Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Third Week: Becoming Trilaminar and Establishing Body Axes
Summary
The first major event of the third week, gastrulation , commences with the formation of a longitudinal midline structure, the primitive streak , in the epiblast near the caudal end of the bilaminar embryonic disc. The cranial end of the primitive streak is expanded as the primitive node ; it contains a circular depression called the primitive pit , which is continuous caudally down the midline of the primitive streak with a trough-like depression called the primitive groove . The primitive pit and groove represent areas where cells are leaving the primitive streak and moving into the interior of the embryonic disc. Some of these cells invade the hypoblast, displacing the original hypoblast cells and replacing them with a layer of definitive endoderm . Others migrate bilaterally from the primitive streak and then cranially or laterally between endoderm and epiblast and coalesce to form the intraembryonic mesoderm . After gastrulation is complete, the epiblast is called the ectoderm . Thus during gastrulation, the three primary germ layers form: ectoderm , mesoderm , and endoderm . Germ layers are the primitive building blocks for formation of organ rudiments .
Formation of the primitive streak also defines for the first time all major body axes . These consist of the cranial-caudal (or head-tail) axis , the dorsal-ventral (or back-belly) axis , and the left-right axis . Before the flat embryonic disc folds up into a three-dimensional tube-within-a-tube body plan, these axes remain incompletely delimited; their definitive form will be better understood after Chapter 4 is studied.
As gastrulation converts the bilaminar embryonic disc into a trilaminar embryonic disc , it brings subpopulations of cells into proximity so they can undergo inductive interactions to pattern layers and specify new cell types. Some of the first cells to move through the primitive streak contribute to the intraembryonic mesoderm by migrating bilaterally and cranially to form the cardiogenic mesoderm . Somewhat later in development, a longitudinal thick-walled tube of mesoderm extends cranially in the midline from the primitive node; this structure, the notochordal process , is the rudiment of the notochord . Migrating bilaterally from the primitive streak and then cranially, just lateral to the notochordal process, are cells that contribute to the paraxial mesoderm . In the future head region, paraxial mesoderm forms the head mesoderm . In the future trunk region, paraxial mesoderm forms the somites , a series of segmental block-like mesodermal condensations. Two other areas of intraembryonic mesoderm form from the primitive streak during gastrulation: intermediate mesoderm and lateral plate mesoderm . The intermediate mesoderm contributes to the urogenital system, and the lateral plate mesoderm contributes to the body wall and the wall of the gut (gastrointestinal system).
During gastrulation, a major inductive event occurs in the embryo: neural induction . In this process, the primitive node induces the overlying ectoderm to thicken as the neural plate , the earliest rudiment of the central nervous system.
During subsequent development, the neural plate will fold up into a neural tube . Neural crest cells arise from the lateral edges of the neural plate during formation of the neural tube. Also during subsequent development, the definitive endoderm will fold to form three subdivisions of the primitive gut: foregut , midgut , and hindgut . The oropharyngeal membrane forms just cranial to the endoderm that will form the foregut, and the cloacal membrane forms just caudal to the endoderm that will form the hindgut. The oropharyngeal and cloacal membranes are formed as ectoderm and endoderm become apposed in localized, midline areas lacking mesoderm.
With formation of endodermal, mesodermal, and ectodermal subdivisions during gastrulation, the stage is set by the end of the third week for formation of the tube-within-a-tube body plan and subsequent organogenesis —the processes by which primitive organ rudiments are established and subsequently differentiated to form all major organ systems.
Clinical Taster
In 2004, a baby girl, Milagros Cerron, was born in Peru with a condition called sirenomelia ( siren and melos are Greek, meaning “nymph limbs”). Because she is one of only three surviving children born with the “mermaid syndrome” (the oldest being 16 years old in 2005), her birth, first birthday, and surgery at 13 months of age received extensive press coverage.
Sirenomelia is a rare condition occurring in 1 in 70,000 births. Most babies born with sirenomelia die within a few days of birth with severe defects in vital organs. The most obvious defect in sirenomelia is a fusion of the two lower limbs at the midline ( Fig. 3.1 ). In Milagros’s case (her name is Spanish for miracles), her lower limbs were fused together from her thighs to her ankles, with her feet deviating from one another in a V-shaped pattern resembling a mermaid’s tail. In the press, she is often referred to as “Peru’s little mermaid.” In addition to fused lower limbs, she was born with a deformed left kidney, a small right kidney that failed to ascend, and anomalies in her terminal digestive, urinary, and genital tracts. These anomalies have resulted in recurrent urinary tract infections.
