2-Histology of the Endocrine Organs

Secretory Ducts

Endocrine glands don’t have any of THESE

Paracrine Secretion

Localized secretion into interstitial fluid for action on nearby cells (ex. – somatostatin in pancreatic d-cells)

Autocrine Secretion

Cells produce molecules that act on themselves or cells of the same type (ex. – growth factors)

Juxtacrine Secretion

Substances anchored to cell membrane bind to receptors on adjacent cells (ex. – delta signaling in developing nervous system)

Endocrine Secretion

Substances released through interstitial space into bloodstream, communicates with far-away cells.

Hydrophilic and Hydrophobic Hormones

• The 2 types of endocrine hormones

• Made by glandular (epithelial) cells, use endocrine routes via binding to cell surfaces or intracellular receptors

Hydrophilic Hormones

• Peptides, glycoproteins, proteins, amino acids (ex.- neurotransmitters)

• Bind to receptor on cell surface

Hydrophobic Hormones

• Cholesterol derived (ex. - steroid hormones)

• Pass through plasma membrane & bind to receptor inside cell

Capilaries

The vascular system transports hormones to target cells/organs.

THIS is how hormones escape the vascular system.

Continuous

Fenestrated

Sinusoid (Discontinuous)

These are the 3 types of capillaries.

Continuous Capillary

Continuous basement membrane and endothelial layer

Allows diffusion of only small molecules & ions (ex. – lung & skeletal muscles)

Fenestrated Capillary

Continuous basement membrane; endothelial cells contain small “fenestrae”

Allows diffusion of larger molecules and proteins (ex. – glomerulus of kidneys; most endocrine glands)

Sinusoid (Discontinuous) Capillary

Discontinuous basement membrane; endothelial cells have large gaps between them (sinusoids)

Allows diffusion of large molecules, cells & cell fragments (ex. – spleen & red bone marrow)

Endocrine Organs

Endocrine glands with a large portion of cells with endocrine functions.

Ex. gonads (testes/ovaries).

Hypothalamus

Secretes hormones that regulate the pituitary gland (anterior and posterior).

Posterior Pituitary

Stores Oxytocin and ADH.

Anterior Pituitary

Secretes hormones that regulate other endocrine organs.

• Thyroid

• Adrenal Cortex

• Testes/Ovaries

• Mammary Glands

Parathyroid Glands

Pancreas

THESE 2 things that are NOT regulated by the hypothalamus/pituitary.

Pituitary (Hypophysis)

• Location:

  • Sella Turcica of sphenoid bone

• Connects to hypothalamus via infundibulum

• Stalk is easily damaged with head trauma

• Tumors:

  • Expand upward

  • Compress optic chiasm (peripheral blindness/tunnel vision)

Neuroectoderm

The embryonic origin of the posterior lobe of the pituitary gland.

Part of diencephalon.

Oral ectoderm

The embryonic origin of the anterior lobe of the pituitary gland.

Outpouching of oropharynx.

Neurohypophysis

• Histologically looks like brain tissue

• Contains axons and neurons, cell bodies in hypothalamus

• Parts:

  • Pars Nervosa

  • Infundibulum

  • Median Eminence

• AKA Posterior lobe of pituitary

Pars Nervosa

Largest part of neurohypophysis

Contains neurosecretory axons

Infundibulum

Part of neurohypophysis

Continuous with median eminence

Connects pituitary to hypothalamus

Median Eminence

Part of neurohypophysis

Contains hypothalamohypophyseal tracts

Adenohypophysis

• Histologically looks dark, epithelial tissue

• Ectodermal origin

• Contains functional secretory epithelial cells

• Parts:

  • Pars Distalis

  • Pars Intermedia

  • Pars Tuberalis

• AKA Anterior lobe of pituitary

Pars Distalis

Part of adenohypophysis

Largest part

Pars Intermedia

Part of adenohypophysis

Thin wall between anterior/posterior pituitary lobes

Pars Tuberalis

Part of adenohypophysis

Makes collar around infundibulum (part of neurohypophysis)

Anterior Lobe of Pituitary

Technically only THIS portion of the pituitary is an endocrine gland.

