Embryology

Fascia development during embryology

The beginning of development of the human fetus and its structures are discussed under the term embryology. With this, we can understand how the adult structure came to be. The fascia starts about two weeks into development of embryonic growth, precisely during the gastrulation phase. This is where the embryo organizes itself from a simple spherical ball of cells, into a multi-layered organism (Purves et al., 1998).  The fascia is progressively folded by the gastrulation process and the rest of the motions of development, into the complex layers of fascia that can be seen in an adult. There is no discontinuity in the layers of fascia (Kumka et al., 2012).

The three layers that are formed are, named from outside: the ectoderm, the mesoderm and the endoderm (see fig 2.1). Each of these layers has distinctive characteristics and gives rise to certain tissues of the body (Kieth et al., 1998).

Figure 2.1 shows the formation of the germ layers
Figure 2.1 shows the formation of the germ layers

The ectoderm gives rise to the brain, the nervous system and the superficial epidermis. The mesoderm is located between the ectoderm and the endoderm and gives rise to somites, which form muscle; the cartilage of the ribs and the vertebrae; the dermis, the notochord, blood, blood vessels, bone and connective tissue. The endoderm gives origin to the epithelium of the digestive system and respiratory system and organs associated with the digestive system such as liver and pancreas (Arnold et al., 2009). Following gastrulation, cells in the body are either organized into sheets of connected cells or as a mesh of isolated cells (mesenchyme) (Hall et al., 1998).

Now from an embryological perspective, the fascial system originates in the mesoderm, although according to some authors this connective network can be partially found in the ectoderm, with particular reference to the cranial and cervical neck (van der Wal et al., 2009; Buckley et al., 2001).  A study performed in accordance with the provisions of the Declaration of Helsinki, 1995 (revised in Edinburgh, 2000), studied 18 mid-term fetuses, comprising 5 between 9 and 12 weeks of gestation, 3 between 15 and 18 weeks of gestation and 10 between 20 and 25 weeks. During observation at 9-12 weeks of gestation, the covering fascia of the longus colli consistently issued a flat, definite fascia medially to the contralateral muscle. During this stage of embryonic development, the bilateral longus colli muscles appear as two bellies of a biventer muscle (Miyake et al., 2011). In a loose connective tissue space between the esophagus and the Prevertebral Lamina of the Deep Cervical Fasciae (PLDCF), the alar fascia develops as a thin bundle of fibers and its lateral ends distribute or connect with lymphatic vessels (Miyake et al., 2011). Around 15 weeks of gestation, the subclavian and common carotid arteries pierce a definite adventitia (the outermost layer of a blood vessel wall) (Gilbert et al., 2000). During this stage, the alar fascia ends laterally at the adventitia. The fascia encloses the thymus, while no visceral fascia surrounds the thyroid gland, larynx, pharynx or esophagus. Nevertheless, by 15 weeks, a mesenchymal condensation becomes clear around the trachea, which could possibly correspond to the development of lymphatic follicles (Hall et al., 2003). No fatty tissues are evident between these fascial structures at the early stages (Downs et al., 2009).

During 18-25 weeks of gestation, a two laminar configuration, which includes the PLDCF and the alar fascia, becomes evident in a fetus because of the appearance of veins and fatty tissue between them (Mikawa et al., 2004). The attachment of the PLDCF to the perichondrium of the vertebral body can be noticed at this stage as well. At the base of the neck, the alar fascia extends laterally to connect with the cupula pleurae and the upper parts of the parietal pleura: the suprapleural membrane is formed by the alar fascia. Lastly, during 27-30 weeks of gestation, the nervous system starts to develop enough to control some body functions.

Additional Information

If you are interested in a more detailed description about the process of embryogenesis from the start, simply stay on this page and continue reading.

Embryogenesis

Embryogenesis is the process where the embryo (a multicellular diploid eukaryote in its earliest stage of development) forms and develops; this is the beginning of life. This process starts with the fertilization of the egg cell, called the ovum, by a sperm cell, called a spermatozoon. Once fertilized, the ovum is referred to as zygote; a single diploid cell, which has two homologous copies of chromosome, usually one from the mother and one from the father (Campbell et al., 2002). Successively, the zygote undergoes mitotic division with no significant growth and cellular differentiation, which leads to development of a multicellular embryo (Stecco et al., 2011).

