lateral borders (misplaced

exocytosis) as well as in the usual site adjacent to capillary basement membrane.

This feature can be used in diagnostic assessments as it is limited to lactotroph-derived tumors.

Anterior pituitary,

immunohistochemical

staining for prolactin.


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Accounting for 15-20% of the anterior pituitary, corticotrophs (arrow)

are large and polygonal in shape, as shown in this micrograph stained to

show ACTH by immunoperoxidase technique. Many corticotrophs possess

an unstained perinuclear vacuole called the enigmatic body, which isderived from secondary lysosomes. Granules in corticotrophs are large

band typically measure 250-700 nm in diameter. Large perinuclear bundles

of intermediate cytokeratin filaments are prominent ultrastructurally and

these become even more prominen in glucocorticoid excess, when they are

visible under the light microscope as pink-staining inclusions (Crookes

hyalin).

Anterior

pituitary,

immuno-histoche-

mical

staining for

ACTH.


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Constituting around 10% of anterior pituitary cells, gonadotrophs

(arrow) are scattered as single cells or small groups throughout the gland as

seen in this section stained for the beta-subunit of FSG by

immunoperoxidease technique. Both FSH and LH may be evident within the

same cell. Ultrastructurally the granules are 150-400 njm in diameter.Following ablation of the ovaries or testes, gonadotrophs develop extensive

cytoplasmic vacuolation. This is due to dilation of the endoplasmatic

reticulum by stored product and caused by the loss of feedback inhibition by

gonadal steroids. Such cells, large rounded and vacuolated on light

microscopy, are called castration cells.

Anterior pituitary,

immunohisto-

chemical staining

for FSH.


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It is also possible to distinguish cells of the anterior pituitary by elecron

microscopy. The electron microphotograph reveals the presence of the dence

core granules in the cytoplasms of adonocytes. Chromophobes fail to stain

because they contain very few granules, and may be lactotrophs, somatotrophs,gonadotrophs, thyrotrophs or corticotrophs by nature.

G

G

Anterior lobe of the

pituitary, E,

12,000


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Posterior pituitary is composed of axons which originate from cells

in the hypothalamus and possess numerous neurosecretory granules

containing either oxytocin or vasopressin together with a carrier proteintermed neurophysin, and ATP. Where axons are adjacent to capillaries

they form fusiform swellings filled with neurosecretory granules (Hering

bodies). The posterior pituitary also contains specialized stellate-shaped

glial cells called pituicytes. In the micrograph the axons are seen a pale

fibrillary background in which the nuclei of pituicytes (P) and small

capillary vessels are present.

Posterior pituitary,

H & E.

P


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Hypothalamic control of anterior pituitary hormone production.

Hypothalamic

neurons secrete

releasing/inhibiting

factors in response

to chemoreceptiveand neural inputs.

These hormones

diffuse into

capillaries at the

median eminenceand are carried to

the anterior pituitary

in the portal vessels.

Astrocyte

foot processes

surrounding the

cappilaries form part

of their diffusion

barrier.

capillary

astrocyte

anteriorpituitarycells

astrocytefootprocess

endothelial

cell

basementmembrane

portal vessel

capillaryat median

eminence

releasing/Inhibitingfactor

hypothalamicneuron

neuralsynapse

Summary of Histology of Pituitary and Hypophysis


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Loca-tion

Gene-ral celltype

Spe-cificcelltype

Hor-mone

Targettissue/organ

Natureof hor-mone

Gra-nulesize

Stain-ing

Func-tions

parsdista-lis ofthepitui-

tary

acido-phils

soma-to-trophs

soma-totro-phin orgrowthhormo-

ne (GH)

espe-ciallymuscleandbone

pro-tein

300-400nm

Orange

G

stimulates

growth,

promotes

protein

synthesis in

bones and

muscles,

influences

carbo-

hydrate and

lipid meta-

bolism.

