| Parte de una serie sobre |
| Crecimiento y desarrollo humano |
|---|
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| Etapas |
| Hitos biológicos |
| Desarrollo y psicología |
| Teorías de las etapas del desarrollo |
El desarrollo embrionario humano , o embriogénesis humana , se refiere al desarrollo y formación del embrión humano . Se caracteriza por los procesos de división celular y diferenciación celular del embrión que ocurre durante las primeras etapas del desarrollo. En términos biológicos, el desarrollo del cuerpo humano implica el crecimiento de un cigoto unicelular a un ser humano adulto . La fertilización ocurre cuando el espermatozoide ingresa con éxito y se fusiona con un óvulo(óvulo). El material genético del espermatozoide y el óvulo luego se combinan para formar una sola célula llamada cigoto y comienza la etapa germinal de desarrollo. [1] El desarrollo embrionario en el ser humano cubre las primeras ocho semanas de desarrollo; al comienzo de la novena semana, el embrión se denomina feto . La embriología humana es el estudio de este desarrollo durante las primeras ocho semanas después de la fertilización. El período normal de gestación (embarazo) es de aproximadamente nueve meses o 40 semanas.
La etapa germinal se refiere al tiempo desde la fertilización hasta el desarrollo del embrión temprano hasta que se completa la implantación en el útero . La etapa germinal tarda alrededor de 10 días. [2] Durante esta etapa, el cigoto comienza a dividirse, en un proceso llamado clivaje . Luego se forma un blastocisto y se implanta en el útero . La embriogénesis continúa con la siguiente etapa de gastrulación , cuando las tres capas germinales del embrión se forman en un proceso llamado histogénesis , y siguen los procesos de neurulación y organogénesis .
En comparación con el embrión, el feto tiene características externas más reconocibles y un conjunto más completo de órganos en desarrollo. Todo el proceso de embriogénesis implica cambios espaciales y temporales coordinados en la expresión génica , el crecimiento celular y la diferenciación celular . Un proceso casi idéntico ocurre en otras especies, especialmente entre cordados .
Etapa germinal [ editar ]
Fertilización [ editar ]
La fertilización tiene lugar cuando el espermatozoide ha entrado con éxito en el óvulo y los dos conjuntos de material genético transportados por los gametos se fusionan, dando como resultado el cigoto (una sola célula diploide ). Esto suele tener lugar en la ampolla de una de las trompas de Falopio . El cigoto contiene el material genético combinado transportado por los gametos masculinos y femeninos, que consta de los 23 cromosomas del núcleo del óvulo y los 23 cromosomas del núcleo de los espermatozoides. Los 46 cromosomas experimentan cambios antes de la división mitótica que conduce a la formación del embrión que tiene dos células.
La fertilización exitosa está habilitada por tres procesos, que también actúan como controles para garantizar la especificidad de la especie. El primero es el de la quimiotaxis que dirige el movimiento de los espermatozoides hacia el óvulo. En segundo lugar, existe una compatibilidad adhesiva entre el esperma y el óvulo. Con el esperma adherido al óvulo, tiene lugar el tercer proceso de reacción acrosómica ; la parte frontal de la cabeza del espermatozoide está cubierta por un acrosoma que contiene enzimas digestivas para descomponer la zona pelúcida y permitir su entrada. [3] La entrada de los espermatozoides hace que se libere calcio que bloquea la entrada a otros espermatozoides. En el óvulo tiene lugar una reacción paralela llamadazona de reacción . Esto ve la liberación de gránulos corticales que liberan enzimas que digieren las proteínas receptoras de los espermatozoides, evitando así la poliespermia . Los gránulos también se fusionan con la membrana plasmática y modifican la zona pelúcida de tal manera que se evita una mayor entrada de espermatozoides.
