FERTILIZATION

= Sequence of events beginning with contact between a sperm and egg, and ending with the _____________________________ and combining of their chromosomes to form a ______________(= fertilized egg).

Movement of sperm through the female reproductive tract:

1.    Deposition of sperm into the vagina during coitus

 

2.    Movement of sperm from vagina into the cervical canal

 

3.    Movement of sperm from the cervical canal into the uterus

 

4.    Movement of sperm from the uterus into the oviducts

 

The process of fertilization:  Click here for diagrammatic overview

1.     Sperm passage through the __________________________ (loosely-packed outer layer of remaining granulosa cells)

·         An enzyme on the sperm head dissolves the cementing material found between the cells of the cumulus.

·         Tiny projections on the surface of the cells, called microvilli, attach to the sperm heads and begin to pull them through.

 

2.     Sperm passage through the _________________________ (tightly-packed inner layer of granulosa cells, attached to the zona pellucida). (See Figure 1)

·         Sperm heads release another enzyme that dissolves the material between the corona cells.

·         Sperm passage is also aided by vigorous beating of the tail

 

3.    Sperm passage through the _________________________ (translucent membrane between the oocyte and the granulosa cells of the follicle)

·         Surface receptors on the zona bind to specific proteins on the head of the sperm, initiating the ______________________ (an enzymatic process):

o        Holes form in the _____________________, which permit the release of several ________________ from the acrosomal space. The enzymes act to break down the proteins and carbohydrates in the zona pellucida, thereby creating a tunnel for the sperm to pass through. The acrosomal cap also dissolves. (See Figure 1 and Figure 2)
 Link to diagram  Link to animated diagram

·         By beating its tail vigorously and moving its head from side to side, the sperm is then able to pass through the zona (a mechanical process).

 

4.    Sperm attachment to the surface of the egg

·         The first sperm to pass through the zona attaches to specific __________________ on the surface of the egg membrane. (See Figure 3)

·         The _________________________ immediately follows, to prevent the entry of more than one sperm: Link to diagrams - be sure to scroll down the linked page

o        Cortical granules are released from the egg into the space between the egg membrane and zona pellucida. These granules contain enzymes that destroy the ____________________________ on the zona membrane surface, as well as inhibitors that block the action of acrosomal enzymes.

o        Then, while the successful sperm is firmly held down on the egg membrane, the egg produces a brief _________________ on its surface, followed by the formation of a hard outer _________________. This pushes all the other sperm away and prevents _______________ (fertilization by more than one sperm).

 

5.    Passage of sperm into egg

·         An opening in the egg membrane appears, and the head and neck (and sometimes tail) of the sperm enter the egg. (See Figure 1 and Figure 4)

·         This causes a rise in intracellular calcium levels and subsequent _______________ of the egg nucleus so that it completes __________________, yielding an ootid and a second polar body.

 

6.    Fusion of sperm and egg nuclei to form a zygote

·         Occurs about ___________ after sperm enters egg.

·         Formation of the zygote marks the completion of fertilization. (See Figure 5) Click for photo

 

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