II. THYROID GLAND = lies adjacent to trachea, posterior to larynx; consists of
two lobes connected by an isthmus and surrounded by a capsule (double layers of CT).
- Develops as an evagination from the wall of the pharynx
- Contains two types of hormone-secreting cells:

1) Follicle Cells = secrete thyroid hormones (T3 and T4)
2) Parafollicular Cells (C-cells) = secrete calcitonin

- Basic structural unit of thyroid = follicle - lined by simple cuboidal epithelium of
follicle cells.
- Colloid = acidophilic glycoprotein filling lumen of follicle, composed of thyroglobulin
(thyroid hormone precursor) which undergoes extracellular
iodination; serves as storage reservoir for thyroid hormones

- SYNTHESIS OF COLLOID AND THYROID HORMONES
1) Synthesis of thyroglobulin
2) Release into follicle lumen
3) Iodination
4) Storage as colloid
5) Endocytosis of colloid in response to secretory stimulus (TSH from
adenohypophysis)
6) Hydrolysis of thyroglobulin (produces T3 and T4)
7) Release of thyroid hormones into capillaries surrounding follicles

- Follicle cells are low cuboidal when hypoactive; high cuboidal/low columnar
when the gland is active; Hyperactive thyroid has little or no colloid in lumen of
follicles. Follicle cells contain large nuclei, basophilic cytoplasm with fine granules,
and numerous mitochondria, RER and lysosomes
- C-cells = lie adjacent to follicles but within basal lamina; larger than follicle
cells with eccentrically placed nuclei, pale-staining cytoplasm, contain numerous
membrane-bound granules (calcitonin)
- T3 and T4 increase metabolic rate and help maintain normal function and growth in
tissues
- Calcitonin decreases blood Ca2+ by inhibiting bone resorption

III. PARATHYROID GLANDS = lie on border of thyroid, usually 4 glands in
humans (range is 2-6); derived from endoderm of pharyngeal pouches (see
pharyngial pouch handout).
- Covered with a thin capsule and also enclosed in fascia of thyroid; septa extend
inward from capsule, divide gland incompletely into lobules; septa carry blood vessels,
lymphatics, nerves
- Glandular epithelium (cord-and-clump type) consists of 2 cell types:

1) Chief Cells = small, round, pale-staining cells with a spherical nucleus and
indistinct cell borders. Secrete Parathyroid hormone
2) Oxyphil Cells = less common, larger with acidophilic cytoplasm and smaller
darker nucleus; increase in number after puberty. May represent nonsecretory chief
cells, but precise function is unknown.

- Parathyroid Hormone = antagonistic to calcitonin, promotes bone resorption,
increases blood calcium levels

IV. ADRENALS = paired glands located at cranial pole of each kidney; composed
of 2 histological regions:
1) Cortex = outer region, secretory cells derived from mesoderm, produce
corticosteroids
2) Medulla = inner region, secretory cells derived from neural crest tissue,
produce catecholamines (epinephrine, norepinephrine)

1) Cortex = composed of 3 zones (outside to inside)

a) Zona glomerulosa = narrow zone immediately beneath capsule (approx. 15%
of cortex), cells arranged into rounded clumps

- consists of pyrimidal or columnar cells with dark-staining spherical nuclei,
acidophilic cytoplasm with some basophilic material, cytoplasm also contains lipid
droplets
- produces mineralocorticoids (aldosterone and deoxycorticosterone) which control
water and salt balance; aldosterone specifically increases kidney tubule resorption of
Na+

b) Zona fasiculata = thickest layer (approx. 78% of cortex)

- consists of irregular cuboidal/polyhedral cells arranged in long radial cords; bordered
by fenestrated capillaries
- cells contain vesicular nuclei and basophilic cytoplasm with numerous lipid droplets
- produces glucocorticoids (cortisol, corticosterone) which regulate carbohydrate, lipid
and protein metabolism

c) Zona reticularis = composes approx. 7% of cortex volume

- cells form anastomosing network, separated by fenestrated capillaries
- cells in this region are smaller, the cytoplasm contains fewer lipid droplets and the
nuclei are deeper-stained
- produces some glucocorticoids and very small amounts of sex hormones
(physiologically relevant?)
- Throughout the cortex, lipid droplets are associated with the synthesis of steroid
hormones

SEE HANDOUT FOR REVIEW OF CORTEX STRUCTURE

2) Medulla = consists of anastomosing cords of cells known as chromaffin
cells (due to specific staining of secretory granules with Chromium salts)
- Secrete epinephrine (adrenaline), norepinephrine
- Secretory cells of medulla innervated by sympathetic preganglionic fibers, so they are
functionally equivalent to sympathetic ganglion cells. Stimulation causes release
of hormone by exocytosis. (Norepinephrine is the neurotransmitter of sympathetic
postganglionic fibers).

V. PINEAL BODY = cone-shaped body attached by a stalk to the roof of the 3rd
ventricle of the brain
- Produces melatonin which exerts a suppressive effect on gonadal function among
other actions
- Pia mater forms capsule around pineal body
- Cell Types:

1) Pinealocytes = epithelial secretory cells (melatonin), irregularly shaped with
branching processes; pale-staining cells with a large nucleus
2) Glial Cells = serve as supporting network; fewer in number, possess
elongated deeply stained nucleus
3) Extracellular Concretions = lamellated bodies of calcified organic matrix;
occur mainly in capsule and septa; unknown function

COMPARATIVE ASPECTS OF THE ENDOCRINE SYSTEM
1. Pituitary -SEE HANDOUT for differences in arrangement in different
vertebrate taxa

- Lamprey = rostral and proximal pars distalis, pars intermedia and pars nervosa
form neurointermediate lobe; no pars tuberalis present
- Teleost = tendency to localize cellular types in specific regions; also have rostral
and proximal pars distalis, neurointermediate lobe, but positions are
reversed from mammalian condition with neurohypophysis anterior,
adenohypophysis posterior; no pars tuberalis
- Anuran = pars tuberalis present so adenohypophysis composed of pars distalis,
pars intermedia and pars tuberalis; some cellular regionalization in pars
distalis; reversed position of adeno- and neurohypophysis
- Reptile = adenohypophysis is well-developed with the same 3 regions as in
anurans; pars distalis divided into cephalic and caudal lobes; best-developed pars
intermedia of any group
- Birds = no pars intermedia present; well-developed pars tuberalis; pars distalis
divided into cephalic and caudal lobes; median eminence forms two specific
regions - each with its own portal connection to pars distalis

2. Pharyngeal Pouch Derivatives - SEE HANDOUT for differences in
origin in the different vertebrate taxa

- Ultimobranchial Bodies = derived from ventral portion of 5th pharyngeal
pouch, present in nonmammalian vertebrates; produce calcitonin; thought to be
homologous to C-cells of mammalian thyroid
- No parathyroids present in fishes, but they are present in all tetrapods

3. Adrenal Gland - SEE HANDOUT for differences in structure and
location of cortical and medullary tissues

- Primitive Condition = diffuse tissue
- Advanced Condition = Discrete glands with discrete regions of cortical and
chromaffin tissue; become associated with cranial pole of kidney

To Lecture 18