ENDOCRINE SYSTEM = composed mainly of ductless glands that secrete
hormones directly into the bloodstream.
- Functions as long-range communication network that regulates homeostasis, works
in conjunction with the nervous system.
- Made up of 3 Components:

1) Separate glands of purely endocrine function. Includes pituitary (hypophysis),
thyroid, parathyroid, adrenals.
2) Scattered masses of endocrine tissue within exocrine glands or other organs.
Includes pancreas, Leydig's cells (testis), corpora lutea (ovary), juxtaglomerular
apparatus in kidney.
3) Isolated endocrine cells - principally cells within the epithelial lining of
digestive and respiratory tracts.

- Endocrine tissue is highly vascularized to facilitate transport of hormones.
- Hormonal communication requires:

1) Receptors in target cell which recognize hormone and bind to it.
2) Mechanism for translating binding of hormone to response (Second Messengers in
the case of peptide hormones).

I. PITUITARY = endocrine "extension" of hypothalamus (the part of the brain
that mediates visceral functions of the body), enclosed in a dense fibrous CT capsule.
- Pituitary composed of 2 major parts:

1) Adenohypophysis = Anterior Pituitary; originates as an invagination from
the roof of the mouth (Rathke's Pouch).
2) Neurohypophysis = Posterior Pituitary; originates as an evagination from
the floor of the diencephalon region of the brain (Infundibulum)

- Pituitary is present in all vertebrates, origin is the same, organization differs
- Adenohypophysis (glandular epithelium component) includes:

1) Pars distalis
2) Pars tuberalis
3) Pars intermedia

- Pars distalis = glandular epithelial cells in cords and clusters surrounding
fenestrated capillaries, forms bulk of pituitary (75%), supported by a delicate reticular
CT stroma.

- CELL TYPES: Chromophils and Chromophobes
1) Acidophils = stain acidophilic; secrete GH and Prolactin; compact dark
nuclei, cell outlines distinct.
2) Basophils = stain basophilic; secrete TSH, FSH, LH, and ACTH; fewer but
larger
nuclei, not as dark staining
3) Chromophobes = pale-staining cells, relatively little cytoplasm; appear to be
chromophils in degranulated, quiescent state; basophilic nuclei; Most Abundant
cell type

- Pars tuberalis = forms collar of cells around infundibular stalk; cells arranged
in groups or short cords - include acidophils, basophils, undifferentiated
chromophobes; secretes gonadotropins
- Pars intermedia = poorly developed (in humans) layer of basophil and
chromophobe cells lying between p. distalis and neurohypophysis. In adult, remnants
of lumen of Rathke's Pouch (= hypophyseal cleft) fills with colloid and appears
as a cyst. Produces Melanocyte Stimulating Hormone (MSH) in amphibians and fish.


NEUROHYPOPHYSIS (Neural Component of Pituitary) includes:

1) Pars nervosa
2) Infundibular stalk

- Both parts are histologically identical. Contain axons from neurosecretory cells in
hypothalamus and glial cells (pituicytes).
- Pars nervosa serves as center for storage and release of oxytocin and vasopressin
(ADH). These are called posterior pituitary hormones since they are released from
axon terminals in pars nervosa (although they are produced in nuclei of
hypothalamus).
- Herring Bodies = accumulations of secretory material, stain basophilic


HYPOTHALAMO - ANTERIOR PITUITARY CONNECTION
- Intimate vascular "connection" occurs between hypothalamus / median eminence
of infundibular stalk and adenohypophysis. Communication by diffusion of
hypothalamic hormones from capillaries in infundibular stalk to cells of
adenohypophysis.
- Diffusion of hypothalamic hormones into glandular epithelium (from portal
vessels) directs cells of the adenohypophysis to produce hormones or inhibit
production of hormones. (see handout)

To Lecture 17