Embryo#Embryos of plants and animals

{{short description|Multicellular diploid eukaryote in its earliest stage of development}}

{{Infobox embryology

| Name = Embryo

| Latin =

| Image = Embryo 7 weeks after conception.jpg

| Caption = A human embryo seven weeks after conception (nine weeks gestational age)

}}

An embryo is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm cell. The resulting fusion of these two cells produces a single-celled zygote that undergoes many cell divisions that produce cells known as blastomeres. The blastomeres (4-cell stage) are arranged as a solid ball that when reaching a certain size, called a morula, (16-cell stage) takes in fluid to create a cavity called a blastocoel. The structure is then termed a blastula, or a blastocyst in mammals.

The mammalian blastocyst hatches before implantating into the endometrial lining of the womb. Once implanted the embryo will continue its development through the next stages of gastrulation, neurulation, and organogenesis. Gastrulation is the formation of the three germ layers that will form all of the different parts of the body. Neurulation forms the nervous system, and organogenesis is the development of all the various tissues and organs of the body.

A newly developing human is typically referred to as an embryo until the ninth week after conception, when it is then referred to as a fetus. In other multicellular organisms, the word "embryo" can be used more broadly to any early developmental or life cycle stage prior to birth or hatching.

Etymology

First attested in English in the mid-14c., the word embryon derives from Medieval Latin embryo, itself from Greek {{lang|grc|ἔμβρυον}} (embruon), lit. "young one",[https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3De%29%2Fmbruon ἔμβρυον] {{webarchive|url=https://web.archive.org/web/20130531161449/http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3De%29%2Fmbruon |date=2013-05-31 }}, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus which is the neuter of {{lang|grc|ἔμβρυος}} (embruos), lit. "growing in",[https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3De%29%2Fmbruos ἔμβρυος] {{webarchive|url=https://web.archive.org/web/20130531041203/http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3De%29%2Fmbruos |date=2013-05-31 }}, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus from ἐν (en), "in"[https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3De%29n ἐν] {{webarchive|url=https://web.archive.org/web/20130531101258/http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3De%29n |date=2013-05-31 }}, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus and βρύω (bruō), "swell, be full";[https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Dbru%2Fw βρύω] {{webarchive|url=https://web.archive.org/web/20130531101318/http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Dbru%2Fw |date=2013-05-31 }}, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus the proper Latinized form of the Greek term would be embryum.

Development

=Animal embryos=

{{About|is a summary of embryonic development in all types of animals, including humans|information specific to human embryonic development|Human embryonic development|section=yes}}

File:Embryonic development of a salamander, filmed in the 1920s.ogv

File:Wrinkledfrog embryos.jpg) of the wrinkled frog (Rana rugosa)]]

File:Mouse and Snake Embryos.jpg and snake embryos]]

In animals, fertilization begins the process of embryonic development with the creation of a zygote, a single cell resulting from the fusion of gametes (e.g. egg and sperm).{{Cite journal|url=https://opentextbc.ca/biology/chapter/24-6-fertilization-and-early-embryonic-development/|title=24.6. Fertilization and Early Embryonic Development – Concepts of Biology – 1st Canadian Edition|website=opentextbc.ca|date=14 May 2015|access-date=2019-10-30|last1=Molnar|first1=Charles|archive-date=2022-05-31|archive-url=https://web.archive.org/web/20220531143151/https://opentextbc.ca/biology/chapter/24-6-fertilization-and-early-embryonic-development/|url-status=live}} The development of a zygote into a multicellular embryo proceeds through a series of recognizable stages, often divided into cleavage, blastula, gastrulation, and organogenesis.{{Cite journal|last=Gilbert|first=Scott F.|date=2000|title=The Circle of Life: The Stages of Animal Development|url=https://www.ncbi.nlm.nih.gov/books/NBK9981/|journal=Developmental Biology. 6th Edition|language=en|access-date=2019-11-07|archive-date=2022-03-24|archive-url=https://web.archive.org/web/20220324172948/https://www.ncbi.nlm.nih.gov/books/NBK9981/|url-status=live}}

Cleavage is the period of rapid mitotic cell divisions that occur after fertilization. During cleavage, the overall size of the embryo does not change, but the size of individual cells decrease rapidly as they divide to increase the total number of cells.{{Cite web|url=http://11e.devbio.com/wt0102.html|title=DevBio 11e|website=11e.devbio.com|access-date=2019-11-07|archive-date=2019-10-30|archive-url=https://web.archive.org/web/20191030192621/http://11e.devbio.com/wt0102.html|url-status=live}} Cleavage results in a blastula.

