Human Tissues

Human Tissues begin in the embryo

Fertilization to week 1

Fertilization = Sperm from Dad meets egg from motherWithin 1 week, cells have divided to form a blastocyst (about 120 cells) with 2 different types of cellsTrophoblast to become the placentaEmbryoblast or inner cell mass to become the baby

Weeks 2-3

Blastocyst imbeds in uterine lining Embryoblast forms a disc of cells with 3 distinct layers of different cells

1. Ectoderm (ecto = outer) forms skin, brain, spinal cord, and sensory receptors

2. Mesoderm (meso= middle) forms muscles, skeleton, blood vessels, heart, blood, and lining of body cavities

3. Endoderm (endo= inside) forms digestive tract, liver, pancreas, trachea, and lungs

Primary germ layers

Cell Renewal

Stem cells are special cells that can become any specific cell type in the bodySome tissues have the ability to renew themselves when cells die (skin, lining of digestive tract)Some tissues do not have the ability to regenerate (nervous tissue and cardiac (heart) cells

Cell Differentiation (becoming different types of cells

About 200 different types of cells in the human bodyDifferent in size, shape, functionDifferentiation is the specialization of cell structure and function

What causes a cell to differentiate?

All cells in the body have the same DNA (instructions)Certain factors can “turn genes on and off”It is when certain genes get turned on that they become differentiatedExample: Hemopoetic stem cells can be any kind of specialized blood cell depending on which genes are turned onStarts at conception and continues through life

Why differentiate?

Divide labor of the body

Good= more efficiency

Bad= Maintenance issues (what happens when cells that don’t renew die off? (No new cells to replace)

Differentiated cells to Tissue

The 200 different types of cells organize into 4 different tissue types: Epithelial, Connective, Muscle, NervousEach tissue type shares structural, biochemical, and functional characteristicsThe 4 different tissue types combine to form the body structures (organs)Cells of tissue are in an extra-cellular matrix of fluid and proteins to hold together and provide environment for cells to communicate and function as a unit.

Biological characteristics of tissue

Each tissue type shows uniformity and regularity of cell shape and size

Each tissue is organized into sheets of single or multiple layers

Cell growth within tissues

Requires growth factors (biochemical signals) from the body.Normal cells divide at the same rate as old cells die off. This is known as cellular turn-overNew cells are attached to the extra cellular matrix to make sure the tissue is aligned in the proper wayCells know to stop dividing when they come in contact with other cells (contact inhibition)Question: What would happen if more cells were made than died or if they didn’t stop dividing due to contact inhibition?

Programmed cell death

Cells are genetically designed to die after a certain amount of time (called apoptosis)Certain cells stay incorporated in the body after they die (skin, lens of the eye)Certain cells are sloughed off or re-absorbed by the body (finger formation in babies in utero)