Histology of the Endocrine System

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Intercellular Communication

Intercellular communication in multicellular organisms is necessary to maintain:

1. Homeostasis of nutrients and vital substances in the cellular environment.

2. Coordination of growth & development.

3. Adaptation to environmental stresses.

Types of Communication Systems

1. Nervous system:

• Fast, specific targets.

2. Endocrine system:

• Slower, more pervasive, less specific.

Endocrine System:

1. Ductless glands.

2. Secretions (hormones) elaborated to interstitium then blood stream and to target organs with specific receptors for the hormone.

3. Over 100 distinct hormones.

4. Effective at very low concentrations.

5. Direct or indirect feedback from the target site regulates further hormone elaboration.

6. Hormones act at different speeds: Peptides = seconds Proteins = hours Steroids = days.

General Characteristics of Endocrine Glands:

1. Simple structure consisting of parenchymal cords or plates and a delicate stroma.

2. Highly vascularized.

Forms of Endocrine Components:

1.Distinct aggregates (endocrine organs) - pituitary, thyroid, parathyroid, adrenal, pineal.

2.Isolated cells - enteroendocrine cells of GI tract.

3. Scattered masses blended with exocrine glands or other organs - Leydig's cells in testis, corpora lutea in ovary, JG cells in kidney, Islets of Langerhans in pancreas.

Embryological Origin from all 3 germ layers:

1. Ectodermal:

• Pituitary, adrenal medulla (neural crest), pineal.

2. Mesodermal:

• Adrenal cortex, testis, ovaries.

3. Endodermal:

• Islets of Langerhans, parenchymal cells of thyroid & parathyroid.

Pituitary Gland

Termed the "master gland" because it has a critical role in producing & regulating secretions of the major endocrine glands. Is attached by infundibular stalk to underside of hypothalamus. Acts in concert with hypothalamus of CNS & through a series of feedback mechanisms to exert precise control over plasma levels of hormones it secretes.

Anterior pituitary (anterior lobe) from upgrowth of oral ectoderm (Rathke's pouch). Posterior pituitary (posterior lobe) from downgrowth of neuroectoderm of CNS (infundibulum of diencephalon).

Pars distalis (anterior lobe) Pars tuberalis - wraps around infund. stalkPars intermedia - mostly vestigial in humans, maybe some MSH (melano. stim. H.)Pars nervosa - neoroendocrine cells.

Anterior lobe or pituitary, adenohypophysis = pars distalis + pars tuberalis + pars intermedia Posterior lobe or pituitary, neurohypophysis = pars nervosa + infundibulum = medial eminence.

Anterior lobe blood supply: internal carotids superior hypophyseal A. breaks into primary plexus in upper stalk portal veins secondary plexus hypophyseal veins cavernous sinus. This system is called the hypothalamic-hypophyseal-portal system and is the primary way in which "regulatory peptides" are distributed to the anterior lobe. In response to these reg. pep., the anterior lobe cells secrete hormones in vascular system and thence to targets. Posterior lobe blood supply is via internal carotids inferior hypophyseal A. capillary plexus in posterior lobe posterior lobe vein.

Anterior Pituitary

Pars distalis - 75% of pituitary, parenchyma, fenestrated vessels & reticular fibers. 6 major hormones:

1. ACTH (adrenocorticotropic hormone).

2. TSH (thyroid stimulation hormone).

3. FSH.

4. LH.

5. Prolactin.

6. GH (growth hormone somatotropin).

Types of Cells in the Pituitary

3 cell types: chromophobes, acidophils & basophils.

Pars Distalis Cells:

Acidophils:

• Somatotrophs and lactotrophs.

Basophils:

• Gonadotrophs and thyrotrophs.

Somatotroph Secretions:

• Growth hormone, which stimulates chondrocyte growth and bone development.

Growth Hormone Deficiency during development:

• Pituitary dwarf

Growth Hormone Overproduction during development:

• Giant.

Growth Hormone Overproduction in adulthood:

• Acromegaly.

Mammotroph Secretions:

• Prolactin production.

TSH Action and Related Cells

1. Thyrotrophs:

• Production of TSH which stimulate biosynthesis & release of thyroxine from thyroid which affects metabolism, growth and calcium regulation.

2. Gonadotrophs:

• FSH promotes growth of ovarian follicles & spermatogenesis. LH stimulation, ovulation & maintains Leydig cells.

3. Corticotrophs:

• ACTH stimulation: release of glucocorticoids from adrenal cortex.

• Beta endorphin: opiate-like peptide of pain.

• MSH and beta lipotropin - lipolysis.

Pars tuberalis has chromophil cells, primarily gonadotrophs. Pars intermedia is rudimentary. Some basophils but mostly chromophobes surrounding colloid filled follicles called Rathke's cysts. Secretions not clear but some MSH and low levels of ACTH.

Neurohypophysis Consists of:

1. Dense capillary network.

2. Highly branched non-secretory glial-like cells called pituicytes whose processes end in close association with capillaries. Appear to serve nutritive function similar to glial cells.

3. 50,000-100,000 unmyelinated axons of neurosecretory cells of hypothalamus.Hypothalamic axons descend and terminate in close association with capillaries, not synapses with other neurons. Neural transmission down the axon signals the release of neurosecretory material in vesicles bound to a carrier protein, neurophysin.

Herring Bodies

• Dense aggregations of neurosecretory material in the axon terminals create dilations called Herring bodies.

