L2 endocrine anatomy Flashcards
Endocrine organs of the neck region
Thyroid
Parathyroid
Thymus
Embryology of the glands of the neck region
Neck endocrine organs are derived from the pharynx
Series of pharyngeal pouches form thymus, para-thyroids & part thyroid
Main part thyroid from median out-growth pharygeal floor
The early embryonic pharynx gives rise to a series of pouches
Function of the thyroid gland
Thyroid hormones increase metabolic rate, generating heat
2 hormones produced by follicular cells:
Thyroxine (T4)
Tri-iodothyronine (T3)
Hormone produced by follicular cells, then stored until required
Parafollicular (thyroid type C) cells produce calcitonin which increase uptake of Ca++ by the skeleton, thus lowering blood Ca++
Thyroid production of Formation & release of hormones
T3 (10%) & T4 (90%) are formed from iodinised tyrosine molecules in cuboidal follicular cells
They are stored outside the follicular cells in the follicular lumen as inactive form “thyroglobulin” which forms “colloid”
When required, released into circulation by follicular cells splitting off T3 or T4
If deficient in TSH or iodine, get build up of thyroglobulin in follicles & enlarged thyroids known as goitre in humans
Anatomy of the thyroid gland
Paired lobes, on lateral trachea, just caudal to larynx
Dark red in colour
May be connected via isthmus on ventral trachea
Species variation
Blood and nerve supply
Blood supply is species dependent
Usually from common carotid a.
Cranial thyroid a.
Caudal thyroid a. in some species (may have different origin)
Drains to internal jugular vein
Parasympathetic nerve supply via vagus
Sympathetic through cranial cervical ganglion (mostly vasomotor)
Histology of the thyroid gland
Consists of follicles lined by simple cuboidal epithelium which produces thyroid hormones
Cells stimulated by TSH (from anterior pituitary)
Active cells are tall cuboidal, with the follicular lumen small as hormone is secreted into the blood stream
Inactive cells are low cuboidal and lumen will be distended with colloid
, parafollicular or ‘C’ (clear) cells
Also, parafollicular or ‘C’ (clear) cells occur either singly, between follicular cells or clumped between follicles
Outer CT capsule and inner fibro-elastic capsule divides thyroid into lobules and contains blood and nerve supply
Thyroid histology
B. Parathyroid Glands
2 pairs small glands
On, or embedded in, thyroid tissue
Referred to as parathyroid III & IV from embryological origin (pharyngeal pouches) or external & internal parathyroid
Small pale pink structures , contrast against brick red of thyroid
Function of Parathyroid Glands
Produce parathyroid hormone (= parathormone) which raises blood calcium levels (opposite effect of calcitonin) by:
Osteoclastic resorption
↑ reabsorption of Ca++ & ↓ reabsorption of phosphate ions in kidneys
↑ Ca++ absorption from intestine
Parathormone is the main regulator of blood calcium levels while calcitonin provides fine adjustment
Dog, cat, small ruminants parathyroid
Recessed or embedded within the thyroid
Dog: parathyroid III may be in fascia of cranial pole of thyroid gland, parathyroid IV usually buried in thyroid parenchyma
Cat: parathyroid III in caudal thyroid, parathyroid IV in cranial thyroid
Watch out for them during surgery!
