▪ Endocrine cells are typically epithelial, at least in origin, and aggregated as cords or clusters.
▪ Characteristics of Endocrine organs ;
a. No duct
b. Fenestrated or Sinusoidal capillaries — higher permeability than continuous capillaries
c. Cells are arranged in 4 ways — groups , cords , networks , follicles ( arranged circularly )
d. Secrete hormones
▪ 3 ways of endocrine secretion ;
1) Autocrine signaling - cells may produce molecules that act on themselves or on cells of the same type
2) Paracrine signaling - produce hormones that act on adjacent target cells or only a short distance away
3) Endocrine signaling - hormones secreted into blood to act on target cells with receptors for those
hormones at a distance from the site of their secretion
▪ Endocrine cells secrete hormones stored in secretory vesicles to act on a target cell
▪ Neuro cell secrete neurotransmitters to another neuro cell to pass ( conduct ) a nerve impulse
▪ Neuro – secretary cells = secrete hormones into blood to act on a target cell
▪ Hormones, like neurotransmitters, are frequently hydrophilic molecules such as proteins, glycoproteins, peptides, or
modified amino acids with receptors on the surface of target cells.
▪ There are 2 types of endocrine cells ;
1) Nitrogen - secreting cells
2) Steroid - secreting cells
Nitrogen - secreting cells Steroid - secreting cells
RER :- synthesize hormone SER :- synthesize cholesterol
Golgi complex :- release hormone Mitochondria :- synthesize steroid
Membrane-bound granules :- storage of granules lipid droplet :- storage of cholesterol
Thyroid gland
▪ located anterior and inferior to the larynx , extending from 2nd – 4th cartilages of trachea
▪ Small gland consists of two lobes united by an isthmus - with four parathyroid glands attached to the posterior
surface
▪ Structure ;
a) Capsule - DCT
b) Parenchyma - follicles + reticular tissue
▪ The thyroid gland is covered by a fibrous capsule from which septa extend into the parenchyma , dividing it into
lobules and carrying blood vessels, nerves, and lymphatics.
▪ The parenchyma of the thyroid is composed of millions of rounded epithelial thyroid follicles of variable diameter,
each with simple epithelium and a central lumen densely filled with gelatinous acidophilic colloid .
▪ Secretary storage is outside the cells , in the colloid of the follicle lumen.
▪ Thyroid colloid contains the large glycoprotein thyroglobulin , the precursor for the active thyroid hormones.
▪ Follicles are densely packed together, separated from one another only by sparse reticular connective tissue ,
although this stroma is very well vascularized with fenestrated capillaries for transfer of released hormone to the
blood.
1
, a) Follicle cavity = colloid — a jelly-like substance
• Inactive - much
• Active - less
b) Simple follicular epithelium cells ;
• Inactive - squamous
• Active - cuboidal or columnar
• Less CT - rich in fenestrated capillaries
▪ Follicular cell - Microvillus , RER , Golgi complex , secreting granules , Pinocytosis vesicles , lysosome
▪ Follicular cells is a Nitrogen - secreting cell
▪ Follicular cells function - production of thyroid hormone
c) Parafollicular cells ;
▪ Between the follicles
▪ Cells are bigger and lightly stained
▪ Secrete calcitonin — Calcitonin functions to reduce calcium levels in the blood by actively reducing breakdown
of bone and inhibiting re-absorption of calcium in digestive system
▪ Well-developed organelles :- Golgi complex , RER , Secreting granules with calcitonin
✓ The follicular cells or thyrocytes , range in shape from squamous to low columnar , their size and other features
varying with their activity, which is controlled by thyroid-stimulating hormone (TSH) from the anterior pituitary.
✓ Active glands have more follicles of low columnar epithelium; glands with mostly squamous follicular cells are
hypoactive.
✓ Thyrocytes have apical junctional complexes and rest on a basal lamina .
✓ Another endocrine cell type, the parafollicular cell or C cell, is also found inside the basal lamina of the follicular
epithelium or as isolated clusters between follicles .
✓ Parafollicular cells are usually somewhat larger than follicular cells and stain less intensely.
✓ Secretion of calcitonin is triggered by elevated blood Ca2+ levels, and it inhibits osteoclast activity , but this function
in humans is less important than the roles of parathyroid hormone and vitamin D in the regulation of normal calcium
homeostasis.
