Embryology

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History of embryology
)

1 - morula, 2 - blastula
1 - blastula, 2 - gastrula with blastopore; orange - ectoderm, red - endoderm

Embryology (from

congenital disorders that occur before birth, known as teratology
.

Early embryology was proposed by Marcello Malpighi, and known as preformationism, the theory that organisms develop from pre-existing miniature versions of themselves. Aristotle proposed the theory that is now accepted, epigenesis. Epigenesis is the idea that organisms develop from seed or egg in a sequence of steps. Modern embryology developed from the work of Karl Ernst von Baer, though accurate observations had been made in Italy by anatomists such as Aldrovandi and Leonardo da Vinci in the Renaissance.[citation needed]

Comparative embryology

Preformationism and epigenesis

preformation
: the idea that semen contains an embryo – a preformed, miniature infant, or homunculus – that simply becomes larger during development.

The competing explanation of embryonic development was

Spallanzani, Enrico Sertoli, and Mauro Ruscóni. According to epigenesis, the form of an animal emerges gradually from a relatively formless egg. As microscopy
improved during the 19th century, biologists could see that embryos took shape in a series of progressive steps, and epigenesis displaced preformation as the favored explanation among embryologists.

Cleavage

Cleavage is the very beginning steps of a developing embryo. Cleavage refers to the many mitotic divisions that occur after the egg is fertilized by the sperm. The ways in which the cells divide is specific to certain types of animals and may have many forms.

Holoblastic

Rotational cleavage
). In holoblastic cleavage, the entire egg will divide and become the embryo, whereas in meroblastic cleavage, some cells will become the embryo and others will be the yolk sac.

Meroblastic

Discoidal cleavage), or centrolecithal (see: Centrolecithal
).

Basal phyla

Animals that belong to the basal phyla have holoblastic radial cleavage which results in radial symmetry (see: Symmetry in biology). During cleavage, there is a central axis that all divisions rotate about. The basal phyla also has only one to two embryonic cell layers, compared to the three in bilateral animals (endoderm, mesoderm, and ectoderm).

Bilaterians

In

germ layers
) from which all the bodily organs and tissues then develop.

Germ layers

  • The innermost layer, or endoderm, give rise to the digestive organs, the gills, lungs or swim bladder if present, and kidneys or nephrites.
  • The middle layer, or mesoderm, gives rise to the muscles, skeleton if any, and blood system.
  • The outer layer of cells, or ectoderm, gives rise to the nervous system, including the brain, and skin or carapace and hair, bristles, or scales.

Drosophila melanogaster (fruit fly)

Drosophila have been used as a developmental model for many years. The studies that have been conducted have discovered many useful aspects of development that not only apply to fruit flies but other species as well.

Outlined below is the process that leads to cell and tissue differentiation.

  1. Nanos (gene)
    .
  2. Gap genes establish 3 broad segments of the embryo.
  3. Pair-rule genes define 7 segments of the embryo within the confines of the second broad segment that was defined by the gap genes.
  4. Segment-polarity genes define another 7 segments by dividing each of the pre-existing 7 segments into anterior and posterior halves using a gradient of Hedgehog and Wnt
    .
  5. Homeotic (Hox) genes use the 14 segments as pinpoints for specific types of cell differentiation and the histological developments that correspond to each cell type.

Humans

Humans are bilateral animals that have holoblastic rotational cleavage. Humans are also deuterostomes. In regard to humans, the term embryo refers to the ball of dividing cells from the moment the zygote implants itself in the uterus wall until the end of the eighth week after conception. Beyond the eighth week after conception (tenth week of pregnancy), the developing human is then called a fetus.

Evolutionary embryology

Evolutionary embryology is the expansion of comparative embryology by the ideas of Charles Darwin. Similarly to Karl Ernst von Baer's principles that explained why many species often appear similar to one another in early developmental stages, Darwin argued that the relationship between groups can be determined based upon common embryonic and larval structures.

