Functional and microanatomy of endocrine glands

what are endocrine organs

Preserve Homeostasis

Ductless organs, derived from the 3 germ layers

Secrete hormones (chemical messengers) directly to blood, lymph or tissue fluid

Great communicator, feedback control mechanisms

Endocrine and nervous systems are integrated (neurohormonal system)

hypothalamus

Coordinates the activity of the pituitary gland through the secretion of peptides and amines

Located at the ventral part of the brain

Regulator of endocrine and nervous system

Regulates temperature, thirst, hunger, sexual behaviour, blood volume etc. maintain homeostasis

Hormones produced are of two types=releasing and inhibitory

nuclei in hypothalamus

Clusters of neurons in the Hypothalamus

Sends axons to the posterior pituitary

Secretes releasing hormones to the anterior pituitary

Intergrade and regulate vital body functions

pituitary gland (hypophysis)

Structurally and functionally integrated with hypothalamus

Suspended below the hypothalamus by a narrow stalk (infundibular or hypophysial stalk)

Lies within bony cavity called the Hypophysial fossa (Sella Turcica) of the sphenoid bone

Small , oval gland

Between rostrally optic chiasm and the caudally mammillary bodies

Controlled by the CNS and feedback from target organs

transverse and cross section of pituitary gland

the two anatomically and fuctionaly distinct lobes of pituitary gland

Adenohypophysis (Anterior lobe)- (oral ectoderm)

Outgrowth of pharynx (root of mouth)- adeno (glandular)

• three parts (or two in some)

Neurohypophysis (Posterior lobe) –(neural ectoderm)

Diencephalon- outgrowth of the brain hence consist of

nervous tissue from hypothalamus

• three parts

development of pituitary gland

During embryonic development, the roof of the mouth bulges upwards

(invaginates) to form, structure known as Rathke’s pouch,

Cells from posterior gland migrate down from brain

Rathke’s pouch, also known as the hypophyseal diverticulum, is an ectodermal outpouching of the stomodeum

Pouch normally closes early in fetal development

main parts of pituitary gland

1, Adenohypophysis

2, Pars Intermedia

3, Neurohypophysis

4, Hypophysial stalk

5, Recess of third ventricle

adenohypophysis

Adenohypophysis (anterior lobe)

• Pars distalis (1)

• Pars intermedia* (2)

• Pars tuberalis (forms the stalk)

*Pars intermedia is rudimentary in humans, lacking in birds

adenohypophysis pars distalis

Largest part of pituitary gland- many cell types

–Chromophils and Chromophobes

Chromophils (Acidophils and Basophils)

• Acidophils (1) Cells that contain the polypeptide hormones-will stain red or orange

• Somatotrophs

• Growth hormone (GH)

• somatotropin

• Lactotrophs

• Prolactin (PRL)

Basophils in pars distalis of adenohypophysis of pituitary gland

• Basophils (2) Cells that contain the glycoprotein hormones-will stain bluish color

Thyrotrophs

• Thyroid stimulating hormone (TSH)

Gonadotrophs

• Follicle stimulating hormone (FSH)

• Luteinizing hormone (LH)

Corticotrophs

• Adrenocorticotropic hormone (ACTH)

Chromophobes in pars distalis of adenohypophysis of pituitary gland

3

• Chromophobes have cytoplasm that stains

very poorly

• • ? Postsecretroy acidophils and basophils

• • ? Stem cell

adenohypophysis - pars intermedia

Closely associated with pars nervosa

This part of the pituitary shows variation in size among species

Pars intermedia is a thin zone of basophilic cells (8)

Primary function is color change regulation

• Melanotrophs

• α-melanocyte stimulating hormone (α-MSH)

adenohypophysis - pars tuberalis

Provides scaffold for the capillary network of the hypophyseal portal system

Capillaries in this gland are fenestrated, to enable passage of hormones from the secretory cells into the bloodstream

Cells form folding sheets (folds) and occasional cysts

neurohypophysis

Median eminence (base of the hypothalamus)

Infundibular stalk (4)- nerve tract

Pars nervosa (3) bulk of the neurohypophysis

Neurohypophysis has a lumen (5) that's continuous with the lumen of the brain's third ventricle

neurohypophysis cont

Neuron cell bodies clustered in the hypothalamus (nuclei)

Send axons to the neurohypophysis

• Axons of hypothalamic neurons (unmyelinated)

• Central gliocytes (pituicytes-neuroglial cells)