Escote [ editar ]
El comienzo del proceso de escisión se marca cuando el cigoto se divide a través de la mitosis en dos células. Esta mitosis continúa y las dos primeras células se dividen en cuatro células, luego en ocho células y así sucesivamente. Cada división tarda de 12 a 24 horas. El cigoto es grande en comparación con cualquier otra célula y se escinde sin ningún aumento general de tamaño. Esto significa que con cada subdivisión sucesiva, aumenta la proporción de material nuclear a citoplasmático. [4] Inicialmente, las células en división, llamadas blastómeros ( blastos en griego para brote), no se diferencian y se agregan en una esfera encerrada dentro de la membrana de glicoproteínas.(denominada zona pelúcida) del óvulo. Cuando se han formado ocho blastómeros , comienzan a desarrollar uniones gap , lo que les permite desarrollarse de manera integrada y coordinar su respuesta a las señales fisiológicas y las señales ambientales. [5]
Cuando las células suman alrededor de dieciséis, la esfera sólida de células dentro de la zona pelúcida se denomina mórula [6]. En esta etapa, las células comienzan a unirse firmemente en un proceso llamado compactación, y la escisión continúa como diferenciación celular .
Blastulación [ editar ]
La escisión en sí es la primera etapa de la blastulación , el proceso de formación del blastocisto . Las células se diferencian en una capa externa de células (denominada colectivamente trofoblasto ) y una masa celular interna . Con una mayor compactación, los blastómeros externos individuales, los trofoblastos, se vuelven indistinguibles. Todavía están encerrados dentro de la zona pelúcida . Esta compactación sirve para impermeabilizar la estructura, conteniendo el fluido que luego segregarán las células. La masa interna de células se diferencian para convertirse en embrioblastos y se polarizan en un extremo. Se cierran entre sí y forman uniones de espacios., que facilitan la comunicación celular. Esta polarización deja una cavidad, el blastocele , creando una estructura que ahora se denomina blastocisto. (En animales que no son mamíferos, esto se llama blástula ). Los trofoblastos secretan líquido en el blastocele. El aumento resultante en el tamaño del blastocisto hace que eclosione a través de la zona pelúcida, que luego se desintegra. [7] [4]
La masa celular interna dará lugar al pre-embrión , [8] el amnios , el saco vitelino y la alantoides , mientras que la parte fetal de la placenta se formará a partir de la capa externa del trofoblasto. El embrión más sus membranas se llama el conceptus , y por esta etapa el conceptus ha alcanzado el útero . La zona pelúcida finalmente desaparece por completo, y las células ahora expuestas del trofoblasto permiten que el blastocisto se adhiera al endometrio , donde se implantará . La formación del hipoblasto y epiblasto., que son las dos capas principales del disco germinal bilaminar, ocurre al comienzo de la segunda semana. [9] O el embrioblasto o el trofoblasto se convertirán en dos subcapas. [10] Las células internas se convertirán en la capa de hipoblasto, que rodeará la otra capa, llamada epiblasto, y estas capas formarán el disco embrionario que se convertirá en el embrión. [9] [10] El trofoblasto también desarrollará dos subcapas: el citotrofoblasto , que está frente al sincitiotrofoblasto , que a su vez se encuentra dentro del endometrio . [9] A continuación, otra capa llamada membrana exocelómica o membrana de Heuser.aparecerá y rodeará el citotrofoblasto, así como el saco vitelino primitivo. [10] El sincitiotrofoblasto crecerá y entrará en una fase llamada etapa lacunar, en la que aparecerán algunas vacuolas que se llenarán de sangre en los días siguientes. [9] [10] El desarrollo de las saco vitelino aperturas con los hypoblastic células planas que forman la membrana exocoelomic, que recubrir la parte interior de la citotrofoblasto para formar el saco vitelino primitivo. Se formará una erosión del revestimiento endotelial de los capilares maternos por las células sincitiotrofoblásticas de los sinusoides donde la sangre comenzará a penetrar y fluir a través del trofoblasto para dar lugar a la circulación uteroplacentaria. [11] [12]Posteriormente se establecerán nuevas células derivadas del saco vitelino entre el trofoblasto y la membrana exocelómica y darán lugar al mesodermo extraembrionario , que formará la cavidad coriónica . [10]
Al final de la segunda semana de desarrollo, algunas células del trofoblasto penetran y forman columnas redondeadas en el sincitiotrofoblasto. Estas columnas se conocen como vellosidades primarias . Al mismo tiempo, otras células migratorias forman en la cavidad exocelómica una nueva cavidad denominada saco vitelino secundario o definitivo, más pequeño que el saco vitelino primitivo. [10] [11]
Implantación [ editar ]
Después de la ovulación , el revestimiento endometrial se transforma en un revestimiento secretor en preparación para aceptar el embrión. Se engrosa, sus glándulas secretoras se alargan y es cada vez más vascular . Este revestimiento de la cavidad uterina (o matriz) ahora se conoce como decidua y produce una gran cantidad de células deciduales grandes en su tejido interglandular aumentado. Los blastómeros en el blastocisto están dispuestos en una capa externa llamada trofoblasto . El trofoblasto luego se diferencia en una capa interna, el citotrofoblasto , y una capa externa, el sincitiotrofoblasto . El citotrofoblasto contiene células epiteliales cuboidales y es la fuente de células en división , y el sincitiotrofoblasto es una capa sincitial sin límites celulares.
