14.3 Cleavage, Gastrulation, and Germ Layers
Key Takeaways
Cleavage is rapid cell division with little growth and produces a morula and then a blastula.
A mammalian blastocyst has an inner cell mass that forms the embryo and a trophoblast that contributes to extraembryonic structures.
Gastrulation forms three germ layers and the archenteron, and in deuterostomes the blastopore becomes the anus.
Ectoderm forms epidermis and the nervous system, mesoderm forms muscle, heart, kidneys, gonads, and the notochord, and endoderm lines the gut and supplies liver, pancreas, and lung tissue.
The neural tube is ectoderm, so the brain and spinal cord are not mesoderm, and cleavage does not enlarge the embryo the way later growth does.
14.3 Cleavage, Gastrulation, and Germ Layers
Fertilization produces a diploid zygote. Everything an animal body will become is still in that one cell, but the cell is not yet a body. Development after fertilization has an order worth memorizing as stages, not as a blur. Cleavage multiplies cells without real growth. Gastrulation rearranges those cells into three germ layers and builds a primitive gut. Organogenesis shapes organs from those layers. Mix the stages and the germ-layer answers fall apart, because a structure that exists only after gastrulation cannot be assigned to cleavage.
Cleavage divides the zygote without enlarging it
Cleavage is a series of rapid mitotic divisions. The cell cycle is short. The embryo skips the growth that adult cells insert between divisions, so the cytoplasm of the zygote is partitioned into smaller cells called blastomeres. The whole embryo stays about the same size. It does not enlarge the way a larva or a fetus enlarges later, when cells grow between divisions and the animal takes in food. More cells, smaller cells, same overall volume: that is the cleavage rule. A stem that says cleavage doubles the embryo's volume before any division has described later growth instead.
The early solid ball is a morula. Further division and fluid accumulation produce a blastula, a hollow ball. The cavity is the blastocoel. The blastula is still one layer of cells arranged around a space, or a simple hollow sphere in animals such as the sea urchin. It does not yet have the three germ layers. Those appear only when cells move during gastrulation.
Yolk changes the pattern, not the growth rule
Yolk changes how completely the egg divides, not whether the embryo grows. Sea urchins and mammals have little yolk, so holoblastic cleavage divides the whole egg. A frog egg has more vegetal yolk, so cleavage is complete but unequal. A bird or reptile egg is mostly yolk, so meroblastic cleavage stays in a blastodisc on top of the yolk. In every pattern the embryo is subdividing cytoplasm, not enlarging the way later growth does.
Mammals add a structural distinction at the hollow-ball stage. Their version is a blastocyst, not a simple one-layer blastula. The inner cell mass will form the embryo. The outer trophoblast contributes to extraembryonic structures, including the chorion that helps form the placenta. In humans, cleavage begins in the oviduct. By the time the embryo is a blastocyst, it has reached the uterus. The two populations of cells are already different jobs: embryo versus extraembryonic contribution. Do not swap them. The trophoblast is not the inner cell mass, and the fluid-filled blastocoel is not a germ layer.
Gastrulation builds three layers and a gut
Gastrulation is a rearrangement, not another round of cleavage. Cells move and change neighbors. The hollow blastula becomes a gastrula with three germ layers. The movements also create the archenteron, the primitive gut. The opening of the archenteron to the outside is the blastopore. Endoderm lines the archenteron. Mesoderm comes to lie between endoderm and the outer ectoderm. After gastrulation, a fate can be stated as a layer. Before gastrulation, that layer does not exist yet.
The choreography depends on the animal, and the result to remember does not. Sea urchin cells tuck inward at the vegetal plate, a movement called invagination. Frog cells roll over the dorsal lip of the blastopore, a movement called involution, while the animal-cap cells spread to cover the outside. Birds and mammals move cells inward through a primitive streak. The anatomy of the opening differs. The exam result is the same: three germ layers, an archenteron, and a blastopore.
