Year One
Semester One: Life Cycles
1.01: All’s Well That Ends Well
What is the relationship between age and fertility?
Males
Levels of circulating testosterone tend to reduce between 50 and 60 whilst levels of
circulating FSH and LH tend to increase. Although sperm production continues (well into
eighties), the level of sexual activity tends to reduce with decreasing testosterone levels.
The testes become softer and smaller in aging men and sperm quality reduces. The
volume of sperm produced reduces, the shape becomes less effective and motility
reduces.
Females
It is estimated that there are over 7 million potential oocytes (the term ‘eggs’ is the same as
oocytes and ova) in a foetus’ ovaries. This reduces to about 1-2 million in the female infant.
By puberty there will only be about 400,000 oocytes remaining. Of the eggs remaining only
about 300-400 will be ovulated. The process of oocyte selection is poorly understood,
during the reproductive cycle a cohort of oocytes is stimulated to begin maturation but only
1 or 2 dominant follicles complete the process and are ovulated. Follicle Stimulating
Hormones (FSH) bind to the receptors in the follicular membranes of oocytes and stimulate
follicular maturation, however as age progresses the remaining oocytes become
increasingly resistant to FSH. Thus plasma FSH levels tend to increase several years
before menopause (menopause is the cessation [end] of menses). Oestrogen levels
decline as fewer follicles mature, resulting in less ovulation or perhaps no ovulation, in
addition the urethral and vaginal epithelium thins.
What factors influence age of first pregnancy?
Social
Approximately 20% of women wait until they are 35 to begin their families.
Contraception is readily available;
More women have careers/in work;
Women are marrying later;
Divorce rate remains high;
Married couples are delaying having children until they are more financially
secure;
Many women don’t realise fertility reduces in late 20’s and early 30’s. At 30 you
have a 20% chance of becoming pregnant per month, by 40 it has dropped to 5%.
Physiological
On average girls begin to experience puberty at around 10.5 years, beginning with breast
budding – small nodules, of varying size. Then pubic hair and breasts develop. Between 12
and 13 is when girls first experience a period (first period: menarche). The whole puberty
process takes approximately 3-4 years. Some girls menstruate as early as 8, others as late
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as 16. Reproductive age is generally defined as between 15 and 44, though there have
been successful pregnancies beyond both extremes.
What is the reproductive cycle and how does fertilization occur?
The female reproductive cycle consists of:
The Ovarian Cycle;
The Menstrual Cycle.
OVARIAN CYCLE
When a female child is born, each ovum is surrounded by a single layer of granulosa cells;
the ovum, with this granulosa cell sheath, is called a primordial follicle. Throughout
childhood, the granulosa cells are believed to provide nourishment for the ovum and to
secrete an oocyte maturation-inhibiting factor that keeps the ovum suspended in its
primordial state in the prophase stage of meiotic division. Then, after puberty, when FSH
and LH from the anterior pituitary gland begin to be secreted in significant quantities, the
ovaries, together with some of the follicles within them, begin to grow.
Enlargement of the ovum
(increases 2-3 fold)
Primoridal follicle
Additional growth of
granulosa layers
Growth of 6-12
follicles each month
Early primary follicle
Spindle cells derived from the ovary
interstitium collect in several layers outside
the granulosa cells, giving rise to a second
mass of cells called the theca.
Theca interna cells
take on epithelioid
characteristics
similar to those of
Primary follicle
granulosa cells
and develop an
ability to secrete
addition steroid
sex hormones.
First meiotic division
completed. Second
meiotic division
commences. Each
of the 23 pairs of
chromosomes loses
Secondary follicle
one of its partners
which becomes
incorporated into a
polar body that is
expelled.
High levels of
oestrogen inhibit FSH
secretion and
promotes large
Graafian follicle secretion of LH.
Ovulation
1. The theca externa (the capsule of the follicle) begins to release proteolytic
enzymes from lysosomes, and these cause dissolution of the follicular capsular
wall and consequent weakening of the wall, resulting in further swelling of the
entire follicle and degeneration of the stigma.
2. Simultaneously, there is rapid growth of new blood vessels into the follicle wall,
and at the same time, prostaglandins (local hormones that cause vasodilation) are
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secreted into the follicular tissues. These two effects cause plasma transudation
into the follicle, which contributes to follicle swelling. Finally, the combination of
follicle swelling and simultaneous degeneration of the stigma causes follicle
rupture, with discharge of the ovum.
MENSTRUAL CYCLE
The cycle is typically 28 days but can range from 22 to 35 days; it is divided into four main
phases.
Copious amounts of glycogen.
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The Menstrual Phase/Menses/Menstruation
Lasts roughly five days. Marked by the degeneration of the endometrial lining of the uterus
– caused by constriction of spiral arteries, reducing blood flow to the endometrium
depriving it of oxygen and nutrients – eventually it erupts and blood fills the uterine cavity.
