Skeletal System Overview

The skeletal system supports the body, aids in movement, protects vital organs, produces blood cells, and stores minerals and fat.

The shaft of a long bone, composed primarily of compact bone, and containing the medullary cavity filled with yellow bone marrow.

The expanded end of a long bone, made of spongy bone containing red bone marrow.

A thin layer of hyaline cartilage covering the epiphyses of long bones, found at joints.

Two types of bone tissue exist: compact bone (highly organized osteons) and spongy bone (with spaces and red bone marrow).

Tiny canals that connect lacunae in compact bone, allowing osteocytes to exchange nutrients and waste.

Mature bone cell that resides in a lacuna and maintains bone tissue.

Thin plates of bone found in spongy bone, providing strength and support.

Cartilage lacks nerves and blood vessels, making it difficult for nutrients and oxygen to reach damaged areas.

Hyaline, fibrocartilage, and elastic cartilage.

Joints, or articulations, connect bones. They are classified as fibrous (immovable), cartilaginous (slightly movable), and synovial (freely movable).

A type of synovial joint that allows movement in all planes, including rotation. Examples include the hip and shoulder joints.

A type of synovial joint that allows movement in only one direction. Examples include the elbow and knee joints.

A bone-forming cell that secretes the organic matrix of bone and promotes the deposition of calcium salts.

Bone repair involves three key cells: osteoblasts (build bone), osteocytes (mature bone cells), and osteoclasts (break down bone).

A process where cartilage is replaced by bone, forming most bones especially long bones like the femur and tibia. It starts with a cartilage model and progresses from the center to the ends.

During endochondral ossification, osteoblasts in the periosteum secrete bone matrix that calcifies, forming a shell of compact bone around the diaphysis of the cartilage model.

The first center for bone formation in endochondral ossification, where blood vessels bring osteoblasts to begin replacing cartilage with bone.

Inside the diaphysis, osteoclasts break down spongy bone during endochondral ossification, creating the medullary cavity which will eventually house bone marrow.

A band of cartilage between the diaphysis and epiphyses in developing long bones, responsible for lengthwise growth of the bone until it ossifies completely.

There are three types of muscle tissue: smooth, cardiac, and skeletal. Each has unique characteristics and functions.

Smooth muscle is found in the walls of hollow internal organs and blood vessels, like the stomach, intestines, and arteries.

Cardiac muscle forms the heart wall. It's unique because it's striated, branched, and has intercalated disks that connect fibers, allowing for rhythmic, coordinated contractions of the heart.

Skeletal muscle attaches to bones and is responsible for voluntary movements like walking, lifting, and facial expressions.

Skeletal muscle fibers are long, tubular, multinucleated cells with striations. They contain specialized structures like myofibrils, sarcomeres, and the sarcolemma.

Contraction of skeletal muscles causes ATP to break down, releasing heat that is distributed throughout the body.

Muscles provide padding for bones and the muscular wall of the abdomen protects internal organs.

A fascicle is a bundle of skeletal muscle fibers, each surrounded by connective tissue. The fascicle itself is also surrounded by connective tissue.

A tendon is a strong, fibrous cord of connective tissue that connects muscle to bone.

Bursae are fluid-filled sacs that provide cushioning and lubrication between tendons and bones.

Muscle cells have three main ways to generate ATP: the creatine phosphate pathway, fermentation, and cellular respiration. The CP pathway is the fastest but short-lived, fermentation is anaerobic, and cellular respiration requires oxygen.

This pathway provides immediate ATP for muscle contraction by transferring a phosphate group from creatine phosphate to ADP. It's the quickest way to generate energy but only lasts for a few seconds.

This anaerobic process converts glucose to lactate, producing a small amount of ATP. It's more sustainable than the CP pathway but less efficient than cellular respiration.

This aerobic process uses oxygen to break down glucose, producing much more ATP than fermentation. It's the main energy producer for sustained muscle activity.

The body prioritizes energy sources based on exercise intensity and duration. For short, intense bursts, it relies heavily on muscle stores (glycogen, triglycerides). For longer, less intense exercise, it increasingly utilizes blood-borne energy sources (glucose, fatty acids).