Pars Nervosa & Infundibulum

THESE are more acidophilic due to large numbers of axons & resemble CNS tissue (diencephalon origin)

Pars Distalis & Pars Tuberalis

THESE are basophilic due to large numbers of epithelial cells (oral ectoderm origin)

Pars Intermedia

THIS can contain a remnant of the embryonic hypophyseal pouch and/or colloid-filled cysts

Anterior Pituitary Gland

• Secretes tropic hormones

  • controlled by hypothalamus

• Regulates:

  • Mammary glands

  • Gonads

  • Thyroid

  • Body growth

Ant. Pit. Tropic Hormones

• GH

• PRL

• ACTH

• FSH

• LH

• TSH

Growth Hormone (somatotropin, GH)

Function:

• Stimulates liver & other organs to secrete insulin-like growth factor I (IGF-I), which stimulates division of progenitor cells located in growth plates & skeletal muscles, resulting in body growth.

Regulation by:

• Growth hormone-releasing hormone promotes secretion of GH by the pituitary

• Somatostatin inhibits secretion of GH

Prolactin (PRL)

Function:

• Promotes mammary gland development

• Initiates milk formation

• Stimulates secretion of casein, lactalbumin, lipids, and carbohydrates into the milk

Regulated by:

• Dopamine inhibits release of prolactin

Adrenocorticotropic Hormone (ACTH)

Function:

• Stimulates secretion of glucocorticoids & gonadocorticoids by the zona fasciculate & zona reticularis of the adrenal cortex

Regulated by:

• Corticotrophin-releasing hormone stimulates synthesis of ACTH

Follicle-Stimulating Hormone (FSH)

Function:

• Stimulates follicular development in the ovary & spermatogenesis in the testis

Regulated by:

• Gonadotropin-releasing hormone stimulates secretion of FSH

Luteinizing Hormone (LH)

Function:

• Regulates final maturation of ovarian follicle, ovulation, & corpus luteum formation

• Stimulates steroid secretion by follicle and corpus luteum

• In males, essential for androgen secretion by the Leydig (interstitial) cells of the testis

Regulated by:

• Gonadotropin-releasing hormone stimulates secretion of LH

Thyrotropic/Thyroid Stimulating Hormone (TSH)

Function:

• Stimulates growth of thyroid epithelial cells

• Stimulates production & release of thyroglobulin and thyroid hormones

Regulated by:

• Thyrotropin-releasing hormone stimulates release of TSH

• Somatostatin inhibits secretion of TSH

Classifications of ant. pit. cells in H & E

• Acidophils (pink)

• Basophils (blue)

• Chromophobes (no stain)

GH & PRL

In Ant. Pit., THESE secreting cells are acidophils.

ACTH, FSH, LH, & TSH

In Ant. Pit., THESE secreting cells are basophils.

Somatotropes

Secrete GH

Acidophils

Lactotropes

Secrete PRL

Acidophils

Thyrotropes

Secrete TSH

Basophils

Gonadotropes

Secrete LH & FSH

Basophils

Melanotropes

Secrete MSH

Chromaphobes

Corticotropes

Secrete ADH

Basophils

Oxytocin

ADH

2 things the post. pit. stores:

• _ in paraventricular nucleus

• _ in supraoptic nucleus

Store & Release Hypothalamic Secretions

The main function of the post. pit. gland is to_

Paraventricular & Supraoptic Nuclei

In the hypothalamus, THIS is where cell bodies of neurosecretory neurons can be found.

Posterior Lobe of the Pituitary

THIS is where secretions from the paraventricular and supraoptic nuclei and their axons are stored.

Pars Nervosa

Axons of the neurosecretory neurons pass through median eminence & infundibulum, they terminate at the fenestrated capillary beds found in THIS.