El sincitiotrofoblasto implanta el blastocisto en el epitelio decidual mediante proyecciones de vellosidades coriónicas , formando la parte embrionaria de la placenta. La placenta se desarrolla una vez que se implanta el blastocisto, conectando el embrión a la pared uterina. La decidua aquí se denomina decidua basalis; se encuentra entre el blastocisto y el miometrio y forma la parte materna de la placenta . La implantación es asistida por enzimas hidrolíticas que erosionan el epitelio . El sincitiotrofoblasto también produce gonadotropina coriónica humana , una hormona que estimula la liberación deprogesterona del cuerpo lúteo . La progesterona enriquece el útero con un revestimiento grueso de vasos sanguíneos y capilares para que pueda oxigenar y sostener el embrión en desarrollo. El útero libera azúcar del glucógeno almacenado de sus células para nutrir al embrión . [13] Las vellosidades comienzan a ramificarse y contienen vasos sanguíneos del embrión. Otras vellosidades, llamadas vellosidades terminales o libres, intercambian nutrientes. El embrión está unido al caparazón trofoblástico por un tallo de conexión estrecho que se desarrolla en el cordón umbilical para unir la placenta al embrión. [10] [14]Las arterias de la decidua se remodelan para aumentar el flujo sanguíneo materno hacia los espacios intervellosos de la placenta, lo que permite el intercambio de gases y la transferencia de nutrientes al embrión. Los productos de desecho del embrión se difundirán a través de la placenta.
A medida que el sincitiotrofoblasto comienza a penetrar en la pared uterina, también se desarrolla la masa celular interna (embrioblasto). La masa celular interna es la fuente de células madre embrionarias , que son pluripotentes y pueden convertirse en cualquiera de las tres células de la capa germinal, y que tienen la potencia para dar lugar a todos los tejidos y órganos.
Disco embrionario [ editar ]
El embrioblasto forma un disco embrionario , que es un disco bilaminar de dos capas, una capa superior llamada epiblasto ( ectodermo primitivo ) y una capa inferior llamada hipoblasto ( endodermo primitivo ). El disco se estira entre lo que se convertirá en la cavidad amnióticay el saco vitelino. El epiblasto está adyacente al trofoblasto y está formado por células columnares; el hipoblasto es el más cercano a la cavidad del blastocisto y está formado por células cuboideas. El epiblasto migra desde el trofoblasto hacia abajo, formando la cavidad amniótica, cuyo revestimiento está formado por amnioblastos desarrollados a partir del epiblasto. El hipoblasto se empuja hacia abajo y forma el revestimiento del saco vitelino (cavidad exocelómica). Algunas células de hipoblastos migran a lo largo del revestimiento interno de citotrofoblasto del blastocele, secretando una matriz extracelular a lo largo del camino. Estas células hipoblásticas y matriz extracelular se denominan membrana de Heuser.(o la membrana exocelómica), y cubren el blastocele para formar el saco vitelino (o cavidad exocelómica). Las células del hipoblasto migran a lo largo de los bordes externos de este retículo y forman el mesodermo extraembrionario; esto altera el retículo extraembrionario. Pronto se forman bolsas en el retículo, que finalmente se fusionan para formar la cavidad coriónica (celoma extraembrionario).