Depending on the species, a blastula or blastocyst stage embryo can appear as a ball of cells on top of yolk, or as a hollow sphere of cells surrounding a middle cavity.{{Cite book|title=An Introduction to Embryology|last=Balinsky|first=Boris Ivan|publisher=W.B. Saunders Company|year=1975|isbn=0-7216-1518-X|edition=Fourth}} The embryo's cells continue to divide and increase in number, while molecules within the cells such as RNAs and proteins actively promote key developmental processes such as gene expression, cell fate specification, and polarity.{{Cite journal|last=Heasman|first=Janet|date=2006-04-01|title=Patterning the early Xenopus embryo|journal=Development|language=en|volume=133|issue=7|pages=1205–1217|doi=10.1242/dev.02304|issn=0950-1991|pmid=16527985|doi-access=free}} Before implanting into the uterine wall the embryo is sometimes known as the pre-implantation embryo or pre-implantation conceptus.{{cite journal |last1=Niakan |first1=KK |last2=Han |first2=J |last3=Pedersen |first3=RA |last4=Simon |first4=C |last5=Pera |first5=RA |title=Human pre-implantation embryo development. |journal=Development |date=March 2012 |volume=139 |issue=5 |pages=829–41 |doi=10.1242/dev.060426 |pmid=22318624|pmc=3274351 }} Sometimes this is called the pre-embryo a term employed to differentiate from an embryo proper in relation to embryonic stem cell discourses.{{cite journal |last1=Jones |first1=DG |last2=Telfer |first2=B |title=Before I was an embryo, I was a pre-embryo: or was I? |journal=Bioethics |date=January 1995 |volume=9 |issue=1 |pages=32–49 |doi=10.1111/j.1467-8519.1995.tb00299.x |pmid=11653031}}

Gastrulation is the next phase of embryonic development, and involves the development of two or more layers of cells (germinal layers). Animals that form two layers (such as Cnidaria) are called diploblastic, and those that form three (most other animals, from flatworms to humans) are called triploblastic. During gastrulation of triploblastic animals, the three germinal layers that form are called the ectoderm, mesoderm, and endoderm. All tissues and organs of a mature animal can trace their origin back to one of these layers.{{Cite journal|last1=Favarolo|first1=María Belén|last2=López|first2=Silvia L.|date=2018-12-01|title=Notch signaling in the division of germ layers in bilaterian embryos|journal=Mechanisms of Development|volume=154|pages=122–144|doi=10.1016/j.mod.2018.06.005|pmid=29940277|issn=0925-4773|doi-access=free|hdl=11336/90473|hdl-access=free}} For example, the ectoderm will give rise to the skin epidermis and the nervous system,{{Cite web|url=https://embryo.asu.edu/pages/ectoderm|title=Ectoderm {{!}} The Embryo Project Encyclopedia|website=embryo.asu.edu|language=en|access-date=2019-11-07}} the mesoderm will give rise to the vascular system, muscles, bone, and connective tissues,{{Cite web|url=https://embryo.asu.edu/pages/mesoderm|title=Mesoderm {{!}} The Embryo Project Encyclopedia|website=embryo.asu.edu|language=en|access-date=2019-11-07|archive-date=2024-09-10|archive-url=https://web.archive.org/web/20240910221203/https://embryo.asu.edu/pages/mesoderm|url-status=live}} and the endoderm will give rise to organs of the digestive system and epithelium of the digestive system and respiratory system.{{Cite journal|last1=Zorn|first1=Aaron M.|last2=Wells|first2=James M.|date=2009|title=Vertebrate Endoderm Development and Organ Formation|journal=Annual Review of Cell and Developmental Biology|volume=25|pages=221–251|doi=10.1146/annurev.cellbio.042308.113344|issn=1081-0706|pmc=2861293|pmid=19575677}}{{Cite journal|last1=Nowotschin|first1=Sonja|last2=Hadjantonakis|first2=Anna-Katerina|last3=Campbell|first3=Kyra|date=2019-06-01|title=The endoderm: a divergent cell lineage with many commonalities|journal=Development|language=en|volume=146|issue=11|pages=dev150920|doi=10.1242/dev.150920|issn=0950-1991|pmid=31160415|pmc=6589075}} Many visible changes in embryonic structure happen throughout gastrulation as the cells that make up the different germ layers migrate and cause the previously round embryo to fold or invaginate into a cup-like appearance.

