Zona pellucida
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The zona pellucida or glass skin , formerly Zona striata called, is a protective shell around the egg ( oocyte ) in all vertebrates , for various glycoproteins is. It is formed by the egg cell and the surrounding cubic follicular epithelial cells ( granulosa cells ) of the primary follicle and lies between the cell membrane of the egg cell and the inner layer of granulosa cells, which is known as corona radiata .
construction
The zona pellucida is an extracellular matrix of glycoproteins , on average about 17 µm thick , in humans from the zona pellucida proteins ZP1, ZP2, ZP3 and ZP4 (in mice only ZP1, ZP2, ZP3). It envelops the growing egg cell and is penetrated by the adjacent granulosa cells of the follicular epithelium with cytoplasmic processes like fingers that are in direct contact with the egg cell ( gap junctions ). The glycoproteins synthesized by the egg cell are released from the cell (after a C-terminal amino acid has been separated off). The proteins ZP2, ZP3 and ZP4 can then form long filament chains, which are mainly linked by ZP1 to form a three-dimensional meshwork ( “cross link” ). The narrow space between the egg cell and the zona pellucida is called the perivitelline space .
function
The zona pellucida is of crucial importance for the actual fertilization . The initial (primary) binding of the sperm cell to the protein ZP3 initiates the acrosome reaction : The hydrolyzing enzymes located in the acrosome of the sperm - essentially the protease acrosin - lead to a selective dissolution of the zona pellucida , which now leads to the secondary binding of the sperm cell to the Protein ZP2 and consequently the entry of the sperm cell into the egg cell. The zona pellucida itself is penetrated by the sperm in a period of about 20 minutes.
When the two germ cells (i.e. the egg and sperm cells) come into contact for the first time, various reactions are initiated at the same time: The part of the zona pellucida in these processes consists in the so-called reaction of the zona pellucida - the immediate change in its structure or its specific membrane receptors. On the other hand, proteases are emptied from vesicles in the edge area of the cytoplasm of the egg cell into the perivitelline space between the egg cell and zona pellucida, which is referred to as a cortical reaction or cortical degranulation and which causes the enzymatic cleavage of ZP2 and thus its inactivation. Both processes together ensure the so-called zona hardening at the moment when more than 16% of the existing ZP2 has been inactivated, which means that no further sperm can enter the egg cell (polyspermia does not occur). The modified zona pellucida also serves to protect the early embryo before implantation.
Fetuin B also plays an important role in regulating the zona pellucida ; the protein inhibits ovastacin , which causes the zona pellucida to harden even before a sperm can penetrate. Female mice without Fetuin-B are - despite normal development and function of the ovaries - sterile.
pathology
For the first time in 2014, an autosomal recessive mutation of a ZP protein and a missing zona pellucida in a Chinese Han family were associated with familial infertility . A homozygous frameshift mutation was found in six infertile family members , which leads to a premature strand breakage of the ZP1 protein by a stop codon and to a missing C-terminal protein end. The normal ZP1 protein comprises 638 amino acids in the synthesis, the portion excreted in the matrix corresponding to amino acids 279 to 549. Deletion of eight base pairs ( nucleotides 1169 to 1176) results in non-homologous transcription from amino acid 390, beginning with threonine instead of isoleucine at position 390 and terminated by a stop codon after position 404: abbreviated "I390fs404X". Due to the mutation, the total protein comprises only 404 amino acids, of which the last 15 amino acids no longer show any homology to the normal protein.
See also
Web links
Individual evidence
- ↑ Fertility: important mechanism found. Report to DocCheck from April 19, 2013.
- ↑ Eileen Dietzel et al: Fetuin-B, a Liver-Derived Plasma Protein Is Essential for Fertilization. In: Developmental Cell. Vol. 25 (1), pp. 106-112, April 4, 2013, doi: 10.1016 / j.devcel.2013.03.001 .
- ↑ Hua-Lin Huang, Chao Lv, Ying-Chun Zhao, Wen Li, Xue-Mei He, Ping Li, Ai-Guo Sha, Xiao Tian, Christopher J. Papasian, Hong-Wen Deng, Guang-Xiu Lu, Hong- Mei Xiao: Mutant ZP1 in Familial Infertility. In: New England Journal of Medicine . 2014, Volume 370, Issue 13 of March 27, 2014, pp. 1220-1226; doi: 10.1056 / NEJMoa1308851