Oxytocin & ADH

THESE hypothalamic secretions are stored in the pars nervosa.

Oxytocin

Promotes contraction of smooth muscle, particularly uterus during labor & myoepithelial cells of the mammary gland during suckling (secreted by paraventricular nucleus)

ADH

Acts on kidneys, regulates water homeostasis and osmolarity of body fluids (secreted by supraoptic nucleus)

AKA vasopressin

Superior Hypophyseal Artery

The Ant. Pit. is supplied by THIS.

Hypothalamohypophyseal portal vv.

Superior hypophyseal a. → Primary capillary plexus → THIS→ Secondary capillary plexus → Systemic Circulation/Target Organs

Abundant blood supply

THIS allows the hypothalamus to regulate cells of the anterior pituitary.

It does so via connecting the pituitary to the hypothalamus.

Portal Veins (Capillaries/Capillary Beds)

THESE are inside the pituitary, brining hypothalamic hormones from the median eminence & infundibulum → pars distalis.

The leaky BBB in the pituitary allows things to pass through these.

Inferior Hypophyseal Artery

The Post. Pit. is supplied with blood via THIS.

Capillary beds of post. pit.

Inferior hypophyseal a. → THIS → Systemic Circulation

Hypothalamic Hormones

THESE reach the post. pit. via neurons, where they are then stored.

Hypothalamic-Hypophysiotropic Hormones

THESE reach the ant. pit. via the hypothalamohypophyseal portal system.

They regulate synthesis & secretion of ant. pit. hormones.

Anterior Pituitary Adenomas

Sx: (compression related)

• Headaches

• Visual field defects

• Increase in skin tags from increased GH production

Common:

• Lactotropes

• Somatropes

Cortex & Medulla

THESE are 2 parts of the adrenal glands that are distinct histologically, functionally, and developmentally.

Cortex

External portion of adrenal gland

Distinct histological & functional layers

Steroid secretion

Originates from mesoderm

90% of glandular tissue

Medulla

Internal portion of adrenal gland

Secretion of catecholamines (epinephrine & norepinephrine)

Originates from neural crest

10% of glandular tissue

Capsule Histology

• CT covering

• Lightly staining

• Few cells

Adrenal

Cortex Histology

• layered appearance

• cells in columns

• central nuclei

• lipid droplets in cytoplasm

Adrenal

Medulla Histology

• cells clumped together

• abundant reticular fibers

Adrenal

Adrenal Cortex Layers (superficial to deep)

Zona Glomerulosa

Zona Fasiculata

Zona Reticularis

Zona Glomerulosa

• Function:

  • Secretion of mineralocorticoids

    esp. aldosterone, which regulates salt balance

• Histo:

  • Closely-packed cords of columnar or pyramidal cells

  • Moderately-staining cytoplasm

Zona Fasiculata

• Function:

  • Secretion of glucorticoids: cortisol & corticosterone

  • Regulation of glucose & fatty acid metabolism

• Histo:

  • Long cords of large cells separated by capillaries

  • Lightly to non-staining cytoplasm due to abundant lipid droplets

Zona Reticularis

• Function:

  • Secretion of androgens esp. dehyroepiandosterone (DHEA)

  • Secretes very small amounts of glucorticoids

• Histo:

  • Small cells arranged in oval clusters or cords separated by capillaries

  • Less lipid content than in zona fasiculata

Adrenal Medulla Histology

• Ovoid clusters of cells

• Pale-staining

• No columns

• Chromaffin cells!

Chromaffin Cells

• Main cell type in adrenal medulla

• Modified sympathetic neurons

• Can secrete Epi or NE!

