insulin in response to meals. Individuals who are overweight and inactive
have an increased chance of developing type II diabetes.
Scattered through several organ systems (e.g., digestive and respira-
tory systems) are enteroendocrine cells,which synthesize peptide hor-
mones for local regulation.
URINARY SYSTEMThe filtration apparatusof the renal glomeruli consists of an expanded
basement membrane and slit pore associated with podocytes.Epithelial
cells of the proximal tubuleare specialized for absorption and ion trans-
port. They remove most of the sodiumandwaterfrom urine, as well as vir-
tually all of the amino acids, proteins, and glucose. The brush borderof the
proximal tubule cells contains proteases. The cells of the distal tubule,
under the influence of aldosterone, resorb sodium and acidify the urine.
Specialized cells of the distal tubule (the macula densa) monitor ion levels
in the urine and stimulate the juxtaglomerular (JG) cellsof the afferent
arterioleto secrete renin, an enzyme that cleaves angiotensinogento a
precursor of angiotensin II. Collecting ductscontain light and dark (inter-
calated) cells; they are sensitive to antidiuretic hormone (ADH)and are
thefinal mechanism for concentrating urine.Transitional epithelium
(allowing for stretch) is found lining the calyces, renal pelvis, ureters, and
urinary bladder.
MALE REPRODUCTIVE SYSTEMThe testes produce spermand testosteroneunder the influence of
luteinizing hormone (LH) and follicle-stimulating hormone (FSH),
secreted by gonadotrophs of the anterior pituitary.The testicular epithe-
lium contains Sertoli cellsand precursors of sperm. Spermatogenesis
involves the following lineage: spermatogonia (germ cells)→(sperma-
tocytogenesis)→primary spermatocytes→secondary spermatocytes→
(completion of meiosis) →spermatids (spermiogenesis) →mature
sperm. Sertoli cellsperform several functions: (1) maintenance of the
blood-testis barrier,(2)phagocytosis,and (3) secretion of androgen-bind-
ing protein andinhibin,as well as Müllerian inhibiting hormonein the
fetus.
Theepididymis,like most of the male duct system, is lined by a pseu-
dostratified epithelium characterized by modified microvilli (stereocilia).
34 Anatomy, Histology, and Cell Biology