Chemistry Reference
In-Depth Information
24
Glycobiology of Fertilization and Early Embryonic Development
Felix A. Habermann and Fred Sinowatz
The preceding chapters have provided detailed information on the glycan struc-
tures of cellular glycoconjugates as well as on the presence and biochemical nature
of glycan receptors (lectins). Biomedical aspects were already covered by dealing
with diseases caused by defects in glycan synthesis/degradation or lectin expres-
sion (for example, Chapter 11 on proteoglycans or Chapter 19 on human lectins).
The emerging connection between glycosylation and disease was further described
in Chapters 22 and 23. This chapter opens the fi nal section of the topic, where
in
vivo
aspects of the sugar code are presented. Naturally, fertilization is the starting
point, thus warranting intense investigation of this process and ensuing develop-
ment. We will start with a bird's eye view on mammalian fertilization and the
description of the major components defi ning the glycophenotype of oocytes.
24.1
Primer to Mammalian Fertilization
In mammals, oocytes are fertilized in the ampulla of the oviduct. At ovulation, the
oocyte is surrounded by the zona pellucida (ZP ) - a transparent, porous and visco-
elastic extracellular glycoprotein matrix (please see below). The ZP itself is enclosed
by several layers of cells of the cumulus oophorus of the ovarian follicle (Figure
24.1a) [1]. These are embedded in an extracellular matrix primarily composed of
hyaluronic acid (for further information on glycosaminoglycans, please see Chapter
11). The oocyte completes the fi rst meiotic division - the pairs of homologous
chromosomes are separated. A haploid (single) set of chromosomes, each still
comprised of two identical sister chromatids, is retained in the oocyte, while the
other one is discarded by extrusion of a small cell, the fi rst polar body. Subse-
quently, the oocyte is arrested at the metaphase stage of the second meiotic divi-
sion, ready to fuse with a sperm (Figure 24.1a).
In vivo
, mammalian sperm acquire
competence to fertilize after ejaculation. In the female reproductive tract, a fi nal
maturation process is initiated, termed capacitation. It denotes a series of changes
on the cell surface and in the cell, which enable the sperm to fertilize the
oocyte.