acidophil

mam-mo-troph

prolac-tine

breast pro-tein

550-700

nm

Orange

G

initiates &

regulates

lactation,

promotes

mammary

develop-ment

Summary of Histology of Pituitary and Hypophysis

Loca Gene Spe Hor Target Nature Gra Stain Func


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Loca-tion

Gene-ral celltype

Spe-cific celltype

Hor-mone

Targettissue/organ

Natureof hor-mone

Gra-nulesize

Stain-ing

Func-tions

parsdista

-lisofthepitui-tary

basophil

gona-do-

trophs

folli-culo-trophs

follicle-

stimu-latinghor-mone

ovary &testis

glyco-pro-

tein

150-200

nm

PAS stimulates

develop-

ment offollicles in

the ovary &

semini-

ferous

tubules in

the testes

basophil

luteo-trophs

lutei-nizinghormone

ovary(corpusluteum)

Glyco-pro-tein

150-200

nm

PAS stimulates

corpus

luteum

develop-

ment &progesteron

secretion:

necessary

for ovula-

tion and

estrogensecretion


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Loca-tion

Gene-ral celltype

Spe-cificcelltype

Hor-

mone

Tar-getorgan

Natureof hor-mone

Gra-nulesize

Stain-ing

Func-tions

parsdista-lis ofthepitui-tary

baso-phil gona-do-troph

intersti-cial cellstimula-ting hor-mone,probablysame asLH

testis glyco-pro-tein

150-200nm

PAS &

alde-

hyde-

fuchsinPAS

stimu-latesLeidigcells toproducetesto-sterone

baso-phil

corti-co-troph

adreno-cortico-tropin

(ACTH)

adre-nalcortex

Poly-pep-tide

200-250

nm

PAS stimulatessynthesis

of adreno-

corticalsteroid

hormones

baso-phil

thyro-troph

thyroid-stimula-

ting hor-mone-TSH

thy-roid

glycoprotein

130-150

nm

controls

thyroxine

production

& release

Loca Gene Spe Hor Tar get Natu Gra Stain Func


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Loca-tion

Gene-ral

Spe-cific

Hor-mone

Tar-getorgan

Natu-re ofhor-mone

Gra-nulesize

Stain-ing

Func-tions

parsinter-media

baso-phil mela-no-troph

APUD

melano-cyte sti-mulatinghormone(MSH)

skin ofloweranimals

poly-pep-tide

200-300nm

PASstimulatesmelano-

cyte

expansion

pars

nervo-sa ofthepitui-tary

neuro

-glia

pitui-

cyte

none storage and

release ofneuro-

hormones

of hypo-

thalamus:

vasopressin,

oxytocin

hypo-thalamus

neuro-secre-toryneu-

rons

neurons

in para-

ventri-

cular

nuclei

oxy-tocin

smoothmuscle,especi-ally

uterus

poly-pep-tide

100-300nm

Gomori stimulates

contraction

of the ute-

rine wall

duringparturition.

L G S H T t N t G St i F


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Loca-tion

Gene-ral

Spe-cific

Hor-mone

Targetorgan

Natureof hor-mone

Gra-nulesize

Stain-ing

Func-tions

hypo-

thalamus

neuro-secre-toryneu-rons

neu-

rons insupra-optical-nuclei

vaso

pres-sin(anti-diure-tichor-mone- ADH

renal

collect-ingtubules,arterio-les

poly-

pep-tide

100-

300nm

Gomori increases

water

absorption

of renal

collecting

tubules,

constricts

arterioles toincease

pressure

neuro-secre-toryneu-rons

neu-rons in

tuberalnuclei

six re-

leasing

factors

(RF);

two

known

inhibi-

tingfactors

parsdistalis

parsdistalis

pep-tide

pep-tide

- causesrelease of

the anterior

pituitary

hormones;

inhibits

release of

the anterior

pituitaryhormones

3rd tentorium


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Diagram shows the

location of the pineal gland.

Output of pineal melatonin

is modulated by light

through nervous pathwayswhich input as sympathetic

innervation to the gland. It is

also thought to influence

gonadal activity

(suppression). The

parenchyma is composed ofpinealocytes and interstitial

cells. Pinealocytes are

resposible for secretion of

serotonin and melatonin,

while the interstisial cells

are believed to astroglia-likecells.

