Levels of Organization and Development Biology

Levels of Organization

• Organisms are the most complex level, encompassing all living things (e.g., trees, humans)
• Organisms are comprised of organ systems, which are groups of organs working together (e.g., digestive system)
• Organs are structures composed of tissues, performing specific functions (e.g., heart)
• Tissues are groups of similar cells with a common function (e.g., muscle tissue)
• Cells are the basic unit of life, carrying out life processes (e.g., plant cell, nerve cell)
• Molecules are groups of atoms, forming the building blocks of cells (e.g., DNA molecule, proteins)
• Atoms are the smallest units of matter (e.g., oxygen, carbon)

Stages of Development of an Organism

• Development is the process where a multicellular organism undergoes changes from a single cell to a complex organism.
• Zygote is the fertilized egg
• Embryo is the organism developing between fertilization and birth
• Embryology is the study of embryonic development
• Developmental Biology is the study of all processes from embryo to adulthood, including regeneration, asexual reproduction, metamorphosis, and stem cell growth.

Two Fundamental Questions in Developmental Biology

• How does the zygote give rise to the adult body?
• This includes pattern formation and morphogenesis
• How does the adult body produce another body?
• This includes differentiation, growth, and reproduction

Considerations to choose a model organism

• Size
• Generation time
• Embryo accessibility
• Organism type and phylogenetic position

Life cycles and the evolution of developmental patterns

• The life cycle is the central unit of biology, encompassing fertilization, cleavage, gastrulation, morphogenesis, and organogenesis of the organism.
• Alteration of life cycles through time, genetics, and development of anatomical structures.

An animal's life cycle

• The stages of development between fertilization and hatching or birth are collectively called embryogenesis.
• Most animals pass through similar stages of development like, fertilization, cleavage, gastrulation, organogenesis, hatching or birth (if applicable), metamorphosis, and gametogenesis.

Cleavage and Gastrulation

• Cleavage involves the rapid mitotic divisions of the zygote into a large number of smaller cells called blastomeres.
• The volume of the zygote stays the same but divides into tens of thousands of cells during cleavage.
• Gastrulation is a significant sequence of cell rearrangements.
• The blastopore forms during gastrulation.

Organogenesis

• Organogenesis is the formation of organs from the germ layers.
• The three germ layers are endoderm, ectoderm, and mesoderm.
• The germ layers interact with one another to form specific organs and tissues at specific sites.

In most species, the organism that hatches from the egg or is born into the world is not sexually mature, but undergoes growth, maturation, and sometimes complete metamorphosis to become a sexually mature adult.

• Gametogenesis is the development of gametes (egg and sperm) that occur after maturation.
• A group of cells are designated from the developing embryo to produce the next generation.

Principles of Development: Life Cycles and Developmental Patterns

• Life cycles are adapted to the non-living environment and interwoven with other life cycles.
• Protostomes form the mouth first and deuterostomes form the anus first.

Principles of experimental Embryology

• It is possible, by means of experimentation alone, to determine the causes of form (Thomas Hunt Morgan, 1898).
• The behavior of a cell in an embryo depends on its surroundings(neighbors).

Three major research programs in experimental embryology

• forces outside the embryo influence its development.
• Forces within the embryo influence and cause the differentiation of cells.
• How cells order themselves into tissues and organs.

How forces outside the embryo influence its development? (Environmental sex determination)

• Recent research has shown sex in alligators, crocodiles, depends on temperature
• Egg temperature determines sex of alligators
• The sex of the echiuroid worm, depends on where the larva settles.

Adaptation of embryos and larvae to their environments (phenotypic plasticity)

• Embryos and larvae adapt to their environments.
• Season-dependent coloration in butterflies is an example.

How do forces INSIDE the embryo encourage cells to differentiate?

• Specification refers to how a cell/tissue is capable of differentiating in a neutral environment.
• Determination refers to how a cell/tissue is capable of differentiating in a new environment, usually irreversible
• Cell differentiation is the development of specialized cell types

Modes of cell type specification

• Autonomous specification
• Conditional specification
• Syncytial specification

How the cells order themselves into tissues and organs?

• Multicellular interactions for organ construction
• Cell migration and establishment of positions for organs
• How specific organs form in particular locations?
• How organ growth and development is coordinated?
• How the different parts of a body develop polarity?

Differential cell affinity

• Cells have different proteins on their surfaces, which determine specific interactions with other cells during development (and therefore organ construction)
• Specific interactions influence cell position.

The embryological origins of the gene theory

• Genes are "form-building elements."(Mendel)
• Development originates from a conflict between gene vs cytoplasm as factors.
• Chromatin plays the major role in inheritance. (Wilson)

Amphibian cloning: The restriction of nuclear potency

• The ultimate test is whether the nucleus of a differentiated cell can direct development.
• Techniques include enucleating the egg (removing the nucleus and thus its genomic components)
• Transferring the intact nucleus from a donor cell into the enucleated recipient egg and activating the process for embryonic development.

What happens when nuclei from more advanced developmental stages are transferred into activated enucleated oocytes?

• Some cells or nuclei lose the ability to direct development as development progresses
• The ability of nuclei from later developmental stages to direct development decreases as somatic cells lose totipotency

Amphibian cloning: The pluripotency of somatic cells

• Can differentiated cell nuclei direct development?
• The nuclei of some differentiated cells can remain totipotent, such as those in the intestinal endoderm.
• A genetic marker could be used to differentiate donor cell nuclei from the host nuclei.

Cloning mammals

• Cloning of mammals from adult cells
• Oocytes are obtained, nuclei are removed (enucleation)
• Donor cell nuclei are inserted into the enucleated oocytes
• The pulses activate the egg and development starts

Can we clone humans unlike other mammals?

• Arguments for and against human cloning given by John Robertson
• Scientific and ethical reasons against human cloning include possibility of low success rate, moral issues involved in creating "spare parts" humans, genetic and developmental uniqueness of clones, and the limitations on procreation rights.

Some of the reasons cited by scientists and ethicists against Robertson's human cloning arguments are:

• Low success rate (high rate of abortion or malformation)
• Ethical issues (the concept of creating "spare parts")
• Genetically distinct individuals (not identical twins)
• Ethical limitations on procreation rights by governments

Differential gene expressions

• Gene expression vary between tissues and organs.
• Gene activity differer based on position or environment