Von Baer's principles

  1. The general features appear earlier in development than do the specialized features.
  2. More specialized characters develop from the more general ones.
  3. The embryo of a given species never resembles the adult form of a lower one.
  4. The embryo of a given species does resemble the embryonic form of a lower one.[1]

Using Darwin's theory evolutionary embryologists have since been able to distinguish between homologous and analogous structures between varying species.

Analogous structures are those that appear to be similar but have no common ancestral derivation.[1]

Origins of modern embryology

Until the birth of modern embryology through observation of the mammalian ovum by Karl Ernst von Baer in 1827, there was no clear scientific understanding of embryology, although later discussions in this article show that some cultures had a fairly refined understanding of some of the principles. Only in the late 1950s when ultrasound was first used for uterine scanning, was the true developmental chronology of human fetus available. Karl Ernst von Baer along with Heinz Christian Pander, also proposed the germ layer theory of development which helped to explain how the embryo developed in progressive steps. Part of this explanation explored why embryos in many species often appear similar to one another in early developmental stages using his four principles.

Modern embryology research

Embryology is central to

cell signalling, its roles in certain diseases and mutations, and its links to stem cell research. Embryology is the key to Gestational Surrogacy
, which is when the sperm of the intended father and egg of intended mother are fused in a lab forming an embryo. This embryo is then put into the surrogate who carries the child to term.

Medical embryology

Medical embryology is used widely to detect abnormalities before birth. 2-5% of babies are born with an observable abnormality and medical embryology explores the different ways and stages that these abnormalities appear in.

teratogens. Common teratogens are alcohol, retinoic acid,[2]
ionizing radiation or hyperthermic stress.

Vertebrate and invertebrate embryology

Many principles of embryology apply to invertebrates as well as to vertebrates. Therefore, the study of invertebrate embryology has advanced the study of vertebrate embryology. However, there are many differences as well. For example, numerous invertebrate species release a larva before development is complete; at the end of the larval period, an animal for the first time comes to resemble an adult similar to its parent or parents. Although invertebrate embryology is similar in some ways for different invertebrate animals, there are also countless variations. For instance, while spiders proceed directly from egg to adult form, many insects develop through at least one larval stage. For decades, a number of so-called normal staging tables were produced for the embryology of particular species, mainly focussing on external developmental characters. As variation in developmental progress makes comparison among species difficult, a character-based Standard Event System was developed, which documents these differences and allows for phylogenetic comparisons among species.[3]

Birth of developmental biology

After the 1950s, with the DNA helical structure being unraveled and the increasing knowledge in the field of molecular biology, developmental biology emerged as a field of study which attempts to correlate the genes with morphological change, and so tries to determine which genes are responsible for each morphological change that takes place in an embryo, and how these genes are regulated.

As of today, human embryology is taught as a cornerstone subject in

medical schools
, as well as in biology and zoology programs at both an undergraduate and graduate level.

History

Ancient Egypt

Knowledge of the placenta goes back at least to ancient Egypt, where it was viewed as the seat of the soul. There was an Egyptian official with the title Opener of the Kings Placenta. An Egyptian text from the time of Akhenaten said that a human originates from the egg that grows in women.[4]

Ancient India

A variety of conceptions on embryology appeared in ancient Asia.[5] Descriptions of the amniotic sac appear in the Bhagavad Gita, Bhagavata Purana,[6] and the Sushruta Samhita. One of the Upanishads known as the Garbhopanisaḍ states that the embryo is "like water in the first night, in seven nights it is like a bubble, at the end of half a month it becomes a ball. At the end of a month it is hardened, in two months the head is formed".[7] The Indian medical tradition in the Ayurveda also has conceptions of embryology from antiquity.[8]

The development of the human embryo is mentioned in the ancient

Buddhist text of Garbhāvakrāntisūtra (1st-4th century CE). It mentions the human gestation period of 38 days. The text describes embryonic development in first three weeks as a liquid part of yogurt and the differentiation of body parts such as arms, leg, feet and head in the third month.[9][7]

