Developmental Zoology (ZOO 228.1.0) Ectodermal derivatives 1 Developmental Stages Ø Early Development Fertilization Cleavage Gastrulation Neurulation Ø Later Development Organogenesis Larval molts Metamorphosis Aging Ø Invertebrates Highly variable depending on phyla Arthropods (insects) Imaginal discs Specialization of segments 2 Developmental Stages Ø Early Development Fertilization Cleavage Gastrulation Neurulation Ø Later Development Organogenesis Larval molts Metamorphosis Aging Ø Vertebrates Complete body plan is apparent at end of neurogenesis Pharyngula All vertebrate embryos similar appearance Major landmarks branchial arches/ visceral arches/ pharyngeal pouches segmented neural tube segmented axial mesoderm somites 3 1
Developmental Stages Ø Early Development Fertilization Cleavage Gastrulation Neurulation Ø Later Development Organogenesis Larval molts Metamorphosis Aging Ø Forming organs gut tube, proctodeum (anus), stomodeum (mouth) pronephros (kidney) dorsal aortas heart liver diverticulum eye anlagen otic vesicles (ears/ tympanic membranes) cartilage deposition 4 Organogenesis The process of organogenesis extends from the neurela stage to birth or hatching. The neurela stage is distinguished by differentiation, which is when unspecialized embryonic cells change into specialized cells destined to form specific tissues or organs. Organogenesis A mass of cells called the neural crest is pinched off the top of the neural tube and then migrates to other parts of the embryo to give rise to neural and other structures. Eventually, the front part of the neural tube thickens and forms the brain; the remainder of the tube becomes the spinal cord. 2
Brain Formation Next to central canal created during neurulation, is the ependyma cell layer Portion of the ependyma of the brain vascularized. Secretes cerebro-spinal fluid. Brain Formation Brain Formation Developing brain 3 regions Prosen-cephalon Forebrain, cerebral hemispheres & the Dieen-cephalon : eyes are forms as outpocketings Mesen-cephalon - Mid brain Rhomben-cephalon Forms Metenchephalon (celebrum & pons), Mylenchephalon (hind brain or brain stem) 3
Eye Development Outpocketings develop in to optic vesicles They proliferate & evaginate through the head mesenchyme & vesicles abut the surface ectoderm. Optic vesicles invaginate laterally, to form an optic cup. Adjacent ectoderm cells changes from cuboidal to columnar. Now surface ectoderm invaginates & pinches off to lie within the entrance to the optic cup Eye Development The raised columnar cells develops into the lens. Outer surface of the optic cup becomes pigmented retina Inner surface becomes photoreceptors neurons of neural retina generates an epithelial iris. Eye Development It is the optic cup that initiates the formation of the lens. 4
Neural Crest Cells So versatile that they could be called a fourth germ layer. They help form many tissues: Melanocytes, adrenal glands, peripheral system neurons including spinal ganglia & cartilage of the head. Parts of the cranial tri-germinal glossopharyngeal & vagus ganglia. Neural Crest Cells They arise along the neural tube starting around 27 hours and are migratory along specific paths. fluorescently labeled cells moving away from the neural tube over 10-12 Neural Crest Cells Neural crest cells can be fluorescently labeled with a specific antibody (HNK-1) 5
Neural Crest Cell Derivatives Origin Origin between ectoderm & neural tube Origin 6
Neural Crest Cells Neural crest cells play a major role in vertebrate evolution because they form so many of the structures in the head and pharyngeal arches that are characteristic of a vertebrate body type. Main cells responsible for changes in craniofacial evolution Bill The avian bill is formed in part from the cranial neural crest migrating from the fore and midbrain region to the branchial arches Cranial Nerves & Ganglia Most of the paired cranial nerves (bundled collections of neurons) have a cell body containing the nucleus and most of the cytoplasm in ganglia (singular ganglion). The third, fourth and sixth have no ganglia. Nerves are made up of neurons extending back to the central nervous system or to some target sensory organ or muscle. 7
Nerves Cranial Nerves The sources of the neurons can be alongside the neuromeres, or from ectoderm just under the epidermis or growth from the developing sensory organ (nose, eye, ear). The olfactory sensory nerve (I) forms axons growing from nasal pits towards cerebral ganglia. Optic nerve (II) forms from sensory retina & later grow back to join to the diencephalon Oculomotor (III) nerves are motor & later innervates the eye muscle. Cranial Nerves of the Hind Brain The trigerminal nerve (V) is both sensory & motor & send branches to eye, maxillary, mandibular processes. The facial nerve (VII) is a mix nerve for the face. It is closer to the auditory nerve (VIII) which innervates the developing ear. Glosso-pharyngeal (IX) sensory nerve can be traced to the 3 rd arch. The Vagus (X) & associated Jugular ganglion is a mixed nerve for 4 th arch. 8
Cranial Nerves of the Hind Brain Spinal accessory nerve (XI) Glossopharangyal ganglion (X) Facial Eye Development The Integument Development Ectoderm & mesoderm make contribution towards integument. Ectoderm forms the outermost layer of the skin; the epidermis, which is several layers thick. Ongoing mitosis in a basal stem cell layer continuously generates new ectodermal cells. Later undergoes keratinization. Epidermal cells filling with keratin undergo apoptosis results in outer cornified layer. 9
The Integument Development In addition, integument is composed of numerous specialized structures composed of both ectoderm & mesoderm 10