Past gastrulation, an embryo continues to develop into a mature multicellular organism by forming structures necessary for life outside of the womb or egg. As the name suggests, organogenesis is the stage of embryonic development when organs form. During organogenesis, molecular and cellular interactions prompt certain populations of cells from the different germ layers to differentiate into organ-specific cell types.{{Cite web|url=https://embryo.asu.edu/pages/process-eukaryotic-embryonic-development|title=Process of Eukaryotic Embryonic Development {{!}} The Embryo Project Encyclopedia|website=embryo.asu.edu|access-date=2019-11-07|archive-date=2022-08-03|archive-url=https://web.archive.org/web/20220803034656/https://embryo.asu.edu/pages/process-eukaryotic-embryonic-development|url-status=live}} For example, in neurogenesis, a subpopulation of cells from the ectoderm segregate from other cells and further specialize to become the brain, spinal cord, or peripheral nerves.{{Cite journal|last1=Hartenstein|first1=Volker|last2=Stollewerk|first2=Angelika|date=2015-02-23|title=The Evolution of Early Neurogenesis|journal=Developmental Cell|volume=32|issue=4|pages=390–407|doi=10.1016/j.devcel.2015.02.004|pmid=25710527|pmc=5987553|issn=1534-5807}}

The embryonic period varies from species to species. In human development, the term fetus is used instead of embryo after the ninth week after conception,{{Cite web|url=https://www.medicinenet.com/embryo_vs_fetus_differences_week-by-week/article.htm|title=Embryo vs. Fetus: The First 27 Weeks of Pregnancy|website=MedicineNet|language=en|access-date=2019-11-07|archive-date=2022-07-23|archive-url=https://web.archive.org/web/20220723175538/https://www.medicinenet.com/embryo_vs_fetus_differences_week-by-week/article.htm|url-status=live}} whereas in zebrafish, embryonic development is considered finished when a bone called the cleithrum becomes visible.{{Cite journal|last1=Kimmel|first1=Charles B.|last2=Ballard|first2=William W.|last3=Kimmel|first3=Seth R.|last4=Ullmann|first4=Bonnie|last5=Schilling|first5=Thomas F.|s2cid=19327966|date=1995|title=Stages of embryonic development of the zebrafish|journal=Developmental Dynamics|language=en|volume=203|issue=3|pages=253–310|doi=10.1002/aja.1002030302|pmid=8589427|issn=1097-0177|doi-access=free}} In animals that hatch from an egg, such as birds, a young animal is typically no longer referred to as an embryo once it has hatched. In viviparous animals (animals whose offspring spend at least some time developing within a parent's body), the offspring is typically referred to as an embryo while inside of the parent, and is no longer considered an embryo after birth or exit from the parent. However, the extent of development and growth accomplished while inside of an egg or parent varies significantly from species to species, so much so that the processes that take place after hatching or birth in one species may take place well before those events in another. Therefore, according to one textbook, it is common for scientists to interpret the scope of embryology broadly as the study of the development of animals.

=Plant embryos=

File:Ginkgo embryo and gametophyte.jpg seed, showing the embryo]]