Ancient Greece

Pre-Socratic philosophers

Many

Anaxagoras of Clazomenae or Alcmaeon of Croton, says that the milk produced by mammals is analogous to the white of fowl egg. Diogenes of Apollonia said that a mass of flesh forms first, only then followed by the development of bone and nerves. Diogenes recognized that the placenta was a nutritional source for the growing fetus. He also said that the development of males took four months, but that the development of females took five months. He did not think the embryo was alive. Alcmaeon also made some contributions, and he is the first person reported to have practiced dissection. One idea, first stated by Parmenides, was that there was a connection between the right side of the body and the male embryo, and between the left side of the body and the female embryo. According to Democritus and Epicurus, the fetus is nourished at the mouth inside the mother and there are comparable teats that supply this nourishment within the mother's body to the fetus.[10]
Discussion on various views regarding how long it takes for specific parts of the embryo to form appear in an anonymous document known as the Nutriment.

Ancient Greeks discussed whether only the male had a seed which developed into the embryo within the female womb, or both the male and the female each had a seed that made a contribution to the developing embryo. The difficulty that one-seed theorists confronted was to explain the maternal resemblance of the progeny. One issue that two-seed theorists confronted was why the female seed was needed if the male already had a seed. One common solution to this problem was to assert that the female seed was either inferior or inactive. Another question was the origin of the seed. The encephalomyelogenic theory stated that the seed originated from the brain or and/or bone marrow. Later came pangenesis, which asserted the seed was drawn from the whole body in order to explain the general resemblance in the body of the offspring. Later on hematogenous theory developed which asserted that the seed was drawn from the blood. A third question was how or in what form the progeny existed in the seed prior to developing into an embryo and a fetus. According to preformationists, the body of the progeny already existed in a pre-existing but undeveloped form in the seed. Three variants of preformationism were homoiomerous preformationism, anhomoiomerous preformationism, and homuncular preformationism. According to the first, the homoiomerous parts of the body (e.g. humors, bone) already exist pre-formed in the seed. The second held that it was the anhomoiomerous parts that were pre-formed. Finally, the third view held that the whole was already a unified organic thing. Preformationism was not the only view. According to epigenesists, parts of the embryo successively form after conception takes place.[11]

Hippocrates

Some of the most well-known early ideas on embryology come from Hippocrates and the Hippocratic Corpus, where discussion on the embryo is usually given in the context of discussing obstetrics (pregnancy and childbirth). Some of the most relevant Hippocratic texts on embryology include the Regimen on Acute Diseases, On Semen, and On the Development of the Child. Hippocrates claimed that the development of the embryo is put into motion by fire and that nourishment comes from food and breath introduced into the mother. An outer layer of the embryo solidifies, and the fire within consumes humidity which makes way for development of bone and nerve. The fire in the innermost part becomes the belly and air channels are developed in order to route nourishment to it. The enclosed fire also helps form veins and allows for circulation. In this description, Hippocrates aims at describing the causes of development rather than describing what develops. Hippocrates also develops views similar to preformationism, where he claims that all parts of the embryo simultaneously develop. Hippocrates also believed that maternal blood nourishes the embryo. This blood flows and coagulates to help form the flesh of the fetus. This idea was derived from the observation that menstrual blood ceases during pregnancy, which Hippocrates took to imply that it was being redirected to fetal development. Hippocrates also claimed that the flesh differentiates into different organs of the body, and Hippocrates saw as analogous an experiment where a mixture of substances placed into water will differentiate into different layers. Comparing the seed to the embryo, Hippocrates further compared the stalk to the umbilical cord.[12]

