Download Reproduction

‫كلية الصيدلة والعلوم الطبية‬
‫قسم التغذية‬
‫« من قيمنا »‬
‫• االلتزام بالعدالة االجتماعية والمسؤولية‬
‫االجتماعية‬
‫نحن كالمسافر في الباخرة أو الطيارة‪،‬‬
‫ه ّمه الغرفة الجميلة أو المقعد المريح‪،‬‬
‫يركب في الدرجة األولى ويأكل أطيب الطعام‪،‬‬
‫ويتصفح الجرائد والمجالت‪،‬‬
‫ينقّل بصره فيما حوله أو تحته من المشاهد‪.‬‬
‫ّ‬
‫ولكن هذا كله أليام السفر‪،‬‬
‫وأيام السفر معدودة‪،‬‬
‫أفما كان خيرا له لو فكّر فيما يريحه‬
‫في إقامته الدائمة في بلده‪.‬‬
Chapter 20 Reproduction (Pages 701-747)
Reproduction is one of life's essential functions.
Sexual reproduction, in which genes from two individuals are combines in
random and novel way with each new generation, offers the further advantage
of introducing great variability into a population. This diversity of genetic
constitution helps to ensure that some members of a population will survive
changes in the environment over evolutionary time.
In sexual reproduction, germ cells, or gametes (sperm and ova) are formed
within the gonads (testes and ovaries) by a process of reduction division, or
meiosis. During meiosis, the normal number of chromosomes in human cells
(46)is halved, so that each gamete receives 23 chromosomes.
Fusion of a sperm cell and ovum (egg cell) in the act of fertilization results in
restoration of the original chromosome number of 46 in the zygote, or fertilized
egg. Growth of the zygote into an adult member of the next generation occurs
by means of mitotic cell divisions.
When this individual reaches puberty, mature sperm or ova will be formed by
meiosis within the gonads so that the life cycle can be continued (Fig. 20.1
page 701)
Fig. 20.1 The human
life cycle.
Numbers in
parentheses indicate
the haploid state (23
chromosomes) or
diploid state (46
chromosomes).
Sex Determination
The first 22 pairs of chromosomes are called autosomal chromosomes.
The 23 rd pair of chromosomes is the sex chromosomes.
The chromosomal sex of the zygote is determined by the fertilizing sperm cell.
Male Accessory Sex Organs (Fig.20.19 page 718)
Female Reproductive System (Fig.20.23 page 722 Fig. 24 & 25 page 723)
The Ovarian Cycle (Fig.20.31 page 727)
Phases of the Menstrual Cycle: (Fig.20.33 page 729 & Table 20.6 page 731)
Fig. 20.4 The
chromosomal sex and the
development of
embryonic gonads.
The very early embryo has
“indifferent gonads” that can
develop into either testes of
ovaries. The testisdetermining factor (TDF) is
a gene located on the Y
chromosome. In the
absence of TDF, ovaries
will develop.
Fig. 20.19 The organs of the male reproductive system
Fig. 20.23 The uterus, the uterine tubes, and ovaries.
Fig. 20.24 The organs of the female reproductive system.
Fig. 20.25 The external female genitalia.
The labia majora and clitoris in a female are homologous
to the scrotum and penis
Fig. 20.31 Stages of ovum and follicle development.
This diagram illustrates the stages that occur in an ovary during the
course of a monthly cycle. The arrows indicate changes with time.
Fig. 20.33 The
cycle of ovulation
and menstruation.
The downward
arrows indicate the
effects of the
hormones.
‫األستاذ‪ :‬متى يحدث الزلزال؟‬
‫الطالب‪ :‬في فصل الشتاء تصاب األرض ببرد‬
‫شديد فتعطس ويحدث زلزال !‬
Effects of Pheromones, Stress, and Body Fat
GnRH stimulates the anterior pituitary to secrete FSH and LH.
The GnRH-releasing neurons of the hypothalamus might be considered the
master regulators of the reproductive system.
The release of GnRH is regulated by feedback effects of ovarian hormones and
by input from higher brain centers.
Because of input to GnRH neurons from the olfactory system, pheromones can
cause the menstrual cycle of roommates to synchronize.
This pheromonal effect in humans is due to the stimulation of olfactory neurons
in the nasal mucosa.
The limbic system of the brain includes regions involved in emotions.