Pineal Gland

light

eye

nervus

opticus

central

sympathetic

pathways

superior cervicalganglion

cerebellum

n.suprachi-

asmaticus

pituitary

gland

optic

chiasm

pineal

gland

3rd

ventricle

tentorium

cerebellicorpus

callosum


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Thyroid possesses three features of special significance:

1) it is unique among the endocrine glands in that it manufactures hormones, stores

them extracellularly and releases for use as required,

2) it is the only endocrine gland that depends on the external environment for raw

materials of its hormones,

3) it possesses one of the richest blood flow compared to adrenal and other glands,

4) thyroid hormones are the only among amino-acid-derived hormones which being

lipid-soluble, diffuse through the cell membrane and binds to intracellular hormone

receptor proteins in the target cells (other amino acid-derived hormones receptorsare present in the cell membrane of the target cells).


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I. General Morphology

1. The thyroid gland consists of right and left lobes

connected across the middle line by a narrowportion, the isthmus (and sometime pyramidal lobe).

2. Its weight is somewhat variable, but is usuallyabout 30 grams.

3. It is slightly heavier in the female, in whom it

becomes enlarged during menstruation andpregnancy.

4. Parenchyma of thyroid gland: follicles +

interfollicular clusters of calcitoninocytes.


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Human Thyroid Gland, H. & E., 162 x.

Stroma: the inner, true, connective tissue (CT) capsule sends in septa to

partially enclose lobules separated by a loose CT, with many blood vessels.

Follicles are the structural units of the thyroid gland. Note variations in shape

(rounded or tubular) and size (0.05 to 0.5 mm in diameter). Close packingwith a thin reticular network between adjacent follicles.

colloid

follicles

bloodvessels

100 m


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Lined by basophilic cuboidal follicular cells, varying in height fromsquamous to low columnar as a simple epithelium on a basal lamina,outside which is an extensive plexus of blood capillaries, and reticular

fibres and fibroblasts. Follicular cells are polarized with respect to thefollicle lumen. Nucleus is placed centrally or basally. Colloid is found in the

lumen of follicles. It is variably acidophil or basophil, and often shrunken

and showing knife chatters.

colloid

follicle

Human Thyroid Gland, H. & E.


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Thyroid follicles:1. In man they vary markedly between 0.02 and 0.9 mm in diameter.

2. Larger follicles filled with colloid have a squamous or cuboidalepithelium.

3. Glands with follicles that have predominantly squamous epithelial

cells are considered to be hypoactive.

4. When the gland is stimulated to synthesize thyroid hormone, thefollicular epithelial cells become columnar and the amount of colloid

is reduced.

5. Each follicle can store several weeks supply of hormone within

colloid.

6. A gland has several million follicles.7. Chemical composition of colloid is a glycoprotein-iodine

complex (thyroglobulin). The follicles release about 100 mg of

hormone daily. Of the several iodinated compounds found in the

gland the 3, 5, 3-triiodothyronine is hormonally the most active.

Human Thyroid Gland A. H. & E.; B. periodic acid-Schiff and hematoxylin.


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In B, the colloid is specifically stained red with the periodic acid-

Schiff method because of the chemical composition of colloid, which

is a glyco-protein-iodine complex (thyroglobulin).

Human Thyroid Gland A. H. & E.; B. periodic acid Schiff and hematoxylin.

In A, the colloid in the lumen of the follicle is not stained.20 m

A B

Follicles

Thyroglobulin

Follicle

cell nuclei

Thyroglobulin


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Thyroid Physiology:

Follicular cells:

(a) Are stimulated by pituitary thyrotrophichormone (TSH) to produce and release twoiodinated amino-acid hormones - tetraiodo-

thyronine (thyroxine/T4) and 3,5,3-triiodo-L-thyronine(T3),

(b) which are stored in the colloid, as component

amino acids of the glycoprotein - thyroglobulin.

(c) The hormones accelerate general and

specific metabolic processes of the body.


