Formation of Germ Layers and Neural Plate in Embryonic Development

Gastrulation is the process that forms the mesoderm and endoderm, which are two of the three germ layers of the embryo. It occurs during the third week of development.

On the 15th day, a groove appears along the longitudinal midline of the germ disc, which is now oval-shaped. The primitive streak, which is located in the center of the germ disc, occupies 50% of its length at this stage. The buccopharyngeal and cloacal membranes are also present.

The groove deepens on the following day, with raised edges extending to about half of the length of the embryo. At the cranial end of the primitive streak, a deep depression called the primitive pit appears, surrounded by a small elevation known as the primitive node. This entire structure is referred to as the primitive streak.

The future head will form near the primitive pit, while the adjacent surface of the primitive streak will form the dorsal portion of the embryo. The appearance of the primitive streak establishes the longitudinal axis and bilateral symmetry of the future adult.

During gastrulation, epiblast cells replace hypoblast cells to form the definitive endoderm on days 14-15. On the 16th day, the epiblast penetrates between the endoderm and epiblast to form the intraembryonic mesoderm.

Epiblastic cells near the primitive streak proliferate and flatten on the 16th day. These cells migrate along the primitive streak, emitting extensions called pseudopodia, and enter the space between the epiblast and the definitive endoderm. This process is known as gastrulation. Some of these migrating cells invade the hypoblast, completely replacing its cells and forming the definitive endoderm, which will give rise to the lining of the digestive tube and its derivatives.

From the 16th day onwards, epiblastic cells migrate along the primitive streak and infiltrate the space between the epiblast and the definitive endoderm, forming the intraembryonic mesoderm. Some of these cells migrate laterally and cranially, while others migrate along the midline. The cells migrating along the midline form the prechordal plate and the notochordal process. The mesodermal cells spread out as a distinct sheet between the epiblast and the endoderm on either side of the midline. Once the intraembryonic mesoderm and definitive endoderm have formed, the epiblast is called the ectoderm. Therefore, all three germ layers (ectoderm, mesoderm, and definitive endoderm) derive from the epiblast.

The primitive streak undergoes withdrawal and disappearance during embryonic development. On the 16th day, it occupies approximately half of the embryo's length. As gastrulation begins, the primitive streak regresses caudally, becoming shorter. By the 22nd day, it represents about 10-20% of the embryo's length, and by the 26th day, it disappears. However, on the 20th day, the primitive streak plays a role in the formation of the caudal eminence, a mass of mesoderm that gives rise to structures derived from the caudal mesoderm and the caudal portion of the neural tube.

The mechanisms of gastrulation are not fully understood, but studies have shown that the migration of epiblastic cells may be influenced by a contractile system mediated by actin microfilaments. The primitive streak may act as a conveyor belt for these cells, and cell proliferation within the primitive streak may also contribute to their movement.

Different regions of the primitive streak are responsible for producing specific components of the extraembryonic, intraembryonic, and definitive endoderm. For example, cells migrating from the caudal portion of the primitive streak give rise to the caudal eminence. Experimental studies using cell marking techniques have been conducted to study the fate of migrating epiblastic cells. These studies involve marking a group of cells with a label or dye, allowing their movements and the movements of their descendants to be tracked.

The "quail-chick" chimera system is commonly used in these studies. Quail cells can be morphologically distinguished, making it easy to identify the descendants of transplanted quail cells in a chick embryo. It is important to note that findings from animal embryology studies can be extrapolated to humans, as early embryonic development follows a similar course in most mammals and vertebrates.

These studies have helped create maps showing the development of specific regions of the epiblast. It has been observed that the posterior region of the primitive streak produces mesoderm that migrates to the lateral parts of the germ disc, while the cranial region produces axial mesosagittal mesoderm and definitive endoderm. Most epiblastic cells, including those migrating along the primitive streak, are pluripotent, meaning they can develop into various cell types depending on their location in the embryo.

The notochord is formed from cells that migrate along the primitive streak. The primitive node buds as a mesodermal tube with a lumen called the notochordal process, caudal to the newly formed prechordal plate. As proliferating cells in the region of the primitive node continue to add to its proximal end, the notochordal process grows in length and the primitive streak regresses. By day 20, the notochordal process has definitively formed, and several transformations occur. First, the ventral floor of the tube fuses with the underlying endoderm. Then, the tube splits along the ventral midline, starting from the region of the primitive pit. This splitting creates an opening at the level of the primitive pit called the neuroenteric canal, temporarily connecting the yolk sac cavity with the amniotic cavity.

