Reproduction 751- The secretion of testosterone is relatively constant
rather than cyclic, and it does not decline sharply at a
particular age.
B. Testosterone promotes the growth of soft tissue and bones
before the epiphyseal discs have sealed; thus, testosterone
and related androgens are anabolic steroids. - Testosterone is secreted by the interstitial Leydig cells
under stimulation by LH. - LH receptor proteins are located in the interstitial tissue.
FSH receptors are located in the Sertoli cells within the
seminiferous tubules. - The Leydig cells of the interstitial compartment and the
Sertoli cells of the tubular compartment of the testes
secrete autocrine regulatory molecules that allow the
two compartments to interact.
C. Diploid spermatogonia in the seminiferous tubules undergo
meiotic cell division to produce haploid sperm. - At the end of meiosis, four spermatids are formed. They
develop into spermatozoa by a maturational process
called spermiogenesis. - Sertoli cells in the seminiferous tubules are required for
spermatogenesis. - At puberty, testosterone is required for the completion
of meiosis, and FSH is required for spermiogenesis.
D. Spermatozoa in the seminiferous tubules are conducted to
the epididymis and drained from the epididymis into the
ductus deferens. The prostate and seminal vesicles add fluid
to the semen.
E. Penile erection is produced by parasympathetic-induced
vasodilation. Ejaculation is produced by sympathetic nerve
stimulation of peristaltic contraction of the male accessory
sex organs.
20.4 Female Reproductive System 722
A. Primordial follicles in the ovary contain primary oocytes
that have become arrested at prophase of the first meiotic
division. Their number is maximal at birth and declines
thereafter.
- A small number of oocytes in each cycle are stimulated
to complete their first meiotic division and become
secondary oocytes. - At the completion of the first meiotic division, the
secondary oocyte is the only complete cell formed. The
other product of this division is a tiny polar body, which
disintegrates.
B. One of the secondary follicles grows very large, becomes a
graafian follicle, and is ovulated. - Upon ovulation, the secondary oocyte is extruded from
the ovary. It does not complete the second meiotic
division unless it becomes fertilized. - After ovulation, the empty follicle becomes a new
endocrine gland called a corpus luteum. - The ovarian follicles secrete only estradiol, whereas the
corpus luteum secretes both estradiol and progesterone.
C. The hypothalamus secretes GnRH in a pulsatile fashion,
causing pulsatile secretion of gonadotropins. This is needed
to prevent desensitization and downregulation of the target
glands.
20.5 Menstrual Cycle 728
A. During the follicular phase of the cycle, the ovarian follicles
are stimulated by FSH from the anterior pituitary.
1. Under FSH stimulation, the follicles grow, mature, and
secrete increasing amounts of estradiol.
2. At about day 13, the rapid rise in estradiol secretion
stimulates a surge of LH from the anterior pituitary.
This represents positive feedback.
3. The LH surge stimulates ovulation at about day 14.
4. After ovulation, the empty follicle is stimulated by LH
to become a corpus luteum, at which point the ovary is
in a luteal phase.
5. The secretion of progesterone and estradiol rises during
the first part of the luteal phase and exerts negative
feedback on FSH and LH secretion.
6. Without continued stimulation by LH, the corpus luteum
regresses at the end of the luteal phase, and the secretion
of estradiol and progesterone declines. This decline
results in menstruation and the beginning of a new cycle.
B. The rising estradiol concentration during the follicular phase
produces the proliferative phase of the endometrium. The
secretion of progesterone during the luteal phase produces
the secretory phase of the endometrium.
C. Oral contraceptive pills usually contain combinations of
estrogen and progesterone that exert negative feedback
control of FSH and LH secretion.20.6 Fertilization, Pregnancy, and
Parturition 734
A. The sperm undergoes an acrosomal reaction, which allows it
to penetrate the corona radiata and zona pellucida.
1. Upon fertilization, the secondary oocyte completes
meiotic division and produces a second polar body,
which degenerates.
2. The diploid zygote undergoes cleavage to form a morula
and then a blastocyst. Implantation of the blastocyst in the
endometrium begins between the fifth and seventh day.
B. The trophoblast cells of the blastocyst secrete human
chorionic gonadotropin (hCG), which functions in the
manner of LH and maintains the mother’s corpus luteum
for the first 10 weeks of pregnancy.
1. The trophoblast cells provide the fetal contribution to
the placenta. The placenta is also formed from adjacent
maternal tissue in the endometrium.
2. Oxygen, nutrients, and wastes are exchanged by
diffusion between the fetal and maternal blood.
C. The placenta secretes chorionic somatomammotropin (hCS),
chorionic gonadotropin (hCG), and steroid hormones.
1. The action of hCS is similar to that of prolactin and
growth hormone. The action of hCG is similar to that of
LH and TSH.
2. The major steroid hormone secreted by the placenta is
estriol. The placenta and fetal glands cooperate in the
production of steroid hormones.
D. Contraction of the uterus in labor is stimulated by oxytocin
from the posterior pituitary and by prostaglandins, produced
within the uterus.