Mednotes

L1 Endocrine Anatomy _ histology of the hypothalomo-pituitary axis

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tagsendocrine, lecture
aliasesL1 Endocrine Anatomy _ histology of the hypothalomo-pituitary axis

20 min read

Overview

This lecture sets up the anatomy and histology that the rest of the endocrinology module rests on. It defines endocrine signalling and where endocrine tissue sits in the body, then narrows to the classical discrete glands and works through the pituitary gland in detail: its three lobes and their hormones, its position in the skull base and its relations on MRI, the histology of the anterior lobe by both H&E and immunohistochemistry, and the tumours that arise from those cell types. It then turns to control from above, covering the hypothalamus and its neurosecretory nuclei, the five hypothalamic to pituitary axes with their feedback loops, and the vascular arrangement (the hypophyseal portal system) that makes that control possible. It closes with how hypothalamic hormones were discovered and how neuronal activity driving pulsatile pituitary secretion is measured today.

Module scope and logistics

  • Anatomy component of the ELM3 Endocrinology Module: 2 lectures and 2 laboratories, taught by Prof. Dave Grattan, Department of Anatomy (Room 207 Scott Building, dave.grattan@otago.ac.nz).
  • Lecture 1 (13/7/26, 14:00): anatomy and histology of the hypothalamo-pituitary axis. Lecture 2 (13/7/26, 15:00): anatomy and histology of the thyroid, parathyroid and adrenal glands.
  • Objective: provide basic anatomical information to support the case-based tutorials, and describe the relevant anatomy, histology and embryology of the major endocrine organs.
  • Focus is on pituitary hormones and pituitary-dependent endocrine glands. Reproductive organs and metabolic hormones are covered in different modules.
  • Lab 1: Thursday 16 July, Histology Classroom (four timeslots: 9-11am, 11am-1pm, 2pm-4pm, 4pm-6pm). Lab 2: Thursday 23 July, run as a virtual tutorial with a 1-hour allocated timeslot but available online throughout the module, essentially an online version of Lab 1, intended as a resource to revise alongside the system-specific tutorials.

What the endocrine system is

  • Endocrine cells synthesise and secrete chemical messengers (hormones) into the bloodstream to regulate the activity of specific target tissues located elsewhere in the body.
  • Primary function: cell-cell communication.
  • No ducts: endocrine glands are “duct-less glands”, in contrast with exocrine glands.
  • Specificity comes from receptors on target cells, in contrast with the nervous system (where specificity comes from the wiring). Because the hormone travels in the blood and reaches everything, the message is a “broadcast” and only receptor-bearing cells respond.
  • Homeostasis: the system acts to “restore normal state”. The lecture uses a thermostat as the analogy for this.

Anatomical distribution of endocrine tissue

Three structural categories:

  • Discrete glands: pituitary, thyroid, parathyroid, adrenal.
  • Clusters of endocrine cells associated with non-endocrine tissue: pancreas, gonad, brain, heart, placenta, adipose tissue.
  • Isolated endocrine cells: GI tract, respiratory tract.

Other points:

  • Endocrine tissue is ductless and of varied structure, defined by function rather than by a common architecture.
  • The literature increasingly treats non-classical tissues as endocrine organs: the slide illustrates this with articles on skin, bone, liver (linking NAFLD and insulin resistance) and muscle as endocrine organs (citations include Endocrine Reviews 40: 1367-1393, 2019; Dermato-Endocrinology 1:5, 250-252, 2009; Current Diabetes Reports 2014, 14:556; Muscle and Exercise Physiology, Elsevier 2019).
  • For this module the focus is the “classical” endocrine glands, predominantly discrete glands for which hormone production is the major function.

The pituitary gland: lobes and hormones

The pituitary gland (hypophysis) has three parts:

  • Anterior lobe (adenohypophysis, pars distalis)
    • growth hormone (GH), also called somatotrophin
    • prolactin (PRL), also called mammotrophin
    • thyroid-stimulating hormone (TSH, thyrotrophin)
    • adrenocorticotrophic hormone (ACTH, corticotrophin)
    • follicle-stimulating hormone (FSH, a gonadotrophin)
    • luteinizing hormone (LH, the other gonadotrophin)
  • Intermediate lobe (pars intermedia)
    • melanocyte-stimulating hormone
  • Posterior lobe (neurohypophysis, pars nervosa)
    • oxytocin
    • vasopressin (anti-diuretic hormone)

Trophic hormones: TSH, ACTH, FSH and LH are grouped together on the slide as the trophic hormones, defined as hormones that “maintain the structure and function of another tissue”. They act on other endocrine glands, which distinguishes them from GH and PRL, which act more directly on non-endocrine targets.

