Human Development & BirthChapter 18

Fertilization

• The uniting of the sperm & egg.

• Results in formation of a zygote.

Sperm Anatomy

• Three parts • Head: contains the

nucleus with the father’s DNA.

• Midpiece: Contains energy producing mitochondria.

• Tail: A flagellum that allows the sperm to swim toward the egg.

Egg Anatomy

• Plasma membrane surrounded by zona pellucida & corona radiata which nourish the egg.

• Zygote receives cytoplasm & organelles only from mother.

Fertilization Steps

• Only one sperm enters the egg.

• Acrosome releases enzymes that aid the sperm’s entrance into the egg.

• The nuclei of the sperm and egg fuse.

• Changes in the egg’s membrane and zona pellucida prevent more sperm from entering the egg.

Processes of Development• Cleavage: Mitotic divisions

of the zygote that do not result in an increase in size.

• Growth: Mitotic divisions accompanied by increase in size.

• Morphogenesis: Migration of cells that results in changing shape of embryo.

• Differentiation: The taking on of a specific structure and function of cells. Nervous system is first to differentiate.

Primitive brainNeural tube

Extra embryonic Membranes

• Not part of embryo.• Chorion: fetal half of

placenta.• Placenta: organ that

provides fetus with nourishment and oxygen & takes away waste.

• Yolk sac: first site of blood formation.

• Allantois: becomes umbilical blood vessels.

• Amnion: contains cushioning fluid.

Development

• Events that occur from fertilization to birth.

• Gestation calculated by adding 280 days from the start of the last menstruation.

Pre-embryonic Development-1st Week

• Zygote: divides into a compact ball of embryonic cells called morula.

• Trophoblast: a layer of cells that surrounds an inner mass of cells—becomes the chorion.

• Cells of the blastocyst and morula have the capability of becoming any tissue.

Twinning

• Identical twins: Result when either the cells of the morula separate or the inner cell mass splits. Identical chromosomes.

• Fraternal twins: Two different eggs fertilized by two different sperm.

Cloning• Therapeutic cloning:

Experimental procedure where cells of embryo are separated and treated to become particular tissues. These are called stem cells.

• To try a simple, interactive cloning activity see: http://www.science.org.au/nova/043/043act.htm

Pre-embryonic Development-2nd Week

• Embryo begins to implant itself in the uterus.

• If embryo implants itself in a location other than the uterus an ectopic pregnancy results. These are not successful pregnancies.

• Embryonic disk, yolk sac, & amnionic cavity form.

2nd week Continued• Primary germ layers

form:• Ectoderm (outer layer)

becomes epidermis, nervous system, and epithelial lining of digestive system.

• Endoderm (inner layer) becomes organ linings of other organs.

• Mesoderm (middle layer) becomes skeletal, muscular system.

Third Week

• Nervous system and heart begin to develop.

Fourth Week

• At 4 weeks body stalk connects tail with chorion.

• Chorionic villi are treelike projection from chorion.

• Umbilical blood vessels appear; umbilical cord fully formed

Fifth Week

• Limb buds appear.• Head enlarges.• Eyes, ears, and

nose can be seen.

6th through 8th Weeks

• Easily recognized as human.

• Neck region develops.

• Reflex actions present.

• All organ systems established.

Placenta

• Two portions—fetal portion (chorionic) and maternal portion (uterine tissue).

• Surface covered with chorionic villi.

• Exchange of materials takes place across placental membrane.

• Maternal & fetal blood rarely mix.• Functions as the digestive

system, lungs, and kidneys of the fetus.

3rd & 4th Month• Head growth slows

down; body increases in length.

• Fingernails, nipples, eyelashes, eyebrows, & hair on head appear.

• Cartilage begins to be replaced by bone.

• Sexual differentiation begins.

• Fetal heartbeat can be heard during the fourth month.

5th-7th Month

• Mother can feel movement.

• Skin covered by a fine down called lanugo.

• Coated with white, cheese like substance called vernix caseosa (protects the skin).

8th-9th Month

• Fetus usually rotates so head is pointed toward cervix.

• Weight gain largely due to accumulation of fat beneath skin.

Premature Birth

• Difficulties associated with premature birth:

• Respiratory distress syndrome

• Retinopathy• Intracranial

hemorrhage• Jaundice• Infection• Circulatory disorders

Development of Sex Organs

• Gonads begin developing during the seventh week of development.

• Development depends on the action of hormones.

• Testosterone stimulates ducts to become male genital ducts.

• No Y chromosome-no testosterone-ovaries develop instead of testes.

• Near the end of development testes descend into scrotum.

Preventable Birth Defects• German measles (Rubella):

blindness, deafness, mental retardation, heart malformations & other problems-can be prevented by vaccination.

• Spina bifida (spinal cord or spinal fluid bulge) Most can be avoided by adequate folate intake.

• Anencephaly (absence of a brain) Most can be avoided by adequate folate intake.

Anencephaly

Preventable Birth Defects Continued…

• Fetal alcohol syndrome-decreased birth rate, height, head size, malformation of head & face. Mental retardation is common.

Birth

• True labor marked by uterine contractions that occur regularly every 15-20 minutes and last 40 seconds or longer.

• Expulsion of mucous plug from uterus.

Parturition: Stage 1

• Cervical canal pulled upward toward the baby’s head.

• Amniotic membrane ruptures if it has not already done so.

• Cervix dilates completely.

Parturition: Stage 2

• Contractions occur every 1-2 minutes and last about 1 minute.

• Mother feels the need to push.

• Baby’s head descends into vagina.

• Ends with the delivery of the baby and the cutting and tying of the umbilical cord.

Parturition: Stage 3

• Placenta becomes dislodged from the uterus and expelled from the vagina.

• Expulsion of placenta marks the end of parturition.

Effects of Pregnancy on the Mother

• Loss of appetite, nausea, vomiting, and fatigue early.

• Weight gain due to breast & uterine enlargement.

• Enlarged uterus compresses the ureters & bladder.

• Sometimes pregnancy induced diabetes can occur.

• Stretch marks appear due to increased steroid levels.

• Darkening in some areas of the skin is common.

Credits:

• Slide 1: http://embryology.med.unsw.edu.au/• Slide 4:

http://www.mercksource.com/ppdocs/us/common/dorlands/dorland/images/fig_o_0024.jpg

• Slide 4: cgee.hamline.edu/.../ dp_cycles_micro_dex.htm • Slide 5: http://www.lsc.org/online_science/prenatal/development.html• Slide 6:

http://embryology.med.unsw.edu.au/wwwhuman/Stages/Stage10.htm• Slide 7: http://www.sunyniagara.cc.ny.us/val/embryonic-mem.jpg• Slide 16: http://www.lsc.org/online_science/prenatal/development.html• Slide 17: http://www.lsc.org/online_science/prenatal/development.htm• Slide 18: http://www.ricktraceyinc.com/placenta2.jpg• Slide 22: http://preemie.info/cms/modules/mylinks/viewcat.php?cid=8• Slide 25: medlib.med.utah.edu/ WebPath/PEDHTML/PED029.html