.
Axons extend from the limbic system to the GnRH neurons of the
hypothalamus. By means of these neural pathways, the secretion of GnRH,
and thus of FSH and LH, can be influenced by stress and emotions.
Considering this, it is not surprising that stress can even cause a cessation of
menstruation or amenorrhea.
Many girls who are very thin or athletic have a delayed menarche.
Women with low body fat can have irregular cycles or amenorrhea.
Functional amenorrhea is the cessation of menstruation caused by inadequate
stimulation of the ovaries by FSH, and LH, which in turn is due to inadequate
release of GnRH from the hypothalamus.
Functional amenorrhea is most often seen in women who are thin and athletic,
as well as women under prolonged stress.
Intense physical exercise can suppress GnRH secretion, and reducing the
exercise program can reverse the amenorrhea produced by rigorous athletic
training.
Leptin, secreted by adipocytes, regulates hunger and metabolism; it also
indirectly affect the GnRH-secreting neurons of the hypothalamus.
Because of this, a sufficient amount of adipose tissue and leptin secretion is
required for ovulation and reproduction, and inadequate adiposity (and leptin
secretion) can produce a functional amenorrhea.
Contraceptive Methods
I. Contraceptive Pill:
About 60 million women worldwide are currently using oral contraceptive.
Usually consist of a synthetic estrogen combined with synthetic progesterone in the form
of pills.
They are taken once each day for 3 weeks after the last day of a menstrual period.
The side effects of earlier versions of the birth control pill have been reduced through the
use of newer generations of progestons (analogue of progesterone).
The newer contraceptive pills are very effective and have a number of
beneficial side effects:
1. Reduced risk for endometrial and ovarian cancer.
2. Reduction in osteoporosis.
The side effects are increased risk for breast cancer, and possibly cervical
cancer.
II. Rhythm Method:
Studies have demonstrated that the likelihood of a pregnancy is close to zero if coitus
occurs more than 6 days prior to ovulation, and that the likelihood is very low if coitus
occurs more than a day following ovulation.
Conception is most likely to result when intercourse takes place 1-2 days prior to
ovulation.
There is no evidence for differences in the sex ratio of babies conceived at these
different times.
Cyclic changes in ovarian hormone secretion also cause cyclic changes in basal body
temperature.
In this method of birth control, a woman measures her oral basal body temperature
upon waking to determine when ovulation has occurred.
On the day of the LH peak, when estradiol secretion begins to decline, there is a
slight drop in basal body temperature.
Starting about 1 day after the LH peak, the basal body temperature sharply rises as a
result of progesterone secretion, and it remains elevated throughout the luteal phase
of the cycle.
The day of ovulation for that month's cycle can be accurately determined by this
method.
Since the day of the cycle on which ovulation occurs is quite variable in many women,
however, the rhythm method is not very reliable for contraception by predicting when
the next ovulation will occur.
Fig. 20.37 Changes in basal body temperature during the menstrual
cycle.
Menopause
Means "pause in the menses".
Refers to cessation of ovarian activity and menstruation.
Occurs at about the age of 50.
The ovaries are depleted of follicles and stop secreting estradiol and inhibin.
The fall in estradiol is due to changes in the ovaries and not in the pituitary.
FSH and LH secretion by the pituitary is elevated because of a lack of negative
feedback from estradiol and inhibin.
Withdrawal of estradiol secretion fro the ovaries is most responsible for the many
symptoms of menopause that include:
1. Vasomotor disturbances that produce "hot flashes" where a fall in core body
temperature is followed by feelings of heat and profuse perspiration.
2. Urogenital atrophy that is atrophy of the urethra, vaginal wall, and vaginal glands
occurs, with loss of lubrication.
3. Increased risk of atherosclerotic cardiovascular disease.
4. Increased progression of osteoporosis.
Fertilization, Pregnancy, and Parturition
Sperm are stored in the epididymis, where they are fully developed yet incapable of
fertilization. This is largely because they are kept in a slightly acidic state, with a
cytoplasmic pH below 6.5.
During the act of sexual intercourse, the male ejaculates an average of 300 million
sperm into the vaginal of the female.
This tremendous number is needed because of the high sperm fatality.
Only about 100 survive to enter each fallopian tube.
During their passage through the female reproductive tract, about 10% of the sperm
gain the ability to fertilize an ovum, this process is called capacitation.