Neurohypophysis:

Produces oxytocin, which:

1. Induces peristaltic contractions of uterine smooth muscle during parturition.

2. Induces contraction of mammary gland myoepithelial cells to eject milk.

Neurohypophysis also produces vasopressin, ADH, antidiuretic H. which:

1. Promotes water resorption through collecting tubules of kidney.

2. Increases blood pressure by promoting contraction of vascular smooth muscle resulting in increased peripheral resistance.

Pineal Gland

• Pineal = epiphysis cerebri attached to diencephalon.

• Pinealocytes produce melatonin and serotonin which may influence reproduction.

• Role in humans is not well understood but is important in seasonal breeding in other animals along with seasonal variations in day length.

• Calcium phosphate and calcium carbonate aggregates are known as brain sand or corpora arenacea.

• Of use clinically on Xray to show deviation from midline by space occupying lesion of CNS.

Thyroid Gland

Thyroid develops from a median downgrowth of the base of the tongue, the thyroid diverticulum. It descends to its normal position but remnants of the attachment to the tongue, the thyroglossal duct, may rarely persist.

Two lobes connected by an isthmus. Thyroid follicular or principal cells line a colloid filled follicle. Each follicle is surrounded by basal lamina & a meshwork of fenestrated capillaries. Outside the follicle but invested in the same basal lamina as the principal cells are the parafollicular or C cells.

Thyroid principal cells synthesize the thyroid hormone precursor thyroglobulin and is stored in the lumen until it is activated. These are the only cells that store their secretory product extracellularly. When activated the contents are endocytosed by the follicle cells and passed through the basal lamina to capillaries.Generally TH is involved with cellular metabolism. Metabolic effects: Increase in basal metabolism Regulates water and ion transport Regulates protein, fat & carbohydrate metabolism.

Growth Promoting Effects:

• Acts synergistically with GH to promote normal growth.

Hypothyroidism:

During development causes cretinism with small stature (dwarf) and stunting of mental ability. In adulthood, myxedema characterized by lethargy, swollen face & mental impairment.Exopthalmic goiter is due to hyperactivity of the thyroid. Goiter is an enlarged thyroid also associated with exopthalmos or protruding eyes due to edema behind the eyes. Other types of goiter are caused by iodine deficiency.

Parathyroid Gland

Parafollicular or C cells - produce calcitonin (thyrocalcitonin) which lowers blood calcium by inhibiting osteoclast activity.

Parathyroid cells:

Chief or principal cells produce parathyroid hormone which has the opposite action of calcitonin. Parathyroid hormone elevates blood calcium levels by stimulating osteoclast activity. Oxyphil cells are acidophilic and their function is unknown by some feel they may become chief cells.

Adrenal Cortex Development

Adrenal cortex (80% of gland) arises from the mesoderm & adrenal medulla (20%) arises from the neural crest.

Adrenal Cortex

Cortex is divided into the zona glomerulosa (peripheral), which produces mineralocorticoids involved with fluid & electrolyte balance. Zona fasiculata, which produces glucocorticoids involved with carbohydrate metabolism, and the zona reticularis produces gonadal-type steroids.

Zona Glomerulosa:

• Secretion of Aldosterone is controlled by the renin-angiotensin system of kidney.

• Aldosterone & deoxycorticosterone both of which are mineralocorticoids that maintain water & electrolyte balance by promoting resorption of sodium from DCT, sweat glands & salivary glands.

Zona Fasiculata:

• Produces glucocorticoids primarily cortisol which down-regulates immune response, modulates carbohydrate metabolism & suppresses immune & inflammatory response.

Zona Reticularis:

• Secretes residual amounts of cortisol & insignificant amounts of several weak androgens. But if there is a tumor, large amounts can be released resulting in virilizing of females and precocious dev. of secondary sex characters in prepubertal males.

Female Pseudohermaphroditism

• XX.

• normal female internal organs (ovaries, Mullerian structures).

• virilized genitalia (clitoral enlargement and labial fusion).

• due to exposure of female fetus to excessive androgens.

• most common cause is Congenital Adrenal Hyperplasia (CAH) may appear more male than female.

Adrenal Medulla

• Adrenal medulla composed of sympathetic ganglion cells & chromaffin (LD 24448) cells derived from neural crest.

• These cells have lost their axons & dendrites and function as secretory cells.

• Produce catecholamines which stim. cardiovascular & respiratory systems, general metabolic activity and raises blood glucose levels.

• Produce epinephrine (adrenaline) fight or flight response.

• Increases blood flow.

• Produce Norepinephrine (noradrenalin).

• Elevates blood pressure via vasoconstriction.

Additional Reading:

Basic Histology

1. Introduction to Histology
2. Basic Cell Physiology
3. Actin, Microtubules, and Intermediate Filaments
4. Mitochondria, Nucleus, Endoplasmic Reticulum, Golgi
5. Epithelium (Epithelial Tissue)
6. Connective and Adipose Tissue
7. Types of Cartilage
8. Osteogenesis
9. Nervous Tissue
10. Muscle Tissue
11. Cardiovascular System
12. Blood and Hematopoiesis
13. Lymphoid Tissue
14. Digestive Tract I: Oral Cavity
15. Digestive Tract II: Esophagus through Intestines
16. Liver, Pancreas, and Gall Bladder
17. Respiratory System
18. Integument
19. Urinary System
20. Endocrine System
21. Male Reproductive System
22. Female Reproductive System
23. Eye and Ear

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