CHAPTER 28 REPRODUCTIVE
SYSTEMS
In the reproductive systems:
Gonads--ovaries or testes--produce gametes (eggs [ova] or sperm)--also
secrete hormones
Ducts--transport & store gametes
Accessory sex organs--produce materials that support gametes
To understand the major function of the
reproductive system, production of gametes, we must first understand a process
of nuclear division that differs from the process of mitosis. In mitosis, the
goal is to produce daughter cells identical to the parent cell, with exactly
the same number and kind of chromosomes. This is not suitable for the
production of gametes, since gametes need to have half the regular number of
chromosomes, one of each of the usual 23 pairs. Only in this way can the 2 gametes
which unite to form a new individual both contribute half of the genetic
material, but still produce an individual with a total of 23 pairs of
chromosomes.
Remember, human somatic cells each
contain 23 pairs of chromosomes (46 total). One
chromosome of each pair came from each parent. The 2 chromosomes of a pair are
called homologous chromosomes.
Homologous pairs 1 - 22 are called autosomes. The 2 members of all of these pairs look the
same and have similar genes.
The 23rd pair are the sex chromosomes. Females have 2 X chromosomes for
this pair; males have one X and one Y.
Somatic cells are all cells of the body
except those that produce eggs and sperm. Somatic cells with their 23 pairs of
chromosomes are said to be diploid. Their number of chromosomes is represented
as the 2n number.
In sexual reproduction, 2 gametes come
together to create a new individual. Each gamete contributes one-half of the
genetic material. Since the total number of chromosomes must be 46, each gamete
must carry only 23 total chromosomes. The process of nuclear division that
reduces the chromosomes in a gamete to half the normal number, or one
chromosome of each homologous pair, is called meiosis. Cells produced by
meiosis are called haploid cells, with the n
number of chromosomes.
The process of meiosis is similar in
some ways to mitosis. However, meiosis involves 2 divisions, instead of 1, and
results in haploid cells whose genetic content is NOT identical to the parent
cell.
The process begins with replication of
chromosomes (same as mitosis). An identical copy of each chromosome is made.
The cell builds a mitotic apparatus to organize division of the chromosomes. In
both meiotic divisions, the same 4 phases occur:
Prophase
Metaphase
Anaphase
Telophase
Meiosis
I---reduction
division---begins after chromosomes are replicated.
Prophase I-----Each chromosome consists
of 2 chromatids connected by a centromere. Duplicated
chromosomes shorten and thicken, nucleolus and nuclear membrane disappear, mitotic apparatus is built (same events as mitosis).
However, a very important difference exists in meiosis. During prophase I,
homologous pairs of chromosomes come into close association with each other
(synapsis). The 4 chromatids make up a tetrad. Portions of one chromatid may be exchanged with portions of another
(crossing-over). This results in genetic recombination, so that the chromosomes
of the gamete are not identical to the chromosomes of the person producing it.
Metaphase I---Chromosomes line up on
the equator of the mitotic apparatus, with homologous pairs of chromosomes
together
Anaphase I-----members of homologous
pairs move to opposite poles. Centromeres do not split, so chromatid
pairs remain together
Telophase I-----one of each homologous
pair reaches opposite poles of the cell and cytokinesis divides the original
cell into 2 cells
Meiosis
II-----equatorial
division-----this process continues in each of the 2 cells left at the end of
meiosis I. With no further replication of DNA, the cell forms a new mitotic
spindle and proceeds through the 4 phases:
Prophase II----same events as prophase
I, except no synapsis or crossing over
Metaphase II-----chromatid
pairs to equator of mitotic spindle
Anaphase II---this time the centromeres
split and one of each chromatid pair has toward
opposite poles of the cell
Telophase II---cell divides into 2
cells
A diploid parent cell can produce 4
genetically different haploid cells. In sperm production, 4 sperm result from
one parent cell. In egg production, as we will explain later, only one complete
egg is produced.
