Formation of germ layers

FERTILIZATION AND STEM CELLS The oocyte (female gamete) is released from the ovary and then "pulled" into the ampulla of the uterine tubes by its finger- like projections, called fimbriae. Sperm (male gametes) travel through the vagina, into the uterus, and continue into the uterine tubes where fertilization of the oocyte occurs.

Fertilization most often takes place in the ampulla of the uterine tube, which is the widest area of the tube. Both gametes are haploid, carrying only half the number of chromosomes as a normal diploid cell. Fertilization restores the cell to a diploid state and denotes the formation of the zygote.

The zygote moves through the uterine tube toward the uterus. At this point, the zygote is a stem cell that will give rise to daughter cells with the maximum capacity for differentiation. It is considered "totipotent" meaning that this cell has the capacity to give rise to cells that can become any type of embryological tissue.

This totipotent cell will differentiate into every type of fetal tissue as well as the support structures for the fetus (such as the placenta.) As development progresses, cells become more and more specialized, and simultaneously lose their potentiality.

III. BLASTOCYST AND IMPLANTATION It takes about 4 days for the zygote to reach the uterus. As the zygote travels through the uterine tubes, the original singe cell divides into a two, four, eight, and sixteen cell unit without increasing in overall size or mass. The sixteen cell unit is approximately the stage at which the embryo enters the uterus.

The cells reach the first branch point in differentiation here. The outer cells form a ring that surrounds the inner cells, which are known as the inner cell mass. The outer cell mass becomes the trophoblast (green cells in Figures), which will contribute to the placenta.

The inner cells form the embryoblast, which will give rise to the embryo (blue cells in Figures.) A cavity (or "cyst") begins to form between the embryoblast and the trophoblast and the embryo is now considered a blastocyst. The blastocyst will penetrate the wall of the uterus and implant into the uterine mucosa.

Cells of the embryoblast now differentiate into two distinct layers. The hypoblast (yellow cells in Figures) layer is most inward and the epiblast (blue cells in Figure) layer lies adjacent to the trophoblast. The epiblast (blue) begins to form a second space called the amniotic cavity.

Simultaneously, the hypoblast forms the primitive yolk sac followed by the secondary yolk sac which will degenerate when fetal circulation is established. The trophoblast continues to invade the uterine wall and establish the foundation for a placenta. The uterus responds to this invasion by altering the vasculature in the uterine tissue adjacent to trophoblast in order to support the growing embryo.

IV. FORMATION OF THE THREE GERM LAYERS The next major stage in the development of the part of the blastocyst that will become the fetus is the formation of three distinctive layers that will go on to form identifiable organ systems. In one half of the epiblast, a primitive streak appears; this event determines the orientation of the developing tissue. The end with the primitive streak will become the head (cephalic) and the other will become the rump (caudal).

Along this streak, the cells begin to proliferate and invaginate, folding inward between the two layers. The resulting mass of tissue forms two new cell layers between the epiblast and hypoblast. The first inward moving cells that displace the hypoblast make the endoderm. The next cells to move inward form the mesoderm. The remaining epiblast cells are now termed the ectoderm layer.

These three germ layers will contribute to all of the cell types in the body. The endoderm gives rise to the lining of the gut, lungs and bladder. The mesoderm becomes muscles, bones, blood cells, spleen, lymphatic tissues, heart, lungs, reproductive and excretory systems.

Derivatives of the ectoderm include the skin, nails, hair, mouth, anus, and nervous system. All three layers give rise to different types of connective tissue. Undifferentiated connective tissue is called mesenchyme regardless of the germ layer it is derived from.

The cells in these three germ layers are still stem cells because, without losing their own generative capacity, they can produce daughter cells that can differentiate into multiple cell types. However, because they have made an irreversible step towards a particular lineage of cell types, they are "pluripotent" (capable of giving rise to multiple cell types) instead of being "totipotent" (capable of giving rise to all cell types.)

NOTOCHORD AND NEURAL TUBE FORMATION At the most medial part of the primitive streak cells begin to proliferate in a linear fashion and migrate toward the cephalic end. They begin to form a chord of cells know as the notochord. The notochord serves as a guide for the axial skeleton.

The ectoderm cells that lie directly over the top of the notochord push up from either side of the notochord and form a layer that curl toward itself until the cells meet in the middle, where they fuse. The resulting cavity lined by these cells is called the neural tube. The neural tube eventually seals completely to encase the brain and spinal cord.

As the edges of the ectoderm layer curl upwards to form the neural tube, another cell type forms at the tip of the curl. These cells differentiate into neural crest cells. Neural crest cells eventually give rise to the dorsal root ganglia, Schwann cells, the autonomic nervous system, meninges, sensory ganglia, bones of the face, teeth, lens of the eyes, melanocytes, adrenal medulla, and many glands. (The neural crest cells are sometimes referred to as the fourth germ layer.)

Along the notochord, mesoderm cells start to form segmented columns that will become somites. The each somite forms a specific segment of muscle, bone, and connective tissue. One of the most notable structures formed from somites is the spinal vertebrae.

During this period, as the notochord, neural tube, and somites are forming, the embryo develops a left and right side. Some tissues and organs are paired and form symmetrically while others form unilaterally.

They act as chemoattractants, growth factors, and growth inhibitors. Signaling molecules guide the migration of tissues to certain places, such as the abdomen, in the embryo. They also guide the orientation indicating the top, bottom, left and right side of the organ.