428 SECTION IVEndocrine & Reproductive Physiology
complete spermatogenesis; this last part of maturation is
called spermiogenesis.
■ Testosterone is the principal hormone of the testis. It is synthe-
sized from cholesterol in Leydig cells. The secretion of testoster-
one from Leydig cells is under control of luteinizing hormone at
a rate of 4 to 9 mg/day in adult males. Most testosterone is
bound to albumin or to gonadal steroid-binding globulin in the
plasma. Testosterone plays an important role in the develop-
ment and maintenance of male secondary sex characteristics, as
well as other defined functions.
■ Ovaries also secrete progesterone, a steroid that has special
functions in preparing the uterus for pregnancy. During preg-
nancy the ovaries secrete relaxin, which facilitates the delivery of
the fetus. In both sexes, the gonads secrete other polypeptides,
including inhibin B, a polypeptide that inhibits FSH secretion.
■ In women, a period called perimenopause precedes menopause,
and can last up to ten years; during this time the menstrual
cycles become irregular and the level of inhibins decrease.
■ Once in menopause, the ovaries no longer secrete progesterone
and 17β-estradiol and estrogen is formed only in small amounts
by aromatization of androstenedione in peripheral tissues.
■ The naturally occurring estrogens are 17 β-estradiol, estrone, and
estriol. They are secreted primarily by the granulosa cells of the
ovarian follicles, the corpus luteum, and the placenta. Their bio-
synthesis depends on the enzyme aromatase (CYP19), which con-
verts testosterone to estradiol and androstenedione to estrone. The
latter reaction also occurs in fat, liver, muscle, and the brain.MULTIPLE-CHOICE QUESTIONS
For all questions, select the single best answer unless otherwise directed.
- If a young woman has high plasma levels of T 3 , cortisol, and
renin activity but her blood pressure is only slightly elevated and
she has no symptoms or signs of thyrotoxicosis or Cushing syn-
drome, the most likely explanation is that
A) she has been treated with TSH and ACTH.
B) she has been treated with T 3 and cortisol.
C) she is in the third trimester of pregnancy.
D) she has an adrenocortical tumor.
E) she has been subjected to chronic stress. - Full development and function of the seminiferous tubules
require
A) somatostatin.
B) LH.
C) oxytocin.
D) FSH.
E) androgens and FSH. - In humans, fertilization usually occurs in the
A) vagina.
B) cervix.
C) uterine cavity.
D) uterine tubes.
E) abdominal cavity.
4. In human males, testosterone is produced mainly by the
A) Leydig cells.
B) Sertoli cells.
C) seminiferous tubules.
D) epididymis.
E) vas deferens.
5. Home-use kits for determining a woman’s fertile period depend
on the detection of one hormone in the urine. This hormone is
A) FSH.
B) progesterone.
C) estradiol.
D) hCG.
E) LH.
6. Which of the following is not a steroid?
A) 17α-hydroxyprogesterone
B) estrone
C) relaxin
D) pregnenolone
E) etiocholanolone
7. Which of the following probably triggers the onset of labor?
A) ACTH in the fetus
B) ACTH in the mother
C) prostaglandins
D) oxytocin
E) placental renin
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