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Mesenchyme is a form of loose connective tissue that develops within an embryo. It consists of a ground substance, made up of proteins and water and being gelatinous in texture, that protects and supports the cells and reticular fibers within. Cells within mesenchyme are undifferentiated, meaning that they can develop into any type of mature cell. From the mesenchyme comes the bone, cartilage, connective tissue, lymphatic system and circulatory system of the fully developed fetus.
During normal development, an embryo grows from a fertilized egg to a zygote to a blastula, then into a grastula, which consists of several layers called germ layers. Most complex vertebrates develop three germ layers: the endoderm, mesoderm and ectoderm. These complex creatures, including humans, are called triploblastic because of the presence of three distinct germ layers. Less complex animals often start with two layers, or even one, as in the case of sponges. Animals that develop from two germ layers are called diploblastic.
The mesenchymal cells arise largely from the mesoderm as the embryo continues to differentiate. Mesenchyme is considered a form of reticular connective tissue because it contains reticular fibers, which are fibers made up of a certain type of collagen known as type III collagen. This form of collagen is found in bone, cartilage, tendons and other connective tissues that develop from the mesenchyme. Reticular fibers also can form into a meshlike supportive network found in soft tissues in the body such as the lymphatic system, which also arises from the mesenchyme during fetal development. The mesenchyme also can differentiate into hematopoietic cells, or cells that produce blood and circulatory tissue, thus developing into the mature circulatory system.
Another related term, "mesenchymal stem cells," sometimes is used interchangeably with the term "marrow stromal cells" to describe undifferentiated stem cells. Referring to these as mesenchymal cells, however, is not entirely accurate, because mesenchyme can differentiate into blood cells, and the stem cells most often called mesenchymal stem cells generally can become bone cells, fat cells or cartilage cells. Another term, considered somewhat more accurate when used to describe this type of stem cell, is "multipotent stromal cells."
During fetal development, a similar type of tissue called ectomesenchyme also develops. Unlike mesenchyme, ectomesenchyme arises a bit later in the development process from a group of cells called neural crest cells. Neural crest cells form in the region of the embryo's cranium and become bones and muscles of the head and neck, as well as forming the branchial or pharyngeal arches, which become gills in fish and develop into the trachea and larynx in mammals.
In sexual reproduction, a zygote is the new cell created when the egg is first fertilized. In multicellular organisms this is the beginning of the embryo. The zygote is always created from a union of two cells called gamete cells. In humans this is the point where the sperm fertilizes the egg.
This one cell has all the tools to grow into a full human. This child, being a boy or girl, will have some genes from the father who contributed the male gamete and some genes from the mother who added the egg or ovum.