The endometrium thins to 1-2mm. Typically up to 50ml of blood is lost. Normally, menstrual
blood does not clot due to the local release of inhibitory (anticoagulant) factors.
Menses may be accompanied by uterine cramps which are due to liberation of
prostaglandins from the endometrium.
With the beginning of menstruation LH, estradiol and progesterone reach their lowest
levels. By means of negative feedback the levels of FSH increase (in fact since day 25 of
previous cycles) and another group of follicles (20 or so) are stimulated into maturing. FSH
binds to receptors in the granulosa cells (nourishing cells) stimulating them to turn into
cuboidal cells.
1 = oocyte;
2 = pellucid zone;
3 = stratum granulosum;
4 = theca folliculi cells
Under the influence of FSH the granulosa cells begin to secrete estradiol (an oestrogen).
Estradiol stimulates LH receptors on the theca cells further increasing secretion of estradiol
(through an enzyme regulated reaction: androgen precursors
estradiol). This up
regulation of LH prepares granulosa and theca cells for progesterone synthesis after
ovulation.
As estradiol levels rise, negative feedback results in a decrease of FSH secretion from the
anterior pituitary.
GnRH
LH
FSH
FSH Stimulates
Ovaries
Growing Follicles
OESTROGEN
Development of female
secondary sex characteristics +
breasts. Increase protein
anabolism. Lower blood
cholesterol. Moderate levels
inhibit release of GnRH, FSH
and LH.
Andrew Gough
PROGESTERONE
Anterior Pituitary
LH Stimulates
Ovulation
Corpus Luteum
RELAXIN
Works with oestrogens to Inhibits contraction of
prepare endometrium for uterine smooth muscle.
implantation. Prepares During labour, increases
mammary glands to
flexibility of pubic
secrete milk. Inhibits
symphysis and dilates
release of GnRH and LH.
uterine cervix.
INHIBIN
Inhibits release of
FSH and to a
lesser extent, LH
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The Endometrium
The endometrium is divided into three histologically and functionally distinct layers. The
deepest or basal layer, the stratum basalis, adjacent to the myometrium, undergoes little
change during the menstrual cycle and is not shed during menstruation. The broad
intermediate layer is characterised by a stroma with a spongy appearance and is called
the stratum spongiosum. The thinner superficial layer, which has a compact stromal
appearance, is known as the stratum compactum. The compact and spongy layers
exhibit dramatic changes throughout the cycle and both are shed during menstruation;
hence they are jointly referred to as the stratum functionalis.
The arrangement of the arterial supply of the endometrium has important influences on
the menstrual cycle. Branches of the uterine arteries pass through the myometrium and
immediately divide into two different types of arteries, straight arteries and spiral
arteries. Straight arteries are short and pass a small distance into the endometrium, then
bifurcate to form a plexus supplying the stratum basalis. Spiral arteries are long, coiled
and thick-walled and pass to the surface of the endometrium giving off numerous
branches which give rise to a capillary plexus around the glands and in the stratum
compactum. Unlike the straight arteries, the spiral arteries are responsive to the
hormonal changes of the menstrual cycle. The withdrawal of progesterone secretion at
the end of the cycle causes the spiral arteries to constrict and this precipitates an
ischaemic phase that immediately precedes menstruation.
The Proliferative Phase
Lasts from day 6-13. A single follicle has become dominant; outgrowing others it secretes
estradiol and inhibin. This reduces FSH secretion. Undeveloped follicles undergo atresia
(they become atretic). The dominant follicle develops becoming ready for ovulation. The
follicle begins to form a blister like bulge due to the increasing outward pressure of the
filling antrum (space within the follicle). As estradiol continues to be produced, LH levels
rise. Small amounts of progesterone are released before ovulation. Oestrogen stimulates
repair of the endometrium, cells of the stratum basalis (B) undergo mitosis, producing the
new stratum functionalis (F). Endometrium thickens to approximately 4-10mm.
Ovulation
By day 11-13, a huge surge in LH triggers ovulation (occurring 30-36 hours after surge).
The oocyte is expelled from the follicle to be converted into a corpus luteum to facilitate
progesterone secretion for rest of cycle. During the LH surge, LHG receptor bind to the LH
and convert the enzymatic machinery of the granulose cells and theca cells to produce
progesterone (hence early progesterone production). An inadequate surge can result in
failed implantation. During the last 20 days (including 6 days of previous cycle) the primary
oocyte has completed meiosis I and is now beginning meiosis II where it will stop at
metaphase. Mittelschmerz is a twinge of pain felt at time of ovulation.
The Post-Prevulatory Phase/Luteal Phase
The period between ovulation and the next menses phase – days 15-28. Characterised by
the dominance of progesterone. Progesterone production occurs rapidly after first 24
hours. If no fertilization and implantation occurs then progesterone production diminishes
rapidly initiating events leading to a new cycle. The corpus luteum (life span: 13-14 days)
degenerates into a corpus albicans (becomes a white fibrous streak). The loss of these
ovarian hormones stimulates the action of GnRH.
