Embryology part 3
Hello again
Introduction to the Embryology in the Mammals
The mammalian egg is microscopic and is nearly without yolk material (Alecithal type) or, in some
forms, has a small amount of yolk that is evenly distributed throughout its cytoplasm (Isolecithal
type).
Just outside the cell membrane of the egg (vitelline membrane) there is a non-cellular layer, the
Zona pellucidn which persists during the early cleavage stages.
Fertilation is internal and takes place in the upper third of the oviduct. Cleavage pattern is of the
holoblastic equal type due to the absence or tiny amount of yolk in the fertilized ovum zygote).
The resulting blastomeres are equal in size and the cleavage furrows follow the same basic planes
previously described for other chordates. The rather solid mass of blastomeres represents a
"morula" which transforms later on to a hollow blastula. Fluid secreted by the cells of the monila
form the focus of a central cavity. The blastomeres that take a peripheral position round this
cavity are arranged in more than one layer at the original animal pole of the egg, but the remaining
blastomeres form one layer only round the blastocoele. This is the mammalian blastula, with
multilayered blastomeres at one side representing the inner cell mass (future material for the
embryo proper) and a single celled layer of blastomeres representing the trophoblast (future
material for the extraembryonic membranes)
1
, Derivatives of the germ layers during the development of A mammal:
1. Ectoderm: Epidermis of skin, hairs and nails, sweat glands, brain. spinal cord, ganglia,
nerves, receptor cells of sense organs, lens of eyes, lining of mouth, nostrils and anus,
enamel of teeth
2. Mesodern: Muscles (smooth, skeletal and cardiac), dermis of skin connective tissue, bone
and cartilage, dentine of teeth, blood and blood vessels, mesenteries, kidneys, testes and
ovaries.
3. Endoderm: Lining of gut, lining of respiratory tracts (trachea, bronchi and lungs), liver,
pancreas, lining of gall bladder, thyroid, parathyroid and thymus glands, urinary bladder
and lining of urethra
Protection of the Embryo
Certain adaptations have evolved in vertebrates to protect the eggs and embryos during the
critical period of development from unfavorable environmental conditions.
In Fishes and Amphibians:
A jelly coat surrounds the eggs of most fishes and amphibians, which is of some value in protecting
against mechanical shock. These eggs, which are large and contain yolk, are laid and typically
develop in water, from which they obtain oxygen, salts and water as required. The embryos
develop a pouch-like outgrowth of the digestive tract, the yolk sac which grows around the yolk.
This sac secretes digestive enzymes, and the digested yoik products pass into the blood vessels of
the sac to the rest of the embryo.
2
Hello again
Introduction to the Embryology in the Mammals
The mammalian egg is microscopic and is nearly without yolk material (Alecithal type) or, in some
forms, has a small amount of yolk that is evenly distributed throughout its cytoplasm (Isolecithal
type).
Just outside the cell membrane of the egg (vitelline membrane) there is a non-cellular layer, the
Zona pellucidn which persists during the early cleavage stages.
Fertilation is internal and takes place in the upper third of the oviduct. Cleavage pattern is of the
holoblastic equal type due to the absence or tiny amount of yolk in the fertilized ovum zygote).
The resulting blastomeres are equal in size and the cleavage furrows follow the same basic planes
previously described for other chordates. The rather solid mass of blastomeres represents a
"morula" which transforms later on to a hollow blastula. Fluid secreted by the cells of the monila
form the focus of a central cavity. The blastomeres that take a peripheral position round this
cavity are arranged in more than one layer at the original animal pole of the egg, but the remaining
blastomeres form one layer only round the blastocoele. This is the mammalian blastula, with
multilayered blastomeres at one side representing the inner cell mass (future material for the
embryo proper) and a single celled layer of blastomeres representing the trophoblast (future
material for the extraembryonic membranes)
1
, Derivatives of the germ layers during the development of A mammal:
1. Ectoderm: Epidermis of skin, hairs and nails, sweat glands, brain. spinal cord, ganglia,
nerves, receptor cells of sense organs, lens of eyes, lining of mouth, nostrils and anus,
enamel of teeth
2. Mesodern: Muscles (smooth, skeletal and cardiac), dermis of skin connective tissue, bone
and cartilage, dentine of teeth, blood and blood vessels, mesenteries, kidneys, testes and
ovaries.
3. Endoderm: Lining of gut, lining of respiratory tracts (trachea, bronchi and lungs), liver,
pancreas, lining of gall bladder, thyroid, parathyroid and thymus glands, urinary bladder
and lining of urethra
Protection of the Embryo
Certain adaptations have evolved in vertebrates to protect the eggs and embryos during the
critical period of development from unfavorable environmental conditions.
In Fishes and Amphibians:
A jelly coat surrounds the eggs of most fishes and amphibians, which is of some value in protecting
against mechanical shock. These eggs, which are large and contain yolk, are laid and typically
develop in water, from which they obtain oxygen, salts and water as required. The embryos
develop a pouch-like outgrowth of the digestive tract, the yolk sac which grows around the yolk.
This sac secretes digestive enzymes, and the digested yoik products pass into the blood vessels of
the sac to the rest of the embryo.
2
