During Which Stage Of Animal Embryo Development Do The Germ Layers Form?

Affiliate 13: Introduction to Animal Reproduction and Evolution

13.2 Development and Organogenesis

Learning Objectives

Past the end of this section, you lot will exist able to:

• Explain how the embryo forms from the zygote
• Discuss the role of cleavage and gastrulation in animal development
• Describe organogenesis

The process by which an organism develops from a single-celled zygote to a multi-cellular organism is complex and well regulated. The regulation occurs through signaling between cells and tissues and responses in the form of differential factor expression.

Early Embryonic Development

Fertilization is the process in which gametes (an egg and sperm) fuse to course a zygote (Figure 13.8). To ensure that the offspring has only 1 complete diploid fix of chromosomes, only i sperm must fuse with one egg. In mammals, a layer chosen the zona pellucida protects the egg. At the tip of the head of a sperm cell is a structure like a lysosome called the acrosome, which contains enzymes. When a sperm binds to the zona pellucida, a series of events, called the acrosomal reactions, take place. These reactions, involving enzymes from the acrosome, allow the sperm plasma membrane to fuse with the egg plasma membrane and permit the sperm nucleus to transfer into the ovum. The nuclear membranes of the egg and sperm break downwards and the 2 haploid nuclei fuse to form a diploid nucleus or genome.

Figure 13.8 Fertilization is the process in which sperm and egg fuse to form a zygote. (credit: scale-bar information from Matt Russell)

To ensure that no more than than one sperm fertilizes the egg, once the acrosomal reactions have place at one location of the egg membrane, the egg releases proteins in other locations to forestall other sperm from fusing with the egg.

The evolution of multi-cellular organisms begins from this single-celled zygote, which undergoes rapid cell partitioning, called cleavage (Figure thirteen.9a), to form a hollow ball of cells chosen a blastula (Figure 13.9b).

Figure 13.9 (a) During cleavage, the zygote rapidly divides into multiple cells. (b) The cells rearrange themselves to class a hollow ball chosen the blastula. (credit a: modification of work by Gray'due south Anatomy; credit b: modification of work past Pearson Scott Foresman; donated to the Wikimedia Foundation)

In mammals, the blastula forms the blastocyst in the next stage of development. Hither the cells in the blastula arrange themselves in two layers: the inner cell mass, and an outer layer called the trophoblast. The inner jail cell mass will proceed to course the embryo. The trophoblast secretes enzymes that allow implantation of the blastocyst into the endometrium of the uterus. The trophoblast will contribute to the placenta and attend the embryo.

Concept in Action

Visit the Virtual Human Embryo projection at the Endowment for Human Development site to click through an interactive of the stages of embryo development, including micrographs and rotating 3-D images.

The cells in the blastula and so rearrange themselves spatially to form three layers of cells. This process is called gastrulation. During gastrulation, the blastula folds in on itself and cells drift to form the three layers of cells (Figure xiii.ten) in a structure, the gastrula, with a hollow infinite that volition become the digestive tract. Each of the layers of cells is chosen a germ layer and will differentiate into different organ systems.

Figure xiii.10 Gastrulation is the procedure wherein the cells in the blastula rearrange themselves to form the germ layers. (credit: modification of work by Abigail Pyne)

The three germ layers are the endoderm, the ectoderm, and the mesoderm. Cells in each germ layer differentiate into tissues and embryonic organs. The ectoderm gives ascension to the nervous system and the epidermis, among other tissues. The mesoderm gives ascent to the muscle cells and connective tissue in the body. The endoderm gives rise to the gut and many internal organs.

Organogenesis

Gastrulation leads to the formation of the 3 germ layers that give rise during further development to the different organs in the animal body. This process is chosen organogenesis.

Organs develop from the germ layers through the procedure of differentiation. During differentiation, the embryonic stem cells limited specific sets of genes that will decide their ultimate prison cell type. For example, some cells in the ectoderm will express the genes specific to skin cells. As a result, these cells will accept on the shape and characteristics of epidermal cells. The process of differentiation is regulated by location-specific chemical signals from the prison cell'south embryonic environs that sets in play a cascade of events that regulates gene expression.

Section Summary

The early stages of embryonic evolution begin with fertilization. The process of fertilization is tightly controlled to ensure that just one sperm fuses with one egg. After fertilization, the zygote undergoes cleavage to form the blastula. The blastula, which in some species is a hollow ball of cells, undergoes a process chosen gastrulation, during which the three germ layers form. The ectoderm gives ascension to the nervous system and the epidermal skin cells, the mesoderm gives rise to the musculus cells and connective tissue in the body, and the endoderm gives ascent to the digestive system and other internal organs. Organogenesis is the formation of organs from the germ layers. Each germ layer gives rise to specific tissue types.

Exercises

1. The process of gastrulation forms the _______.
   1. blastula
   2. zygote
   3. organs
   4. germ layers
2. Which of the following gives rise to the skin cells?
   1. ectoderm
   2. endoderm
   3. mesoderm
   4. none of the above
3. What do you call up would happen if multiple sperm fused with i egg?

Answers

1. D
2. A
3. If multiple sperm fused with one egg, a zygote with a multiple ploidy level (multiple copies of the chromosomes) would course, and and then would die.

Glossary

blastocyst:the construction formed when cells in the mammalian blastula split up into an inner and outer layer

gastrulation:the process in which the blastula folds over itself to course the three germ layers

inner cell mass:the inner layer of cells in the blastocyst, which becomes the embryo

organogenesis:the process of organ formation during development

trophoblast:the outer layer of cells in the blastocyst, which gives rising to the embryo's contribution to the placenta

zona pellucida:the protective layer effectually the mammalian egg

Source: https://opentextbc.ca/biology/chapter/13-2-development-and-organogenesis/

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