HUMAN EMBRYOLOGY

After fertilization of the egg, let the division begin!

Cleavage follows fertilization A zygote is formed when the egg is fertilized. Cleavage is a series of rapid mitotic divisions (without cell growth). The two-celled zygote divides repeatedly until a ball of 32 cells is formed. This is the morula - 32 cells.

Continued divisions make the hollow blastula. These few cells are pluripotent (have the potential to become ANY of the 220 types of cells in the human body). These are embryonic stem cells.

Gastrulation At the end of the cleavage stage, cells making up the blastula move about in a very purposeful and orchestrated manner with the help of surface proteins to recognize each other. The gastrula, which consists of 3 “germ layers” is formed. Layers - Endoderm, Mesoderm, & Ectoderm

Germ Layers Endoderm – Inner Layer; becomes the inner lining of most systems: digestive, respiratory, etc. Mesoderm – Middle Layer; becomes muscles, bones, some skin tissue, connective tissues, the reproductive and urinary systems and other internal organs. Ectoderm – Outer Layer; becomes the nervous system, tooth enamel and the epidermis. It also f forms the lining of mouth, anus, nostrils, sweat glands, hair and nails.

Gastrulation

Human Development The gestation period lasts 266 days from fertilization to birth. Organogenesis (development of the organs and organ systems) begins with the nervous system.