All of the approximately 200
different types of body cells can be classified into four basic or primary
tissue types: epithelial, connective, muscle, and nervous (Table 4.1). These
basic tissue types are often described by their embryonic origin. The embryo is
essentially a three-layered tubular structure (Fig. 4.17). The outer layer of
the tube is called the ectoderm; the middle layer, the mesoderm; and the inner
layer, the endoderm. All of the adult body tissues originate from these three cellular
layers. Epithelium has its origin in all three embryonic layers, connective
tissue and muscle develop mainly from the mesoderm, and nervous tissue develops
from the ectoderm.