In order for the sperm to become capacitated, they must be present in the female tract
for at least 7 hours.
During this time, the pH of sperm cytoplasm is increased (made alkaline) by exposure
to the increasing pH of the female reproductive tract (from 4 to pH 7) and by extrusion
of H+ from the sperm cytoplasm.
The capacitated sperm are guided in their passage up the oviduct toward the ovum by
chemotaxis (attraction toward particular chemicals) and thermotaxis (attraction toward
the warmer temperatures higher in the oviduct).
A woman usually ovulates only one ovum a month.
A total of less than 450 ova during the reproductive years.
Each ovulation releases a secondary oocyte arrested at metaphase of the second
meiotic division.
The secondary oocyte enters the uterine tube surrounded by its zona pellucida (a thin
transparent layer of protein and polysaccharides) and corona radiate of granulose cells
(Fig.20.37 page 734)
Fig. 20.37 The
process of
fertilization.
As the head of the
sperm cell encounters
the gelatinous corona
radiata of the
secondary oocyte, the
acrosomal vesicle
ruptures and the sperm
cell digests a path for
itself by the action of
the enzymes released
fro the acrosome.
When the plasma
membrane of the
sperm cell contacts the
plasma membrane of
the ovum, they
become continuous,
and the nucleus of the
sperm cell moves into
the cytoplasm of the
ovum.
Fertilization
Normally occurs in the uterine tubes.
Each sperm contains a large, enzyme-filled vesicle above its nucleus, known as an
acrosome.
The interaction of sperm with particular molecules in the zona pellucida triggers an
acrosomal reaction.
The reaction involves the progressive fusion of the acrosomal membrane with the
plasma membrane of the sperm, creating pores through which the acrosomal enzymes
can be released by exocytosis.
Enzymes includes protein digesting enzyme and hyaluronidase, allow the sperm to
digest a path though the zona pellucida to the oocyte.
Fertilization acting through a second messenger (IP3), stimulates the endoplasmic
reticulum of the oocyte to release its stored Ca++.
This causes a rise in cytoplasmic Ca++ that creates a Ca++ wave.
This wave activates the fertilized egg cell, causing numerous structural and metabolic
changes.
Some of these changes prevent polyspermy i.e. preventing other sperm from fertilizing
the same oocyte; only one sperm can fertilize an egg cell.
The Ca++ wave activates proteins that allow the cell cycle to continue meiosis II.
Within 12 hours after fertilization, the nuclear membrane in the ovum disappears, and
the haploid number of chromosomes in the ovum is joined by the haploid number of
chromosomes from the sperm cell.
A fertilized egg, or zygote, containing the diploid number of chromosomes (46) is thereby
formed.
Twins can be:
1. Monozygotic twins (identical twins) are derived from a single zygote that splits and
becomes two embryos.
2. Dizygotic twins (fraternal twins) are derived from two different zygotes produced by
two ovulated oocytes fertilized by two different sperm.
The sperm cell contributes more than the paternal set of chromosomes to the zygote.
Recently, it was demonstrated that the centrosome of the human zygote is derived from
the sperm cell and not from the oocyte.
The midpiece of the sperm brings some mitochondria into the oocyte during fertilization,
but these do not contribute to the embryo. The paternal mitochondria, with their contents
of mitochondrial DNA, are quickly eliminated in the zygote by a process of autophagy.
All of the mitochondrial DNA in a person is inherited from the mother’s oocyte.
Secondary oocyte that has been ovulated but not fertilized does not complete its second
meiotic division, but instead disintegrated 12-24 hours after ovulation.
Fertilization cannot occur if intercourse takes place later than 1 day following ovulation.
Sperm can survive up to 3 days in the female reproductive tract.
Fertilization can occur if intercourse takes place within a 3-day period prior to the day of
ovulation.
‫تعلمت‪:‬‬
‫أنه خير لإلنسان أن يكون‬
‫كالسلحفاة في الطريق‬
‫الصحيح‪...‬‬
‫على أن يكون غزاالً في‬
‫الطريق الخطأ‪.‬‬
Cleavage and Blastocyst Formation (Fig.20.42 & Fig.20.44)
At about 30-36 hours after fertilization, the zygote divides by mitosis into 2
smaller cells.
This division is called cleavage.
