This document summarizes the major endocrine organs and their functions. It describes the classification of endocrine organs into central, peripheral, mixed and diffuse systems. It then details the key tissues and cells involved in hormone production for each organ, including the hypothalamus, hypophysis, thyroid gland, parathyroid glands, adrenal glands and pancreas. For each organ, it explains the histology, hormone production and regulation via feedback loops with other endocrine organs like the hypothalamus and hypophysis.

1. Endocrine organs

2. Endocrine organ classification
 Central organs:
hypothalamus, hypophysis, pineal gland
 Peripheral organs:
thyroid gland, parathyroid glands, adrenals
 Mixed organs:
pancreas, gonads, placenta
 Diffuse neuroendocrine or APUD system

3. Endocrine gland tissue composition
 Nerves tissue:
hypothalamus
neurohypophysis
pineal gland
adrenal medulla
 Epithelial tissue:
adenohypophysis
thyroid gland
parathyroid glands
adrenal cortex
⇒
⇐

4. Endocrine organs are ductless glands
 Contain rich network of
blood sinusoidal fenestrated capillaries
lymphatic capillaries
 Release hormones into
the blood, lymph, and intracellular spaces
Hormone chemistry:
steroids
proteins
glycoproteins
amino acid derivatives

5. Endocrine system is regulating and controlling body system
Hormones

act on specific target cells

interact with cell receptors (surface or nuclear)

alter the target cell biological activity
Control
mechanisms:
(a) endocrine
(b) paracrine
(c) autocrine

6. Hypothalamus
 Contains nuclei of neurosecretory cells
 Nuclei with large cells:
Paraventricular nuclei
Supraoptic nuclei
 Nuclei with small cells:
Dorsomedial nuclei
Ventromedial nuclei
Arcuate or Infundibular nuclei
⇒

7. Neurosecretory cells secrete hormones
Neurosecretory cell axons

leave the hypothalamus

terminate on blood capillaries

form the axovasal synapses in
the neurohemal organs
⇔

8. Neurohemal organs contain axovasal synapses
 Pars nervosa
 Median eminence

9. Hypothalamus nuclei with large neurosecretory cells
 Paraventricular nuclei ⇒ oxytocin
 Supraoptic nuclei ⇒ ADH
Neurosecretory cell axons
 pass through the pituitary stalk
(hypothalamohypophysial tract)
 enter the pars nervosa
 form axovasal synapses

10. Hypothalamus nuclei with small neurosecretory cell
Secrete adenohypophyseal tropic hormones:
 releasing (liberins)
 inhibiting (statins)
 Neurosecretory cell axons:
form axovasal synapses in the median eminence
on capillaries of the primary capillary network

11. Hypophysis (pituitary gland)
 lies beneath the brain
 is connected with the hypothalamus
hypothalamohypophysial
neuroendocrine system

12. Hypophysis anatomical subdivision
Neurohypophysis (nerves tissue)
 Pars nervosa (posterior lobe)
 Infundibulum
Adenohypophysis (epithelial tissue)
 Pars distalis (anterior lobe)
 Pars intermedia
 Pars tuberalis

13. Hypophysis development
 Neuroectoderm ⇒ neurohypophysis
 Ectodermal epithelium of the oral cavity
⇓
adenohypophysis

14. Pars nervosa
 is not endocrine gland
 is the neurohemal organ
 stores and releases hormones
from the hypothalamus nuclei

15. Pars nervosa histology
 unmyelinated nerve fibers
 glial cells – pituicytes
 blood capillaries
⇔

16. Pars nervosa nerve fibers
 are axons of the neurosecretory cells
from supraoptic and paraventricular nuclei
of the hypothalamus
 form hypothalamohypophyseal tract
 convey hormones to the pars nervosa
 form axovasal synapses on capillaries

17. Herring bodies
 are dilated axon terminals
 accumulate secretory granules
 release hormones to the bloodstream

18. Pars nervosa hormones
 ADH
 Oxytocin

19. Adenohypophysis
 is true endocrine gland
 produces and releases hormones
 Hormone chemistry:
small proteins
glycoproteins

20. Adenohypophysis histology
 endocrine epithelial cells (glandulocytes)
 loose connective tissue trabeculae
 sinusoidal fenestrated capillaries
cells are arranged in
adenomeres

21. Pars distalis cell types
 Acidophils
 Basophils
 Chromophobes
acidophils and basophils
contain secretory granules

22. Pars distalis cell hormones
 Acidophils
 GH (STG)
 PL (LTH)
 Basophils
 TSH
 LH and FSH
 ACTH

23. Pars distalis chromophobes lack secretory granules
 are partially degranulated secretory cells
 some of them are stem cells
⇔

24. Adenohypophysis pars intermedia
Endocrine cells
 surround the colloid-filled cysts
pseudofollicles
 secrete hormones
 MSH
 LPH

25. Adenohypophysis vascularization
 superior hypophyseal artery
⇓
 primary capillary network
in the median eminence
⇓
 portal veins
⇓
 secondary capillary network
in the adenohypophysis
⇓
 hypophyseal veins

26. Adenohypophysis portal system
Scanning EM

27. Portal system transports hypothalamic hormones
 neurosecretory cells release
 liberins
 statins
 in primary capillary network
⇓
 portal veins
⇓
 secondary capillary network
of the adenohypophysis

