Embryology Lec 3 PDF

Embryology lec 3 Dr : Amira Madany  Egg membranes There are 3 types of egg membranes Primary egg membrane: Formed by the egg itself inside the ovary ; eg. Zona pellucida in mammals. Secondary egg membrane: Formed by the ovarian tissue before the egg leave the ovary. Coro...

Embryology lec 3 Dr : Amira Madany  Egg membranes There are 3 types of egg membranes Primary egg membrane: Formed by the egg itself inside the ovary ; eg. Zona pellucida in mammals. Secondary egg membrane: Formed by the ovarian tissue before the egg leave the ovary. Corona radiata around egg of mammals. Tertiary egg membrane: Formed during passage of the egg in the oviduct ; eg. Egg of toad and egg of bird.  Types of ova According to the amount of yolk Alecithal type egg: not contain yolk like that of human Microlecithal type egg: contains few amount of yolk like that of Amphioxus Mesolecithal type egg: contains moderate amount of yolk like that of toad. Macrolecithal type egg: contains large amount of yolk like that of birds, insects, and reptiles  Types of ova According to the distribution of yolk Isolecithal type egg: the yolk is equally distributed in the cytoplasm as in the egg of amphioxus Telolecithal type egg: the yolk is concentrated in one pole known as the vegetal pole, for example egg of toad (modertelly telo.) and egg of birds (heavy telo.) Centrolecithal type egg: the yolk is concentrated in the center of cytoplasm as in the egg of insects. Egg of Human Egg of Amphioxus Egg of Frog Egg of Bird  Fertilization is the union between the nucleus of sperm (1n) and nucleus of ovum (1n) to give zygote (2n). Types of Fertilization External fertilization: occurs in water living animals like fishes and amphibians. The embryo develops inside very thin eggshell. Internal fertilization: occurs in land living animals like reptiles, birds and mammals. In reptiles and birds, the embryo develops inside eggshell while in mammals; the embryo develops inside the uterus. Significance of fertilization restore the diploid number of chromosomes Keep the species Stimulate the egg for the subsequent division(cleavage) Stimulate the ovum to complete the 2nd meiotic division Accept new genetic characters for the embryos  Cleavage Successive mitotic division of the zygote to produce a ball of cells(blastomeres) known as blastula. Planes of cleavage Meridional plane: it is a vertical division from the animal pole to the vegetal pole and passes through the center, for example the 1st , 2nd and 4th cleavage of amphioxus. Vertical plane: similar to the meridional but not pass through the center, for example the 1st , 2nd and 4th cleavage of toad. Horizontal plane: it is the plane which divides the zygote into unequal upper half and lower half i.e this plane not pass through the center, for example the 3rd cleavage of amphioxus and toad. Equatorial plane: it is a horizontal division but pass through the center. Pattern of cleavage Holoblastic (complete) cleavage :where the axes of cleavage pass from one pole to another. It is of two types: Equal holoblastic cleavage: where the most of cleavage plane pass through the center of zygote as in amphioxus. Unequal holoblastic cleavage: where the planes of cleavage not pass through the center of zygote as in frog. Discoidal (meroblastic) or incomplete cleavage which confined to the germinal disc of the egg as in that of birds. Superficial cleavage It is confined to the outer cytoplasm rim of the egg as in insects.  Embryonic Development of Amphioxus Ova: Microlecithal and Isolecithal type Sperm: with long tail and rounded head Fertilization: External in water Cleavage: Equal Holoblastic type Cleavage of Amphioxus  Fate map of blastula of Amphioxus Presumptive ectoderm Presumptive neural plate Presumptive mesoderm Presumptive notochordal plate Presumptive endoderm TISSUE LAYER WILL BECOME… Brain and nervous system Epidermis of skin Ectoderm Hair and nails Corona and lens of eye Skeletal system (bones, cartilage, notochord) Muscular system Mesoderm Circulatory system Excretory system Reproductive system Organs: Endoderm Liver Pancreas  Gastrulation of Amphioxus In the beginning of the gastrulation, the endodermal cells become plate like. The mitotic activity in the primitive ectoderm cells will increase (Epiboly). Whereas the endodermal blastomeres will not show mitotic activity. The endodermal cells of the vegetal pole will show invagination The notochordal cells should roll to the middle of the root of the archenteron. In the same way the mesodermal cells present on the ventral lip of the blastopore should roll inside. The movement of cells inside is called involution At the end of gastrulation, disappearance of blastocoel and formation of gastrocoel (archentron) Gastrulation of Amphioxus Neurulation of Amphioxus  Neurulation of Amphioxus The presumptive neural ectoderm cells along the mid-dorsal line become flat and thickened to form a neural plate which sinks inwards below the level of lateral epidermal ectoderm. The lateral edges of the neural plate grow upwards and meet in the mid-dorsal line to from a neural tube, enclosing a canal, the neural canal or neurocoel.  Neurulation of Amphioxus Formation of notochord The cells which were invaginated from the dorsal lip of the blastopore lie in the mid-dorsal roof of the archenteron. They evaginate dorsally at the anterior end of embryo and become separated from the endoderm. This evagination of notochordal material also continues caudally and ultimately forms a solid, cylindrical cord of cells, which is called the notochord. It lies below the neural tube and between the mesodermal somites. It extends in the entire length of the body.  Neurulation of Amphioxus Formation of mesoderm The cells in the dorso-lateral roof of the archenteron are presumptive mesodermal cells, they lie on either side of the notochord. Mesoderm cells form two lateral bands, which also get separated from the endoderm by dorsal evagination. In each mesodermal band a longitudinal groove appears. The groove deepens and opens widely into the archenteron..