Lecture 8: hypothalamus, pituitary, pineal

What did Berthold's 1849 rooster experiment show?

He removed the testes from 6 roosters and reimplanted them in the abdomen of 2. The implants connected only to blood vessels (not nerves), so some molecule must travel in the blood (the first endocrine experiment).

By definition, a hormone is

A chemical signal carried by the bloodstream. The same molecule can also act as a neurotransmitter or paracrine signal.

What are the 3 chemical classes of hormones?

Amino acid derivatives; peptide hormones; lipid derivatives.

Which hormones are derived from tyrosine?

Thyroxine (T4), epinephrine, norepinephrine, and dopamine.

Which hormone is derived from tryptophan?

Melatonin (secreted by the pineal gland).

Which gland secretes epinephrine and norepinephrine?

The suprarenal (adrenal) medulla.

Which structure secretes dopamine (in this lecture's classification)?

The hypothalamus.

Peptide hormones are

Chains of amino acids. Many begin as inactive precursor proteins called prohormones.

Which peptide hormones does the hypothalamus make?

ADH (vasopressin), oxytocin, and the regulatory (releasing/inhibiting) hormones.

Which peptide hormones does the pituitary make?

TSH, LH, FSH, ACTH, growth hormone (GH), MSH, and prolactin (PRL).

Peptide hormone sources: heart / kidney / pancreas / parathyroid / thyroid C cells / adipose

Heart: ANP and BNP. Kidney: erythropoietin (EPO). Pancreas: insulin and glucagon. Parathyroid: PTH. C cells: calcitonin. Adipose: leptin. (Reproductive organs: inhibin.)

Lipid derivative hormones are

Hormones with carbon rings and side chains built from fatty acids or cholesterol.

Which lipid derivatives come from arachidonic acid (eicosanoids)?

Leukotrienes, prostaglandins, thromboxanes, and prostacyclins.

Which steroid hormones are made by the gonads / suprarenal cortex / kidneys?

Gonads: androgens, estrogens, progestins. Suprarenal cortex: mineralocorticoids, glucocorticoids, androgens. Kidneys: calcitriol.

What is the relationship between the hypothalamus and pituitary?

Together they control the other endocrine glands and form the interface between the nervous and endocrine systems.

Where are the hypothalamus and pituitary located?

The hypothalamus is part of the brain, immediately superior to the pituitary. The pituitary sits in the sella turcica of the sphenoid bone.

What is a nucleus in the hypothalamus?

A cluster of neuron cell bodies within the CNS. Each hypothalamic nucleus is associated with particular functions.

Which hypothalamic nuclei make oxytocin and vasopressin?

The supraoptic nucleus and the paraventricular nucleus.

Vasopressin is also called

Antidiuretic hormone (ADH).

What are the 3 ways the hypothalamus coordinates hormone release?

1) Direct control of endocrine organs via the sympathetic nervous system. 2) Direct secretion of hormones into the posterior pituitary via axonal projections. 3) Secretion of regulatory hormones that act on the adenohypophysis.

Another name for the posterior pituitary

Neurohypophysis.

Another name for the anterior pituitary

Adenohypophysis.

Which hormones are released from the neurohypophysis?

Oxytocin and vasopressin (ADH). They are made in the hypothalamus and released from axon terminals.

What are Herring bodies?

Swollen, eosinophilic axon terminals of hypothalamic neurons in the neurohypophysis. They store and release oxytocin and vasopressin into the extracellular space (secretory granules visible at high magnification).

What are pituicytes?

Glial cells of the neurohypophysis; their basophilic nuclei are visible between the Herring bodies.

How do oxytocin and ADH get into the blood?

They diffuse from the Herring bodies into nearby fenestrated capillaries.

What type of hormone is stored in the neurohypophysis granules?

Peptide hormones (oxytocin and vasopressin).

Which hormone does the pars intermedia release?

Melanocyte-stimulating hormone (MSH).

When is the pars intermedia functional?

