Human Development Human development takes about 9 months (280 days) This 9 month period is divided into 3 trimesters: 1 st trimester:0 3 months 2 nd trimester:4 6 months 3 rd trimester:7 9 months 8 weeks old 1 st Trimester 2 nd Trimester 18 weeks old16 weeks old 3 rd Trimester 24 weeks old32 weeks old Human development is most easily disturbed during the 1 st trimester of pregnancy This is unfortunate because most of the critical events of development occur before the woman knows that she is pregnant 1 st Trimester Events: 1. Fertilization occurs in the fallopian tube when a single sperm penetrates an ovum and the 2 nuclei fuse to form a zygote remember both the sperm and egg contain 23 chromosomes, thus the zygote (fertilized egg) contains 46 chromosomes Fraternal vs. Identical Twins: Fraternal twins are produced when two ova are released from the ovary and they are both fertilized by 2 different sperm Fraternal Twins they may or may not be the same sex Identical twins are produced when a single fertilized ovum splits very early in development they contain exactly the same genetic material and are always the same sex 2. Cleavage is the series of rapid cell divisions (mitosis) of the zygote that take place after fertilization The cells divide so rapidly during cleavage that they do not have time to grow before the next division occurs thus they get smaller and smaller with each division A few days after fertilization, a solid ball of cells (= morula) has been formed About 5 days after fertilization, the morula has developed into a hollow ball of cells (= blastocyst) and enters the uterus 3. Implantation occurs about 6 days after fertilization, the blastocyst implants itself in the endometrium of the uterus 4. Gastrulation and formation of the embryonic membranes about 8 days after fertilization, the blastocyst begins to grow rapidly some of the cells of the blastocyst undergo gastrulation to form the embryo, while other cells form the embryonic membranes that will protect and nourish the developing embryo Gastrulation is the migration of outer cells of blastocyst to form three primary tissue layers (ectoderm, mesoderm, and endoderm) Structures formed from Ectoderm brain, spinal cord, and nerves epidermis of the skin lens and cornea of eye Structures formed from Mesoderm skeletal, cardiac and smooth muscles cartilage and bone blood and blood vessels dermis of skin Structures formed form Endoderm lining of the digestive tract lining of the urinary bladder lining of the respiratory tract Four embryonic membranes form from the blastocyst a. Yolk sac contains cells that migrate to the ovaries or testes of the developing embryo These cells will eventually develop into ova or sperm Yolk Sac b. Amnion membranes that surrounds the embryo, creating a cavity that becomes filled with amniotic fluid Amniotic fluid functions as a shock absorber for the fetus Cells within the amniotic fluid can be examined by a procedure called amniocentesis to detect certain genetic disorders (Downs syndrome, sickle cell anemia, hemophilia) c. Chorion membrane that interacts with the endometrium to form the placenta the placenta is the physical link between mother and fetus despite what many people think, the blood of the mother and child do not mix nutrients and O 2 from the mother diffuse from her capillaries to capillaries of the developing embryo fetal wastes and CO 2 diffuse in the opposite direction Within the placenta there are fingerlike extensions of the chorion called chorionic villi these villi have the same genetic makeup as the fetus when physicians take a sample from these villi (= chorionic villi sampling), they are able to detect genetic abnormalities chorionic villi sampling has an advantage over amniocentesis in that it can be performed as early as 8 weeks of gestation amniocentesis cannot be performed until 14 16 weeks As the placenta grows, it begins to secrete the hormone human chorionic gonadotropin (hCG) This hormone stimulates the continued production of estrogen and progesterone High levels of estrogen and progesterone are needed to maintain pregnancy hCG is believed to cause morning sickness its levels are highest in the 1 st trimester, when morning sickness typically occurs d. Allantois membrane that forms the umbilical cord. The umbilical cord connects the developing embryo/fetus to the placenta At birth, the umbilical cord is cut. The scar that remains on the babys abdomen is called the umbilicus (= navel) 5. Neurulation about 3 weeks after fertilization, the hollow dorsal nerve cord is formed from the ectoderm by a process called neurulation this nerve cord will eventually become enclosed by the vertebral column 6. Organogenesis at about 4 weeks, the body organs begin to form in a process called organogenesis 7. Morphogenesis during the 2 nd month, the various parts of the body develop their adult shape by a process called morphogenesis By the end of the 2 nd month (8 weeks), the body plan of a human embryo is complete From this point on (8 weeks until birth), the developing human is referred to as a fetus By the end of the first trimester: the eyes are almost fully developed but the eyelids are still fused nose develops a bridge the external ears are present the limbs are fully formed and nails develop heartbeat can be detected the fetus