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By the end of this section, you will be able to:
  • Describe the process of organogenesis
  • Identify the anatomical axes formed in vertebrates

Gastrulation leads to the formation of the three germ layers that give rise, during further development, to the different organs in the animal body. This process is called organogenesis    . Organogenesis is characterized by rapid and precise movements of the cells within the embryo.

Organogenesis

Organs form from the germ layers through the process of differentiation. During differentiation, the embryonic stem cells express specific sets of genes which will determine their ultimate cell type. For example, some cells in the ectoderm will express the genes specific to skin cells. As a result, these cells will differentiate into epidermal cells. The process of differentiation is regulated by cellular signaling cascades.

Scientists study organogenesis extensively in the lab in fruit flies ( Drosophila ) and the nematode Caenorhabditis elegans . Drosophila have segments along their bodies, and the patterning associated with the segment formation has allowed scientists to study which genes play important roles in organogenesis along the length of the embryo at different time points. The nematode C.elegans has roughly 1000 somatic cells and scientists have studied the fate of each of these cells during their development in the nematode life cycle. There is little variation in patterns of cell lineage between individuals, unlike in mammals where cell development from the embryo is dependent on cellular cues.

In vertebrates, one of the primary steps during organogenesis is the formation of the neural system. The ectoderm forms epithelial cells and tissues, and neuronal tissues. During the formation of the neural system, special signaling molecules called growth factors signal some cells at the edge of the ectoderm to become epidermis cells. The remaining cells in the center form the neural plate. If the signaling by growth factors were disrupted, then the entire ectoderm would differentiate into neural tissue.

The neural plate undergoes a series of cell movements where it rolls up and forms a tube called the neural tube    , as illustrated in [link] . In further development, the neural tube will give rise to the brain and the spinal cord.

Illustration shows a flat sheet. The middle of the sheet is the neural plate, and the epidermis is at either end. The neural plate border separates the neural tube from the epidermis. During convergence the plate folds, bringing the neural folds together. The neural folds fuse, joining the neural plate into a neural tube. The epidermis separates and folds around the outside.
The central region of the ectoderm forms the neural tube, which gives rise to the brain and the spinal cord.

The mesoderm that lies on either side of the vertebrate neural tube will develop into the various connective tissues of the animal body. A spatial pattern of gene expression reorganizes the mesoderm into groups of cells called somites with spaces between them. The somites, illustrated in [link] will further develop into the ribs, lungs, and segmental (spine) muscle. The mesoderm also forms a structure called the notochord, which is rod-shaped and forms the central axis of the animal body.

 Embryo resembles a segmented earthworm with a bulging head.
In this five-week old human embryo, somites are segments along the length of the body. (credit: modification of work by Ed Uthman)

Vertebrate axis formation

Even as the germ layers form, the ball of cells still retains its spherical shape. However, animal bodies have lateral-medial (left-right), dorsal-ventral (back-belly), and anterior-posterior (head-feet) axes, illustrated in [link] .

Illustration shows a fish dissected by lines into anterior (front) and posterior (rear) ends and dorsal (top) and ventral (bottom) surfaces.
Animal bodies have three axes for symmetry. (credit: modification of work by NOAA)

How are these established? In one of the most seminal experiments ever to be carried out in developmental biology, Spemann and Mangold took dorsal cells from one embryo and transplanted them into the belly region of another embryo. They found that the transplanted embryo now had two notochords: one at the dorsal site from the original cells and another at the transplanted site. This suggested that the dorsal cells were genetically programmed to form the notochord and define the axis. Since then, researchers have identified many genes that are responsible for axis formation. Mutations in these genes leads to the loss of symmetry required for organism development.

Animal bodies have externally visible symmetry. However, the internal organs are not symmetric. For example, the heart is on the left side and the liver on the right. The formation of the central left-right axis is an important process during development. This internal asymmetry is established very early during development and involves many genes. Research is still ongoing to fully understand the developmental implications of these genes.

Section summary

Organogenesis is the formation of organs from the germ layers. Each germ layer gives rise to specific tissue types. The first stage is the formation of the neural system in the ectoderm. The mesoderm gives rise to somites and the notochord. Formation of vertebrate axis is another important developmental stage.

Questions & Answers

What is Staining?
Fazal Reply
what is biology
PEACE Reply
Biology is the study of life
Tijani
what is biology
Ysabella Reply
biology is a study of living things
PEACE
Biology is a diverse branch of science that deals with mostly living things
Emmanuel
yes
Swapnil
What happen when inhibit the transcription?
Swapnil
what is the effect of not doing sexual intercourse
SUZAN Reply
what is the mechanism of cellular respiration
Rita Reply
what is enzyme
garry Reply
They are organic catalysts that alter the rate of chemical reactions in the body.
Iyadi
what is a cell
Praize Reply
Basic Functional Unit of Life
Pascal
what is biology
Mordi Reply
biology is the study of living organisms and their interactions with one another and their environments
Ysabella
which of the following event does not occur during some stages of interface?
Bangha Reply
What is microfilaments
KHalid Reply
What is multicellular organisms
Ovie Reply
these are organisms with more than two cells
Bangha
the process when a male toad fertilizer a female eggs is called what?
Ahrebe Reply
Fertilization
Gyamfi
how did unicellular organisms form plants and animals or is it that different unicellular organisms formed plants and animald
YXNG Reply
yes
James
thanks
YXNG
name the components of faeces
Damali
undigested carbohydrate, fibre
Sandra
what are unicellular organisms..?
Sackson
they have only one cell
Sandra
faeces contains many undigested food materials, after the food has been digested then it will be absorbed in the blood stream for assimilation.,......... but the remains toxic materials are stored in the rectum these toxic materials are the faeces and it contains bile salts, the polysaccharides .
James
nice.
Sandra
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James
unicellular organisms are the ones with only single cell.
James
thanks for your answers guys.
Sackson
Ok
Richard
what is class bryophyta
Emefa Reply
how many stages do we have in glycolysis?
Damali
10 stages
Elisha
the presence of a membrane enclosed nuclosed is a characteristics of what
Addai Reply
eukaryotic cell
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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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