Flowering plants (angiosperms) create embryos after the fertilization of a haploid ovule by pollen. The DNA from the ovule and pollen combine to form a diploid, single-cell zygote that will develop into an embryo.{{Cite web|url=https://www.britannica.com/science/seed-plant-reproductive-part|title=seed {{!}} Form, Function, Dispersal, & Germination|website=Encyclopædia Britannica|language=en|access-date=2019-11-09|archive-date=2022-07-11|archive-url=https://web.archive.org/web/20220711150310/https://www.britannica.com/science/seed-plant-reproductive-part|url-status=live}} The zygote, which will divide multiple times as it progresses throughout embryonic development, is one part of a seed. Other seed components include the endosperm, which is tissue rich in nutrients that will help support the growing plant embryo, and the seed coat, which is a protective outer covering. The first cell division of a zygote is asymmetric, resulting in an embryo with one small cell (the apical cell) and one large cell (the basal cell).{{Cite web|url=http://biology.kenyon.edu/courses/biol114/Chap12/Chapter_12A.html|title=Chapter 12A. Plant Development|website=biology.kenyon.edu|access-date=2019-11-09|archive-date=2021-03-08|archive-url=https://web.archive.org/web/20210308084505/http://biology.kenyon.edu/courses/biol114/Chap12/Chapter_12A.html|url-status=live}} The small, apical cell will eventually give rise to most of the structures of the mature plant, such as the stem, leaves, and roots.{{Cite journal|last1=Hove|first1=Colette A. ten|last2=Lu|first2=Kuan-Ju|last3=Weijers|first3=Dolf|date=2015-02-01|title=Building a plant: cell fate specification in the early Arabidopsis embryo|journal=Development|language=en|volume=142|issue=3|pages=420–430|doi=10.1242/dev.111500|issn=0950-1991|pmid=25605778|doi-access=free}} The larger basal cell will give rise to the suspensor, which connects the embryo to the endosperm so that nutrients can pass between them. The plant embryo cells continue to divide and progress through developmental stages named for their general appearance: globular, heart, and torpedo. In the globular stage, three basic tissue types (dermal, ground, and vascular) can be recognized. The dermal tissue will give rise to the epidermis or outer covering of a plant,{{Cite web|url=https://www.ck12.org/book/CK-12-Biology-Advanced-Concepts/section/13.23/|title={{!}} CK-12 Foundation|website=www.ck12.org|access-date=2019-11-09|archive-date=2024-09-10|archive-url=https://web.archive.org/web/20240910221053/https://flexbooks.ck12.org/cbook/ck-12-advanced-biology/section/13.23/primary/lesson/Dermal-Tissue-of-Plants-Advanced-BIO-ADV/|url-status=live}} ground tissue will give rise to inner plant material that functions in photosynthesis, resource storage, and physical support,{{Cite web|url=https://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookglossG.html#ground%20system|title=GLOSSARY G|website=www2.estrellamountain.edu|access-date=2019-11-09|archive-date=2022-06-14|archive-url=https://web.archive.org/web/20220614073946/https://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookglossG.html#ground%20system|url-status=dead}} and vascular tissue will give rise to connective tissue like the xylem and phloem that transport fluid, nutrients, and minerals throughout the plant.{{Cite web|url=https://biologydictionary.net/vascular-tissue/|title=Vascular Tissue|date=2018-05-21|website=Biology Dictionary|language=en-US|access-date=2019-11-09|archive-date=2022-09-09|archive-url=https://web.archive.org/web/20220909043434/https://biologydictionary.net/vascular-tissue/|url-status=live}} In heart stage, one or two cotyledons (embryonic leaves) will form. Meristems (centers of stem cell activity) develop during the torpedo stage, and will eventually produce many of the mature tissues of the adult plant throughout its life. At the end of embryonic growth, the seed will usually go dormant until germination.{{Cite journal|last=Penfield|first=Steven|date=2017-09-11|title=Seed dormancy and germination|journal=Current Biology|language=en|volume=27|issue=17|pages=R874–R878|doi=10.1016/j.cub.2017.05.050|issn=0960-9822|pmid=28898656|doi-access=free|bibcode=2017CBio...27.R874P }} Once the embryo begins to germinate (grow out from the seed) and forms its first true leaf, it is called a seedling or plantlet.{{Cite web|url=https://forages.oregonstate.edu/regrowth/how-does-grass-grow/developmental-phases/vegetative-phase/germination-and-seedling|title=Germination and Seedling Emergence|date=2016-03-28|website=Forage Information System|language=en|access-date=2019-11-09|archive-date=2022-06-16|archive-url=https://web.archive.org/web/20220616125456/https://forages.oregonstate.edu/regrowth/how-does-grass-grow/developmental-phases/vegetative-phase/germination-and-seedling|url-status=live}}

