Biology AOS 1

Stages of Prenatal Development

Fertilization, Zygote, 2-cell stage, 4-cell stage, 16-cell stage, Morula, Blastocyst, Gastrula, Embryo, Foetus

When does life begin development

At fertilization and progresses through several key stages.

Zygote Age

Day 0

Morula Age

Day 3

Blastocyst Age

Day 5-6

Gastrula Age

Day 7+

Embryo Age

Week 3-8

Foetus Age

Week 9-Birth

Zygote Stage Structure

1 cell: A diploid cell formed when the nuclei of an ovum and sperm fuse during fertilisation.

Zygote Stage DNA and Division Process.

This cell contains a full set of DNA (46 chromosomes) and begins dividing by mitosis.

Morula Stage Structure and Origin

A solid ball of 16-32 undifferentiated cells (blastomeres) resulting from the cleavage of the zygote.

Blastocyst Stage Structure

A hollow structure of ~107 cells with two layers.

Blastocyst Stage 3 Core Components

Trophoblast (Outer layer): Will become placenta. Inner Cell Mass: Becomes the embryo. Blastocoel: fluid-filled cavity.

Gastrula Stage Origin

Gastrulation results in single layered blastocyst developing into three layered gastrula. Occurs after blastocyst implantation into the uterine wall.

Gastrula Stage Process

The inner cell mass of the blastocyst folds and gives rise to embryonic germ layers.

Embryo Stage Definition

Most major structural development happens now, also between weeks 3-8, also known as the embryonic period.

Embryo Stage Process

Forms critical organs and structures.

Foetus Stage Definition

Growth and functional development occur. Most cells are specialised.

One of 3-part Blastocyst (Trophoblast)

Trophoblast: Develops into structures that support the embryo, such as the placenta.

One of 3-part Blastocyst (Blastocoel)

Blastocoel: Fluid-filled cavity that supports cell movement and growth.

One of 3-part Blastocyst (Inner Cell Mass)

Inner Cell Mass: A cluster of pluripotent cells that will become the embryo.

Inner Cell Mass becoming 3 Germ Layers

Around day 14, inner cell mass (ICM) undergoes gastrulation forming three primary germ layers. Ectoderm (outer layer), Mesoderm (middle layer), Endoderm (inner layer).

Cells in Germ Layers

Cells are pluripotent stem cells-they can give rise to all the cell types of the body (except placenta). Foundation for all future tissues and organs in the individual.

Mesodermal Stem Cells

Form muscle cells, red blood cells, osteocytes (bone cells).

Endodermal Stem Cells

Forms cells for internal organs: Alveolar, lungs, stomach (digestive tract).

Ectodermal Stem Cells

Forms skin cells (hair, nails) and neurons.

Stem Cell Definition

A type of cell that is capable of differentiating into one of a range of specialized cells within an organism.

Stem Cell Classification

According to potency (ability to differentiate to other types of cells). Higher the potency, greater the differentiation potential of the stem cells.

Stem Cells Ability

Self-renew: Divide to produce identical stem cells. Differentiate: Develop into specialised cell types.

Stem Cells and Development

Stem cells play essential roles in growth, tissue formation, and repair.

Totipotent Stem Cells

Can differentiate into all possible cell types. Embryonic stem cells in zygote to morula are only cells considered totipotent.

Pluripotent Stem Cells

Can differentiate into almost any cell type. Embryonic stem cells that form ICM of blastocyst are pluripotent.

Multipotent Stem Cells

Can differentiate into a variety of closely related types of cells. Hematopoietic stem cells in bone marrow are multipotent. They produce a variety of blood cells, but all are varieties of blood cells.

Unipotent Stem Cells

Can only produce one type of cell, their own. All somatic cells are unipotent, e.g. skin or muscle cells are able to self-renew, which allows for tissue repair.

Areas with Stem Cells

2 main areas. Embryonic stem cells and Adult stem cells.

Embryonic Stem Cells

Found in an embryo prior to uterine implantation (zygote to inner cell mass of blastocyst). Pluripotent (can differentiate into all specialized cell types). Develops into any cell type in the body. Easily grown and maintained under proper conditions. High Ethical Concerns - Requires destruction of a human embryo, which some view as a potential life.

