核内体

内体是膜包裹的囊泡结构,有初级内体(early endosome)和次级内体(late endosome)之分, 初级内体通常位于细胞质的外侧，次级内体常位于细胞质的内侧，靠近细胞核。

内体的主要特征是酸性的、不含溶酶体酶的小囊泡, 其内的受体与配体是分开的。

一般认为初级内体是由于细胞的内吞作用而形成的含有内吞物质的膜结合的细胞器, 通常是管状和小泡状的网络结构集合体。

次级内体中的pH呈酸性, 且具有分拣作用，能够分选与配体结合的受体，让它们再循环到细胞质膜表面或高尔基体反面网络,次级内体中的受体和配体不再偶联在一起，所以次级内体又被称为CURL(compartment of uncoupling of receptor and ligand),意思是受体与配体非偶联的区室。

有学者将与溶酶体酶运输小泡融合的次级内体称为前溶酶体, 因为此时的次级内体中有前体酶的存在。

内体膜上具有A TPase-H+质子泵，利用H+质子的浓度，保证了内部pH的酸性。

初级内体和次级内体是可以区别的，因为它们的密度、pH和酶的含量不相同。

但是次级内体是如何产生的还不太清楚。

(即内吞的小泡，与溶酶体结合之前)In biology, an endosome is a membrane-bound compartment inside cells, roughly 300-400 nm in diameter when fully mature[1]. Many endocytotic vesicles derived from the plasma membrane are transported to an endosome and fuse with it. Some endocytosed material passes through endosomes on its way to lysosomes. Endosomes are in part responsible for the sorting of endocytosed material before transport to lysosomes. This allows some material to be returned to the plasma membrane.Many endocytic vesicles originate at the cell surface as clathrin-coated pits. As clathrin assembles under a patch of plasma membrane the clathrin-coated pit soon (about a minute) pinches off from the surface and forms a clathrin-coated endocytotic vesicle. Soon after forming, the clathrin coated vesicles release their associated clathrin and become competent to fuse with early endosomes. Extracellular materials trapped in the endocytic vesicles can either be passed into the endosomal compartment or returned to the surface.Some materials are specifically endocytosed by receptor-mediated endocytosis. Some extracellular molecules bind to transmembrane receptor proteins that efficiently accumulate in coated pits. One physiologically important example of receptor-mediated endocytosis is the main mechanism by which cholesterol is taken up by cells, particularly liver cells.In some people the cholesterol receptor is defective, uptake of cholesterol from the blood into liver cells is slow and cholesterol accumulates in the blood. This is thought to be the cause of damage to blood vessel walls as increased cholesterol levels have also observed in cases of atherosclerosis. As the arterial damage may have caused the cholesterol damage, rather than vice versa, it is unlikely to cause this until further evidence is shown. This receptor defect, if it did lead to blood vessel damage, would lead to observations of strokes and heart attacks at a young age.One great risk with very high LDL levels is that LDL is vulnerable to free radical damage and that damaged LDL molecules could clump together and form arterial blockages, especially if encountering the sticky platelets of already-damaged blood vessel walls.Short signal peptides have been identified that target certain transmembrane proteins into clathrin-coated pits. A set of proteins called adaptins bind the signal peptides. The signal for the cholesterol receptor is the tetrapeptide Asn-Pro-Val-Tyr.Microscopy indicates that in some cells the endosomal compartments is a network of membranous tubes and vesicles extending all the way to the cell nucleus. The deep end of the endosomalcompartment is called the late endosome compartment. It may take 5-15 minutes for materials to be transported from the cell surface through early endosome compartments and on to the late endosome. The endosomal compartment is usually acidic due to the action of a proton-pumping ATPase of the endosomal membrane. Many receptors involved in endocytosis of extracellular substances change their conformation at low pH and release their bound substance. The empty receptor proteins can then be sorted back to the cell surface.Some materials that reach the late endosomes are degraded in lysosomes. Parts of the late endosomal compartment may become lysosomes or temporarily fuse with lysosomes in order to transfer endoctosed materials into the lysosomes. In epithelial cells a special process called transcytosis allows some materials to enter one side of a cell and exit from the opposite side. For example, the GI tract of babies can take protective antibody proteins from breast milk and via transcytosis, transport the antibodies into the blood stream.A cell surface transferrin receptor binds the iron transport protein transferrin in another example of receptor-mediated endocytosis. In the acidic endosome, the iron is released from transferrin and then the iron-free transferrin (still bound the transferrin receptor) returns from the early endosome to the cell surface.。