Embryology and Body Cavities Lecture 3

Tissue Development Zygote (fertilized egg) undergoes rapid cell divisions called cleavage Forms a hollow ball of cells called the blastula Blastula

Tissue Development Continued The blastula INVAGINATES (folds inward at one point) Called Gastrulation The opening is called the blastopore The center is the primitive gut or Archenteron blastopore Archenteron Gastrula

Embryonic Development

Tissue Development Protostomes-mouth forms from blastopore – Mollusks, Annelids, Arthropods Deuterostomes- anus forms from blastopore – Echinodermata, Chordata

Germ Layers Germ layers forms tissues, organs, and systems 3 Types of Germ Layers 1.Ectoderm 2.Mesoderm 3.Endoderm

3 Germ Layers 1.Ectoderm- outer most layer – Gives rise to: Epidermis (skin) Nervous system

2. Mesoderm-middle layer – Gives rise to: Muscles Skeleton Blood System

Endoderm- inner most layer – Gives rise to: Digestive Organs Lungs Bladder

Three Types of Organisms 1. Multicellular-have no tissue layers at all – Sponges 2.Diploblastic-have only Ectoderm and Endoderm – Cnidarians (jellyfish, sea anemones) 3.Triploblastic-have all three layers – Mollusks, arthropods, echinoderms, chordates, etc.

Benefit of Having a Body Cavity? Provides room for organ development Provides more surface area for gas exchange and nutrient absorption Provides for storage area

Benefits Cont’d Can act as a hydrostatic skeleton Allows for waste removal Allows for increased body size

Triploblastic Organisms Have three Coelom (body cavity) patterns – Acoelomate – Pseudocoelomate – Coelomate

Acoelomates Have a solid mass of Mesoderm cells between the Endoderm and Ectoderm layers Ex. Flatworms

Pseudocoelomates The cavity is not entirely lined by mesoderm Ex. Nematode (worm)

Coelomates The body cavity is completely surrounded by mesoderm. Ex. Human

Triploblastic Body Plans AcoelomatePseudocoelomateCoelomate