Pituitary

PITUITARY GLAND (Hypophysis)

PITUITARY AND HYPOTHALAMUS

are morphologically and functionally linked in the endocrine and neuroendocrine glands.

they play central roles in a number of regulatory feedback system

“master organs” of the ES

PITUITARY GLAND

a pea-sized, compound endocrine gland, centrally located at the base of the brain.

Sellaturcica– saddle-shaped depression of the sphenoid bone where lies the pituitary gland

develops in the embryo partly from oral ectoderm and partly from the developing brain

A short stalk, the infundibulum, and a vascular network connect the pituitary gland to the hypothalamus.

2 FUNCTIONAL COMPONENTS

Anterior lobe (adenohypophysis)

- the glandular epithelial tissue

- derived from an evagination of the ectoderm of the oropharynx toward the brain (Rathke’s pouch)

Posterior lobe (neurohypophysis)

- the neural (secretory) tissue

- derived from a downgrowth of neuroectoderm of the floor of the third ventricle (diencephalon) of the developing brain.

ANTERIOR LOBE3 derivatives of Rathke’s pouch:

Pars distalis (distal lobe)

- comprises the bulk of the anterior lobe; arises from the thickened anterior wall of the pouch

Pars intermedia (intermediate lobe)

- a thin remnant of the posterior wall of the pouch that abuts the pars distalis

Pars tuberalis (tuberal lobe)

- develops from the thickened lateral walls of the pouch and forms a collar or sheath around the infundibulum

Posterior lobe3 components:

Neural lobe (Pars nervosa, Infundbular process)

- lying behind the anterior pituitary in the sellaturcicaPituitary stalk (Infundibular stem)

- where axons run from the brain aboveMedian Eminence (Infundibulum)

- a funnel shape extension of the hypothalamus; where the infundibulum with its stalk is attached.

PITUITARY GLAND

infundibular stalk (IS); pars nervosa (PN); pars distalis (PD); pars intermediate (PI); and pars tuberalis (PT)

I. ADENOHYPOPHYSIS (Anterior Pituitary)

The cells are organized in clumps and cords separated by fenestrated sinusoidal capillaries of relatively large diameter.

Have cells that respond to signals from the hypothalamus and synthesize and secrete a number of pituitary hormones (4+2)

4 hormones (“tropic hormones”)ACTH, TSH, FH, LH

regulate the activity of cells in other endocrine glands throughout the body

Other 2 hormones (not tropic)GH, PRL

act directly on target organs that are not endocrine in nature

regulates other endocrine glands and some non-endocrine tissues

Hormones of the Anterior PituitaryHormone Function

Growth hormone (somatotrophin, GH)

Stimulates liver and other organs to synthesize and secrete insulin-like growth factor I (IGF-II) to stimulate division of progenitor cells located in growth plates and in skeletal muscles for body growth

Prolactin (PRL) Promotes mammary gland development

Adrenocorticotropic hormone (ACTH)

Maintains structure and stimulates secretion of glucocorticoids and gonadocorticoids by the zonafasciculata and zonareticularis of the adrenal cortex

Follicle-stimulating hormone (FSH)

Stimulates follicular development in the ovary and spermatogenesis in the testis

Luteinizing hormone (LH) Regulates final maturation of ovarian follicle, ovulation and corpus luteum formation; stimulates steroid secretion by follicle and corpus luteum; In males, essential for maintenance of and androgen secretion by the Leydig (interstitial) cells of the testis

Thyrotropic hormone (TSH Stimulates growth of thyroid epithelial cells; stimulates production and release of thyroglobulin and thyroid hormones

Pars distalisaccounts for 75% of the adenohypophysis and is covered by a thin fibrous capsule.

main components are cords of epithelial cells interspersed with fenestrated capillaries

Fibroblasts are present and produce reticular fibers supporting the cords of hormone-secreting cells.

Common stains suggest two broad groups of cells in the pars distalis based on staining affinity: chromophils and chromophobes.

chromophils- are secretory cells in which hormone is stored in cytoplasmic granules.

- are also called basophils and acidophils according totheir affinity for basic

and acidic dyes

chromophobes - stain weakly, with few or no secretory granules, and also represent a heterogeneous group, including stem and undifferentiated progenitor cells as well as any degranulated cells present.

Acidophils include the somatotropic and mammotropic cells

Basophilic cells are the gonadotropic, corticotropic, and thyrotropic cells

Hormones produced by the pars distalis have widespread functional activities; they regulate almost all other endocrine glands, milk secretion, melanocyte activity, and the metabolism of muscle, bone, and adipose tissue.

Pars distalis: acidophils, basophils, and chromophobes

(a,b):acidophil cells (A), basophils (B), and chromophobes (C)(c):acidophil cells (A), basophils (B), and chromophobes (C); Cords of acidophils and basophils vary in distribution and number in different regions of the pars distalis, but are always closely associated with capillaries and sinusoids (S) in the second capillary plexus of the portal system.

