Download document

© 2011 Pearson Education, Inc.
Sperm Structure and Function
1.
The head contains the nucleus and
the enzyme-filled acrosome.
– Acrosome enzymes allow the
sperm to penetrate the egg's
barriers.
2.
The neck encloses a centriole.
– The centriole will fuse with a
second centriole, contributed by
the egg, to form the
centrosome.
3.
The midpiece is packed with
mitochondria, which produce the
ATP necessary for movement.
4.
The tail has a flagellum that acts as
a propeller.
© 2011 Pearson Education, Inc.
Egg Structure and Function
• Egg cells are relatively large (nutrient storage) and nonmotile.
• Quantity of nutrients varies across species.
– The relatively small mammalian egg only has to supply
nutrients for early development, as embryos start to obtain
nutrition through the placenta shortly following fertilization.
– Egg-laying species produce larger eggs; the yolk of the egg is
the embryo’s sole source of nutrition prior to hatching.
© 2011 Pearson Education, Inc.
Other Egg Structures
• The eggs of many species contain
cytoplasmic determinants that control the
early events of development.
• Many eggs also contain cortical granules,
small enzyme-filled vesicles that are
activated during fertilization.
• The vitelline envelope, a fibrous, mat-like
sheet of glycoproteins, surrounds the egg.
– Mammals have an unusually thick
vitelline envelope called the zona
pellucida.
• Some species have a jelly layer, a thick,
gelatinous matrix around the vitelline
envelope for further protection.
© 2011 Pearson Education, Inc.
Fertilization
• Fertilization occurs when a haploid sperm and egg cells fuse,
forming a diploid zygote (a fertilized egg).
• Many conditions must be met before a zygote can form:
– Gametes must be in the same place at the same time.
– Gametes must recognize and bind to each other.
– Gametes must fuse together.
– Fusion must trigger the onset of development.
• Sea urchins are a model system for studying fertilization; they
produce large numbers of gametes and undergo external
fertilization.
© 2011 Pearson Education, Inc.
Fertilization in Sea Urchins
• When a sperm head contacts the jelly layer, enzymes from the
sperm’s acrosome digest through the egg's jelly layer and vitelline
envelope.
• The plasma membranes of sperm and egg fuse when the sperm
head contacts the surface of the egg cell.
• Fertilization is complete when the sperm nucleus, mitochondria,
and centriole enter the egg, and the sperm and egg nuclei fuse to
form the zygote nucleus.
© 2011 Pearson Education, Inc.
Gametes from the Same Species Recognize Each Other
• Bindin is a protein on the head of sea urchin sperm that binds to the
surface of sea urchin eggs.
• Bindin acts as a “key,” such that a sperm binds only to eggs of the
same species.
© 2011 Pearson Education, Inc.
Identifying the Egg Cell Receptor for Sperm
• By treating sea urchin egg cells with a protease (an enzyme that
cleaves peptide bonds), researchers isolated the fragment of the egg
cell receptor that binds to sperm.
• Fertilizin is a compound on the surface of sea urchin egg cells that
is involved in binding sperm. It acts as the “lock” to bindin’s “key.”
• Each species has its own version of fertilizin and bindin, making
the lock-and-key interaction both necessary for fertilization and
species specific.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Why Does Only One Sperm Enter the Egg?
• Animals employ different mechanisms to avoid polyspermy,
fertilization by more than one sperm.
• In sea urchins, fertilization stimulates the creation of a physical
barrier.
– After fertilization, a Ca2+-based signal is rapidly induced and
propagated throughout the egg, resulting in the formation of a
fertilization envelope, which keeps away additional sperm.
• In mammals, the cortical granules release enzymes that modify egg
cell receptors, preventing binding by additional sperm.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Cleavage
• Cleavage
– is the set of rapid cell divisions that take place in animal
zygotes immediately after fertilization.
– first step in embryogenesis, the process that makes a singlecelled zygote into a multicellular embryo.
– partitions the egg cytoplasm
• The cells created by cleavage divisions are called blastomeres.
• When cleavage is complete the embryo consists of a mass of
blastomere cells called a blastula (in mammals: blastocyst).
© 2011 Pearson Education, Inc.
