Human Biology Quiz

Students should be able to discuss stem cell function, different types of stem cells, and understand medical applications of stem cells.

Cell proliferation and cell death are balanced throughout the life of multicellular organisms.
Homeostasis is the balance between cell death and proliferation.
Cell death includes apoptosis and necrosis.
Development begins with rapid proliferation of embryonic cells.
Embryonic cells differentiate to produce specialized cells, forming adult tissues and organs.
The human body consists of 10¹⁴ cells and 200 different cell types.
All cells originate from a single cell.
As cells differentiate, the rate of proliferation decreases, and cells arrest in the G₀ phase of the cell cycle.
This is important to maintain cells in adult tissues and organs, as cells may be lost due to injury or programmed cell death.
Some tissues have a high rate of turnover, requiring continual cell division.

Some differentiated cells retain the ability to divide and re-enter the cell cycle.
- Examples include fibroblasts (loose connective tissue and skin) and endothelial cells (blood vessels).
- Some internal organs, such as the liver, also exhibit some capacity for regeneration.
Most differentiated cells are unable to proliferate.
Proliferation occurs from less differentiated cells, specifically self-renewing stem cells.

Most fully differentiated cells cannot divide; they are terminally differentiated.
Stem cells are replaced by proliferation of populations of less differentiated, self-renewing cells.
Stem cells play a crucial role in the maintenance of most tissues and organs throughout the lifetime of the organism.
Stem cells have the capacity to proliferate and replace differentiated cells.
Stem cells have the key property to divide to produce one stem cell and one differentiated cell.

HSCs were first identified in 1961 by McCulloch and Till.
HSCs originate from bone marrow.
They proliferate and differentiate into multiple blood cells.
HSCs are well characterized.
Blood cells have limited lifespans (less than a day to a few months), and over 100 billion blood cells are replaced daily in humans.
All blood cells originate from a single population of HSCs in the bone marrow.
HSC descendants continue to proliferate.
HSC descendants are committed to specific differentiation pathways.
HSC differentiation is influenced by various factors.
Proliferation stops when cells reach their fully differentiated state.

Intestinal epithelial cells are exposed to a harsh environment and have a short lifespan (only a few days), repeatedly dying and shedding into the digestive tract.
The intestine is lined with a single layer of epithelial cells, crucial for digestion and nutrient absorption.
The renewal of intestinal epithelium is a continuous process throughout life.
New cells are formed from the continuous (but slow) division of stem cells at the bottom of intestinal crypts (intestinal crypts).
Stem cells give rise to transit-amplifying cells, which then rapidly divide.
Transit-amplifying cells differentiate into three cell types of the colon surface epithelium.
Each crypt contains approximately 6 self-renewing stem cells.

Skin stem cells (SCs) are responsible for the continuous renewal of skin and hair.
Skin and hair are exposed to harsh environmental factors, such as UV radiation from sunlight.
Cells are continuously renewed throughout life.
Skin consists of three major cell lineages (epidermis, hair follicles, and sebaceous glands), each maintained by its own stem cells.
The epidermis is a multilayer epithelium with a turnover every 2 weeks.
Cells are replaced by epidermal stem cells in a single basal layer.
Stem cells give rise to transit-amplifying cells—which divide in the basal layer before differentiating and moving outward to the surface.
Hair is produced from stem cells in hair follicles located in the bulge.
Bulge SCs give rise to transit-amplifying cells.
Transit-amplifying cells proliferate and differentiate to form the hair shaft.
Sebaceous gland cells are produced from stem cells located at the base of the sebaceous gland.

Skeletal muscle is composed of large, multinucleated cells called muscle fibers.
Skeletal muscle normally comprises a stable tissue with little cell turnover.
However, it regenerates rapidly in response to injury or exercise.
Regeneration originates from muscle stem cells, also known as satellite cells.
Satellite cells are located in the basal lamina of muscle fibers.
Satellite cells are normally arrested in the G₀ phase.
Upon activation by injury or exercise, they proliferate.
Activated satellite cells give rise to progeny that undergo several divisions, differentiating and fusing to form new muscle fibers.

Stem cells have the ability to repair damaged tissue, potentially used to replace damaged tissue and treat disorders such as diabetes or degenerative diseases (e.g., muscular dystrophy, Parkinson's, or Alzheimer's).
One well-established clinical application is bone marrow transplantation.
Adult stem cells can be harvested, separated, activated, and returned to the patient.

Adult stem cells that renew organs are multipotent.
Cells differentiate and give rise to many but limited cell types.
Hematopoietic stem cells (HSCs) can produce different types of blood cells but not any other type of cell, like brain cells.
Embryonic stem cells are pluripotent.
Embryonic stem cells can differentiate into any cell type in the body.
Induced pluripotent stem cells (iPS cells) are pluripotent and are derived from adult somatic cells converted to pluripotent stem cells.