Fetal Stem Cell

Three to five days after fertilization, the microscopic hollow sphere has about 50 cells and resembles a golf ball more than a human form. Still in the fallopian tube, it is now called a blastocyst (sometimes “blastomcre”)
and is also totipolent. although this level of plasticity decreases rapidly.

   The blastocyst from a live-day-old human embryo can give rise to only a limited range of cells. his now pluripoent (a word derived from the Latin terms plures. meaning ‘many or several.” and pofrns. meaning “powerful”). Still in the fallopian tube, it will now pass through several stages and form three layers—the endoderm, the mesoderm, and the ectoderm. This stage of development is called the blastuha, when two kinds of cells—the inner cell mass (1CM) and the trophoblast—develop. Surrounding the cell is the zona pellucida, the outside cell membrane. The 1CM will form the tissues of the embryo. The trophoblast will become the placenta and choronic membrane, and will direct implantation into the mother’s uterus. By the
end of the first week, the blastocyst implants in the uterus 

    Here an amazing feat of communication takes place. The cells begin to talk to each other in an intricate manner. They touch each other or in some way signal to tell cells to differentiate into certain other types of cells. Timing is critical in establishing the basic body plan for what shall become the head and what shall become the tail. This process is called embryonic induction and occurs in three stages: primary. secondary. and tertiary. Primary induction leads to the next stage. the gastrula, in which three germ layers are
formed that become the differentiated tissues shown in Figure

• Ectoderm—the outer layer that gives rise to the outer epithelium. including hair, nails, and skin: the sense     organs; and the brain and spinal cord. Epithelium forms from the epidermis and other stru.1urcs.
• Mesoderm—the middle layer that gives rise to bones, muscle, connective tissue, the circulatory system. and most of the excretory and
reproductive systems.
• Endoderm—the inner layer that gives rise to the epithelial linings. those cells that form the linings of the body cavity. including the digestive tract, most of the respiratory tract, the urinary bladder, liver. pancreas. and some endocrine glands.

   The process of the development of the three layers is called gastrulation.
Secondary induction involves a complex communication pattern to initiate the formation of the rudimentary brain and nervous system: tertiary induction regulates the development of body organs and other structures.
The fertilized egg will continue dividing until it produces a complete organism capable of life in the uterus, or in utero. More than 200 kinds of cells will develop from a single, totipotent cell—the zygote or fertilized
egg. These cells include neurons (nerve cells); myocytes (muscle cells); epithelial (skin) cells; bl(xxl cells such as erythrocytes, monocytes, and lymphocytes; osteocytes (bone cells); and chondrocytes (cartilage
cells). Other cells essential for embryonic development include the extraembryonic tissues, placenta, and umbilical cord. 

   The blastocyst in vitro (i.e.. in the laboratory) develops in a similar and predictable way. After fertilization, the embryo develops as follows:

• Day I—Development begins 18 to 24 hours after fertilization.
• Day 2—The zygote undergoes its Iirs cleavage to produce a two- cell embryo. 24 to 25 hours after fertilization.
• Day 3—An eight-cell mass called a modular that resembles a blackberry develops 72 houm after fertilization. Now the embryo begins to control its own development, and the mother’s influences arc reduced.
• Day 4—The embryo’s cells hold close to each other in a process called compaction.

• Day 5—The cavity of the blastocyst is complete. The inner cell mass begins to separate from the outer cells, which become the trophectoderm that surrounds the blastocyst. This is the first sign of differentiation.
• Tmplantatlon—After about the fifth day. the human embryo makes a connection with the mother. This connection has been impossible to make in the laboratory, or even to monitor as it happens in the mother in vivo. However, scientists have studied implantation using the four processes of analysis of gene expression in small samples, in vitro fertilization, analysis of the clones of cultured embryos. and study of genetic markers. In humans, implantation occurs when the irophobla.st cells invade the uterine tissue, forming a syn-claustrophobia.s— something like a mcga-ccIl. The fetus secretes procolytic enzymes that dissolve the uterine epithelial cells and degrade the exracellular matrix. Implantation protects the embryo and provides its metabolic needs.