Gastrulación [ editar ]
The primitive streak, a linear band of cells formed by the migrating epiblast, appears, and this marks the beginning of gastrulation, which takes place around the seventeenth day (week 3) after fertilisation. The process of gastrulation reorganises the two-layer embryo into a three-layer embryo, and also gives the embryo its specific head-to-tail, and front-to-back orientation, by way of the primitive streak which establishes bilateral symmetry. A primitive node (or primitive knot) forms in front of the primitive streak which is the organiser of neurulation. A primitive pit forms as a depression in the centre of the primitive node which connects to the notochord which lies directly underneath. The node has arisen from epiblasts of the amniotic cavity floor, and it is this node that induces the formation of the neural plate which serves as the basis for the nervous system. The neural plate will form opposite the primitive streak from ectodermal tissue which thickens and flattens into the neural plate. The epiblast in that region moves down into the streak at the location of the primitive pit where the process called ingression, which leads to the formation of the mesoderm takes place. This ingression sees the cells from the epiblast move into the primitive streak in an epithelial-mesenchymal transition; epithelial cells become mesenchymal stem cells, multipotent stromal cells that can differentiate into various cell types. The hypoblast is pushed out of the way and goes on to form the amnion. The epiblast keeps moving and forms a second layer, the mesoderm. The epiblast has now differentiated into the three germ layers of the embryo, so that the bilaminar disc is now a trilaminar disc, the gastrula.
The three germ layers are the ectoderm, mesoderm and endoderm, and are formed as three overlapping flat discs. It is from these three layers that all the structures and organs of the body will be derived through the processes of somitogenesis, histogenesis and organogenesis.[15] The embryonic endoderm is formed by invagination of epiblastic cells that migrate to the hypoblast, while the mesoderm is formed by the cells that develop between the epiblast and endoderm. In general, all germ layers will derive from the epiblast.[10][14] The upper layer of ectoderm will give rise to the outermost layer of skin, central and peripheral nervous systems, eyes, inner ear, and many connective tissues.[16] The middle layer of mesoderm will give rise to the heart and the beginning of the circulatory system as well as the bones, muscles and kidneys. The inner layer of endoderm will serve as the starting point for the development of the lungs, intestine, thyroid, pancreas and bladder.
Following ingression, a blastopore develops where the cells have ingressed, in one side of the embryo and it deepens to become the archenteron, the first formative stage of the gut. As in all deuterostomes, the blastopore becomes the anus whilst the gut tunnels through the embryo to the other side where the opening becomes the mouth. With a functioning digestive tube, gastrulation is now completed and the next stage of neurulation can begin.
Neurulation[edit]
Following gastrulation, the ectoderm gives rise to epithelial and neural tissue, and the gastrula is now referred to as the neurula. The neural plate that has formed as a thickened plate from the ectoderm, continues to broaden and its ends start to fold upwards as neural folds. Neurulation refers to this folding process whereby the neural plate is transformed into the neural tube, and this takes place during the fourth week. They fold, along a shallow neural groove which has formed as a dividing median line in the neural plate. This deepens as the folds continue to gain height, when they will meet and close together at the neural crest. The cells that migrate through the most cranial part of the primitive line form the paraxial mesoderm, which will give rise to the somitomeres that in the process of somitogenesis will differentiate into somites that will form the sclerotomes, the syndetomes,[17] the myotomes and the dermatomes to form cartilage and bone, tendons, dermis (skin), and muscle. The intermediate mesoderm gives rise to the urogenital tract and consists of cells that migrate from the middle region of the primitive line. Other cells migrate through the caudal part of the primitive line and form the lateral mesoderm, and those cells migrating by the most caudal part contribute to the extraembryonic mesoderm.[10][14]
The embryonic disc begins flat and round, but eventually elongates to have a wider cephalic part and narrow-shaped caudal end.[9] At the beginning, the primitive line extends in cephalic direction and 18 days after fertilization returns caudally until it disappears. In the cephalic portion, the germ layer shows specific differentiation at the beginning of the 4th week, while in the caudal portion it occurs at the end of the 4th week.[10] Cranial and caudal neuropores become progressively smaller until they close completely (by day 26) forming the neural tube.[18]
Development of organs and organ systems[edit]
Organogenesis is the development of the organs that begins during the third to eighth week, and continues until birth. Sometimes full development, as in the lungs, continues after birth. Different organs take part in the development of the many organ systems of the body.