Production of Thyroid Hormone - promoted by TSH
① Production of thyroglobulin ;
* synthesis in the rough ER and glycosylation in the Golgi apparatus , then the glycoprotein is released from
thyrocytes into the follicular lumen
② Uptake of iodide ;
* from blood by Na/I symporters (NIS) in the thyrocytes’ basolateral cell membranes. Decreased levels of
circulating iodide trigger synthesis of NIS , increasing iodide uptake and compensating for the lower plasma
concentration. An apical iodide/chloride transporter (also called pendrin) pumps I – from thyrocytes into the
colloid
③ Iodination of tyrosyl residues ;
2
▪ Characteristics of Endocrine organs ;
a. No duct
b. Fenestrated or Sinusoidal capillaries — higher permeability than continuous capillaries
c. Cells are arranged in 4 ways — groups , cords , networks , follicles ( arranged circularly )
d. Secrete hormones
▪ 3 ways of endocrine secretion ;
1) Autocrine signaling - cells may produce molecules that act on themselves or on cells of the same type
2) Paracrine signaling - produce hormones that act on adjacent target cells or only a short distance away
3) Endocrine signaling - hormones secreted into blood to act on target cells with receptors for those
hormones at a distance from the site of their secretion
▪ Endocrine cells secrete hormones stored in secretory vesicles to act on a target cell
▪ Neuro cell secrete neurotransmitters to another neuro cell to pass ( conduct ) a nerve impulse
▪ Neuro – secretary cells = secrete hormones into blood to act on a target cell
▪ Hormones, like neurotransmitters, are frequently hydrophilic molecules such as proteins, glycoproteins, peptides, or
modified amino acids with receptors on the surface of target cells.
▪ There are 2 types of endocrine cells ;
1) Nitrogen - secreting cells
2) Steroid - secreting cells
Nitrogen - secreting cells Steroid - secreting cells
RER :- synthesize hormone SER :- synthesize cholesterol
Golgi complex :- release hormone Mitochondria :- synthesize steroid
Membrane-bound granules :- storage of granules lipid droplet :- storage of cholesterol
Thyroid gland
▪ located anterior and inferior to the larynx , extending from 2nd – 4th cartilages of trachea
▪ Small gland consists of two lobes united by an isthmus - with four parathyroid glands attached to the posterior
surface
▪ Structure ;
a) Capsule - DCT
b) Parenchyma - follicles + reticular tissue
▪ The thyroid gland is covered by a fibrous capsule from which septa extend into the parenchyma , dividing it into
lobules and carrying blood vessels, nerves, and lymphatics.
▪ The parenchyma of the thyroid is composed of millions of rounded epithelial thyroid follicles of variable diameter,
each with simple epithelium and a central lumen densely filled with gelatinous acidophilic colloid .
▪ Secretary storage is outside the cells , in the colloid of the follicle lumen.
▪ Thyroid colloid contains the large glycoprotein thyroglobulin , the precursor for the active thyroid hormones.
▪ Follicles are densely packed together, separated from one another only by sparse reticular connective tissue ,
although this stroma is very well vascularized with fenestrated capillaries for transfer of released hormone to the
blood.
1
, a) Follicle cavity = colloid — a jelly-like substance
• Inactive - much
• Active - less
b) Simple follicular epithelium cells ;
• Inactive - squamous
• Active - cuboidal or columnar
• Less CT - rich in fenestrated capillaries
▪ Follicular cell - Microvillus , RER , Golgi complex , secreting granules , Pinocytosis vesicles , lysosome
▪ Follicular cells is a Nitrogen - secreting cell
▪ Follicular cells function - production of thyroid hormone
c) Parafollicular cells ;
▪ Between the follicles
▪ Cells are bigger and lightly stained
▪ Secrete calcitonin — Calcitonin functions to reduce calcium levels in the blood by actively reducing breakdown
of bone and inhibiting re-absorption of calcium in digestive system
▪ Well-developed organelles :- Golgi complex , RER , Secreting granules with calcitonin
✓ The follicular cells or thyrocytes , range in shape from squamous to low columnar , their size and other features
varying with their activity, which is controlled by thyroid-stimulating hormone (TSH) from the anterior pituitary.
✓ Active glands have more follicles of low columnar epithelium; glands with mostly squamous follicular cells are
hypoactive.
✓ Thyrocytes have apical junctional complexes and rest on a basal lamina .
✓ Another endocrine cell type, the parafollicular cell or C cell, is also found inside the basal lamina of the follicular
epithelium or as isolated clusters between follicles .
✓ Parafollicular cells are usually somewhat larger than follicular cells and stain less intensely.
✓ Secretion of calcitonin is triggered by elevated blood Ca2+ levels, and it inhibits osteoclast activity , but this function
in humans is less important than the roles of parathyroid hormone and vitamin D in the regulation of normal calcium
homeostasis.
Production of Thyroid Hormone - promoted by TSH
① Production of thyroglobulin ;
* synthesis in the rough ER and glycosylation in the Golgi apparatus , then the glycoprotein is released from
thyrocytes into the follicular lumen
② Uptake of iodide ;
* from blood by Na/I symporters (NIS) in the thyrocytes’ basolateral cell membranes. Decreased levels of
circulating iodide trigger synthesis of NIS , increasing iodide uptake and compensating for the lower plasma
concentration. An apical iodide/chloride transporter (also called pendrin) pumps I – from thyrocytes into the
colloid
③ Iodination of tyrosyl residues ;
2