SCROTUM--loose skin and superficial
fascia that cover and support testes. The cremaster muscle(a continuation of the internal oblique) can raise or
lower the scrotum and testes, regulating the temp of the testes, which must be
about 3o lower than body
temp.
TESTES (testicles)--male
gonads--develop inside the abdominal cavity of the embryo and descend into the
scrotum, passing through the inguinal canal, before birth. Inside the testes
are tiny tightly coiled tubes called the seminiferous
tubules, where sperm are produced. Spermatogenic
cells in various stages of development line the tubules. Also in the tubules
are sustentacular (Sertoli)
cells, which nourish the developing sperm.
Outside the tubules are interstitial (Leydig) cells, which secrete testosterone.
Spermatogenesis--process by which
haploid spermatozoa are produced. In humans, spermatogenesis takes a total of
about 75 days (2 1/2 months). The seminiferous
tubules are lined with diploid cells called spermatogonia,
which developed from primordial germ cells early in development. Some spermatogonia remain near the basement membrane and undergo
mitosis, producing more spermatogonia. Other spermatogonia lose contact with the basement membrane and
differentiate into primary spermatocytes, which are
still diploid. At this point the primary spermatocytes
enlarge and meiosis begins.
Meiosis I (Reduction division) takes
place in primary spermatocytes and the end result is
the production of 2 secondary spermaticytes.
Meiosis II (Equatorial division)---takes place in the 2 secondary spermatocytes
and each produces 2 spermatids (a total of 4 spermatids). With no further replication of DNA, the
chromosomes line up on the equator and the chromatids separate and migrate to
opposite poles. This forms spermatids, which are
truly haploid. Two of these will have an X chromosome for the 23rd
chromosome and 2 will have a Y. (Sperm determine sex of the baby, because all
eggs will have an X.)
Each primary spermatocyte
produces 4 spermatids. In the process of spermiogenesis, the spermatids
mature into spermatozoa. Each spermatid embeds in a sustentacular cell and develops:
Spermiation is the release of the spermatozoan from its sustentacular
cell into the lumen of the tubule. From here spermatozoa migrate to the ductus epididymis, where they
must mature for 10-14 days. Then they progress to the ductus
(vas) deferens, where they may be stored for up to several months.
Spermatozoa mature at the rate of 300
million per day.
FSH (follicle-stimulating hormone) from the anterior pituitary
stimulates the sustentacular cells
LH (luteinizing hormone) from the anterior
pituitary stimulates the interstitial cells of the testes to produce the 2
major androgens, testosterone and dihydrotestosterone.
These hormones are synthesized from cholesterol. Testosterone is the principal
male hormone. Effects:
1. Development of the male reproductive organs and descent of the testes
before birth
2. At puberty brings about further development of male sex organs and
male secondary sex characteristics
3. Sexual function, spermatogenesis and sex drive (libido)
4. Metabolism--stimulates protein synthesis, leading to heavier muscle
and bone mass
Seminiferous tubules (where sperm development
occurs)
Straight tubules
Rete testis
Epididymis--comma-shaped organ along the
posterior border of the testis. It consists of the head, the body and the tail.
Here sperm spend 10-14 days and undergo maturation
Ductus (vas) deferens--formed at the end of
the tail of the epididymis. On each side the ductus deferens enters the pelvic cavity through the
inguinal canal as a part of the spermatic cord and loops down behind the
urinary bladder, where it joins the duct from the seminal vesicle to form the
ejaculatory duct. As the ductus deferens ascends
toward the inguinal canal, it travels in the spermatic cord, which contains:
Ductus
(vas) deferens
Artery & vein (gonadal or testicular)
Lymphatics
Nerves
Cremaster
muscle
Ejaculatory duct--formed posterior to
the urinary bladder as the terminal part of the ductus
deferens, the ampulla,
and the duct from the seminal vesicle unite
Urethra--terminal duct of both the male
reproductive and the male urinary systems. It passes through the prostate gland
and into the penis, ending at the external urethral orifice. In the male, the
urethra is about 8 inches long and consists of 3 parts:
1. Prostatic urethra
2. Membranous urethra
3. Penile urethra
These secrete most of the liquid
portion of semen. Their secretions together act to protect
and activate sperm, increasing chances for fertilization.
