CHAPTER 7: FERTILIZATION

Sperm cells and their characteristics

The acrosome (head of a sperm cell)  has enzymes used to digest and enter the outer layers of the egg.

Sperm cells eliminate most of their cytoplasm. Their function is to transmit their haploid nucleus (male pronucleus) to the egg. In many animals the sperm also transmits a centriole.

Sperm cells move with a flagellum that has an axoneme of tubulin and dynein. Dynein uses the energy of ATP that is produced by mitochondria at the base of the flagellum.

The LINC protein keeps together the acrosome, the nucleus and the flagellum.

Egg and its characteristics

The egg contains all the materials needed for early development and growth:

• Proteins serve as source of energy and amino acids

• Ribosomes and tRNA are necessary for the embryo to make its own proteins

• mRNA for making new proteins (repressed until after fertilization)

• Morphogenetic factors direct cell differentiation and are localized in the egg

• Protective chemicals (UV filters, molecules with bad taste, antibodies)

In many species, sperm entry happens before egg meiosis is completed. In most mammals (including humans), this happens at metaphase II.

The plasma membrane of the egg

fuses with the sperm and controls the flow of ion signals.

The ECM of the egg

   is needed for sperm-egg recognition

Inside the plasma membrane of the egg, its cortex

forms microvilli that contain actin microfilaments, and also contains cortical granules with digestive enzymes.

Many invertbrate eggs have an

external jelly layer, which attracts or activates sperm.

Chemoattraction of sperm

by soluble molecules secreted by the egg

Exocytosis of the acrsoome

to release its enzymes

Binding of sperm

to viteline envelope or zona pellucida

Passage of sperm through

viteline enevelop or zona pellucida

Fusion

of plasma membranes of sperm and egg.

Fertilization in mammals

The female reproductive tract regulates the transport and maturity of the gametes. Fertilization happens in the ampulla of the oviduct.

Translocation of the egg

• The fimbriae of the oviduct recognize the ECM of the cumulus cells and pick up the oocyte-cumulus complex, allowing it to enter the oviduct.

• Cilia and muscle contractions move this complex toward the ampulla.

Translocation of the sperm

• Sperm motility (activity of the flagellum) allows sperm cells to enter the uterus.

• Uterine muscle contractions move sperm to the oviduct, where the sperm cells slow down.

• Sperm cells swim against the flow (rheotaxis) of liquid that moves from the ampulla to the uterus.

During this trek the sperm cells mature.

Capacitation

Newly ejaculated sperm cells cannot fertilize the egg until they spend some time in the female reproductive tract, which changes the sperm cells.

The capacitation of the sperm cell is initiated by removal of cholesterol from its plasma membrane.

The new properties of capacitated sperm are:

• Ability to follow signals from the egg.

• Ability to undergo the acrosome reaction.

• Ability to fuse with the egg membrane.

Capacitation of sperm cells may need contact with ?

epithelial cells at the isthmus of the oviduct. This contact also extends the active life of the sperm cells.

At the end of capacitation, the sperm cell is?

hyperactivated (the flagellum beats faster), releasing the sperm cell from the epithelium of the oviduct.

Another directional cue for the sperm is ?

progesterone from the cumulus cells, which hyperactivates and attracts sperm.

Elevated calcium will next trigger the acrosome reaction.

Contact recognition

The acrosome reaction of the sperm happens when it enters the cumulus.

The mouse zona pellucida contains three glycoproteins (ZP1, ZP2, and ZP3). Humans have an additional glycoprotein called ZP4.

Sperm that have undergone the acrosome reaction bind to ZP2; unreacted sperm bind to ZP3, undergo the acrosome reaction, and then bind to ZP2.

The release of acrosome enzymes allows the sperm to digest a path through the zona pellucida and meet the egg membrane.

Gamete fusion

Fusion happens at the equatorial region of the sperm and depends on a sperm protein called Izumo.

Izumo binds to the oocyte protein Juno and together they recruit other oocyte proteins required for fusion.

Prevention of polyspermy

In sea urchins, the fast block changes the membrane potential from negative to positive after the first sperm enters, blocking the entry of a second sperm. Mammals do not have a fast block.

Sea urchins have a second mechanism called the slow block. Mammals also have a slow block:

• A cortical granule reaction is triggered in the egg by increased calcium, releasing digestive enzymes like ovostacin, which clips ZP2, blocking sperm binding.

• A “zinc ion spark” from the egg inhibits cumulus enzymes needed for sperm capacitation and acrosome enzymes.

• In addition, Juno is released from the egg, making the egg plasma membrane unable to bind a second sperm.

Fusion of the genetic material and activation of the egg

The DNA and the histones of both pronuclei are demethylated, removing most epigenetic markers.

The sperm releases phospholipase C-zeta (PLC_ZETA). This enzyme produces inositol triphosphate (IP₃), which triggers Ca²⁺ waves that induce cortical reaction, resumption of meiosis, mRNA activation and DNA synthesis