The blastopore becomes the anus in deuterostomes
What the blastopore becomes is a phylogenetic split. In deuterostomes, the blastopore becomes the anus, and the mouth forms later at the other end of the gut. Echinoderms, including sea urchins and sea stars, and chordates, including humans and other vertebrates, are deuterostomes. In protostomes, the blastopore becomes the mouth. Mollusks, annelids, and arthropods are protostomes. A sea urchin and a human share the deuterostome fate of the blastopore even though their gastrulation movements look different. A stem that calls every animal's blastopore the mouth has described protostomes only.
Organogenesis uses the three layers
Organogenesis builds organs from the germ layers. It is not a synonym for cleavage, and it is not the moment the blastopore first opens. The clearest early organ is the neural tube. Dorsal ectoderm thickens into a neural plate, the plate folds, and the folds meet. The anterior neural tube becomes the brain. The rest becomes the spinal cord. The tube itself is ectoderm. The notochord, a mesodermal rod underneath the plate, helps signal that ectoderm to fold, but the notochord does not turn into the brain. Neural crest cells leave the edges of the folding plate, migrate, and stay ectoderm in origin. They contribute peripheral ganglia, pigment cells, and parts of the adrenal medulla.
Mesoderm beside the neural tube forms somites, which contribute vertebrae, skeletal muscle, and dermis. Epidermis is ectoderm. Heart and vessels, kidneys, and gonads are mesoderm. Endoderm supplies the gut lining and the epithelial tissues of the liver, pancreas, and lungs. One organ can mix layers. Intestinal lining is endoderm, while intestinal muscle is mesoderm. Brain and spinal cord are ectoderm. Skeletal muscle, heart, kidney, gonad, and notochord are mesoderm.
| Germ layer | Structures formed from it |
|---|---|
| Ectoderm | Epidermis of the skin; nervous system, including the neural tube that becomes the brain and spinal cord; neural crest derivatives |
| Mesoderm | Muscle, skeleton, heart and blood vessels, kidneys, gonads, and the notochord |
| Endoderm | Lining of the gut; epithelial tissues of the liver, pancreas, and lungs |
The adrenal gland shows why the layer and the hormone chapter meet. The adrenal cortex, source of cortisol and aldosterone, is mesoderm. The adrenal medulla, source of epinephrine, comes from neural crest and is therefore ectoderm. The gland looks like one organ. Its two hormone regions do not share one germ layer.
Warning
The brain and spinal cord come from the neural tube, which is ectoderm, not mesoderm. Cleavage multiplies cells without enlarging the embryo the way later growth does. The blastopore of a deuterostome becomes the anus, not the mouth.
How is cleavage different from later growth of the embryo?
Cleavage immediately sorts cells into ectoderm, mesoderm, and endoderm and opens the blastopore.
Cleavage is a long pause with no division, during which the embryo doubles in volume before any blastula forms.
Cleavage builds the brain from mesoderm and enlarges the embryo the way later fetal growth does.
Cleavage is rapid division with little growth, so the zygote becomes a morula and then a blastula of smaller cells.
A sea urchin and a human are both deuterostomes. During gastrulation, what does the blastopore become?
The neural tube, because the blastopore folds inward from mesoderm and becomes the spinal cord.
The anus, because the mouth forms later in echinoderms and chordates.
The trophoblast, because the blastopore is the outer wall of the mammalian blastocyst.
The mouth, because every deuterostome is defined by the blastopore becoming the mouth.
Which match of germ layer to structures is correct?
Endoderm forms skeletal muscle and the notochord, and the trophoblast is another name for the inner cell mass.
Mesoderm forms the brain and spinal cord, and ectoderm forms the heart, blood vessels, and kidneys.
Ectoderm forms the epidermis and the neural tube, mesoderm forms muscle, heart, kidneys, gonads, and the notochord, and endoderm lines the gut and forms the epithelial tissues of the liver, pancreas, and lungs.
All body organs form directly from blastocoel fluid, and germ layers appear only in protostomes.
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