Reproduction
601
As a result of changes in the amount and pattern of hor-
mone secretions, reproductive function changes markedly
during a person’s lifetime and may be divided into the stages
summarized in
Table 17–1
.
General Principles of Gametogenesis
At any point in gametogenesis, the developing gametes are
termed
germ cells.
These cells undergo either mitosis or the
type of cell division known as meiosis (described later). Because
the general principles of gametogenesis are essentially the same
in males and females, they are introduced in this section, with
features specifi c to the male or female described later.
The fi rst stage in gametogenesis is proliferation of the
primordial germ cells by mitosis. With the exception of the
gametes, the DNA of each nucleated human cell is contained
in 23 pairs of chromosomes, giving a total of 46. The two cor-
responding chromosomes in each pair are said to be homol-
ogous to each other, with one coming from each parent. In
mitosis,
the 46 chromosomes of the dividing cell are repli-
cated. The cell then divides into two new cells called daughter
cells. Each of the two daughter cells resulting from the divi-
sion receives a full set of 46 chromosomes identical to those
of the original cell. Thus, each daughter cell receives identical
genetic information during mitosis.
In this manner, mitosis of primordial germ cells, each
containing 46 chromosomes, provides a supply of identical
germ cells for the next stages. The timing of mitosis in germ
cells differs greatly in females and males. In the female, mito-
sis of germ cells in the ovary occurs primarily during fetal
development. In the male, some mitosis occurs in the embry-
onic testes to generate the population of germ cells present at
birth, but mitosis really begins in earnest at puberty and usu-
ally continues throughout life.
The second stage of gametogenesis is
meiosis,
in which
each resulting gamete receives only 23 chromosomes from a
46-chromosome germ cell, one chromosome from each homol-
ogous pair. Because a sperm and an ovulated egg each has only
23 chromosomes, their union at fertilization results once again
in a cell with a full complement of 46 chromosomes.
The process of meiosis is depicted in
Figure 17–2
(let-
ters are keyed to the text). Meiosis consists of two cell divi-
sions in succession. The events preceding the fi rst meiotic
division are identical to those preceding a
mitotic
division.
During the interphase period that precedes a mitotic division,
chromosomal DNA is replicated. Thus, after DNA replication,
an interphase cell has 46 chromosomes, but each chromosome
consists of two identical strands of DNA, termed sister chro-
matids, which are joined together by a centromere (a).
As the fi rst meiotic division begins, homologous chromo-
somes, each consisting of two identical sister chromatids, come
together and line up adjacent to each other. Thus, (b) 23 pairs
of homologous chromosomes (called
bivalents
) are formed.
(c) The sister chromatids of each chromosome condense into
thick, rodlike structures. Then, (d) within each homologous
Gonads
Sex hormones
Secrete
sex hormones
Gametogenesis
Anterior pituitary
Secretes FSH and LH
FSH and LH
Hypothalamus
Secretes GnRH
GnRH
(in hypothalamo-pituitary
portal vessels)
+
Reproductive tract
and other organs
Respond to sex hormones
Begin
Figure 17–1
General pattern of reproduction control in both males and females.
GnRH, like all hypothalamic hypophysiotropic hormones, reaches
the anterior pituitary via the hypothalamo-pituitary portal vessels.
The arrow within the box marked “gonads” denotes the fact that
the sex hormones act locally, as paracrine agents, to infl uence the
gametes.
-
indicates negative feedback inhibition.
+
indicates
estrogen stimulation of FSH and LH in the middle of the menstrual
cycle in women (positive feedback).
Table 17–1
Stages in the Control of Reproductive
Function
1.
During the initial stage, which begins during fetal life
and ends in the fi rst year of life (infancy), GnRH, the
gonadotropins, and gonadal sex hormones are secreted at
relatively high levels.
2.
From infancy to puberty, the secretion rates of these
hormones are very low, and reproductive function is quiescent.
3.
Beginning at puberty, hormonal secretion rates increase
markedly, showing large cyclical variations in women during
the menstrual cycle. This ushers in the period of active
reproduction.
4.
Finally, reproductive function diminishes later in life,
largely because the gonads become less responsive to the
gonadotropins. The ability to reproduce ceases entirely in
women.
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