Aristotle

Some embryological discussion appears in the writings of Aristotle's predecessor

On the Parts of Animals, On Respiration, and On the Motion of Animals. Means by which we know Aristotle studied embryology, and most likely his predecessors as well, was through studying developing embryos taken out from animals as well as aborted and miscarried human embryos. Aristotle believed that the female supplied the matter for the development of the embryo, formed from the menstrual blood whereas the semen that comes from the male shapes that matter. Aristotle's belief that both the male and female made a contribution to the actual fetus goes against some prior beliefs. According to Aeschylus and some Egyptian traditions, the fetus solely develops from the male contribution and that the female womb simply nourishes this growing fetus. On the other hand, the Melanesians held that the fetus is solely a product of the female contribution. Aristotle did not believe there were any external influences on the development of the embryo. Against Hippocrates, Aristotle believed that new parts of the body developed over time rather than all forming immediately and developing from then on. He also considered whether each new part derives from a previously formed part or develops independently of any previously formed part. On the basis that different parts of the body do not resemble each other, he decided in favor of the latter view. He also described development of fetal parts in terms of mechanical and automatic processes. In terms of the development of the embryo, he says it begins in a liquid-like state as the material secreted by the female combines with the semen of the male, and then the surface begins to solidify as it interacts with processes of heating and cooling. The first part of the body to differentiate is the heart, which Aristotle and many of his contemporaries believed was the location of reason and thinking. Aristotle claimed that vessels join to the uterus in order to supply nourishment to the developing fetus. Some of the most solid parts of the fetus cool and, as they lose moisture to heat, turn into nails, horns, hoofs, beaks, etc. Internal heat dries away moisture and forms sinews and bones and the skin results from drying of the flesh. Aristotle also describes the development of birds in eggs at length. He further described embryonic development in dolphins, some sharks, and many other animals. Aristotle singularly wrote more on embryology than any other pre-modern author, and his influence on the subsequent discussion on the subject for many centuries was immense, introducing into the subject forms of classification, a comparative method from various animals, discussion of the development of sexual characteristics, compared the development of the embryo to mechanistic processes, and so forth.[14]

Later Greek embryology

Reportedly, some

Wisdom of Solomon (7:2) also has the embryo formed from menstrual blood. Soranus of Ephesus also wrote texts on embryology which went into use for a long time. Some rabbinic texts discuss the embryology of a female Greek writer named Cleopatra, a contemporary of Galen and Soranus, who was said to have claimed that the male fetus is complete in 41 days whereas the female fetus is complete in 81 days. Various other texts of less importance also appear and describe various aspects of embryology, though without making much progress from Aristotle. Plutarch has a chapter in one of his works titled "Whether was before, the hen or egg?" Discussion on embryological tradition also appears in many Neoplatonic traditions.[16]

Next to Aristotle, the most impactful and important Greek writer on biology was Galen of Pergamum, and his works were transmitted throughout the Middle Ages. Galen discusses his understanding of embryology in two of his texts, those being his On the Natural Faculties and his On the Formation of the Foetus.[17] There is an additional text spuriously attributed to Galen known as On the Question of whether the Embryo is an Animal. Galen described embryological development in four stages. In the first stage, the semen predominates. In the second stage, the embryo is filled with blood. In the third stage, the main outlines of the organs have developed but various other parts remain undeveloped. In the fourth stage, formation is complete and has reached a stage where we can call it a child. Galen described processes that played a role in furthering development of the embryo such as warming, drying, cooling, and combinations thereof. As this development plays out, the form of life of the embryo also moves from that like a plant to that of an animal (where the analogy between the root and umbilical cord is made). Galen claimed that the embryo forms from menstrual blood, by which his experimental analogy was that when you cut the vein of an animal and allow blood to flow out and into some mildly heated water, a sort of coagulation can be observed. He gave detailed descriptions of the position of the umbilical cord relative to other veins.[18]

