10Important steps after fertilization: zygote continues down the oviductZygote undergoes cell division: 2, 4, 8 cells. No increase in the overall size of the zygotes= CleavageIt is now called an embryo and will remain an embryo until bone cells begin to form weeks later!
123. 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.
13Implantation: 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!
14Gastrulation: 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 respirationMesoderm forms the middle layer and gives rise to muscles, bones, and structures associated with reproduction
15Primary membranes develop from the germ layers and support, nourish and protect the embryo. Chorion- is involved in gas exchangeAmnion- Provides a liquid environment for the developing embryoAllantois- collects waste from the embryoYolk sac- surrounds the yolk.What do you think the purpose of the yolk is?Nourishes the embryo during development
17InvaginationImmediately after gastrulation, the body axis of the embryo begins to appearcells that will form the nervous system fold into a neural tube (which will eventually form the spinal cordThe mesoderm forms the notochord which will eventually form the vertebrae
18Neural Development: ~week 3 Ecto= nervousMeso= notochordOnce the folds fuse around the notochord the embryo is now called a NEURULA
19The First Trimester: Weeks 1-12 By week 4 limbs, eyes and spine have begun to formAt weeks 8-9 bone cells begin to form, now the neurula is called a fetus~ 100mm long by the end of the first trimester
20Remember 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!
21The Second Trimester Weeks 13-24 You can hear the fetus’ heartbeat with a stethoscopeSkeleton is formingBegin to feel movement of the babyBy the end the baby is 300mm longMost organs formed but still developing
22The Third Trimester: Weeks 25-38 Size increases rapidly Fetus moves around a lotImmune system developsBy the 8th month, eyes are openMaternal nutrition VIPNormally 525mm long
24The Placenta: Blood-rich organ present only during pregnancy Forms from the chorionFully formed by week 10Produces estrogen and progesterone…any ideas why?Prevents new follicles from forming, maintains the endometrium
25So, because levels of progesterone and estrogen are high, does menstruation occur during pregnancy? No, because high levels of estrogen INHIBIT FSH!
26Embryo gets all its nutrients and oxygen from the placenta Umbilical cord- gets rid of wastes and connects the fetus to the placentaSee figure on page 511SEE THIS: (feeding and the third trimester)
27Teratogens: 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 bloodEx: Smoke=constrict blood flow to fetusEx: alcohol= FASEX: x rays= genetic defects
29Birth:Prostoglandins= hormones that cause the release of another VIP hormone called oxytocin- Both these hormones cause the uterus to contract which starts labour
30Dilation 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).
31Once 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.
32Caesarean 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 babiesDone if baby is ‘rump’ first or if complications arise
341. 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 different2. Identical TwinsForm from the one eggEgg splits in to during early embryonic developmentThus twins are the same sex, with the same genetic materialStill unknown the entire mechanism which causes this to happen