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 – 4 rd – 6 th month  third trimester – 7 th 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 #1-14 and hand in