Reproduction and Development

In asexual reproduction one parent makes offspring that are genetically identical to it. Animals do it in four main ways (see Georgia Tech):

• Fission: the body splits into two individuals of about the same size, as in some sea anemones and coral polyps.
• Budding: an outgrowth separates as a new individual. A hydra's bud grows into an adult and breaks away.
• Fragmentation: the body breaks and each part regrows. An arm broken off a sea star can grow into a new sea star.
• Parthenogenesis: an egg develops without being fertilized. Male honeybees, the drones, come from unfertilized eggs.

In sexual reproduction two gametes fuse. Offspring are “unpredictably dissimilar” to their parents and to each other, while asexual organisms “breed true” (see Alberts). The mixing may help a species survive in a changing environment, clear out harmful mutations and bring helpful ones together. But sex has a cost: the resources spent on it are large (see Alberts).

Gametes are made by meiosis, which halves the chromosome number and shuffles the DNA (see Alberts). In humans meiosis cuts 46 chromosomes to 23, and egg and sperm each bring 23 so the embryo has 46 (see MedlinePlus Genetics). See how meiosis works in Cell Division.

The two gametes are built very differently: the egg is large and does not move, while the sperm is small and swims (see Alberts).

Where do they meet? In external fertilization, animals release eggs and sperm into water, usually by spawning; the water keeps the gametes from drying out (see Ask A Biologist). In internal fertilization, egg and sperm meet inside the body. It evolved partly as animals moved onto land, to protect gametes from drying out (see Ask A Biologist).

Many sperm can reach one egg, but only one should fertilize it (see Alberts). In sea urchins the egg has two blocks to polyspermy (see Gilbert):

• Fast block: sodium ions flow in and the egg's membrane potential rises. Sperm cannot fuse with it while it lasts, about a minute.
• Mammals: their eggs do not appear to use this electrical fast block, and rely on slower blocks instead (see Wozniak and Carlson).
• Slow block: a wave of calcium sets off the cortical granule reaction. The vitelline envelope lifts off the egg and hardens into the fertilization envelope.

The fertilized egg, the zygote, begins cleavage: fast mitotic divisions that cut the egg into smaller cells, the blastomeres, without growing. The egg divides into halves, quarters, eighths and on; a frog egg becomes about 37,000 cells in 43 hours (see Gilbert).

In the mouse, the cells pack tightly together after the 8-cell stage. At 16 cells the embryo is a morula, with inner and outer cells. The outer cells pump in fluid to make a cavity, the blastocoel, giving the blastocyst (see Gilbert). In other animals the hollow ball is called a blastula (see Gilbert).

Then comes gastrulation. Cells move so that the endoderm and mesoderm go inside and the ectoderm spreads over the outside (see Gilbert). The new inner gut cavity is the archenteron (see Gilbert).

Each of the three germ layers builds its own parts of the body (see Gilbert):

• Ectoderm: the epidermis of the skin, the nervous system and pigment cells.
• Mesoderm: the kidneys, gonads, bones, heart and blood cells.
• Endoderm: the lining of the digestive tube and the respiratory system.

The endoderm also forms the liver, pancreas, thyroid and thymus (see Gilbert).

In neurulation, part of the ectoderm becomes the neural plate, which rolls up into the neural tube, the future brain and spinal cord. An embryo doing this is a neurula. Cells at the edges, the neural crest, travel through the body and form nerve cells, pigment cells and the cartilage of the face (see Gilbert).

A stem cell can divide to make more of itself and can also make specialized cells. Potency is how many kinds of cell it can make (see UC Irvine):

• Totipotent cells make every cell type of the body plus the extraembryonic tissues, such as the placenta (see UC Irvine). The zygote is the clearest example (see Stem Cells in Neuroendocrinology).
• Pluripotent: Cells of the inner cell mass, embryonic stem cells and iPS cells can make every cell type of the body, but not the placenta (see UC Irvine) (see Nobel Prize 2012).
• Multipotent cells make more than one cell type. A blood-forming stem cell makes all red and white blood cells and platelets (see UC Irvine).

Adult stem cells usually make the cell types of the tissue they live in (see NIH).

Do specialized cells lose genes? In 1962 John Gurdon showed they do not: a tadpole intestinal cell's nucleus, put into a frog egg, could still direct a whole tadpole. In 2006 Shinya Yamanaka turned mature mouse cells into pluripotent stem cells by adding a few genes. They are called induced pluripotent stem cells, iPS cells, and the two scientists shared the 2012 Nobel Prize (see Nobel Prize 2012).

In 1924 Hans Spemann and Hilde Mangold moved a small piece of a newt embryo, the dorsal lip of the blastopore, into another embryo. The host grew a second body, with its own neural plate made of host cells. They named the piece the organizer (see Gilbert).

Later, work on fruit flies found the genes that lay out the body plan and its segments (see Nobel Prize 1995). Hox genes specify which body segment is which, from head to tail (see Gilbert). Edward Lewis found that these genes sit on the chromosome in the same order as the segments they control (see Nobel Prize 1995).

Some animals rebuild their bodies after hatching. In frog metamorphosis, thyroid hormones, mainly T3, drive the change. The tail's cells die by apoptosis, the gills are lost, legs and lungs grow, the gut shortens and the eyes move forward. Tadpoles fed thyroid gland change early, and tadpoles without a thyroid grow into giant tadpoles (see Gilbert).

A butterfly goes from egg to larva, the caterpillar, which molts as it grows, then to a pupa that does not feed, then to the adult. Most larval cells die, and clusters called imaginal discs build the adult's organs. Two hormones control this, 20-hydroxyecdysone and juvenile hormone; juvenile hormone keeps the larva a larva (see Gilbert).

A human sperm and egg each carry 23 chromosomes; at fertilization they form a zygote with 46 (see MedlinePlus Genetics). The zygote divides into a blastocyst, which implants in the wall of the uterus. Its inner cells become the embryo and its outer cells the membranes around it (see MedlinePlus).

Doctors count the weeks of pregnancy from the first day of the last menstrual period. On that count the embryonic period starts in week 5, when the brain, spinal cord and heart begin to form, and after the 10th week the embryo is called a fetus (see MedlinePlus).

Most of what we know about development came from experiments on newts, frogs, flies, sheep and mice.