For 3 months before her first surgery to separate her fused legs, saline-filled bags were inserted to stretch the skin to allow it to cover her legs once they were separated. She recovered quickly from surgery, and over the course of the next 15 years she undergoes many other surgeries to correct her digestive, urinary, and reproductive organs. Remarkably, a few months after her first surgery, she was able to run around the school playground with her classmates and take ballet lessons.
Overview of Gastrulation: Forming Three Primary Germ Layers and Body Axes
Animations are available online at StudentConsult .
Primitive Streak Forms at Beginning of Third Week and Marks Three Body Axes
On about day 15 of development, a thickening containing a midline groove forms along the midsagittal plane of the embryonic disc, which has now assumed an oval shape ( Fig. 3.2 ). Over the course of the next day, this thickening, called the primitive streak , elongates to occupy about half the length of the embryonic disc, and the groove, called the primitive groove , becomes deeper and more defined. The cranial end of the primitive streak is expanded into a structure called the primitive node . It contains a depression, called the primitive pit , which is continuous caudally with the primitive groove.
With formation of the primitive streak, major body axes are identifiable ( Fig. 3.3 ). The primitive streak forms at the caudal midline of the embryonic disc. Thus, the cranial-caudal axis can be identified. Because the primitive streak occupies the midline, when the epiblast is viewed looking down at it from inside the amniotic cavity and facing its cranial end, what lies to the left of the primitive streak represents the left side of the embryo, and what lies to the right represents its right side. At the time of primitive streak formation, the future dorsal-ventral axis of the embryonic disc is roughly equivalent to its ectoderm-endoderm axis. Later, with body folding and formation of the tube-within-a-tube body plan (covered in Chapter 4 ), the dorsal-ventral axis becomes better defined.
Formation of the primitive streak also heralds the beginning of gastrulation . During gastrulation, epiblast cells move toward the primitive streak, enter the primitive streak, and then migrate away from the primitive streak as individual cells (see Fig. 3.3 ). The movement of cells through the primitive streak and into the interior of the embryo is called ingression .
In the Research Lab
Induction of Primitive Streak
Experiments suggest that the primitive streak is induced by cell-cell interactions at the caudal end of the embryonic disc. Although the exact tissue interactions are disputed, it is clear that extraembryonic tissues induce the adjacent epiblast to form primitive streak (see Fig. 3.3 ), and that this process of induction continues as the extraembryonic endoderm (hypoblast) migrates from caudal to cranial.
Misexpression studies (gain-of-function and loss-of-function; covered in Chapter 5 ) in both mouse and chick suggest that Tgfβ and Wnt family members induce the primitive streak (see Fig. 3.3A,B ). In chick, Vg1 (a Tgfβ family member) in conjunction with Wnt8a (formerly called Wnt8c) induces the epiblast to express another Tgfβ family member—nodal. Nodal in turn, along with fibroblast growth factor 8 (Fgf8; and likely other Fgfs), causes epiblast cells to de-epithelialize and form the primitive streak. Finally, inhibition of endogenous Bmp signaling (through its antagonist chordin; covered in Chapter 4 and 5 )) also seems to be required for primitive streak formation.
In mouse, Wnt3 and its downstream target brachyury (a T-box–containing transcription factor) are expressed in both future cranial and caudal prestreak epiblast (see Fig. 3.3B ). During subsequent development, Wnt3 is downregulated cranially by signals from a specialized region of extraembryonic endoderm called the anterior visceral endoderm , and it is upregulated caudally (note: “anterior” in the mouse is equivalent to cranial in the human). Finally, expression of Wnt3, brachyury, and nodal becomes consolidated within the primitive streak. Loss-of-function mutations of genes expressed by the anterior visceral endoderm (e.g., Dand5 [also known as Cerberus-like 2] and lefty1—both inhibitors of Tgfβ and Wnt signaling) result in formation of extra primitive streaks. Moreover, embryos with loss-of-function mutations of nodal (or its co-factor cripto) fail to form a primitive streak. Further studies in mouse (using injection chimeras; covered in Chapter 5 ) reveal that formation of the primitive streak involves signaling of Tgfβ family members from extraembryonic tissues (as in chick).
