Early Development

Welcome to this lesson today on early development. Today you are going to be learning about the development of the three germ layers which eventually give rise to organs and tissues of an embryo. Specifically, you will look at:

1. Cleavage
2. Gastrulation
3. Cell Differentiation & Morphogenesis

1. Cleavage

Before an embryo can even develop, fertilization has to occur. Fertilization is the beginning of life when an egg and a sperm combine nuclei to form a single celled zygote. A zygote is composed of one cell and is an individual's first cell. In order to develop into an embryo the zygote has to undergo divisions. Those divisions will turn this one cell into a ball of many cells, which will then later specialize and develop into an embryo.

Cleavage are the rounds of cell division and the process of cleavage happens while the zygote is moving towards the uterus within the fallopian tubes. As the zygote is moving down the fallopian tubes it will undergo a series of divisions and eventually implant in the endometrium of the uterus once it reaches it.

Each time a new cell is added to this ball of cells through the process of cleavage, it is called a blastomere. That blastomere will then take on a part of that cell's cytoplasm and will play a crucial role in how these cells will specialize and develop into an embryo. Eventually, through this process of cleavage we'll end up with a morula which is a cluster of 16 cells that reaches the uterus.

• Cleavage
• A general term to describe the splitting or breaking apart of an object or molecule; in embryology this is when the zygote starts to divide into multiple cells.
• Blastomere
• After the morula forms, each new cell that divides is called a blastomere.

2. Gastrulation

Gastrulation is a process that occurs after cleavage that rearranges the cells of the morula into three tissues layers called germ layers. The three germ layers in a developing embryo are the ectoderm, mesoderm, and endoderm. The ectoderm is the outer layer, while the mesoderm is the middle layer and the endoderm is the inner layer. The ball of 16 cells gets rearranged into these different germ layers, which will differentiate into all of our tissues and organ systems.

• Germ Layers
• The first three layers of tissue that develops in the embryo as it starts to form (ectoderm, mesoderm, and endoderm).

3. Cell Differentiation & Morphogenesis

Each layer is going to differentiate through a process called cell differentiation which is when the cells become specialized. This process of cell differentiation is called morphogenesis, which is when the tissues and organs form as the cells began to become specialized. One cell might develop into a nerve cell, which composes the nervous system or one type of cell might develop into a cell that becomes a part of your skin.

Generally, cells of the ectoderm will develop into the nervous system, sense organs, pituitary gland, epidermis and hair. The mesoderm cells will specialize into cartilage, bone, muscle, connective tissue, cardiovascular system, lymphatic system, urinary system, respiratory system, and the outer layer of the digestive tube. Endoderm cells will specialize into becoming the lining of the digestive tube and lining of the airways.

• Cell Differentiation
• The process of cells taking on specific forms and functions, also called specialization of cells.
• Morphogenesis
• When an organism develops a specific shape.
• Ectoderm
• The outer germ layer that forms the nervous system, skin, melanocytes and lenses of the eyes.
• Mesoderm
• The middle germ layer that forms muscles (cardiac, smooth, skeletal), mesenchyme, blood vessels, and mesenteries.
• Endoderm
• The innermost germ layer that forms the digestive tract, respiratory tract, endocrine glands, auditory systems, and urinary bladder.

This lesson has been an overview on early development and the three germ layers. Specifically, this lesson covered the process of cleavage, gastrulation, cell differentiation & morphogenesis.

Keep up the learning and have a great day!