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(d) Electron radioautography has shown the sites in the

sequence of hormone production by the follicular cells:

i. Iodide concentration - basal part of the follicular cell.

ii. Iodide oxidation - throughout the cell.

iii. (ii) Synthesis of thyroglobulin - basal cell, granular ER, Golgi body, by

vesicle to the lumen.

iv. In the luminal thyroglobulin, tyrosine residues are iodinated, then pairs

condense.

v. Cellular retrieval of thyroglobulin from colloid storage cell's apical

region by endocytosis.vi. Transport to lysosomes, where cathepsins degrade the large modified

molecule.

vii. Release of freed iodothyronines - out of the base of the cells into the

blood.

viii. Binding to intracellular proteins after entering cytoplasm andslow use over a period of several days to weeks (both T3 and T4bind to nuclear thyroid hormone receptor proteins, but T3 binds with amuch greater affinity than does T4, which accounts for the greaterbiological activity of T3).


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Diagram of the synthesis and iodination of thyroglobulin (A) and release of thyroid hormone

(B).


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Biological Effects of Triiodothyronine and Thyroxine.

1) Normal thyroid function is essential for the normal growth, development, and

tissue metabolism of the body.

2) T3 and T4: stimulate transcription of many genes

that encode various types of proteins, resulting in a

generalized increase in cellular metabolism that may

be as great as twice resting rate;

a) stimulate carbohydrate metabolism,

b) decrease synthesis of cholesterol, phospholipids,

and triglycerides but increase synthesis of fattyacids and the uptake of various vitamins,

c) T3 and T4 also increase the growth rate in the

young, facilitate mental processes, and stimulate

endocrine gland activity.


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CLINICAL CORRELATES:

Increased thyroid hormone production:

a) decreases body weight,

b) increases heart rate,

c) increases metabolism, respiration,

muscle function, and appetite.

d) hormone production increases from 5 to 15times normal.

E i t f th id h


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Excessive amounts of thyroid hormone(hyperthyroidism) cause:

a) muscle tremor and weakness,

b) tiredness,

c) impotence in men,

d) frequent menstrual bleeding in women.

e) excessive appetite and thirst,

f) weight loss,

g) rapid respiration, sweating, heat intolerance (due to an increase

d basal metabolic rate),

h) increase in heart rate (tachycardia)

i) emotional disturbance and nervousness

k) bulging (protrusion) of the eyes (exophthalmos) - occurs due to

an increase in orbital support tissue.

l) increases of the size of the thyroid gland two to three times above

normal

Th id Gl d H l i Diff


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Thyroid Gland, Hyperplasia, Diffuse

In thyroxic hyperplasia a number of follicular cells increases, papillary folds

of acinar epithelium develop. In addition each epithelial cell is large

columnar and the edges of colloid are scalloped, indicating active removalof stored colloid for processing into thyroxine.

colloid in lumen of follicle papillary infoldings


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CLINICAL CORRELATIONS:

Overactivity of the thyroid(hyperthyroidism) may be due to a numberof factors.

In Graves disease, the thyroid isoveractive even with low levels of THS, dueto an immunologic disturbance in which an

immunoglobulin has an effect similar tothat of TSH (autoimmune IgG antibodiesbind to TSH receptors which stimulatethyroid follicular cells).


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Deficiency of thyroid hormone:

a) from the time of birth results in:

1a) a dwarf child who is mentally retarded (cretinism).

2a) slow heart rate, muscular weakness, and

gastrointestinal disturbances.

Thyroid hormone given to infants at an early stage of

cretinism can alleviate the symptoms.

b) i d lt


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Patients with severe hypothyroidism may develop myxedema, which ischaracterized by bagginess under the eyes and swollen face that is due to

nonpitting edema of the skin, infiltration of excess glycosaminoglycans, and

proteoglycans into the extracellular matix.

b) in adults:

1b) mental slowing,

2b) cold intolerance,

3b) reflex changes,4b) skin changes

5b) fatigue, sleeping for up to 16 hours per day,

6b) muscular sluggishness,

7b) slowed heart rate, decreased cardiac output and blood volume

8b) failure of body functions,

9b) constipation,

10b) loss of hair growth.

