Cell signaling, embryo development

Signals

Molecules produced by cells or come from the environment

Ligand

Signaling molecules that will bind to receptors

Reception

Proteins that bind ligand

Binding is specific (particular ligand binds particular receptor like lock and key)

Only cells that express the receptor will recieve the signal

• cell types express different genes

How does reception occur

membrane proteins for hydrophilic signaling molecules

within a cell for lipid soluble signaling molecules (hydrophobic)

3 types of membrane receptors

Ion channels: signaling molecule (ligand) binds a channel protein, causing channel to change shape between open/close

enzyme linked receptors:Ligand binding causes shape change to activate/inactivate enzymatic activity of receptor protein

G-protein coupled receptors:G-protein inactive when all 3 subunits bound to receptor & alpha subunit bound to GDP

G-protein active when subunits separate & alpha subunit bound to GTP

Transduction: steps between reception and response

Signal can be passed from one protein to another in a multi-step pathway to reach the final cellular response

can include secondary messenger(small non-protein molecule produced or released by cell in response to extracellular signal)

Phosphorylation cascades

Phosphorylation cascades are chains of kinases

• transduction step

• one kinase phosphorylates another and they phosphorylate each other

Transduction amplification

Amplification: small amount of ligand can lead to activation of large number of ’downstream’ proteins via amplification

Transduction enzymes can amplify signal by carrying out multiple reactions

Secondary messengers can amplify signal as many copies can be produced/released

Phosphorylation amplifies

One kinase can phosphorylate as many copies as necessary and the chain increases, with lots of activated kinases at the end

Response

change in cell state (like gene expression)

Common responses to signals are to
activate/inactivate a target protein
or
Turn many target genes on/off via transcriptional regulation

• Response: can vary in different cell types

Transduction pathway can be different in different cells based on what components are expressed in each cell type

Pathways can have complicated interactions leading to complex interpretation of multiple signals and sophisticated response patterns

Examples of different responses for different cell types

Cell type A: signal leads to response 1

Cell type B: both signal needed to lead to response

Cell type C: signal leads to response 2 and 3

Pathways can branch

Cell type D: both signals needed to lead to response

Pathways can merge

(displaying different cells receive signals differently)

Development

The process by which a single-celled zygote becomes a multicellular organism with specialized structures and functions.

Embryogenesis

The early stages of development from fertilization to the formation of the basic body plan.

Fertilization

The fusion of a sperm cell and an egg cell to form a zygote, restoring the diploid chromosome number.

Zygote

The single diploid cell resulting from fertilization, marking the beginning of embryonic development.

Cleavage

Rapid mitotic cell divisions of the zygote without an increase in overall size, leading to a multicellular embryo.

Blastula

A hollow sphere of cells formed during early embryonic development, containing a fluid-filled cavity called the blastocoel.

Gastrulation

The process in which the blastula reorganizes into a three-layered structure (ectoderm, mesoderm, endoderm), establishing the basic body plan

Ectoderm

The outer germ layer that gives rise to the nervous system, epidermis, and sensory organs.

Mesoderm

The middle germ layer that forms muscles, bones, connective tissues, blood vessels, and the circulatory system.

Endoderm

The inner germ layer that develops into the digestive and respiratory tracts, liver, and pancreas.

Neurulation

The process by which the neural tube forms, giving rise to the central nervous system (brain and spinal cord).

Organogenesis

The development of organs from the three germ layers, beginning after gastrulation and continuing throughout fetal development.

Induction

The process by which one group of cells influences the development of another group through chemical signaling.

Cells can begin as once cell, however due to

gene regulation, different gene types are present

Morphogenesis

The biological process that controls the shape of tissues, organs, and the overall body structure.

Differentiation

The process by which unspecialized stem cells develop into specialized cells with distinct functions.

(two different cell types now)

Apoptosis in Development

Programmed cell death that helps shape organs, remove unnecessary structures, and ensure proper development.

Hox Genes

A group of regulatory genes that determine body plan and segmentation along the anterior-posterior axis.

Placenta

A temporary organ that facilitates nutrient, gas, and waste exchange between the mother and developing fetus.

Extraembryonic Membranes

Structures that support embryonic development, including the amnion (fluid protection), chorion (gas exchange), yolk sac (nutrients), and allantois (waste storage).

Teratogens

Environmental agents (e.g., drugs, chemicals, infections) that can disrupt development and cause birth defects.

Fetal Development Stages

• First Trimester: Major organs begin to form; highest risk of developmental abnormalities.

• Second Trimester: Growth and refinement of features; fetal movements become detectable.

• Third Trimester: Rapid growth, brain development, and final organ maturation.

Stem Cells in Development

Undifferentiated cells capable of dividing and differentiating into specialized cell types.

Totipotent Stem Cells

Stem cells that can differentiate into any cell type, including embryonic and extraembryonic tissues (e.g., zygote).

transcription factors can

help define cell types

Pluripotent Stem Cells

Stem cells that can differentiate into any cell type except extraembryonic tissues (e.g., embryonic stem cells).

Multipotent Stem Cells

Stem cells that can differentiate into a limited range of cell types within a specific lineage (e.g., hematopoietic stem cells).

Cell Migration in Development

The movement of cells to their appropriate locations during embryonic development to form tissues and organs.

Genetic and Environmental Factors in Development

Genes provide the blueprint for development, while environmental influences (e.g., nutrition, toxins) can alter developmental outcomes.

Fetal Circulation Adaptations

Specialized structures like the ductus arteriosus (bypasses lungs) and foramen ovale (shunts blood between atria) that support fetal circulation before birth.

Parturition (Birth Process)

The process of childbirth, initiated by hormonal and mechanical signals leading to uterine contractions and delivery.

Postnatal Development

The continued growth, maturation, and refinement of physiological systems after birth, including neural and immune development.