the endocrine system

Kristina C. Erasmo, M.D.

System of glands that involve the release of extracellular signaling molecules known as hormones

Do not possess any excretory ducts

Integration and control system of the body

Substance that is carried by the blood to target organs/tissues whose cells have the appropriate receptors for the substance

Function as “chemical signals”

Hypothalamus Pituitary gland Pineal gland Thyroid gland Parathyroid gland Adrenal gland Pancreas

At base of brain, behind the optic chiasm

Forms the floor and part of the wall of the 3rd ventricle

Part of the diencephalon

Functions:o Control numerous bodily functions

o Thirst, hunger, satietyo Temperatureo Sexual behavioro Circadian rhythms

o Produces several hormoneso Oxytocino Antidiuretic hormone o Neurohormones

The cell bodies of the neurons form numerous aggregations (nuclei)

Supraoptic nucleuso Above and lateral to optic chiasm

Paraventricular nucleuso In the lateral wall of 3rd ventricle

Oxytocino Stimulates contractions of the uterus and ejection of

milk Vasopressin (antidiuretic hormone/ADH)

o Regulates water retention (kidneys)o ↑ urine osmolarity (↑ concentration), ↓ urine excretion

Neurohormones (hypophysiotropic hormones) – secretory neurons

Ovoid body attached to inferior surface of hypothalamus

Lodged in the hypophyseal fossa of the sella turcica of the sphenoid bone

Neurohypophysis (posterior pituitary)o From neural ectoderm

Adenohypophysis (anterior pituitary)o From oral ectoderm

Greater part is formed by the axons of the secretory neurons whose cell bodies are in the SVN and PVN of hypothalamus

Proximal portions of these axons form hypothalamo-hypophyseal tract

Distal portions form bulk of the posterior lobe of pituitary

Not a gland Stores and secretes oxytocin and ADH

(“posterior pituitary hormones”) 2 parts:

o Pituitary stalk (infundibulum, infundibular stem, infundibular stalk, hypophyseal stalk)o Attached to the hypothalamus

o Pars nervosa (posterior lobe, infundibular process)o Expanded inferior continuation of the pituitary stalk

Produces “anterior pituitary hormones”o Somatotropin, prolactin, thyrotropin, corticotropin,

FSH, LH 3 parts:

o Pars distalis (anterior lobe)o Pars tuberalis (pars infundibularis)o Pars intermedia (intermediate lobe)

Forms a sleeve around the pituitary stalk of neurohypophysis

Where vessels of hypophyseoportal system and arteries that supply anterior and posterior lobe run through

Thin, poorly developed region of adenohypophysis

Contain secretory granuleso Melanocyte-stimulating

hormone (MSH)o β-endorphin

Comprises 70% of pituitary gland

Nearly all hormones produced by pituitary gland come from this lobe (“anterior pituitary hormones”)

Consists of anastomosing and irregularly arranged cords and clusters of secretory epithelial cells that surround fenestrated capillaries

2 types of secretory cells:o Chromophils

o Acidophils (alpha cells)

o Basophils (beta cells)o Chromophobes

65 % of epithelial secretory cells in pars distalis Consists of follicular cells and undifferentiated

stem cells

35 % of epithelial secretory cells of pars distalis Acidophils

o Somatotrophso Mammotrophs

Basophilso Thyrotrophso Corticotrophso Gonadotrophs

Secretes Target Organ

Acidophils

Somatotrophs Most abundant (50%)

Growth hormone (somatotropin)

Non-specific

Mammotrophs 20% Prolactin Mammary glands

Basophils

Thyrotrophs 5% TSH (thyroid-stimulating hormone, thyrotropin)

Thyroid gland

Corticotrophs 20% ACTH (adrenocorticotropic hormone, corticotropin)

Adrenal cortex

Gonadotrophs 5% Gonadotropins – luteinizing hormone and follicle-stimulating hormone

LH: Ovary, Leydig cellsFSH: Ovary, Sertoli cells

Hypothalamic control over anterior gland of pituitary glando Corticotropin-releasing hormone (CRH)o Thyrotropin-releasing hormone (TRH)o Growth hormone-releasing hormone (GRH)o Growth hormone-inhibiting hormone (GIH,

somatostatin)o Gonadotropin-releasing hormone (GnRH)o Prolacting-inhibiting hormone (PIH)

Small, conical structure Base is attached to

posterior portion of the roof of the 3rd ventricle of the brain by two short stalks

Well-developed in children

Starts to involute at puberty

Enveloped by thin CT capsule derived from the pia mater

CT septa arise from the capsule and divide the organ into irregular lobules

Lobules contain cells arranged in short cords or clusters surrounded by fenestrated capillaries

CT septa contain blood vessels and unmyelinated nerve fibers

Cells:o Pinealocytes (95 %)

o Modified neuronso Produce melatonin

o Glial cells (Interstitial cells)

Brain sand (Psammoma bodies)o Calcified bodies with a concentric lamellar structureo Increase in number with age

Largest endocrine gland Unpaired gland that lies

on the anterior aspect of the neck

Left and right lateral lobes Isthmus – horizontal

bridge of glandular tissue which connects both lobes

Enclosed by two capsules

CT septa divide the organ into lobules

CT infiltrated with lymphocytes and some lymphoid nodules

Each lobule contains follicles

Follicleso Irregular spherical,

cystic structureso Lined by simple

cuboidal epitheliumo Cavity contains colloid

(homogenous, gel-like material)

Epithelial cells:o Follicular cells (principal cells)

o Majorityo Synthesize thyroglobulin

o Parafollicular cells (C or clear cells)o 0.1 %o Synthesize calcitonin – inhibits bone resorption (directly

suppressing osteoclasts), thus lowering blood calcium levels

Two pairs (superior and inferior), yellowish-brown, tiny ovoid bodies

Attached to the posterior surface of the lateral lobes of the thyroid gland

PARATHYROID GLAND

Chief cells (principal cells)o Majorityo Secrete parathyroid

hormone (PTH) Oxyphil cells (acidophil

cells)o Non-secretoryo Unknown function

Paired, flat, pyramidal organs

Rest on the upper pole of their correspondin kidneys

Enveloped by thick CT capsule

Cortex o Outer layero Completely surrounds

medullao Mesodermal origin

Medullao Inner layero Ectodermal origin

80 – 90 % of the adrenal gland Produce adrenocortical hormones (steroid

hormones)o Zona glomerulosa – aldosterone o Zona fasciculata – cortisol o Zona reticularis – androgens (DHEA and

androstenedione)

Comprises 10-20 % of adrenal gland

Central part: medullary veins that drain the entire gland

Consists of parenchymal cells arranged in groups or cords surrounded by sinusoids and richly supplied with nerves (associated with pre-ganglionic sympathetic neurons)

Chromaffin cells: secrete cathecolamines o Epinephrine (adrenaline)o Norepinephrine (noradrenaline)o Dopamine

Islets of Langerhanso Small aggregations of pale-staining cells scattered

among the dark-staining cells of the exocrine portion of the pancreas

o Each islet surrounded by thin layer of fine reticular fibers

o Provided by rich supply of capillarieso Millions but only 2 % of volume of pancreas

β cellso Secrete insulin

α cellso Secrete glucagon

δ cellso Secrete somatostain – inhibits secretion of glucagon,

insulin and pancreatic polypeptide F cells

o Secrete pancreatic polypeptide – slows absorption of food from the intestines