Polarity in embryogenesis

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An oocyte with poles depicted Oocyte Poles.jpg
An oocyte with poles depicted

In developmental biology, an embryo is divided into two hemispheres: the animal pole and the vegetal pole within a blastula.


The animal pole consists of small cells that divide rapidly, in contrast with the vegetal pole below it. In some cases, the animal pole is thought to differentiate into the later embryo itself, forming the three primary germ layers and participating in gastrulation. In the frog Xenopus laevis the animal pole is heavily pigmented while the vegetal pole remains unpigmented. [1]

The vegetal pole contains large yolky cells that divide very slowly, in contrast with the animal pole above it. In some cases, the vegetal pole is thought to differentiate into the extraembryonic membranes that protect and nourish the developing embryo, such as the placenta in mammals and the chorion in birds.

The development of the animal-vegetal axis occurs prior to fertilization. [2] Sperm entry can occur anywhere in the animal hemisphere. [3] The point of sperm entry defines the dorso-ventral axis - cells opposite the region of sperm entry will eventually form the dorsal portion of the body. [2] [4]

In the frog Xenopus laevis , a pigment pattern provides the oocyte with features of a radially symmetrical body with a distinct polarity. The animal hemisphere is dark brown, and the vegetal hemisphere is only weakly pigmented. The axis of symmetry passes through on one side the animal pole, and on the other side the vegetal pole. The two hemispheres are separated by an unpigmented equatorial belt. Polarity has a major influence on the emergence of the embryonic structures. In fact, the axis polarity serves as one coordinate of geometrical system in which early embryogenesis is organized. [5]


The animal pole draws its name from its liveliness relative to the slowly developing vegetal pole. Hence the vegetal pole is named for its relative inactivity relative to the animal pole.

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Developmental biology is the study of the process by which animals and plants grow and develop. Developmental biology also encompasses the biology of regeneration, asexual reproduction, metamorphosis, and the growth and differentiation of stem cells in the adult organism.

Embryology Branch of biology studying prenatal biology

Embryology is the branch of biology that studies the prenatal development of gametes, fertilization, and development of embryos and fetuses. Additionally, embryology encompasses the study of congenital disorders that occur before birth, known as teratology.


Blastulation is the stage in early animal embryonic development that produces the blastula. The blastula (from Greek βλαστός is a hollow sphere of cells surrounding an inner fluid-filled cavity. Embryonic development begins with a sperm fertilizing an egg cell to become a zygote, which undergoes many cleavages to develop into a ball of cells called a morula. Only when the blastocoel is formed does the early embryo become a blastula. The blastula precedes the formation of the gastrula in which the germ layers of the embryo form.

Gastrulation Stage in embryonic development in which germ layers form

In developmental biology, gastrulation is a phase early in the embryonic development of most animals, during which the blastula is reorganized into a multilayered structure known as the gastrula. Before gastrulation, the embryo is a continuous epithelial sheet of cells; by the end of gastrulation, the embryo has begun differentiation to establish distinct cell lineages, set up the basic axes of the body, and internalized one or more cell types including the prospective gut.

Blastocyst Structure formed around day 5 of mammalian embryonic development

The blastocyst is a structure formed in the early development of mammals. It possesses an inner cell mass (ICM) which subsequently forms the embryo. The outer layer of the blastocyst consists of cells collectively called the trophoblast. This layer surrounds the inner cell mass and a fluid-filled cavity known as the blastocoel. The trophoblast gives rise to the placenta. The name "blastocyst" arises from the Greek βλαστός blastos and κύστις kystis. In other animals this is called a blastula.

<i>Drosophila</i> embryogenesis Embryogenesis of the fruit fly Drosophila, a popular model system

Drosophila embryogenesis, the process by which Drosophila embryos form, is a favorite model system for genetics and developmental biology. The study of its embryogenesis unlocked the century-long puzzle of how development was controlled, creating the field of evolutionary developmental biology. The small size, short generation time, and large brood size make it ideal for genetic studies. Transparent embryos facilitate developmental studies. Drosophila melanogaster was introduced into the field of genetic experiments by Thomas Hunt Morgan in 1909.


A blastocoel, also spelled blastocoele and blastocele, and also called blastocyst cavity is a fluid-filled cavity that forms in the blastula (blastocyst) of early amphibian and echinoderm embryos, or between the epiblast and hypoblast of avian, reptilian, and mammalian blastoderm-stage embryos.

Organogenesis is the phase of embryonic development that starts at the end of gastrulation and continues until birth. During organogenesis, the three germ layers formed from gastrulation form the internal organs of the organism.

Embryonic development

In developmental biology, embryonic development, also known as embryogenesis, is the development of an animal or plant embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm cell, (spermatozoon). Once fertilized, the ovum becomes a single diploid cell known as a zygote. The zygote undergoes mitotic divisions with no significant growth and cellular differentiation, leading to development of a multicellular embryo after passing through an organizational checkpoint during mid-embryogenesis. In mammals, the term refers chiefly to the early stages of prenatal development, whereas the terms fetus and fetal development describe later stages.