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If there is fertilization and the zygote implants into the endometrium then human chorionic
gonadotrophin (hCG) sustains the corpus luteum for another 6-7 weeks.
FERTILIZATION
Conception is the fertilization of an ovum by a spermatozoon, this typically occurs in the
ampulla or isthmus of the uterine tubes.
Movement of the ovum along
the tube is mediated by gentle
peristaltic action of the
longitudinal
and
circular
smooth muscle layers of the
oviduct wall; this is aided by a
current of fluid propelled by
the action of the ciliated
epithelium lining the tube. The
mucosal lining of the Fallopian
tube is thrown into a labyrinth
of
branching, longitudinal
folds, a feature that is most
prominent in the ampulla,
At the time of ovulation, the infundibulum moves so as to which is the usual site of
overlie the site of rupture of the follicle; finger-like fertilisation.
projections called fimbriae extending from the end of the
tube envelop the ovulation site and direct the ovum into
the tube.
Contractions of the uterine musculature help to accelerate sperm to its destination. This is
stimulated by prostaglandins in seminal fluid and oxytocin secreted from the woman’s
posterior pituitary gland during her orgasm. The journey can take anything from 30 minutes
to 2 hours. Dozens of sperm must reach the ovum because a single sperm cannot
penetrate the corona radiata.
The oocyte is much larger than a spermatozoon because it’s suspended in metaphase II of
meiosis II. The acrosome (structure at head) of a spermatozoon contains enzymes such as
hyaluronidase which breaks bonds between adjacent follicle cells. Dozens of sperm need
to release this enzyme to enable fertilization. The acrosomal head ruptures when a
spermatozoon binds to a sperm receptor on the zona pellucida. The enzyme acrosin then
helps the spermatozoon make a path towards the surface of the oocyte. They then fuse
together. The spermatozoon is absorbed into the oocyte cytoplasm. This results in
inactivation of the sperm receptors. The zona pellucida hardens and helps prevent
polyspermy – which leads to a zygote unable to develop. Meiosis II also completes.
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Nuclear materials of ovum and spermatozoon both swell forming pronuclei and migrate to
the centre where they fuse, amphimixis, with the formation of the 46 chromosome zygote.
How does a pregnancy test work?
The urine test detects the hormone: human chorionic gonadotrophin (produced in the
syncytiotrophoblast – outer layer – of growing placenta), hCG, and the levels rise sharply in
the early period of pregnancy. However the structure of LH and hCG are closely related
(both LH and hCG share an -subunit – a peptide formed by chromosome 6, found on
some hormones) and a test must take account for this overlap in structure; hence the
concentration of hCG must be high to evoke a positive test result to avoid a false-positive
result. The test is an immunoassay test. There are several antibody binding sites on the
hormone hCG and its free subunits. Once one antibody binds to one site, a second (tracer)
antibody binds to a distant site and is labelled with a blue dye.
How does the oral contraceptive pill work?
There are two types of oral contraceptive pill.
Combined Oral Contraceptive Pill
Oestrogen and Progesterone.
Progesterone Only Pill
The ‘minipill’ – may be started immediately
after delivery as it has no effects on
lactation. Must be taken same time of the
day each day.
Progesterone suppresses LH secretion and in turn ovulation. Thickens cervical mucus and
alters fallopian tube peristalsis – makes the endometrium hostile to implantation and the
cervix relatively impermeable.
Oestrogen suppresses FSH secretion. Also acts as an adjuvant to progesterone by
increasing the number of progesterone receptors.
What are the general symptoms of pregnancy?
Symptoms include:
Tiredness;
Indigestion;
Constipation;
Sore breasts;
Sickness;
*Abdominal Pains*.
Gastrointestinal Changes
Earliest symptoms of pregnancy include nausea/morning sickness. It appears a main
reason is to do with elevated hCG and progesterone levels causing relaxation of the
smooth muscle in the GI tract and stomach. The transit time from stomach and small bowel
increases significantly by 15-30%. The heartburn/gastric reflux are associated with the
increased emptying time and the sphincter muscle at the gastroesophageal junction and
increase intra-abdominal pressure as gestation increases. Constipation is also common;
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this is due to unfamiliar pressure on the bowel system and also by reduced motility
elsewhere in bowel and increased absorption of water.
Breast Changes
The breasts grow in size rapidly during the first 8 weeks of pregnancy as well as the
nipples becoming larger and more mobile.
Tiredness
One possibility is anaemia, a condition arising due to reduced oxygenation of the blood.
Whilst the numbers of red blood cells do increase during pregnancy the plasma volume
also does and so it can become diluted. Also there are the normal reasons such as
increased weight to support and also a pregnant woman may find her sleep patterns are
poor due to other symptoms she is experiencing.
