TOPIC 16.3 – FERTILIZATION,

PREGNANCY AND BIRTH

Note:

At 4 days a zygote becomes an embryo At 8 weeks an embryo is called a fetus

Fertilization and Early Development Definitions (there are 13)

Cleavage – equal division of a cell without growth in the zygote

Blastocyst – an outer circle of cells and an inner mass of cells which grows into the embryo

Implantation – when the embryo attaches to the endometrium

chorion – produces homan chorionic gondotropic hormone (hCG)

Is an outer structure helping hold the embryo, part of the placenta

amnion – fluid-filled extraembryonic structure hCG – embryonic hormone that maintains the

corpus luteum for the first 3 months of pregnancy

amniotic cavity – fluid-filled space surrounding the embryo

extraembryonic coelum – body cavity between the amnion and chorion

yolk sac – forms during development of most vertebrates, is a membranous sac

placenta – where the exchange of nutrients and waste occurs between mom and fetus

chorionic villi – projections of the chorion

allantois – extraembryonic structure that contributes to the blood vessels in the placenta

umbilical cord – connects the fetus to the placenta

Fertilization and Implantation A fertilized ovum takes 3-5 days to move

into the uterus While it moves it undergoes cleavage –

the zygote becomes progressively smaller

On about day 6 the zygote reaches the uterus and has become a blastocyst

A blastocyst has 2 layers, one develops into the extraembryonic structures and one that develops into the embryo

Implantation occurs when the blastocyst connects to the endometrium in the uterus

Within a Pregnant Female… Problem : to prevent menstruation high

levels of progesterone and estrogen levels must be maintained BUT high levels of progesterone and estrogen cause a negative-feedback loop on the LH and FSH. We can’t have that happen either because lower LH would cause the corpus luteum to deteriorate and the endometrium would be shed. So…

The blastocyst creates hormones itself

The outer layer of the blastocyst becomes 2 structures

1) chorion

2) amnion the chorion produces hCG which

maintains the corpus luteum The corpus luteum in turn produces

estrogen and progesterone which cause the endometrium to be maintained

Pregnancy tests look for hCG in the urine

The amniotic cavity forms between the amnion and embryo and insulates and protects it

Extraembryonic coelom is the cavity between the chorion and amnion

The yolk sac forms under the embryo and is the site of red blood cell formation and is a part of the primitive digestive tract

Let’s Draw!

Draw figure 3 on page 531

The placenta is made from some cells from the embryo and some from the endometrium

The placenta produces estrogen and progesterone – high progesterone stops ovulation so a woman cannot get pregnant while pregnant

The placenta has many blood vessels in it to allow the exchange of waste into the mother and nutrients and oxygen to move into the fetus

The chorionic villi allow vessels of the fetus to be exposed to maternal blood

The allantois gives blood vessels to the placenta

The umbilical cord connects the embryo with the placenta

Embryonic and Fetal Development Definitions (8 of them)

gastrulation – the process by which a two-layer structure turns into a three-layer structure

gastrula – state of embryonic development when the embryo is made of three layers

ectoderm – outer layer of cells in an embryo

mesoderm – middle layer of cells in an embryo

endoderm – inner layer of cells in an embryo

first trimester – from conception to the end of the third month

second trimester – 4rd – 6th month third trimester – 7th month till birth

Embryonic and Fetal Development

During the first trimester, about week 2, the blastocyst reorganizes into a disk made of 2 layers

The 2 layers divide and differnentiate into a three-layered structure called a gastrula which is called gastrulation

The three layers are called the ectoderm (outside), mesoderm (middle) and endoderm (inner)

Each layer becomes specific parts of the embryo

Read table 1 on page 532 to see what the layers of gastrula form

During the first trimester most of the major organs have begun developing

During the second trimester the baby grows, hair covers it in its entirety and the cartilage skeleton is replaced by bone

During the third trimester the baby grows

Read through table 2 on page 534 as a class

Read Human Sex Determination on page 535 as a class

Effects of Environmental Agents on Embryonic Development

Mothers need to make good choices while they’re pregnant

Spina bifida can is the failure of the spinal cord to develop properly – women often take folic acid supplements to counteract that

Smoking mothers often have smaller babies

Teratogens are chemicals and microbes that can cause abnormalities in utero

They can cross the placental membrane and harm the fetus

Read through table 3 on page 537 Read through Fetal Alcohol Spectrum

Disorder and be sure you can answer questions 1-3 of Understanding the Issue

Birth Definitions (4)

Parturition – giving birth Relaxin – hormone produced by the

placenta before labour to cause the ligaments to loosen in the pelvis

Oxytocin – hormone from the pituitary gland which causes the uterus to contract

Prolactin – hormone produced by the pituitary gland and associated with milk production

Birth

Labour or partuition begins with contractions

The cervix thins and dilates which causes the amnion into the birth canal

The amnion bursts which is called “the water breaking” and lubricates the canal

As the cervix dilates, contractions move the baby through the canal

After the baby has been born the placenta also moves through the canal

There is huge variations in birth times – from a few hours to a few days

Birth Hormones

Labour can be induced by giving prostaglandins or pitocin (synthetic form of oxytocin) – this stimulates contractions

Relaxin is made by the placenta close to labour and causes the ligaments in the pelvis to loosen and the cervix to soften

Oxytocin, made by the pituitary, causes the uterine contractions

Prostaglandins also appear in the blood – may help with uterine contractions

Lactation

Breasts are developed during puberty by estrogen and progesterone

Milk production is begun by high levels of estrogen and progesterone during pregnancy

Prolactin, a hormone created in the pituitary stimulates the breast to make fluids

Estrogen stimulates the production of prolactin but prolactin is blocked by high levels of progesterone so milk is not made until after birth

After birth progesterone levels decrease so that the prolactin can work and cause first colostrum and then milk to be made

The colostrum (similar to breast milk without the fat) and breast milk allow transfer of antibodies from the mother to the baby as well as nutrients

Breast Feeding

A baby’s sucking stimulates nerve endings in the areola in the breast which send signals to the pituitary and causes oxytocin to be released

Oxytocin moves to the breasts and uterus and causes weak contractions in the breast to move the milk into the ducts

In the uterus the oxytocin causes weak contractions to shrink the uterus back to its pre-birth size

Read the paragraph on page 541 as a class

To Do:

With a partner or on your own do the case study on page 542 and 543

Do questions 1-5 – not to be handed in Review – page 548-549 #1-14 and hand

in