Development & Tissues

Embryonic Development: Embryogenesis




How an animal embryo develops:

Syngamy: embryogenesis begins when the female haploid (n) gamete (egg cell plural: ova; singular: ovum) is fertilized by a male haploid (n) gamete (sperm cell plural: spermatozoa; singular: spermatozoon) and thus becomes a diploid (2n) zygote.

Cleavage: the zygote then undergoes a series of mitotic cell divisions to become a dense multicellular cluster that is still the same size of the original zygote. Each cell is called a blastomere.

Morula: the first four (24) cell divisions produce sixteen cells called a morula.

Blastula: After the seventh cleavage (27) about 128 blastomeres (cells) exist as a spherical layer of cells surrounding a hollow, fluid-or-yolk-filled cavity termed a blastoceol.

Gastrula: cells migrate to the interior of the blastula so that the two  (diploblastic) or three ( triploblastic) germinal layers are aligned in their correct positions.

Archenteron: it is a primitive gut which eventually can become the lumen of the alimentary canal. It is formed during gastrulation when the spherical embryo invaginates (sort of like a double-walled cup) and the blastoceol is reduced. The cells lining the inside gives rise to the endoderm and/or mesoderm while the blastomeres (cells) on the outside become the ectoderm.

Blastopore: the opening into the archenteron. In triploblastic organisms the distinction between protostomes and deuterostomes is based on the direction in which the mouth (stoma) develops in relation to the blastopore. In protostomes ("mouth first") the blastopore becomes the mouth; in deuterostomes ("mouth second") it becomes the anus.

Germ Layers


See also: Germ layers 
 * Ectoderm develops into epidermal and neural tissues.
 * Mesoderm displaces blastoceol and develops into muscle, endoskeleton, and connective tissues.
 * Endoderm develops into feeding tissues, gut, and accessory organs.

Two Clades: Triploblastic protostomes with spiral determinate cleavage
 * Lophotrocazoa: cillia
 * Ecdysozoa: molt



Tissue Types
Links:

Images From Tissue Slides Viewed in Class