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

• 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.