Download 9 Human Development

Human
Development
Hormones and the
menstrual cycle
 The menstrual cycle is approximately 28
days.
 The menstrual cycle is regulated by
hormones produced by the pituitary
gland.
The cycle begins
 Follicle – stimulating hormone (FSH)
from the pituitary gland stimulates the
follicle inside the ovary to develop
 This developing follicle secretes
estrogen
The cycle continues
 The estrogen secreted by the developing
follicle stimulates the lining of the uterus
to thicken.
 Estrogen also stimulates the pituitary
gland to produce luteinising hormone
(LH).
 The release of LH causes ovulation.
 Ovulation is the release of the egg into
the fallopian tubes.
 After the release of the egg the corpus
luteum will develop from the follicle.
The Corpus Luteum
 The corpus luteum produces both
progesterone and estrogen.
 Progesterone increases the uterine lining
and causes FSH and LH to drop.
 This drop in FSH and LH prevents the
release of another egg until progesterone
decreases again.
If the egg is not fertilized
 The corpus luteum will break down causing the
levels of progesterone to drop.
 This drop in progesterone causes the uterine
lining to break down and menstruation to
occur.
 Menstruation continues for 4 – 7 days until
progesterone reaches a certain level, which
causes the release of FSH by the pituitary, and
the cycle starts again.
If the egg is fertilized –
Pregnancy
 When sperm is deposited into the female
cervix, they travel up the uterus into the
oviduct.
 Fertilization occurs in the oviducts when
one sperm enters the mature egg and
their nuclei fuse to form a zygote
After Fertilization
 After fertilization the zygote travels down
the oviduct towards the uterus.
 As the zygote travels, it begins the
process of mitosis, with a series of
divisions. By the time it reaches the
uterus, 24 – 36 hours after fertilization, it
is an almost hollow ball of cells called a
blastocyst.
The Blastocyst
 The outer cells of the blastocyst will
become the placenta while the inner cell
mass will become the embryo.
Implantation
 6 – 10 days after fertilization the embryo
attaches itself to the thickening lining of
the uterus in a process called
implantation.
 Once implanted the embryo sends a
hormonal signal that keeps the corpus
luteum producing progesterone for the
next three months
Embryo Development
 In the blastocyst stage, the cells are all
similar to each other. However, in the
second week of pregnancy the cells
begin to specialize. This process
produces a gastrula and is called
gastrulation.
Gastrulation
 During gastrulation the cells in the embryo
become arranged in 3 layers called germ
layers.
 The cells move to specific positions to produce
the layers called the;
 Endoderm – forms the skin and nervous
system
 Mesoderm – forms the kidneys, skeleton,
muscles, blood vessels and gonads
 Ectoderm – forms the lungs and lining of
the digestive tract
The outer cells
 Between the tenth and fourteenth day of
pregnancy the outer portions of the
embryo form four important parts;
 1. Yolk Sac – supplies nutrients for the first 2
months of development
 2. Amnion – fluid filled sac that protects the
embryo
 3. Allantois – removes waste
 4. Chorion – surrounds everything
The Placenta
 The chorion contains many finger-like
projections that contain blood vessels.
 These projections extend into the uterine
wall to form the placenta.
 Once the placenta has formed it takes
over the role of the yolk sac by providing
nutrients to the embryo.
 The placenta also replaces the corpus
luteum in maintaining the high levels of
progesterone necessary to sustain
pregnancy.
 The placenta is the lifeline of the embryo
by delivering nutrients and oxygen and
ensuring that wastes are removed.
 The embryo is attached to the placenta
by the umbilical cord.
Gestational Background
 The human gestation period is about 40
weeks
 It is divided into three trimesters, each
approximately 3 months long
First Trimester
(1 – 12 weeks)
 At 4 weeks, limbs, eyes and spine begin
to form
 At 9 weeks the first bone cells form and
the embryo is called a fetus
 At 12 weeks, all major organs have
begun to develop (liver, brain, stomach,
heart)
First Trimester…
 The fetus is 100 mm long
 Has a noticeable head and limbs
 Sex can be identified using ultrasound
Second Trimester
(13 – 25 weeks)
 At 16 weeks the skeleton begins to form,
the brain grows rapidly and the nervous
system begins to form
 The mother begins to feel movements as
the fetus uses its new muscles (around
20 weeks)
Second Trimester…
 At 24 weeks the fetus is 300 mm long
 The fetus becomes more active and all
organs are formed but not fully
developed
Third Trimester
(26 – 40 weeks)
 A rapid increase is size during this time
 Vital brain tissue is built and the nervous
system develops
 By the 8th month, the fetus can open its
eyes
Birth
 During pregnancy, high progesterone
levels have maintained the pregnancy
 A sharp frop in levels of progesterone
and estrogen causes the muscles of the
uterus to contract
 The pituitary gland secretes oxytocin,
which stimulates the uterus to contract
and opens the birth canal
Three Stages of Birth
1. Dilation Stage:
-
-
Lasts about 2-20 hours
Uterine contractions and oxytocin cause
the cervix to dilate
Amnion breaks and fluid is released (does
not always occur in this stage, some babies
have been born still in the amniotic sac)
Early labour
2. Expulsion Stage:
- Lasts about 0.5-2 hours
- Contractions become so forceful that
the baby is pushed through the cervix
into the birth canal
- The head of the baby rotates making it
easier to pass through the birth canal
3. Placental Stage:
- The placenta and umbilical cord are
expelled from the uterus
- Usually happens between 10-15
minutes after the baby is born
Risk Factors during
Development
 The fetus receives all of its nutrients and
oxygen from the mother’s blood.
 Anything that the mother east, drinks or
inhales from her environment end up in
her blood
 The first trimester is the critical period
for the developing embryo
Risk Factors during
Development continued
 Cigarette smoke constricts fetal blood
vessels preventing it from getting enough
oxygen
 Alcohol affects the functioning of the
fetus’ brain and central nervous system
as well as physical development (FAS)
Risk Factors during
Development…
 Other factors such as radiation,
pollutants such as PCB’s and mercury
can lead to genetic mutations
 The mothers age is also a risk, with
women under 20 having increased risk of
delivering prematurely, and women over
45 having 5% chance of having a baby
with chromosomal abnormalities