Embryonic Development and Gastrulation

Early embryogenesis and gastrulation

Early embryogenesis is the sequence of events that starts with fertilization and turns a single cell into a multicellular embryo with organized germ layers. Stage names must be learned precisely, because exam items test their exact definitions. The zygote is the single diploid cell formed when a sperm fuses with an ovum.

The morula is a compact, solid mass of cells produced by cleavage. The blastocyst is a hollow sphere of cells enclosing a fluid-filled blastocoel. The ovum is the unfertilized egg. After birth the offspring is called a neonate. The developing embryo in the later stages of pregnancy is called a fetus.

Gastrulation converts the blastula into a multilayered embryo with ectoderm, mesoderm and endoderm. It is driven by coordinated cell movements, and four types are standard:

• Invagination — infolding of an epithelial sheet, as in endoderm formation in sea urchin embryos.
• Involution — an expanding outer layer turns inward and spreads over the internal surface of the remaining external cells.
• Ingression — individual cells leave the surface and migrate into the interior of the embryo.
• Delamination — one cellular sheet splits into two nearly parallel sheets.

Bottle cells are cells that change shape by apical constriction, becoming flask-shaped, and they drive invagination of the blastopore lip. This mechanism is classically described in amphibian embryos, where it initiates formation of the archenteron. Birds and mammals do not use bottle cells as the characteristic gastrulation mechanism, since birds and mammals form a primitive streak instead.

Each germ layer gives rise to specific organs. Endoderm forms the gut lining and its derivatives, so the liver, which arises from a hepatic diverticulum of the foregut, is endodermal. Ectoderm forms the nervous system, so the cerebellum, which develops from the neural tube, is ectodermal. Mesoderm forms skeletal and muscular tissue, so the femur, derived from somites and lateral plate mesoderm, is mesodermal.

Mammalian extraembryonic tissues must also be distinguished. The hypoblast, or primitive endoderm, gives rise to the extraembryonic endoderm that forms the wall of the yolk sac. The syncytiotrophoblast is the multinucleated layer that forms the placental syncytium, not the yolk sac. The epiblast gives rise to the embryo proper and its three germ layers.

Segmentation genes in Drosophila are also tested by class. Pair-rule genes such as fushi tarazu are expressed in seven stripes, each covering alternate segments, so they are pair-rule genes. Gap genes act earlier to define broad regions, and segment polarity genes subdivide each segment.