Adult Stem Cells

Undifferentiated cells that are found in certain tissues throughout the life of an individual (e.g. bone marrow, skin, live). Multipotent (form a limited number of cell types) or Unipotent. Maintain and repair old or damaged tissues. Limited range of cells to develop into. Difficult to isolate and grow in lab conditions. Low Ethical Concerns - Cells are taken from consenting adult donors without harming the donor.

Critical Periods Definition

Periods of time during an organism’s development when it is more susceptible to developmental abnormalities.

Organ Period Timing

Different organs have different critical periods depending on when they start and finish forming.

Embryonic Period

From weeks 3-8. It is the most critical for structural development.

Tetratogens

Substances that can cause birth defects or miscarriages if exposure occurs during the critical periods (e.g. drugs, alcohol, viruses).

Stem Cells in Regenerative Medicine

Stem Cell Therapy is a field of regenerative medicine, involving using stem cells to replace, regenerate, or engineer human cells, tissues, or organs, to restore normal function.

Aim of Stem Cell Therapy

To treat and prevent diseases by manipulating stem cell differentiation.

Sources of Stem Cells

Embryonic Stem Cells, Adult Stem Cells, and Induced Pluripotent Stem Cells

Induced Pluripotent Stem Cells

Adult somatic cells that are reprogrammed in the lab. Pluripotent. Engineered to behave like ESCs for research and therapy. Can be grown in lab after reprogramming, similar to ESCs. Low Ethical Concerns - Avoids embryo use, but concerns exist about genetic modification and long term safety.

Cell Origins

All cells arise from pre-existing cells through the cell cycle.

Binary Fission

Form of asexual reproduction used by prokaryotic cells (e.g. bacteria). Process is in 3 stages: R.E.D.

Stage 1: Replication

Chromosome attaches to plasma membrane at approximately the midpoint of the cell and is copied resulting in two identical circular chromosomes.

Stage 2: Elorigation

Parent cell elongates, separating the now two identical chromosomes.

Stage 3: Division

Once the two chromosomes are located on opposite sides of the cell, a septum forms down the middle of the cell giving rise to the new cell walls.

Eukaryotic Cell Cycle

The cell cycle is the life cycle of a cell, consisting mostly of normal cell functions and growth, with division (mitosis and cytokinesis) occurring as one part of the cycle. The cell passes through a series of discrete stages or phases, which results in the production of two genetically identical daughter cells.

Main stages of the Eukaryotic Cell Cycle

Interphase (G1(G0), S, G2) and M+C Phase (Mitosis and Cytokinesis).

Interphase Stage

Cells spend majority (95%) of their time in interphase. Genetic material in nucleus is in form of chromatin fibres (chromosomes aren’t visible). Chromatin is a substance within a chromosomes consisting of DNA and protein. Major proteins in chromatin are histones, which help package DNA in a compact form that fits cell nucleus.

G1 Stage

In 24H cell cycle, 11 hours long. Cell grows larger, nearly doubling in size. Organelles are copied to equip daughter cells for survival. Growth conserves cell size post division - each new cell is about the same size as the original parent cell before it started growing. Longest phase; most cells observed here under microscope.

G0 Stage

Some cells exit the cycle from G1 to G0. G0 can be temporary (cells re-enter cycle later) or permanent (e.g. neurons). Permanent G0 explains why nerve cell damage is difficult to repair.

S Stage

In 24H cell cycle, 8 hours long. DNA replication occurs, creating identical copies of each chromosome. Single chromosomes become double chromosomes (2 sister chromatids joined at centromere). DNA replication is semi-conservative; each with one original (parent) strand and one evenly synthesized strand. In humans: Before S phase: 46 single chromosomes. After S phase: 46 double chromosomes (92 chromatids total). Sister chromatids carry identical copies of DNA (genetic information) passed to daughter cells after mitosis.

G2 Stage

In 24H cell cycle, 4 hours long. Final stage of interphase. Cell undergoes another growth period. Energy stores increase to prepare for mitosis. Protein synthesis increases, particularly for proteins involved in cell division. Metabolic activity is high as the cell gears up for mitosis.

Mitosis Definition

The stage of the cell cycle where the nucleus of a diploid parent cell divides into two genetically identical nuclei.

The 4 processes that involve Mitosis.

Tissue Repair