Proembryo, or
pro-embryo, is a
botanicalBotany, plant science, phytology, or plant biology is a branch of biology and is the scientific study of plant life and development...
term that has been used for centuries, to denote the series of cells that are formed after fertilization within the
ovuleOvule literally means "small egg." In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: The integuments forming its outer layer, the nucellus , and the megaspore-derived female gametophyte in its center...
of a flowering plant, before formation of the
embryoPlant embryogenesis is the process that produces a plant embryo from a fertilised ovule by asymmetric cell division and the differentiation of undifferentiated cells into tissues and organs. It occurs during seed development, when the single-celled zygote undergoes a programed pattern of cell...
.
More recently, this same term has been given a more extensive meaning, with reference to human egg cells (
ovaAn ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
) that have been fertilized but not yet implanted into a
uterusThe uterus is a major female hormone-responsive reproductive sex organ of most mammals, including humans. It is within the uterus that the fetus develops during gestation. The term uterus is used consistently within the medical and related professions; the Germanic term, womb is more common in...
. This more recent meaning is synonymous with "preembryo" or "pre-embryo."
Preembryo in context of human development
A preembryo is a human
conceptusConceptus denotes the embryo and its adnexa or associated membranes The conceptus includes all structures that develop from the zygote, both embryonic and extraembryonic...
at least between fertilization and
implantationImplantation is an event that occurs early in pregnancy in which the embryo adheres to the wall of uterus. At this stage of prenatal development, the embryo is a blastocyst...
, and is known too as a proembryo. A conceptus between fertilization and implantation is also frequently classified as an embryo.
Even after implantation begins, a preembryo (and proembryo) may exist up until formation of the
primitive streakThe primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development.-Formation:The formation of the primitive streak is one of the first signs of gastrulation. It is characterized as a furrow in the midline of the embryonic disk at the...
. Implantation begins at about six days after fertilization, and lasts for about a week, during which time formation of the primitive streak occurs.
Use of the terms preembryo and proembryo, in the context of human development, has drawn criticism from opponents of embryo research, and from scientists who have considered this categorization invalid or unnecessary. One rationale that has been advanced for distinguishing an early fertilized human conceptus from an embryo is that there is a potential for the conceptus to split into identical twins prior to implantation, and so (the argument goes) the conceptus cannot be regarded before implantation as a single human being. However, the conceptus before implantation exhibits self-actuated activity, which has led to the assertion that it is an embryo.
In the United States, a report by the
National Institutes of HealthThe National Institutes of Health is an agency of the United States Department of Health and Human Services and is the primary agency of the United States government responsible for biomedical and health-related research. It consists of 27 separate institutes and centers which includes the Office...
stated that a conceptus could be both a pre-implantation embryo and a pre-embryo at the same time. However,
Ann KiesslingDr. Ann A. Kiessling is an American reproductive biologist and is currently one of the leaders in human parthenogenic stem cell research at The Bedford Stem Cell Research Foundation. She also has an appointment in the Department of Surgery at Harvard Medical School .Dr...
(a leader in stem cell research) has written that those categories are inaccurate.
Ontological status
The illustration shows the ontological status of the preembryo,
embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
, and
fetusA fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth. The plural is fetuses....
:
The colors represent the maternal and paternal genetic contributions.
See also
- Beginning of pregnancy controversy
Controversy over the beginning of pregnancy usually occurs in the context of the abortion debate. Depending on where pregnancy is considered to begin, some methods of birth control or infertility treatment might be considered abortifacient...
- In vitro fertilization
- Pregnancy
Pregnancy is the carrying of one or more offspring, known as a fetus or embryo, inside the uterus of a female. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets. Human pregnancy is the most studied of all mammalian pregnancies. Obstetrics is the surgical field...
- Stem cell controversy
The stem cell controversy is the ethical debate centered only on research involving the creation, usage and destruction of human embryos. Most commonly, this controversy focuses on embryonic stem cells.- Background :...