Cell Type Stain Affinity

% of Total Cells

Hormone Produced

Main Physiologic Activity

Somatotropic cell

Acidophilic

50 Somatotropin (growth hormone, GH)

Acts on growth of long bones via insulin-like growth factors synthesized in liver

Mammotropic cell (or actotropic cell)

Acidophilic

15-20

Prolactin (PRL) Promotes milk secretion

Gonadotropic cell

Basophilic

10 Follicle-stimulating hormone (FSH) luteinizing hormone (LH) in the same cell type

FSH promotes ovarian follicle and development and estrogen secretion in women and spermatogenesis in men. LH promotes ovarian follicle maturation and progesterone secretion in women and interstitial cell androgen secretion in men.

Thyrotropic cell

Basophilic

5 Thyrotropin (TSH) Stimulates thyroid hormone synthesis, storage, and liberation

Corticotropic cell

Basophilic

15-20

Adrenal corticotropin (ACTH)Lipotrophins

Stimulates secretion of adrenal cortex hormones Lipid metabolism regulation

5 Secretory Cell Types of the pars distalis

Hormones of the pars distalis and their targets

Pars tuberalis

a funnel-shaped region surrounding the infundibulum of the neurohypophysis

Highly vascular region containing veins of the hypothalamohypophyseal system

Most of its cells are basophilic gonadotropic cells that secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH). 

Pars Intermedia

a thin zone of basophilic cells between the pars distalis and the pars nervosa of the neurohypophysis, which is often invaded by these basophils

develops from the dorsal wall of the hypophyseal pouch and usually contains colloid-filled cysts that represent remnants of that structure's lumen

Pars intermedia

The pars intermedia (PI) lies between the pars distalis (PD) and the pars nervosa (PN), with many of its basophilic cells (B) usually invading the latter. Remnants of the embryonic hypophyseal pouch's lumen are usually present in this region as colloid-filled cysts (C) of various sizes.

Presence of different-sized follicles filled with colloid (CF)

Control of Secretion in the Adenohypophysis

The activities of the cells of the anterior pituitary are controlled primarily by peptide hormones produced by specialized neurons in certain hypothalamic nuclei and stored in their axons that run to the median eminence

Most of these hormones are hypothalamic-releasing hormones; liberated from the axons, they are transported by capillaries to the pars distalis where they stimulate hormone synthesis and/or release.

Two of the hypothalamic factors, however, act to inhibit hormone release by specific cells of the pars distalis (hypothalamic-inhibiting hormones)

Hormone Function

Thyrotropin-releasing hormone (TRH)

Stimulates synthesis and release of both thyrotropin (TSH) and prolactin

Gonadotropin-releasing (GnRH) Stimulates the release of both FSH and LH hormone

Somatostatin Inhibits release of both somatotropin (GH) and thyrotropin (TSH)

Growth hormone–releasing hormone(GHRH)

Stimulates synthesis and release of somatotropin (GH)

Dopamine (prolactin-inhibiting hormone)

Inhibits release of prolactin

Corticotropin releasing hormone (CRH)

Stimulates synthesis of POMC and release of both -lipotropin (-LPH) and corticotropin (ACTH)

Hypothalamic hormones regulating the anterior pituitary

Negative feedback loops affecting anterior pituitary secretion

Hypothalamic thyrotropin-releasing hormone (TRH) stimulates secretion of thyrotropin (TSH), which stimulates synthesis and secretion of thyroid hormone (TH). In addition to their effects on target organs, TH inhibits TSH secretion from the pars distalis and TRH secretion from the hypothalamus by negative-feedback.

II. Neurohypophysis (Posterior Pituitary)

an extension of the CNS

consists of the pars nervosa (which doesn’t have secretory cells) and the infundibular stalk

is composed of neural tissue, containing some 100,000 unmyelinated axons of secretory neurons situated in the supraoptic and paraventricular nuclei of the hypothalamus

presence of highly branched glial cells called pituicytes that resemble astrocytes and are the most abundant cell type in the posterior pituitary; serves a supporting role in the CNS

NOT an endocrine gland; stores and releases secretory products from the hypothalamus

Pars nervosa: Neurosecretory bodies and pituicytes

The pars nervosa of the posterior pituitary consists of modified neural tissues containing unmyelinated axons supported and ensheathed by glia cells called pituicytes (P), the most numerous cell type present. The axons run from the supraoptic and paraventricular hypothalamic nuclei and have swellings called neurosecretory (Herring) bodies (NB) from which either oxytocin or vasopressin is released upon neural stimulation. The released hormones are picked up by capillaries (C) for distribution throughout the body.

Hormone Major Function

Oxytocin Stimulates activity of the contractile cells around the ducts of the mammary glands to eject milk from the glands; stimulates contraction of smooth muscle cells in the pregnant uterus

Antidiuretic hormone (ADH; vasopressin)

Decreases urine volume by increasing water permeability by collecting ducts of the kidney; decreases the rate of perspiration in response to dehydration; increases blood pressure by stimulating contractions of smooth muscles cells in the wall of arterioles

Hormones of the Posterior Pituitary