what happens after fertilization? a sperm enters an ovum, and the nuclei combine to form one with 46...
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What happens after fertilization?
A sperm enters an
ovum, and the nuclei
combine to form one
with 46 chromosomes.
More free powerpoints at www.worldofteaching.com
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Fertilization: Four Major Steps
1. Sperm contacts the egg
2. Sperm or its nucleus enters the egg
3. Egg becomes activated and developmentalchanges begin
4. Sperm and egg nuclei fuse
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Words to know…Fuse- to physically join together Ovum – egg cell (female gamete)Cleavage – process of cell division
during developmentDifferentiation – the process of
forming different kinds of cells from similar cells of the early embryo
Embryo – an organism in an early stage of development
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Words to Know continued
Morula – solid ball of cells formed from cleavage
Blastula – hollow ball of cells formed from cleavage
Gastrula – a hollow ball of cells with an “in pushing” and 3 layers (germ layers)
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Fertilization
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The Nuclei Fuse Together
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What happens now?Development of the zygote, the
study of which is known as embryology or developmental biology.
The zygote undergoes a series of mitotic cell divisions called cleavage.
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Stages of Development
The stages of development are: Fertilized ovum (zygote) 2-cell stage 4-cell stage 8-cell stage Morula Blastula Early Gastrula Late Gastrula
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Cleavage (divide via mitosis) forms the 2 cell stage
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They cleave again to form the 4 cell stage
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And again to form the 8 cell stage…
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And eventually form a Morula
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Next it becomes a blastula
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And next, a gastrula
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1. Sperm and ovum
2. Zygote (fertilized ovum)
3. 2-cell stage
4. 4-cell stage
5. Morula
6. Blastula
7. Gastrula
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What Happens After Cleavage Begins?
Organogenesis is the formation of the organs. Organo = organs
genesis = creationArises from the layering of cells
that occurs during gastrula stage
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Differentiation/OrganogenesisThe layers are germ layers; they have
specific fates in the developing embryo: Endoderm
The innermost layerGoes on to form the gut
MesodermIn the middleGoes on to form the muscles,
circulatory system, blood and many different organs
Ectoderm The outermostGoes on to form the skin and nervous
system
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Late Gastrula
Mesoderm
Endoderm
Ectoderm
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Differentiation of Primary Germ Layers (from the gastrula)
Ectoderm Mesoderm Endoderm
Nervous system
Skeleton Digestive tract
Epidermis of skin
Muscles Respiratory system
Circulatory system
Liver, pancreas
Gonads Bladder
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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
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Embryonic Membranes Continued
The yolk sac encloses the yolk in vertebrates with yolk-rich eggs
The allantois is an outgrowth of the gutIn humans, there is no yolk sac,
but the yolk aids in formation of red blood cell
In reptiles and birds, it stores nitrogenous wastes
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Human Prenatal Development
Gestation lasts 266 days from fertilization to birth
Development begins in the oviductAbout 24 hours after fertilization, the
zygote has divided to form a 2-celled embryo
The embryo passes down the oviduct by cilia and peristalsis
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Human Prenatal Development
The zona pellucida has dissolved by the 5th day, when the embryo enters the uterus
The embryo floats free for several days, nourished by fluids from glands in the uterine wallAt this point, it is called a blastocyst
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Fertilization of a Human Egg
(1) The sperm release enzymes that help disperse the corona radiata and bind to the zona pellucida.
(2) The outer sperm head layer is sloughed off, exposing enzymes that digest a path through the zona pellucida.
(3) The sperm fuses with the egg cell membrane, causing the zona pellucida to become impenetrable to other sperm.
4) The tail separates from the sperm head, and the male pronucleus enlarges and travels to the female pronucleus in the center of the cell. Chromosomes merge to form a
fertilized egg.
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Implantation The embryo implants in the wall of the uterus on about the 7th
day of development
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12-day Human Embryo
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Where does this all take place?
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The PlacentaThe placenta is the site of nutrient,
gas, and waste exchangeIt secretes hormones that maintain
pregnancy
Trophoblast cells release human chorionic gonadotropin (hCG) which signals the corpus luteum to enlarge and produce progesterone
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The Placenta
The placenta develops from the embryonic chorion and maternal uterine tissue
Chorionic villi are formed from the chorion, and project into the endometrium of the uterus
The umbilical cord, containing two umbilical arteries and one umbilical vein connects the embryo and the placenta
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Development of the Placenta
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Human Fetus at
Ten Weeks