Junctional complexes of epithelial cells.

Three cuboidal epithelial cells, emptied of their contents, show the four major types of junctional complexes between cells. The tight junction (zonula occludens) and adherent junction (zonula adherens) are typically close together and each forms a continuous ribbon around the cell's apical end. Multiple ridges of the tight, occluding junctions prevent passive flow of material between the cells, but are not very strong; the adhering junctions immediately below them serve to stabilize and strengthen these circular bands around the cells and help hold the layer of cells together. Both desmosomes and gap junctions make spotlike plaques between two cells. Bound to intermediate filaments inside the cells, desmosomes form very strong attachment points which supplement the role of the zonulae adherens and play a major role to maintain the integrity of an epithelium. Gap junctions, each a patch of many connexons in the adjacent cell membranes, have little strength but serve as intercellular channels for flow of molecules. All of these junctional types are also found in certain other cell types besides epithelia.

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EMBRYOLOGY

EMBRYOGENESIS	The development of an embryo from the initial fertilization of an egg.
PRIMARY PERIOD	The first week of development.
EMBRYONIC PERIOD	Second to eight weeks of development.
FETAL PERIOD	Continues from ninth week of intranatal development till the birth of child.

EVENTS OF EMBRYOGENESIS
FERTILIZATION	Fusion of male and female sexual cells with formation of unicellular embryo (zygote) with diploid number of chromosomes. It occurs in ampullar part of uterine tube.
CLEAVAGE OR FRAGMENTATION	A series of rapid division which occurs after fertilization. Human cleavage is: (a) complete (cleavage is characterized by complete division of the mass of the fertilized egg to form the blasomeres); (b) unequal (cleavage in which the dark and light blastomeres are unequal in size).
- Blastocyst	The germinal vesicle that consists of a hollow sphere of cells which results from the process of cleavage, and is formed at the late morula stage when fluid passes into the intercellular spaces between the inner and outer layers of cells.
1. trophoblast	A layer of tissue formed by cells that make up the outer covering of the blastocyst.
2. embryoblast	The group of dark blastomeres, which is the part of human blastocyst.
3. blastocoele	The cavity of the blastocyst that separates the trophoblast from the inner cell mass except for the small area where they are in contact.
GASTRULATION	Differentiation of germ layers.
- Germ layers
1. ectoderm	A superficial germ layer developing on the outside of the embryo.
2. endoderm	The inner layer of the embryo, giving rise to the primitive intestine and the yolk sac.
3. mesoderm	The primary germ layer formed between the ectoderm (outermost layer) and endoderm (innermost layer) of the embryo.
HISTOGENESIS, ORGANOGENESIS, SYSTEMOGENESIS

CLEAVAGE

Series of rapid divisions.

Human cleavage is:

- complete (cleavage is characterized by complete division of the mass of the fertilized egg to form the blastomeres);

- unequal (cleavage in which the dark and light blastomeres are unequal in size).

Дата добавления: 2015-10-30