Download Endocrine System - Dr. Haar

Endocrine System
Jack L. Haar, Ph.D.
Learning objectives
1. Identify endocrine organs and cells at the light microscopic level.
2. Correlate the embryological development of the pituitary gland with its
adult structure and function.
3. Describe the structure of endocrine organs.
4. Name the secretory cells in endocrine organs, identify their secretory
products and explain how their secretion is regulated.
I.
General information
A. Endocrine versus exocrine glands
Exocrine
Endocrine
Method of product distribution
Duct (usually)
Ductless
Site of product release
Epithelial surface
Intercellular
Fluid or Blood
Method of product release
Mero-, Apo-, Holocrine
Merocrine, Diffusion
Site of product action
Local
Long distance, usually
Secretory product
Mucus, Lipid, Cells, Enzymes Polypeptides, Steroids
Hormones
Cell polarity
High
None (usually)
B.
Endocrine glands of the human body
Histology of the Pituitary Gland
(Hypophysis)
II.
Pituitary gland origins
A. Stomodeal ectoderm
1. Adenohypophysis
B. Evagination of diencephalon
1. Neurohypophysis
III.
Pituitary organization
A. Located in sella turcica of sphenoid bone below hypothalamus, separated
from it by diaphragm sellae through which the pituitary stalk, infundibulum
passes
B. Surrounded by dense connective tissue capsule
C. Extensive capillary beds associated with gland
D. Subdivisions:
IV.
Adenohypophysis
A. Pars distalis
1.
Structure
a.
Forms the majority of the pituitary gland
b.
Glandular appearance-cells arranged in cords, not
polarized, protein secreting
c.
Supported by reticular fiber framework
d.
2.
Fenestrated capillaries
Cell types
a.
Chromophobes
i.
ii.
b.
No secretory granules
Reserve cells or degranulated cells
Chromophils
i.
Contain secretory granules with protein hormone
ii.
Acidophils: granule contents have affinity for acid
dyes (eosin)
iii.
Basophils: granule contents have affinity for basic
dyes (hematoxylin)
3.
Acidophils
a.
Smaller cells, more numerous
b.
Types
i.
Somatotropes (75%)
1) Secrete somatotropin (growth hormone, GH)
2) Hormone has sequence homology with prolactin
3) Indirectly stimulates epiphyseal plate
4)
Gigantism/Dwarfism; Acromegaly
2.
Mammotropes (25%)
1. Secrete prolactin
2. Stimulates milk secretion
3. Galactorrhea, continuous milk production (tumor)
4. Basophils
a.
Larger than acidophils
b.
Granules stain with PAS
c.
Types
i.
Thyrotropes
1)
ii.
Secrete thyroid stimulating hormone (TSH)
Gonadotropes
1)
2)
iii.
Secrete leutinizing hormone (LH) and
follicle stimulating hormone (FSH)
Same cell secretes both hormones having
similar structure
Corticotropes
1)
2)
Secrete adrenocorticotropic hormone
(ACTH), lipotropin and melanocyte
stimulating hormone (MSH) and endorphin
Primary sequence of each hormone found
within a larger precursor protein-proopiomelanocortin
Hormone
General Cell Type
Specific Cell Type
GH
Prolactin
TSH
FSH/LH
ACTH
Acidophil
Acidophil
Basophil
Basophil
Basophil
Somatotrope
Mammotrope
Thyrotrope
Gonadotrope
Corticotrope
B. Pars Tuberalis
1.
Consists of a collar of cells surrounding the infundibulum
2.
Contains gonadotropes only
C. Pars Intermedia (2% of the adenohypophysis)
1.
Consists of cells surrounding the cystic remains of Rathke’s pouch
and cells which have migrated into the pars nervosa
2.
Composed of corticotropes only
D. Control of adenohypophysis secretion (primarily pars distalis)
IV.
1.
Hypothalamic releasing or inhibitory hormones (factors) are
synthesized by neurons located in the hypothalamus and are
transported down the axon
2.
Axons end at a capillary bed located in a brain region just above
the infundibulum (median eminence) and the hypothalamic factors
are secreted into this capillary bed
3.
The capillary bed drains into hypophyseal portal vessels which
travel down the infundibulum carrying the hypothalamic factors;
portal vessels connect/end in capillaries within the pars distalis
4.
Hypothalamic factors exit capillaries in the pars distalis and either
stimulate or inhibit the release of hormones from the acidophils
and basophils in the pars distalis
5.
Negative feedback occurs at multiple levels for control
Neurohypophysis
A.
Components
1.
Infundibulum
2.
a.
Connects hypophysis with hypothalamus
b.
Composed of axons from supraoptic and paraventricular
nuclei of the hypothalamus forming the hypothalamichypophyseal tract
Pars nervosa
a.
Axons and terminals of hypothalamic-hypophyseal tract
terminating in association with capillaries – Herring bodies
b.
Pituicytes
i.
ii.
iii.