28. Adenohypophysis is regulated by hypothalamic hormones
Liberins and statins
 leave the secondary capillaries
 stimulate or inhibit adenohypophysis cells
⇔

29. Thyroid gland
 is located
in anterior portion of the neck
on the thyroid cartilage and upper trachea
 consists of
two lateral lobes
isthmus
pyramidal lobe (may or may not be)

30. Thyroid histologic composition
 follicles
 connective tissue stroma
 capillaries

31. Follicles are the thyroid morpho-functional units
 are spherical cyst-like compartments
 consist of
 basal lamina
 epithelial cells - thyrocytes
 colloid

32. Thyroid follicles are surrounded by rich capillary network
 capillaries are
fenestrated sinusoidal
⇔

33. Thyrocytes or follicular cells
 are epithelial cells
 arise from the endoderm
 are cuboidal in shape
 rest on the basal membrane
 are in contact with the colloid

34. Thyrocytes secrete protein hormones
 Thyroxine (tetraiodothyronine) – T4
 Triiodothyronine – T3
Hormones regulate
 cell and tissue basal metabolism
 cell growth and cell differentiation

35. Colloid takes part in hormone synthesis
Colloid contains
 thyroglobulin - iodinated glycoprotein
 several enzymes
Thyroglobulin is inactive storage form
of the thyroid hormones

36.  synthesis and secretion of thyroglobulin
 concentration of iodide, oxidation to iodine,
and release to the colloid
 formation of T3 and T4 hormones by iodination
 resorption of colloid by endocytosis
 release of hormones from cells into circulation
resorption vacuoles ⇒
⇐

37. Hyperfunction and hypofunction of the thyroid gland
Hyperfunction Hypofunction
Follicular cell shape Columnar Flat
Follicle diameter Smaller Larger
Colloid volume Decreases Increases
Resorption vacuoles More in number Less in number

38. Thyrocytes are the adenohypophysis-dependent cells
 are stimulated by TSH from the pars distalis
 release of T3 and T4 is regulated by
a simple feed-back system ⇒

39. Parafollicular cells or C-cells
 arise from the neural crest
 occur in follicle wall or interfollicular spaces
 contain numerous secretory granules

40. Parafollicular cells secrete protein hormone calcitonin
 lowers the blood calcium level
⇓
 suppresses bone resorption
 stimulates bone calcification
 are the adenohypiphysis-independent cells
 are regulated by the blood calcium level

41. Parathyroid gland
 are two pairs: superior and inferior
 associated with the thyroid

42. Parathyroid gland histology
 epithelial endocrine cells
 loose connective tissue
 sinusoidal fenestrated capillaries
⇔

43. Endocrine cells are arranged in cords
 Principal or chief cells ⇒ parathyroid hormone (PTH)
 Oxyphil cells (unknown function)

44. Parathyroid hormone is antagonist to calcitonin
 increases the blood calcium level
⇓
 stimulates bone resorption
 reduces the kidney calcium excretion
 activates the intestine calcium absorption

45. Parathyroid gland is the adenohypophysis-independent
 is directly regulated by the blood calcium level
Simple feedback system:
 low calcium levels
⇓
stimulate PTH secretion
 high calcium levels
⇓
inhibit PTH secretion

46. Adrenal (suprarenal) glands
 are paired glands
 are located at the kidney superior poles
 are subdivided in cortex and medulla

47. Adrenal gland blood vessels
 Short cortical arteries
⇓
cortical capillaries
⇓
medullary capillaries
⇓
medullary veins
 Long cortical arteries
⇓
medullary capillaries
⇓
medullary veins

48. Adrenal gland vascularization
⇔

49. Adrenal cortex
 arises from the mesoderm
 consists of epithelial cells
 secretes steroid hormones
 is adenohypophysis-dependent

50. Adrenal cortex zonation
 Zona glomerulosa
 Zona fasciculata
 Zona reticularis

51. Zona glomerulosa
 cells are arranged in ovoid clusters
 secretes mineralocorticoids (aldosterone)
 is regulated by renin-angiotensin system
and ACTH

52. Aldosterone effects
 stimulates
sodium resorption and potassium excretion
in the kidney, salivary glands and sweat glands

53. Zona fasciculata
 cells are arranged in long straight cords
⇔

54. Zona fasciculata cell ultrastructure
is characteristics of steroid-secreting cells:
 well-developed SER
 numerous mitochondria with tubular
and vesicular cristae
 well-developed Golgi complex
 numerous lipid droplets

55. Zona fasciculata secretion
 secretes glucocorticoids (cortisol, corticosterone)
 hormone effects:
regulate gluconeogenesis and glycogenesis
depress immune and inflammatory response

56. Regulation of Zona fasciculata activity
 is under control of ACTH

57. Zona reticularis
 cells are arranged in anastomosing cords
⇔

58. Zona reticularis secretion
 secretes weak androgens
(dehydroepiandrosterone)
 is under ACTH control

59. Adrenal medulla
 arises from the neural crest
 consists of modified neural cells
 secretes catecholamine hormones
 is adenohypophysis-independent
⇔

60. Adrenal medulla cells
 are called chromaffin cells
 secrete norepinephrine and epinephrine
 are regulated by preganglionic sympathetic nerve fibers
 glucocorticoids induce norepinephrine to epinephrin
conversation

61. Adrenal medulla hormone effects
 rise in blood pressure
 dilation of the coronary vessels
 increase in the heart rate
⇔

62. The END
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