The ventral midline splitting of the notochordal tube transforms it into a flattened, medioventral bar of mesoderm known as the notochordal plate. On days 22-24, the notochordal plate completely separates from the endoderm and retracts back into the space between the ectoderm and endoderm, becoming a full cylindrical median structure called the notochord. Some cells of endodermal origin may be incorporated into the notochord during these processes. The initial rudiments of the vertebral bodies will subsequently appear around the notochord, which will form the nucleus pulposus of the intervertebral discs. However, the notochordal-derived nucleus pulposus cells will degenerate and be replaced by adjacent mesodermal cells during later stages of development. Therefore, the notochord does not contribute to the formation of the bony elements of the vertebral column. The notochordal cells trapped in the center of the intervertebral disc will disappear. However, the notochord plays a crucial role in inducing the formation of the vertebral bodies, and failure of this induction can lead to various developmental anomalies of the vertebral column.

In the fourth week of development, two small depressions appear in the ectoderm. One is located at the cranial end of the embryo, near the prechordal plate, and the other is at the caudal end, behind the primitive streak. The ectoderm in these areas fuses tightly with the underlying endoderm, forming a bilaminar membrane without the intervention of mesoderm. The cranial membrane is known as the buccopharyngeal membrane, while the caudal one is the cloacal membrane. These membranes will later become the blind ends of the digestive tube. The buccopharyngeal membrane disappears in the fourth week, creating the opening to the oral cavity. The cloacal membrane disappears later, in the seventh week, forming the opening of the anus and the urogenital tract.

The mesoderm, which consists of the paraxial, intermediate, and lateral mesoderm, is formed by the migration of cell groups from the primitive streak. As the primitive streak regresses, the mesoderm cells condense into cylindrical bars and sheets on either side of the notochord. This process starts in the cephalic portion of the embryo and progresses caudally from the third week to the fourth week. The paraxial mesoderm, located immediately lateral to the notochord, forms a pair of cylindrical bars. The intermediate mesoderm, located laterally to the paraxial mesoderm, forms another pair of less pronounced cylindrical bars. The remaining lateral mesoderm forms a flat sheet called the lateral mesodermal lamina.

These mesodermal structures will give rise to specific adult structures. The paraxial mesoderm will differentiate into the axial skeleton, voluntary musculature, and a portion of the skin dermis. The intermediate mesoderm will form the urinary system and a portion of the genital system. The lateral mesodermal lamina divides into two layers: the ventral layer called the splanchnopleura, which covers the visceral organs derived from the endoderm, and the dorsal layer called the somatopleura, from which the internal lining of the body wall, a portion of the limbs, and the majority of the dermis will form.

Later, the somitomeres develop into segmented mesodermal blocks called somites. The first seven pairs of somitomeres do not form somites. The first somites appear on the 20th day in the region of the future base of the skull, developed from the 8th, 9th, and 10th pairs of somitomeres. The rest of the somites form progressively, cranio-caudally, about 3-7 per day, until the 30th day. In humans, approximately 42-44 pairs of somites are formed, located laterally to the notochord from the occipital region to the tail of the embryo. Some caudal somites disappear, resulting in a total of approximately 37 pairs.

The somites play a crucial role in the overall development and segmental organization of the body. They give rise to the majority of the axial skeleton, voluntary musculature of the neck and body wall, and a portion of the dermis of the neck and trunk. The first four pairs of somites contribute to the formation of the occipital parts of the skull, bones around the nose, eyes, and internal ears, extrinsic muscles of the eyeball, and muscles of the tongue. The next eight pairs form the cervical vertebrae, associated muscles, and a portion of the skin of the neck. The following 12 pairs form the thoracic vertebrae, musculature and bones of the thoracic wall, a portion of the thoracic dermis, and a portion of the abdominal wall. Cell groups from the cervical and thoracic somites invade the upper limb buds to form their musculature. The five lumbar somites form the abdominal dermis, abdominal muscles, and lumbar vertebrae, while the five sacral somites form the sacrum with its associated dermis and musculature. Cell groups from the lumbar somites invade the lower limb buds to form their musculature. The three remaining coccygeal somites form the coccyx.

The formation of the neural plate is the first step in the development of the central nervous system. The neural plate appears on the 18th day of development and is a thickened structure located along the medio-sagittal axis. The narrow caudal portion of the neural plate will give rise to the spinal cord, while the wide cephalic portion will develop into the brain. The process of gastrulation plays a significant role in the induction of the central nervous system, with inducing substances from the underlying mesodermic structures activating specific genes in the epiblastic cells to form the neural plate.

More details about the development of the neural system will be covered in a dedicated lesson on special embryology.

References

1. Anatomia Omului, Volumul I: Embriologie; Semestrul I; Prof. univ. dr. med. Andrei Motoc; Editura „Victor Babeș”, Timișoara, 2018

2. Embriologie - Sinteze didactice; Romeo Chiriac, Anca Indrei ;Editura Fundatiei Chemarea, Iasi, 1997

3. Embriologie Umană; Lidia Chircor, Loredana Surdu;Editura Ex Ponto, Constanța, 2014

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