Embryology of Endocrine Glands and Tissues

Discrete Endocrine Glands – discrete tissues devoted to endocrine function

distinct from Diffuse Endocrine System, which are single cells or clusters of cells scattered among non-endocrine cells
(a.k.a. ectopic endocrine system, entero-endocrine or chromaffin system, diffuse neuroendocrine system)

Origin of Endocrine System

(1) Hypophysis (4^th week) – from ectoderm

Adenohypophysis – Derived from a dorsal evagination of the ectoderm of the oropharynx (stomodeum) toward the brain (Rathke’s pouch); eventually meets and partially surrounds neurohypophysis

Neurohypophysis – Derived from a downgrowth (the future infundibulum) of neuroectoderm of the floor of the third ventricle (diencephalon); pituicytes differentiate from infundibular cells

Pituitary Abnormalities: Rare; most common is missing pituitary

(2) Hypothalamus (4^th week) – from ectoderm

floor of diencephalon immediately cranial to infundibulum thickens, folds back caudally and finally comes to lie just dorsal to (on top of) the infundibulum
nerve fibers of hypothalamic neurons grow into infundibulum and terminate in the capillary beds of the pars nervosa
late in the fetal period cytodifferentiation occurs and small amounts of hormone are produced

(3) Pineal Gland (7^th-8^th week) – from ectoderm

develops from neuroectoderm of posterior portion of roof of diencephalon–remains attached to brain by a short stalk
the pineal gland is the ephysis of the diencephalon

Does not have photoreceptors in people; function is to secrete melatonin, which controls circadian rhythms

(4) Adrenal Glands (5^th-6^th week) – from mesoderm and ectoderm

Cortex – derived from mesodermal mesenchyme lining posterior abdominal wall medial to genital ridge and lateral to dorsal mesentary

fetal cortex (inner mass of cells) functions during fetal period but degenerates soon after birth

produces D -5,3-b -hydroxy steroids (100-200 mg/day)

differentiation of permanent cortex (outer mass of cells) begins during late fetal period

Zona glomerulosa and fasciculata are present at birth; Zona reticularis appears near end of 3^rd year after birth.

Medulla – neuroectodermal cells migrate from the neural crest of the trunk region into the region of celiac ganglia and on to the developing suprarenal cortex – these cells populate the central region of the gland

this migration continues to birth
epinephrine and norepinephrine are produced and secreted by different cell types

epinephrine by cells with small granules

norepinephrine by cells with large granules

(5) Thyroid (24^th day) – from endoderm (except parafollilular cells (C cells), from neural crest)

develops from foramen cecum – endoderm in midline of floor of pharynx between 1st and 2nd pharyngel pouches, just caudal to median tongue bud

foramen cecum thickens to form unpaired thyroid primordium that elongates and invaginates Þ thyroid diverticulum

diverticulum continues to grow caudally, becomes thyroglossal duct; tip expands and bifurcates to form thyroid Þ two thyroid lobes connected by isthmus

thyroglossal duct connects the gland with its site of origin (the foramen cecum) at the base of the tongue

by 7^th week, duct has regressed; however, distal part persists in 50% of people as the pyramidal lobe of the thyroid

Follicular Cells – secrete T₄ and T₃, arise from a downward growth of endodermal epithelium

by 10^th week contain colloid
by 12^th-13^th week synthesize non-iodinated thyroglobulin
by 16^th week synthesize T₃

Parafollicular Cells (C cells) – secrete calcitonin, arise from neural crest cells (ultimobranchial (telopharyngeal) bodies) that migrate into the developing thyroid gland

Thyroid Abnormalities: Many, since it migrates from place of origin; can have thyroid tissue from back of tongue down

(6) Parathyroid Gland (5^th week) – from ectoderm

inferior parathyroid glands and thymus derive from 3^rd branchial pouch; superior parathyroid glands derive from 4^th

normally, the inferior parathyroids separate from the thymus and come to lie below the superior parathyroids
start producing PTH by 7^th week (when bones start ossifying)
oxyphil cells appear a few years before puberty

(7) Pancreas (Islets of Langerhans) (10^th-11^th week) – from endoderm

dorsal and ventral pancreatic buds rotate and fuse to create pancreas
the islets bud off the pancreatic duct system and form clusters of endocrine cells; alpha, then beta, then delta

insulin appears in the 10^th week and glucagon by the 15^th week

Pancreatic Abnormalities:

missing
strangulation of duodenum because of malrotation of dorsal pancreatic bud