Gastrulation and Development

To go from simple cells to organ systems 5 overall stages of embryonic development must occur. Fertilisation is the first stage and results in the formation of a zygote. After fertilisation, a process known as cleavage occurs and this is followed by gastrulation where distinct tissue types are starting to be observed. Following gastrulation is the process known as neurulation whereby the nervous system is developing. The final stage is organogenesis and this stage is the development of distinct organs which then make up organ systems in mammalians.

**Fertilisation and cleavage:**

Once a zygote is formed, the zygote undergoes cleavage where several cycles of mitotic cell divisions occur, multiplying the number of cells whilst the cells become smaller. This leads to the formation of a 16 celled structure known as a morula. This structure will then form into a hollow ball of cells called a blastocyst, in mammals. The hollow cavity of the blastocyst is known as blastocoel and the movement and invagination of this cavity marks the next stage of development known as gastrulation.

The process of gastrulation is vital for tissue and organ formation, allowing cells to proliferate and differentiate into specific tissue types. Gastrulation starts with the formation of a structure called the primitive streak where epiblast cells, from the bilaminar disk, migrate and differentiate resulting in the formation of three germ layers: **ectoderm**, **endoderm**, and **mesoderm** in a structure called a gastrula. Each of the germ layers differentiates into different tissue types. Endoderm is the internal layer and gives rise to epithelial tissue which lines organ systems such as the digestive and respiratory systems. The ectoderm is the outer layer of the gastrula and aids in the formation of the nervous system, the skin and the epithelial tissue present in sensory organs. Finally, the mesoderm, the middle layer, forms connective tissue which maintains tissue and organ structures. The mesoderm layer can give rise to mesenchymal cells which are involved in the formation of blood vessels, bone, connective and muscle tissues. Through the proliferation, differentiation and movement of these layers, different tissue types are formed. The 4 main tissue types found in all mammals include connective, muscle, nervous and epithelial tissue and all originate from the process of gastrulation.

**Neurulation:**

The process of neurulation is essential for the formation of the central nervous system (CNS) and spinal cord. This process occurs mainly in the ectoderm layer of the gastrula where further differentiation results in the notochord forming. The formation of the notochord causes cells of the ectoderm to thicken and proliferate forming a neural plate that experiences lengthening and folding, forming a neural tube. As the neural tube is formed, a group of cells named neural crest cells are present and are essential in the development of the peripheral nervous system (PNS) and also form glial cells and other cells that contribute to the nervous system.

**Morphogenesis:**

Many complex processes support gastrulation such as cell signalling cascades and transcription factors. Another key process observed during both gastrulation and organogenesis is a process called morphogenesis. Morphogenesis influences the development of the specific shape of an organism's tissues and organs and is involved in the migration and shape of cells which consequently affect the tissue formed. This process is observed in the cytoskeleton, extracellular matrix (ECM) and between cell-to-cell adhesions[2].

Within the ECM, morphogenesis regulates the movement of the mesoderm, where fibronectin is involved and this, in turn, supports the epithelial tissue. Cell adhesion molecules (CAM) are essential in the arrangement of cells and adhesion occurs when the same type of CAM is present between cells. This is observed in gastrulation when the blastula becomes a gastrula structure. During this process, cells within the blastula, inner cell mass (ICM), separate, differentiate and proliferate to form the 3 germ layers. Morphogens, signal molecules involved in morphogenesis, can influence the types of CAM and therefore cells interact with different cell groups thus influencing the tissues that are developed. Also, morphogens present in the cytoskeleton regulate actin and myosin activity influencing the shape of cells and neighbouring cells.

1. ↑ Campbell, N. A. 2017. Biology: A Global Approach, Global Edition. Pearson Education
2. ↑ Sadler, T. W. 2019. *Langman’s medical embryology*. 14th ed. Philadelphia: Wolters Kluwer
3. ↑ Ansari, A. and Pillarisetty, L. S. 2022. Embryology, Ectoderm.*StatPearls*.  Treasure Island (FL): StatPearls Publishing
4. ↑ Seigel, GJ. Agranoff, BW. Albers, RW. et al. 1990. Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th ed. Philadelphia: Lippincott-Raven
5. ↑ Leptin, M. 2005. Gastrulation movements: the logic and the nuts and bolts. *Dev Cell* 8(3), pp. 305-320. doi: 10.1016/j.devcel.2005.02.007