Animal Development - Biology for Science - Lecture Notes, Study notes for Biology. Allahabad University

Biology

Description: These are the lecture notes of Biology for Science. Key important points are: Animal Development, Multicellular Flowering Plants, Growth, Differentiation, Morphogenesis, Development of Multicellular Organisms, Single-Celled Zygote, Tissue-Specific Proteins
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Chapt. 47 Animal Development
NOTE: You should review growth, differentiation, and morphogenesis (i.e., development) in
angiosperms (multicellular flowering plants) from Chapt. 35; see the chapter outline for Chapt. 35
for those details.
NOTE: You are strongly encouraged to examine and know the details in figures listed from Chapt.
21, since they illustrate concepts that are assumed as background for Chapt. 47.
Growth, differentiation, and morphogenesis also occur during the development of multicellular
organisms…
E.g., from a single-celled zygote (about the size of a period on a printed page) to a fully mature
adult human
Cell division alone would simply result in a growing mass of identical cells
Development produces cells of different types, arranged in a particular three-spatial dimensional
pattern and appearing in a particular temporal pattern [Fig. 21.4]
All of the autosomal cells of a given organism share the same genetic material (the organism’s
genome)
Differentiation and morphogenesis result from differences in gene expression among cells, i.e.,
different portions of the common genome are expressed in different cells
Differentiation occurs as tissue-specific proteins are produced, some of which are transcription
factors
E.g., skeletal muscle cells [Fig. 21.10]
Transcription factors = regulatory proteins that can “switch on” developmental cascades by
causing gene expression
E.g., stem cells used in medical research and treatment [Fig. 21.9]
This example also illustrates the critical nature of the environment for a cell’s differentiation
The environment determines which genes are expressed
The internal and external environments influence gene expression
E.g., differences in the chemical constitution of a cell’s cytoplasm received from the parent cell
cause divergent differentiation in the daughter cells [Fig. 21.11 & 47.24]
E.g., induction by signals from other cells causes selective gene expression [Fig. 21.11 & 47.25]
Consider this classic example from Hans Spemann and Hilde Mangold (1920s)
A piece from the dorsal side of a nonpigmented newt gastrula was transplanted to the
ventral side of a pigmented gastrula
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A secondary embryo developed on the primary embryo’s ventral side
The secondary embryo’s tissues were largely derived from the primary embryo’s gastrula,
indicating that induction from the cells of the small piece of transplanted non-pigmented
gastrular tissue “triggered” or “switched on” the developmental cascade that caused the
development of the secondary embryo
As specific genes are expressed, owing to the particular environment a cell experiences, tissue-specific
proteins are produced that cause changes in a differentiating cell
E.g., a tube, such as the neural tube in vertebrates, may form from cells in a single layer becoming
wedge shaped [Fig. 47.19]
In this example, tissue-specific proteins including those forming microfilaments and
microtubules, cause the cells to change shape
A major difference in morphogenesis in plants and animals is that only in animals do some cells
change position within the developing organism
In this example, cell shape and positional changes result in a sheet of cells becoming narrower and
longer [Fig. 47.20]
As cells change shape and position, embryologists have used dyes to create fate maps of regions of
cells [Fig. 47.23a] and individual cells [Fig. 47.23b]
Developmental biologists have also discovered that molecular cues convey positional information to
cells, informing cells of their positions relative to other cells in the developing body
For example, cell-specific gene expression in this chick’s wing depended and continues to depend
upon cells’ positions relative to other cells in 3D [Fig. 47.26]
Vertebrate limbs, like a chick’s wing, begin as bumps of tissue known as limb buds
Two main organizer regions of cells send chemical signals that form concentration gradients
that define two of the main spatial axes of the developing limb
The apical ectodermal ridge (AER) defines the proximal-distal axis
The zone of polarizing activity (ZPA) defines the anterior-posterior axis
Development isn’t restricted to embryonic and juvenile states; it occurs throughout the lifetime of an
organism…
E.g., in all organisms some cells are continually being replaced (e.g., red blood cells in humans)
E.g., in humans one’s behavior changes throughout one’s lifetime
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