Chemistry Reference
In-Depth Information
Figure 24.1
Fertilization in mammals. (a) Sche-
matic diagram of the stages of sperm-oocyte
interaction: (1) binding of a sperm to the ZP,
(2) acrosome reaction, (3) transit of the sperm
through the ZP, (4) sperm-oocyte fusion, (5)
sperm entry and oocyte activation, (6) cortical
reaction, and (7) zona reaction blocking poly-
spermy; (I) cells of the cumulus oophorus from
the ovarian follicle, (II) ZP, (III) mature fertiliz-
able oocyte arrested at the metaphase of the
second meiotic division, (IV) perivitelline space
and (V) fi rst polar body. The image panel on
the right side illustrates how the structure of
the ZP and the presence of glycan epitopes can
be visualized by staining with plant lectins and
confocal laser scanning microscopy. (b) Optical
section through the center of a canine ZP,
which is intensely stained with wheat germ
agglutinin (WGA; for lectin acronyms and sugar
specifi city, please see Table 18.1). In compari-
son, the extracellular matrix between the
cumulus cells surrounding the ZP is weakly
stained. Please note that the oocyte and
cumulus cells themselves are not visible. Scale
bar = 100
m. (c) The surface of a bovine ZP
stained with
Sambucus nigra
agglutinin ( SNA ).
The maximum intensity projection was com-
puted from a stack of 60 serial optical sections.
The cumulus cells have been mechanically
removed prior to staining. The insert shows the
lattice-like structure and the pores at higher
magnifi cation.
μ
Scale
bars = 100
and
10
m
μ
(insert).
What happens next is that the sperm have to pass through the layers of cumulus
cells and the ZP. Towards this end, sperm are equipped with appropriate bio-
chemical means. In detail, they have a cap-like secretory vesicle at the tip of their
head, called the acrosome. This organelle is fi lled with inactive hydrolases (proen-
zymes) such as proacrosin (for information on acrosin, a protease with a lectin
site to target enzymatic activity, please see Table 19.2) and hyaluronidase. The
content of the acrosome is released by the so-called acrosome reaction only when
the sperm head specifi cally recognizes and binds to the ZP. Acrosomal proen-
zymes are now activated and enable the sperm to cut its way through the ZP to
the oocyte. Necessarily, specifi c contact formation between sperm and the ZP
depends on structural complementarity and, in particular, on sugar- encoded infor-
mation. After entering the perivitelline space beneath the ZP, a sperm eventually
succeeds in binding to and fusing with the oocyte plasma membrane (Figure 24.1 a).
