LG: What happens after fertilization?

From one cell to many A fertilized egg goes through an early stage of development called an embryo. EMBRYO

The value of sexual reproduction Sexual reproduction introduces genetic variation –genetic recombination during meiosis –crossing over mixing of alleles across homologous chromosomes –random fertilization which sperm fertilizes which egg? Driving evolution –Variation for evolution

Variation from random fertilization Sperm + Egg = ? –any 2 parents will produce a zygote with over 70 trillion (2 23 x 2 23 ) possible diploid combinations

Sexual reproduction creates variations Sexual reproduction allows us to maintain both genetic similarity & differences. Martin & Charlie Sheen, Emilio Estevez Michael & Kirk Douglas Baldwin brothers

Regents Question

Fertilization: Four Major Steps 1.Sperm contacts the egg 2.Sperm or its nucleus enters the egg 3.Egg becomes activated and developmental changes begin 4.Sperm and egg nuclei fuse

Fertilization

The Nuclei Fuse Together

What happens now? Development of the zygote The zygote undergoes a series of mitotic cell divisions called cleavage. The stages of development are: Fertilized ovum (zygote)  2-cell stage  4-cell stage  8-cell stage  Morula  Blastula  Early Gastrula  Late Gastrula

Twins Dizygotic or fraternal twins occur when two separate eggs are ovulated. May be of different sexes. Monozygotic or identical twins occur when a single egg is fertilized but dividing cells break into two groups and develop into two individuals. Genetically identical (clones)

Cleavage (divide via mitosis) forms the 2 cell stage

They split again to form the 4 cell stage

And again to form the 8 cell stage…

And eventually form a Morula Morula – solid ball of cells formed from cleavage

Next it becomes a blastula Blastula – hollow ball of cells formed from cleavage

And next, a gastrula Gastrula – a hollow ball of cells with an “in pushing” and 3 layers (germ layers)

The Regents Diagram… 1.Sperm and ovum (fertilization) 2.Zygote (fertilized ovum) 3.2-cell stage 4.4-cell stage 5.Morula 6.Blastula 7.Gastrula Cleavage: process of cell division (Mitosis) during development

Regents Question

Differentiation (Organogenesis) Organogenesis is the formation of the organs (Organo = organs, genesis = creation) Arises from the layering of cells that occurs during gastrula stage The layers are germ layers; they have specific fates in the developing embryo: –Endoderm The innermost layer  form the gut –Mesoderm In the middle  form the muscles, circulatory system, blood and many different organs –Ectoderm The outermost  form the skin and nervous system

Specialized cells perform specific functions. Cells develop into their mature forms through the process of cell [differentiation]. Cells differ because different combinations of genes are expressed. Outer: skin cells Middle: bone cells Inner: intestines

Late Gastrula Mesoderm Endoderm Ectoderm

Differentiation of Germ Layers EctodermMesoder m Endoder m Nervous system SkeletonDigestive tract Epidermis of skin MusclesRespirator y system Circulatory system Liver, pancreas GonadsBladder

Early Human Development Summary Meiosis makes sperm in males and ovum in females Sperm and ovum unite nuclei to form a zygote Zygote undergoes cleavage and becomes gastrula with 3 germ layers

Extraembryonic Membranes

Human Prenatal Development Gestation lasts 266 days from fertilization to birth Development begins in the oviduct –About 24 hours after fertilization, the zygote has divided to form a 2-celled embryo –The embryo passes down the oviduct by cilia and peristalsis –By the 5th day, the embryo enters the uterus –The embryo floats free for several days, nourished by fluids in the uterine wall At this point, it is called a blastocyst

Implantation The embryo implants in the wall of the uterus on about the 7th day of development

Implantation can also occur in uterine tube, cervix, or the abdominal cavity. Implantation anywhere outside the uterus is called an ectopic pregnancy. It is possible for fetus to grow in the abdominal cavity, but growth inside the uterine tube causes the tube to rupture, resulting in severe bleeding.

As early as days after fertilization, the blastocyst begins to secrete human chorionic gonadotropin or hCG. hCG keeps the corpus luteum active until the placenta can produce estrogens and progesterone. The presence of hCG is the basis for pregnancy tests.

Development of the Placenta

Functions of the placenta: 1 Transfer gasses 2 Transport nutrients 3 Excretion of wastes 4 Hormone production – temporary endocrine organ – estrogen and progesterone 5 Formation of a barrier – incomplete, nonselective – alcohol, steroids, narcotics, anesthetics, some antibiotics and some organisms can cross

UMBILICAL CORD Connecting link between fetus and placenta. Transports oxygen and nutrients to fetus from the placenta and returns waste products from the fetus to the placenta. Contains: 2 arteries and 1 vein Contains NO pain receptors.

Regents Question Identify the structure in the uterus where the exchange of material between the mother and the developing fetus takes place. _____________________________________

Regents Question

Placenta Food & gases diffuse across blood vessels

Gestation period Divided into three trimesters. During first trimester individual starts out as a zygote, then morula, blastocyst, and after implantation, is called an embryo. Embryonic phase of development lasts from fertilization until the 8 th week of gestation, when it becomes a fetus. By day 35 the heart is beating, and eye and limb buds are present.

TRIMESTERS Trimester 1st= week’s 1-13 (2.5-3 inches, 0.5-1oz.) 2nd= week’s 14 – 26 (11-14 inches, lbs.) 3rd= week’s 27 and on (38-40 WEEKS) (7-7.5 inches, lbs.)

Human fetal development 7 weeks4 weeks

Regents Question

Human Fetus at Ten Weeks

Human fetal development 10 weeks

Human fetal development 12 weeks20 weeks

Human fetal development The fetus just spends much of the 2 nd & 3 rd trimesters just growing …and doing various flip-turns & kicks inside amniotic fluid Week 20

Human fetal development 24 weeks (6 months; 2nd trimester) fetus is covered with fine, downy hair called lanugo. Its skin is protected by a waxy material called vernix

Human fetal development 30 weeks (7.5 months) umbilical cord

Getting crowded in there!! 32 weeks (8 months) The fetus sleeps 90-95% of the day & sometimes experiences REM sleep, an indication of dreaming

Birth positive feedback

Intestine Placenta Umbilical cord Wall of uterus Vagina Cervix Birth (36 weeks) Bladder

Labor (parturition) Stage one – the period from the onset of true labor contractions until the cervix is completely dilated at 10 cm. The uterine contractions cause the cervix to dilate, and the amniotic sac may rupture. Usually lasts 6 – 24 hours depending on the number of previous deliveries.

Stage 2 Period from maximal cervical dilation until the birth of the baby Lasts minutes to an hour Contractions become more intense and frequent.

Stage 3 The expulsion of the placenta Usually occurs within 15 minutes after the birth of the baby, but can range from 5 to 60 minutes.

Words You Must know Prophase Genetic material Chromosome Centromere Centrioles Nuclear membrane Metaphase Spindle fibers Homologous chromosomes Mitosis Meiosis Somatic cells Gametes Asexual reproduction Identical Variation Interphase

Anaphase Telophase/cytokinesis Sexual reproduction Cell division Sperm Egg Autosomes Sex chromosomes Diploid (2n) Haploid (n) Fertilization Zygote Crossing over Maternal Paternal Gametogenesis Polar bodies Replication Placenta Umbilical cord Differentiation Cleavage