The First Morphogenetic Events During Mammalian Development
Pp. 6-35 (30)
Manal E. Elsawaf
The question of how could a single cell develop into an animal has been
asked for centuries. Since the seventeenth century, epigenesis and preformation
theories have been two persistent ways seeking to explain the development of
individual organic form. Nowadays, it is proved that both zygote’s genome and
cytoplasmic determinants control development. Cell division, cell differentiation and
morphogenesis then take place. To build an animal’s body, fertilization is the first step
where a diploid number of chromosomes is restored. Fusion of egg and sperm activates
the egg. Three stages then follow; cleavage, gastrulation and organogenesis. Cleavage
pertains to the repeated mitotic division of a zygote into smaller cells, blastomeres.
More cleavage results in a solid ball of cells called morula. With further cleavage a
hollow ball of cells, the blastocyst, is produced. Gastrulation is a necessary event in
developing a multicellular animal. During this process, the embryonic cells are
rearranged to form a three layered embryo. Accordingly, cells acquire new positions
enabling them to interact with cells that were initially far away from them. Many
inductive interactions then occur to start neurulation and organogenesis. Early in
vertebrate organogenesis, the notochord which forms in mesoderm leads to neural plate
induction from the covering ectoderm. This neural plate forms the neural tube that will
become the central nervous system. All other organs develop from folds, splits and
condensations of cells. Thorough understanding of early mammalian development has
initiated the era of embryonic stem cell generation and its use in medicine.
Acrosome reaction, Amniotic cavity, Blastocyst, Capacitation,
Cleavage, Compaction, Ectoderm, Endoderm, Epigenesis, Fertilization, Folding,
Gastrulation, Implantation, Mammalian development, Mesoderm, Morphogenesis, Neurulation, Notochord, Organogenesis, Pluripotency, Preformation, Primitive streak,
Stem cells, Yolk sac.
Faculty of Medicine, Tanta University, Egypt.