Pituitary, also known as Hypophysis, is a small gland suspended from the brain by the hypophyseal stem. It is located in a cavity of the sphenoid bone called "turkish saddle". 1
Pituitary is composed of two lobes: Anterior lobe (or Adenohypophysis) and Posterior lobe (or Neurohypophysis). 2 These two areas have different embryologic origins: The anterior lobe (adenohypophysis) develops from the ectoderm of the pharyngeal roof (oral cavity) that is to pocket the called Rathke's pouch. On the other side, posterior lobe (neurohypophysis) is derived from the floor of the diencephalon.
The anterior lobe (adenohypophysis) shows three main subdivisions 3
- Pars distalis, occupies the main part of the anterior lobe.
- Pars intermedia, is located between anterior and posterior lobes.
- Pars tuberalis, is located in the infundibulum (hypophyseal stalk)
The posterior lobe (neurohypophysis) shows two subdivisions 3
- Pars nervosa, occupies the main part of the neurohypophysis
- Neural stalk, is located in the infundibulum (Hypophyseal stalk)
Both parts of the pituitary shows different structure. While the anterior pituitary shows a highly cellular structure, the neurohypophysis shows a fibrous structure. 4 5
ADENOHYPOPHYSIS (ANTERIOR LOBE)
At microscope the Pars distalis of the anterior lobe of the pituitary we can see bundles and cords of epithelial glandular cells polygonal shaped. Among these clumps, can be seen walls of connective tissue highly vascularized.
Using hematoxylin -eosin stain these cells can be differentiated into two main groups: 6 7 8
- Chromophil (those cells which are colored with this stain technique). Chromophil cells can be subdivided into two groups:
- Acidophil: These cells are characterized by a dense pink stained cytoplasm (eosinophil).
- Basophil: The cytoplasm of this types of cells is stained in light purple.
- Chromophobe (those cells which don't stain in this technique).
According to the secreted hormones we can differentiate up to five cells types on this area: 9
- Somatotrophic. Secretes GH (Growth Hormone).
- Lactotrophic. Secretes PRL (Prolactin).
- Thyrotrophic . Secretes TSH (Thyroid Stimulating Hormone).
- Gonadotrophic. Secretes FSH and LH (Follicle Stimulating Hormone and Luteinizing Hormone).
- Corticotrophic. Secretes ACTH (Adrenal Cortico Trophic Hormone).
In order to differentiate these cells types on the microscope we need to use special staining techniques like immunohistochemical ones. Using Hematoxylin-eosin stained slides these five cell types are seen as chromophiles. In this way Somatotrophic and Lactotrophic are Acidophil, while Thyrotrophic, Gonadotrophic and Corticotrophic are Basophil. 9
In humans, Pars intermedia is significant in fetal life, when secretes MSH (Melanocyte Stimulating Hormone), which increases the number of this type of cells. In adults only remains in this area some dispersed follicular structures filled with protein rich fluid without know function. 5
Pars tuberalis is formed by cords of cell, among blood vessels. Most of the cells of this area are chromophobes. There is no known endocrine function of the cells of this area.
The vascular network of the adenohypophysis not only works for the supply of nutrients and oxygen, but also as a delivery system of hormones of the hypothalamus that control the endocrine secretion of the adenohypophysis.
The structure of the adenohypophyseal vascular network is a bit complex and consists of two capillary plexus connected by several veins. 10
At the hypophyseal stalk we can see the primary plexus, which is feeded by the upper hypophyseal arteries. This plexus drains in several veins (portal veins) that down up to the pars distalis of the anterior lobe of the pituitary (adenohypophysis) where develop the secondary plexus. 10 7 This system is called "hypophyseal portal system"
Adenohypophysis secretes several hormones that control several organic functions. Most of them control secretion of other endocrine glands. 12
In this way:
- Thyrotrophic cells secretes TSH (Thyroid Stimulating Hormone), that regulates the secretion of the thyroidal hormones at the thyroidal follicles activating their synthesis and secretion.
- Corticotrophic cells secretes ACTH (Adrenal Cortico Trophic Hormone), that regulates the secretion of the hormones of the area fasciculata and area reticularis of the adrenal cortex stimulating the secretion of cortisol.
- Gonadotrophic cells secretes FSH (Follicle Stimulating Hormone) and LH (Luteinizing Hormone) which acts on ovary in females and on testis on males. On the ovary, FSH stimulates the development of the ovarian follicles and the secretion of estrogens, while LH induces follicle maturation, ovulation, formation of the "corpus luteum" and estrogen secretion. On the testis FSH stimulates spermatogenesis acting on Sertoli cells, while LH stimulates Leydig cells to produce androgens (testosterone).