Patristics

The question of embryology is discussed among a number of

Augustine, who instead viewed that the animation of life began on the 40th day in males and the 80th day in females but not prior. Before the 40th day for men and 80th day for women, the embryo was referred to as the embryo informatus, and after this period was reached, it was referred to as the embryo formatus. The notion originating from the Greeks that the male embryo developed faster remained in various authors until it was experimentally disproven by Andreas Ottomar Goelicke in 1723.[23]

Various patristic literature from backgrounds ranging from

Monophysite Philoxenus of Mabbug claimed that the soul was created in the body forty days after conception. In his De opificio mundi, the Christian philosopher John Philoponus claimed that the soul is formed after the body. Later still, the author Leontius held that the body and soul were created simultaneously, though it is also possible he held that the soul pre-existed the body.[24]

Some Monophysites and Chalcedonians seemed to have been compelled into accepting synhyparxis in the case of Jesus because of their view that the incarnation of Christ resulted in both one hypostasis and one nature, whereas some Nestorians claimed that Christ, like us, must have had his soul formed after the formation of his body because, per Hebrews 4:15, Christ was like us in all ways but sin. (On the other hand, Leontinus dismissed the relevance of Hebrews 4:15 on the basis that Christ differed from us not only in sinfulness but also conception without semen, making synhyparxis another of Christ's supernatural feats.) They felt comfortable holding this view, under their belief that the human nature of Jesus was separate from the divine hypostasis. Some Nestorians still wondered, however, if the body united with the soul in the moment the soul was created or whether it came with it only later. The Syriac author

Moses Bar Kepha claimed, for Christological reasons as a Monophysite, that only synhyparxis was acceptable. He claimed that Genesis 2:7 has no temporal sequence and that Exodus 21:22 regards the formation of the body and not the soul and so is not relevant. To argue against methyparxis, he reasoned that body and soul are both present at death and, because what is at the end must correspond to what is also at the beginning, conception must also have body and soul together.[24]

Embryology in Jewish tradition

Many Jewish authors also discussed notions of embryology, especially as they appear in the

Philo of Alexandria in the 1st century. Some Talmudic texts discuss magical influences on the development of the embryo, such as one text which claims that if one sleeps on a bed that is pointed to the north–south will have a male child. According to Nachmanides, a child born of a cold drop of semen will be foolish, one born from a warm drop of semen will be passionate and irascible, and one born from a semen drop of medium temperature will be clever and level-headed. Some Talmudic discussions follow from Hippocratic claims that a child born on the eighth month could not survive, whereas others follow Aristotle in claiming that they sometimes could survive. One text even says that survival is possible on the seventh month, but not the eighth. Talmudic embryology, in various aspects, follows Greek discourses especially from Hippocrates and Aristotle, but in other areas, makes novel statements on the subject.[15]

Embryology in the Islamic tradition

Passing reference to embryological notions also appear in the

Qur'an (22:5), where the development of the embryo proceeds in four stages from drop, to a blood clot, to a partially developed stage, to a fully developed child.[25] The notion of clay turning into flesh is seen by some as analogous to a text by Theodoret that describes the same process.[26] The four stages of development in the Qur'an are similar to the four stages of embryological development as described by Galen. In the early 6th century, Sergius of Reshaina devoted himself to the translation of Greek medical texts into Syriac and became the most important figure in this process. Included in his translations were the relevant embryological texts of Galen. Anurshirvan founded a medical school in the southern Mesopotamian city of Gundeshapur, known as the Academy of Gondishapur, which also acted as a medium for the transmission, reception, and development of notions from Greek medicine. These factors helped the transmission of Greek notions on embryology, such as found in Galen, to enter into the Arabian milieu.[27] Very similar embryonic descriptions also appear in the Syriac Jacob of Serugh's letter to the Archdeacon Mar Julian.[19]

Embryological discussions also appear in the Islamic legal tradition.[28]