PARAFOLLICULAR CELLS


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PARAFOLLICULAR CELLS

OF THE THYROID GLAND

In thyroid between the follicular cells and the basal lamina (BL), and sometimes

outside the BLs, lie occasional C cells (clear/parafollicular cells), having no directaccess to the lumen, and no colloid droplets, but with small argyrophil, secretory

granules.

Histophysiology of C-cells:(a) they are APUD cells of neural crest origin,

(b) and produce the polypeptide calcitoninfor the reduction of highplasma Ca2+ and phosphate levels by inhibiting bone resorption

by osteoclasts and decreasing calcium and phosphate

reabsorbtion by renal tubules. It may increase the rate of osteoidmineralization.

(c) although diffuse, in sum they form a gland antagonistic to the

action of the parathyroids.

(d) the secretion of calcitonin results from elevation of blood calcium

concentration above normal levels.


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PARAFOLLICULAR CELLS (C CELLS)

Thyroid gland, H. & E.

Parafollicular cells are located between follicular cells

or in the interstitial connective tissue.

C

C

C

G l M h l d Mi t t f C ll


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General Morphology and Microstructure of C-cells.

1. Although C-cells are 2 to 3 times larger than follicular cells, they

account for only about 0.1% of the epithelium.

2. Parafollicular cells may be found intimately associated with thyroid

follicles or as isolated or interstitial clusters of cells (the latter location,

which is less common for human thyroid, explains the archaic name

parafollicular cells). They are not readily found in routine thicker

sections of the thyroid gland. The photomicrographs above are from1.5 m plastic sections.

3. Although parafollicular cells appear, at the light microscopic level, to

be in intimate contact with thyroid colloid, they are, in fact, separated

from colloid by thin intervening processes of adjacent thyroid follicular

cells.

4. Ultrastructurally they contain dense-core neurosecretory granules that

are characteristic of neuroendocrine cells.

PARAFOLLICULAR CELLS (C CELLS)


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H. & E.,

A. 119 x;

B. & C. 169 x.

PARAFOLLICULAR CELLS (C CELLS)Thyroid gland

C

B

A

A 100 mB & C 50 m

Parafollicular

cells

Thyroid

follicles

Parathyroid

gland

Thyroid

follicular cells

Colloid

parafollicular

cells

Follicle filled

with colloid

Parafollicularcells

Mast cells

Interstitial

aggregation of

parafollicular

cells

Parathyroid gland Human H & E


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Parathyroid gland, Human, H. & E.

Stroma: Each of the four (may range from 2 to 12) rounded or ovoid bodies has a

fine connective tissue capsule and delicate, incomplete septa which divide the glandin lobules. . These septa carry vessels, nerves and many fat cells.

Parenchyma: supported on fine reticular fibres are many fenestrated blood capillariesand sheets and cords or plates of chief cells (small, 7-10 m diameter; some dark,

some light: contain glycogen, lipofuscin pigment, and argyrophil secretory granules;

form occasional small follicles.) and oxyphil cells (larger, acidophilic, and often occur in

clumps; no secretory granules). The most abundant type is the chief (or principal) cell,which is functionall im ortant.

A

C

C

O

Adherent to the true

capsule of the thyroid,

the small parathyroid

glands (4 to 5 mm in

diameter) and are

usually found on the

posterior surface of the

thyroid gland.

P h id H H & E


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Parathyroid, Human, H. & E.

Chief cells: with prominent nucleus, round and centrally located, and cytoplasm thatstains variably and may be light or dark depending upon its secretory activity.

The second type, oxyphilic (acidophilic or eosinophilic) cells, occurs in fewer numbers

in small clumps or nests among chief cells. Larger than chief cells. Oxyphilic cells

usually arise after puberty and increase in number beyond the age of 40 but their

specific function is unknown. May be derived from chief cells.

