This term was put forward by A. Engler and K. Planter in 1897 to be related tosomatic cellIs different.
Somatic cells will eventually die, and only germ cells have the chance to survive to the next generation.Organisms mainly rely on germ cells to continue andReproduction。extendednatural selectionThe structure of each kind of organism provides the best conditions for the survival of its germ cells.
germ cellIt can be divided into spores andgameteTwo types:
Spore
It is an uncooperative germ cell, usually asexualMeiosisormitosisArising from protozoaSporidaeAnd plants.
gamete
It is a germ cell, also called sex cell, which can develop only after being mated to a zygote, and is produced by meiosis or mitosis.Gamete producing cellGametocyte。This is undifferentiatedPrimordial germ cell, which can be differentiated intospermAnd egg cells.
Schematic diagram of primordial germ cell production
There are already signs of germ cell differentiation in unicellular populations.asVolvoxFour smaller cells of Haematococcus sp. of the family Haematococcus lose the ability to divide and specialize in movement and metabolism, called vegetative individuals, and the other 28 cells have the ability to divide, called reproductive individuals;Volvox is found in most smallVegetative cellA few large germ cells appeared between.
Polar substance, which is rich inRibonucleic acidThe small particles of RNA are called polar particles.afterbe fertilized、cleavageThe cells containing polar particles are called polar cells, which are the primordial germ cells of Drosophila melanogaster.If the polar substance at the back is injected into the front of the egg, the cells that are scheduled to develop into somatic cells can develop into germ cells.
Types of germ cells
A similar situation can be seen in paraascaris equi eggs.There is evidence that amphibian germ cells may be determined in the same way.
But we still don't know that in mammalsembryoWhat factors determine the development of certain cells into germ cells.It is only known that after the germ cell is determined, it needs to migrate to the gonad and differentiate there.
produce
Insect germ cell
aboutShiwangandXu NaidengFor example, cells are mainly a structural component, but in the 19th century, other scholars have emphasized the physiological functions of cells, especially the developmental and nutritional functions.With the deepening understanding of cells and their components (especially nuclei), the meaning of the concept of "cell theory" has gradually changed.Schneiden's theory directly promoted the differentiationplant cellA very active research work.
In 1852, Remak (1815 – 1865) pointed out that the frog egg is a cell, and new cells in the developing frog embryo are generated by the division of pre-existing cells.He emphatically refuted the theory of free cell formation.On this point, he was supported by Weierke (1855), who pointed out that from the perspective of many normal and pathological animal and human tissues, each cell is divided from the pre-existing cell.He asserted: "As a general principle, the development of any kind is not from the beginning, so (we must) deny the theory of (natural) genesis, whether in the development history of individual parts or in the development history of the whole organism" (Virchow, 1858:54).Kolliker and some other botanists also reached the same conclusion at the same time. However, due to the authority of Xu Naideng, this conclusion was delayed to be generally accepted in the botanical community.DarwinIn 1868 (II: 370), it was still uncertain whether free cell formation was possible.Later, the motto of Weierke "cells come from (pre-existing) cells" was finally accepted by everyone, but at that time, the details of cell (especially nuclear) division process were not clear (see below“mitosis”）。
With this new interpretation of cells, the time is ripe to reconsider the fertilization process.
If all parts of the body are composed of cells, then the gonad（ovaryandtestis）Whether it is the same, what is the "seed substance" of male and female, whether it is also composed of cells, and what is the difference between female and male sex cellsThis kind of questions that have been conceived for a long time were certainly not raised at the beginning, but they must be the logical result of the cell theory.
