Human Human DevelopmentDevelopment

Hormones and the Hormones and the menstrual cyclemenstrual cycle

The menstrual cycle is approximately 28 The menstrual cycle is approximately 28 days.days.

The menstrual cycle is regulated by The menstrual cycle is regulated by hormones produced by the hormones produced by the pituitary pituitary glandgland..

The cycle beginsThe cycle begins

Follicle – stimulating hormone (FSH)Follicle – stimulating hormone (FSH) from the pituitary gland stimulates the from the pituitary gland stimulates the follicle inside the ovary to developfollicle inside the ovary to develop

This developing follicle secretes This developing follicle secretes estrogenestrogen

The cycle continuesThe cycle continues

The estrogen secreted by the developing The estrogen secreted by the developing follicle stimulates the lining of the uterus follicle stimulates the lining of the uterus to thicken.to thicken.

Estrogen also stimulates the pituitary Estrogen also stimulates the pituitary gland to produce gland to produce luteinising hormone luteinising hormone (LH).(LH).

The release of LH causes The release of LH causes ovulation.ovulation. Ovulation Ovulation is the release of the egg into is the release of the egg into

the fallopian tubes.the fallopian tubes. After the release of the egg the After the release of the egg the corpus corpus

luteumluteum will develop from the follicle. will develop from the follicle.

The Corpus LuteumThe Corpus Luteum

The corpus luteum produces both The corpus luteum produces both progesterone and estrogen.progesterone and estrogen.

Progesterone increases the uterine lining Progesterone increases the uterine lining and causes FSH and LH to drop.and causes FSH and LH to drop.

This drop in FSH and LH prevents the This drop in FSH and LH prevents the release of another egg until progesterone release of another egg until progesterone decreases again.decreases again.

If the egg is not fertilizedIf the egg is not fertilized

The corpus luteum will break down causing the The corpus luteum will break down causing the levels of progesterone to drop.levels of progesterone to drop.

This drop in progesterone causes the uterine This drop in progesterone causes the uterine lining to break down and menstruation to lining to break down and menstruation to occur.occur.

Menstruation continues for 4 – 7 days until Menstruation continues for 4 – 7 days until progesterone reaches a certain level, which progesterone reaches a certain level, which causes the release of FSH by the pituitary, and causes the release of FSH by the pituitary, and the cycle starts again.the cycle starts again.

If the egg is fertilized – If the egg is fertilized – PregnancyPregnancy

When sperm is deposited into the female When sperm is deposited into the female cervix, they travel up the uterus into the cervix, they travel up the uterus into the oviduct.oviduct.

Fertilization occurs in the oviducts when Fertilization occurs in the oviducts when one sperm enters the mature egg and one sperm enters the mature egg and their nuclei fuse to form a zygotetheir nuclei fuse to form a zygote

After FertilizationAfter Fertilization

After fertilization the zygote travels down After fertilization the zygote travels down the oviduct towards the uterus.the oviduct towards the uterus.

As the zygote travels, it begins the As the zygote travels, it begins the process of process of mitosismitosis, with a series of , with a series of divisions. By the time it reaches the divisions. By the time it reaches the uterus, 24 – 36 hours after fertilization, it uterus, 24 – 36 hours after fertilization, it is an almost hollow ball of cells called a is an almost hollow ball of cells called a blastocyst.blastocyst.

The BlastocystThe Blastocyst

The outer cells of the blastocyst will The outer cells of the blastocyst will become the placenta while the inner cell become the placenta while the inner cell mass will become the embryo.mass will become the embryo.

ImplantationImplantation

6 – 10 days after fertilization the embryo 6 – 10 days after fertilization the embryo attaches itself to the thickening lining of attaches itself to the thickening lining of the uterus in a process called the uterus in a process called implantationimplantation..

Once implanted the embryo sends a Once implanted the embryo sends a hormonal signal that keeps the corpus hormonal signal that keeps the corpus luteum producing progesterone for the luteum producing progesterone for the next three monthsnext three months

Embryo DevelopmentEmbryo Development

In the blastocyst stage, the cells are all In the blastocyst stage, the cells are all similar to each other. However, in the similar to each other. However, in the second week of pregnancy the cells second week of pregnancy the cells begin to specialize. This process begin to specialize. This process produces a produces a gastrula gastrula and is called and is called gastrulation.gastrulation.

GastrulationGastrulation

During gastrulation the cells in the embryo During gastrulation the cells in the embryo become arranged in 3 layers called become arranged in 3 layers called germ layers. germ layers.

The cells move to specific positions to produce The cells move to specific positions to produce the layers called the;the layers called the; Endoderm – Endoderm – forms the skin and nervous forms the skin and nervous

systemsystem Mesoderm – Mesoderm – forms the kidneys, skeleton, forms the kidneys, skeleton,

muscles, blood vessels and gonadsmuscles, blood vessels and gonads Ectoderm – Ectoderm – forms the lungs and lining of the forms the lungs and lining of the

digestive tractdigestive tract

The outer cellsThe outer cells

Between the tenth and fourteenth day of Between the tenth and fourteenth day of pregnancy the outer portions of the pregnancy the outer portions of the embryo form four important parts;embryo form four important parts; 1. Yolk Sac – supplies nutrients for the first 2 1. Yolk Sac – supplies nutrients for the first 2

months of developmentmonths of development 2. Amnion – fluid filled sac that protects the 2. Amnion – fluid filled sac that protects the

embryoembryo 3. Allantois – removes waste 3. Allantois – removes waste 4. Chorion – surrounds everything4. Chorion – surrounds everything

The PlacentaThe Placenta

The chorion contains many finger-like The chorion contains many finger-like projections that contain blood vessels.projections that contain blood vessels.

These projections extend into the uterine These projections extend into the uterine wall to form the placenta.wall to form the placenta.

