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39.4 – Fertilization and Development
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39–4 Fertilization and Development
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Fertilization
Sperm swim through the uterus into the Fallopian tubes.
The egg has a protective layer w/ binding sites for sperm.
Once attached, the sperm releases enzymes that break down the protective layer of the egg.
The sperm nucleus enters the egg, and chromosomes from the sperm and egg are brought together.
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39–4 Fertilization and Development
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Early Development
While still in the Fallopian tube, the zygote begins to undergo mitosis.
Four days after fertilization, the embryo is a solid ball of about 64 cells (a morula).
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Implantation
The morula grows and becomes a hollow structure with an inner cavity called a blastocyst which attaches to the uterine wall.
The embryo secretes enzymes that digest a path into it. Blastocyst cells specialize due to the activation of genes.
The inner cell mass (a cluster of cells) develops within the inner cavity of the blastocyst.
The embryo will develop from these cells, while the other cells will differentiate into tissues that surround the embryo.
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39–4 Fertilization and Development
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Fertilization
Fertilization and Implantation
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Gastrulation
The inner cell mass of the blastocyst gradually sorts itself into two layers, which then give rise to a third layer.
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39–4 Fertilization and Development
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Mesoderm
Amniotic cavity
Primitive streak
Ectoderm
Endoderm
Early Development
The third layer is produced by a process of cell migration known as gastrulation.
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Early Development
The result of gastrulation is the formation of three cell layers – the ectoderm, the mesoderm, and the endoderm.
Amniotic cavity
Primitive streak
Ectoderm
Endoderm
Mesoderm
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Early Development
The ectoderm develops into the skin and nervous system.
The endoderm forms the digestive lining and organs.
Mesoderm cells differentiate into internal tissues and organs.
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Early Development
Neurulation is the development of the nervous system.
Shortly after gastrulation is complete, a block of mesodermal tissue begins to differentiate into the notochord.
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Neural crest Neural fold
Notochord
Early Development
As the notochord develops, the neural groove changes shape, producing neural folds.
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Neural crest Neural tube
Ectoderm
Notochord
Early Development
Gradually, these folds move together to create a neural tube from which the spinal cord and the nervous system develop.
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Extraembryonic Membranes
As the embryo develops, membranes (amnion and chorion) form to protect and nourish the embryo.
Fluid-filled amniotic sac cushions and protects the developing embryo.
Uterus
Amnion
Fetus
Amniotic sac
Placenta
Umbilical cord
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Early Development
Fetal portion of placenta
Maternal portion of placenta
Maternal artery
Maternal vein
Umbilical vein
Umbilical arteries
Umbilical cord
Amnion
Chorionic villus
The chorionic villi and uterine lining form the placenta.
The placenta connects the mother and developing embryo.
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Early Development
After eight weeks, the embryo is called a fetus.
After three months, most major organs and tissues are formed. The umbilical cord also forms.
The umbilical cord connects the fetus to the placenta.
The inner cell mass contains embryonic stem cells, unspecialized cells that can differentiate into nearly any specialized cell type.
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Later Development
4–6 months after fertilization:
The heart can be heard with a stethoscope.
Bone replaces cartilage that forms the early skeleton.
During the last 3 months, fetus doubles mass, heart and lungs completely develop.
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Childbirth
The mother’s posterior pituitary gland releases the hormone oxytocin.
These muscles begin rhythmic contractions (labor).
The opening of the cervix expands until it is large enough for the head of the baby to pass through it.
The amniotic sac breaks.
Contractions force the baby out through the vagina.
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Childbirth
The baby now begins an independent existence.
Its systems quickly adapt to life outside the uterus, supplying its own oxygen, excreting waste on its own, and maintaining its own body temperature.
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Multiple Births
Multiple Births
If two eggs are released during the same cycle and fertilized by two different sperm, fraternal twins result.
A single zygote may split apart to produce two embryos, which are called identical twins.
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Early Years
Early Years
The first two years of life are called infancy. It is a period of rapid growth and development.
Childhood lasts from infancy until puberty.
Adolescence begins with puberty and ends with adulthood.
Puberty produces a growth spurt that will conclude in mid-adolescence.
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Adulthood
Adulthood
Development continues during adulthood.
Adults reach their highest levels of physical strength and development between the ages of 25 and 35.
Most people begin to show signs of aging in their 30s.
Around age 65, most body systems become less efficient, making homeostasis more difficult to maintain.
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