Anatomy of Endocrine system

Introduction, Pituitary gland and Thyroid gland

Prepared by
Dr. Payal Jain
 Endocrine System

I. Introduction
A. Considered to be part of animals communication system
1. Nervous system uses physical structures for communication
2. Endocrine system uses body fluids to transport messages (hormones)

II. Hormones
A. Classically, hormones are defined as chemical substances produced by ductless
glands and secreted into the blood supply to affect a tissue distant from the gland,
but now it is understood that hormones can be produced by single cells as well.
1. epicrine
a. hormones pass through gap junctions of adjacent cells without
entering extracellular fluid
2. paracrine
a. hormones diffuse through interstitial fluid (e.g. prostaglandins)
3. endocrine
a. hormones are delivered via the bloodstream (e.g. growth hormone
 Different endocrine glands with cell
Organ Division arrangement Cell arrangement/morphology Hormone
Hypophysis Adenohypophysis

Pars distalis Cells in cords around large-bore

capillaries:
Acidophils Growth hormone,
prolactin
Basophils ACTH, TSH, FSH,
LH

Pars intermedia Mostly basophilic cells around ACTH, POMC

cystic cavities
Pars tuberalis Narrow sleeve of basophilc cells LH
around infundibulum

Neurohypophysis
Pars nervosa Nerve fibers and supporting cells Oxytocin and
(pituicytes) vasopressin
(produced in
hypothalamus)
Infundibulum Nerve fibers (traveling from
hypothalamus to pars nervosa)
Pancreas Islet of Langerhans Irregularly arranged cells with Insulin, glucagon
many capillaries
 Follicles: Simple cuboidal to Principal
columnar epithelium in spherical cells: T3 and T4
Thyroid
shells around colloid Parafollicular cell:
Thyrocalcitonin
Densely packed cords of PTH
Parathyroid polygonal cells (chief cells and
oxyphilic cells)
Adrenal Cortex
Zona glomerulosa Columnar cells in rounded Aldosterone
clusters
Zona fasiculata Large, pale-staining polygonal Glucocorticoids
cells in columns (Cortisone)
Zona reticularis Round cells in irregular cords Gonadocorticoids
(DHEA)

Medulla Chromaffin cells= large round Norepinephrine

cells with centrally located and epinephrine
nucleus with prominent nucleus,
often cytoplasmic granules. Note
large veins in center of medulla.
III. Pituitary Gland (hypophysis cerebri)
A. Has two distinct parts
1. anterior lobe (adenohypophysis)
2. posterior lobe (neurohypophysis)
B. Located in a bony recess (sella turcica) at the base of the brain
C. Connected to the brain by the hypothalamus and a portal blood supply
1. vein draining the hypothalamus breaks up into a capillary bed within the
anterior pituitary
2. route by which releasing factors from the hypothalamus travel to cause
release of hormones from the anterior pituitary
Adenohypophysis - based on grouping of all regions composed of glandular tissue
This includes the...

pars distalis
pars intermedia
pars tuberalis
Neurohypophysis - based on grouping of all regions composed of neural or
neurosecretory tissue
This includes the . . .
median eminence
infundibular stalk
pars nervosa ( infundibular process)
The Adenohypophysis

Pars distalis: This region of the pituitary gland is organized as cords or clusters
of cells supported by a reticular connective tissue. With routine staining two
types of cells can be observed: (1) chromophils which stain readily and are either
red (acidophiles), blue or purple (basophiles) depending on the type of secretory
material present, and (2) chromophobes which do not take up the stain and
thus appear unstained or rather clear. Chromophobes may be chromophils that
have lost their secretory granules or chromophils that have not accumulated large
numbers of secretory granules. Use of specific antibodies against the protein
secretory products has allowed the identification of the different cells.
The cells of the pasr distalis are:
Somatotrophs secrete growth hormone which affects many cells
Mammotrophs secrete prolactin which controls milk production during lactation
Corticotrophs secrete ACTH which controls secretion of cortisol by cells in the
adrenal cortex
Thyrotrophs secrete TSH which controls secretion of thyroid gland
Gonadotrophs secrete FSH and LH which control development of follicles and
ovulation in the ovary.
Pars intermedia: With routine histological staining, the cells in the pars
intermedia stain blue-purple and thus are basophilic.
Cells secrete ACTH, MSH, endorphins and lipotrophins.

Pars tuberalis: This region is an extension of the glandular pituitary gland and its
cells resemble those of the pars intermedia and pars distalis. The specific function
of the cells in the pars tuberalis, however, is not clear.