Once the placenta has formed it takes Once the placenta has formed it takes over the role of the yolk sac by providing over the role of the yolk sac by providing nutrients to the embryo.nutrients to the embryo.

The placenta also replaces the corpus The placenta also replaces the corpus luteum in maintaining the high levels of luteum in maintaining the high levels of progesterone necessary to sustain progesterone necessary to sustain pregnancy.pregnancy.

The placenta is the lifeline of the embryo by The placenta is the lifeline of the embryo by delivering nutrients and oxygen and ensuring delivering nutrients and oxygen and ensuring that wastes are removed.that wastes are removed.

The embryo is attached to the placenta by The embryo is attached to the placenta by the umbilical cord.the umbilical cord.

Gestational BackgroundGestational Background

The human gestation period is about 40 The human gestation period is about 40 weeksweeks

It is divided into three trimesters, each It is divided into three trimesters, each approximately 3 months longapproximately 3 months long

First Trimester First Trimester (1 – 12 weeks)(1 – 12 weeks)

At 4 weeks, limbs, eyes and spine begin to At 4 weeks, limbs, eyes and spine begin to formform

At 9 weeks the first bone cells form and the At 9 weeks the first bone cells form and the embryo is called a embryo is called a fetusfetus

At 12 weeks, all major organs have begun At 12 weeks, all major organs have begun to develop (liver, brain, stomach, heart)to develop (liver, brain, stomach, heart)

First Trimester…First Trimester…

The fetus is 100 mm longThe fetus is 100 mm long

Has a noticeable head and limbsHas a noticeable head and limbs

Sex can be identified using ultrasoundSex can be identified using ultrasound

Second Trimester Second Trimester (13 – 25 weeks)(13 – 25 weeks)

At 16 weeks the skeleton begins to form, At 16 weeks the skeleton begins to form, the brain grows rapidly and the nervous the brain grows rapidly and the nervous system begins to formsystem begins to form

The mother begins to feel movements as The mother begins to feel movements as the fetus uses its new muscles (around the fetus uses its new muscles (around 20 weeks)20 weeks)

Second Trimester…Second Trimester…

At 24 weeks the fetus is 300 mm longAt 24 weeks the fetus is 300 mm long

The fetus becomes more active and all The fetus becomes more active and all organs are formed but not fully organs are formed but not fully developeddeveloped

Third TrimesterThird Trimester(26 – 40 weeks)(26 – 40 weeks)

A rapid increase is size during this timeA rapid increase is size during this time

Vital brain tissue is built and the nervous Vital brain tissue is built and the nervous system developssystem develops

By the 8By the 8thth month, the fetus can open its month, the fetus can open its eyeseyes

BirthBirth During pregnancy, high progesterone During pregnancy, high progesterone

levels have maintained the pregnancylevels have maintained the pregnancy

A sharp frop in levels of progesterone A sharp frop in levels of progesterone and estrogen causes the muscles of the and estrogen causes the muscles of the uterus to contractuterus to contract

The pituitary gland secretes oxytocin, The pituitary gland secretes oxytocin, which stimulates the uterus to contract which stimulates the uterus to contract and opens the birth canaland opens the birth canal

Three Stages of BirthThree Stages of Birth

1.1. Dilation Stage:Dilation Stage:- Lasts about 2-20 hoursLasts about 2-20 hours- Uterine contractions and oxytocin cause Uterine contractions and oxytocin cause

the cervix to dilatethe cervix to dilate- Amnion breaks and fluid is released (does Amnion breaks and fluid is released (does

not always occur in this stage, some babies not always occur in this stage, some babies have been born still in the amniotic sac)have been born still in the amniotic sac)

- Early labourEarly labour

2. Expulsion Stage:2. Expulsion Stage:

- Lasts about 0.5-2 hours- Lasts about 0.5-2 hours

- Contractions become so forceful that - Contractions become so forceful that the baby is pushed through the cervix the baby is pushed through the cervix into the birth canalinto the birth canal

- The head of the baby rotates making it - The head of the baby rotates making it easier to pass through the birth canaleasier to pass through the birth canal

3. Placental Stage:3. Placental Stage:

- The placenta and umbilical cord are - The placenta and umbilical cord are expelled from the uterusexpelled from the uterus

- Usually happens between 10-15 - Usually happens between 10-15 minutes after the baby is bornminutes after the baby is born

Risk Factors during Risk Factors during DevelopmentDevelopment

The fetus receives all of its nutrients and The fetus receives all of its nutrients and oxygen from the mother’s blood. oxygen from the mother’s blood.

Anything that the mother east, drinks or Anything that the mother east, drinks or inhales from her environment end up in inhales from her environment end up in her bloodher blood

The first trimester is the The first trimester is the critical periodcritical period for the developing embryofor the developing embryo

Risk Factors during Risk Factors during Development continuedDevelopment continued

Cigarette smoke constricts fetal blood Cigarette smoke constricts fetal blood vessels preventing it from getting enough vessels preventing it from getting enough oxygenoxygen

Alcohol affects the functioning of the Alcohol affects the functioning of the fetus’ brain and central nervous system fetus’ brain and central nervous system as well as physical development (FAS)as well as physical development (FAS)

Risk Factors during Risk Factors during Development…Development…

Other factors such as radiation, Other factors such as radiation, pollutants such as PCB’s and mercury pollutants such as PCB’s and mercury can lead to genetic mutationscan lead to genetic mutations

The mothers age is also a risk, with The mothers age is also a risk, with women under 20 having increased risk of women under 20 having increased risk of delivering prematurely, and women over delivering prematurely, and women over 45 having 5% chance of having a baby 45 having 5% chance of having a baby with chromosomal abnormalitieswith chromosomal abnormalities