Development of an organism happens through fertilization, cleavage, blastulation, gastrulation, organogenesis, and metamorphosis into an adult. Each species of
animal has a slightly different journey through these stages, since some stages might be shorter or longer when compared to other species, and where the offspring develops is different for each animal type (e.g., in a hard egg shell, uterus, soft egg shell, on a plant leaf, etc.).
Fertilization In humans, the process of
fetal development starts after
sperm fertilizes an
egg and they fuse together, kickstarting
embryonic development. The fusion of egg and sperm into a
zygote changes the surrounding membrane to not allow any more sperm to penetrate the egg, so multiple fertilizations can be prevented. Fusion of a zygote also activates the egg so it can begin undergoing cell division. Each animal
species might not have specifically a sperm and an egg, but two gametes that contain half of the species' typical genetic material and the membranes of these gametes fuse to start creating an offspring.
Cleavage Not long after successful fertilization by sperm, the zygote undergoes many
mitotic divisions, which are also non-sexual cell divisions.
Cleavage is the process of cell division, so the starting zygote becomes a collection of identical cells which is a morula and contains cells called blastomeres. Cleavage prepares the zygote to become an embryo, which is from 2 weeks to 8 weeks after conception (fertilization) in humans.
Blastulation After the zygote has become an embryo, it continues dividing into a hollow sphere of cells, which is a
blastula. These outer cells form a single epithelial layer, the blastoderm, that essentially encases the fluid-filled inside that is the blastocoel. The figure to the right shows the basic process that is modified in different species. Blastulation differs slightly in different species, but in mammals, the eight-cell stage embryo forms into a slightly different type of blastula, called a blastocyst. Other species such as
sea stars,
frogs,
chicks, and
mice have all the same structures in this stage, yet the orientation of these features differs, plus these species have additional types of cells in this stage.
Gastrulation After blastulation, the single-layered blastula expands and reorganizes into multiple layers, a gastrula (seen in the figure to the right).
Reptiles,
birds and
mammals are triploblastic organisms, meaning the gastrula comprises three
germ layers; the endoderm (inner layer), mesoderm (middle layer), and ectoderm (outer layer).
Organogenesis In the figure below, human germ cells are able to differentiate into the specific organs and tissues they become later on in life. Germ cells are able to migrate to their final locations to rearrange themselves and some organs are made of two germ layers; one for the outside, the other for the inside. The figure above shows how the development of a
pig,
cow,
rabbit, and
human offspring are similar when compared to one another. This figure shows how the germ layers can become different organs and tissues in evolutionarily higher life-forms and how these species essentially develop very similarly. Additionally, it shows how multiple species develop in a parallel manner but branch off to develop more specific features for the organism such as hooves, a tail, or ears.
Neurulation In developing
vertebrate offspring, a
neural tube is formed through either
primary or secondary neurulation. Some species develop their spine and nervous system using both primary and secondary neurulation, while others use only primary or secondary neurulation. In human fetal development, primary neurulation occurs during weeks 3 and 4 of gestation to develop the brain and spinal cord. Then during weeks 5 and 6 of gestation, secondary neurulation forms the lower sacral and coccygeal cord.
Primary Neurulation The diagram to the right illustrates primary neurulation, which is the process of cells surrounding the neural plate interacting with neural plate cells to proliferate, converge, and pinch off to form a hollow tube above the
notochord and mesoderm. This process is discontinuous and can start at different points along the cranial-caudal axis necessary for it to close. In more advanced organisms like
amphibians,
birds and
mammals;
Secondary Neurulation In secondary neurulation, caudal and sacral regions of the spine are formed after primary neurulation is finished. This process initiates once primary neurulation is finished and the posterior neuropore closes, so the tail bud can proliferate and condense, then create a cavity and fuse with the central canal of the neural tube. Secondary neurulation occurs in the small region starting at the
spinal tail bud up to the posterior neuropore, which is the open neural folds near the tail region that don't close through primary neurulation. As canalization progresses over the next few weeks, neurons and ependymal cells (cells that create cerebral spinal fluid) differentiate to become the tail end of the spinal cord. Next, the closed neural tube contains neuroepithelial cells that immediately divide after closure and a second type of cell forms; the neuroblast.
Neuroblast cells form the mantle layer, which later becomes the
gray matter, which then gives rise to a marginal layer that becomes the
white matter of the spinal cord. This young organism is the larva and is the intermediate form before
metamorphosing into an adult. The juvenile phase is different in plants and animals, but in plants juvenility is an early phase of plant growth in which plants can't flower. In animals, the juvenile stage is most commonly found in social mammals, such as
wild dogs,
monkeys,
apes,
lions,
wolves, and more. In humans,
puberty marks the end of this stage and
adolescence follows. Some species begin puberty and reproduction before the juvenile stage is over, such as in female non-human primates. The larval and pupal stages can be seen in the figure to the right.
Metamorphosis The process of an organism's body undergoing structural and physical changes after birth or hatching to become suitable for its adult environment is
metamorphosis. For example,
amphibian tadpoles have a maturation of liver enzymes,
hemoglobin, and eye pigments, in addition to their nervous, digestive, and reproductive systems being remodeled. In all species,
molting and
juvenile hormones appear to regulate these changes. In
dogs, small breeds (e.g.,
Yorkshire Terrier,
Chihuahua,
Cocker Spaniel, etc.) physically mature faster than large breeds (e.g.,
Saint Bernard,
Great Dane,
Golden Retriever, etc.), so adulthood is reached anywhere from 12 to 24 months or 1 to 2 years. In contrast, many insect species have long larval stages and the adult stage is only for reproduction. The silkworm
moths don't have mouthparts and don't feed, so they have to consume enough food during the larval stage for energy to survive and mate. Senescence can be induced by un-repaired DNA damage (e.g., from radiation, old age, etc.) or other cellular stress and also is the state of being old.
Ontogenetic allometry Most organisms undergo
allometric changes in
shape as they grow and
mature, while others engage in
metamorphosis. Even reptiles (non-avian sauropsids, e.g.,
crocodilians,
turtles,
snakes, and
lizards), in which the offspring are often viewed as miniature adults, show a variety of ontogenetic changes in
morphology and
physiology. ==See also==