The rate of cleavage is thereafter accelerated.
A 2nd cleavage occurs 40 hrs after fertilization producing 4 cells.
A 3rd cleavage occurs 50-60 hrs after fertilization produces a ball of 8 cells
called morula.
3rd day: Morula enters the uterus.
4th day: continued cleavage produces morula consisting of
32-64 cells.
The embryo remains unattached to the uterine wall for the next 2 days, during
which time it undergoes changes that convert it into a hollow structure called a
blastocyst.
The blastocyst consists of 2 parts:
1. An inner cell mass, which will become the fetus.
2. A surrounding chorion, which will become part of the placenta. The cells that
form the chorion are called trophoblast cells.
6th day: the blastocyst attaches to the uterine wall, with the side containing the
inner cell mass positioned against the endometrium.
The trophoblast cells produce enzymes that allow the blastocyst to "eat its way"
into the thick endometrium. This begins the process of implantation (or
nidation).
7th-10th day: the blastocyst is completely buried in the endometrium.
Approximately 75% of all lost pregnancies are due to a failure of implantation,
and consequently are not recognized as pregnancies.
Fig. 20.42 Fertilization, cleavage, and the formation of a blastocyst.
A diagram showing the ovarian cycle, fertilization, and the events of the first week
following fertilization. Implantation of the blastocyst begins between the 5th and 7th day
and is generally complete by the 10th day.
Fig. 20.43 Scanning
electron micrographs
of preembryonic
human development.
A human ovum fertilized
in vitro is seen at:
(a) the 4-cell stage.
(b) Cleavage at the
16-cell stage.
(c) Formation of a
morula.
(d) Formation of
blastocyst.
Fig. 20.44 Implantation of the blastocyst.
(a) The blastocyst attached to the endometrium on about the 6th day.
(b) Implantation of the blastocyst at the 9th or 10th day.
Embryonic Stem Cells and Cloning
Only the fertilized egg cell and each of the early cleavage cells are totipotent
which refers to their ability to create the entire organism if implanted into a
uterus.
The nuclei of adult somatic cells can be reprogrammed to become totipotent if
they are transplanted into egg cell cytoplasm. This is called somatic nuclear
transfer.
Through this transfer, the cloning of an entire adult organism is possible. This
cloning is called reproductive cloning.
The reproductive cloning has been accomplished in sheep, cattle, cats, and
other animals.
The possible use of this technique to clone humans has been widely
condemned by many scientists and others for many reasons, including the low
probability of producing healthy children.
Implantation of the Blastocyst and Formation of the Placenta
If fertilization does not take place, the corpus luteum begins to decrease its
secretion of steroid about 10 days after ovulation.
This withdrawal of steroids causes necrosis and sloughing of the endometrium
following day 28 of the cycle.
If fertilization and implantation have occurred, these events must obviously be
prevented to maintain the pregnancy.
Chorionic Gonadotropin
The blastocyst saves itself from being eliminated with the endometrium by
secreting a hormone that indirectly prevents menstruation.
Even before the 6th day when implantation occurs, the trophoblast cells of the
chorion secrete chorionic gonadotropin (hCG).
hCG is identical to LH in its effect and therefore is able to maintain the corpus
luteum.
The secretion of estradiol and progesterone is maintained and menstruation is
normally prevented.
The secretion of hCG declines by the 10th week of pregnancy.
hCG is required for only the 5-6 weeks of pregnancy, because the placenta
becomes an active steroid hormone-secreting gland.
5th-6th week, the mother's corpus luteum begins to regress.
Fig. 20.45 The
secretion of human
chorionic
gonadotropin
(hCG).
This hormone is
secreted by
trophoblast cells during
the first trimester of
pregnancy, and it
maintains the mother’s
corpus luteum for the
5½ weeks. After that
time, the placenta
becomes the major
sex-hormoneproducing gland,
secreting increasing
amounts of estrogen
and progesterone
throughout pregnancy.
‫األستاذ‪ :‬ما هي الثورة يا تالميذ؟‬
‫الطالب‪ :‬هي زوجة الثور يا أستاذ!‬
Chorionic Membranes (Fig.20.46)
Between days 7-12, as the blastocyst becomes completely embedded in the
endometrium, the chorion becomes a 2-cell-thick structure that consists of an
inner cytotrophoblast layer and an outer syncytiotrophoblast layer.
