Metazoan Body Plans: Organization and Characteristics

• Metazoan body plans dictate body organization, the arrangement of organs, and the polarity axes and their interrelation

Body Plan Basics

• Diploblastic organisms possess endoderm and ectoderm
• Triploblastic organisms adds in mesoderm and bilateral symmetry enabling a head-tail axis
• Diploblastic organisms are not monophyletic

Metazoan Characteristics

• Metazoans that lack bilateral symmetry are diploblastic and parahyletic
• Metazoans collagen is a major component of the extracellular matrix forming fibrillar structures that provide mechanical resistance to tissue stretching
• Metazoan meiosis produces gametes rather than spores
• Male gametes have a structure consisting of a conical head which contains the nucleus and acrosome, an intermediate piece with mitochondria and centrioles, and a flagellum
• Metazoans include Placozoans, Sponges, and Eumetazoans

Placozoans and Sponges

• Placozoans are marine benthic organisms with two cell layers, reproducing through fission
• Sponges have an ectoderm called pinacoderm, an outer layer of pinacocytes and amoebocytes
• Sponges have an endoderm called choanoderm is an inner layer of choanocytes
• Choanocytes are cells whose function is nutrition
• The apical part of the choanocyte has a flagellum surrounded by a microvilli collar
• Water passes through the collars where suspended food particles are filtered and then phagocytosed at the base of the choanocyte
• Sponges possess calcareous or siliceous spicules shaping the sponge structure
• Sponges lack true epithelia due to the absence of Cell Adhesion Molecules(CAM), adherent junctions, and basal laminae

Development

• Segmentation involves a mobile blastula attaching to a substrate
• Gastrulation forms a gastrula with two layers

Choanoflagellates and Metazoans

• Strong links between Choanoflagellates and Metazoans are supported by evidence
• Choanoflagellates are morphologically similar to sponge choanocytes
• Choanocytes are observed in other animals such as Cnidarians, Platyhelminthes, and Echinoderms, but are absent in other protists, plants, or eumycetes
• Genetic data suggest Choanoflagellates and Animals may be sister taxa; Choanoflagellates share activation genes and adhesion proteins previously exclusive to animals

Eumetazoans

• Eumetazoans include Diploblastic Cnidarians, Ctenophores, and Bilaterians
• Eumetazoans possess collagen in the basal lamina beneath the epithelial layer with connective tissue
• Desmosomes creates strong membrane junctions are present with CAMs

True Epithelium

• Communicating (gap junctions) contains connexins enabling the passage of small molecules and continuity between cells
• The digestive cavity is differentiated and digestion products are distributed

Cnidarians

• The nervous system uses chemical synapses for cell communication
• Cnidarians are the sister group to Bilaterians
• Cnidarians have radial symmetry and multiple symmetry axes
• Cnidocytes are stinging cells clustered on the ectoderm of polyp or medusa forms, used to paralyze prey _ Cnidocytes contain a cnidocil that is excited by contact with prey releasing an operculum
• The filament, with spines, penetrates the epidermis and releases venom
• Cnidarians have a specific life cycle with alternating polyp/medusa phases
• In Cnidarian development segmentation yields a blastocyst, a ciliated single-cell layer surrounding a blastocoele
• Gastrulation leads to an inner layer: the endoderm, enclosing an archenteron
• The larval stage has a ciliated larva with two layers, the planula for musculature of ectoendodermal origin
• The specialization of cells like cnidocytes, secretory, muscle, and nerve cells give them a less passive lifestyle

Bilaterian Metazoans

• Non-Bilaterians are usually sessile and lack bilateral symmetry, a nervous system, or a through-gut, also called alimentary canal.
• Bilaterians symmetry is defined by anterior-posterior and dorsal-ventral axes
• Bilaterians the anterior end has a head (tête) with a ventral side oriented forwards
• The through-gut has two openings: mouth and anus

Bilaterians cont...