*Abdominal Pains*
Mild abdominal pain can be normal during early pregnancy. As progesterone softens
ligaments and relaxes muscles in the body, it may cause the round ligaments which
suspend the enlarging uterus, to stretch and cause pain. These feel like pulling sensations
in the right and left lower quadrants of the abdomen and are very annoying; however the
pain subsides during the second and third trimesters. Pain is often more pronounced on
right due to usual dextro(-to the right)rotation of the gravid (carrying developing young)
uterus.
Preeclampsia and eclampsia
About 5 per cent of all pregnant women experience a rapid rise in arterial blood pressure to
hypertensive levels during the last few months of pregnancy. This is also associated with
leakage of large amounts of protein into the urine. This condition is called preeclampsia or
toxaemia of pregnancy. It is often characterized by excess salt and water retention by the
mother's kidneys and by weight gain and development of oedema and hypertension in the
mother. In addition, there is impaired function of the vascular endothelium, and arterial
spasm occurs in many parts of the mother's body, most significantly in the kidneys, brain,
and liver. Both the renal blood flow and the glomerular filtration rate are decreased, which
is exactly opposite to the changes that occur in the normal pregnant woman. The renal
effects also include thickened glomerular tufts that contain a protein deposit in the
basement membranes.
During normal placental development, the trophoblasts invade the arterioles of the uterine
endometrium and completely remodel the maternal arterioles into large blood vessels with
low resistance to blood flow. In patients with preeclampsia, the maternal arterioles fail to
undergo these adaptive changes, for reasons that are still unclear, and there is insufficient
blood supply to the placenta. This, in turn, causes the placenta to release various
substances that enter the mother's circulation and cause impaired vascular endothelial
function, decreased blood flow to the kidneys, excess salt and water retention, and
increased blood pressure.
Eclampsia is an extreme degree of preeclampsia.
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How does an egg implant outside of the uterus?
An ectopic pregnancy presents a major problem for women of child bearing age. It is a
result of a flaw that allows the conceptus to implant and mature outside the endometrial
cavity, which ultimately ends in death of the foetus. The condition can be life-threatening.
In the USA ectopic pregnancy accounts for 9% of all pregnancy related deaths. This
abnormally implanted gestation grows and draws its blood supply from the site of abnormal
implantation. As the gestation develops organ rupture becomes a potential because only
the uterine cavity is designed to expand and accommodate a foetus.
Essentially anything hampering the migration of the embryo (a conceptus up to 8 weeks) to
the endometrial cavity could predispose a woman to ectopic pregnancy.
Pelvic Inflammatory Disease – Cause: antecedent (existed before) infection
caused by chlamydia – resulting in asymptomatic (not producing indications)
cervicitis – inflammation of the cervix. Gonorrhoea causes inflammation of mucous
membrane. A history of salpingitis (inflammation of the fallopian tube) increases
risk four fold.
History of ectopic pregnancy.
Prior tubal surgery
Age 35-44, a three fold risk compared to 15-24 years, this may be due to reduced
myoelectrical activity at fallopian tubes.
Salpingitis isthmica nodosum – protrusions into the fallopian tubes from the tubal
epithelium in the myosalpinx (muscular lining of uterine tube).
80% of ectopic pregnancies are in the ampulla followed in turn by the isthmic segment,
then the fimbriae. Abdominal ectopic pregnancies are rare, occurring in about 1.2% of
cases. Ovarian and cervical sites represent 0.2% each.
Symptoms:
Pain;
Absence of menstruation (amenorrhea);
Vaginal Bleeding;
Only 50% of patients present this triad of symptoms.
What happens in the first trimester of pregnancy?
Starts from fertilization and lasts 12 weeks (three months = trimester).
Cleavage
A series of cell divisions that sub-divides the cytoplasm of the zygote.
There is the initial division forming two cells – the pre-embryo and is
completed 30 hours after fertilization. Subsequent divisions occur every
10-12 hours. After three days the morula stage is reached and there is a
clump of blastomeres (these cells produced by mitotic divisions).
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There is a hollow cavity called the blastoceole, the trophoblast is the outer layer – the
insulator and supplier of nutrients. The inner cell mass is a group of cells at one end of the
blastocyst – this will form the embryo.
Implantation
The blastocyst releases enzymes that erode through the zona pellucida, which is shed in a
process known as hatching, from here on also: hCG begins to be secreted from the
trophoblast. The blastocyst becomes fully exposed to the glycogen rich fluid contents of the
uterine secreted by its endometrium. The blastocyst enlarges and as it obtains more
nutrients. When fully formed the blastocyst contacts the endometrium and implantation
occurs. On contact, the trophoblast cells divide rapidly increasing the thickness of the
trophoblast layer, cells closest to the interior remain intact – cellular trophoblast; however
the cell membranes of cells closer to the endometrium break down and nuclei filled
cytoplasm is left – the syncytial (sin-SISH-al) trophoblast, which erodes a path through the
uterine epithelium by secreting (that special enzyme) hyaluronidase. When the uterine
lining heals the blastocyst becomes detached from the uterine cavity and has burrowed in
– the functional layer is where development occurs. Generally implantation is into the
fondus. The syncytial trophoblast continues to expand and erodes glands within the
endometrium releasing nutrients – these nutrients are then distributed to the underlying
cellular trophoblast and inner cell mass. The nutrients assist embryo development.
