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A human begins life as a fertilized ovum. This single cell gives rise to the millions of cells that form the human body. In the first few days following fertilization, the developing embryo consists of a ball of cells. This implants on the wall of the uterus and begins to grow further, supported by nutrients and blood from the mother.

As the developing embryo grows in the first few weeks, there is increasing complexity from differentiation of the cells into specialized tissues to form specific organs. This differentiation is directed by genetic factors inherited via the chromosomes from both mother and father. Most organs are formed between 5 and 8 weeks of life. After that, there is continued growth and development to the time of delivery of the baby, which typically occurs following 38 to 42 weeks of gestation in utero.

Embryology Simplified

Fertilized  ------->      Cellular division,      ------->   Adult
   ovum              differentiation, and growth             human

The three major embryologic categories of cells, called the germ cell layers, are:

  • Ectoderm: forms the epithelium that covers the body, and gives rise to cells in the nervous system

  • Endoderm: forms the gastrointestinal tract and associated structures involved in digestion

  • Mesoderm: forms the connective tissues and "soft" tissues such as bone, muscle, and fat

After birth, some cells within the body continue to proliferate, while others do not and remain or are lost in the aging process. Aging results from the inability of cells to maintain themselves or replace themselves.

The following discussion will introduce you to the types of cells and tissues that constitute the human body. Examples of the major cell types, along with the organs they compose, will be demonstrated with histologic sections.

The genes that direct cellular proliferation and development in embryologic life are "turned off" or suppressed once appropriate growth has been achieved. However, when some of these genes are "turned on" inappropriately because of mutations or alterations (oncogenes), or when the genes that suppress growth (tumor suppressor genes) become faulty later in life, then the result can be neoplasia.