Fertilization and the zygote

The ovum is always asymmetric and has an animal pole, which is the future ectoderm and mesoderm, and a vegetal pole, future endoderm (Gilbert et al., 2000). Different protective envelopes with different layers always cover the cell. The first envelope is in contact with the membrane of the egg and is made of glycoproteins; known as the vitelline membrane. Fertilization or fecundation is the process, where the fusion of gametes occurs to produce a new organism (Gilbert et al., 2000).

Cleavage and morula

Cleavage process
Figure 2.2 shows the cleavage process from zygote to the blastula

The term cleavage indicates the division of cells in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant circumferential growth, producing a cluster of cells the same size as the original zygote (Sherk et al., 2006). Cleavage ends with the formation of the blastula. At least four initial cell divisions occur, resulting in a dense ball of at least 16 cells, called the morula, consisting of 16 undifferentiated cells. The cells derived from cleavage, up to the blastula stage, are defined as blastomeres (Campbell et al., 2002). Cleavage can be holoblastic (total) or meroblastic (partial). This depends on the amount of yolk (part of an egg that feeds the developing embryo) in the egg.

Holoblastic cleavage occurs mostly in animals with little amount of yolk in their eggs, such as humans who receive nourishment as embryos from the mother, through placenta (Drasdo et al., 2000). Meroblastic cleavage occurs in animals whose eggs have more yolk (birds and reptiles).The end of the cleavage is defined as mid-blastula transition and coincides with the onset of zygotic transcription.

Blastula formation

cleavage has produced 128 cells, the embryo is called a blastula. This is a hollow sphere of cells, referred to as blastomeres, surrounding an inner fluid-filled cavity called the blastocele, which is formed during an early stage of embryonic development in animals (Encyclopedia Britannica, 2013). Then the next structure formed is the blastocyst, a mass of inner cells that are distinct from the blastula. The blastocyst arises from the morula in the uterus, 5 days after fertilization (Encyclopedia Britannica, 2013). The morula becomes the blastocyst through cellular differentiation after entering into the uterus from the fallopian tube. The morula’s cells can be distinguished into two types: an inner cell mass growing on the interior of the blastocel and trophoblast cells growing on the exterior (Niakan et al., 2012). The early embryo undergoes cell differentiation and structural changes to become the blastocyst. Successively, this is then prepared for implantation into the uterine wall 6 days after fertilization. This process marks the end of the germinal stage and the beginning of the embryonic stage of development (Sherk et al., 2006). The blastocoel is a fluid-filled cavity which contains amino acids, proteins, growth factors sugars, ions and some other components, that are necessary for cellular differentiation. The blastocoel also has the function to allow blastomeres to move during the process of gastrulation (Fleming et al., 2000).

Formation of germ layers

Fertilization leads to the formation of a zygote. Successively, cleavage and mitotic cell divisions transform the zygote into a hollow ball of cells (blastula). This early embryonic form undergoes gastrulation, forming a gastrula with either two or three layers, called germ layers. In all vertebrates, these progenitor cells differentiate into all adult tissues and organs (Gilbert et al., 2000).

As stated above, three layers are formed, named from outside: the ectoderm, the mesoderm and the endoderm. Each of these layers has distinctive characteristics and gives rise to certain tissues of the body (Keith et al., 1998).The ectoderm gives rise to the brain, the nervous system and the superficial epidermis. The mesoderm is located between the ectoderm and the endoderm and gives rise to somites, which form muscle; the cartilage of the ribs and the vertebrae; the dermis, the notochord, blood, blood vessels, bone and connective tissue. The endoderm gives origin to the epithelium of the digestive system and respiratory system and organs associated with the digestive system such as liver and pancreas (Arnold et al. 2009). Following gastrulation, cells in the body are either organized into sheets of connected cells or as a mesh of isolated cells (mesenchyme) (Hall et al., 1998).

