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Embryonic Development

For additional background information, see Purves et. al., pages 883-896.
Most sea urchin images are from Gilbert, SF (1988). Developmental Biology (2nd ed.) Sinauer Assoc., Inc., Sunderland, MA
Most chick images are from Mathews,WW (1976).  Atlas of Descriptive Embryology (2nd ed.) Macmillan Publishing Co., Inc, New York

Go to sea urchin
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The development of a fertilized egg (called a zygote) into a functional, multicellular organism is a dynamic process that is tightly orchestrated in both time and space, and requires multiple interactions between developing cells and tissues. Many of the events during embryogenesis are controlled by gradients of different proteins or other factors formed within the developing embryo. The presence or absence of one of these factors at the wrong place or at the wrong time can have dramatic (and disastrous) effects on the developing organism.

In general, the embryonic development of multicellular organisms can be subdivided into a number of different stages:

In today's experiment, we will study developing embryos of sea urchins and chickens; these organisms have been used to study various aspects of developmental processes. We will examine the fertilization and the early cleavage reactions in the sea urchin, and gastrulation and organogenesis in the chicken.

The websites below have some wonderful time-lapse movies of developing embryos:

(FishScope-Time-lapse and Confocal Images of Fish Development)

(Time-lapse images of developing human, pig, chicken and fish embryos)

(Embryo Images-Normal and abnormal mammalian development)

Fertilization and Cleavage in the Sea Urchin

Embryonic development begins with the fertilization of an egg by a single sperm to form a diploid zygote. Fertilization is not a single event, but rather, the meeting of sperm and egg triggers a series of complex processes that result in the formation of the zygote and start the developmental pathway of the organism. The events in fertlization include-

The eggs of the female sea urchin are fertilized externally. When a sperm of the same sea urchin species contacts an egg, the sperm is activated by the jelly coat surrounding the egg. The sperm head is capped by a vessicle containing digestive enzymes, called the acrosome, and these enzymes are released from the acrosome upon contact with the jelly coat of the egg.

An acrosomal process made up of the protein actin extends out of the head of the sperm through the egg jelly coat and makes contact with the vitelline envelop of the egg. The acrosomal process of the sperm is coated with a protein called bindin that is recognized by specific binding receptors on the membrane of the vitelline envelop of the egg. (Different species of sea urchins have different forms of bindin and the vitelline envelop has species-specific bindin receptors.)

Contact of acrosomal 
process with egg microvillus

Formation of fertilization cone 
as microvilli fuse with sperm
Sperm internalization

The reaction between the acrosomal bindin with the vitelline envelop bindin receptors attaches the spem to the egg and stimulates the formation of a depression in the egg membrane. The fusion of the sperm and egg membranes allows entry of the sperm nucleus.

The fusion of sperm and egg membranes triggers a series of events that result in blocks to polyspermy or the entry of additional sperm into the fertilized egg. Within a tenth of a second of sperm entry, there is a large influx of Na+ ions into the egg that results in depolarization of the egg membrane; this change in membrane potential is called the fast block to polyspermy.

Within 20 -30 seconds of fertilization, the egg releases Ca2+ that is sequestered in organelles into the cytoplasm; this increase in Ca2+ causes the corticle vessicles to fuse with the egg plasma membrane. These vessicles are filled with enzymes that break the bonds between the vitelline envelope and the plasma membrane. As these bonds are digested, water fills the space between the plasma membrane and the vitelline envelop; the envelop to rises and forms the fertilization membrane.   This process is shown in the images above.  This membrane hardens, thus blocking the entry of additional sperm. The figure below shows an unfertilized sea urchin egg and a newly formed sea urchin zygote; the fertilization membrane can be seen as a faint "halo" surrounding the fertilized egg.

The release of Ca2+ into the cytoplasm of the egg also changes the metabolism in the egg. Prior to fertlization, the egg is metabolically dormant; Ca2+ release activates transcription and protein synthesis in the fertilized egg and the nucleus of the egg completes meiosis by undergoing its second meiotic division. Fusion of the sperm nucleus with that of the egg occurs approximately 1 hour after fertilization in the sea urchin.

The first steps of the developmental pathway is a rapid series of cell divisons called cleavage. The cytoplasm of the egg contains nutrients and other factors such as proteins and mRNAs that play a significant role in early embryonic development. These developmental factors are are not distributed evenly throughout the egg cytoplasm, but rather concentration gradients of these factors are formed in the egg. These developmental factors are not divided evenly among all the daughter cells when cleavage begins; this uneven distribution of factors yields positional information to the daughter cells in the developing embryo allowing each daughter cell to enter its own specific developmental pathway.

The positions of the mitotic spindles during cell division determine the cleavage plane; the mitotic spindles in of successive cell division form at right angle to each other producing a radial cleavage pattern in the sea urchin.  Notice the symmetry of the cells in the 8- and 16-cell stages of the sea urchin embryo in the images above.

