Zona pellucida

The zona pellucida or glass skin, formerly Zona striata, is a protective shell around the egg ( oocyte ) that of the cubic follicular ( granulosa ) of the Primärfollikels is formed. It is located between the cell membrane of the oocyte and the follicular epithelial cells, which are collectively referred to as the corona radiata.

Construction

The zona pellucida is a thick 17 microns in average glycoprotein matrix, consisting in humans of the zona pellucida proteins ZP1, ZP2, ZP3 and ZP4 (with mouse only ZP1 - ZP3 ). It is penetrated by finger-like projections ( microvilli ) of the follicular epithelial cells, which thus allow the diet of the egg. The proteins are synthesized from the oocyte, and discharged to the removal of a C-terminal amino acid moiety from the cell. The proteins ZP2, ZP3 and ZP4 then form a long filament chains, which are mainly linked by ZP1 ( "cross- link" ).

Function

Of crucial importance, the zona pellucida is in the actual fertilization. The initial (primary) binding of the sperm cell to the protein ZP3 initiates the acrosome reaction a: the present in the acrosome of the sperm hydrolyzing enzymes - mainly the protease acrosin - lead to a selective dissolution of the zona pellucida of what is now the secondary binding of sperm to the ZP2 protein and consequently enables the entry of sperm into the egg. The zona pellucida itself is penetrated by the sperm in a span of about 20 minutes.

At the first touch of the two germ cells (ie, the egg and sperm cells ) are simultaneously different reactions initiated: the proportion of the zona pellucida in these processes is the so-called reaction of the zona pellucida - the immediate change in their structure or their specific membrane receptors. On the other hand, are emptied from vesicles in the peripheral region of the cytoplasm of the egg cell proteases into the perivitelline space between the egg and the zona pellucida, which as cortical reaction or cortical degranulation - is called and causes the enzymatic cleavage of ZP2 and thus its inactivation. Both processes together ensure the so-called zona hardening in the moment in which more than 16 % of the existing ZP2 was inactivated, which no other sperm can enter the egg ( it does not come to polyspermy ). Furthermore, the modified Zona pellucida is the protection of the early embryo prior to implantation.

An important role in the regulation of the zona pellucida has also fetuin B; the protein inhibits this Ovastacin, which can harden even before the penetration of a sperm, the zona pellucida. Female mice without fetuin -B are - despite normal development and function of the ovaries - barren.

Pathology

For the first time an autosomal recessive mutation of a ZP- protein and the absence of zona pellucida has been associated with familial infertility in 2014 at a Chinese Han family. In this case, a homozygous frameshift mutation was found in six infertile family members, which leads to premature Strangabbbruch of ZP1 protein by a stop codon and a missing C -terminal protein end. The normal ZP1 protein comprises 638 amino acids in the synthesis, the precipitated in the matrix portion of amino acids 279-549 corresponds. By deletion of eight base pairs (nucleotides 1169-1176 ) are from amino acid 390, a non-homologous transcription, starting with threonine instead of isoleucine at point 390 and ending with a stop codon after position 404: abbreviated " I390fs404X ". By mutation of the full protein thus has only 404 amino acids, of which the last 15 amino acids having no homology to the normal protein more.