Oxyphil cells: occurs in small clumps and in fewer numbers. These cells usually

have small densely staining heterochromatin and an oxyphilic cytoplasm whoseperimeter is usually well defined.

C

C

O

3 Functions


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(d) unlike most other endocrine glands, no specific pituitary trophic

hormone is involved in its control.

3.Functions

(a) Secretory granules of chief cells are the polypeptide hormone,

parathormone/PTH, which is important in calcium and phosphatemetabolism acting mainly on bone and the kidney:

1a) it is released in response to low blood Ca2+,

2a) acts on osteoclasts and macrophages to increase boneresorption,

3a) it also removes calcium phosphate directly from bone matrix, and,

via intermediary factors, from osteoblasts;.

(b) in the kidney:

1b) PTH: promotes the tubular reabsorption ofcalcium

2b) inhibits the renal tubular reabsorption of phosphate - aphosphaturic action;

(c) promotes conversion of 25-hydroxyvitamin D to 1,25 -dihydroxyvitamin D (this metabolite increases calcium absorbtion by

the gut);

Clinical Correlates:


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Parathyroid glands are essential for life whereas calcitonin of

the thyroid appears to provide a complementary mechanism

for fine adjustment of blood calcium level and is not essentialfor life.

In the absence of parathyroid hormone, there is a pronounced

decrease in blood calcium resulting in tetany, abnormal

twitching, the intense, involuntary spasm of skeletal muscle.caused by changes in excitability at the neuromuscular

junction, and death.

Dietary addition of calcium and especially administration of

parathyroid hormone relieves the abnormal spasms,

preventing death of the organism.Abnormal levels of calcium may result in abnormal deposition

of calcium in the kidneys and muscle. Abnormally increased

blood levels of calcium occur at the expense of bone, which

may fracture as a result.

Clinical Correlates:


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Adrenal Gland

Cortex: Zona glomerulosa (narrow

subcapsular zone of cortex) secretesmineralocorticoids, mainly aldosterone.

Zona fasciculata (broad, yellow mid-zone of

cortex) secretes glucocorticoids, mainly cortisol

and corticosterone.

Zona reticularis (narrow inner zone of

cortex) secretes mainly androgenic steroids.

Adrenal medulla (central, enclosed by

cortex, brown) is neuroendocrine and secretes

epinephrine and norepinephrine

(noradrenaline).

cortexmedulla

z.reticularis

z.fasciculata

z.glomerulosa

capsule


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The zona glomerulosa is composed of small compact cells (C)

arranged in clumps and separated by stroma composed

largely of thin-walled capillaries. The cells contain scanty lipid

droplets associated with well developed SER and

comparatively little RER.

Adrenal

Cortex,

H & E.


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Zona fasciculata contains cells arranged in vertical

columns, which are usually 2-3 cells wide, the

columns being separated by capillaries (Cap).

Adrenal Cortex, H & E.


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The inner zona reticularis produces androgenic steroids and some glucocorticoids, butnormally only in small amounts. The inner zone of adrenal cortex is thinner than the zonafasciculata, but thicker than the zona glomerulosa. It is composed of cells with eosinophyliccytoplasm arranged in an anastomosing network of clumps and columns with a capillarynetwork closely apposed to the cell membranes. A characteristic feature of this layer whenstained with H & E is the present of brown pigment (lipofuscin). To the naked eye the layer

appears pale brown, whereas the zona fasciculata is bright yellow.

Adrenal Cortex,

Zona Reticularis.H & E.


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Because of their high catecholamine content, adrenal medullary cellsdevelop an intensive brown color when exposed to air or to a strongoxidizing agent, such as potassium dichromate, due to the formation ofbrown pigment when the amines are oxydized. This is the basis of theirantiquated name chromaffincells.Adrenal medullary cells (E- and N-cells) show their large nuclei and finelygranular cytoplasm.

Adrenal

Medulla,

H & E

Adrenal Medulla TEM x20 000


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Large dense-core

granules

corresponding tonorepinephrine (N)

and epinephrine (E)

granules.

Adrenal Medulla, TEM, x20,000

Histology of Endocrine SystemMK

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