Central Adrenomedullary Vein

• Abundant smooth muscle in tunica media! (unique)

• Synchronous contraction of the sm in THIS decreases adrenal gland volume (increasing Epi and NE release)

Pancreatic Islets (of Langerhans)

• Endocrine epithelioid tissue

• Surrounded by exocrine tissue (acinar cells/ducts)

• Thin layer reticular tissue separates THESE from the surrounding cells

• Shares embryo origin with acinar cells = Endoderm

Islets of Langerhans Histology

• ovoid clusters of cells

• pale-staining (compared to surrounding acinar cells)

• small secretory granules in cytoplasm

• fenestrated capillaries around the cells!

Pancreatic Islets Cell Types

• Alpha (secrete glucagon) (peripheral)

• Beta (secrete insulin) (central)

• Delta (secrete somatostatin) (sparse)

• PP or F (secrete pancreatic polypeptide) (very sparse)

Glucagon

Insulin

Immunofluorescence of islets!

The cells in red are THIS.

The cells in green are THIS.

Thyroid

• origin is endoderm (from inferior pharynx)

  • Migrates inferiorly

• made of 2 lobes, connected via isthmus

• thyroglossal duct may remain (connects THIS to foramen cecum of tongue)

  • if it atrophies, it leaves pyramidal lobe

Thyroid Follicle Histology

• Ovoid/spherical

• Acidophilic

• Surrounded by simple cuboidal epithelium

• Capillaries surround

Thyroid Lumen Histology

• Pale-staining

• Acidophilic

• Filled with colloid (thyroglobulin storage)

Thyroid Follicular Epithelium

• 2 cell types:

  • Follicular (principle) cells

  • Parafollicular cells

Stores its own product

The thyroid gland is unique in terms of endocrine glands because it _ in an inactive form (thyroglobulin).

Follicular cells (thyrocytes)

Location/Features:

• Directly surrounding lumen; spherical nuclei & cuboidal to columnar shape

Location:

• Thyroglobulin, T3 and T4 secretion

Thyroid

C Cells/Parafollicular Cells

Location/Features:

• In basal lamina of follicular cells

• Pale staining and flat to cuboidal in shape

• Do NOT contact colloid

• Small clusters

• Fenestrated capillaries between follicles

Function:

• Calcitonin secretion

Thyroid

Thyroid Lumen

Location/Features:

• Large & central in follicles; contains colloid, principally comprised of thyroglobulin

Function:

• Storage for thyroglobulin

Thyroid Histological Characteristics (2 things)

Inactive Follicle:

• Cuboidal or squamous cells

• No resorption of colloid

• Thyroglobulin is being stored

Active Follicle:

• Columnar cells

• Resorption of colloid

• Thyroglobulin being converted to thyroid hormones

Follicular cells can be either secretory or absorptive depending on whether they are active or inactive

Thyroid: Graves Disease

• Hyperthyroidism

  • Hypertrophy of follicular cells

• Follicular cells become columnar

  • Colloid areas in contact with apical portions of follicular cells decrease

• High circulating T3 and T4

  • Negative feedback on Ant. Pit. = Normal TSH Levels

Thyroid: Hashimoto’s Disase

• Autoimmune thyroiditis

• Extensive lymphocyte infiltrate

• Atrophy of follicles with reduced or absent colloid

• Usually T3 & T4 are low, prompting increase in TSH release from pituitary (increased TSH)

Parathyroid Glands

• Originate from 3rd & 4th pharyngeal pouches (endoderm)

• 4 lentil-sized glands on posterior surface of lateral thyroid lobes (2 pairs, superior/inferior)

• Each is surrounded by a CT capsule

Parathyroid Cell Types

• Principal Cells

• Oxyphil Cells

Principal Cells

• Small polygonal cells

• Round basophilic nuclei

• Most common cell type

• Secrete PTH

Parathyroid

Oxyphil Cells

• Pale-staining

• occur more frequently in older individuals

• typically lighter-staining than principal cells

• secrete low levels of PTH

Parathyroid

CT in Parathyroid

• Forms a capsule around the entire gland as well as septa between cell clusters

• fibroblast nuclei may be visible

Can see fenestrated capillaries in parathyroid histo too!