Question

Write the time sequence in which the following structures begin to develop: notochord, gastrula, inner cell mass, fetus, zygote, morula.

Short answer

The correct sequence of development is: Zygote, Inner cell mass, Morula, Gastrula, Notochord, Fetus.

Step-by-step solution

Identify the initial stage

The zygote is the initial stage of development and is formed by the fusion of sperm and egg during fertilization.

Identify the second stage

After fertilization, the zygote undergoes cell division and forms the inner cell mass. This mass is a group of cells within the early embryo that will develop into the body of the organism.

Identify the third stage

Continued cell division of the inner cell mass leads to the formation of morula, a solid mass of cells.

Identify the forth stage

The morula undergoes further cell division and differentiation, leading to the formation of gastrula, a three-layered structure which includes the ectoderm, mesoderm, and endoderm.

Identify the fifth stage

After gastrulation, the notochord develops from the middle layer, or mesoderm of the gastrula. The notochord is a rod-shaped structure that is the precursor to the vertebral column.

Identify the final stage

Subsequent to these developments, the embryo progresses and grows into a fetus in the womb.

Key concepts

Zygotic Stage

The zygotic stage marks the beginning of embryonic development. It starts when the sperm fertilizes the egg, resulting in the formation of a single cell known as a zygote. This cell contains a complete set of DNA from both parents, which provides the genetic blueprint for the future organism.

• Formation: This is the very first step in the development sequence.
• Key Process: Fertilization, where genetic material from the sperm and egg combine to create a new organism.
• Significance: Establishes the genetic identity of the embryo.

Once formed, the zygote begins a series of rapid cell divisions, a process known as cleavage, setting the stage for the development of all the other structures.

Gastrulation

Gastrulation is a critical phase in early embryonic development where the simple ball of cells undergoes significant morphological changes. This process transforms the embryonic cells into a multi-layered structure called the gastrula.
During gastrulation, three distinct layers form:

• Ectoderm: The outermost layer, which will give rise to the skin and nervous system.
• Mesoderm: The middle layer, responsible for forming muscles, bones, and the circulatory system.
• Endoderm: The innermost layer, which develops into the digestive tract and other internal organs.

The formation of these layers is crucial as it sets the groundwork for organogenesis, the development of organs in the embryo.

Notochord Formation

Notochord formation is a pivotal event during development that originates from the mesodermal layer of the gastrula. The notochord is an elongated rod-like structure that serves as the primary axial support during early development.

• Role: Acts as a signaling center critical for the development of surrounding structures.
• Function: Dictates the formation of the neural tube, a precursor to the central nervous system.
• Fate: Ultimately, it becomes part of the vertebral column, contributing to the intervertebral discs in vertebrates.

This process not only provides structural support but also plays a key role in the patterning and differentiation of tissues along the embryo's body axis.

Inner Cell Mass

The inner cell mass (ICM) is a group of cells inside the early embryo that play a crucial role in further development. Formed shortly after the zygotic stage, these cells are destined to become the body of the developing organism.
After initial cell divisions, the embryo forms a structure known as the blastocyst, where the ICM resides.

• Composition: Consists of pluripotent cells which can develop into any cell type in the body.
• Potential: These cells will differentiate and develop into the entire organism except the placenta.
• Significance: Ensures the proper formation and differentiation of tissues and organs as development progresses.

Morula Stage

Following several rounds of cell division, the early embryo forms a compact mass known as the morula. This stage is named after the Latin word 'morus', which resembles the mulberry fruit due to its solid appearance.
The morula is an important transitional phase that precedes the formation of the blastocyst.

• Structure: A solid ball of 16-32 cells resulting from continuous division of the zygote.
• Development: The cells within begin to experiment with compacting and communicating, setting the stage for differentiation.
• Transition: Morphs into the blastocyst, which will soon house the inner cell mass.

This stage is essential in preparing the embryo for implantation into the uterine wall and further developmental processes.