Blood[edit]
Haematopoietic stem cells that give rise to all the blood cells develop from the mesoderm. The development of blood formation takes place in clusters of blood cells, known as blood islands, in the yolk sac. Blood islands develop outside the embryo, on the umbilical vesicle, allantois, connecting stalk, and chorion, from mesodermal hemangioblasts.
In the centre of a blood island, hemangioblasts form the haematopoietic stem cells that are the precursor to all types of blood cell. In the periphery of a blood island the hemangioblasts differentiate into angioblasts the precursors to the blood vessels.[19]
Heart and circulatory system[edit]
The heart is the first functional organ to develop and starts to beat and pump blood at around 22 days.[20] Cardiac myoblasts and blood islands in the splanchnopleuric mesenchyme on each side of the neural plate, give rise to the cardiogenic region.[10]:165This is a horseshoe-shaped area near to the head of the embryo. By day 19, following cell signalling, two strands begin to form as tubes in this region, as a lumen develops within them. These two endocardial tubes grow and by day 21 have migrated towards each other and fused to form a single primitive heart tube, the tubular heart. This is enabled by the folding of the embryo which pushes the tubes into the thoracic cavity.[21]
Also at the same time that the endocardial tubes are forming, vasculogenesis (the development of the circulatory system) has begun. This starts on day 18 with cells in the splanchnopleuric mesoderm differentiating into angioblasts that develop into flattened endothelial cells. These join to form small vesicles called angiocysts which join up to form long vessels called angioblastic cords. These cords develop into a pervasive network of plexuses in the formation of the vascular network. This network grows by the additional budding and sprouting of new vessels in the process of angiogenesis.[21] Following vasculogenesis and the development of an early vasculature, a stage of vascular remodelling takes place.
The tubular heart quickly forms five distinct regions. From head to tail, these are the infundibulum, bulbus cordis, primitive ventricle, primitive atrium, and the sinus venosus. Initially, all venous blood flows into the sinus venosus, and is propelled from tail to head to the truncus arteriosus. This will divide to form the aorta and pulmonary artery; the bulbus cordis will develop into the right (primitive) ventricle; the primitive ventricle will form the left ventricle; the primitive atrium will become the front parts of the left and right atria and their appendages, and the sinus venosus will develop into the posterior part of the right atrium, the sinoatrial node and the coronary sinus.[20]
Cardiac looping begins to shape the heart as one of the processes of morphogenesis, and this completes by the end of the fourth week. Programmed cell death (apoptosis) at the joining surfaces enables fusion to take place.[21]In the middle of the fourth week, the sinus venosus receives blood from the three major veins: the vitelline, the umbilical and the common cardinal veins.
During the first two months of development, the interatrial septum begins to form. This septum divides the primitive atrium into a right and a left atrium. Firstly it starts as a crescent-shaped piece of tissue which grows downwards as the septum primum. The crescent shape prevents the complete closure of the atria allowing blood to be shunted from the right to the left atrium through the opening known as the ostium primum. This closes with further development of the system but before it does, a second opening (the ostium secundum) begins to form in the upper atrium enabling the continued shunting of blood.[21]
A second septum (the septum secundum) begins to form to the right of the septum primum. This also leaves a small opening, the foramen ovale which is continuous with the previous opening of the ostium secundum. The septum primum is reduced to a small flap that acts as the valve of the foramen ovale and this remains until its closure at birth. Between the ventricles the septum inferius also forms which develops into the muscular interventricular septum.[21]
Digestive system[edit]
The digestive system starts to develop from the third week and by the twelfth week, the organs have correctly positioned themselves.