Plants that produce spores instead of seeds, like bryophytes and ferns, also produce embryos. In these plants, the embryo begins its existence attached to the inside of the archegonium on a parental gametophyte from which the egg cell was generated.{{Cite web|url=https://www.anbg.gov.au/bryophyte/life-cycle-in-nutshell.html|title=Life Cycle - in a nutshell - bryophyte|website=www.anbg.gov.au|language=en|access-date=2019-11-14|archive-date=2022-04-18|archive-url=https://web.archive.org/web/20220418220626/https://www.anbg.gov.au/bryophyte/life-cycle-in-nutshell.html|url-status=live}} The inner wall of the archegonium lies in close contact with the "foot" of the developing embryo; this "foot" consists of a bulbous mass of cells at the base of the embryo which may receive nutrition from its parent gametophyte.{{Cite web|url=https://www.britannica.com/science/plant-development|title=Plant development - Nutritional dependence of the embryo|website=Encyclopedia Britannica|language=en|access-date=2019-11-14|archive-date=2022-07-12|archive-url=https://web.archive.org/web/20220712151801/https://www.britannica.com/science/plant-development|url-status=live}} The structure and development of the rest of the embryo varies by group of plants.{{Cite journal|url=https://opentextbc.ca/biology2eopenstax/chapter/bryophytes/|title=Bryophytes – Biology 2e|website=opentextbc.ca|date=5 March 2018|access-date=2019-11-14|last1=Clark|first1=Mary Ann|archive-date=2022-05-03|archive-url=https://web.archive.org/web/20220503161011/https://opentextbc.ca/biology2eopenstax/chapter/bryophytes/|url-status=dead}}

Since all land plants create embryos, they are collectively referred to as embryophytes (or by their scientific name, Embryophyta). This, along with other characteristics, distinguishes land plants from other types of plants, such as algae, which do not produce embryos.{{Cite web|url=http://formosa.ntm.gov.tw/seaweeds/english/a/a1_01.asp|title=What are seaweeds?|website=formosa.ntm.gov.tw|access-date=2019-11-09|archive-date=2019-11-20|archive-url=https://web.archive.org/web/20191120024418/http://formosa.ntm.gov.tw/seaweeds/english/a/a1_01.asp|url-status=dead}}

Research and technology

= Biological processes =

Embryos from numerous plant and animal species are studied in biological research laboratories across the world to learn about topics such as stem cells,{{Citation|title=Chapter 4 - Of Mice and Men: The History of Embryonic Stem Cells|date=2014-01-01|url=http://www.sciencedirect.com/science/article/pii/B9780124115514000040|work=Stem Cells (Second Edition)|pages=69–100|editor-last=Mummery|editor-first=Christine|publisher=Academic Press|doi=10.1016/B978-0-12-411551-4.00004-0|isbn=9780124115514|access-date=2019-11-14|editor2-last=van de Stolpe|editor2-first=Anja|editor3-last=Roelen|editor3-first=Bernard A. J.|editor4-last=Clevers|editor4-first=Hans|archive-date=2022-04-18|archive-url=https://web.archive.org/web/20220418220700/https://www.sciencedirect.com/science/article/pii/B9780124115514000040|url-status=live}} evolution and development,{{Cite journal|last1=Martín-Durán|first1=José M.|last2=Monjo|first2=Francisco|last3=Romero|first3=Rafael|date=2012|title=Planarian embryology in the era of comparative developmental biology|journal=The International Journal of Developmental Biology|volume=56|issue=1–3|pages=39–48|doi=10.1387/ijdb.113442jm|issn=1696-3547|pmid=22450993|doi-access=free}} cell division,{{Cite journal|last1=Kumar|first1=Megha|last2=Pushpa|first2=Kumari|last3=Mylavarapu|first3=Sivaram V. S.|date=July 2015|title=Splitting the cell, building the organism: Mechanisms of cell division in metazoan embryos|journal=IUBMB Life|volume=67|issue=7|pages=575–587|doi=10.1002/iub.1404|issn=1521-6551|pmc=5937677|pmid=26173082}} and gene expression.{{Cite journal|last1=Jukam|first1=David|last2=Shariati|first2=S. Ali M.|last3=Skotheim|first3=Jan M.|date=2017-08-21|title=Zygotic Genome Activation in Vertebrates|journal=Developmental Cell|volume=42|issue=4|pages=316–332|doi=10.1016/j.devcel.2017.07.026|issn=1878-1551|pmc=5714289|pmid=28829942}} Examples of scientific discoveries made while studying embryos that were awarded the Nobel Prize in Physiology or Medicine include the Spemann-Mangold organizer, a group of cells originally discovered in amphibian embryos that give rise to neural tissues,{{Cite web|url=https://embryo.asu.edu/pages/spemann-mangold-organizer|title=Spemann-Mangold Organizer {{!}} The Embryo Project Encyclopedia|website=embryo.asu.edu|access-date=2019-11-14|archive-date=2022-04-02|archive-url=https://web.archive.org/web/20220402153712/https://embryo.asu.edu/pages/spemann-mangold-organizer|url-status=live}} and genes that give rise to body segments discovered in Drosophila fly embryos by Christiane Nüsslein-Volhard and Eric Wieschaus.{{Cite web|url=https://www.nobelprize.org/prizes/medicine/1995/7713-the-nobel-prize-in-physiology-or-medicine-1995/|title=The Nobel Prize in Physiology or Medicine 1995|website=NobelPrize.org|language=en-US|access-date=2019-11-14|archive-date=2022-04-02|archive-url=https://web.archive.org/web/20220402211514/https://www.nobelprize.org/prizes/medicine/1995/7713-the-nobel-prize-in-physiology-or-medicine-1995/|url-status=live}}

