EMBRYOLOGY: HISTORY, FIELD OF STUDY AND BRANCHES - BIOLOGY - 2025

The embryology (from the Greek: embryon = fruit in the uterus; logos = treaty), animals (including humans), is the study of all matters relating to the development, since the formation of the zygote to birth.

Development begins when an egg is fertilized by a sperm, forming a zygote. Eggs and sperm are gametes. They are formed by gametogenesis in the ovaries of females and the testes of males.

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The production of gametes happens through a process of cell division called meiosis. In this process, four cells, or gametes, are formed that have half the chromosomes (N = haploid) that a somatic cell has (2N = diploid). The zygote has half the chromosomes from the mother and the other half from the father. Hence, it is diploid.

Knowledge of how normal development of the embryo and fetus occurs, and of the causes of infant defects at birth, are helpful in increasing the likelihood of normal development. For example, it is now possible to correct certain defects of the fetus by surgery.

History of embryology

Embryology in antiquity and up to the Middle Ages

In the year 3000 a. C., the Egyptians thought that the god of the sun, Aten, created a germ in the woman, a seed in the man, and gave life to the baby inside the woman.

In the year 1416 a. C., a Hindu treatise on embryology, written in Sanskrit, described that, one day after the sexual encounter, an embryo (Kalada) is formed, which is followed by the formation of a vesicle (after 7 nights), a firm mass (after one month), the head (after two months) and the limbs (after three months).

Pythagoras (570–495 BC), proposed that the father provided the essential characteristics of the offspring, which is known as "spermism." Hippocrates, 460–377 a. C., stated that the development of the chicken embryo can be similar to that of man.

Aristotle (384–322 BC) wrote a treatise on the embryos of chickens and other animals. Due to this he is considered the founder of embryology.

Claudius Galenus (129–216 BC) wrote a treatise on the formation of the fetus, describing structures such as the placenta, amnion, and allantois.

Samuel-el-Yehudi, ~ 200 AD, described the development of the embryo by distinguishing six stages, from a shapeless embryo to a fetus.

Embryology from the Renaissance to the 18th century

Leonardo da Vinci (1452–1519), by dissecting the uterus of a pregnant woman, made very precise drawings of the fetus.

William Harvey (1578–1657), believed that sperm entered the uterus and metamorphosed, transforming into an egg and then an embryo.

Marcello Malpighi (1628–1694) and Jan Swammerdam (1637–1680), through microscopic observations, provided information that they postulated supported the preformism theory, which proposed that semen contained whole human beings.

Regnier de Graaf (1641–1643), dissected and observed the ovaries of various species of mammals, including humans, describing the corpus luteum (Graafian follicle).

Casper Friedrich Wolff (1733–1794), in his 1759 publication Theory of Generation, argued that the organs of the body do not exist before gestation, but are formed in stages from undifferentiated material.

Lázaro Spallanzani (1729–1799), carried out in vitro fertilization tests in amphibians, and insemination in dogs, concluding that oocytes and semen are necessary to initiate the development of an individual.

Heinrich Christian Pander (1794–1865), observed the early development of chicken embryos, describing the three germ layers: ectoderm, mesoderm, endoderm.

Modern embryology

Karl Ernst von Baer (1792–1876), asserted that semen contained millions of moving cells, which he called spermatozoa. In addition, he discovered the oocytes of the mammalian ovaries, the zygote in the fallopian tubes, and the blastocyst in the uterus. Because of this, he is considered the founder of modern embryology.

Hans Spemann (1869–1941), introduced the concept of induction in the development of the embryo, according to which the identity of certain cells influences the development of other cells in their environment. Spermann received the Nobel Prize in Physiology and Medicine in 1935.

Patrick Steptoe (1913–1988), and Robert Edwards (1925–), were the gynecologists and scientists who made possible the birth of Louise Brown in 1978, the first baby produced by in vitro fertilization.

Edward Lewis (1918–2004), Christiane Nüsslein-Volhard (1942–), and Eric F. Wieschaus (1947–) were awarded the Nobel Prize in physiology and medicine in 1995 for their discovery of genes that control embryonic development.

Ian Wilmut (1944–) and his colleagues were the first to transfer the nucleus of a differentiated adult cell to produce a clone of a mammal, the sheep named Dolly, which was born in 1996.

Branches of embryology

Embryology is divided into general embryology, systemic embryology, descriptive embryology, comparative embryology, experimental embryology, chemical embryology, and teratology.

General embryology

Study of development from fertilization and zygote formation, through blastocyst formation and implantation, embryoblast formation, until embryo formation. These events span eight weeks and are divided into pre-embryonic and embryonic periods.

Systemic embryology

Study of the development of organs and systems during the embryo stage.

Descriptive embryology

Study, from direct observation and description, of the development stages of the embryo.

Comparative embryology

Comparison of the development of embryos of different species of animals. This branch is related to comparative and integrative biology, which gave rise in the nineties to the biology of evolutionary development, known as evo-devo.

Experimental embryology

Experiments with laboratory animals (rats, mice, amphibians, etc.) to study embryonic development.

Chemical embryology

Biochemical study of the blastocyst, the embryo and the fetus until the moment of birth.

Teratology

Study of the effect of infectious agents, chemical substances, radiation and other external factors that alter fetal morphology and function.

Human embryology

In humans, three stages of prenatal development have been described: 1) pre-embryo period, from conception to the second week; 2) period of embryo formation, from the second to the eighth week; 3) fetal period, from the ninth week to birth.

In general, the prenatal development of the human being involves the formation of: 1) embryo; 2) placenta; 3) membranes of the fetus; 4) body cavities and diaphragm; 5) muscular, skeletal, respiratory, cardiovascular, digestive, urinary, reproductive, and nervous systems; 6) head and neck; 7) eyes and ears.