Trophoblastic extensions grow around the endometrial capillaries and soon maternal blood
percolates through these extensions known as lucnae. Blood flow increases as
endometrial veins and arteries are penetrated, roughly at day 9.
As separation between the inner cell mass and trophoblast increases a fluid filled chamber
called the amniotic cavity forms. When the amniotic cavity first appears, the cells of the
inner cell mass are arranged into an oval that is two cells thick. By day 12 a third layer
begins to form through gastrulation – specific cells are moving towards a central area
known as the primitive streak, this layer of poorly organised cells is known as the
mesoderm layer and is sandwiched between the ecto- and endo- (cells closest to the
blastoceole) derm layers. These layers are known as germ layers. The oval three layered
sheet becomes known as the embryonic disc. The germ layers all have very distinct fates.
For instance the endoderm layer contributes the urinary bladder and mesoderm layer
contributes all components of the cardiovascular system.
Ectoderm
Mesoderm
Endoderm
Primitive streak
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Germ layers also participate in the formation of some extraembryonic membranes:
The Yolk Sac – Begins life as a layer of cells spread out around the outer edges of
the blastoceole to form a complete pouch. As gastrulation proceeds, the
mesodermal cells migrate around this pouch and complete the formation. Blood
cells form within this sac and it soon becomes an important place for blood cell
generation.
The Amnion – The ectodermal layer increases in size and the ectodermal cells
spread all over the amniotic cavity. Mesodermal cells form a second layer, the
amniotic cavity contains amniotic fluid.
The Allantois – Begins life as an out pocketing of the endoderm near the base of
the yolk sac. The free endodermal tip then grows towards the edge of the
blastocyst, surrounded by a mass of mesodermal cells, and will later form the
urinary bladder!
The Chorion – consists of two layers: an outer formed by the ectoderm or
trophoblast and an inner by the mesoderm. Blood vessels develop inside the
mesoderm of the chorion. This is the first step towards a functional placenta. By
the third week the mesoderm extends along the core of each trophoblastic villus
forming chorionic villi in contact with maternal tissue.
Blood starts flowing through these villi in the third week when the embryonic heart
starts to beat. As the villi enlarge more maternal blood vessels are eroded and
blood now moves more slowly through the complex lucnae formed by the syncytial
trophoblastic extensions known as lucnae. Maternal blood re-enters maternal veins
and there is no mixing of maternal and foetal blood due to the trophoblast.
Placentation
At first the entire blastocyst is surrounded by chorionic villi and the chorion continues to
th
enlarge. By the 4 week the embryo, amnion and yolk sac are suspended within an
expansive, fluid filled chamber. The body stalk, the connection between embryo and
chorion, contains the distal portions of the allantois and blood vessels that carry blood to
and from the placenta. The narrow connection between the endoderm of the embryo and
the yolk sac becomes known as the yolk stalk. The placenta does not continue to enlarge
indefinitely. Regional differences in placental organization begin to develop as placental
expansion creates a prominent bulge in the endometrial surface. The relatively thin portion
of the endometrium no longer participates in nutrient exchange and chorionic villi in this
region disappear – this becomes known as the decidua capsularis. Placental functions are
now concentrated in the deeper region known as the decidua basalis. The rest of the
uterine endometrium, which has no contact with the chorion, is called the decidua
parietalis. As the end of the trimester approaches the foetus moves farther apart from the
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placenta but remains in contact by means of the umbilical cord, which contains the
allantois, placental blood vessels and the yolk sac.
Hormones releases by the placenta include:
hCG – Acts like LH promoting secretion of progesterone, which maintains the
endometrial lining.
A number of hormones which convert mammary glands to an active status.
Relaxin – increase the flexibility of the pubic symphysis, dilation of cervix and
suppresses oxytocin by the hypothalamus and delays the onset of labour
contractions.
Progesterone and Oestrogen – Progesterone maintains the endometrial lining and
continue the pregnancy. Towards the end of the pregnancy oestrogen stimulates
labour and delivery.
Embryogenesis
Shortly after gastrulation the body of the embryo separates from the rest of the embryonic
disc. Internal organs start to form. Folding and differential growth of the embryonic disc
produces a bulge that projects into the amniotic cavity. This projection is known as the
head fold. Similar movements lead to the tail folds. The embryo is now physically and
developmentally distinct from the embryonic disc and extraembryonic membranes. The first
trimester establishes the base for organogenesis.
What pregnancy services are available?