= Assisted reproductive technology =

Creating and/or manipulating embryos via assisted reproductive technology (ART) is used for addressing fertility concerns in humans and other animals, and for selective breeding in agricultural species. Between the years 1987 and 2015, ART techniques including in vitro fertilization (IVF) were responsible for an estimated one million human births in the United States alone.{{Cite web|url=https://www.pennmedicine.org/updates/blogs/fertility-blog/2018/march/ivf-by-the-numbers|title=IVF by the Numbers – Penn Medicine|website=www.pennmedicine.org|language=en-US|access-date=2020-04-15|archive-date=2020-04-24|archive-url=https://web.archive.org/web/20200424230537/https://www.pennmedicine.org/updates/blogs/fertility-blog/2018/march/ivf-by-the-numbers|url-status=live}} Other clinical technologies include preimplantation genetic diagnosis (PGD), which can identify certain serious genetic abnormalities, such as aneuploidy, prior to selecting embryos for use in IVF.{{Cite journal|last1=Basille|first1=Claire|last2=Frydman|first2=René|last3=El Aly|first3=Abdelwahab|last4=Hesters|first4=Laetitia|last5=Fanchin|first5=Renato|last6=Tachdjian|first6=Gérard|last7=Steffann|first7=Julie|last8=LeLorc'h|first8=Marc|last9=Achour-Frydman|first9=Nelly|date=July 2009|title=Preimplantation genetic diagnosis: state of the art|journal=European Journal of Obstetrics, Gynecology, and Reproductive Biology|volume=145|issue=1|pages=9–13|doi=10.1016/j.ejogrb.2009.04.004|issn=1872-7654|pmid=19411132}} Some have proposed (or even attempted - see He Jiankui affair) genetic editing of human embryos via CRISPR-Cas9 as a potential avenue for preventing disease;{{Cite web|url=https://www.npr.org/sections/health-shots/2019/02/01/689623550/new-u-s-experiments-aim-to-create-gene-edited-human-embryos|title=New U.S. Experiments Aim To Create Gene-Edited Human Embryos|website=NPR.org|language=en|access-date=2020-04-15|archive-date=2024-09-10|archive-url=https://web.archive.org/web/20240910221054/https://www.npr.org/sections/health-shots/2019/02/01/689623550/new-u-s-experiments-aim-to-create-gene-edited-human-embryos|url-status=live}} however, this has been met with widespread condemnation from the scientific community.{{Cite journal|last1=Cyranoski|first1=David|last2=Ledford|first2=Heidi|date=2018-11-26|title=Genome-edited baby claim provokes international outcry|journal=Nature|language=en|volume=563|issue=7733|pages=607–608|doi=10.1038/d41586-018-07545-0|pmid=30482929|bibcode=2018Natur.563..607C|s2cid=53768039|doi-access=free}}{{Cite magazine|url=https://time.com/5550654/crispr-gene-editing-human-embryos-ban/|title=Experts Are Calling for a Ban on Gene Editing of Human Embryos. Here's Why They're Worried|magazine=Time|language=en|access-date=2020-04-15|archive-date=2022-05-03|archive-url=https://web.archive.org/web/20220503161032/https://time.com/5550654/crispr-gene-editing-human-embryos-ban/|url-status=live}}

