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endocrine system histology overview pituitary thyroid parathyroid adrenal pancreas histology

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H&E histology endocrine glands pituitary thyroid parathyroid adrenal cortex medulla labeled

Endocrine system: step-by-step strict histology guide

Your PDF mainly covers hypothalamus-pituitary, thyroid, parathyroid, and adrenal glands. It lists pancreas and gonads but does not explain their histology.

1. General histology of endocrine glands

Definition

Endocrine glands are ductless glands. Their secretory products, called hormones, enter:

Interstitial connective tissue
Fenestrated capillaries
Bloodstream
Target cells that have specific receptors

General microscopic features

Most endocrine glands show:

Cells arranged in cords, clumps, nests, or follicles
Very rich vascular supply
Many fenestrated capillaries/sinusoids
Little connective tissue stroma
No excretory ducts

Cell organization

There are two basic histological patterns:

Pattern	Examples	Microscopic appearance
Follicular	Thyroid	Epithelial cells surround a lumen containing colloid
Cords/clumps	Pituitary, parathyroid, adrenal, pancreatic islets	Endocrine cells lie close to capillaries

Hormone types

Peptide/protein hormones: stored in granules and released by exocytosis. Example: insulin, GH, PTH.
Steroid hormones: synthesized on demand; cells have abundant smooth ER and lipid droplets. Example: cortisol, aldosterone.
Amino-acid derivatives: example: catecholamines and thyroid hormones.

2. Hypothalamus

The hypothalamus is nervous tissue with neurosecretory neurons. It connects the nervous system with the endocrine system.

Histology

Contains large neurosecretory cells, especially in the:
- Supraoptic nucleus
- Paraventricular nucleus
These cells synthesize hormones and transport them down their axons.
Axons travel through the infundibulum to the posterior pituitary.

Main products

Hormones synthesized in hypothalamic nuclei and released from posterior pituitary

ADH/vasopressin, predominantly synthesized in the supraoptic nucleus
Oxytocin, predominantly synthesized in the paraventricular nucleus

Releasing/inhibiting hormones for anterior pituitary

TRH
CRH
GnRH
GHRH
Somatostatin
Dopamine, which inhibits prolactin secretion

3. Pituitary gland or hypophysis

Location

Lies in the sella turcica of the sphenoid bone.
Connected to hypothalamus by the infundibulum.
Covered by a thin connective-tissue capsule.

Two major parts

Part	Name	Tissue type
Anterior pituitary	Adenohypophysis	Glandular epithelial tissue
Posterior pituitary	Neurohypophysis	Nervous tissue

4. Adenohypophysis: anterior pituitary

The adenohypophysis consists of:

Pars distalis: largest part, about 75%
Pars intermedia
Pars tuberalis

A. Pars distalis

Microscopic appearance

Cells are arranged in cords and nests.
Cords are separated by abundant fenestrated sinusoidal capillaries.
On H&E, three staining groups are recognized:

Cell group	Staining	Appearance
Acidophils	Eosinophilic	Pink/red cytoplasm
Basophils	Basophilic	Blue/purple cytoplasm
Chromophobes	Pale	Weakly stained cells

B. Acidophils

Histological appearance

Strongly eosinophilic, pink cytoplasmic granules.
Usually smaller in number than chromophobes plus basophils together, though proportions vary with stain and physiological state.

Types and hormones

Cell	Hormone	Major action
Somatotroph	Growth hormone, GH	Body growth and metabolism
Lactotroph or mammotroph	Prolactin, PRL	Mammary-gland development and milk production

Important identification point: Acidophils are more numerous during states with increased somatotroph or lactotroph activity. Lactotrophs enlarge and increase in pregnancy and lactation.

C. Basophils

Histological appearance

Cytoplasm stains blue-purple with hematoxylin or other basic stains.
Often less obvious in routine H&E than acidophils.

Types and hormones

Cell	Hormone
Thyrotroph	TSH
Gonadotroph	FSH and LH
Corticotroph	ACTH and related POMC-derived peptides

D. Chromophobes

Histological appearance

Pale, poorly stained cytoplasm.
Often seen as small cells with a visible nucleus and little cytoplasmic staining.

Meaning

Chromophobes may be:

Degranulated acidophils or basophils after secretion
Resting endocrine cells
Progenitor/stem cells

E. Pars intermedia

Histological appearance

Thin zone between pars distalis and pars nervosa.
May contain colloid-filled cysts or follicles, remnants of Rathke pouch.
More prominent in some animals than in adult humans.

