Endocrine histology

• Endocrine glands

  • Lack ducts
  • Secretory products pass directly into sinusoids (sinusoidal capillaries) and then into the general circulatory system
• Sinusoids


  • Larger than other capillaries
  • Walls conform to the shape of surrounding tissue
  • Endothelial cells have open pores
  • Basal lamina surrounding endothelial cells is discontinuous

  
• Components of the endocrine system


  • Endocrine organs
  • Groups of endocrine cells within non-endocrine organs
  • Single endocrine cells within non-endocrine organs
  • Cells of the Diffuse NeuroEndocrine System (DNES)
• Endocrine organs


  • Hypophysis cerebri (pituitary gland)
  • Epiphysis cerebri (Pineal gland)
  • Thyroid gland
  • Parathyroid gland
  • Adrenal gland

  
• Hypophysis cerebri


  Releases hormones which may influence the activities of other endocrine glands
• Hypophysis cerebri


  • Suspended from the hypothalamus

  
• Parts of the hypophysis cerebri

  • Adenohypophysis (glandular part)
  • Neurohypophysis (neural part)

  
• Adenohypophysis


  • Arises embryologically from epithelium of pharynx
  • Composed of three parts: Pars distalis, Pars intermedia, Pars tuberalis

  
• Cell types in the pars distalis

  • Acidophils
  • Basophils
  • Chromophobes
• Acidophils


  • Contain eosinophilic (pink) granules
  • Two types: Somatotrophs and Lactotrophs
• Somatotrophs


  Stain positive for somatotropin (growth hormone)
• Lactotrophs


  • Stain positive for prolactin
  • Cell size and staining intensity increase during pregnancy and lactation
• Types of basophils in the pars distalis


  • Gonadotrophs
  • Thyrotrophs
  • Corticotrophs
• Chromophobes


  • Poorly-stained cells
  • Three types: Post-secretory acidophils and basophils, Follicular cells, Stellate cells

  
• Pars intermedia


  • Located adjacent to pars nervosa of neurohypophysis
  • Separated from pars distalis by the hypophysial cleft
  • Poorly vascularized
  • Cells often form small, colloid-filled follicles

  
• Melanotrophs


  • Main cell type in pars intermedia
  • Stain positive for alpha melanocyte-stimulating hormone and beta lipotropin
• Other cell types in pars intermedia

  • Corticotrophs
  • Follicular cells
  • Stellate cells
  • Cells lining the hypophysial cleft

  
• Pars tuberalis


  • Surrounds median eminence and infundibulum of neurohypophysis
  • Contains hypophysial portal venules and clusters of epithelial cells

  
• Cell types in pars tuberalis

  • Pars tuberalis cells
  • Gonadotrophs
  • Thyrotrophs

  
• Neurohypophysis


  • Has three parts: Median eminence, Infundibulum, Pars nervosa
  • Contains axons and central gliocytes (pituicytes)

  
• Epiphysis cerebri (Pineal gland)

  • Dorsal evagination of the roof of the diencephalon
  • Secretes melatonin, involved in circadian rhythms, sexual behaviour, reproduction, and thermoregulation
• Pinealocytes


  • Originate from neural epithelium
  • Axons terminate on other pinealocytes and sinusoidal capillaries
• Corpora arenacea (brain sand)

  Intercellular calcium deposits found in old animals, function/significance unknown
• Thyroid gland


  Surrounded by a capsule, subdivided into lobules containing thyroid follicles, parafollicular cells, and sinusoidal capillaries
• Thyroid follicles


  Round structures lined by follicular epithelial cells, containing colloid (gel-like substance containing thyroid hormones)
• Thyroid follicular epithelial cells


  Height depends on physiological condition, resting cells are simple squamous/cuboidal, stimulated cells are columnar
• Ultrastructure of stimulated thyroid follicular epithelial cells


  • Basally located nucleus surrounded by abundant rough endoplasmic reticulum and numerous mitochondria
  • Large Golgi complex between nucleus and apical cell surface
  • Apical cytoplasm contains secretory vesicles, endocytic vesicles, lysosomes, and phagolysosomes

  
• Parafollicular cells (C cells)


  • Occur singly or in groups between thyroid follicular epithelial cells or surrounded by their own basal lamina
  • Contain numerous mitochondria, well developed rough endoplasmic reticulum, large Golgi complex, and numerous secretory vesicles (calcitonin)

  
• Parathyroid glands


  Surrounded by a connective tissue capsule, cells are arranged in clusters
• Cell types in the parathyroid gland


  • Principal (chief) cells
  • Oxyphil cells

  
• Principal (chief) cells


  • Active (dark) cells secrete parathyroid hormone, have dark acidophilic cytoplasm and dark nuclei due to condensed chromatin
  • Inactive (light) cells have pale cytoplasm and nuclei
• Endocrine organs


  • Hypophysis
  • Epiphysis cerebri (Pineal gland)
  • Thyroid gland
  • Parathyroid gland
  • Adrenal gland

  
• Parathyroid glands


  • Surrounded by a connective tissue capsule
  • Cells forming the gland are arranged in clusters
• Parathyroid cell types


  • Principal (chief) cells – inactive/active
  • Oxyphil cells
• Active (dark) principal cells


  • Secrete parathyroid hormone
  • Nucleus is dark due to condensed chromatin
  • Dark acidophilic cytoplasm
• Active principal cells (ultrastructure)


  • Abundant rER
  • Well-developed Golgi complexes
  • Numerous mitochondria
  • Numerous secretory granules
• Inactive principal cells (ultrastructure)

  • Poorly developed Golgi complexes
  • Few secretory granules
  • Glycogen present in some species, e.g. cats
• Oxyphil cells


  • Large acidophilic cells
  • Occur singly & in clusters in horse & ruminants. Rare in other species
• Oxyphil cells


  Function is unclear, occur mainly in older animals (horse and bovine)
• Adrenal gland structure


  • Surrounded by a dense irregular connective tissue capsule
  • Cells forming adrenal gland are arranged in clusters or cords