Location, relations and imaging

  • The hypothalamus lies at the base of the diencephalon, immediately posterior to the optic chiasm, and is divided into two symmetrical halves by the third ventricle. The pituitary hangs below it, attached by the infundibular stalk.
  • On the hemisected brain, landmarks around the hypothalamus are the hypothalamic sulcus, anterior commissure, optic chiasm and mammillary bodies.
  • Sagittal MRI shows the same structures: hypothalamus, optic chiasm, infundibulum, anterior gland and posterior gland.
  • Coronal MRI shows the third ventricle, optic chiasm, infundibulum, pituitary gland, internal carotid artery (ICA) and cavernous sinus. The prompt on this slide is “What might happen with a pituitary tumour?”, pointing at the gland’s close relationship to the optic chiasm above and the cavernous sinus laterally.
  • In the skull base, the pituitary (hypophysis cerebri) sits directly above the sphenoidal sinus, related to the intercavernous sinus, basilar sinus/plexus, internal carotid artery and optic nerve, with the nasal conchae and meatuses anterior and inferior to it.
  • Transsphenoidal surgery exploits this: an endoscope and curette are passed through the nose, through the sphenoid sinus and sphenoid bone, to reach a pituitary tumour.

Warning

The transcript flags three points where the slide itself does not supply the meaning: on the sagittal skull-base drawing the labels “Tensor Palat” and “Levator Palat” are cut off at the image edge (presumably tensor palatini and levator palatini); on the repeated hypothalamo-pituitary vascular schematic a vertical red line is added down the centre with no labelled meaning given; and on the coronal MR anatomy slide the bottom-left colour anatomical drawing is entirely unlabelled, so its individual structures are not identified.

Blood supply and the hypophyseal portal system

  • Superior hypophyseal arteries (from the internal carotid and posterior communicating arteries) supply the median eminence and the infundibular stalk.
  • Inferior hypophyseal arteries (from the internal carotid artery) supply the posterior lobe.
  • The anterior lobe has no direct arterial supply. Its blood arrives second-hand, through the portal vessels, which is the anatomical basis for hypothalamic control of anterior pituitary secretion.
  • Route shown on the vascular schematic, in order: neuroendocrine cell bodies in the hypothalamus, beside the third ventricle, send axons down the stalk; the superior hypophyseal artery supplies a capillary bed in the median eminence; releasing factors are secreted into that bed; long portal vessels carry them down the stalk to the trophic hormone secreting cells of the anterior pituitary; those cells secrete their hormones into the circulation. The inferior hypophyseal artery supplies the posterior pituitary directly, and a short portal vessel also runs between the two lobes. Venous drainage carries hormone secretion from both lobes away into the systemic circulation.
  • Consequence for measurement: hypothalamic hormones can only be measured in the pituitary portal blood, because they are diluted beyond detection once they reach the systemic circulation.

Histology of the anterior pituitary

General architecture (a “typical” endocrine tissue):

  • Loosely arranged cords of cells.
  • Extensive fenestrated capillaries and sinusoids. A GH-GFP transgenic mouse (green = growth hormone cells, red dye = vasculature; LeTissier et al., Frontiers in Neuroendocrinology 2012) shows the dense capillary network running among the hormone-producing cells.

Traditional H&E classification (chromophils and chromophobes):

  • Acidophils (stain with eosin, which is acidic; pink): 40%. Hormones: GH, PRL.
  • Basophils (stain with haematoxylin, which is basic; purple): 10%. Hormones: TSH, ACTH, LH, FSH.
  • Chromophobes (little or no cytoplasmic stain): 50%, hormone content not identifiable by H&E.
  • Nucleic acids in the cell nucleus also attract haematoxylin, so nuclear staining is not a guide to cell type.
  • “Chromophobe” is a relative term, not all or nothing: staining reflects the amount of stored hormone.

