histology and embryology of thyroid gland

1. Thyroid gland

3. • Thyroid gland is found in the neck,
• below the thyroid cartilage
• (which forms the laryngeal prominence, or
"Adam's apple").

5. • It produces thyroid hormones, the principal
ones being triiodothyronine (T3)
and thyroxine which can sometimes be
referred to as tetraiodothyronine (T4).
• These hormones regulate the growth and rate
of function of many other systems in the body.
• T3 and T4 are synthesized
from iodine and tyrosine.

6. • The thyroid also produces calcitonin, which
plays a role in calcium homeostasis.

7. • Hormonal output from the thyroid is regulated
by thyroid-stimulating hormone (TSH)
produced by the anterior pituitary,
• which itself is regulated by thyrotropinreleasing hormone (TRH) produced by
the hypothalamus.

9. • The thyroid gland is a butterfly-shaped organ
and is composed of two cone-like lobes or
wings, lobus dexter (right lobe) and lobus
sinister (left lobe), connected via the isthmus.

11. • The organ is situated on the anterior side of
the neck, lying against and around the larynx
and trachea, reaching posteriorly
the oesophagus and carotid sheath.

13. • It starts cranially at the oblique line on
the thyroid cartilage (just below the laryngeal
prominence, or 'Adam's Apple'), and extends
inferiorly to approximately the fifth or
sixth tracheal ring.
• It is difficult to demarcate the gland's upper
and lower border with vertebral levels
because it moves position in relation to these
during swallowing.

14. Histology
• The gland consists of many indistinct lobules
containing follicles and many blood vessels
enmeshed in fine connective tissue
• The follicles contain a homogeneous,
acidophilic material called colloid
• The follicles are lined by simple cuboidal
epithelium but the epithelium may be
columnar or squamous depending upon the
function activity of the gland

21. Parathyroid gland
• The parenchyma of the gland consists of irregular
masses of cords of cells, separated by blood
vessels embedded in loose connective tissue
containing many fat cells
• The cords of cells contain two types of cells Chief
or principal cells are most common and a few
oxyphil cells. The chief cells have prominent
nuclei and relatively little cytoplasm. The oxyphil
cells are larger and more darker stained
cytoplasm.

29. • Oxyphil cells:
● Slightly larger than chief cell (12 microns),
acidophilic cytoplasm due to mitochondria
● No secretory granules
● First appear at puberty as single cells, then
pairs, then nodules at age 40

31. Development
• The thyroid gland is the first endocrine gland
to develop in the embryo.
• It begins to form about 24 days after
fertilization.
• It appears as a median endodermal thickening
in the floor of the primordial pharynx between
tuberculum impar and copula.

32. • Ayat 1. Coronal section of five week embryo.
Tuber impar and lat lingual swellings (first arch). Copula
second arch. Hypobranchial eminence third and fourth
arches.
• Ayat 2. Coronal section of five month old fetus.
Anterior and posterior parts of the tongue has developed.

33. • The site from which it
originated persists as the
foramen cecum at the base of
the tongue.

35. • This thickening soon forms a
small out pouching downward –
the thyroid diverticulum.

37. • Diagram showing the developing thyroid
diverticulum.
• On the right the lateral view of the
developing embryo during 4th week is
shown.
• On left the developing pharynx has been
shown during 4th week.

38. • 5 week
5 month

39. • Ventral view of developing pharynx and
developing tongue.
• In Week Five, Foramen Cecum is present
between Tuberculum Impar and Copula.
• In Month Five, It is between anterior two third
and posterior third of tongue at the tip of
sulcus terminalis.

41. • As the embryo and tongue grow, the
developing thyroid gland descends in
the neck in front of pharyngeal gut,
passing ventral to the developing
hyoid bone and laryngeal cartilages.

42. • Left lateral view of developing embryo during fifth
week of intrauterine life. Look at the thyroglossal
duct developing from foramen cecum and the
formation of thyroid gland from its lower end.

43. • During this migration for a short
time the developing thyroid gland
is connected to the tongue by a
narrow tube, the thyroglossal
duct.
• This duct later disappears.

44. • At first the thyroid diverticulum is hollow but
it soon becomes solid and divides into right
and left branches.
• It reaches its final position in front of trachea,
just below the cricoid cartilage, during 7th
week.

45. • When it reaches the position it occupies in the
adult, the gland has assumed its definitive
shape and has a small median isthmus and
two lateral lobes.
• The isthmus lies anterior to the developing
second and third tracheal rings.

47. • By this time the thyroglossal duct has
normally degenerated and disappeared.
• The proximal opening of the thyroglossal duct
persists as a small blind pit, the foramen
cecum of the tongue.

49. • The proximal opening of the thyroglossal duct
persists as a small blind pit, the foramen
cecum of the tongue.
• In about 50% of people thyroglossal duct
persists as a pyramidal lobe extending
superiorly from the isthmus.

51. Histogenesis
• Thyroid primordium consists of a solid mass of
endodermal cells.
• This cellular aggression breaks up into a
network of epithelial cords because of the
invasion of the surrounding vascular
mesenchyme.

52. • By tenth week cords have formed small
cellular groups.
• A lumen soon forms in each cell cluster and
the cells become arranged in a single layer
around a lumen.
• These are thyroid follicles.
• During eleventh week colloid begins to appear
in thyroid follicles.

54. • Ultimobranchial bodies loose their
connections with pharynx and migrate toward
thyroid gland.
• Cells of ultimobranchial bodies disseminate
within gland.
• These cells are parafollicular or C-cells.

56. • They are placed between follicular cells and
their basement membrane.
• This is the reason they are called parafollicular
cells because they are outside the follicles.
• They are called C-cells because they secrete
calcitonin.
• C-cells actually are derived from neural crest
cells of pharyngeal arches.

58. Thyroglossal Cyst
• A thyroglossal cyst may be found at any point
along the migratory path followed by thyroid
gland in the midline of neck.
• A thyroglossal cyst is a cystic remnant of
thyroglossal duct.

59. • In 50 per cent cases such cyst is located close
to hyoid bone.
• It may also be found at the base of tongue or
close to thyroid cartilage.

61. Thyroglossal Fistula
• Sometimes the cyst is connected to outside by
a fistulous canal.
• Then it is called thyroglossal fistula.
• The fistula may be primary when it is present
at birth. It may be secondary thyroglossal
fistula when a cyst ruptures and communicate
outside at later stage.

62. Aberrant Thyroid Tissue
• Thyroglossal tissue may be found anywhere
along the path of the descent of thyroid gland.
• It is most commonly found in the base of
tongue, just behind foramen cecum.
• It is subject to the same diseases as thyroid
gland itself.