SEMINAL VESICLES--paired and lie
posterior to the bladder. They secrete an alkaline fluid that neutralizes the
acid pH of the vagina. It also contains fructose, which provides energy for the
sperm, and prostaglandins, which contribute to sperm motility and viability. 60% of the volume of semen.
PROSTATE GLAND--just inferior to the
bladder and surrounds the superior urethra. The prostate secretes a fluid that
contributes to sperm motility and viability and makes up about 25% of
semen.
BULBOURETHRAL GLANDS (Cowper's
glands)--paired and lie beneath the prostate--secrete an alkaline fluid and
mucus
SEMEN--mixture of sperm and the
secretions of the accessory sex glands. Average volume in an ejaculation is
2.5-5 ml with 50-150 million sperm per ml. A male is considered infertile if
the number of sperm falls below 20 million per ml. Only a tiny fraction ever
reach the ovum and the action of many sperm is required for one to penetrate
the ovum. pH is slightly alkaline. Semen provides
sperm with nutrients, a transportation medium and enzymes that activate sperm
after ejaculation.
PENIS--used to introduce sperm into the
vagina. It contains 3 masses of erectile tissue which contains blood sinuses.
Arteries leading in dilate and blood is trapped in the sinuses as veins are
compressed, producing erection of the penis. Ejaculation is the propulsion of
semen through the urethra. The smooth muscle sphincter of the bladder prevents
urine from leaving the bladder during ejaculation.
OVARIES--female gonads in the upper
pelvic cavity, held in place by ligaments. A single layer of germinal
epithelium covers the surface of the ovary. Within the ovary are ovarian
follicles containing oocytes in various stages of
development. The cells that form these follicles can be stimulated to secrete
the female hormones.
OOGENESIS--formation of haploid ova.
The process is similar to spermatogenesis, although only one viable ovum is
produced from one oogonium. During embryonic
development, primordial germ cells migrate to the ovaries and become oogonia, precursors of eggs. The diploid oogonia develop into diploid primary oocytes
before birth. These cells begin reduction division, but do not complete it at
this time. Each primary oocyte is surrounded by one
layer of follicular epithelial cells and the structure is called a primordial
follicle.
Each ovary contains about 200,000
primordial follicles at birth and no more will be formed. Beginning at puberty,
several primordial follicles respond each month to FSH and become primary
follicles. Additional layers of follicular cells appear and are now called granulosa cells because they are involved with a developing
follicle. Most of the developing primary follicles soon degenerate, but one per
month proceeds, under the influence of FSH and later LH. Within this lucky follicle, meiosis resumes
and Meiosis I is completed. This stage in oogenesis
produces one secondary oocyte, which receives most of
the cytoplasm, and a small polar body, which is discarded.
The secondary oocyte
continues to develop within what is now called a secondary follicle and
proceeds to metaphase of meiosis II, where it remains until after ovulation and
fertilization. If the secondary oocyte is not
fertilized, meiosis II will not be completed. As the follicle continues to
develop, it is called a mature or Graafian follicle.
At ovulation the secondary oocyte is discharged from the follicle (ovulation) and
enters the Fallopian tube. If fertilization occurs, the secondary oocyte proceeds with Meiosis II and produces an ovum and
another small polar body. All polar bodies disintegrate, so only 1 ovum is
produced from 1 oogonium.