Glial cells, highly polymorphic
Also present in infundibulum
In humans, may accumulate pigment granules
resembling Herring bodies
E. Neurohypophyseal secretion
1.
Unlike the adenohypophysis, no hormone synthesis occurs in the
pars nervosa, rather, the hormones (oxytocin and vasopressin)
are stored in axon terminals (Herring bodies) after being
synthesized in and transported from the supraoptic and
paraventricular nuclei of the hypothalamus.
2.
Hormones include:
a.
Oxytocin
i.
ii.
b.
3.
Synthesized primarily in the paraventricular nucleus
Stimulates smooth muscle contraction
Antidiuretic hormone (ADH/vasopressin)
i.
Synthesized primarily in the supraoptic nucleus
ii.
Increases water retention
Release of hormone (neurosecretion) results from nervous activity
(action potentials) in the hypothalamic neurons causing release
analogous to release of neurotransmitter
Endocrine Organs II
Thyroid Gland
I. Origin and Location
II. Functions
A.
Synthesizes thyroxine (tetraiodothyronine, T4) and triiodothyronine (T3)
for regulation of metabolic rate
B.
Synthesizes calcitonin, lowers blood calcium levels
III. Gross Structure
A.
Bilobed gland connected by an isthmus
anterior to the larynx
B.
Lobes lateral to trachea extending from
thyroid cartilage to the fifth or sixth tracheal ring.
C.
Extensive vascular supply
IV. Histology
A. Composed of spherical follicles
1. Follicular or principal cells; cuboidal epithelium, however, height varies
between squamous and columnar depending on activity of gland
2. Center of follicle contains colloid
i. Colloid is a non-specific term used for any accumulation of
proteinaceous, gelatin-like material, which in this case, refers to
the material contained within the follicle, specifically, the
protein thyroglobulin
ii. Thyroglobulin (glycoprotein) secreted by follicle cells
iii. Thyroid hormones contain modified tyrosine amino acids in the
primary sequence of thyroglobulin, the entire protein complex
representing a storage form of thyroid hormones
3. Fenestrated capillaries
B. Parafollicular cells (C cells, clear cells)
1. Neural crest origin
2. Occur singly within follicular epithelium or in small clusters between
follicles
3. Contain typical protein synthetic organelles and secretory granules,
no polarity
4. In some species, but not human, is known to secrete calcitonin
i. Inhibits bone resorption, lowering Ca++ levels
ii. Antagonist of parathyroid hormone
V. Ultrastructure of follicle cells
A. Thyroid hormone synthesis and secretion involves release of larger precursor
molecule (thyroglobulin) onto an epithelial surface (follicle lumen) then reuptake and cleavage of the precursor to release the hormone; organelles and
their distribution in the follicle cells reflect this polarity of function, typical of
exocrine glands
B. Basal lamina
C. Apical microvilli
D. Junctional complexes
E. Synthetic features and secretory features
1. Extensive smooth ER
2. Golgi
3. Transport/secretory vesicles
F. Resorptive and degradative features
1. Pseudopodia to form resporption vaculoles
2. Numerous lysosomes and phagosomes
VI. Histophysiology of follicle cells
A. Synthesis and secretion of thyroid hormones occur in three stages
1. Synthesis, secretion and storage of thyroglobulin in the follicle lumen
2. Iodination of tyrosines on thyroglobulin in the follicle lumen and
rearrangement to form thyroid hormones which remain intercalated in
the primary sequence of thyroglobulin
3. Re-uptake (resorption) of iodinated thyroglobulin, hydrolysis and
release of free thyroid hormones (T3 and T4) from the follicle cell
B.
Active and inactive follicles
!.
Active follicle–follicle cells are cuboidal to columnar and are
involved with both secretion and resorption of thyroglobulin.
2.
Inactive follicle–follicle cells are squamous, reflecting the paucity
of secretory organelles and the lack of synthetic and uptake activity.
C. Regulation
1. Thyroid stimulating hormone (TSH)
2. Feedback loop with pituitary and hypothalamus
VII. Diseases of the Thyroid
A. Hyperthyroidism (Grave’s Disease
B. Hypothyroidism
1. Myxedema (adult)
2. Cretinism (childhood)
C. Iodine Deficiency Goiter
Parathyroid Gland
General Organization and Action
Embryology and Gross Anatomy
Usually 2 pair (superior and inferior) located on posterior surface of
thyroid gland embedded in connective tissue capsule of thyroid
6x1x2mm in size. Ellipsoid, dark tan to yellowish in color
Number may vary (2-6), can be associated with thymus or isolated in the
mediastinum
Functions
Increase blood Ca++ levels via single hormone, parathyroid hormone
(Parathormone)
Parathyroid hormone (PTH) acts on osteoclasts, renal tubular cells and
indirectly on intestinal epithelial cells
Microscopic Anatomy
A. Stroma
1. Thin connective tissue capsule from which septae (trabeculae)
subdivide gland into lobes
2. Various amounts of adipose cells, increasing with age; may account
for 60-70% of the gland in the elderly
3. Fenestrated capillaries
B. Parenchyma
1. Chief (principal) cells
i. Major cell type, arranged in cords or clumps
ii. Secrete PTH
2. Oxyphilic cells
i.