Before becoming pregnant a woman should speak to their GP about any regular
medication they are taking. Once a woman believes she is pregnant she should see her
physician to maximise pre-natal care and to minimise risks of birth defects and
complications. This should be done by the tenth week. Blood screening, starting pre-natal
vitamin supplements and early detection of problems are better accomplished sooner
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rather than later.
What is spotting and what are its causes?
Spotting or bleeding during pregnancy can be serious and should always be investigated.
About a quarter of women experience some spotting or bleeding in early pregnancy. About
half of these women miscarry.
Spotting is very light bleeding, similar to what you may have at the beginning or end of a
period. It can very in colour from pink to red to brown.
Causes:
There is greater blood supply to cervix so there may be spotting after a smear test,
internal exam or sex.
Implantation – burrowing egg into the wall of the uterus.
Miscarriage or ectopic pregnancy, especially likely if there is abdominal pain or
cramping.
Infections – vaginal infection or STI. This can cause an inflamed cervix susceptible
to bleeding.
Placental problems or premature labour, e.g. placenta previa (low-lying placenta),
placental abruption (placenta separates from uterus), late miscarriage or
premature labour (classed as mid pregnancy – 37 weeks).
Normal labour – a mucus discharge that’s tinged with blood after 37 weeks is most
likely just a sign that the mucus plug has dislodged and the cervix is beginning to
soften or dilate, in preparation for pregnancy.
Sex Hormones.
Hormone
Details
Follicle Stimulating Hormone (FSH)
Origin: anterior pituitary
Endocrine target: Testes and ovaries.
Function: Promotes follicle development in
females and, in combination with luteinising
hormone, stimulates the secretion of oestrogens
by ovarian cells. In males, FSH stimulates
sustentacular cells, specialised cells in the tubules
where sperm differentiate.
Regulation: Inhibited by inhibin. Regulated by
GnRH.
Luteinising Hormone (LH)
Origin: anterior pituitary
Endocrine target: Testes and ovaries.
Function: Induces ovulation. Promotes the
secretion by the ovaries of oestrogens and the
progestins (such as progesterone). In males it
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stimulates the production of sex hormones by the
interstitial cells of the testes.
Regulation: Regulated by GnRH.
Progesterone
Origin: ovary (corpus luteum); placenta
Function: Preparation of uterus for pregnancy;
maintenance of pregnancy; development of
alveolar system in mammary glands.
Regulation: The corpus luteum is formed under
LH stimulation and the lipids contained within it
are used to form progesterone. Placenta
produces sufficient amounts of progesterone to
maintain the endometrial lining and continue
pregnancy.
Oestrogen
Origin: ovary, testis, and placenta
Function: Stimulating bone and muscle growth.
Development
of
secondary
female
sex
characteristics: Hair growth; voice pitch; broad
pelvis; mons pubis (fatty layer overlying pubic
symphysis). Affecting CNS – e.g. affect sex drive
in hypothalamus. Initiating growth and repair of
endometrium.
Regulation: FSH and LH stimulate.
Inhibin
Origin: Sustentacular cells of the testes; follicular
cells of ovaries
Endocrine target: anterior pituitary gland.
Function: Inhibit secretion of FSH.
Regulation: Stimulated by FSH from anterior
pituitary gland.
Human Chorionic Gonadotrophin
Hormone (hCG)
Origin: Corpus luteum
Function: Maintains the integrity of the corpus
luteum and promotes continued secretion of
progesterone – appears in blood stream soon
after implantation. The presence of hCG reduces
after 3-4 months however by now the placenta
actively
secretes
both
oestrogens
and
progesterones.
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Relaxin
Semester One: Life Cycles
Origin: Corpus luteum and placenta
Function: Increase the flexibility of the pubic
symphysis; permitting the pelvis to expand during
delivery. Causes dilation of cervix and suppresses
the release of oxytocin by the hypothalamus –
delaying onset of labour contractions.
What is spermatogenesis?
SOME ANATOMY
The testes is divided
into lobules (division or
lobe) and seminiferous
tubules are distributed
throughout.
THERE ARE THREE PROCESSES
Mitosis
Spermatogonia (primitive male reproductive cells) undergo cell divisions through life. One
daughter cell from each division is pushed towards the lumen of the seminiferous
(producing sperm) tubule. These cells then differentiate into primary spermatocytes.
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Meiosis
In the seminiferous tubules meiotic divisions take place. The spermatocytes become
spermatids – undifferentiated (no distinguishable characteristics) male gametes.
Interphase between divisions I and II is short with no replication.
Four haploid gametes are produced known as spermatids. In anaphase I the tetrads (2
pairs of matched chromosomes – where crossing over may take place) separate. The
daughter cells receive both the maternal and paternal chromosome. Increasing variety
results from the random assortment of parental and maternal chromosomes. Cytokinesis
does not occur – ensuring therefore that nutrients can be supplied and that cells develop in
synchrony.