In developmental biology, cleavage is the division of cells in the early embryo. The process follows fertilization, with the transfer being triggered by the activation of a cyclin-dependent kinase complex. The zygotes of many species undergo rapid cell cycles with no significant overall growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a compact mass called the morula. Cleavage ends with the formation of the blastula.


Epiboly describes one of the five major types of cell movements that occur in the Gastrulation stage of embryonic development of some organisms. Epibolic movement is the way in which a layer epithelial cells spreads. This can be achieved in multiple ways.

In the field of developmental biology, regional differentiation is the process by which different areas are identified in the development of the early embryo. The process by which the cells become specified differs between organisms.

Convergent extension The morphogenetic process in which an epithelium narrows along one axis and lengthens in a perpendicular axis.

Convergent extension (CE), sometimes called convergence and extension (C&E), is the process by which the tissue of an embryo is restructured to converge (narrow) along one axis and extend (elongate) along a perpendicular axis by cellular movement. An example of this process is where the anteroposterior axis becomes longer as the lateral tissues move in towards the dorsal midline. This process plays a crucial role in shaping the body plan during embryogenesis and occurs during gastrulation, neurulation, axis elongation, and organogenesis in both vertebrate and invertebrate embryos. In chordate animals, this process is utilized within a vast population of cells; from the smaller populations in the notochord of the sea squirt (ascidian) to the larger populations of the dorsal mesoderm and neural ectoderm of frogs (Xenopus) and fish. Many characteristics of convergent extension are conserved in the teleost fish, the bird, and very likely within mammals at the molecular, cellular, and tissue level.

Fish development

The development of fishes is unique in some specific aspects compared to the development of other animals.

Symmetry breaking in biology is the process by which uniformity is broken, or the number of points to view invariance are reduced, to generate a more structured and improbable state. That is to say, symmetry breaking is the event where symmetry along a particular axis is lost to establish a polarity. Polarity is a measure for a biological system to distinguish poles along an axis. This measure is important because it is the first step to building complexity. For example, during organismal development, one of the first steps for the embryo is to distinguish its dorsal-ventral axis. The symmetry-breaking event that occurs here will determine which end of this axis will be the ventral side, and which end will be the dorsal side. Once this distinction is made, then all the structures that are located along this axis can develop at the proper location. As an example, during human development, the embryo needs to establish where is ‘back’ and where is ‘front’ before complex structures, such as the spine and lungs, can develop in the right location. This relationship between symmetry breaking and complexity was articulated by P.W. Anderson. He speculated that increasing levels of broken symmetry in many-body systems correlates with increasing complexity and functional specialization. In a biological perspective, the more complex an organism is, the higher number of symmetry-breaking events can be found. Without symmetry breaking, building complexity in organisms would be very difficult.

Vegetal rotation

Vegetal rotation is a morphogenetic movement that drives mesoderm internalization during gastrulation in amphibian embryos. The internalization of vegetal cells prior to gastrulation was first observed in the 1930s by Abraham Mandel Schechtman through the use of vital dye labeling experiments in Triturus torosus embryos. More recently, Winklbauer and Schürfeld (1999) described the internal movements in more detail using pregastrular explants of Xenopus laevis.

The Spemann-Mangold organizer is a group of cells that are responsible for the induction of the neural tissues during development in amphibian embryos. First described in 1924 by Hans Spemann and Hilde Mangold, the introduction of the organizer provided evidence that the fate of cells can be influenced by factors from other cell populations. This discovery significantly impacted the world of developmental biology and fundamentally changed the understanding of early development.

Dorsal lip

The dorsal lip of the blastopore is a structure that forms during early embryonic development and is important for its role in organizing the germ layers. The dorsal lip is formed during early gastrulation as folding of tissue along the involuting marginal zone of the blastocoel forms an opening known as the blastopore. It is particularly important for its role in neural induction through the default model, where signaling from the dorsal lip protects a region of the epiblast from becoming epidermis, thus allowing it to develop to its default neural tissue.

A developmental signaling center is defined as a group of cells that release various morphogens which can determine the fates, or destined cell types, of adjacent cells. This process in turn determines what tissues the adjacent cells will form. Throughout the years, various development signaling centers have been discovered.


  1. Wolpert, Lewis; Tickle, Cheryll; Martinez Arias, Alfonso (2015). Principles of Development (5th ed.). Oxford University Press. p. 146. ISBN   9780198709886 . Retrieved 12 October 2015.
  2. 1 2 Gilbert SF. Developmental Biology. 6th edition. Sunderland (MA): Sinauer Associates; 2000. Early Amphibian Development. Available from: https://www.ncbi.nlm.nih.gov/books/NBK10113/
  3. Wolpert, Lewis; Tickle, Cheryll; Martinez Arias, Alfonso (2015). Principles of Development (5th ed.). Oxford University Press. p. 149. ISBN   9780198709886 . Retrieved 12 October 2015.
  4. Angerer, Lynne M.; Angerer, Robert C. (February 2000). "Animal–Vegetal Axis Patterning Mechanisms in the Early Sea Urchin Embryo". Developmental Biology. 218 (1): 1–12. doi:10.1006/dbio.1999.9553. PMID   10644406.
  5. P. Hausen, M. Riebesell: The Early Embryonic Development of Xenopus Laevis - An Atlas of the Histology ISBN   0387537406

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