Development and Differentiation What do you see in this picture? Let’s take a closer look

When does the heart begin to beat? When can a fetus move its arms and legs?

Fertilization has three functions: transmission of genes restoration of the diploid number of chromosomes initiation of development

Fertilization Millions of sperm enter the vagina and swim into the oviducts Why are there so many sperm for the one egg you may ask? Suggestions?

There are Millions of sperm for a couple of reasons: Assures survival in the acidic female reproductive tract ensures that some will enter the right oviduct that the ovum was released from.

Steps in Fertilization: Contact between sperm and egg Entry of sperm into the egg Fusion of egg and sperm nuclei Activation of development

Union of the male and female gametes gives rise to a zygote. the sperm acrosome or “head” penetrates the egg and releases enzymes that stimulates the egg to become impenetrable.

Important steps after fertilization: zygote continues down the oviduct Zygote undergoes cell division: 2, 4, 8 cells. No increase in the overall size of the zygotes= Cleavage It is now called an embryo and will remain an embryo until bone cells begin to form weeks later!

Human Development before Implantation:

3. By the time the embryo reaches the uterus it has formed the Blastocyst- a hollow ball of cells formed by mitotic division . The inner cell mass will develop into the baby, whereas the other cells called the trophoblast will become the membranes that nourish the embryo.

Implantation: Occurs at the end of the first week. This is when the embryo has finished it’s journey enters the uterus and attaches to the endometrium (which we remember is the uterine lining!) The trophoblast (the outside layer of the blastocyst) secretes human chorionic gonadotropin (HGC) which pregnancy kits are designed to pick up!

Gastrulation: Happens during the second week of development involves a series of cell migrations to positions where they will form the three primary cell layers. Ectoderm forms the outer layer and gives rise to the skin, brain, and nervous system . Endoderm forms the inner layer and gives rise to organs and tissues associated with digestion and respiration Mesoderm forms the middle layer and gives rise to muscles, bones, and structures associated with reproduction

Primary membranes develop from the germ layers and support, nourish and protect the embryo. Chorion- is involved in gas exchange Amnion- Provides a liquid environment for the developing embryo Allantois- collects waste from the embryo Yolk sac- surrounds the yolk. What do you think the purpose of the yolk is? Nourishes the embryo during development

Invagination Immediately after gastrulation, the body axis of the embryo begins to appear cells that will form the nervous system fold into a neural tube (which will eventually form the spinal cord The mesoderm forms the notochord which will eventually form the vertebrae

Neural Development: ~week 3 Ecto= nervous Meso= notochord Once the folds fuse around the notochord the embryo is now called a NEURULA

The First Trimester: Weeks 1-12 By week 4 limbs, eyes and spine have begun to form At weeks 8-9 bone cells begin to form, now the neurula is called a fetus ~ 100mm long by the end of the first trimester

Remember at 8 weeks of age the baby has developed it’s sexual reproductive organs, so the doctor can tell whether the baby is a boy or a girl!

The Second Trimester Weeks 13-24 You can hear the fetus’ heartbeat with a stethoscope Skeleton is forming Begin to feel movement of the baby By the end the baby is 300mm long Most organs formed but still developing

The Third Trimester: Weeks 25-38 Size increases rapidly Fetus moves around a lot Immune system develops By the 8th month, eyes are open Maternal nutrition VIP Normally 525mm long

The Placenta: Blood-rich organ present only during pregnancy Forms from the chorion Fully formed by week 10 Produces estrogen and progesterone…any ideas why? Prevents new follicles from forming, maintains the endometrium

So, because levels of progesterone and estrogen are high, does menstruation occur during pregnancy? No, because high levels of estrogen INHIBIT FSH!

Embryo gets all its nutrients and oxygen from the placenta Umbilical cord- gets rid of wastes and connects the fetus to the placenta See figure 15.19 on page 511 SEE THIS: (feeding and the third trimester) http://www.pbs.org/wgbh/nova/miracle/program.html

Teratogens: Any agent that causes structural abnormality to the fetus. Whatever a mother ingests or inhales can end up in her blood and therefore the baby’s blood Ex: Smoke=constrict blood flow to fetus Ex: alcohol= FAS EX: x rays= genetic defects

Birth: Prostoglandins= hormones that cause the release of another VIP hormone called oxytocin - Both these hormones cause the uterus to contract which starts labour

Dilation stage- cervix widens, amnion breaks and the amniotic fluid is released (2-20 hrs). The cervix widens to approx. 10 cm. 2. Expulsion- contractions push baby into the birth canal. (0.5-2 hrs) 3. Placental Stage- after baby is born, placenta and umbilical cord are expelled from uterus (called afterbirth).

Once baby is delivered and is breathing normally, the umbilical cord is cut and now the baby will ingest food for nutrients and rid its own waste.

Caesarean Section: Known as a c-section a form of childbirth in which a surgical incision is made through a mother's abdomen and uterus to deliver one or more babies Done if baby is ‘rump’ first or if complications arise

Twins- A Unique Occurrence

1. Fraternal Twins 2. Identical Twins Form when more than one egg is released from the ovaries. Thus the twins can be different sexes because their genetic make ups are different 2. Identical Twins Form from the one egg Egg splits in to during early embryonic development Thus twins are the same sex, with the same genetic material Still unknown the entire mechanism which causes this to happen