M = myofibroblasts;
SA = spermatogonia type A;
SB = spermatogonia type B;
S1 = primary spermatocytes;
S3 = spermatids;
S4 = spermatozoa;
St = Sertoli cells.
Spermiogenesis
Each spermatid matures into a single spermatozoon, or sperm. Developing spermatocytes
undergoing meiosis and spermatids undergoing spermiogenesis are not free in the
seminiferous tubules. Instead they are surrounded by the cytoplasm of the
sustentacular/Sertoli (support – protect sperm because their antigens would initiate an
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immune response) cells. Spermatids gradually develop the appearance of mature
spermatozoa. At spermination a spermatozoon loses its attachment to the sustentacular
cell and enters the lumen of the seminiferous tubule. The whole process of spermination
takes 9 weeks.
What happens during labour?!
The goal of labour is parturition, the forcible expulsion of the foetus. During true labour,
each labour contraction begins near the top of the uterus and sweeps in a wave toward the
cervix. The contractions are strong and occur at regular intervals.
During most of the months of pregnancy, the uterus undergoes periodic episodes of weak
and slow rhythmical contractions called Braxton Hicks contractions. These contractions
become progressively stronger toward the end of pregnancy; then they change suddenly,
within hours, to become exceptionally strong contractions that start stretching the cervix
and later force the baby through the birth canal, thereby causing parturition. This process is
called labour, and the strong contractions that result in final parturition are called labour
contractions.
Based on experience with other types
of physiological control systems, a
theory has been proposed for
explaining the onset of labour. The
positive feedback theory suggests
that stretching of the cervix by the
foetus’s head finally becomes great
enough to elicit a strong reflex
increase in contractility of the uterine
body. This pushes the baby forward,
which stretches the cervix more and
initiates more positive feedback to
the uterine body. Thus, the process
repeats until the baby is expelled.
There are two known types of positive feedback which increase uterine contractions during
labour:
1. Stretching of the cervix causes the entire body of the uterus to contract, and this
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contraction stretches the cervix even more because of the downward thrust of the
baby's head.
2. Cervical stretching also causes the pituitary gland to secrete oxytocin, which is another
means for increasing uterine contractility.
Dilation Stage
Begins with the onset of true labour, as the cervix dilates and the foetus begins to move
toward the cervical canal. This stage is highly variable in length but typically lasts 8 or more
hours. At the start of this stage, the labour contraction last up to half a minute and occur t
intervals once every 10-30 minutes. Frequency steadily increases. The amniochorionic
membrane breaks at some point (waters break).
Expulsion Stage
The cervix is pushed open by the approaching foetus and dilation it completed.
Contractions reach maximum intensity occurring at 2-3 minute intervals and lasting a full
minute. Expulsion continues until the foetus has emerged from the vagina; in most cases
this lasts less than 2 hours. The arrival of the newborn is called delivery.
Placental Stage
Muscle tension builds in the walls of the partially empty uterus and the organ gradually
decreased in size. The uterine contraction tears the connections between the endometrium
and the placenta. In general, within an hour of delivery, the placental stage ends with the
ejection of the placenta accompanied by a loss of blood, however as maternal blood
volume has increased greatly during pregnancy, this blood loss can easily be tolerated.
Placental production
Oestrogen Relaxin
Foetal Growth
Distortion of the
myometrium
Maternal
oxytocin release
at posterior
pituitary
Increased
excitability of
uterine
musculature
Increased
prostaglandin
production
positive
feedback
Foetal oxytocin
release
Labour Contractions
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What are the female and male sexual functions?
MALES
The most important source of sensory nerve signals for initiating the male sexual act is the
glans penis. The glans contains an especially sensitive sensory end-organ system that
transmits into the central nervous system that special modality of sensation called sexual
sensation. The slippery massaging action of intercourse on the glans stimulates the
sensory end-organs, and the sexual signals in turn pass through the pudendal nerve, then
through the sacral plexus into the sacral portion of the spinal cord, and finally up the cord
to undefined areas of the brain.
Impulses may also enter the spinal cord from areas adjacent to the penis to aid in
stimulating the sexual act. For instance, stimulation of the anal epithelium, the scrotum,
and perineal structures in general can send signals into the cord that add to the sexual
sensation.
Psychic Element of male sexual stimulation
Appropriate psychic stimuli can greatly enhance the ability of a person to perform the
sexual act. Simply thinking sexual thoughts or even dreaming that the act of intercourse is
being performed can initiate the male act, culminating in ejaculation. Indeed, nocturnal
emissions during dreams occur in many males during some stages of sexual life,
especially during the teens.
Integration of the male sexual act in the spinal cord
The male sexual act results from inherent reflex mechanisms integrated in the sacral and
lumbar spinal cord, and these mechanisms can be initiated by either psychic stimulation
from the brain or actual sexual stimulation from the sex organs, but usually it is a
combination of both.