Respiratory system[edit]
The respiratory system develops from the lung bud, which appears in the ventral wall of the foregut about four weeks into development. The lung bud forms the trachea and two lateral growths known as the bronchial buds, which enlarge at the beginning of the fifth week to form the left and right main bronchi. These bronchi in turn form secondary (lobar) bronchi; three on the right and two on the left (reflecting the number of lung lobes). Tertiary bronchi form from secondary bronchi.
While the internal lining of the larynx originates from the lung bud, its cartilages and muscles originate from the fourth and sixth pharyngeal arches.[22]
Urinary system[edit]
Kidneys[edit]
Three different kidney systems form in the developing embryo: the pronephros, the mesonephros and the metanephros. Only the metanephros develops into the permanent kidney. All three are derived from the intermediate mesoderm.
Pronephros[edit]
The pronephros derives from the intermediate mesoderm in the cervical region. It is not functional and degenerates before the end of the fourth week.
Mesonephros[edit]
The mesonephros derives from intermediate mesoderm in the upper thoracic to upper lumbar segments. Excretory tubules are formed and enter the mesonephric duct, which ends in the cloaca. The mesonephric duct atrophies in females, but participate in development of the reproductive system in males.
Metanephros[edit]
The metanephros appears in the fifth week of development. An outgrowth of the mesonephric duct, the ureteric bud, penetrates metanephric tissue to form the primitive renal pelvis, renal calyces and renal pyramids. The ureter is also formed.
Bladder and urethra[edit]
Between the fourth and seventh weeks of development, the urorectal septum divides the cloaca into the urogenital sinus and the anal canal. The upper part of the urogenital sinus forms the bladder, while the lower part forms the urethra.[22]
Reproductive system[edit]
Integumentary system[edit]
The superficial layer of the skin, the epidermis, is derived from the ectoderm. The deeper layer, the dermis, is derived from mesenchyme.
The formation of the epidermis begins in the second month of development and it acquires its definitive arrangement at the end of the fourth month. The ectoderm divides to form a flat layer of cells on the surface known as the periderm. Further division forms the individual layers of the epidermis.
The mesenchyme that will form the dermis is derived from three sources:
- The mesenchyme that forms the dermis in the limbs and body wall derives from the lateral plate mesoderm
- The mesenchyme that forms the dermis in the back derives from paraxial mesoderm
- The mesenchyme that forms the dermis in the face and neck derives from neural crest cells[22]
Nervous system[edit]
Late in the fourth week, the superior part of the neural tube bends ventrally as the cephalic flexure at the level of the future midbrain—the mesencephalon. Above the mesencephalon is the prosencephalon (future forebrain) and beneath it is the rhombencephalon (future hindbrain).
Cranial neural crest cells migrate to the pharyngeal arches as neural stem cells, where they develop in the process of neurogenesis into neurons.
The optical vesicle (which eventually becomes the optic nerve, retina and iris) forms at the basal plate of the prosencephalon. The alar plate of the prosencephalon expands to form the cerebral hemispheres (the telencephalon) whilst its basal plate becomes the diencephalon. Finally, the optic vesicle grows to form an optic outgrowth.
Development of physical features[edit]
Face and neck[edit]
This section needs expansion. You can help by adding to it. (November 2017) |
From the third to the eighth week the face and neck develop.
Ears[edit]
The inner ear, middle ear and outer ear have distinct embryological origins.
Inner ear[edit]
At about 22 days into development, the ectoderm on each side of the rhombencephalon thickens to form otic placodes. These placodes invaginate to form otic pits, and then otic vesicles. The otic vesicles then form ventral and dorsal components.
The ventral component forms the saccule and the cochlear duct. In the sixth week of development the cochlear duct emerges and penetrates the surrounding mesenchyme, travelling in a spiral shape until it forms 2.5 turns by the end of the eighth week. The saccule is the remaining part of the ventral component. It remains connected to the cochlear duct via the narrow ductus reuniens.
The dorsal component forms the utricle and semicircular canals.