Large cells which may appear singly or in clumps
ii. Heterochromatic nucleus with intensely eosinophilic cytoplasm
due to large numbers of mitochondria
iii. No secretory granules
iv. Function unknown
Adrenal Gland
I.
General Organization
A. Cortex
B. Medulla
II.
Cortex
A. Zona glomerulosa (about 15%)
1. Located immediately beneath capsule
2. Cells appear as round clusters
3. Secretes mineralcorticoids, such as aldosterone
B. Zona Fasiculata (about 80%)
1. Middle layer and largest cortical zone
2. Cells arranged in rows perpendicular to the capsule with alternating
sinusoidal capillaries
3. Cells appear highly vacuolated due to presence of numerous lipid
(cholesterol) droplets in the cytoplasm
4. Secretes glucocorticoids: corticosterone, cortisol, androgens and
estrogens
C. Zona reticularis (about 5%)
1. Occupies deepest layer of the cortex
2. Cells arranged as anastomosing cords
3. Same secretions as Zona fasciculate
D. Cytological features
1. Large numbers of cytoplasmic lipid droplets
2. Large amounts of sER
3. Mitchondria with tubular cristae
4. Secretion by diffusion; no storage of products, which are released
upon demand
III.
Adrenal Medulla
A. Derived from neural crest; represents sympatheic ganglion
B. Cells arranged as cords or clumps
C. Chromaffin cells
1. Synthesize and release epinephrine (80%) and norepinephrine (20%)
from different cell populations
2. Product stored in dense core granules with chromgranins
3. Secretion by merocrine mode
4. Innervated by sympathetic, pregangionic fibers
5. Parenchymal cells of medulla can be regarded as modified
sympathetic post-ganglionic neurons
Pineal Gland (Epiphysis Cerebri)
Structure
I.
Conical-shaped gland, 5-8 mm in length and 3-5 mm in width; develops from the
roof of the diencephalon and remains attached by a short pineal stalk
II.
Surrounded by a capsule composed of pia mater
A.
Connective tissue septa derived from the pia mater penetrate the gland
and subdivide it into indistinct lobules.
B.
III.
Sympathetic axons and blood vessels enter the gland with the septa.
Cells
A.
Pinealocytes
1.
Major cell type, represent modified neurons
2.
Euchromatic nucleus, spherical to ovoid, with a prominent
nucleolus
3.
Cytoplasm not evident with conventional stains; however, silver
staining reveals that the cell generally has two or more extensions
similar to neuronal processes.
B.
4.
Processes end in association with capillaries
5.
Secrete melatonin, an indoleamine hormone
Interstitial cells
1.
Minor cell type, similar to astrocytes in the brain
2.
Nucleus is elongated and more heterochromatic than that of
pinealocytes.
3.
Possess long processes with intermediate filaments
4.
Located among groups of pinealocytes and in the connective
tissue septae
IV.
Corpora araneacea (brain sand)
A.
Globular, basophilic accumulations of calcium phosphates and
carbonates in the interstitial space
B.
Radio-opaque and, thus, often used as indicators of midline deflection of
the brain resulting from pathological conditions
Secretion
I.
Major hormone secreted is melatonin which regulates diurnal (circadian) lightdark cycles and seasonal rhythms.
II.
Melatonin is secreted during darkness; secretion is inhibited by light.
III.
Retinal stimulation by light is relayed to the pineal via sympathetic innervation
from the superior cervical ganglion.
Endocrine organs laboratory
You are responsible for identifying the following structures in the Digital Histology CD
package.
Pituitary gland (hypophysis)
Adenohypophysis
Pars distalis
Acidophils
Basophils
Capillary
Chromophobes
Pars intermedia
Basophils
Colloid
Remnants of Rathke's pouch
Pars tuberalis
Basophils
Neurohypophysis
Infundibulum
Pars nervosa
Axons
Capillary
Herring bodies
Pituicyte
Thyroid gland
Capillary
Colloid
Follicle
Active follicle
Inactive follicle
Follicle cells
Parafollicular cell (clear cell)
Secretory granules
Stroma
Parathyroid gland
Capillary
Oxyphil cell
Principal cell
Adrenal gland
Adrenal cortex
Zona fasciculata
Zona glomerulosa
Zona reticularis
Adrenal medulla
Chromaffin cell
Secretory granule
Capsule
Capillaries
Veins
Pineal gland
Blood vessels
Capsule
Connective tissue septum
Interstitial cell
Pia mater
Pinealocyte
VIRTUAL SLIDE LINKS
Pituitary
Thyroid Gland
Adrenal Gland