• Bilaterians exhibit a mesoderm results in bilateral symmetry
• The nervous system has a around a cephalic ganglion or nerve chain
• Cephalization concentrates sensory organs
• Hox genes coordinate regional axis
• Gastrulation involves positioning mesoderm between ectoderm and endoderm, which can form a coelom
• Bilateral symmetry enables greater evolutionary innovation

Amphibians axes

• In amphibians the establishment of mesoderm in gastrulation leads to body plan
• The posterior pole forms a groove for invagination
• The anterior pole has a mesodermal advancement
• The dorsal-ventral axis has a gradient of mesoderm differentiation and neural tube formation, as well as development of the axial skeleton

Bilateral Symmetry

• Though not always visible bilateral symmetry can be seen early in development
• Prior to amphibian fertilization, radial symmetry is present but, following fertilization, bilateral symmetry arises following the formation of a gray crescent
• Symmetrization rotation turns 30° aligning the pigmented cap with sperm entry
• In drosophila before fertilization, the anterior oocyte face connects with nurse cells forming mesoderm, the dorsal side flattens as the ventral side rounds, setting the stage
• Bilateral symmetry is responsive to external signals and cascades of genetic determinants that differentially express genes according to body parts to establish body plan

Drosophila axis determination

• Molecularly, the anterior-posterior axis in Drosophila is determined by the Bicoid gene
• Bicoid mRNAs localize to the anterior and are translated after fertilization
• In segmentations, Bicoid protein establishes an AP axis gradient acting as FT for zygotoc genes and determining Hox genes

Embryonic Development

• The blastoderm embryo expresses homeotic genes
• Extensive colored surfaces signify high expression segments
• Lighter coloration signifies lower parasegment expression
• Spatial colinearity is established where gene order match spatial expression

Drosophila axis determination cont...

• The drosophila AP axis is progressively determined through maternal effect genes and zygotic effect genes, like homeotic (Hox) genes
• Hox genes are responsible for differentiating the body along the AP axis
• Homeotic genes contribute to body plan organization
• Hox genes are homologous and derived from gene duplication
• Homeotic genes were detected study of mutations resulting in homeosis where a body part is replaced by another from elsewhere

Hox Genes in Evolution

• Hox gene number varies over time where Metazoans have fewer that function differently from axis placement
• Bilaterian colinearity is observed in its ancestor

Bilaterians: Diversity and Evolution

• Protostomes: The blastopore becomes the mouth, anus forms secondarily
• Protostomes are hyponeurians with a ventral nervous system
• They have an outer exoskeleton
• Spiralians: spiral cleavage
• The segmentation is a spiral blastula
• Plathelminthes: a Rouphozoaire that feeds via pharyngeal aspiration
• Ecdysozoans grow discontinuously via ecdysone, developing successive chitin cuticles
  • This clade includes Nematoids that lack flagella
• Euarthropods form articulated appendages with divided exoskeleton into sclerites

Bilaterians: Deuterostomians

• The blastopore becomes the anus and the mouth forms secondarily
• Ambulacrarians consists of the Echinoderms and Hemichordates
• The larval pluteus possesses bilateral symmetry, while the adult has pentaradial symmetry
• Through-gut has two openings: mouth and anus
• The body plan has one set of hox genes

Chordates

• Chordates have epineurians forming dorsal neural tubes
• Chordates have a dorsal spinal cord at least in early development
• Cephalochordates: are amphioxus, they retain chorda and exhibits colinearity of HOX genes
• Vertebrates: Possess HOX duplication and Cyclostomes are lamprey and myxine

Vertebrates and the Jaw

• Myxine: retains chorda, adults degenerate vertebrae
• Lamproie; retains chord, are adults and vertebrae
• Passing from amphioxus to vertebrates is complex as it leads to skull formation, ME, and special cells like osteoblasts, odontoblasts facilitated by by HOX multiplication
• Gnathostomes: Have multiplated homeotic complex forming unique gene families in 4 paralogous complexes
  • Chondrichthyes: cartilaginous skeleton
  • Osteichthyes: bone skeleton