`
Ox parathyroid
Ox
Parathyroid III usually cranial to thyroid; parathyroid IV usually embedded in thyroid or between it & tracheal wall
Horse parathyroid
Parathyroid III much further caudal, near thoracic inlet
Parathyroid IV close to thyroid
Histology of the Parathyroid Gland
Thin CT capsule Dense plexus of sinusoidal capillaries 2 cell types: Chief cells (=C) synthesize & secrete parathormone prominent nucleus, little cytoplasm Oxyphil cells (=O) less numerous large & clumped dense nucleus & eosinophilic cytoplasm with fine granules (Fibrous septa = S)
Thymus Gland
Large lymphoid organ, regresses with age
Thoracic and cervical parts
Produces immunocompetent T lymphocytes
Produces several hormones inc. thymosine, a hormone which stimulates lymphopoiesis
Histology of thymus gland
Capsule (C) Cortex (Cx) Packed with maturing T cells Medulla (M) Lobes & lobules Interlobular septa (S) Hormone producing epithelial cells line capsule, coat septa & capillaries
Ox: thymus gland
extensive in calf, both cervical and thoracic component
horse thymus
less extensive than calf, may have small cervical portion
Dog thymus
in precardial mediastinum
Pig thymus
extensive cervical and thoracic portions
lis the endocrine organs of the abdomen (7)
Adrenal glands Pancreas Ovaries Testes Placenta Gastrointestinal tract Kidneys
A. Adrenal glands
Embryology
Dual origin
Cortex from coelomic epithelium, secretes steroids inc. mineralocorticoids, glucocorticoids & some sex hormones (similar origin to gonads)
Medulla from neural crest cells, secretes adrenaline (= epinephrine) and noradren-aline (= norepinephrine
Anatomy of the Adrenal Glands
Small white/yellow bodies, flattened dorsoventrally
Lie in retroperitoneal space close to thoracolumbar junction
Usually craniomedial to kidneys, on either side of aorta, hence R adrenal often more cranial than L adrenal
Phrenico-abdominal vein bisects the adrenal gland ventrally
Anatomy of the Adrenal Glands 2
Blood supply from nearby vessels
Venous drainage via hilum & into vena cava
Nerves mainly sympathetic into medulla
Cortex controlled by ACTH from adenohypophysis (glucocorts.) & renin-angiotensin system (mineralocorts.)
Accessory masses occur, usually on surface cortex but can be separate on nearby vessels or in organs
Histology of the adrenal glands
Dense fibrous capsule Support for delicate collagenous framework supporting secretory cells Cortex Medulla Large vein centre of medulla
Histology of the adrenal glands Cortex with 3 layers (from outer to inner)
Zona glomerulosa
Zona fasciculata
Zona reticularis
(Sometimes 4th layer, zona intermedia, containing undifferentiated cells)
Zona glomerulosa
Rounded clusters of cells
Mineralocorticoids, esp. aldosterone
Zona fasciculata
Parallel cords secretory cells
Glucocorticoids, esp. cortisol
Zona reticularis
Irregular branching cords & clusters, large capillaries
Small amounts androgens & glucocorticoids
Adrenal medulla
Clusters of cells around capillaries & venules
Venules drain cortex to central medullary vein
Secretory cells also known as chromaffin cells
Secrete adrenaline > noradrenaline
Nerves to medulla pre-ganglionic sympathetic nn. & cells of medulla functionally equivalent to post-ganglionic neurons
Ox comparative anatomy adrenal glands
Left adrenal gland C-shaped
Right adrenal gland heart-shaped with caudal vena cava running over it
Horse adrenal glands
Close to kidneys
V compressed dorso-ventrally
Brown marsupial mouse or Antechinus
Males die after single breeding season
Sexual rampage for two weeks then overcome by a range of infections & starvation
Deficit in glucocorticoid feedback mechanism results in very high free cortisol levels
This impairs immunocompetency (but results in lots of matings)
B. Endocrine pancreas
Review Digestion notes for gross anatomy
Canine pancreas has 2 lobes, one in mesoduodenum and other in greater omentum near greater curvature of stomach
Larger component is exocrine pancreas, which opens via ducts into duodenum
Embryologically develops as out-pouching of gut
Endocrine cells migrate away from ducts & cluster around capillaries as Islets of Langerhans
Pancreas functions
Endocrine
Islets of Langerhans
Insulin & glucagon
Exocrine
Enzymes, electrolytes & water
From pancreatic acini duodenum
Anatomy of the pancreas
Diffuse, lobulated,pink
Islets of Langerhans
Small masses of pale endocrine cells Scattered through the exocrine pancreas Demarcated by reticular fibres About 1 million islets per pancreas
Islet cells
Rich blood and nerve supply With special stains, different cell types can be distinguished Alpha Peripheral 25%, glucagon Beta Throughout 60%, insulin Delta 10%, somatostatin