Later, it became increasingly clear that only when the role of cells in the process of fertilization was clarified, could a truly viable genetic theory be established.It is in these decades that the concept of germ cell has come to the fore.
combination
Sperm capacitation
Half of the sperm containsY chromosome(23, Y), half containingX chromosome(23, X). Although the ejaculated sperm has the ability to move, it has no ability to pass through the radiation crown and the zona pellucida around the egg.This is because there is a glycoprotein on the surface of the sperm head that can prevent the release of acrosin.In the process of sperm running in the uterus and fallopian tube, the glycoprotein is degraded by enzymes in the secretion of female genital duct, thus obtaining the fertilization ability. This phenomenon is calledEnable（capacitation）。The fertilization ability of sperm in the female genital canal can generally last for 1 day.
Maturation of eggs
The egg discharged from the ovary is in the second maturation divisionmetaphase, withFimbria oviductLiquid flow enteringFallopian tube, onbe fertilizedThe second mature division was completed only when.If unfertilizedovulationAfter 12 to 24 hours, it degenerates.
be fertilized
The essence of animal and plant fertilization is the mutual fusion of the germ cells (gametes) of the male parent and the female parent. These two gametes have made the same contribution to the formation of new zygotes, and the key process is the mutual fusion of the nuclei of the two gametes. These views have been gradually established by about 1844 and are generally accepted by relevant scholars.People's attention began to shift to the nucleus.
Is the nucleus just a mass of amorphous embryo like material, as the epigenetic theorists imagine, and perhaps it only stimulates the development process of the egg cell when it is fused.Or even though the nucleus is small, it has a tight structure. The microscopic structure invisible to the naked eye is the key to the very precise and specific development process after fertilization.If the nucleus is only regarded as the initiator of cell development and cell division, it will be considered that it will be dissolved after completing this task and will be re formed before new cell division or at least before gamete formation.
Since the cytologists in the second half of the 19th century received education and training from physiologists or embryologists, their focus was on development, so they did not need to care about the continuity of the nucleus.They seldom go beyond the genetic problem of how the traits are passed from parents to offspring.
implantation
Schematic diagram of ovulation, fertilization, cleavage and implantation
The embryo is gradually embeddedendometriumProcess scale ofEmbryo implantation(implantation), also known asimplantation（imbed）。Implantation is a special reproductive activity of mammals.Implantation starts about 5 – 6 days after fertilization and is completed on 11 – 12 days.
Studies have shown that blastocysts produceLamininAnd the receptor protein on the endometrium promote the blastocyst to adhere to the endometrium, and the blastocyst and endometrium form randomlymicrovilliStaggered phenomenon,Trophoblast cellAnd intimaepithelial cellsInterphase formationDesmosomeAnd other special fixing structures.During implantation, the trophoblast on the side of the inner cell group first contacts the endometrium, and secretes protease to digest the endometrial tissue in contact with it. The blastocyst is gradually embedded into the functional layer of the endometrium along the gap of the digested tissue (as shown in the Schematic Diagram of Ovulation, Fertilization, Cleavage and Implantation).
After implantation, the original floating blastocyst is closely attached to the uterine wall, and then embedded in the uterine wall, so as to obtain maternal nutrition and protection, and establish the structural relationship between mother and child.
The implantation site of blastocyst is usually in the uterine body and bottom, and the posterior wall is more than the anterior wall.stayEndocrine disorder，SalpingitisUnder the influence of adhesion, stenosis and other factors, the blastocyst fails to enter the uterus and is implanted outside the uterusectopic pregnancyIt often occurs in the fallopian tube and occasionally on the surface of the ovaryBroad ligament of uterus, mesentery, etc.[1]
Research progress
Announce
edit
December 24, 2014, byUniversity of CambridgeThe research team formed by other universities and colleges published an article in the American electronic science journal, saying that the experiment of stably producing primordial germ cells from human "universal cells" was successful.The research team will divide and develop ES cells from fertilized eggs（embryonic stem cell）And bysomatic cellInducedIPS cells（Inducible multifunctional stem cells）It is used as a universal cell and successfully developed into a primordial germ cell (the predecessor of sperm and egg).
Although there have been successfully produced report cases so far, through this study, experts found the important role of the genetic factor named "sox17" for the first time, and made the stable production of this primordial germ cell a reality.[2]
