The hormones PTH, calcitriol (1,25-dihydroxyvitamin D), and FGF23 play a major role in regulating the levels of serum calcium (Ca++) and phosphate (Pi).

Vitamin D is produced in the skin from 7-dehydrocholesterol when exposed to UVB light or obtained through the diet. It undergoes hydroxylation in the liver to become 25-hydroxycholecalciferol and is further activated by renal 1α-hydroxylase to form 1,25-dihydroxyvitamin D.

1,25-dihydroxyvitamin D facilitates the absorption of calcium and to a lesser extent, phosphate in the intestines.

The flux of calcium and phosphate into and out of the bones is determined by the rates of bone formation by osteoblasts and bone resorption by osteoclasts.

The PTH/PTHrP receptor is present on osteoblasts, not osteoclasts. PTH can have both anabolic and catabolic effects on bone, depending on the dosage and timing of administration. PTH promotes bone resorption by increasing the production of M-CSF and RANKL in osteoblasts.

1,25-dihydroxyvitamin D binds to the VDR in osteoblasts, promoting osteoclast differentiation through RANKL and aiding in bone mineralization by maintaining appropriate levels of serum calcium and phosphate.

After studying this chapter, you should be able to answer the following questions:
1. Explain the structure and composition of the pituitary gland and its relationship to magnocellular and parvicellular hypothalamic neurons.
2. Discuss the synthesis, transportation, and release of antidiuretic hormone (ADH) and oxytocin by magnocellular neurons.
3. Create a diagram illustrating the components and feedback loops of a typical endocrine axis, including central input, hypothalamic releasing factors, pituitary hormones, and a peripheral endocrine gland. Understand the concept of a set point.
4. List the different cell types in the adenohypophysis and the tropic hormones they produce, noting any hormones that share a common subunit.
5. Compare the axes of somatotropes and lactotropes with the classic endocrine axes and explain their differences.
6. Explain the actions of growth hormone (GH) and insulin-like growth factor I (IGF-I) in regulating growth, as well as the role of growth hormone in the fasted state.
7. Describe the role of prolactin in initiating and maintaining lactation.

The pituitary gland, also known as the hypophysis, is a complex endocrine structure located at the base of the forebrain. It weighs approximately 0.5 g and consists of the adenohypophysis, which is an epithelial component, and the neurohypophysis, which is a neural structure. The adenohypophysis comprises five different cell types that secrete six hormones, while the neurohypophysis releases several neurohormones. The hypothalamus and negative and positive-feedback loops regulate all endocrine functions of the pituitary gland.
The major physiological hormones regulating serum [Ca++] and [Pi] are PTH, 1,25-dihydroxyvitamin D (calcitriol), and FGF23.
4.
Vitamin D is synthesized from 7-dehydrocholesterol in skin in the presence of UVB light or acquired in the diet. It is hydroxylated to 25-hydroxycholecalciferol in the liver and activated by renal 1α-hydroxylase to 1,25-dihydroxyvitamin D.
5.
1,25-Dihydroxyvitamin D promotes intestinal Ca++ absorption and modestly increases Pi absorption.
6.
The flux of Ca++ and Pi into and out of bone is determined by the relative rates of osteoblastic
Bhattacharyya N, et al. Fibroblast growth factor 23: state of the field and future directions. Trends Endocrinol Metab. 2012;23:610-618.
Boyce BF. Advances in the regulation of osteoclasts and osteoclast functions. J Dent Res. 2013;92:860-867.
Christakos S, et al. Vitamin D: metabolism, molecular mechanism of action, and pleiotropic effects. Physiol Rev. 2016;96:365-408.
bone formation and osteoclastic bone resorption.
7.
The PTH/PTHrP receptor is expressed on osteoblasts, not on osteoclasts. PTH has both anabolic and catabolic actions in bone depending on the dose and timing of administration. PTH promotes bone resorption by upregulation of M-CSF and RANKL in osteoblasts.
8.
1,25-Dihydroxyvitamin D binds to the VDR in osteoblasts to support osteoclast differentiation via RANKL and promotes bone mineralization by maintaining appropriate serum [Ca++] and [Pi].
Upon completion of this chapter the student should be able to answer the following questions: 1.
Describe the structure and composition of the pituitary gland and its structural and functional relationship to magnocellular and parvicellular hypothalamic neurons.
2.
Discuss the mechanisms by which the neurohormones antidiuretic hormone (ADH) and oxytocin are synthesized, transported, and released by magnocellular neurons.
3.
Diagram a basic scheme illustrating the components and feedback loops of a typical endocrine axis, including central input, hypothalamic releasing factors, pituitary hormones, and a peripheral endocrine gland. Explain the concept of a set point.
4.
List the endocrine cell types of the adenohypophysis and the tropic hormones they produce, noting hormones that share a common subunit.
5.
Contrast the axes of somatotropes and lactotropes with the classic endocrine axes and explain how they differ.
6.
Discuss the actions of growth hormone (GH) and insulinlike growth factor I (IGF-I) inthe regulation of growth, and the role of growth hormone in the fasted state.
7.
Describe the role of prolactin in the initiation and maintenance of lactation.
he pituitary gland (also called the hypophysis) is a small (≈0.5 g in weight) yet complex endocrine structure at the base of the forebrain (
Fig. 41.1 ). It is composed of an epithelial component called the adenohypophysis and a neural structure called the neurohypophysis. The adenohypophysis is composed of five cell types that secrete six hormones. The neurohypophysis releases several neurohormones. All endocrine functions of the pituitary gland are regulated by the hypothalamus and by negativeand positive-feedback loops.