L2 endocrine anatomy

Question 1

Endocrine organs of the neck region

Answer 1

Thyroid
Parathyroid
Thymus

Question 2

Embryology of the glands of the neck region

Answer 2

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

Question 3

The early embryonic pharynx gives rise to a series of pouches

Answer 3

Question 4

Question 5

Function of the thyroid gland

Answer 5

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++

Question 6

Thyroid production of Formation & release of hormones

Answer 6

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

Question 7

Anatomy of the thyroid gland

Answer 7

Paired lobes, on lateral trachea, just caudal to larynx
Dark red in colour
May be connected via isthmus on ventral trachea
Species variation

Question 8

Blood and nerve supply

Answer 8

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)

Question 9

Histology of the thyroid gland

Answer 9

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

Question 10

, parafollicular or ‘C’ (clear) cells

Answer 10

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

Question 11

Thyroid histology

Answer 11

Question 12

B. Parathyroid Glands

Answer 12

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

Question 13

Function of Parathyroid Glands

Answer 13

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

Question 14

Dog, cat, small ruminants parathyroid

Answer 14

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!
`

Question 15

Ox parathyroid

Answer 15

Ox

Parathyroid III usually cranial to thyroid; parathyroid IV usually embedded in thyroid or between it & tracheal wall

Question 16

Horse parathyroid

Answer 16

Parathyroid III much further caudal, near thoracic inlet

Parathyroid IV close to thyroid

Question 17

Histology of the Parathyroid Gland

Answer 17

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)

Question 18

Thymus Gland

Answer 18

Large lymphoid organ, regresses with age
Thoracic and cervical parts
Produces immunocompetent T lymphocytes
Produces several hormones inc. thymosine, a hormone which stimulates lymphopoiesis

Question 19

Histology of thymus gland

Answer 19

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

Question 20

Ox: thymus gland

Answer 20

extensive in calf, both cervical and thoracic component

Question 21

horse thymus

Answer 21

less extensive than calf, may have small cervical portion

Question 22

Dog thymus

Answer 22

in precardial mediastinum

Question 23

Pig thymus

Answer 23

extensive cervical and thoracic portions

Question 24

lis the endocrine organs of the abdomen (7)

Answer 24

Adrenal glands
Pancreas
Ovaries
Testes 
Placenta
Gastrointestinal tract
Kidneys

Question 25

A. Adrenal glands

Answer 25

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

Question 26

Anatomy of the Adrenal Glands

Answer 26

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

Question 27

Anatomy of the Adrenal Glands 2

Answer 27

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

Question 28

Histology of the adrenal glands

Answer 28

``` Dense fibrous capsule Support for delicate collagenous framework supporting secretory cells Cortex Medulla Large vein centre of medulla ```

Question 29

Histology of the adrenal glands Cortex with 3 layers (from outer to inner)

Answer 29

Zona glomerulosa Zona fasciculata Zona reticularis (Sometimes 4th layer, zona intermedia, containing undifferentiated cells)

Question 30

Zona glomerulosa

Answer 30

Rounded clusters of cells | Mineralocorticoids, esp. aldosterone

Question 31

Zona fasciculata

Answer 31

Parallel cords secretory cells | Glucocorticoids, esp. cortisol

Question 32

Zona reticularis

Answer 32

Irregular branching cords & clusters, large capillaries | Small amounts androgens & glucocorticoids

Question 33

Adrenal medulla

Answer 33

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

Question 34

Ox comparative anatomy adrenal glands

Answer 34

Left adrenal gland C-shaped | Right adrenal gland heart-shaped with caudal vena cava running over it

Question 35

Horse adrenal glands

Answer 35

Close to kidneys | V compressed dorso-ventrally

Question 36

Brown marsupial mouse or Antechinus

Answer 36

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)

Question 37

B. Endocrine pancreas

Answer 37

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

Question 38

Pancreas functions

Answer 38

Endocrine Islets of Langerhans Insulin & glucagon Exocrine Enzymes, electrolytes & water From pancreatic acini  duodenum

Question 39

Anatomy of the pancreas

Answer 39

Diffuse, lobulated,pink

Question 40

Islets of Langerhans

Answer 40

``` Small masses of pale endocrine cells Scattered through the exocrine pancreas Demarcated by reticular fibres About 1 million islets per pancreas ```

Question 41

Islet cells

Answer 41

``` 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 ```