- Somatotrophic cells secretes GH (Growth Hormone) which produces body growth stimulating liver hepatocytes to secrete IGF1 (Insuline like Growth Factor 1. IGF1 acts in different tissues, mainly bone, and skeletal muscle tissue.
- Lactotrophic cells secretes PRL (Prolactin), which stimulates synthesis and secretion of milk at the mammary gland.
At the same time, all this hypophyseal endocrine secretion is controlled by hypothalamic hormones, called hypophysiotrophic hormones. Neurons located on hypothalamic nuclei release their hypophysiotrophic, through their axons, to the hypophyseal portal system at the primary plexus level, then down through the portal veins up to the secondary plexus to achieve their target cells in the pars distalis of the adenohypophysis. 11
In this way: 13
- TRH (Thyrotrophic Releasing Hormone) is secreted by neurons located in ventromedial, dorsal and paraventricular nuclei of the hypothalamus. TRH activates Thyrotrophic cells to secrete TSH.
- CRH (Corticotrophic Releasing Hormone) is secreted by neurons located in arciform, medial paraventricular and periventricular nuclei of the hypothalamus. CRH activates Corticotrophic cells to secrete ACTH.
- GnRH (Gonadotrophic Releasing Hormone) is secreted by neurons located in arciform, dorsal, ventromedial and paraventricular nuclei of the hypothalamus. GnRH activates Gonadotrophic cells to secrete FSH and LH.
- GRH (Growth Hormone Releasing Hormone) is secreted by neurons located in arciform nucleus of the hypothalamus. GRH activates Somatotrophic cells to secrete GH.
- GIH (Growth Hormone Inhibiting Hormone)(Somatostatin) is secreted by neurons located in arciform, periventricular and paraventricular nuclei of the hypothalamus. GIH inhibits the GH secretion of the Somatotrophic cells.
- PIH (Prolactin Inhibiting Hormone) (Dopamine) is secreted by neurons located in the arciform nucleus of the hypothalamus. PIH inhibits the PRL secretion of the Lactotrophic cells.
NEUROHYPOPHYSIS (POSTERIOR LOBE)
Overview (( 15))
Cells that secretes their hormones in the neurohypophysis (Posterior lobe of the Pituitary) actually are neurons which are located in the hypothalamus. These neurons send their axons through the neural stalk up to the pars nervosa of the neurohypophysis where liberate hormones on the capillary. These hormones are transported inside vesicles along the axon up to the terminals where they are liberated. Along the axons there are some temporal reservoirs, called Herring bodies, which store hormone filled vesicles. The own neurohypophyseal cells are the Pituicytes. They are astroglial related cells that can be stained with GFAP (Glial Fibrilar Acid Protein).
Histology 14 16
At microscope the pars nervosa of the neurohypophysis show a very fibrous structure with some sparse cells among the fibers. Using Hematoxylin-eosin satin these cells are show as irregularly shaped cells with oval shaped nuclei, the Pituicytes. Among the fibers can be seen oval shaped, poor stained structures called Herring bodies.
Functional correlations 17
Two hormones are secreted at the neurohypophysis OX (Oxitocine) and ADH (Anti Diuretic Hormone). Both hormones are synthesized by hypothalamic neurons.
- OX is synthesized by neurons located at supraoptical and paraventricular hypothalamic nuclei. This hormone activates contraction of myoepithelial cells around of the excretory ducts of the mammary gland in order to secrete milk. It, also stimulates smooth muscle cells of the uterus during menstruation, orgasm and childbirth.
- ADH is synthesized by neurons located at supraoptical and paraventricular hypothalamic nuclei. This hormones reduces the volume of the urine by increasing water reabsorption at kidney collecting tubule. It, also, increases arterial pressure by stimulating smooth muscle cells of the arterial wall.
- Ersochenko, V.P, "diFIore's Atlas of Histology with Functional Correlations" 10th ed., Liippincott, Williams & Wilkins, Baltimore 2005.
- Gartner L.P & Hiatt, J.L. "Color Atlas of Histology" 4th ed., Liippincott, Williams & Wilkins, Baltimore 2006.
- Geneser, F., "Histología, sobre bases biomoleculares" 3rd ed, Editorial médica Panamericana, Barcelona 2003.
- Kierszenbaum, A.L., "Histology and Cell Biology, An introduction to Pathology", Mosby publishing, St Louis 2002.
- Ross M.H. & Pawlina, W., " Histología. Texto y Atlas Color con Biología Celular y Molecular" 5th ed., Editorial médica Panamericana, Barcelona 2007.
- Young, B. & Heath, J.W, "Weather's Histología funcional, texto y atlas en color" 4th ed., Elsevier Science, Barcelona 2002.
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