The Neurohypophysis
Pars nervosa: This region consists of unmyelinated nerve axons (cell bodies are in
the hypothalamus) and supportive cells called pituicytes.
Secretes ADH (antidiuretic hormone) which is synthesized by neurons in the
supraoptic nucleus of the hypothalamus.
Secretes vasopressin which is synthesized by neurons in the paraventricular
nucleus of the hypothalamus.
Function - Control of Secretion

Adenohypophysis.
The cells in the adenohypophysis secrete two classes of hormones: (1) direct acting and (2) trophic. Direct
acting hormones include growth hormone (GH) and prolactin from the pars distalis, and melanocyte
stimulating hormone (MSH) from the pars intermedia. Trophic hormones include adrenocorticotrophic
hormone (ACTH), thyroid stimulating hormone (TSH), follicle stimulating hormone (FSH) and luteinizing
hormone (LH).
Secretion of these hormones is controlled by specific releasing hormones in the hypothalamus. Most of the
releasing hormones are stimulatory in their action except for the one for prolactin which is inhibitory and the
one for growth hormone which has both inhibitory and stimulatory releasing hormones. Releasing hormones
are produced in the median eminence of the hypothalamus and reach the adenohypophysis via a portal system
of veins known as the pituitary portal system.

Neurohypophysis.
The cells in the neurohypophysis secrete only direct acting hormones : (1) antidiuretic hormone (ADH) also
known as vasopressin secreted by neurons in the supraoptic nucleus in the hypothalamus and (2) oxytocin
secreted by neurons in the paraventricular nucleus in the hypothalamus. After synthesis in the hypothalamus,
these hormones move down the axons of the hypothalamohypophyseal tract through the infundibular stalk
and terminate near blood vessels in the pars nervosa. Accumulations of these hormones bound to specific
glycoproteins can be observed along the axons of the hypothalamophypophyseal tract and in the pars
nervosa. These "accumulations" often called Herring bodies represent a storage form of the hormone. Release
of these hormone stores is determined by impulses in the axons of the hypothalamophypophyseal tract
originating in the hypothalamus. Such a mechanism of secretion controlled by nerve impulses is called
"neurosecretion".
THYROID
I. Gross Anatomy
The thyroid gland is located dorsolateral to the trachea, close to the larynx. It has two
lobes that are connected by a narrow isthmus.
II. Histology

The thyroid gland is composed of follicles and interfollicular connective tissue. The
capsule, classified as loose areolar connective tissue, surrounds the mass of thyroid follicles
and sends smaller pieces of connective tissue into the gland to surround the individual
thyroid follicles. Near the thyroid gland and embedded in the same connective tissue
capsule is the parathyroid gland.
Sometimes patches of lymphocytes can be observed in the thyroid/parathyroid glands.
Thyroid follicles consist of a layer of simple epithelium surrounding a gel-like pinkish
material called colloid. The principal cell is the most numerous cell present in the simple
epithelial layer and is responsible for secreting the thyroid hormones as well as
thyroglobulin, a glycoprotein.
Thyroid hormones are stored extracellularly as part of the thyroglobulin which is the main
component of the colloid.
The size of follicles and the height of principal cells varies even within one section of the
gland. Squamous principal cells indicate a relatively inactive gland whereas cuboidal to
columnar cells indicate more activity in removing the hormone from the stored form.
In addition to principal cells there is another type of functional cell in the thyroid
gland. This is the parafollicular cell which may be found as single cells in the epithelial
lining of the follicle or in groups in the connective tissue between follicles. They usually
appear as large, clear cells since they do not stain well with hematoxylin and eosin. They
are sometimes called parafollicular cells based on their location and clear cells (C
cells) based on their appearance of their cytoplasm.
Parafollicular cells secrete calcitonin, a hormone that lowers the level of calcium in the
blood.
Mechanism of Secretion of T3 and T4 (thyroxin).
Under the influence of increased TSH from the pituitary gland, principal cells
concentrate iodine by active transport. At the same time they
synthesize thyroglobulin and secrete it into the lumen of the thyroid follicle

The iodination reaction, catalyzed by the enzyme peroxidase, is carried out

on the large thyroglobulin molecule at the luminal surface of the principal
cell. Various combinations of iodinated and non-iodinated tyrosine are
possible. If the two molecules of tyrosine are both fully iodinated, the
hormone resulting upon cleavage is T4 but if one of the tyrosines has only one
iodine, then the hormone that results is T3. In the circulation T4 is
converted to T3 which appears to be the active form of the hormone.

Under the influence of rising TSH levels, the principal cells take up colloid by
pinocytosis, the vesicles fuse with lysosomes which hydrolyze throglobulin
releasing T3 and T4 (thyroxin) which diffuse into the blood and lymph

V. Parafollicular Cells
Secrete calcitonin which inhibits osteoclasts from resorbing bone resulting in
decrease in calcium in the blood
Controlled by the level of calcium in the blood
Thank You