Vertebrate Development & Evolution

Theme 1: Evolution and Developmental Biology

• Lecture Lz3 discusses the embryonic development of the vertebrate blueprint (bauplan).

EVO-DEVO

• EVO-DEVO combines evolutionary biology and developmental biology.
• It studies adaptation through variation, replication, inheritance, selection, development, and convergence.

Key Stages in Ontogeny of Vertebrates

• Fertilization involves the fusion of egg and sperm, resulting in a zygote.
• Cleavage is the process of cell divisions, forming a blastocoel.
• Gastrulation leads to the formation of three germ layers: ectoderm, mesoderm, and endoderm, forming the archenteron.
• Organogenesis involves folding to create organs, described as a 'tube in tube' structure
• Growth leads to reach adult form

Key Stages in Evolution

• Evolution progresses from a single cell to multicellular organisms.
• Diploblasts have two layers: ectoderm and endoderm and a gastrovascular cavity.
• Triploblasts have three layers: ectoderm, mesoderm, and endoderm in a process called archenteron.
• The quadruplo'blast stage features the development of the neural crest.

Life Cycle of the Frog

• Gametogenesis produces sperm and oocytes.
• Fertilization follows the stage of gametogenesis.
• Cleavage results in a morula and then a blastula.
• Gastrulation and organogenesis lead to larval stages and eventual maturity.

Cleavage and Blastula stage

• Cleavage is defined as the multiplication of cells in a developing embryo.
• Mesolecithal refers to the average yolk that is unequally distributed.
• Isolecithal refers to where little yolk are equally distributed .
• Holoblastic cleavage undergoes whole egg cell divisions.
• Holoblastic cleavage is observed across the animal kingdom.
• A blastula forms a blastocoel (the cleavage cavity).
• Morula is the formation prior to blastula

Vertebrate Phylotypic Stage

• The pharyngula stage involves the development of gill slits, somites, heart, and tail.

Diploblast Development

• Diploblasts are early ancestral species with ectoderm and endoderm layers.

Triploblast Development

• Triploblasts are more derived species forming ectoderm, endoderm, and mesoderm layers.

Cleavage Cell Divisions and Yolk

• Holoblastic leads to whole egg cell divisions.
• Meroblastic leads to part of the egg undergoes cleavage.
• Isolecithal eggs have little yolk distributed equally.
• Mesolecithal eggs have an average yolk distributed unequally.
• Telolecithal eggs have a large yolk distributed unequally.
• Rotational holoblastic cleavage found in mammals which are isolecithal.
• Radial holoblastic cleavage found in mesolecithal eggs such as in frogs.
• Discoidal meroblastic cleavage found in telolecithal eggs such as in chicks.

Blastocyst Formation in Mammals

• The process involves whole egg cell divisions typical of holoblastic and isolecithal eggs.

Key stages of blastocyst formation in humans

• Formation and hatching of morula which is a sold ball of cells.
• The formation of blastula which has in inner cell mass.
• The attachment of the blastula in the uterine wall which occurs about 7-10 days after fertilization.

Implantation in Humans

• The trophoblast provides nutrition to the embryo.

Embryoblast layers

• The two cell layers that the embryoblast forms include the epiblast and hypoblast.
• The epiblast forms the ectoderm, which later develops into the amnion.
• The hypoblast forms the endoderm, which becomes the yolk sac.
• Both the epiblast and hypoblast contribute to the embryo.

Gastrulation

• Gastrulation is considered the most important stage in life.
• During gastrulation, three germ layers are formed: ectoderm, mesoderm, and endoderm.

Germ Layers

• Ectoderm forms surface ectoderm (epidermis) and the neural tube (central nervous system).
• Mesoderm forms the notochord, somites, intermediate mesoderm, and lateral mesoderm.
• Endoderm forms the lining of the GI tract, respiratory system, liver, pancreas, testis, and ovarium.

Extra-Embryonal Structures

• The yolk sac is the first extraembryonic membrane in birds, found surrounding the embryo within the egg.
• Mesoderm and ectoderm extend beyond the embryo and surrounds the embryo.
• Mesodermal and endodermal tissues forms the allantois, which is sac that hold metabolic wastes.

Amnion, Chorion, and Allantois

• The amnion surrounds the embryo and prevents dehydration while helps absorb shocks.
• The chorion is the outer membrane, providing protection and a respiratory surface.
• The allantois stores waste and works as another respiratory surface in the embryo.

Yolk Sac

• The yolk sac is formed from yolk cells and provides nutrition and hematopoiesis.
• A second yolk sac is formed from the hypoblast.

Neuralation (Amphibians)

• The neural plate folds to become the neural tube.

Neurulation

• The surface ectoderm becomes the epidermis.
• The neural tube will become the central nervous system.
• Derivates of the somite mesoderm includes vertebrae, muscles and dermis.

Organogenesis

• Somites develop into sclerotome, myotome, and dermatome.

Neural Crest development

• Neural crest cells differentiate into neurons, Schwann cells, and pigment cells.

Neural Crest Development Results

• Neural crest has been found in both vertebrate and jawless-vertebrate ancestors.
• Multipotent neural crest with long-range, direct migration.
• Peripheral Nervous System has been known to form.