Immunohistochemical classification (antibody to the hormone itself):

  • An anti-prolactin antibody stains lactotrophs brown, scattered among unstained cells.
  • somatotrophs, GH, 50% (acidophil group)
  • mammotrophs (lactotrophs), PRL, 20% (acidophil group)
  • corticotrophs, ACTH, 20% (grouped with the basophils, though intermediate in staining)
  • thyrotrophs, TSH, 5% (basophil group)
  • gonadotrophs, LH and FSH, 5% (basophil group)
  • Key reconciliation: all cells contain some secretory product, so immunohistochemistry accounts for 100% of cells. Chromophobes are presumably degranulated cells, which could mean either low synthesis/inactive cells or highly secretory, very active cells that have released their stores.

Pituitary adenoma

  • Pituitary adenomas are classified on H&E by the same staining categories as normal cells: acidophilic (pink), chromophobic (pale, minimal staining) and basophilic (purple).
  • Immunohistochemistry identifies the hormone actually produced: an acidophilic tumour staining widely positive for prolactin is a prolactinoma.

Hypothalamic control of the pituitary: the axes

The hypothalamus controls the pituitary through five axes (hypothalamic factor, then anterior pituitary hormone, then target tissue and its product):

  1. GnRH stimulates (+) FSH and LH, which act on the gonads for germ cell development; the gonads secrete hormones (female: estrogen and progesterone; male: testosterone).
  2. GHRH stimulates (+) and somatostatin (SS) inhibits (-) growth hormone, which acts on the liver and other cells to secrete IGF-I. GH also acts on many organs and tissues, affecting protein synthesis and carbohydrate and lipid metabolism.
  3. TRH stimulates (+) TSH, which acts on the thyroid, which secretes thyroxine and triiodothyronine.
  4. Dopamine (DA) inhibits (-) prolactin, which acts on the breasts for breast development and milk production, and in males may facilitate reproductive function.
  5. CRH stimulates (+) ACTH, which acts on the adrenal cortex, which secretes cortisol.

Feedback:

  • Long loop feedback: the hormone secreted by the peripheral target gland feeds back on the hypothalamus, as shown by the gonadal steroids feeding back to the GnRH level.
  • Short loop feedback: the pituitary hormone itself feeds back on the hypothalamus, as shown by prolactin feeding back on the dopamine neurons that inhibit it.
    (Figure source: Fig 11-17, Vander, 10th edn., 2006, p 365.)

Hypothalamic neurosecretory neurons

  • Hypothalamic neurons are arranged into anatomically named clusters, or nuclei. Those shown include the paraventricular nucleus, medial preoptic nucleus and arcuate nucleus, with the optic chiasm and mammillary body as landmarks.
  • Axons from these nuclei project down to the median eminence, which sits above the pituitary, where they release their products into the portal capillary bed for transport to the adenohypophysis. The neurohypophysis is supplied separately by the inferior hypophyseal artery, and veins drain both sides.
  • Tuberoinfundibular dopamine (TIDA) neurons: cell bodies in the arcuate nucleus with a dense axonal terminal field in the median eminence at the base of the third ventricle. This is the anatomical pathway by which dopamine reaches the anterior pituitary to inhibit prolactin (Brown et al., Endocrinology 2012; mouse).

Measuring hypothalamic and pituitary activity

Portal blood sampling:

  • Because hypothalamic hormones can only be measured in pituitary portal blood, portal and peripheral sampling must be compared. The recording shows small sharp GnRH pulses in portal blood and larger, broader LH pulses in peripheral blood, each LH pulse following a GnRH pulse, demonstrating that pulsatile GnRH release drives pulsatile LH release.

Nobel Prize in Physiology or Medicine, 1977 (Karolinska Institutet):

  • One half awarded jointly to Roger Guillemin and Andrew Schally for their discoveries concerning “the peptide hormone production of the brain”.
  • The other half to Rosalyn Yalow for “the development of radioimmunoassays of peptide hormones”.
  • The scale of the isolation work: Guillemin’s group collected more than 5 million sheep brains over several years and handled more than 50 tons of hypothalamic fragments; Schally’s group procured hundreds of thousands of hypothalami, with about a million pig hypothalami donated by Oscar Mayer & Co.