What Role Do Cytoplasmic Determinants Play?
Cytoplasmic determinants are found in specific locations within
the egg cytoplasm, so they end up in specific populations of
blastomeres.
will result in differentiation of cells
© 2011 Pearson Education, Inc.
Cleavage in Mammals
• Cleavage occurs in the mammalian oviduct (fallopian tubes), which connects
the ovary, where the egg matures, to the uterus, where the embryo develops.
• Cleavage results in a blastocyst, a specialized blastula consisting of two
populations of cells:
– The external, thin-walled hollow trophoblast surrounds the inner cell mass
(ICM).
• After the blastocyst embeds in the uterine wall, a mixture of trophoblast and
maternal cells form the placenta, an organ which provides nourishment and
waste removal for the developing embryo.
• The ICM contains the cells that undergo gastrulation and develop into the
embryo.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Gastrulation
• During gastrulation, extensive and highly organized cell
movements radically rearrange the embryonic cells into a structure
called the gastrula.
• Gastrulation results in the formation of embryonic tissue layers. A
tissue is an integrated set of cells that function as a unit.
• Most early embryos have three primary tissue layers: ectoderm,
mesoderm, and endoderm.
• These embryonic tissues are called germ layers because they give
rise to adult tissues and organs.
© 2011 Pearson Education, Inc.
Gastrulation in Frog Embryos
• The frog blastula contains a fluid-filled space called the blastocoel.
• Gastrulation begins with the formation of an opening called a blastopore.
• Cells from the periphery move inward through the blastopore, forming a tubelike structure that will become the gut.
© 2011 Pearson Education, Inc.
Germ Layers
• Ectoderm forms the outer
covering of the adult body and
the nervous system.
• Mesoderm gives rise to muscle,
most internal organs, and
connective tissues such as bone
and cartilage.
• Endoderm produces the lining
of the digestive tract or gut,
along with some of the
associated organs.
© 2011 Pearson Education, Inc.
The Completion of Gastrulation
In addition to establishing the embryonic tissue layers,
gastrulation has another major outcome: The major body axes
become visible.
• At the end of gastrulation, the three embryonic tissues are
arranged in layers, the gut has formed, and the major body axes
have become visible.
© 2011 Pearson Education, Inc.
Organogenesis
• Organogenesis is the process of tissue and organ formation that
begins once gastrulation is complete and the embryonic germ layers
are in place.
• During organogenesis, cells proliferate and become differentiated,
meaning that they become a specialized cell type.
– Differentiated cells have a distinctive structure and function
because they express a distinctive suite of genes.
© 2011 Pearson Education, Inc.
Organizing the Mesoderm into Somites
• Early in organogenesis, the rod-like notochord appears in the
dorsal mesoderm.
– notochord characterizes chordates, which includes humans and
other vertebrates.
• The notochord functions as a key organizing element during
organogenesis.
– In many chordates, as organogenesis continues, the notochord
cells undergo apoptosis.
© 2011 Pearson Education, Inc.
Neural Tube Formation
• Signals from the notochord trigger reorganization of the dorsal
ectodermal cells, leading to neural tube formation.
• The neural tube is the precursor to the brain and spinal cord.
© 2011 Pearson Education, Inc.
Somite Formation
• Once the neural tube forms, the
mesodermal cells become
organized into blocks of tissues
called somites, which form on
both sides of the neural tube
down the length of the body.
© 2011 Pearson Education, Inc.
Somite Formation
• Once the neural tube forms, the
mesodermal cells become
organized into blocks of tissues
called somites, which form on
both sides of the neural tube
down the length of the body.
© 2011 Pearson Education, Inc.
Somite Maturation and Determination
• Somite cells form a variety of
structures, but are initially not
determined, meaning they can
become any of the somitederived elements of the body.
• As the somite matures, somite
cells become irreversibly
determined, and will
eventually differentiate into a
specific cell type based on
their location within the
somite.
© 2011 Pearson Education, Inc.
The Process of Determination
• In the process of determination, somite cells differentiate in response to
signals from nearby tissues.
• These signals diffuse away from cells in the notochord, the neural tube,
and nearby ectoderm and mesoderm to act on specific populations of
target cells in the somite.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.