|
PRIMORDIAL FOLLICLE |
APPEARED BEFORE
BIRTH |
CONTAINS PRIMARY
OOCYTE IN INACTIVE STATE |
NO HORMONES |
|
PRIMARY FOLLICLE |
SEVERAL BEGIN
DEVELOPMENT EACH MONTH, ONLY ONE GOES ON TO COMPLETE IT (USUSALLY) |
CONTAINS PRIMARY
OOCYTE BUSY COMPLETING MEIOSIS I |
SECRETES ESTROGENS |
|
SECONDARY FOLLICLE |
FROM PRIMARY
FOLLICLE THAT CONTINUES TO DEVELOP |
CONTAINS SECONDARY
OOCYTE WORKING ON MEIOSIS II |
SECRETES ESTROGENS |
|
MATURE (GRAAFIAN)
FOLLICLE |
DEVELOPS FROM
SECONDARY FOLLICLE---FINAL MATURATION AND RUPTURE DUE TO LH |
CONTAINS SECONDARY
OOCYTE IN METAPHASE OF MEIOSIS II |
SECRETES ESTROGENS |
|
CORPUS LUTEUM |
DEVELOPS FROM
REMAINING GRANULOSA CELLS AFTER FOLLICLE RUPTURES |
OOCYTE GONE DUE TO
OVULATION |
SECRETES ESTROGENS
AND PROGESTERONE |
OVULATION RELEASES A SECONDARY
OOCYTE. THERE ARE 2 POSSIBILITIES AS TO
WHAT HAPPENS NEXT:
COMPLETES
DETERIORATES
MEIOSIS II
DEVELOPS INTO BABY PASSES OUT UNNOTICED
UTERINE (FALLOPIAN) TUBES--these extend
laterally from the uterus to each ovary and transport ova. The distal end of
each tube forms the funnel-shaped infundibulum, which
ends in a fringe of finger-like projections called fimbriae, which help sweep
the ovum into the tube. The ovum is moved along the tube by peristalsis and the
beating of cilia of the surface of cells lining the tube. Fertilization
normally occurs high in the tube and the zygote arrives in the uterus about 7
days after ovulation. Unfertilized secondary oocytes
disintegrate.
UTERUS--shaped like an inverted pear,
the uterus functions as:
Pathway for sperm
Site of menstruation
Site of implantation of zygote
Site of development of embryo and fetus
Means of expulsion of fetus (labor)
Parts of the uterus:
Fundus--dome-shaped portion above the tubes
Body--main central portion
Cervix--narrow inferior portion
The normal position of the uterus is
bent forward over the urinary bladder (anteflexion).
Several ligaments hold the uterus in place:
Broad ligaments--attach uterus to sides of pelvic cavity
Uterosacral ligaments--connect uterus to
sacrum
Cardinal ligaments--maintain normal flexed position
Round ligaments---attach uterus to abdominal wall and labia majora
The uterine wall consists of 3 layers
of tissue:
Perimetrium--visceral peritoneum
Myometrium--3 layers of smooth muscle
Endometrium--highly vascular inner lining
which is in 2 layers:
Stratum basalis--deeper permanent layer which
forms each month a new:
Stratum functionalis--this layer will support
the fetus if pregnancy occurs--if not it is shed in menstruation
VAGINA--muscular tube-like organ which:
Serves as a passage for menstrual flow
Serves as a passage for childbirth
Receives semen from the penis during intercourse
It is located between the urinary
bladder and the rectum. it is lined with mucous
membrane and maintains a low pH to retard growth of harmful bacteria.
VULVA--external genitalia of the female
and consists of:
Mons pubis--adipose tissue covered by skin located over the symphysis pubis
Labia majora--folds of skin that extend
inferiorly and posteriorly from the mons pubis
Labia minora--2 smaller folds of mucous membrane
located medial to the labia majora
Clitoris--small mass of erectile tissue located at the anterior junction
of the labia minora--homologous to the penis
Vestibule--cleft between the labia minora--contains
the external vaginal orifice and the external urethral orifice. The paraurethral (Skene's) glands and
the greater vestibular (Bartholin's) glands also open
into the vestibule.
The perineum is the diamond-shaped area
between thighs and buttocks in both males and females that contains external genitalia
and the anus.
MAMMARY GLANDS--modified sudoriferous glands that produce milk. They lie over the pectoralis major and serratus
anterior muscles. Each mammary gland consists of 15-20 lobes separated by
adipose tissue. Lobes are divided into lobules, which contain clusters of
milk-secreting tissue called alveoli.