Modern approach, in vivo fibre photometry (Han et al., eLife 2025; “Multi-dimensional oscillatory activity of mouse GnRH neurons in vivo”, Han, Yeo, Kim, Zhou and Herbison):

  • Method: GnRH-cre mice are given AAV-GCaMP6s so that the genetically encoded calcium biosensor is expressed in GnRH neurons, and a fibre optic cannula is implanted bilaterally into the arcuate nucleus.
  • GCaMP mechanism: a GFP barrel carries calmodulin (CaM) and the M13 peptide; binding of Ca²⁺ causes a conformational change that increases GFP fluorescence, so fluorescence reports neuronal activity.
  • Recording setup: 490 nm and 405 nm LEDs feed an optical coupler/splitter, light travels down the fibre to the freely moving mouse in a behaviour box, and the returning signal goes to a photoreceiver and then acquisition software.
  • Result over 24 hours (11:00 to 11:00): intermittent large sharp transients up to about 130% ΔF/F occur throughout, during both the light period and the shaded 19:00 to 07:00 period, with no clear difference in pulse pattern between them.
  • Result over a 0 to 120 minute window: each large GnRH neuron activity transient (around 40 min and around 105 min) is followed a short time later by a rise and peak in peripheral LH, then a decline, showing that bursts of GnRH neuron activity precede and drive pulsatile LH secretion.
  • The same fibre photometry approach is applied to prolactin-sensitive neurons: activity of Prlr+ neurons in the medial preoptic area (MPOA) recorded in a freely moving mouse (Clarkson et al., Science Advances, 2026).

Warning

The transcript flags that on the final slide the panel beside the “1 z-score” and “5 s” scale bars contains no visible data trace, so no result is recorded for the Prlr+ MPOA neuron recording. This may be because the trace is part of an animation not captured in the static PDF.

Self-test

  1. Define a hormone as the lecture does, and state how an endocrine cell delivers it to its target.
  2. Explain where specificity comes from in endocrine signalling, and contrast this with the nervous system.
  3. Explain what is meant by calling endocrine glands “ductless”, and what the thermostat analogy conveys about their role.
  4. List the three anatomical categories of endocrine tissue, with the examples given for each.
  5. Explain which glands the module focuses on and what makes them “classical” endocrine glands.
  6. List the hormones secreted by each of the three lobes of the pituitary.
  7. Define a trophic hormone and state which anterior pituitary hormones were grouped as trophic, and why GH and PRL sit outside that group.
  8. Describe the route and instruments used in transsphenoidal pituitary surgery.
  9. Describe the position of the hypothalamus and how it is divided.
  10. List the structures labelled on the sagittal MRI of the hypothalamo-pituitary region.
  11. Predict what might happen with a pituitary tumour, based on the structures the coronal MR anatomy slide shows adjacent to the gland.
  12. State the three H&E categories of anterior pituitary cells with their proportions, the stain each takes up and the hormones attributed to each chromophil group.
  13. Explain why “chromophobe” is described as a relative term rather than an absolute one.
  14. List the five immunohistochemically defined anterior pituitary cell types with their hormone and proportion, and state which stain as acidophils and which as basophils.
  15. Explain how immunohistochemistry resolves the problem posed by the 50% of cells that appear as chromophobes on H&E.
  16. Describe two histological features that make the anterior pituitary a “typical” endocrine tissue, and the evidence given for the second.
  17. Distinguish the three H&E subtypes of pituitary adenoma, and describe how a prolactinoma is confirmed.
  18. Describe the arterial supply of the pituitary, naming each artery, its origin and the region it supplies.
  19. Explain the implication of the anterior lobe having no direct arterial supply, and describe in order the route a hypothalamic releasing factor takes to reach an anterior pituitary cell.
  20. For each of the five axes shown, name the hypothalamic factor, whether it stimulates or inhibits, the pituitary hormone, the target tissue and what that tissue secretes.
  21. Distinguish long loop from short loop feedback, using the examples given in the lecture.
  22. Name three hypothalamic nuclei shown on the neurosecretory neuron diagram and state where their axons project.
  23. Describe the tuberoinfundibular dopamine pathway and explain what it does.
  24. Explain why hypothalamic hormones can only be measured in portal blood, and describe the relationship demonstrated between GnRH and LH pulses.
  25. State who received the 1977 Nobel Prize in Physiology or Medicine in this field and for what work.
  26. Describe how GCaMP fibre photometry is used to record GnRH neuron activity in vivo, including the biosensor mechanism.
  27. State what the 24-hour photometry recording and the 120-minute GnRH/LH recording each demonstrated.
  28. Integrative: a patient has a prolactin-secreting pituitary adenoma. Using the anatomy and histology in this lecture, explain which cell type it arises from, how it would appear on H&E and on immunohistochemistry, which hypothalamic input normally restrains that cell type and by what route, and what local structures the expanding mass could compromise.

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