Fertilization In Sea Urchin (CSIR NET Developmental Biology)

Fertilization In Sea Urchin

- Malay Nayak

1.      What is sea urchin?

·         Spiny water animal

·         Phylum ECHINODERMATA

·         Roughly spherical sea animal

Sea Urchin

 

2. Why Sea Urchin ?

The cell-surface events that take place during fertilization have been studied most extensively in sea urchins, members of the phylum Echinodermata .

·  Sea urchin gametes are easy to collect,

·  fertilization occurs outside the animal body.

·  As a result, researchers can observe fertilization and subsequent development simply by combining eggs and sperm in seawater in the laboratory.

Sea Urchin

FERTILIZATION

Source: Campbell Biology

1 Sperm of Sea Urchin

•  The sperm head consists of a haploid nucleus and an acrosome.  
• The acrosome is derived from the Golgi apparatus and contains enzymes needed to digest extracellular coats surrounding the egg.

•  The midpiece of the sperm contain mitochondria and the centriole that generates the microtubules of the flagellum.

• Energy for flagellar motion comes from mitochondrial ATP and a dynein ATPase in the flagellum.

2. Egg of Sea Urchin

•        The female gamete is an egg with a haploid nucleus.
•          The egg has a large mass of cytoplasm storing ribosomes and nutritive proteins.
•         Some mRNAs and proteins that will be used as morphogenetic factors are also stored in the egg.
•         Surrounding the egg cell membrane is an extracellular layer often used in sperm recognition. It is called vitelline envelope.
•         A Jelly Coat is present outside of the egg.

Structure of the sea urchin egg at fertilization. 

(source: Gilbert Developmental Biology

3. Steps of Fertilization

• Sperm Attraction:

 sperm are attracted toward eggs of their species by chemotaxis—that is, by following a gradient of a chemical secreted by the egg.

 once sperm are spawned into seawater, their pH is elevated to about 7.6, resulting in the activation of the dynein ATPase. The splitting of ATP provides the energy for the flagella to wave, and the sperm begin swimming vigorously.

 The ability to move does not provide the sperm with a direction. In echinoderms, direction is provided by small chemotactic peptides called sperm-activating peptides (SAPs). One such SAP is resact, a 14-amino acid peptide that has been isolated from the egg jelly of the sea urchin Arbacia punctulata

source: Gilbert Developmental Biology

 

 

• The acrosome reaction

A second interaction between sperm and egg jelly results in the acrosome reaction.

The acrosome reaction in sea urchins is initiated by contact of the sperm with the egg jelly. Contact causes the exocytosis of the sperm’s acrosomal vesicle. The proteolytic enzymes and proteasomes (protein-digesting complexes) thus released digest a path through the jelly coat to the egg cell surface.

source: Gilbert Developmental Biology

species-specific binding event must occur once the sperm has penetrated the egg jelly and its acrosomal process contacts the surface of the egg . The acrosomal protein mediating this recognition in sea urchins is an insoluble, 30,500-Da protein called bindin.

Biochemical studies have confirmed that the bindins of closely related sea urchin species have different protein sequences. This finding implies the existence of species-specific bindin receptors (ERB1) on the egg vitelline envelope.

These bindin receptors are thought to be aggregated into complexes on the vitelline envelope, and hundreds of such complexes may be needed to tether the sperm to the egg. The receptor for sperm bindin on the egg vitelline envelope appears to recognize the protein portion of bindin on the acrosome  in a species-specific manner.

• Fusion of the egg and sperm cell membranes

Once the sperm has traveled to the egg and undergone the acrosome reaction, the fusion of the sperm cell membrane with the egg cell membrane can begin . Sperm-egg fusion appears to cause the polymerization of actin in the egg to form a fertilization cone.

Fusion is an active process, often mediated by specific “fusogenic” proteins. In sea urchins, bindin plays a second role as a fusogenic protein

Sodium ions diffuse into the egg and cause depolarization, a decrease in the membrane potential, the charge difference across the plasma membrane . The depolarization occurs within about 1–3 seconds after a sperm binds to an egg. By preventing additional sperm from fusing with the egg’s plasma membrane, this depolarization acts as the fast block to polyspermy.

•  The Cortical Reaction

Although membrane depolarization in sea urchins lasts for only a minute or so, there is a longer-lasting change that prevents polyspermy. This slow block to polyspermy is established by vesicles in the outer rim, or cortex, of the cytoplasm. Within seconds after a sperm binds to the egg, these vesicles, called cortical granules, fuse with the egg plasma membrane. Contents of the cortical granules are released into the space between the plasma membrane and the surrounding vitelline layer, a structure formed by the egg’s extracellular matrix. Enzymes and other granule contents then trigger a cortical reaction, which lifts the vitelline layer away from the egg and hardens the layer into a protective fertilization envelope.

Formation of the fertilization envelope requires a high concentration of calcium ions (Ca2+) in the egg.

• Egg Activation

Fertilization initiates and speeds up metabolic reactions that bring about the onset of embryonic development, “activating” the egg. There is, for example, a marked increase in the rates of cellular respiration and protein synthesis in the egg following the entry of the sperm nucleus. Soon thereafter, the egg and sperm nuclei fully fuse, and cycles of DNA synthesis and cell division begin.

What triggers activation of the egg?

source: Gilbert Developmental Biology

 A major clue came from experiments demonstrating that the unfertilized eggs of sea urchins and many other species can be activated by an injection of Ca2+. Based on this discovery, researchers concluded that the rise in Ca2+ concentration that causes the cortical reaction also causes egg activation. Further experiments revealed that artificial activation is possible even if the nucleus has been removed from the egg. This further finding indicates that the proteins and mRNAs required for activation are already present in the cytoplasm of the unfertilized egg. Not until about 20 minutes after the sperm nucleus enters the sea urchin egg do the sperm and egg nuclei fuse. DNA synthesis then begins. The first cell division, which occurs after about 90 minutes, marks the end of the fertilization stage.

Reference: Developmental Biology by Gilbert

                  Campbell Biology                

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