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Embryonic Development – Anatomy & Physiology


Embryos have different stages depending on organisms for instance, in humans, it is a newly developing being up to the ninth week of development. In organisms with multiple cells, the term ’embryo’ broadly describes the life cycle or early stage of development before hatching or birth. The embryonic development process starts with fertilization, where a zygote is created. A zygote is a single cell that results from the union of both male and female gametes. Zygotes develop into a multicellular embryo, proceeding into a sequence of identifiable stages including cleavage, blastula, gastrulation, and organogenesis. An embryo of good quality must show the required kinetics and simultaneous division. Normal developing embryos have cell division taking place every eighteen to twenty hours.

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Germinal Stage

The entrance of the spermatozoon into the ovum and the fusion of the two sets of genetic materials contained in the gametes form the zygote, which is a single diploid cell. The process is referred to as fertilization and happens at the ampulla part of the fallopian tube. The zygote is made up of amalgamated genetic materials of both the female and male gametes, consisting of 23 chromosomes from the nucleus of sperm, and 23 chromosomes from the nucleus part of the ovum (Gardner & Balaban, 2016). The 46 (23+23) chromosomes go through changes before mitotic division, leading to the formation of the embryo with 2 cells (Wang et al., 2018). Three processes that enable fertilization to happen include chemotaxis, adhesive compatibility between the egg and sperm, and acrosomal reaction.

Cleavage Stage of Development

A single cell of a zygote goes through rapid cell division among organisms with multiple cells. Cleavage refers to the fast, numerous rounds of cell division that take place with a single zygote. The cleavage process produces more than a hundred cells, resulting in a new embryo termed as blastula (Wang et al., 2018). At the cleavage stage, cell division takes place without increasing the mass of the cells. For example, multiple cells result after the division from one large single cell. Cleavage happens either partially or fully, commonly referred to as holoblastic, or meroblastic respectively. Yolk amount and distribution influence the cleavage type that a zygote undergoes (Gardner & Balaban, 2016). Isolecithal cells are cells that are evenly and fairly distributed throughout the cell. In holoblastic cleavage, blastomeres and zygotes completely divide during the process of cleavage.

Blastula Formation

Blastulation is a stage in embryonic development where blastula is produced. The blastula is a hollow sphere of cells that surround the blastocoel (an inner cavity filled with fluid). When a zygote undergoes numerous cleavages, a ball of cells named morula develops. The early embryo develops into blastula only when the blastocoel forms. The morula becomes blastula after the seventh cleavage produces 128 cells. At this stage, Wang et al. (2018) point that blastocyst is formed among mammals, with a distinct inner mass from the surrounding blastula. Blastocyst and blastula have different fates, with a similarity in terms of their structure. The process of cell differentiation also takes place during this stage i.e., into the outer and inner layers. The inner cells undergo differentiation, forming polarize and embryoblast, which closes together to form gap junctions.


During the gastrulation stage, the blastula reorganizes into numerous layers called the gastrula. Before the process, the embryo is initially an unending sheet of cells. However, as gastrulation ends, the embryo begins differentiating, establishing specific cell lineages (Gardner & Balaban, 2016). The second process following differentiation involves setting up basic body axes such as posterior-anterior, and internal types of cells e.g. the gut. Gastrulation occurs after cleavage and blastula formation and begins when primitive streak forms (Wang et al., 2018). The primitive streak, a line band made up of cells at the mid-embryo, forms by the migrating epiblast cells. Their formation takes the form of bilateral symmetry, giving the embryo the orientations of the front-to-back and head-to-tail.

Composition of an Entire Organism

Every embryo’s germ layer gradually gives rise to various distinct cells, tissues, and organs which make up a whole organism as illustrated. The ectoderm for instance gradually forms cells of numerous internal organs and glands such as the thyroid, intestines, bladder, lungs, and pancreas. The mesoderm (mid-layer) eventually forms the cells of the muscles, kidneys, blood, heart, and bones. The ectoderm layer forms nervous system cells, eye cells, epidermal cells, and other connective tissues (Wang et al., 2018). The primitive gut forms in the last gastrulation phase, eventually developing into the gastrointestinal tract. Blastopore forms on one side of the embryo and deepens to finally develop into the anus. The tiny hole (blastopore) continues tunneling through the embryo to the remaining side, forming an opening that eventually becomes the mouths. The gastrulation process is known for completing with a working digestive tube.


Neurulation commences when notochord stimulates the formation of the Central Nervous System (CNS). The stimulation signals the ectoderm above it to a flat and thick neural plate (Wang et al., 2018). The plate folds upon itself, forming a neural tube that later becomes the brain and spinal cord, gradually forming the CNS. Two processes form various parts of the neural tube and they include; primary neurulation, and secondary neurulation (Gardner & Balaban, 2016). In the former, the neural plates fold inwards till the edges meet, hence fusing, while in secondary neurulation, the formation of tubes takes place with the solid precursor’s interior hollowing out (Wang et al., 2018). Primary neurulation also refers to the process where the flat neural plate folds into a cylinder-like neural tube.

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By definition, organogenesis refers to the process where the 3 layers of the germ tissue develop into internal organism organs. The differentiation process results in the formation of the endoderm, ectoderm, and mesoderm. The differentiation process takes place when cells with less specialization become more specialized and can happen numerous times as the zygote develops into a full organism. The embryo’s stem cells express specific genes that later determine their type of cell. For instance, ectoderm cells help in expressing the genes to cells in the skin (Gardner & Balaban, 2016). As a result, these cells later undergo differentiation, becoming epidermal cells.

Environmental and Generic Dangers to Development of the Embryo

The activities that happen in the embryo are fundamental for different cells, tissue, organs, and organ systems of the body. Defects resulting from genes or hazardous exposures from the environment during the embryonic stage have fatal effects on the organism. According to Gardner & Balaban (2016), such risks may lead to the eventual death of the embryo. In case the embryo survives and grows as a fetus, chances of it having birth defects are usually high. Examples of the environmental dangers include; consumption of strong drinks such as alcohol by the mother. Exposing an embryo to such strong drinks from the mother’s blood results in fetal alcohol spectrum disorder (Wang et al., 2018). Diagnostics X-ray: Expectant mothers going for radiation therapy risk both their lives and the embryos. Radiation is linked to the damaging of DNA and causing mutation in germ cells of the embryo.


The embryogenesis process involves the division of cells and differentiation, leading to embryo development. A human being (mammals) develops a zygote (a single-cell), resulting from the fertilization of an ovum by a sperm. The zygote goes through cleavage, where the number of cells increases in the zona pellucid. The degeneration of zona pellucid allows the embryo to increase volume, which proceeds for four to six days. It leads to the formation of blastocyst, and cells that undergo differentiation into trophoblast and an inner cell mass. At the gastrulation stage, the primitive streak forms, and the process reorganizes the embryonic layers.


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