ART techniques are also used to improve the profitability of agricultural animal species such as cows and pigs by enabling selective breeding for desired traits and/or to increase numbers of offspring.{{Cite journal|last=Blondin|first=P.|date=January 2016|title=Logistics of large scale commercial IVF embryo production|journal=Reproduction, Fertility, and Development|volume=29|issue=1|pages=32–36|doi=10.1071/RD16317|issn=1031-3613|pmid=28278791}} For example, when allowed to breed naturally, cows typically produce one calf per year, whereas IVF increases offspring yield to 9–12 calves per year.{{Cite web|url=https://ag4impact.org/sid/genetic-intensification/livestock-breeding/embryo-transfer/|title=Agriculture for Impact Embryo Transfer|language=en-US|access-date=2020-04-15|archive-date=2020-07-31|archive-url=https://web.archive.org/web/20200731013759/https://ag4impact.org/sid/genetic-intensification/livestock-breeding/embryo-transfer/|url-status=dead}} IVF and other ART techniques, including cloning via interspecies somatic cell nuclear transfer (iSCNT),{{Cite book|last=Fletcher|first=Amy Lynn|title=Mendel's Ark|chapter=Bio-Interventions: Cloning Endangered Species as Wildlife Conservation|date=2014|pages=49–66|editor-last=Fletcher|editor-first=Amy Lynn|publisher=Springer Netherlands|language=en|doi=10.1007/978-94-017-9121-2_4|isbn=978-94-017-9121-2}} are also used in attempts to increase the numbers of endangered or vulnerable species, such as Northern white rhinos,{{Cite news|first=Ian|last=Sample|url=https://www.theguardian.com/environment/2019/sep/11/scientists-use-ivf-procedures-to-help-save-near-extinct-rhinos|title=Scientists use IVF procedures to help save near-extinct rhinos|date=2019-09-11|work=The Guardian|access-date=2020-04-15|language=en-GB|issn=0261-3077|archive-date=2022-05-03|archive-url=https://web.archive.org/web/20220503161041/https://www.theguardian.com/environment/2019/sep/11/scientists-use-ivf-procedures-to-help-save-near-extinct-rhinos|url-status=live}} cheetahs,{{Cite web|url=https://www.cnn.com/2020/02/24/us/cheetah-cubs-ivf-scn-trnd/index.html|title=Two cheetah cubs were born for the first time by IVF. The breakthrough offers hope for the threatened species|first=Alicia|last=Lee|website=CNN|date=25 February 2020|access-date=2020-04-15|archive-date=2022-05-03|archive-url=https://web.archive.org/web/20220503161032/https://www.cnn.com/2020/02/24/us/cheetah-cubs-ivf-scn-trnd/index.html|url-status=live}} and sturgeons.{{Cite journal|last1=Fatira|first1=Effrosyni|last2=Havelka|first2=Miloš|last3=Labbé|first3=Catherine|last4=Depincé|first4=Alexandra|last5=Iegorova|first5=Viktoriia|last6=Pšenička|first6=Martin|last7=Saito|first7=Taiju|date=2018-04-16|title=Application of interspecific Somatic Cell Nuclear Transfer (iSCNT) in sturgeons and an unexpectedly produced gynogenetic sterlet with homozygous quadruple haploid|journal=Scientific Reports|language=en|volume=8|issue=1|pages=5997|doi=10.1038/s41598-018-24376-1|pmid=29662093|pmc=5902484|bibcode=2018NatSR...8.5997F|issn=2045-2322 |bibcode-access=free }}