5. Hypothalamic-hypophyseal portal system

This system controls the adenohypophysis.

Sequence

Hypothalamic neurons release releasing or inhibiting hormones into the primary capillary plexus at the median eminence.
Hormones enter hypophyseal portal veins.
Portal veins carry them to the anterior pituitary.
They enter the secondary capillary plexus in pars distalis.
They regulate secretion by anterior pituitary endocrine cells.

Histology point: The anterior pituitary receives regulatory hypothalamic hormones through portal blood vessels, not through direct axonal endings.

6. Neurohypophysis: posterior pituitary

The neurohypophysis includes:

Pars nervosa
Infundibulum
Hypothalamic neurosecretory axons

Histological appearance

Pale-staining tissue compared with the pars distalis.
Contains many unmyelinated axons.
Contains capillaries.
Contains supporting glial cells called pituicytes.
Pituicyte nuclei are oval or round and are often the most visible cellular feature.

Herring bodies

Dilated accumulations of neurosecretory axons.
Store secretory granules containing ADH or oxytocin.
Seen as eosinophilic swellings in the posterior lobe.

Important correction

The posterior pituitary does not synthesize ADH or oxytocin. It stores and releases hormones synthesized by neurons in the hypothalamus.

7. Thyroid gland

Basic structural unit: thyroid follicle

The thyroid is the only classic endocrine gland in which hormone is stored extracellularly.

Histological structure

Each follicle has:

A central lumen
Colloid inside the lumen
A lining of simple cuboidal follicular cells
A basement membrane
Rich capillary network outside the follicle

Colloid

Homogeneous, eosinophilic material in H&E sections.
Contains thyroglobulin.
Thyroid hormone precursor is stored within the colloid.

Follicular cells or thyrocytes

Function

Follicular cells synthesize:

T4, thyroxine or tetraiodothyronine
T3, triiodothyronine

Activity-dependent appearance

Functional state	Epithelial appearance	Follicle/colloid
Low activity	Low cuboidal to squamous cells	Large follicles with abundant colloid
Normal activity	Cuboidal cells	Moderate colloid
High activity	Tall cuboidal to columnar cells	Reduced colloid with scalloped edges

Scalloping of colloid

During active thyroid hormone release, the edge of colloid may look irregular due to resorption vacuoles. This is called scalloping.

Parafollicular cells or C cells

Location

Located at the basal part of follicular epithelium.
Found within the follicular basement membrane.
Do not contact the follicular lumen.
Usually pale and difficult to recognize on routine H&E.

Function

Produce calcitonin.
Calcitonin reduces blood calcium mainly by reducing osteoclast-mediated bone resorption.

Slide identification: thyroid

Look for:

Numerous round or oval follicles
Pink homogeneous colloid
Cuboidal follicular epithelium
Very vascular interfollicular connective tissue

8. Parathyroid glands

Location

Usually four small glands on the posterior surface of the thyroid.

General histology

Thin connective-tissue capsule.
Cells arranged in cords, clusters, and anastomosing strands.
Numerous capillaries between cell cords.
Adipose tissue increases with age.

Main cells

A. Chief cells

Histological appearance

Most numerous cells.
Small, polygonal cells.
Round central nuclei.
Pale or lightly eosinophilic cytoplasm.

Function

Produce parathyroid hormone, PTH.

PTH action

PTH raises serum calcium by actions on:

Bone
Kidney
Indirectly, intestine through activation of vitamin D

B. Oxyphil cells

Histological appearance

Larger than chief cells.
Strongly eosinophilic cytoplasm.
Small, dense nucleus.
Often in clusters.
Increase after puberty and with age.
Cytoplasm contains many mitochondria.

Function

Their exact physiological role is less clear than that of chief cells, but they can express PTH-related activity.

Slide identification: parathyroid

Look for:

Solid cellular tissue, not follicles
Cords of chief cells
Eosinophilic oxyphil-cell groups
Rich capillary network
Adipocytes, especially in adult glands

Distinguishing thyroid from parathyroid

Feature	Thyroid	Parathyroid
Main pattern	Follicles	Cords/clusters
Lumen	Present, with colloid	No follicles or colloid
Main cell	Follicular cell	Chief cell
Key secretion	T3/T4	PTH

9. Adrenal or suprarenal gland

General structure

The adrenal gland has:

Outer connective-tissue capsule
Cortex
Medulla

The cortex and medulla are histologically and functionally distinct.

