522 VERTEBRATE LIFE AND ORGANIZATION 



epithelial cells surrounding blood vessels. Three kinds of cells can be 

 distinguished by the shape and staining properties of their granules: 

 acidophils, basophils and chromophobes. The intermediate lobe con- 

 tains basophil cells smaller than those of the anterior lobe, some with 

 and some without granules. The posterior lobe is composed of many 

 nonmyelinated nerve fibers and branching cells (pituicytes) which con- 

 tain brownish cytoplasmic granules. 



The posterior lobe contains two hormones, oxytocin and vaso- 

 pressin. The latter is also known as antidiuretic hormone, or ADH. The 

 brilliant work of Vincent du Vigneaud, for which he was awarded the 

 Nobel Prize in 1955, has led to the isolation of these two hormones, 

 the determination of their molecular structure, and their synthesis. Each 

 is a peptide containing nine amino acids, seven of which are identical in 

 the two. It is of considerable interest that these two substances, with 

 cjuite different physiologic properties, differ only in two amino acids. 

 Oxytocin stimulates the contraction of the uterine muscles and is some- 

 times injected after childbirth to contract the uterus. Vasopressin causes 

 a contraction of smooth muscles; its contraction of the muscles in the 

 wall of arterioles causes a general increase in blood pressure. It also 

 regulates the reabsorption of water by the cells of the distal convoluted 

 tubules and Henle's loop in the kidney. Most investigators agree that 

 these two hormones are not produced in the posterior lobe, but are se- 

 creted by neurosecretory cells in the supraoptic and paraventricular 

 nuclei of the brain. They then pass along the axons of the hypothalamic- 

 hypophysial tract, and are stored and released by the posterior lobe. 

 An injury of these brain nuclei, of the posterior lobe, or of the con- 

 necting nerve tracts may lead to a deficiency of ADH and the condi- 

 tion known as diabetes insipidus. In this disease the patient's kidneys 

 have a lessened ability to reabsorb water and his urine volume in- 

 creases from the normal one or two liters to 10 to 25 liters per day. 

 He suffers from excessive thirst and drinks copiously. A comparable 

 condition can be produced in experimental animals by severing the 

 hypothalamic-hypophysial tract by electrolytic lesions accurately placed 

 with a microelectrode. Injection of ADH relieves all of the symptoms 

 but the injections must be repeated every few days. 



The intermediate lobe of the pituitary secretes a hormone, Inter- 

 medin, which darkens the skin of fishes, amphibians and reptiles by 

 dispersing the pigment granules in the chromatophores. The skin of a 

 frog becomes darkened in a cool, dark environment and light-colored 

 in a warm, light place (Fig. 30.8). Hypophysectomy produces a perma- 

 nent blanching of the skin, and injection of intermedin causes darken- 

 ing. The location of the pigment in the chromatophore is controlled 

 directly by the amount of intermedin present, not by nerves. The 

 pituitaries of birds and mammals are rich in intermedin but there is no 

 known function for this hormone in these animals; it does not affect 

 their pigmentation. 



The anterior lobe of the pituitary secretes the following hormones, 

 all of which are proteins: growth hormone (somatotropin), thyro- 

 tropin, adrenocorticotropin (ACTH), follicle-stimulating hormone (FSH), 