= Cryoconservation of plant and animal biodiversity =

Cryoconservation of genetic resources involves collecting and storing the reproductive materials, such as embryos, seeds, or gametes, from animal or plant species at low temperatures in order to preserve them for future use.{{Cite web |url=http://www.fao.org/3/a0399e/A0399E06.htm |title=II. Use of cryopreservation and reproductive technologies for conservation of genetic resources |first1=Sipke Joost |last1=Hiemstra |first2=Tette |last2=van der Lende |first3=Henri |last3=Woelders |first4=Bart |last4=Panis |first5=Maurizio |last5=Lambardi |work=The Role of Biotechnology in Exploring and Protecting Agricultural Genetic Resources |publisher=Food and Agriculture Organization of the United Nations |date=2006 |editor-first1=John |editor-last1=Ruane |editor-first2=Andrea |editor-last2=Sonnino |access-date=2020-04-15 |archive-date=2022-05-03 |archive-url=https://web.archive.org/web/20220503161025/https://www.fao.org/3/a0399e/A0399E06.htm |url-status=live }} Some large-scale animal species cryoconservation efforts include "frozen zoos" in various places around the world, including in the UK's Frozen Ark,{{Cite web |title=The Frozen Ark |url=https://www.frozenark.org/ |url-status=live |archive-url=https://web.archive.org/web/20240414160526/https://www.frozenark.org/ |archive-date=Apr 14, 2024 |website=frozenark.org}} the Breeding Centre for Endangered Arabian Wildlife (BCEAW) in the United Arab Emirates,{{Cite web|url=http://www.bceaw.ae/|title=Breeding Centre for Endangered Arabian Wildlife|website=bceaw.ae|access-date=2020-04-15|archive-date=2020-01-28|archive-url=https://web.archive.org/web/20200128163136/http://www.bceaw.ae/|url-status=live}} and the San Diego Zoo Institute for Conservation in the United States.{{Cite web|url=https://institute.sandiegozoo.org/resources/frozen-zoo%C2%AE|title=Frozen Zoo®|date=2016-01-26|website=San Diego Zoo Institute for Conservation Research|language=en|access-date=2020-04-15|archive-date=2024-09-10|archive-url=https://web.archive.org/web/20240910221053/https://science.sandiegozoo.org/resources/frozen-zoo%C2%AE|url-status=live}}{{Cite web|url=https://www.smithsonianmag.com/science-nature/san-diegos-frozen-zoo-180971276/|title=San Diego's Frozen Zoo Offers Hope for Endangered Species Around the World|website=Smithsonian Magazine|language=en|access-date=2020-04-15|archive-date=2024-09-10|archive-url=https://web.archive.org/web/20240910221053/https://www.smithsonianmag.com/science-nature/san-diegos-frozen-zoo-180971276/|url-status=live}} As of 2018, there were approximately 1,700 seed banks used to store and protect plant biodiversity, particularly in the event of mass extinction or other global emergencies.{{Cite web|url=http://www.independent.co.uk/life-style/gadgets-and-tech/features/seed-vault-doomsday-svalbard-norway-milennium-kew-biodiversity-bank-gene-a8237221.html|title=A vast crypt was built to protect humans from the apocalypse. But doomsday might already be here|date=2018-03-04|website=The Independent|language=en|access-date=2020-04-15|archive-date=2020-11-09|archive-url=https://web.archive.org/web/20201109032059/http://www.independent.co.uk/life-style/gadgets-and-tech/features/seed-vault-doomsday-svalbard-norway-milennium-kew-biodiversity-bank-gene-a8237221.html|url-status=live}} The Svalbard Global Seed Vault in Norway maintains the largest collection of plant reproductive tissue, with more than a million samples stored at {{cvt|-18|C}}.{{Cite web|url=https://www.croptrust.org/our-work/svalbard-global-seed-vault/|title=Svalbard Global Seed Vault|website=Crop Trust|language=en-US|access-date=2020-04-15|archive-date=2019-01-02|archive-url=https://web.archive.org/web/20190102082044/https://www.croptrust.org/our-work/svalbard-global-seed-vault/|url-status=live}}

Fossilized embryos

Fossilized animal embryos are known from the Precambrian, and are found in great numbers during the Cambrian period. Even fossilized dinosaur embryos have been discovered.{{cite news | url=https://www.bbc.com/news/science-environment-22085535 | title=Dinosaur embryo fossils reveal life inside the egg | work=BBC News | access-date=8 August 2015 | author=Morelle, Rebecca | author-link=Rebecca Morelle | url-status=live | archive-url=https://web.archive.org/web/20150924135744/http://www.bbc.com/news/science-environment-22085535 | archive-date=24 September 2015 }}

Notes

External links

[http://php.med.unsw.edu.au/embryology/index.php?title=Main_Page UNSW Embryology - Educational website]
[https://web.archive.org/web/20060206140431/http://raven.zoology.washington.edu/embryos/ A Comparative Embryology Gallery]