Stages of male sexual act
Penile Erection – erection is caused by parasympathetic impulses that pass from the
sacral portion of the spinal cord through the pelvic nerves to the penis. Nitric oxide is
released, believed to assist in vascular dilation. For erection to occur the arterial blood flow
is rapid and the venous outflow is partially occluded. High blood pressure causes
ballooning of erectile tissue.
Lubrication – parasympathetic stimulation causes urethral glands and the bulbourethral
glands to secrete mucous. This mucus flows through the urethra during intercourse to aid
in the lubrication during coitus (sexual intercourse).
Emission and ejaculation – the culmination of the sexual act. When the stimulus
becomes extremely intense the reflex centres of the spinal cord begin to emit sympathetic
impulses from T-12 and L-2 – this initiates emission, the forerunner of ejaculation. The vas
deferens and ampulla contract causing expulsion of sperm into the internal urethra. Then
contractions of the coat of the prostate gland followed by the seminal vesicles expel
prostatic and seminal fluid into the urethra also. The filling of the internal urethra with
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semen elicits sensory signals that are transmitted through the pudendal nerves to the
sacral regions of the cord giving a sense of fullness in the internal genital organs. Sensory
signals further excite rhythmical contraction of the ischiocavernosus and bulbocavernosus
muscles that compress the bases of the penile erectile tissue. A wave-like sequence of
contractions results in ejaculation of semen from the urethra. Rhythmical contractions of
the pelvic muscles and even some of the body trunk cause thrusting movements of the
pelvis and penis which also help propel the semen into the deepest recesses of the vagina
and perhaps even into the cervix. At termination of this, the male orgasm, sexual
excitement is terminated within 1 to 2 minutes and erection ceases, a process called
resolution.
FEMALES
Thinking sexual thoughts can lead to female sexual desire, and this aids greatly in the
performance of the female sexual act. Such desire is based largely on a woman's
background training as well as on her physiological drive, although sexual desire does
increase in proportion to the level of sex hormones secreted. Desire also changes during
the monthly sexual cycle, reaching a peak near the time of ovulation, probably because of
the high levels of oestrogen secretion during the prevulatory period.
Local sexual stimulation in women occurs in more or less the same manner as in men
because massage and other types of stimulation of the vulva, vagina, and other perineal
regions can create sexual sensations. The glans of the clitoris is especially sensitive for
initiating sexual sensations.
As in the male, the sexual sensory signals are transmitted to the sacral segments of the
spinal cord through the pudendal nerve and sacral plexus. Once these signals have
entered the spinal cord, they are transmitted to the cerebrum. Also, local reflexes
integrated in the sacral and lumbar spinal cord are at least partly responsible for some of
the reactions in the female sexual organs.
Female erection and lubrication
Located around the introitus (entrance, i.e. to the vagina) and extending into the clitoris is
erectile tissue almost identical to the erectile tissue of the penis. This erectile tissue, like
that of the penis, is controlled by the parasympathetic nerves that pass through the nervi
erigentes from the sacral plexus to the external genitalia. In the early phases of sexual
stimulation, parasympathetic signals dilate the arteries of the erectile tissue, probably
resulting from release of acetylcholine, nitric oxide, and vasoactive intestinal polypeptide
(VIP) at the nerve endings. This allows rapid accumulation of blood in the erectile tissue so
that the introitus tightens around the penis; this aids the male greatly in his attainment of
sufficient sexual stimulation for ejaculation to occur.
Parasympathetic signals also pass to the bilateral Bartholin's glands located beneath the
labia minora and cause them to secrete mucus immediately inside the introitus. This
mucus is responsible for much of the lubrication during sexual intercourse, although much
is also provided by mucus secreted by the vaginal epithelium and a small amount from the
male urethral glands. This lubrication is necessary during intercourse to establish a
satisfactory massaging sensation rather than an irritative sensation, which may be
provoked by a dry vagina. A massaging sensation constitutes the optimal stimulus for
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evoking the appropriate reflexes that culminate in both the male and female climaxes.
Female Climax
When local sexual stimulation reaches maximum intensity, and especially when the local
sensations are supported by appropriate psychic conditioning signals from the cerebrum,
reflexes are initiated that cause the female orgasm, also called the female climax. The
female orgasm is analogous to emission and ejaculation in the male, and it may help
promote fertilization of the ovum. Indeed, the human female is known to be somewhat
more fertile when inseminated by normal sexual intercourse rather than by artificial
methods, thus indicating an important function of the female orgasm. There is evidence to
suggest that the rhythmic contraction of the perineal muscles increases uterine and
fallopian tube motility. It’s also possible that the dilation of the cervical canal may be due to
orgasm. In addition to the possible effects of the orgasm on fertilization, the intense sexual
sensations that develop during the orgasm also pass to the cerebrum and cause intense
muscle tension throughout the body. But after culmination of the sexual act, this gives way
during the succeeding minutes to a sense of satisfaction characterized by relaxed
peacefulness, an effect called resolution.
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