Animal Development: From Zygote to Organism

• Between fertilization and birth, a developing organism is known as an embryo, which is a staggering concept
• One critical distinction between organisms and machines lies in the fact that every multicellular organism must steadily function even as it constructs itself.
• Multicellular organisms arise through the process of development, and in almost all instances, it commences with a single, fertilized egg, or zygote
• The zygote undergoes mitotic divisions
• Traditionally, the study of animal development is called embryology, development extends beyond birth and adulthood
• Animal development is characterized by the differentiation of the fertilized egg into various cell types and the construction of functionally integrated organs
• Development can be studied from molecules to ecosystems, is the route from genotype to phenotype
• Development includes fertilization, cleavage, gastrulation, organogenesis, metamorphosis, regeneration, and senescence
• Key questions in science involve how cell types form and differ, how cells organize into functional organs, how organs determine their size, how organisms make reproductive cells, how organisms regenerate, how organisms integrate environmental cues, and how development changes to create new organism types

Questions of Developmental Biology

• Animal development sparks fundamental questions about body formation and anatomical differences
• Developmental biologists seek to understand how genotype influences sexual anatomy
• Developmental biologists ask how the globin genes express only in red blood cells and at specific times
• Development generates cellular diversity and order, ensuring life's continuity across generations

Categories of questions scrutinized by developmental biologists

• Differentiation addresses how a single fertilized egg gives rise to hundreds of cell types
• Morphogenesis explores how cells organize into functional structures through division, migration, and death
• Growth investigates how cells know when to stop dividing to ensure proper size and proportions
• Reproduction examines how germ cells transmit instructions for organism creation from one generation to the next
• Regeneration explores how some organisms can regenerate body parts and how stem cells retain this capacity
• Environmental integration studies how organisms integrate environmental cues for proper development
• Evolution analyzes how changes in development create new body forms and heritable changes for survival

Developmental biology and its importance

• Developmental biology's questions are crucial in molecular biology, physiology, cell biology, genetics, anatomy, cancer, neurobiology, immunology, ecology, and evolutionary biology
• An understanding of development is essential for other biology areas
• Advances in molecular biology and cell imaging help answer developmental questions
• Developmental biologists come from diverse backgrounds like cell biology, genetics, biochemistry, immunology, and even fields like anthropology, engineering, physics, and art

The Cycle of Life

• Animals, fungi, and plants develop through an embryo, translating genotype to phenotype to form the adult
• Developmental biology studies organism building and processes
• A generalizable animal life cycle is a major triumph of descriptive embryology
• Modern biology examines temporal changes in gene expression and anatomical organization along this life cycle
• Each animal develops through: fertilization, cleavage, gastrulation, organogenesis, birth, metamorphosis, and gametogenesis
• Development between fertilization and hatching/birth is called embryogenesis

Key stages of development

• Fertilization is the fusion of mature sex cells (gametes) that stimulate egg development and create a new individual. The genome is created, instructing the embryo to develop like its parents
• Cleavage is rapid mitotic divisions after fertilization that divide the zygote cytoplasm into smaller blastomeres, forming a sphere called a blastula
• Gastrulation involves the blastomeres moving and changing positions, creating three germ layers (endoderm, ectoderm, mesoderm)
• Organogenesis involves germ layers interacting and rearranging to produce tissues and organs. Chemical signals cause organ formation at specific sites
• Some cells migrate to final locations, including blood cells, lymph cells, pigment cells, and gametes
• The organism undergoes metamorphosis to sexual maturity and is called a larva, which can be different from the adult and may last longest for feeding or dispersal. Some adults have short lives solely for reproduction
• In many species, certain cells (germ cells) produce the gametes, unlike somatic cells
• Germ cells migrate to the gonads and differentiate into gametes in a process that are not completed until the organism has become physically mature
• Gametes release for fertilization, with adult organism undergoes senescence

An Example: A frog's Life

• All animal life cycles are variations of the main one
• Figure 1.1 shows developmental history of the leopard fog.

Gametogenesis and Fertilization

• Life cycles are connected. In frogs, gametogenesis and fertilization are seasonal, controlled by photoperiod and temperature, pituitary secretions mature eggs and sperm
• Fertilization in frogs is external, the eggs secured to vegetation, and fertilization accomplishes sex and reproduction
• Sperm entry allows cytoplasm movement, critical for body axis determination
• Fertilization activates molecules needed for cell cleavage and gastrulation

Cleavage and Gastrulation

• During frog cleavage, egg volume remains constant as it divides into tens of thousands of cells
• Gastrulation starts at sperm entry's opposite point with a blastopore, which marks the dorsal side
• Migrating blastopore cells create mesoderm and endoderm; outside cells become ectoderm and enclose the embryo
• At gastrulation's end, the ectoderm, endoderm and mesoderm are separated

Organogenesis

• Organogenesis starts in frogs as mesoderm cells condense into a notochord
• Notochord cells send signals, which instructs ectodermal cells above to become nervous system cells
• The embryo becomes a neurula, and neural precursor cells form the neural tube and neural tube and notochord induce continued organogenesis
• Mesodermal tissue segments into somites, and the embryo develops a mouth, anus, and tadpole structure

Metamorphosis and Gametogenesis

• Tadpole hatches and feeds until yolk is exhausted
• Frog metamorphosis transforms a tadpole larva to land-living adult
• Organs modify
• Hindlimbs and forelimbs appear after the paddle tail recedes
• tadpole skull transform to a bony skull of the young frog
• Horny teeth are replaced, the mouth and jaw change, the fly-catching tongue forms, lengthy intestine shortens, gills regress, and lungs enlarge
• Thyroid hormones trigger metamorphosis
• Rana metamorphosis must occur before ponds freeze
• Gametogenesis commences as metamorphosis ends, eggs take 3 years to mature and sperm develops faster
• Competent germ cells undergo meiosis, fuse in fertilization, and start development, continuing the circle of life

Comparative Embryology

• Comparative embryology deals with questions like how the heart forms and how bird wings and human hands relate
• Study of comparative developmental anatomy was begun by Aristotle
• Some animals are born from eggs (oviparity); some are born live (viviparity); some produce eggs that hatch inside (ovoviviparity).
• Embryos form through: holoblastic cleavage or meroblastic cleavage
• It was only until 1651 that William Harvey concluded that all animals from eggs
• Harvey saw the blastoderm of the chick embryo and noticed blood tissue forms before the heart

Epigenesis and Preformationism

• With Malpighi one of the debates in embryology began
• Epigenesis said that the organs of the embryo are formed from scratch in each generation, versus preformationism that they appear and are in miniature within the egg or sperm.
• Malpighi supported, which said unincubated chick eggs has structure
• Malpighi showed chick egg already had significant structure, advocating that all adult organs prefigured in miniature within the sperm/egg and are not "constructed" but "unrolled"
• Preformationism aligned with 18th-century values
• This would mean embryonic development simply required growth of existing structures with no extra force
• Early microscopes revealed chick neural grooves, somites, and arteries and veins