Cellular Basis of Primitive Streak Formation
Studies in chick have revealed the cellular basis of primitive streak formation ( Fig. 3.4 ). Four major processes are involved: cell migration , oriented cell division , progressive delamination from the epiblast, and convergent extension (covered later in this chapter). As just covered, during formation of the primitive streak, cells are induced from the epiblast by the caudal extraembryonic region. As induction occurs, these cells delaminate (de-epithelialize or undergo an epithelial-to-mesenchymal transition) from the epiblast and migrate cranially and medially. Analyses of labeled clones of cells show that cells are displaced mainly cranially as they undergo division, suggesting that their division plane is preferentially oriented in the medial-lateral plane, so that daughters are displaced cranially. As extraembryonic endoderm migrates cranially, progressively more cranial epiblast cells along the midline are induced to delaminate, extending the cranial end of the primitive streak more cranially. Finally, cells within the forming streak merge medially, and consequently the streak extends craniocaudally to accommodate the merging cells. Thus, convergent extension contributes to the later aspects of primitive streak formation and elongation.
Formation of Definitive Endoderm
On day 16, epiblast cells lateral to the primitive streak begin to move into the primitive streak, where they undergo an epithelial-to-mesenchymal transition (EMT) . An epithelium consists of a sheet of regularly shaped (often cuboidal) cells tightly interconnected to one another at their lateral cell surfaces; a mesenchyme consists of much more irregularly shaped (often stellate) and loosely connected cells. During EMT, epiblast cells often elongate and become flask or bottle shaped ( Fig. 3.5 ), detaching from their neighbors as they extend foot-like processes called pseudopodia (as well as thinner processes called filopodia and flattened processes called lamellipodia ), which allow them to migrate through the primitive streak into the space between the epiblast and the hypoblast (or into the hypoblast itself). This collective movement of cells through the primitive streak and into the interior of the embryo to form the three primary germ layers constitutes gastrulation . The first ingressing epiblast cells invade the hypoblast and displace its cells, so that the hypoblast eventually is completely replaced by a new layer of cells—the definitive endoderm (see Fig. 3.5A ). The definitive endoderm gives rise to the lining of the future gut and gut derivatives.
Formation of Intraembryonic Mesoderm
Starting on day 16, some epiblast cells migrating through the primitive streak diverge into the space between epiblast and nascent definitive endoderm to form a third germ layer—the intraembryonic mesoderm ( Figs. 3.5B,C , 3.6 ). These cells migrate bilaterally from the primitive streak and initially form a loose mat of cells between epiblast and endoderm. Shortly thereafter, the mat reorganizes to form four main subdivisions of intraembryonic mesoderm: cardiogenic mesoderm , paraxial mesoderm , intermediate mesoderm (also called nephrotome ), and lateral plate mesoderm . In addition, a fifth population of mesodermal cells migrates cranially from the primitive node at the midline to form a thick-walled midline tube called the notochordal process .
During the third week of development, two faint depressions form in the ectoderm: one at the cranial end of the embryo and the other at the caudal end behind the primitive streak. Late in the third week, the ectoderm in these areas fuses tightly with the underlying endoderm, excluding the mesoderm and forming bilaminar membranes. The cranial membrane is called the oropharyngeal membrane , and the caudal membrane is the cloacal membrane . The oropharyngeal and cloacal membranes later become the blind ends of the gut tube. The oropharyngeal membrane breaks down in the fourth week to form the opening to the oral cavity, whereas the cloacal membrane disintegrates later, in the seventh week, to form the openings of the anus and the urinary and genital tracts (covered in Chapters 14 through 16 ).
Formation of Ectoderm
Once formation of the definitive endoderm and intraembryonic mesoderm is complete, epiblast cells no longer move toward and ingress through the primitive streak. The remaining epiblast now constitutes the ectoderm , which quickly differentiates into the central neural plate and the peripheral surface ectoderm . However, the embryo develops in a cranial-to-caudal sequence, so that once epiblast is no longer present cranially, for some time it still will be present caudally where cells continue to move into the primitive streak and undergo ingression ( Fig. 3.7 ). Eventually, the process of gastrulation is complete. At that time, formation of the three definitive germ layers of the trilaminar embryonic disc —ectoderm, mesoderm, and definitive endoderm—will be complete throughout the disc. Thus, all three germ layers derive from epiblast during gastrulation (note: some textbooks call the epiblast the primitive ectoderm, but because epiblast gives rise to mesoderm and endoderm as well as ectoderm, the term epiblast is a more appropriate one).
