Fertilisation

Question 1

Define fertilisation.

Answer 1

Fertilisation is process whereby the sperm and egg—collectively called gametes—fuse together to form a zygote, and begin the development of a new individual.

Question 2

What are the four major events of conception?

Answer 2

1. Contact and recognition between sperm and egg.
2. Regulation of sperm entry into the egg. Only one sperm nucleus can ultimately unite with the egg nucleus. This is usually accomplished by allowing only one sperm to enter the egg and actively inhibiting any others from entering.
3. Fusion of the genetic material of sperm and egg.
4. Activation of egg metabolism to start development.

Question 3

Outline the main structures of the sperm, and their function.

Answer 3

• Acrosomal vesicle: derived from the cell’s Golgi apparatus and contains enzymes that digest proteins and complex sugars.
• Cell membrane
• Nucleus
• Centriole
• Mitochondria: provide energy needed to move the flagellum.
• Axoneme: the major motor portion of the flagllum. A structure formed by microtubules emanating from the centriole at the base of the sperm nucleus. The core consists of two central microtubules surrounded by a row of 9 doublet microtubules.
• Tail
• End piece

Question 4

Outline the main strutures of the oocyte, and their function.

Answer 4

• Cell membrane: must be capable of fusing with the sperm cell membrane and regulate the flow of certain ions during fertilisation.
• Vitelline envelope: an extracellular matrix that forms a fibrous mat around the egg, and is involved in sperm-egg recognition. Contains several different glycoproteins that adhere to the cell membrane.
• Zona pellucida: a thick matrix and seperate envelope.
• Cumulus: layer of cells made up of ovarian follicular cells, outer layer of corona radiata.
• Corona radiata: innermost cumulus cells, immediately adjacent to the zona pellucida.
• Cortex: thin layer of gel-like cytoplasm lying immediately beneath the cell membrane.
• Cortical granules: memrane-bound, Golgi-derived structures in the cortex that contain proteolytic enzymes, mucopolysaccharides, adhesive glycoproteins, and hyalin protein. These help prevent polyspermy.

Question 5

List the components of the ovum cytoplasm and their function. [5]

Answer 5

1. Nutritive proteins: supply of energy and amino acids, accomplished through hoarding yolk protein.
2. Ribosomes and tRNA: needed to make early structural proteins and enzymes.
3. Messenger RNAs: for proteins in the early stages of development.
4. Morphogenic factors: molecules that direct differentiation of cells, including transcription factors and paracrine factors.
5. Protective chemicals: many eggs contain UV filters and DNA repair enzymes, some contain distasteful molecules and bird eggs contain antibodies.

Question 6

What are the five basic steps of sperm-egg interaction?

Answer 6

1. The chemoattraction of the sperm to the egg by soluble molecules secreted by the egg.
2. The excocytosis of the acrosomal vesicle of the sperm to release its enzymes.
3. The binding of the sperm to the extracellular matrix (vitelline envelope or zona pellucida) of the egg.
4. The passage of the sperm through the extracellular matrix.
5. Fusion of egg and sperm cell membranes.

Sometimes step 2 and 3 can be reversed, as is the case in mammalian fertilisation, and the sperm binds to the extracellular matrix before releasing the contents of the acrosome.

Question 7

Many marine organisms release their gametes into the environment. How can the sperm and egg meet in such a dilute concentration?

Answer 7

In many species, sperm are attracted towards eggs of their own species by chemotaxis- that is, following the chemical gradient secreted by the eggs, e.g. the sea urchin Arbacia puctulata secreted resact. Upon binding the chemical the sperm becomes activated, increasing cyclic GMP and Ca²⁺, activatig dyenin ATPase that stimulates flagellar movemet.

Thus resact can act as a guide in the general direction that the egg is in.

Question 8

How is polyspermy prevented? Outline this process.

Answer 8

The fast block to polyspermy (found in sea urchins in frogs, probably not most mammals) is achieved by changing the electric potential of the egg cell membrane from -70 mV to +20 mV; sperm cannot fuse to eggs with a positive resting potential.

This brief potential shift is not enough to block polyspermy permanently, and the bound sperm are not removed. The cortical granule reaction is a slower, mechanical block to polyspermy that becomes active around a minute after the first sperm-egg fusion. Upon sperm entry, the cortical granules fuse with the egg cell membrane and release their contents into vitelline envelope.

• Cortical granule serine protease: cleaves