Middle ear[edit]
The tympanic cavity and eustachian tube are derived from the first pharyngeal pouch (a cavity lined by endoderm). The distal part of the cleft, the tubotympanic recess, widens to create the tympanic cavity. The proximal part of the cleft remains narrow and creates the eustachian tube.
The bones of the middle ear, the ossicles, derive from the cartilages of the pharyngeal arches. The malleus and incus derive from the cartilage of the first pharyngeal arch, whereas the stapes derives from the cartilage of the second pharyngeal arch.
Outer ear[edit]
The external auditory meatus develops from the dorsal portion of the first pharyngeal cleft. Six auricular hillocks, which are mesenchymal proliferations at the dorsal aspects of the first and second pharyngeal arches, form the auricle of the ear.[22]
Eyes[edit]
The eyes begin to develop from the third week to the tenth week.
Limbs[edit]
This section needs expansion. You can help by adding to it. (November 2017) |
At the end of the fourth week limb development begins. Limb buds appear on the ventrolateral aspect of the body. They consist of an outer layer of ectoderm and an inner part consisting of mesenchyme which is derived from the parietal layer of lateral plate mesoderm. Ectodermal cells at the distal end of the buds form the apical ectodermal ridge, which creates an area of rapidly proliferating mesenchymal cells known as the progress zone. Cartilage (some of which ultimately becomes bone) and muscle develop from the mesenchyme.[22]
Clinical significance[edit]
Toxic exposures in the embryonic period can be the cause of major congenital malformations, since the precursors of the major organ systems are now developing.
Each cell of the preimplantation embryo has the potential to form all of the different cell types in the developing embryo. This cell potency means that some cells can be removed from the preimplantation embryo and the remaining cells will compensate for their absence. This has allowed the development of a technique known as preimplantation genetic diagnosis, whereby a small number of cells from the preimplantation embryo created by IVF, can be removed by biopsy and subjected to genetic diagnosis. This allows embryos that are not affected by defined genetic diseases to be selected and then transferred to the mother's uterus.
Sacrococcygeal teratomas, tumours formed from different types of tissue, that can form, are thought to be related to primitive streak remnants, which ordinarily disappear.[9][10][12]
First arch syndromes are congenital disorders of facial deformities, caused by the failure of neural crest cells to migrate to the first pharyngeal arch.
Spina bifida a congenital disorder is the result of the incomplete closure of the neural tube.
Vertically transmitted infections can be passed from the mother to the unborn child at any stage of its development.
Hypoxia a condition of inadequate oxygen supply can be a serious consequence of a preterm or premature birth.
See also[edit]
- Embryo loss
- Aorta-gonad-mesonephros
- CDX2
- Development of the reproductive system
- Development of the urinary system
- Developmental biology
- Drosophila embryogenesis
- Embryomics
- Gonadogenesis
- Human tooth development
- List of human cell types derived from the germ layers
- Potential person
- Recapitulation theory
Additional images[edit]
Representing different stages of embryogenesis
Early stage of the gastrulation process
Phase of the gastrulation process
Top of the form of the embryo
Establishment of embryo medium
Spinal cord at 5 weeks
Head and neck at 32 days
References[edit]
- ^ Shrek, S (1 December 2013). "Prenatal Development Definition and Patient Education". Archived from the original on 1 December 2013. Retrieved 21 April 2020.
- ^ "germinal stage". Mosby's Medical Dictionary, 8th edition. Elsevier. Retrieved 6 October 2013.
- ^ "acrosome definition - Dictionary - MSN Encarta". Archived from the original on 2009-10-31. Retrieved 2007-08-15.
- ^ a b Forgács, G.; Newman, Stuart A. (2005). "Cleavage and blastula formation". Biological physics of the developing embryo. Cambridge University Press. p. 27. ISBN 978-0-521-78337-8.
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External links[edit]
 | Wikimedia Commons has media related to Human embryos. |
- Photo of blastocyst in utero
- Slideshow: In the Womb
- Online course in embryology for medicine students developed by the universities of Fribourg, Lausanne and Bern