The inner cell mass also develops 2-cell layer:
1.The ectoderm which will form the nervous system and skin.
2.The endoderm which will form the gut and its derivatives.
3.Middle embryonic layer (mesoderm) is not yet seen at this stage. It will form
the following systems: musculskeletal, cardiovascular, urinary, lymphatic,
reproductive, and the dermis of the skin.
The embryo at this stage is a 2-layer-thich disc separated from the
cytotrophoblast of the chorion by an amniotic cavity.
The syncytiotrophoblast envades the endometrium, and secretes proteindigesting enzymes that create numerous blood-filled cavities in the maternal
tissue.
The cytotrophoblast forms villi that grow into pools of venous blood producing
chorion frondosum.
This occurs only on the side of the chorion that faces the uterine wall.
As the embryonic structures grow, the other side of the chorion bulges into the
cavity of the uterus, loses its villi, and takes on a smooth appearance.
The chorionic membrane is derived from the zygote that inherited paternal
genes that produce proteins foreign to the mother, then "why the mother's
immune system doesn't attack the embryonic tissues?"
The placenta, it seems, is an "immunologically privileged site."
Fig. 20.46 The extraembryonic membranes.
(a) After the synctiotrophoblast has created
blood-filled cavities in the endometrium, these
cavities are invaded by extensions of the
cytotrophoblast.
(b) These extensions, or villi, branch extensively
to produce the chorion frondosum.
The developing embryo is surrounded by a
membrane called the amnion.
Formation of Placenta and Amniotic Sac (Fig.20.47&Fig.20.48)
As the blastocyst implants in the endometrium and the chorion develops, the
cells of the endometrium also undergo changes.
These changes are called the decidual reaction which includes: cellular growth
and the accumulation of glycogen.
The maternal tissue in contact with the chorion frondosum is called decidua
basalis.
Chorion frondosum (fetal tissue) and decidua basalis (maternal tissue) form the
functional unit known as the placenta.
The disc-shaped human placenta is continuous at its outer surface with the
smooth part of the chorion, which bulges into the uterine cavity.
Immediately beneath the chorionic membrane is the amnion, which has grown
to envelop the entire embryo.
The embryo, together with its umbilical cord, is located within the fluid-filled
amniotic sac.
Amniotic fluid is formed initially as an isotonic secretion.
The volume is increased and the concentration changed by urine from the
fetus.
Amniotic fluid also contains cells that sloughed off from the fetus, placenta, and
amniotic sac.
All are derived from the fertilized ovum; therefore they have the same genetic
composition.
Many genetic abnormalities (such as Down syndrome) can be detected by
amniocentesis.
Amniocentesis is a procedure usually performed at about the 16th week of
pregnancy.
Amniocentesis is performed by aspiration of amniotic fluid and examination of
the cells obtained.
Major structural abnormalities can often be detected by ultrasound.
Fig. 20.47 The
aminiotic sac and
placenta.
Blood from the
embryo is carried to
and from the chorion
frondosum by
umbilical arteries and
veins. The maternal
tissue between the
chorionic villi is known
as the decidua
basalis, this tissue,
together with the
chorionic villi, forms
the functioning
placenta. The space
between chorion and
amnion is obliterated,
and the fetus lies
within the fluid-filled
amniotic sac.
Fig. 20.48
Amniocentesis.
In this procedure,
amniotic fluid containing
suspended cells is
withdrawn for
examination. Various
genetic diseases can be
detected prenatally by this
means.
Smile:
A curve that can
set a lot of
things
straight
‫ال يكذب من يثق بنفسه ‪--------------‬‬
‫وال يخون من يعتز بنفسه ‪------------‬‬
Exchange of Molecules Across the Planceta (Fig.20.49)
The umbilical arteries deliver fetal blood to vessels within the villi of the chorion
frondosum of the placenta.
This blood circulates within the villi and returns to the fetus via the umbilical
vein.
Maternal blood is delivered to and drained fro the cavities within the decidua
basalis that are located between the chorionic villi.
In this way, maternal and fetal blood is brought close together but never mixes
within the placenta.
The placenta serves as a site for the exchange of gases and other molecules
between the maternal and fetal blood.
O2 and nutrient molecules diffuse from mother to fetus, and CO2 and waste
products diffuse in the opposite direction.