Morphogenetic changes (i.e., shape-generating events) occur in each of these germ layers to form the primitive organ rudiments. Thus, we often speak about ectodermal, mesodermal, and endoderm derivatives. In reality few organ rudiments form from only one germ layer; rather two or more layers often collaborate (e.g., the gut tube is derived from endoderm and mesoderm). Formation of organ rudiments during the formation of the tube-within-a-tube body plan (see Chapter 4 ) is followed by the transformation of organ rudiments into organ systems, that is, the process of organogenesis ; organogenesis is the major topic of most of the remaining chapters of this textbook.
In the Research Lab
Cellular Basis of Gastrulation
The cellular basis of gastrulation has been studied in a large variety of animal models. During gastrulation, cells undergo four types of coordinated group movements, called morphogenetic movements: epiboly (spreading of an epithelial sheet), emboly (internalization), convergence (movement toward the midline), and extension (lengthening in the cranial-caudal plane). The last two movements occur in conjunction with one another as a coordinated movement and are called convergent extension . Thus, convergent extension involves cell rearrangement to narrow the medial-lateral extent of a population of cells and concomitantly increase its cranial-caudal extent. Morphogenetic movements are generated by a combination of changes in cell behaviors . These behaviors include changes in cell shape, size, position , and number . These changes are often associated with changes in cell-to-cell or cell-to-extracellular matrix adhesion .
Changes in cell shape involve cell flattening (from columnar or cuboidal to squamous), cell elongation or shortening (from cuboidal to columnar or from columnar to cuboidal), and cell wedging (from columnar to wedge shaped). Changes in cell size may involve an increase in cell volume ( growth ) or a decrease. Changes in cell position involve the active (i.e., migration ) or passive displacement of cells from one region of an embryo to another, and changes in cell number may involve an increase ( mitosis ) or a decrease ( apoptosis , also called programmed cell death ).
Both epiboly and emboly are involved in human gastrulation as cells move toward, into, and through the primitive streak. Epiboly involves the spreading of a sheet of cells, generally on the surface of an embryo. Epiblast cells undergo epiboly to move toward and into the primitive streak. Emboly involves the movement of cells into the interior of an embryo and is also called internalization . Emboly can involve the movement of individual cells or sheets of cells. Movement of cells through the primitive streak and into the interior involves a type of emboly called ingression —the internalization of individual cells undergoing an EMT.
Epithelial-to-mesenchymal transition involves changes in both cell-to-cell adhesion and cell shape , with the latter mediated by changes in the cytoskeleton . During EMT, epiblast cells within the primitive streak shift their predominant adhesive activity from cell-to-cell to cell-to-substratum (basement membranes and extracellular matrix). One gene responsible for repressing epithelial characteristics in the mesenchymal cells of the streak is snail, a zinc-finger transcription factor. Under its influence, expression of certain cell-to-cell adhesion molecules such as E-cadherin ceases, whereas expression of cytoskeletal proteins, such as vimentin, is induced. In addition, the cytoskeleton is altered by expression of members of the Rho family of GTPases such as RhoA and Rac1. These are required to regulate actin organization and the development of lamellipodia of gastrulating cells within the primitive streak. When GTPases are disrupted, cells accumulate and die within the space between epiblast and hypoblast. Similarly, loss-of-function mutations of a variety of adhesion and cytoskeletal molecules disrupt EMT. These include N-cadherin, a cell-cell adhesion molecule, and β-catenin, a cytoplasmic component of the cadherin/catenin adhesion complex, as well as afadin, an actin filament–binding protein. In addition to changes in adhesion and cytoskeleton, Fgf signaling plays a role in EMT. In loss-of-function mutations of fibroblast growth factor receptor 1 (Fgfr1), involuting cells lose their ability to ingress and, consequently, they accumulate within the primitive streak.
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here