A quick energy source using creatine phosphate to generate ATP, lasting only about 15 seconds. It's like a small, immediate energy burst.

The heavy breathing after strenuous exercise, necessary to metabolize lactate and restore the body to its original energy state.

A protein in muscle cells that directly delivers oxygen to mitochondria, enhancing energy production. It's like a local delivery service for oxygen.

Sex cells (sperm and egg) responsible for reproduction. They are haploid, containing half the number of chromosomes compared to other body cells.

Cell division that creates two identical daughter cells with the same number of chromosomes as the parent cell. Used for growth and repair.

Cell division that creates four daughter cells with half the number of chromosomes. Used for producing gametes (sperm and egg).

The period of physical and hormonal changes that lead to sexual maturity.

Male and female reproductive organs, respectively. They produce gametes and sex hormones.

Meiosis takes place in the reproductive organs, specifically in the testes of males during sperm production (spermatogenesis) and the ovaries of females during egg production (oogenesis).

A zygote is the first cell formed when a sperm cell fertilizes an egg cell. It contains a complete set of 46 chromosomes, half from the sperm and half from the egg.

Long, coiled tubes within the testes where spermatogenesis, the production of sperm, occurs.

Semen, the fluid containing sperm, provides the energy and environment for sperm motility. It contains fructose, which is a source of energy for the sperm, and prostaglandins, which help the sperm move towards the egg.

The placenta is an organ that connects the developing fetus to the uterine wall, facilitating nutrient and gas exchange between the mother and fetus. It also produces hormones that regulate the pregnancy.

The placenta produces human chorionic gonadotropin (HCG), which helps maintain the corpus luteum, the structure in the ovary that produces progesterone.

HCG stimulates the corpus luteum to produce increasing amounts of progesterone. This progesterone shuts down the hypothalamus and anterior pituitary, preventing the development of new follicles. It also maintains the uterine lining, preventing menstruation.

Eventually, the placenta takes over the production of progesterone and estrogen. The corpus luteum becomes unnecessary and regresses.

Birth control pills contain synthetic estrogen and progesterone. When taken regularly, they prevent ovulation and thicken the cervical mucus, inhibiting sperm from reaching the egg.

There are two main phases: the follicular phase, where a follicle develops and releases an egg, and the luteal phase where the corpus luteum forms and secretes hormones.

Menopause marks the end of the ovarian cycle, typically between ages 45 and 55. The ovaries no longer release eggs and stop producing estrogen and progesterone.

Increased estrogen levels during this stage cause the endometrium (lining of the uterus) to thicken and become glandular, preparing it for a potential pregnancy.

Estrogen and progesterone are responsible for the development of female secondary sex characteristics, like breast development and body shape changes. They also play crucial roles in the menstrual cycle and pregnancy.

A series of events that occurs within the uterus over approximately 28 days, driven by the hormones estrogen and progesterone. It includes menstruation, the proliferative phase, and the secretory phase.

A chain of proteins within a cell that relay signals from the outside to the inside, triggering a response like cell cycle regulation.

Genes that stimulate cell growth and division, acting like accelerators for the cell cycle.

Genes that inhibit cell growth and division, acting like brakes to prevent uncontrolled cell growth.

Points in the cell cycle where progress is halted until specific conditions are met, ensuring accurate replication and preventing errors.

Factors outside the cell, like growth factors or hormones, that influence the cell cycle, ensuring proper growth and division.

The cell cycle's first major checkpoint. If passed, the cell commits to dividing. If not, it enters G0, performing normal functions without division. Requires growth signals like growth factors.

The cell cycle pauses here to verify DNA replication. If DNA is damaged, it prevents the M stage from starting, allowing time for repair. This ensures that the damaged DNA isn't passed on to daughter cells.

This checkpoint occurs between metaphase and anaphase. It ensures chromosomes are properly attached to the spindle fibers, guaranteeing accurate distribution to daughter cells.

A protein that can stop the cell cycle at the G1 checkpoint if DNA is damaged, preventing the propagation of mutations. It can also initiate apoptosis if the DNA can't be repaired.

Mitosis is a type of cell division that creates two identical daughter cells from a single parent cell. It's essential for growth, development, and repair.

