When is organogenesis complete in humans

Organs develop from the germ layers through the process of differentiation. During differentiation, the embryonic stem cells express specific sets of genes that will determine their ultimate cell type.

For example, some cells in the ectoderm will express the genes specific to skin cells. As a result, these cells will take on the shape and characteristics of epidermal cells.

The early stages of embryonic development begin with fertilization. The process of fertilization is tightly controlled to ensure that only one sperm fuses with one egg.

After fertilization, the zygote undergoes cleavage to form the blastula. The blastula, which in some species is a hollow ball of cells, undergoes a process called gastrulation, during which the three germ layers form. The ectoderm gives rise to the nervous system and the epidermal skin cells, the mesoderm gives rise to the muscle cells and connective tissue in the body, and the endoderm gives rise to the digestive system and other internal organs.

Organogenesis is the formation of organs from the germ layers. Each germ layer gives rise to specific tissue types. Learning Objectives By the end of this section, you will be able to: Explain how the embryo forms from the zygote Discuss the role of cleavage and gastrulation in animal development Describe organogenesis.

Concept in Action Visit the Virtual Human Embryo project at the Endowment for Human Development site to click through an interactive of the stages of embryo development, including micrographs and rotating 3-D images. Answers D A If multiple sperm fused with one egg, a zygote with a multiple ploidy level multiple copies of the chromosomes would form, and then would die. Glossary blastocyst: the structure formed when cells in the mammalian blastula separate into an inner and outer layer.

By week five, the buds of tissue which will become the limbs are in place. The structures which will become the skeleton, nervous system , and circulatory system of the face, neck, and jaws are in place. A five-week-old embryo has the early developmental structures of the esophagus, stomach, intestine, liver, and pancreas. The heart is already functioning, and continues to develop and change over this period of time.

The respiratory system begins developing, as do blood vessels, blood cells, nervous and endocrine organs. Clearly, the most crucial organs of the human form are developing during organogenesis. Essentially, the earlier the injury to these developing buds of tissue, the more severe the ultimate defect. Photographs of human embryos at five stages of gestation reproduced from the collection of the Congenital Anomaly Research Center, Kyoto University Graduate School of Medicine, courtesy of Dr.

Kohei Shiota, Ms. Chigako Uwabe, and Dr. Shigehito Yamada. Shown from left to right are Carnegie Stages 9, 10, 13, 17, and 23 during the period of the embryo. The Stage 9 embryo has initiated neurulation, which is largely completed by Stage 10 with the exception that the cranial and caudal ends of the neural tube remain open as the neuropores.

By Stage 13, body folding has established the tube-within-a-tube body plan of the early embryo, with a distinct head, trunk, and tail, and paddle-like limb buds. By Stage 17, the developing eyes are readily identifiable, and the limb buds now have bulbous distal plate-like structures that will form the hands and the feet.

By Stage 23—the last stage in the period of the embryo—all external structures have taken on morphologies similar to those of the adult. Periods of development broadly define the structure of the developing organism at three different times during pregnancy. The embryo contains the rudiments of the organs of the fetus; the link between the structure of the embryo and fetus is more intuitive, but, for example, the paddle-like limb buds present in the early embryo are non-functional and have a very different structure than that of the upper and lower limbs of the fetus, which at birth are fully functional.

Just as the egg contains the precursor cells for the rudiments of the embryo, the embryo contains the precursor cells for all of the tissues and organs of the fetus. Precursor cells—regardless of their period in development—are highly susceptible to disruption by teratogenic exposures, which are capable of adversely altering their survival, rate of proliferation, migratory activity, ability to differentiate, or to function properly.

In contrast to periods of development, phases of development define not the structure of the developing organism, but the unique developmental events that are occurring at that time. Four major phases are recognized in the prenatal development of humans: gametogenesis; fertilization, cleavage, and blastulation; gastrulation and formation of the tube-within-a-tube body plan ; and organogenesis , with cellular and tissue differentiation and rapid growth.

The gametes are generated during gametogenesis in the ovaries of the female and the testes of the male. During gametogenesis, germ cells from each of the prospective parents as they were developing in utero—first identifiable in the yolk sac, an extraembryonic membrane broadly attached to the ventral side of the early embryo migrate to the developing gonads, divide mitotically, and then initiate meiosis, which is completed postnatally after the onset of puberty, resulting in the generation of haploid eggs and sperm.

During the second phase, fertilization, cleavage, and blastulation , the egg and sperm produced during gametogenesis fuse soon after coitus or shortly after in vitro fertilization is initiated to produce a diploid zygote, which rapidly begins a series of mitotic divisions that is, undergoes cleavage to produce a solid ball of cells called a morula. As the morula passes down the oviduct toward the uterus, it forms an internal cavity, transforming into a hollow cyst-like structure called the blastocyst.

The latter is capable of implanting into the wall of the uterus, initiating in utero development of the embryo. During implantation, the blastocyst differentiates two cell regions: the outer trophoblast, consisting of the cells that invade the uterine wall and contribute to the formation of the placenta, and the inner cell mass, the source of the embryo and its extraembryonic membranes.

As implantation is occurring, the embryo initiates the third phase, gastrulation , a process in which three distinct cell layers the germ layers form, each of which gives rise to specific derivatives. For example, the ectoderm forms the nervous system, the mesoderm forms most of the muscle and bone of the embryo, and the endoderm forms the lining of the gut gastrointestinal tract.

The three germ layers established during gastrulation consist of three uniform layers stacked upon each other like pancakes. This essentially two-dimensional stack becomes sculpted into a three-dimensional embryo having a tube-within-a-tube body plan and containing rudiments of all of the major organ systems.

How this body plan is achieved is referred to as primary morphogenesis and it involves localized changes in the shape, size, position, and numbers of cells in the three germ layers, generating tissue movements such as thickening, folding, delamination, and fusion.

As a result of primary morphogenesis, specific organ rudiments are generated as well as an embryonic body that is now largely separated from its surrounding extraembryonic membranes. For example, the outer tube of the tube-within-a-tube body plan is the future body wall; it is generated by the expansion, folding, and fusion of the edges of the two-dimensional embryo.