During cleavage, DNA synthesis and cell divison proceed rapidly. There is no cell growth and little gene expression. The embryo remains the same size overall, but the number of cells in the embryo increases (also note the presence of the fertilization membrane surrounding the embryo). These early cell divisions in the embryo are sometimes called reduction divisions. Cleavge in the sea urchin embryo produces daughter cells of similar size and are said to undergo complete cleavage or holoblastic cleavage (contrast this to the process in the chicken embryo, below). The first cell division occurs approximately 60-90 minutes after fertilization.

Cleavage continues such that the embryo becomes a ball of smaller and smaller cells until the cells surround a central cavity called the blastocoel. Blastulation occurs at different stages in different organisms; in the sea urchin, the blastula, as the embryo is now called, occurs at the 128-cell stage (7 divisions or ~ 6 hours after fertilization). The individual cells of the embryo are known as blastomeres.

After formation of the blastula, this ball of cells becomes an animal composed of multiple tissue layers in a process known as gastrulation. Gastrulation results in the formation of the embryonic germ layers-

The sea urchin blastula is one cell layer thick; the beginning of gastrulation (~12-20 hours after fertilization) is seen when vegetal pole of the embryo begins to flatten. Blastomeres from the vegetal pole break free, and begin to migrate into the blastocoel as seen in the photograph above (early gastrula). These cells become the primary mesenchyme cells which eventually give rise to the mesoderm.

As can be seen in the photographs above, other cells at the vegetal pole begin to migrate into the blastocoel forming an invagination. The cells that invaginate become the endoderm and form the archenteron, or primitive gut. At the tip of the archenteron, several more cells break free and enter the blastocoel becoming the secondary mesenchyme cells; these mesenchyme cells are also destined to become part of the mesoderm. The archenteron continues to form until it makes contact with the ectoderm layer at the animal pole of the embryo. At the position where archenteron contacts the ectoderm, the mouth forms and at the opening in the vegetal pole (the blastopore) created by the invagination of blastocoels, the anus forms.

Where and when the mouth forms is an important evolutionary distinction in animals that results in two distinct lineages-

For more information on sea urchins and development:

Chicken Embryonic Development

Vertebrate embryos develop in an aqeous environment; fish and amphibians lay their eggs are laid directly in the water. Land animals, such as reptiles and birds use shelled-eggs while mammals use a uterus. Within this shell or uterus, the developing embryo is surrounded by fluid in a special structure formed by a membrane called the amnion.

The part of the chicken egg commonly called the yolk is actually the entire egg cell or ovum swollen with nutrients and developmental factors (which is properly called yolk). The egg white is a protein rich solution that surrounds the ovum providing additional nutrients for the developing embryo.

Cleavage of the fertilized egg is restricted to a small disc of cytoplasm located at the animal pole of the egg cell. After fertilization, incomplete cleavage (or meroblastic cleavage) of the yolk-filled egg cell results in the formation of a cap of developing cells called the blastodisc that rests on the undivided portion of the original egg cell (contrast this with complete cleavage in the sea urchin above).

Cleavage continues at the animal pole of the cell and the cells of the blastodisc sort into upper (epiblast) and lower (hypoblast) cell layers; the cavity between these layers is the blastocoel.

(19-22 hours incubation)

Gastrulation in the chicken embryo begins 6-8 hours after development begins, and as in the sea urchin, involves the movement of cells from the surface of the embryo to interior positions.   In stage 2, epiblast cells move to the middle of the blastodisc, detach from the epiblast layer and move inward toward the vegetal pole. The movement of epiblast cells toward the middle and downward produces a groove in the embryo called the primitive streak that becomes the anterior-posterior axis of the developing embryo (stage 5, above).

The cells that enter the primitive streak move back laterally producing a middle layer of cells in the cavity between the epiblast and hypoblast cell layers. Cells of the primitive streak become the mesoderm, The hypoblast layer, lying on top of the yolk, forms the endoderm, while the epiblast layer becomes the ectoderm. After the mesoderm has formed, lateral folds form in all three primary germ layers; the developing embryo is pinched off from the yolk. The archenteron is formed from the invagination of endoderm during the formation of the folds.

Stage 8 (26-29 hours incubation)

During organogenesis in the chicken (~ 24-30 hours after deveolpment begins), the organs that begin to take shape first are the neural tube and notochord (stage 8, above) which are characteristic of all chordates. The notocord forms from the dorsal mesoderm located just sbove the archenteron. The neural tube develops from cells of the dorsal ectoderm called the neural plate; the neural plate rolls itself into a tube and becomes the brain and spinal chord. The notochord stretches along the anterior-posterior axis of the embryo. Cells of the mesoderm to the sides of the notochord separate into discrete blocks called somites. Cells from the somites give rise to the vertebrae of the backboneand the muscles associated with the axial skeleton. Lateral to the somites, the mesoderm separates into layers to form the lining of the body cavity called the coelom.

(72 hours incubation)

During organogenesis, the ectoderm gives rise to the epidermis and associated glands, the nervous system (see neural tube formation above), and structures such as the lens of the eye. Mesoderm develops into the notochord, the coelom, muscles, skeleton, and most of the circulatory system. The endoderm forms the lining of the digestive tract, and organs such as the liver, pancreas and lungs that arise from folds of the archenteron.

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