See also

References

Citations

  1. ^
    OCLC 945169933.{{cite book}}: CS1 maint: location missing publisher (link
    )
  2. .
  3. .
  4. ^ Joseph Needham, A History of Embryology, Cambridge 1959,pp. 19-25.
  5. ^ Andreeva & Streavu (eds.), Transforming the Void: Embryological Discourse and Reproductive Imagery in East Asian Religions, Brill 2015.
  6. ^ The translation of one of the relevant texts may be accessed here.
  7. ^ a b John Wallingford, "Aristotle, Buddhist scripture and embryology in ancient Mexico: building inclusion by re-thinking what counts as the history of developmental biology", Development 2021.
  8. ^ Needham, Joseph (1959). A history of embryology. New York: Abelard-Schuman.
  9. ^ Kritzer, Robert. "Tibetan Texts of Garbhāvakrāntisūtra: Differences and Borrowings".
  10. ^ Joseph Needham, A History of Embryology, Cambridge 1959, pp. 27-31.
  11. ^ a b James Wilberding, "Plato's Embryology," Early Science and Medicine 2015.
  12. ^ Joseph Needham, A History of Embryology, Cambridge 1959, pp. 31-37.
  13. ^ Cera Lawrence. "On the Generation of Animals, by Aristotle". The Embryo Project Encyclopedia. 2010. Accessible.
  14. ^ Joseph Needham, A History of Embryology, Cambridge 1959, pp. 37-60.
  15. ^ a b c Samuel Kottek, "Embryology in Talmudic and Midrashic Literature", Journal of the History of Biology 1981.
  16. ^ James Wilberding, Forms, Souls, and Embryos: Neoplatonists on Human Reproduction, Routledge 2017.
  17. ^ Michael Boylan, Galen's Conception Theory, Journal of the History of Biology 1986.
  18. ^ Joseph Needham, A History of Embryology, Cambridge 1959, pp. 60-74.
  19. ^ a b Yousef Kouriyhe. "Jakob von Sarug (451-521): Brief an den Erzdiakon Mar Julian — Edessa — 451-521 (Syrisch) — Mekka II — TUK_0955". Corpus Coranicum. Available.
  20. ^ Sebastian Brock (translator), Saint Ephrem: Hymns on Paradise, St. Vladimir's Seminary Press 1990, p133.
  21. ^ Peter Kitzler, "Tertullian and Ancient Embryology in De carne Christi 4,1 and 19,3−4," Journal of Ancient Christianity 2014.
  22. ^ W.V. Harris, "Child Exposure in the Roman Empire," Journal of Roman Studies 1994.
  23. ^ Joseph Needham, A History of Embryology, Cambridge 1959, pp. 75-77.
  24. ^ a b Dirk Krausmuller, "When Christology intersects with embryology: the viewpoints of Nestorian, Monophysite and Chalcedonian authors of the sixth to tenth centuries", Byzantinische Zeitschrift 2020.
  25. ^ Joseph Needham, A History of Embryology, Cambridge 1959, pp. 74-82.
  26. ^ Emmanouela Grypeou. "Theodoret von Kyrrhos: Kompendium häretischer Erdichtungen V.9 — Syrien — ca. 450 n.Chr. (Griechisch) — Mekka II — TUK_1235". Corpus Coranicum. Available.
  27. ^ Michael Marx. "Galen von Pergamon (129-199): Galen De Semine I, 8 — Kleinasien und Rom — 2. Jh. n.Chr. (Griechisch) — Mekka II — TUK_0986". Corpus Coranicum. Available.
  28. ^ Ghaly, Mohammed. "Human Embryology in the Islamic Tradition The Jurists of the Post-formative Era in Focus," Islamic Law and Society (2014).

Sources

  • "Germ layer." Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 6 Nov. 2009 <https://www.britannica.com/EBchecked/topic/230597/germ-layer>.
  • Welton, John. "| Circle Surrogacy". Circle Surrogacy, Full Service, Worldwide Surrogate-Parenting Agency, www.circlesurrogacy.com/about/what-is-surrogacy.

Further reading

External links