Mitosis proceeds through four main stages: prophase, metaphase, anaphase, and telophase. Each stage involves specific changes to the chromosomes and the cell's structure.

Cytokinesis is the division of the cytoplasm, following nuclear division (mitosis), to create two separate daughter cells.

Mitosis produces two identical daughter cells with the same number of chromosomes as the parent cell. Meiosis produces four daughter cells with half the number of chromosomes, creating gametes.

Meiosis increases genetic variation through crossing over (exchange of genetic material between chromosomes) and independent assortment (random distribution of chromosomes into gametes).

Homologous chromosomes are pairs of chromosomes that look alike and carry genes for the same traits. They come from each parent.

During meiosis I, homologous chromosomes come together and line up side by side in a process called synapsis. This keeps the four chromatids close together.

Crossing-over is an exchange of genetic material between the nonsister chromatids of a homologous pair. It occurs during prophase I of meiosis I.

Independent assortment describes how homologous chromosomes line up randomly at the equator during metaphase I. This leads to genetic variation in the offspring.

Environmental factors like radiation, chemicals, or viruses can cause chromosomes to break. Usually, the broken ends rejoin. However, sometimes they rejoin in a different pattern, leading to changes in chromosome structure.

A part of a chromosome breaks off and is lost. This can lead to abnormalities even if only one chromosome in a pair is affected.

A segment of a chromosome is copied more than once. This results in multiple copies of the same genes on a chromosome.

A segment of a chromosome is flipped and reconnected. This may not always cause problems, as all genes are still present.

A segment of a chromosome moves to a different, nonhomologous chromosome. This is a common cause of Down syndrome.

Williams syndrome is a disorder caused by a deletion in chromosome 7. It results in facial features like a turned-up nose, wide mouth, and large ears. People with Williams syndrome often have excellent verbal and musical abilities but struggle with academic skills.

In 5% of Down syndrome cases, a translocation from chromosome 21 to chromosome 14 is the culprit. A translocation is like a piece of a chromosome changing places.

Meiosis I separates homologous chromosome pairs, resulting in two haploid daughter cells. Meiosis II separates sister chromatids, producing four haploid daughter cells.

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis, leading to an abnormal number of chromosomes in the gametes.

Trisomy 21 is a genetic condition caused by an extra copy of chromosome 21, leading to Down syndrome.

A translocation occurs when a portion of one chromosome breaks off and attaches to a different chromosome.

Deletions involve the loss of a chromosome segment, leading to missing genetic information. Duplications occur when a chromosome segment is copied, resulting in extra genetic material.

The final stage of meiosis II where the chromosomes reach the poles of the cell, the nuclear envelope reforms around each set of chromosomes, and cytokinesis divides the cytoplasm, resulting in four haploid daughter cells.

The failure of homologous chromosomes to separate properly during meiosis I or of sister chromatids to separate during meiosis II, leading to an abnormal number of chromosomes in daughter cells.

A condition where a particular chromosome is present in three copies instead of two in a cell (2n + 1), like Down Syndrome (trisomy 21).

A condition where a particular chromosome is present in only one copy instead of two in a cell (2n − 1).

An autosomal trisomy, the most common autosomal trisomy, causing a person to have three copies of chromosome 21. Characteristics include short stature, eyelid fold, flat face, and intellectual disability.

Autosomes are the non-sex chromosomes that carry genes for most of the traits in an organism. There are 22 pairs of autosomes. Sex chromosomes determine the sex of an individual, either XX (female) or XY (male).

Alternative forms of a gene that can occupy the same locus (position) on a chromosome. For example, freckles and non-freckles.

In metaphase of mitosis, individual chromosomes line up at the metaphase plate. In metaphase I of meiosis, homologous chromosomes pair up and align at the metaphase plate.

Both processes involve the separation of sister chromatids. Both involve the phases prophase, metaphase, anaphase, and telophase. Both result in the formation of two daughter cells.

Meiosis I involves homologous chromosome pairing, crossing over, and independent assortment, which are absent in mitosis. In meiosis I, the daughter cells are haploid, while in mitosis, they are diploid. Meiosis I produces four daughter cells, while mitosis produces two daughter cells.