begins to move, but it cannot be felt by the mother Events during the 2 nd trimester: Month 4 head is very large in proportion to body face takes on human features hair appears on the head Month 5 head is less disproportionate to rest of the body fine hair covers the body fetal movements commonly felt by the mother Month 6 eyelids separate and eyelashes form skin is wrinkled and pink substantial weight gain occurs alveoli begin to produce surfactant Events during the 3 rd trimester: Month 7 fetus assumes an upside-down position fetus is capable of surviving if delivered prematurely Month 8 testes descend into the scrotum bones of head are soft chances of survival much greater if premature birth does not occur until the end of the 8 th month Month 9 fine hair that covered body is shed nails extend to tips of fingers and maybe even beyond Labor and Birth Labor is the process by which the fetus is expelled from the uterus into the vagina and then outside the body Events of labor: 1. About 280 days after fertilization occurs, progesterone levels in the mothers bloodstream fall 2. Two hormones produced by the mother, estrogen and oxytocin, causes contractions of the uterus to begin Uterine contractions occur in waves that start at the top of the uterus and move downward true labor begins when uterine contractions occur at regular intervals, usually producing pain As the interval between contractions shortens, the contractions intensify, eventually rupturing the amniotic sac, releasing the amniotic fluid These contractions also cause the cervix to dilate (open up) birth occurs when the cervix has completely dilated and the contractions force the fetus through the birth canal (cervix and vagina) 3. Expulsion of the placenta About 10 minutes after the birth of the baby, the placenta (= afterbirth) is expelled by final contractions of the uterus The afterbirth is made up of the remains of the placenta and amnion If fetal or maternal distress prevents a vaginal birth, the baby may be delivered by cesarean section The baby is not completely developed at birth the brain continues to grow and develop until the age of 10 years this is why childhood diseases or accidents have the potential to cause permanent mental impairment Effect of Chemicals and Drugs on the Developing Embryo Any drug or chemical taken by the mother during pregnancy can affect her developing child because it can easily pass through the placenta 1. Thalidomide a sedative used in Europe in the 1960s as a remedy for morning sickness was found to cause deformities in the arms and legs (they were greatly reduced and looked like flippers) if it was taken by the mother during the time the limbs were forming (days 26 56) 2. Alcohol the number 1 cause of fetal defects The most common result of maternal alcohol intake is fetal alcohol syndrome (FAS) Symptoms of FAS include: slow growth before and after birth small head with narrow eye slits defective heart and other organs damage of the CNS behavioral problems such as hyperactivity and poor attention span 3. Cigarette Smoking causes low infant birth weight, higher fetal and infant mortality, ectopic pregnancy (embryo implants in fallopian tubes), cardiac defects and anencephaly (born without a cerebrum) 4. X-Rays exposure may cause mental retardation and skeletal deformities 5. Cocaine mental retardation, attention and orientation problems, crib death, premature birth, malformed or missing organs, strokes and seizures 6. Prescription Drugs pregnant women should always clear any drug (even OTC drugs) with her doctor before she takes them Genetic Abnormalities: 1. Down Syndrome child is born with an extra 21 st chromosome Characteristics of Down Syndrome include: Mental retardation,short height, stubby fingers,distinctive facial features,and malformation of the heart, ears, hands and feet an estimated 1 in 700 infants is born with Down Syndrome a womans chances of having a child with the disorder increase with age For example, the chance is 1 in 1205 for a 25 year old woman, but 1 in 365 for a 35 year old woman 2. Fragile X Syndrome occurs when a small portion of the tip of the X chromosome is susceptible to breakage it is the leading cause of mental retardation among newborns and affects more males than females Males with Fragile X Syndrome often have abnormal faces including long faces, large ears, and prominent jaw 3. Red/Green Color Blindness a sex linked trait in which individuals see reds and greens as shades of gray The gene for color vision is located on the X chromosome since females have 2 X- chromosomes, only females with 2 defective genes will be color blind since males have only 1 X- chromosome (the other is Y), they will be color blind if they inherit only 1 defective gene red/green color blindness is more common in males than females 4. Hemophilia a sex linked trait in which the blood fails to clot after an injury As with red/green color blindness, the gene for blood clotting is found on the X chromosome 5. Sickle Cell Anemia condition caused by a defective gene that produces an abnormal form of hemoglobin When this hemoglobin gives up O 2, it changes shape causing distortion of the RBCs this abnormally shaped RBCs tend to rupture easily and tend to get stuck in blood vessels, cutting off the blood supply to a part of the body