Region	Embryological origin	Major hormones
Cortex	Mesoderm	Steroid hormones
Medulla	Neural crest	Catecholamines

10. Adrenal cortex

The cortex has three zones. From outside to inside, remember:

GFR: Glomerulosa, Fasciculata, Reticularis

Zone	Arrangement	Main product
Zona glomerulosa	Rounded clusters/arches	Aldosterone
Zona fasciculata	Straight radial cords	Cortisol
Zona reticularis	Anastomosing network	Androgens

A. Zona glomerulosa

Histology

Immediately beneath capsule.
Thin outer layer.
Small cells arranged in rounded groups, arches, or curved cords.
Cytoplasm is less vacuolated than zona fasciculata.

Hormone

Produces mineralocorticoids, mainly aldosterone.

Regulation

Mainly controlled by the renin-angiotensin-aldosterone system and serum potassium.
ACTH has only a minor or transient influence, not the principal long-term control.

B. Zona fasciculata

Histology

Widest cortical zone.
Cells arranged in long straight radial cords, usually one or two cells thick.
Cords are separated by sinusoidal capillaries.
Cells have pale, foamy cytoplasm due to lipid droplets removed during routine tissue processing.
These cells are called spongiocytes.

Hormones

Produces glucocorticoids, mainly cortisol.

Regulation

Controlled primarily by ACTH.

C. Zona reticularis

Histology

Inner cortical zone adjacent to medulla.
Cells form irregular, branching, anastomosing cords.
Smaller, darker cells than in zona fasciculata.
Lipofuscin pigment may give cytoplasm a brownish-yellow appearance.

Hormones

Produces adrenal androgens, especially DHEA and androstenedione.
Also contributes small amounts of glucocorticoids.

Regulation

ACTH-dependent.

Why cortical cells appear pale

Adrenal cortical cells produce steroid hormones. Therefore, they have:

Abundant smooth ER
Numerous mitochondria with tubular cristae
Lipid droplets

Lipid is dissolved during ordinary paraffin processing, creating a vacuolated appearance.

11. Adrenal medulla

Histology

Central part of gland.
Cells occur in nests, cords, and clumps.
Numerous wide capillaries and venules.
Large central medullary vein may be visible.
The medulla receives blood that has passed through the cortex.

Chromaffin cells

Modified postganglionic sympathetic neurons.
Called chromaffin cells because their granules react with chromium salts.
Cytoplasm may be pale basophilic on H&E.

Main cell types

Cell	Main secretion
Epinephrine-producing chromaffin cells	Epinephrine/adrenaline
Norepinephrine-producing chromaffin cells	Norepinephrine/noradrenaline

Histological and functional point

Cortical blood containing glucocorticoids reaches the medulla. Cortisol promotes conversion of norepinephrine to epinephrine through phenylethanolamine N-methyltransferase.

12. Practical slide identification from your PDF

Slide 1: Pituitary, low power

Identify:

Pars distalis: darker, highly cellular anterior lobe
Pars intermedia: narrow zone, possibly with cystic spaces
Pars nervosa: paler posterior lobe

Slide 2: Pars distalis, high power

Identify:

Sinusoidal capillaries
Acidophils: pink cells
Basophils: blue-purple cells
Chromophobes: pale cells

Slide 3: Adrenal gland, low power

Draw and label:

Capsule
Zona glomerulosa
Zona fasciculata
Zona reticularis
Medulla
Central vein, if visible

Slide 4: Adrenal gland, higher power

Identify cortical zones by arrangement:

Zona glomerulosa: rounded groups
Zona fasciculata: straight pale radial cords
Zona reticularis: dark branching network

Slide 5: Thyroid with parathyroid

Label:

Thyroid follicles
Colloid
Follicular cells
Interfollicular capillaries
Parafollicular cells, if visible
Parathyroid tissue
Chief cells
Oxyphil cells, if present
Capsule/septa

13. High-yield one-line revision

Pituitary anterior lobe: cords of acidophils, basophils, and chromophobes around sinusoids.
Pituitary posterior lobe: unmyelinated axons, pituicytes, and Herring bodies.
Thyroid: follicles filled with colloid and lined by follicular cells.
Parathyroid: chief-cell cords with oxyphil cells and adipocytes, but no follicles.
Adrenal cortex: GFR, glomerulosa makes aldosterone, fasciculata makes cortisol, reticularis makes androgens.
Adrenal medulla: chromaffin cells make catecholamines.

A useful external histology reference is the SIU endocrine histology guide, which confirms the core cord-versus-follicle organization of the endocrine organs.

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