Human Development and Aging Lecture Review

Fundamentals of Reproduction and Fertilization

• The Zygote: Fertilization is the process where a sperm and an egg combine to form a zygote.

• Male Sperm Production:

  • Sperm are produced within specific cells in the male anatomy.

  • Sperm undergo maturation and are stored in the epididymis.

  • Mitochondria and ATP: Sperm contain mitochondria because they require $ATP$ to function and maintain motility.

• Hormonal Control in Males:

  • One hormone specifically controls the production of testosterone.

  • A different hormone in males controls the production of sperm.

• Female Anatomy and Egg Development:

  • Eggs are produced in the ovaries.

  • The follicle within the ovary contains an immature egg, which undergoes a maturation process every month.

  • Implantation: Once fertilized, the egg is implanted in the uterus.

Stages of Development: Pre-Embryonic and Embryonic

• Timeline Overview:

  • Pre-embryonic development: Occurs during Week $1$.

  • Embryonic development: Spans from Week $2$ to Week $8$.

• Pre-Embryonic Stages:

  • Fertilization occurs to form the zygote.

  • The zygote undergoes cleavage, progressing to the $2$-cell stage and further.

The Extraembryonic Membranes and Placenta

• Chorion: The outermost membrane; it develops projections known as chorionic villi.

• Amnion: A membrane that encloses the embryo in a fluid-filled sac.

• Amniotic Fluid: The fluid within the amnion that cushions and protects the developing fetus.

• Allantois: Involved in the formation of the umbilical cord and bladder.

• Yolk Sac: In humans, it is the first site of blood cell formation.

• Umbilical Cord: Connects the developing embryo/fetus to the placenta.

• Placenta:

  • Acts as the source of progesterone and estrogen during pregnancy.

  • Hormonal Functions:

    1. Provides negative feedback to the hypothalamus and pituitary to prevent new follicles from maturing.

    2. Maintains the endometrium to prevent menstruation from occurring.

  • Blood Separation: The blood of the mother and the blood of the fetus never mix.

  • Diffusion: Gas, nutrient, and waste exchange occur across the chorionic villi via diffusion between maternal blood vessels and fetal vessels.

• Endometrium: The lining of the uterus where the placenta develops.

Primary Germ Layers and Organogenesis

• Ectoderm (Outer Layer):

  • Gives rise to the epidermis of the skin.

  • Forms the epithelial lining of the oral cavity and the rectum.

  • Develops into the entire nervous system.

• Mesoderm (Middle Layer):

  • Develops into the skeleton and the muscular system.

  • Forms the dermis of the skin.

  • Creates the cardiovascular system, urinary system, and reproductive system.

  • Forms the outer layers of the respiratory and digestive systems.

  • Includes the formation of the notochord.

• Endoderm (Inner Layer):

  • Forms the epithelial lining of the digestive tract and the respiratory tract.

  • Develops the associated glands of the digestive and respiratory systems.

  • Forms the epithelial lining of the urinary bladder.

Timeline of Embryonic and Fetal Development

• Third Week:

  • The nervous system is the first organ system to appear.

  • The heart begins to develop during the third and fourth weeks.

• Fourth and Fifth Weeks:

  • The future umbilical cord connects the embryo to the chorion.

  • Chorionic villi are established.

  • The umbilical cord fully forms to connect the embryo to the placenta.

• Sixth Through Eighth Weeks:

  • The embryo begins to take on a human appearance.

  • The nervous system is sufficiently developed to allow for reflex actions.

  • At the end of the eighth week, the embryo is approximately $38\,mm$ ($1.5\,in.$) long.

  • All major organ systems have been established by this point.

• Third and Fourth Months:

  • Skull Development: The skull features six large membranous areas called fontanels.

    • These fontanels allow the head to compress slightly to pass through the birth canal.

    • They allow for the rapid growth of the brain during infancy.

    • The fontanels typically close by the age of $2\,years$.

  • Sex Determination: Male and female biological sex can be distinguished during the third month.

  • Heartbeat: The heartbeat can be heard using a Doppler device during the fourth month.

• Fifth Through Seventh Months:

  • Movement: The mother begins to feel fetal movement.

  • Lanugo: The translucent, wrinkled skin is covered by a fine downy hair.

