57. Embryonic connective tissue.

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Embryonic connective tissue is primarily found in the developing embryo and is classified into two main types:

Mesenchymal Connective Tissue (Mesenchyme):

• Origin and Development:
  All connective tissues originate from embryonic mesenchyme, a primitive type of connective tissue. Mesenchyme develops predominantly from the mesoderm germ layer, although it can also arise from other germ layers, such as the ectoderm and endoderm, during embryonic development. This tissue is primarily present in the embryo, situated between and within developing tissues and organs. In adults, remnants of mesenchymal tissue can be found in areas such as the dental pulp.

• Structure and Composition:
  Mesenchymal connective tissue is characterized by a prominent ground substance that is translucent and viscous, providing a supportive matrix around the cells. This ground substance facilitates the diffusion of small molecules, which is essential for nutrient and waste exchange during early development. The tissue consists of a loose network of collagen and reticular fibers interspersed among mesenchymal cells.

• Mesenchymal Cells:
  The primary cell type found in mesenchymal tissue is the mesenchymal cell, which is undifferentiated and possesses stem cell properties. These cells have the potential to differentiate into various types of connective tissues, including bone, cartilage, and adipose tissue, as well as cells of the lymphatic and circulatory systems. The mesenchymal matrix is rich in hyaluronan and water, creating an ideal environment for cell migration, growth, and differentiation.

• Location and Development:
  Mucous connective tissue is found during fetal development, starting a few weeks after fertilization. It is most commonly located in the umbilical cord (known as Wharton's Jelly) and in the pulp of developing teeth in young children.

• Structure and Composition:
  Mucous connective tissue is structurally similar to mesenchyme but is characterized by a more gelatinous consistency due to a higher content of ground substance, which is rich in hyaluronan. This ground substance gives the tissue its jelly-like appearance. The primary cells found in mucous connective tissue are fibroblasts, which are surrounded by a sparse network of collagen fibers, with fewer elastic and reticular fibers observed under electron microscopy.

• Properties and Function:
  Unlike mesenchymal tissue, mucous connective tissue is less plastic and has a lower capacity for differentiation. Its primary function during development is to provide mechanical support and cushioning, particularly in the umbilical cord, where it helps protect blood vessels from compression

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