The placenta is the only link between the fetus and outside world.
The placenta has a very high metabolic rate.
It utilizes about a third of all the O2 and glucose supplied by the maternal blood.
The rate or protein synthesis is higher in the placenta than in liver.
The placenta produces a great variety of enzymes capable of converting
hormones and exogenous drugs into less active molecules.
Therefore potentially dangerous molecules in the maternal blood are often
prevented from harming the fetus.
Fig. 20.49 The circulation of blood within the placenta.
Maternal blood is delivered to and drained from the spaces between the chorionic villi.
Fetal blood is brought to blood vessels within the villi by branches of the umbilical artery
and is drained by branches of the umbilical vein.
Endocrine Functions of the Placenta (Table 20.7 &Fig.20.50)
The placenta secretes both steroid hormones (progesterone and estrogens)
and protein hormones (hCG and hCS).
I. Pituitary-like Hormones from the Placenta
Chorionic gonadotropin (hCG) is important in:
1. Maintaining the mother's corpus luteum for the first 5 &1/2
weeks of pregnancy.
2. May help to prevent immunological rejection of the implanting
embryo.
Chorionic somatomammotropin (hCS) acts together with growth hormone from
the mother's pituitary to produce a diabetic-like effect in the pregnant woman.
Their effects are:
1.Lipolysis and increased plasma fatty acid concentration.
2.Glucose-sparing by maternal tissues and increased blood glucose
concentrations.
3.Polyruria that produces a degree of dehydration and thirst.
This diabetic-like effect in the mother helps to ensure a sufficient supply of
glucose for the placenta and fetus, which use glucose as their primary energy
source.
II. Steroid Hormones from the Placenta
After the first 5 &1/2 weeks of pregnancy, when the corpus luteum regresses, the
placenta becomes the major sex-steroid-producing gland.
The blood concentration of estrogens, as a result of placental secretion, rises to levels
more than 100 times greater than those existing at the beginning of pregnancy.
The placenta also secretes large amounts of progesterone, changing the
estrogen/progesterone ratio in the blood from 100:1 at the beginning of pregnancy to
close to 1:1 toward full-term.
The placenta is "incomplete endocrine gland" because it cannot produce estrogen and
progesterone without the aid of precursors supplied to it by both the mother and the
fetus.
The placenta lacks the enzymes needed to convert progesterone into androgens.
Therefore, androgens produced by the fetus are needed as substrates for the placenta
to convert into estorogen.
The placenta produces estrogen by cooperating with the steroid-producing tissues
(principally the adrenal cortex) in the fetus.
Fetus and placenta thus form a single functioning system in terms of steroid hormone
production called the fetal-placental unit.
Fig. 20.50 Interactions between the embryo and placenta produce the
steroid hormones.
The secretion of progesterone and estrogen from the placenta requires a
supply of cholesterol from the mother’s blood and the cooperation of fetal
enzymes that convert progesterone to androgens.
‫تعلمت‬
‫أن مفتاح الفشل هو محاولة‬
‫إرضاء‬
‫كل شخص تعرفه‬
‫تعلمت‬
‫أن العمل الجيد‬
‫أفضل‬
‫من الكالم الجيد‬
‫أهبل بيقول ألخوه ‪ :‬تصدق هسه اكتشفت إني‬
‫عبقري ألني خلصت لعبة التركيب بسنتين !‬
‫قال األخ‪ :‬ما تحس ان سنتين كتير !‬
‫قال ‪ :‬ال كاتبين على الكرتونة من ‪ 7-4‬سنوات !‬
‫ها ها ها‬
Labor and Parturition (Fig.20.51)
Powerful contractions of the uterus are needed to expel the fetus in the
sequence of events called labor.
These uterine contractions are known to be stimulated by 2 agents:
1. Oxytocin: produced in the hypothalamus and released by the
posterior pituitary, and also produced by the uterus.
2. Prostaglandins: produced within the uterus and have paracrine
functions.
Labor can be induced artificially by injections of oxytocin or by insertion of
prostaglandins into the vagina as suppository.
Although labor is known to be stimulated by oxytocin and prostaglandins, the
factors responsible for the initiation of labor are incompletely understood.
In all mammals, labor is initiated by activation of fetal adrenal cortex.
