Early Embryogenesis
Embryogenesis:
— formation of body structures & organs (organogenesis)
— requires cell division (proliferation) and cell differentiation (specialization)
— produces the great variety of cell types and extracellular products found in the body.
Cell specialization:
— selective gene expression (and resultant protein production) is the ultimate explanation for
the cell differentiation process during embryogenesis.
— genetic expression by a particular cell depends on the cell’s previous genetic history (commitment lineage) and its current cellular environment (intercellular communications).
Cell differentiation is the result of cells expressing some genes and suppressing others
within a common genome. Cells differ because they produced different proteins/peptides.
Proteins & peptides are:
— structural components (cytoskeleton or extracellular structures)
— enzymes (controlling cell metabolism)
— secretory products (e.g., hormones; digestive enzymes; etc.)
— channels & pumps (passage of molecules across membranes)
— receptors (communication, etc.)
Periods:
Embryonic Period — defined as the time from fertilization to the earliest (primordial) stages
of organ development (about 30 days in dog, cat, sheep, pig; almost 60 days in horse, cattle, human).
Fetal Period — the time between the embryonic period and parturition (the end of gestation), during which organs grow and begin to function.
Fertilization:
— union of a haploid oocyte and a haploid spermatozoon, producing a diploid zygote
(a pleuripotent cell capable of developing into a new individual)
— fertilization begins with gamete fusion (zygote formation)
— fertilization ends with the initiation of zygote cell division (the start of cleavage)
Fertilization related details:
— fusion of a spermatozoon with an oocyte takes place in the uterine tube, near the ovary
— the spermatozoon must bind to a specific glycoprotein on the zona pellucida surrounding
the oocyte [this species recognition process prevents union with foreign sperm];
http://vanat.cvm.umn.edu/vanatpdf/EmbryoLectNotes.pdf
Veterinary Developmental Anatomy (Veterinary Embryology)
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