• No neuron cell bodies

• Secretory vesicles along the axons = Herring bodies

• Oxytocin and Antidiuretic hormone (ADH) = vasopressin - promotes fluid reabsorption by kidneys (ADH)

histology of neurohypophysis

9 pars nervosa

8 pars intermedia

3 cavity of Rathke's pouch

2 blood vessel

7 pars distalis

anterior pituitary hormones

Hypothalamus produces peptides and amines that influence the pituitary gland to produce tropic hormones* e.g. corticotropin which in turn influences production of cortisol by peripheral target tissues

They are produced by neurosecretory cells in the several hypothalamic nuclei

Most tropic hormones are produced and secreted by the anterior pituitary gland

Adenohypophysis produces several hormones designated by acronyms as growth hormone (GH), follicle-stimulating hormone (FSH)

*trope, turning-because they turn on endocrine glands or support

hypophysial portal system

Blood vessels in the brain that connects the hypothalamus with the adenohypophysis

It begins at the base of the hypothalamus

Arterial blood reaching hypothalamus branches to capillaries

Here joins venous blood and makes small portal vein that pass through the stalk into the adenohypophysis

Here they branch to anterior capillaries which in turn drain into the venous system

Adenohypophysis

1- Neurohormones from the hypothalamus are released to the Hypothalamic-pituitary portal system

2-Neurohormones bind to receptors on endocrine cells

3- Regulate their hormonal secretion (releasing or inhibiting)

HYPOTHALAMIC -HYPOPHYSIAL TRACT

Neurohypophysis

Neurohypophysis has cell bodies that originate in the hypothalamus

Secretes two neurohormones -ADH and oxytocin

These neurohormones are placed in vesicles and transported and stored each terminal stores either vasopressin or oxytocin

Hormones conveyed along axons and released into neurohypophyseal capillary bed

neurohypophysis

Oxytocin: Stimulates uterine contractions, milk let-down (hormone of love!)

Anti-diuretic hormone(ADH) or Vasopressin: Stimulates vasoconstriction and

promotes fluid reabsorption by the kidneys; constricts vessels to raise blood

pressure (arterioles)

Synthesized in the hypothalamus but released into the bloodstream in the pars

nervosa

Neurohypophysis is connected to hypothalamus by neural pathway, whereas adenohypophysis is connected by a vascular link

Unlike the neurohypophysis, which releases hormones that are synthesized by the hypothalamus, adenohypophysis itself synthesizes the hormones that it releases into the blood

Neurohypophysis of the pituitary does not have a portal system. Neurohormones are deposited directly into the capillaries

epiphysis cerebri (pineal gland)

Part of the epithalamus, located mid brain

Small organ, shaped like a pinecone

Attached to the caudal end of the roof of the third ventricle and directly before the rostral colliculi

Only endocrine gland directly influenced by the external environment via the retina

Secretes the hormone melatonin (derived from aa tryptophan)

A vestigial eye (Many fish, and amphibians have a median pineal eye and/or parietal eye/third eye)

In lower vertebrates, the pineal gland is directly photosensitive

histology of epiphysis (pineal gland)

Pineal gland contains pinealocytes (12), astrocytes (3)*

Pinealocytes are responsible for secreting melatonin

Pinealocytes are arranged like cords, follicles

Human pineal gland contains characteristic, extracellular concretions called corpora arenacea (brain sand)

role of pineal gland

Secretes a hormone called Melatonin- is the hormone of darkness

Melatonin is important for regulating biological rhythms

Circadian rhythms & season effects on breeding, sleep etc. also

innate immune response

Master biological clock that serves as the pacemaker for circadian

rhythms is located in the hypothalamic suprachiasmatic nucleus

Its rhythm controls the rhythm of melatonin secretion by the pineal

gland by a polysynaptic pathway

retiono-pineal pathway

Light enters the retina

Suprachiasmatic nuclei

Paraventricular nuclei

Intermediolateral cell column of the spinal cord

Cervical ganglia

Pineal gland

Your eyes detect light, which sends signals through a specific pathway in your brain that ultimately controls melatonin production in the pineal gland. This is how light regulates your sleep-wake cycle.

The Pathway Step-by-Step:

1. Light enters your eye and hits the retina, where special cells detect it

2. Suprachiasmatic nucleus (SCN) - This is your brain's "master clock" located just above where the optic nerves cross. It receives the light information and acts as the central timekeeper for your body's circadian rhythms

3. Paraventricular nucleus (PVN) - The SCN sends signals here, which acts as a relay station

4. Spinal cord - Specifically the intermediolateral cell column, which carries the signal down from your brain

5. Superior cervical ganglion (SCG) - A cluster of nerve cells in your neck that receives the signal from the spinal cord

6. Pineal gland - Finally, the signal reaches this small gland deep in your brain, which produces and releases melatonin