Formation of gastrula and gastrulation

The early phase in the embryonic development is defined as gastrulation. During this process, the single-layered blastula is reorganized into a three-layered structure, the gastrula. Gastrulation takes place after cleavage and the formation of the blastula and primitive streak. Gastrulation is followed by organogenesis, which is when individual organs develop within the newly formed germ layers mentioned above (Hall et al., 1998). The molecular mechanism and timing of the gastrulation process is different in various organisms. Nevertheless, some common features can be outlined; these are:

  • A change in topological structure of the embryo, from simply connected surface to a non-simply connected surface

  • The differentiation of cells into one of three types (endodermal, mesodermal and ectodermal)

  • The digestive function of a large number of endodermal cells (Harrison et al., 2011)

Gastrulation patterns show large variations throughout the animal kingdom. However, five basic types of cell movements, occurring during gastrulation unify these patterns: invagination, involution, ingression, delamination and epiboly (Gilbert et al., 2010). A short summery of these are:

  • Invagination: infolding of cell sheet into embryo, forming the mouth, anus and archenteron

  • Involution: inturning of cell sheet over the basal surface of an outer layer

  • Ingression: migration of individual cells into the embryo

  • Delamination: splitting or migration of one sheet into two sheets

  • Epiboly: expansion of one cell sheet over the other cells (Gilbert et al., 2010)

Nervous system formation: neural groove, tube and notochord

Neural development is one of the earliest systems to begin and the last to be completed after birth. This development generates the most complex structure within the embryo. The primitive streak is a structure that forms in the blastula during the early stages of embryonic development (Mikawa et al., 2004). The presence of this structure will establish bilateral symmetry, determine the site of gastrulation and initiate germ layer formation (Mikawa et al., 2004). The primitive streak extends through a midline and creates the left-right and cranial-caudal body axes and begins the process of gastrulation (Downs et al., 2009). In front of the primitive streak, two longitudinal ridges can be distinguished, one on each side of the middle line formed by the streak. These two ridges are named neural folds (Gilbert et al., 2000). Between these folds a shallow median groove is located, the neural groove, which gradually deepens as the neural folds become elevated. Lastly, the folds meet and merge in the middle line and convert the groove into a closed tube, the neural tube (Dellaud et al., 2008). Following the fusion of the neural folds over the anterior end of the primitive streak the blastopore does not open anymore on the surface but instead opens into the closed canal of the neural tube. Thus a transitory communication (neurenteric canal) is established between the neural tube and the primitive digestive tube (Mikawa et al., 2004).  The fusion of the neural folds occurs in the region of the hindbrain, towards the end of the 3rd week. The front opening of the tube then closes at the anterior end of the future brain and forms a recess that is in contact with the overlying ectoderm. Before the neural groove is closed, a ridge of ectodermal cells appears along the margin of each neural fold; this is called neural crest and from it the spinal and cranial nerve ganglia and the ganglia of the sympathetic nervous system (SNS) are developed (Gilbert et al., 2006). By the upward growth of the mesoderm, the neural tube is finally separated from the ectoderm (Campbell et al., 2002).

Formation of early septum

The extension of the mesoderm takes place through the embryonic and extra-embryonic areas of the ovum. One of these can be seen immediately in front of the neural tube, where the mesoderm extends forward in the form of two crescentic masses. On this area, the ectoderm and endoderm come into contact with each other and constitute a thin membrane, called the buccopharyngeal membrane. This forms a septum between the primitive mouth and the pharynx (Kieth et al., 1998).

Somitogenesis

Somitogenesis is the process where somites (primitive segments) are produced. This process begins with the formation of somitomeres marking the future somites in the pre-somitic mesoderm (Maroti et al., 2012). This gives origin to more parts of somites with the same appearance.  During this process, somites form from the paraxial mesoderm. This tissue undergoes convergent extension as the embryo gastrulates. The notochord extends from the base of the head to the tail; with it thick bands of paraxial mesoderm extend (Gilbert et al., 2010).  Somites are specified according to their location, as the segmental paraxial mesoderm from which they form it itself determined by position along the anterior-posterior axis before somitogenesis (Maroti et al., 2012). The cells within each somite are specified based on their location within the somite. Moreover, they retain the ability to become any kind of somite-derived structure until relatively late in the process of somitogenesis (Gilbert et al., 2010).

Organogenesis

During organogenesis the three germ layers (ectoderm, endoderm and mesoderm) develop into internal organs of the developing organism (Kieth et al., 1998). Internal organs start developing in humans within the 3rd week in utero. During the process of organogenesis, the germ layers differ by three processes: folds, splits and condensation. Vertebrates all differentiate from the gastrula the same way. They develop a neural crest that differentiates into many different structures (bones, muscles and peripheral nervous system) (Schultz et al., 1996).

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