Crucial stages of embryological development

Formation of the embryo, placenta and membranes of the fetus

Once the zygote is formed, it begins to divide through mitosis, and increases the number of cells without increasing their size. The cells of the zygote are called blastomeres. When 12 cells are reached, the morula is formed. Then this forms the blastocyst, which is a hollow sphere filled with fluid.

The blastocyst has an internal cell mass at one pole. It is surrounded by a thin layer of cells called the trophoblast, which is responsible for attaching it to the uterine wall, eventually forming the fetal part of the placenta.

The amniotic and chorionic cavities surround the embryo. Its walls form the membranes of the fetus. The inner cell mass forms, by gastrulation, the disc of a bilaminar embryo, formed by the epiblast (later ectoderm) and the hypoblast (later endoderm). The ectoderm differentiates and forms a third layer: the mesoderm.

The mesoderm forms the bones, the connective tissue, the cartilage, the cardiovascular, lymphatic and reproductive systems, the kidneys, the dermis of the skin, among other structures. The ectoderm forms the nervous system. The endoderm forms the gastrointestinal tract, the lungs, and the respiratory tract.

By eight weeks, most organs and systems have already formed, but are immature.

Formation of body cavities and diaphragm

In the fourth week, the embryo is three-dimensional in shape and exhibits a folding as a result of the formation of the intestinal tube. A coelom, or closed cavity, forms inside the embryo originated by the somatic and visceral layers of the lateral plate of the mesoderm.

The somatic mesodermal layer forms the parietal serous membrane, while the splanchnic mesodermal layer forms the visceral serous membrane. When the embryo folds, the union with the chorionic cavity is lost and a cavity forms that goes from the pelvic region to the thoracic region.

The coelom gives rise to the pericardial, pleural and peritoneal cavities. The transverse septum divides the cavity in two: the thoracic cavity and the abdominal cavity (or the peritoneum). However, communication between both cavities is maintained through pericardioperitoneal canals, which have their own membranes.

The newly named membranes divide the thoracic cavity into the pericardial cavity and the pleural cavity, and are called pleuropericardial folds. From the twenty-first day to the eighth week, cavities are formed.

The diaphragm is formed mainly from the transverse septum and pleuroperitoneal membranes. The transverse septum originates, at the cervical level, around the twenty-second day. It receives its innervation from the C3 – C5 spinal nerves.

Training of the muscular, skeletal, respiratory and cardiovascular systems

Most of the muscle originates from the paraxial mesoderm. Three types of skeletal muscle are formed, smooth and cardiac. Skeletal muscle comes from somites, the somatopleuric layer of the lateral plate, and the neural crest. The smooth muscle of the viscera. The gastrointestinal tract and the heart muscle of the splanchnic mesoderm.

The mesoderm makes up most of the bones and cartilage. The sclerotome cells form the individual vertebrae. In the development of the skull, two parts are formed: neurocranium and viscerocranium. The ribs are formed from the ossification of cartilaginous precursors. The ossification of the long bones marks the end of the embryonic period.

The development of the respiratory system is divided into five stages: 1) embryonic, initial bud and branching; 2) pseudoglandular, full branching; 3) canicular, terminal bronchial tubes; 4) saccular, terminal sacs and capillaries come into contact; 5) alveolar, 8 months, complete development of the blood-air barrier.

The development of the cardiovascular system begins with the formation of the heart tube. Then septation occurs, separation into atria, ventricles, and great vessels. Septation involves the formation of two septa, which are not completely closed until birth.

Formation of the digestive, urinary, reproductive and nervous systems

The development of the digestive system begins when the germ layers of the early embryo fold laterally and cephalocaudally. This pushes the yolk membrane into the embryo, thus forming the intestinal tube, which is divided into anterior (future pharynx), middle (future esophagus), and posterior (future duodenum, intestine, colon, and anal canal).

The urinary and reproductive systems could be considered as one because they have a common embryological origin and they share common channels. Both systems develop from the intermediate mesoderm, which forms the urogenital crest, divided into the nephrogenic cord and the crest of the gonads.

The nephrogenic cord gives rise to the pronephros, mesonephros, and metanephros, which are involved in the formation of the kidneys. The genital system develops from the crest of the gonads. The development of the female or male reproductive system depends on the pair of sex chromosomes.

The nervous system develops in the third week from the ectoderm. Initially, the neural tube is formed, the folds of which form the neural crest. A spinal cord is formed that has three layers: neuroepithelial, mantle, marginal zone. Subsequently, the vesicles of the telencephalon, diencephalon, midbrain, metancephalon, and midbrain are formed.

Head, neck, eye and ear development

Most of the head of the head and neck are formed from the pharyngeal arches, sacs and grooves, as well as from the pharyngeal membranes. These structures form the pharyngeal apparatus and give the embryo its distinctive appearance in the fourth week of development.

The pharyngeal arches are formed by mesomeric mesoderm and cells of the neural crest, which are differentiated, respectively, into: 1) muscles and arteries; 2) bone and connective tissue. The pharyngeal sacs consist of invaginations of the endoderm that border the foregut.

The pharyngeal sulci consist of invaginations of the ectoderm. It is located between the pharyngeal arches. The pharyngeal membranes are made up of ectoderm, mesoderm, and endoderm. They are located between the pharyngeal arches.

The ear consists of: inner ear, middle ear, outer ear. By the fourth week, the inner ear develops from the otic plate of the ectoderm, which invaginates forming the utricular and saccular portions. The middle and external ears are derived from the first pharyngeal arches and from neuroglial cells.

The eyes originate from the optic vesicle, which forms from the lateral part of the forebrain at the beginning of the fourth week.

References

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