Transcript The Modification of a Germ Cell to Form a

Structure of the Gametes
Gilbert Ch. 7 pp. 175-180
Structure of the Gametes: Sperm
• Parts of mature sperm:
• Head
– Haploid nucleus
– Little cytoplasm
– Acrosome
• Neck/Midpiece
– Mitochondria
– Centriole
• Tail (or propulsion system)
– Some species - ameboid motion
– Most sperm are propelled by flagella
• Formed by microtubles
Highly
Specialized
Cell Type!
Gilbert - figure 19.19 - Sperm maturation
Figure 7.2(1) The Modification of a Germ Cell to Form a Mammalian Sperm
Figure 7.2(2) The Modification of a Germ Cell to Form a Mammalian
Sperm
Figure 7.2(3) The Modification of a Germ Cell to Form a Mammalian Sperm
Flagella structure
• Must allow sperm to travel long
distances, using plenty of energy
• Axoneme: motor portion
– Microtubules in a 9+2 configuration
• 2 central microtubules, 9 doublets
• Made up of the protein tubulin
• Dyenin molecules attach to microtubules and
provide motor activity by hydrolysis of ATP
• Allows filaments to slide and flagellum to bend
Sperm Capacitation
• Upon release, mammalian sperm are
able to move, but do not yet have the
capacity to bind an egg
• Must enter the female reproductive tract
to complete the last step of the
maturation process (Capacitation) and
acquire the ability to bind the egg
Structure of Gametes: The egg
• Ovum (mature egg) stores all material
for beginning of growth and
development
• Unlike sperm, the egg conserves and
acquires more cytoplasm as it matures
• Synthesizes and stores proteins (like
yolk) as reservoirs for the developing
embryo
• The components of the egg vary from
species to species
Structure of the gametes: The egg
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PARTS OF THE EGG:
Cytoplasm - many components
Haploid nucleus
Cell membrane
– will fuse with sperm plasma membrane
• Vitelline envelope
– Contains glycoproteins essential for species
specificity & sperm binding
• Zona pellucida (mammals) extra coating
made of Extracellular matrix
Structure of the Gametes: The egg
(cont’d)
• Cumulus (mammals): layer of cells that nurture the
egg
– Innermost layer is called Corona Radiata
• Cortex
– Beneath the cell membrane
– Gel-like cytoplasm - may help sperm entry into the cell
• Cortical granules
– Inside cortex
– Membrane bound vesicles (like the acrosome in sperm)
– Help prevent polyspermy
• Egg jelly (some species)
– Attract/activate sperm
Sea urchin egg at fertilization
Eggs are protected by elaborate envelopes
• Vitelline envelope: a glycoprotein layer covers the plasma membrane of
all eggs. This acts to protect the egg.
• Eggs that are deposited in water have a jelly-like coating that surrounds
the egg (frogs eggs)
• Eggs that are deposited on land have particularly elaborate envelopes.
The eggs of birds have a vitelline envelope, a fibrous layer, an
outer layer of albumin (egg white), and a shell composed of
calcium carbonate. The outer envelopes are synthesized in the
oviduct after the egg has been fertilized.
Egg Cytoplasm
• Proteins: energy, amino acids
• mRNA
– To provide early instructions for development
• Ribosomes and tRNA
– To aid in protein synthesis early in development
• Morphogenetic factors
– Molecules that effect differentiation of various cell
types (can be localized to specific areas of the cell)
• Protective Chemicals
– UV filters, DNA repair enzymes, antibodies (birds)
Egg maturation at the time of fertilization in various species
HUMANS
Hamster Eggs Before Fert.