It is mostly nonfunctional in adult humans but functions in fetuses, young children, and pregnant women.

How do you identify the pars intermedia on histology?

Densely packed basophilic cells plus cysts filled with eosinophilic colloid (a characteristic feature).

What are releasing hormones (-RH) and inhibiting hormones (-IH)?

Regulatory hormones secreted by the hypothalamus that stimulate (-RH) or inhibit (-IH) cells in the adenohypophysis.

What does the hypophyseal portal system do?

It delivers releasing and inhibiting hormones directly from the hypothalamus to the adenohypophysis.

What is a portal system?

A second capillary bed placed between the arteries supplying a tissue and the veins that carry blood back to the heart.

What cell classes are in the adenohypophysis?

Basophils (hematoxylin-stained granules), acidophils (eosin-stained granules), and chromophobes (non-secretory supporting cells), along with fenestrated capillaries.

Which anterior pituitary hormones do acidophils release?

Growth hormone (GH, from somatotropes) and prolactin (PRL, from lactotropes).

Which anterior pituitary hormones do basophils release?

FSH and LH (gonadotropes), ACTH (corticotropes), and TSH (thyrotropes).

List the 6 anterior pituitary hormones.

GH, PRL, FSH, LH, ACTH, TSH.

Match the cell to its hormone: somatotrope, lactotrope, gonadotrope, corticotrope, thyrotrope

Somatotrope: GH. Lactotrope: PRL. Gonadotrope: FSH and LH. Corticotrope: ACTH. Thyrotrope: TSH.

Can H&E staining tell you which cell releases which anterior pituitary hormone?

No. H&E only shows acidophil vs basophil vs chromophobe, not which specific hormone is made.

What are chromophobes?

Non-secretory supporting cells of the adenohypophysis (they do not stain strongly).

Hypothalamic regulation of the thyroid axis

Hypothalamus releases TRH, which stimulates TSH release from the anterior pituitary, which stimulates the thyroid gland. (Standard textbook pathway; check against your slide diagram.)

Hypothalamic regulation of the adrenal cortex axis

Hypothalamus releases CRH, which stimulates ACTH release from the anterior pituitary, which stimulates the adrenal cortex. (Standard textbook pathway; check against your slide diagram.)

Hypothalamic regulation of the gonads

Hypothalamus releases GnRH, which stimulates FSH and LH release from the anterior pituitary, which act on the gonads. (Standard textbook pathway; check against your slide diagram.)

Hypothalamic regulation of GH and prolactin

GH is controlled by GHRH (stimulating) and GHIH/somatostatin (inhibiting). Prolactin is mainly controlled by inhibition from dopamine (PIH). (Standard textbook pathway; check against your slide diagram.)

Which hormone drives uterine contractions in labor, and why is this unusual?

Oxytocin. Oxytocin also drives more oxytocin release, a rare positive feedback loop that gradually strengthens contractions.

Besides labor, what else does oxytocin influence?

Pair bonding, social awareness, memory, and metabolism (hormones are multipurpose).

Which hormone does the pineal gland secrete?

Melatonin.

Where is the pineal gland and how does it get light information in humans?

Along the midline of the brain. Light information is routed from the normal eyes to the pineal gland.

Pineal gland capsule and subdivisions

Encased in pia mater and loosely split into lobules by connective tissue septa (which carry blood vessels).

What is corpus arenacea?

Calcified deposits ("brain sand") commonly found in the pineal gland; useful for finding the midline on brain imaging.

What are pinealocytes?

The secretory cells of the pineal gland; they stain strongly only in their nuclei.

What are pineal interstitial cells?

Pineal glial cells; smaller and more darkly stained than pinealocytes.

Neurohypophysis vs adenohypophysis: how does each receive its hormone signals from the hypothalamus?

Neurohypophysis: via axons from hypothalamic neurons that release oxytocin/ADH directly. Adenohypophysis: via releasing/inhibiting hormones delivered by the hypophyseal portal system.