Alveoli
Secondary tubules
Mammary ducts
Lactiferous sinuses (storage area)
Lactiferous ducts
Nipple
The pigmented area surrounding the
nipple is the areola.
Lactation is the synthesis, secretion
and ejection of milk. Prolactin is the main hormone
of lactation, with estrogens and progesterone also involved. Oxytocin causes milk ejection ("let-down").
Read about breast cancer p. 1054
This really includes 2 cycles:
·
The
ovarian cycle, a series of events in the ovary leading to maturation of an ovum
·
The
uterine (menstrual) cycle, a series of changes in the endometrium
of the uterus.
These must be coordinated so that the
uterus is in the proper state to receive a fertilized egg at the exact time the
egg would arrive. Both cycles are controlled by hormones of the hypothalamus
and anterior pituitary:
Gonadotropin-releasing hormone--stimulates
release of FSH and LH
FSH--stimulates initial development of ovarian follicles and secretion
of estrogens
LH--stimulates maturation of follicles, ovulation and secretion of
hormones by the corpus luteum
The ovaries produce 2 major hormones:
A. Estrogens--synthesized from cholesterol and at least 6 different
forms are found. Effects:
1. Promote development and maintenance of female reproductive
structures--in the uterus causes growth of a new
stratum functionalis each month
2. Increase protein anabolism
3. Lower blood cholesterol level
B. Progesterone--works with estrogens to prepare the endometrium
to receive a fertilized ovum and the mammary glands for milk secretion
Also, 2 additional hormones are produced:
C. Inhibin--inhibits secretion of FSH and LH
toward the end of the uterine cycle
D. Relaxin--produced by the corpus luteum and the placenta during pregnancy, it relaxes the symphysis pubis and pelvic ligaments
The female reproductive cycle may range
from 24-35 days, with 28 days being the average.
1. Menstrual phase--first 5 days of the
cycle. 50-150 ml of blood, tissue fluid, mucus and sloughed-off epithelial
cells from the endometrium are discharged. This
begins because the corpus luteum tapers off
secretion. Estrogen and progesterone levels drop, causing small arteries of the
uterus to constrict so that the stratum functionalis
dies and is shed. During the menstrual phase rising levels of FSH are already
causing the development of several primary follicles in the ovaries. By the end
of day 5 about 20 primary follicles are developing and secreting estrogens.
2. Preovulatory
phase--time between menstruation and ovulation--day 6-13 in a 28-day cycle.
Early in this phase one secondary follicle begins to outgrow all the others.
This one matures into a vesicular ovarian (Graafian)
follicle and forms a bulge on the surface of the ovary. LH secretion is
increasing late in this phase.
In the uterus, estrogens are
stimulating the regrowth of the stratum functionalis. The preovulatory
phase is also called the proliferative phase of the
uterus.
In the ovaries, both the menstrual and preovulatory phase together are called the follicular phase
because ovarian follicles are growing and developing.
OVULATION--this is not considered a
phase in the cycle. It is the rupture of the Graafian
follicle and release of the secondary oocyte into the
pelvic cavity, usually on day 14 in a 28-day cycle. At the time of ovulation
the secondary oocyte is in metaphase of Meiosis II.
The fimbriae and movements of the Fallopian tube draw the oocyte
into the tube.
A surge of LH causes ovulation to occur
(this is what is measured in tests to predict ovulation). After ovulation the
follicle collapses and remaining follicular cells change to form the corpus luteum, which secretes estrogens and progesterone.
3. Postovulatory phase--days
15-28--time between ovulation and menstruation. It is also called the luteal phase.
Progesterone secreted by the corpus luteum prepares the endometrium
to receive a fertilized ovum. If fertilization and implantation do not occur,
the corpus luteum degenerates to the form the corpus albicans and stops secreting estrogens and progesterone.
This brings on menstruation and a new cycle begins.
If fertilization and implantation do
occur, the corpus luteum is maintained by hCG produced by the placenta, and
secretes estrogens and progesterone until the placenta takes over later in
pregnancy.