Everything about Embryo totally explained
An
embryo (from
Greek:
ἔμβρυον, plural
ἔμβρυα, lit. "that which grows," from en- "in" + bryein "to swell, be full") is a
multicellular diploid eukaryote in its earliest stage of
development, from the time of first
cell division until
birth,
hatching, or
germination. In humans, it's called an embryo from the moment of
fertilisation until the end of the 8th week, whereafter it's instead called a
fetus.
Development
The development of the embryo is called embryogenesis. In organisms that
reproduce sexually, once a
sperm fertilizes an
egg cell, the result is a
cell called the
zygote that has all the
DNA of two parents. The resulting embryo derives 50 percent of its genetic makeup from each parent. In
plants,
animals, and some
protists, the zygote will begin to divide by
mitosis to produce a multicellular organism. The result of this process is an embryo.
In animals
In animals, the development of the zygote into an embryo proceeds through specific recognizable stages of
blastula,
gastrula, and
organogenesis. The blastula stage typically features a fluid-filled cavity, the
blastocoel, surrounded by a sphere or sheet of cells, also called
blastomeres.
During
gastrulation the cells of the blastula undergo coordinated processes of cell division, invasion, and/or migration to form two (
diploblastic) or three (
triploblastic) tissue layers. In triploblastic organisms, the three
germ layers are called
endoderm,
ectoderm and
mesoderm. However, the position and arrangement of the germ layers are highly species-specific, depending on the type of embryo produced. In
vertebrates, a special population of embryonic cells called the
neural crest has been proposed as a "fourth germ layer", and is thought to have been an important novelty in the evolution of head structures.
During organogenesis, molecular and cellular interactions between germ layers, combined with the cells' developmental potential or
competence to respond, prompt the further differentiation of organ-specific cell types. For example, in
neurogenesis, a subpopulation of ectoderm cells is set aside to become the
brain,
spinal cord and
peripheral nerves. Modern developmental biology is extensively probing the molecular basis for every type of organogenesis, including
angiogenesis (formation of new blood vessels from pre-existing ones),
chondrogenesis (cartilage),
myogenesis (muscle),
osteogenesis (bone), and many others.
Generally, if a structure pre-dates another structure in evolutionary terms, then it often appears earlier than the other in an embryo; this general observation is sometimes summarized by the phrase "
ontogeny recapitulates phylogeny." For example, the backbone is a common structure among all vertebrates such as
fish,
reptiles and
mammals, and the backbone also appears as one of the earliest structures laid out in all vertebrate embryos. The
cerebrum in humans, which is the most sophisticated part of the
brain, develops last. This rule isn't absolute, but it's recognized as being partly applicable to development of the human embryo.
Embryos of plants and animals
- Plants: In botany, a seed plant embryo is part of a seed, consisting of precursor tissues for the leaves, stem (see hypocotyl), and root (see radicle), as well as one or more cotyledons. Once the embryo begins to germinate — grow out from the seed — it's called a seedling. Plants that don't produce seeds, but do produce an embryo, include the bryophytes and ferns. In these plants, the embryo is a young plant that grows attached to a parental gametophyte.
- Animals: The embryo of a placental mammal is defined as the organism between the first division of the zygote (a fertilized ovum) until it becomes a fetus. In humans, the embryo is defined as the product of conception from implantation in the uterus through the eighth week of development. An embryo is called a fetus at a more advanced stage of development and up until birth or hatching. In humans, this is from the eighth week of gestation.
The human embryo
Growth
Week 1-3 5-7 days after fertilization, the blastula attaches to the wall of the uterus (
endometrium). When it comes into contact with the endometrium it performs
implantation. Implantation connections between the mother and the embryo will begin to form, including the
umbilical cord. The embryo's growth centers around an axis, which will become the spine and spinal cord. The brain, spinal cord, heart, and gastrointestinal tract begin to form.
Week 4-5 Chemicals produced by the embryo stop the woman's
menstrual cycle. Neurogenesis is underway, showing brain activity at about the 6th week. The heart will begin to beat around the same time. Limb buds appear where the arms and legs will grow later. Organogenesis begins. The head represents about one half of the embryo's axial length, and more than half of the embryo's
mass. The brain develops into five areas, along with vertebra and bones beginning to form. The heart starts to beat and blood starts to flow. The
NIH defines the embryonic stage as the beginning of developed human form
Footnotes
Further Information
Get more info on 'Embryo'.
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