Corticosteroids secreted by the fetal adrenal cortex then stimulate the placenta
to convert progesterone into estrogens.
This is significant because progesterone inhibits activity of myometrium, while
estrogens stimulate the ability of the myometrium to contract.
The initiation of labor in human and other primates is more complex.
Progesterone levels do not fall because the human placenta cannot covert
progesterone into estrogens, it can only make estrogen when it is supplied with
androgens from the fetus.
The fetal adrenal lacks a medulla, but the cortex itself is composed of 2 parts.
The outer part secretes cortisol, as does the adult adrenal cortex. The inner
part, called the fetal adrenal zone, secretes the androgen
dehydroepiandrosterone sulfate (DHEAS).
Once the DHEAS from the fetus travels to the placenta, it is converted into
estrogens. The rising secretion of estrogens stimulates the uterus to:
1. Produce receptors for oxytocin which makes the myometrium more sensitive
to the hormone.
2. Produce receptors for prostaglandins which make the myometrium more
sensitive to these molecules.
3. Produce gap junctions between myometrial cells in the uterus that help to
synchronize and coordinate the contractions of the uterus.
The chain of events may be set in motion by the placenta, through its secretion
of corticotrophin-releasing hormone (CRH).
The CRH produced by the placenta stimulates the anterior pituitary to secrete
ACTH.
There are CRH receptors in the fetal adrenal gland, suggesting that CRH
produced by the placenta can itself stimulate adrenal secretion.
Thus, CRH from the placenta directly and indirectly stimulates the fetal adrenal
cortex to secrete cortisol and DHEAS.
The secretion of cortisol from the fetal adrenal cortex helps to:
1. Promote maturation of the fetus's lungs.
2. Stimulates the placenta to secrete CRH.
The secretion of CRH results in a positive feedback loop that also increases
secretion of DHEAS.
The placenta can then convert the increased amount of DHEAS into increased
amounts of estriol.
The estriol, in turn, activates the myometrium to become more sensitive to
oxytocin and prostaglandins.
Thus, the chain of events that culminates in parturition may be set in motion by
the placenta's secretion of CRH.
How this "placental clock" is timed, however, is not currently understood.
The oxytocin concentration of the mother's plasma rises during the night but not
during the day.
The uterus also produces oxytocin which may act as a paracrine regulator
along with prostaglandins to stimulate contractions and supplement the actions
of the oxytocin released by the posterior pituitary.
The concentration of oxytocin receptors in the myometrium increases
dramatically as a result of estrogen stimulation making the uterus more
sensitive to oxytocin.
These effects culminate in parturition, or childbirth.
Following delivery of the baby, oxytocin is needed to:
1. Maintain the muscle tone of the myometrium and to reduce hemorrhaging
from uterine arteries.
2. Promoting the involution (reduction in size) of the uterus.
Fig. 20.51 Labor in
humans.
The fetal adrenal gland
secretes DHEAS and
cortisol upon stimulation by
CRH and ACTH. In turn,
cortisol stimulates the
placenta to secrete CRH,
producing a positive
feedback loop. The DHEAS
is converted by the placenta
into estriol, which is needed,
together with prostaglandins
and oxytocin, to stimulate
the myometrium of the
mother’s uterus to undergo
changes leading to labor.
The plus signs emphasize
activation steps critical to
this process.
Lactation (Fig.20.52, 53,54, & 55)
Each mammary gland is composed of 15-20 lobes divided by adipose tissue.
The amount of adipose tissue determines the size and shape of the breast but
has nothing to do with the ability of a woman to nurse.
Each lobe is subdivided into lobules which contain the glandular alveoli that
secrete the milk of a lactating female.
The clustered alveoli secrete milk into a series of secondary tubules.
These tubules converge to form a series of mammary ducts, which in turn
converge to form a lactiferous duct that drains at the tip of the nipple.
The lumen of each lactiferous duct expands just beneath the surface of the
nipple to form an ampulla, where milk accumulates during nursing.
The changes that occur in the mammary glands during pregnancy and the
regulation of lactation provide excellent examples of hormonal interactions and
neuroendocrine regulation.
Growth and development of the mammary glands during pregnancy requires
the permissive actions of insulin, cortisol, and thyroid hormones.
In the presence of adequate amounts of these hormones, high levels of
progesterone stimulate the development of the mammary alveoli and estrogen
stimulates proliferation of the tubules and ducts.
The production of milk proteins, including casein and lactoalbumin, is
stimulated after parturition by prolactin, a hormone secreted by the anterior
pituitary.
The secretion of prolactin is controlled primarily by PIH (prolactin-inhibiting
hormone) produced by hypothalamus.
High levels of estrogen causes:
1. The secretion of PIH.
2. Direct action on the mammary glands to block their stimulation by prolactin.
During pregnancy, the high levels of estrogen prepare the breast for lactation
but prevent prolactin secretion and action.
Fig. 20.52 The structure of the breast and mammary glands.
Fig. 20.53 The
hormonal control
of mammary
gland
development and
lactation.
Notice that milk
production is
prevented during
pregnancy by
estrogen inhibition of
prolactin secretion.
This inhibition is
accomplished by the
stimulation of PIH
(prolactin-inhibiting
hormone) secretion
from the
hypothalamus.
After parturition, when the placenta is expelled as the after birth, declining
levels of estrogen are accompanied by an increase in the secretion of prolactin
and milk production is therefore stimulated.
If a woman does not wish to breast-feed her baby she may take oral estrogens
to inhibit prolactin secretion.
The act of nursing helps to maintain high levels of prolactin secretion via a
neuroendocrine reflex.
Sensory endings in the breast, activated by the stimulus of suckling, relay
impulses to the hypothalamus and inhibit secretion of PIH.
Suckling, thus results in the reflex secretion of high levels of prolactin that
promotes the secretion of milk from the alveoli into the ducts.
The stimulus of suckling also results in the reflex secretion of oxytocin from the
posterior pituitary.
Oxytocin is produced in the hypothalamus and stored in the posterior pituitary;
its release results in the milk-ejection reflex, or milk letdown. This is because
oxytocin stimulates contraction of the lactiferous ducts as well as of the uterus.
Fig. 20.54 Milk production and the milk-ejection reflex.
Lactation occurs in 2 stages: milk production (stimulated by prolactin) and milk ejection
(stimulated by oxytocin). The stimulus of sucking triggers a neuroendocrine reflex that
results in increased secretion of oxytocin and prolactin.
Breast-feeding supplements the immune protection given to the infant by its
mother.
While the fetus is in utero, IgG antibodies cross the placenta from the maternal
to the fetal blood.
Infants that are breast-fed also receive IgA antibodies from the mother's milk.
Both IgG and IgA antibodies provide passive immune protection to the baby for
the first 3-12 months.
In addition, the mother's milk contains cytokines, lymphocytes, and antibodies
that may promote the development of the baby's system of active immunity.
The passive immunity provided by maternal antibodies may be significant in
protecting the baby from a variety of infections, since the ability of the baby to
produce its own antibodies is not well developed for several months after birth.
Breast-feeding, acting through reflex inhibition of GnRH secretion, can also
inhibit the secretion of gonadotropins from the mother's anterior pituitary and
thus inhibit ovulation.
Breast-feeding is thus a natural contraceptive mechanism that helps to space
births. This mechanism appears to be most effective in women with limited
caloric intake and in those who breast-feed their babies at frequent intervals
throughout the day and night.
Fig. 20.55 Maternal antibodies that protect the baby.
Circulating IgG antibodies cross the placenta and protect the baby for 3 moths to 1 year
after birth, This passive immunity is supplemented by IgA antibodies in the baby’s
intestine obtained from the mother’s milk. This protection lasts longer for babies weaned
at a later age. Notic the inability of the baby to produce a large amount of its own
antibodies until it is several months of age. .
Forgive
everyone
for
everything
‫”يجب أن تعرف متى تغلق هاتفك‬
‫ومتى تفتحه‬
‫ويفترض أن يكون الهاتف عبدا ً لك !‬
‫واليفترض أن تكون أنت عبدا ً‬
‫لهاتفك“‬
‫هذه هي كلمات ” مارتن كوبر“‬
‫أول من إخترع الهاتف المحمول ”‬
‫الموبايل“‬
‫تم إنجازه وإعالن إختراعه بتاريخ‪:‬‬
‫‪3/4/1973‬‬
‫كان طول الجهاز ‪ 25‬سم ووزنه‬
I don’t forgive people
because I’m weak.
I forgive them because
I am strong enough
to understand
that people make mistakes….