  • Vernix Caseosa: A white, cheese-like substance that coats the skin to protect it from the amniotic fluid.

  • Viability: The eyelids open. A baby born at this stage may survive. The youngest recorded surviving premature baby was born at $21\,weeks$ ($5\,months$).

The Process of Labor and Birth

• Uterine Contractions: The uterus contracts throughout the pregnancy.

• Braxton Hicks Contractions: Stronger, more frequent contractions near the end of pregnancy that are often referred to as "false labor."

• Stage 2 of Birth:

  • Uterine contractions occur every $1\,to\,2\,minutes$ and last for approximately $1\,minute$ each.

  • The mother experiences a strong desire to push.

  • Episiotomy: A surgical incision sometimes made to enlarge the vaginal orifice to speed up childbirth.

• Stage 3 of Birth:

  • Afterbirth: The delivery of the placenta and its membranes.

  • This occurs approximately $15\,minutes$ after the baby is born.

  • The delivery of the placenta completes the third stage of parturition.

Biological Aging and Gerontology

• Life Stages: Development is a continuous process through infancy, childhood, adolescence, and adulthood.

• Aging: Defined as the progressive changes that occur from infancy until death.

• Gerontology: The scientific study of aging.

Effects of Aging on Physiological Systems

• Integumentary System:

  • Skin becomes thinner and less elastic, leading to sagging and wrinkling.

  • Adipose tissue in the subcutaneous layer decreases, making elderly individuals feel colder.

  • Sweat glands become less active, reducing tolerance to heat.

  • Decrease in melanocytes and hair follicles leads to gray hair and pale skin.

  • "Age spots" appear on the skin.

• Cardiovascular System:

  • The heart muscle weakens, and the maximum heart rate decreases.

  • The elasticity of the arteries decreases, which often results in increased blood pressure.

• Immune System:

  • General decline in immune function.

  • The thymus gland shrinks significantly, reaching less than $5\%$ of its original size.

  • There is a notable decrease in antibody production.

• Digestive System:

  • Saliva secretion decreases, allowing more bacteria to adhere to teeth, increasing decay.

  • Reduced blood flow to the liver results in less efficient metabolism of toxins and drugs.

  • Because of reduced metabolism, older individuals often require less medication to maintain therapeutic blood levels.

• Respiratory System:

  • Often affected by cardiovascular problems.

  • Decreased elasticity of lung tissue leads to reduced ventilation.

• Excretory System:

  • Kidneys become smaller and blood supply to them is reduced.

  • Filtering of waste becomes less efficient.

  • It becomes difficult to maintain salt and water balance, leading to faster dehydration.

  • Urinary incontinence increases, particularly in women.

  • Prostate enlargement in men can lead to difficult urination.

• Nervous System:

  • Lifestyle factors such as calorie-restricted diets have been shown in animals to result in fewer Alzheimer-like brain changes.

  • Maintenance of brain health is aided by mental exercise, physical exercise, and adequate sleep.

• Sensory Systems:

  • Increased stimulation is required for taste, smell, and hearing.

  • Anosmia: A total inability to smell, affecting about $15\%$ of the elderly.

  • Hearing loss typically begins after age $50$.

  • Presbyopia: Difficulty focusing on near objects, commonly starting at age $40$.

  • Increased prevalence of cataracts and other eye disorders.

• Musculoskeletal System:

  • Muscle Mass: There is a decrease in both the size and number of muscle fibers. By age $90$, most people have lost $50\%$ of the muscle mass they had at age $20$. Regular exercise can slow this decline.

  • Bone Density: Bones shrink in size and density. Posture changes and vertebral compression lead to a loss of height.

  • Osteoporosis: A common disease in the elderly; women lose bone mass more rapidly than men, especially following menopause.

• Endocrine System:

  • Thyroid gland activity declines, leading to a lower basal metabolic rate.

  • Body cells become less sensitive to insulin.

Reproductive Aging and Hormonal Changes

• Male Reproductive Aging:

  • Testosterone levels peak in the $20s$.

  • Low testosterone is linked to weight gain, decreased sex drive, loss of muscle mass, osteoporosis, fatigue, and depression.

  • Testosterone Replacement Therapy: Potential side effects include acne, prostate enlargement, and the production of too many red blood cells.

• Female Reproductive Aging:

  • Menopause: Typically occurs between the ages of $45\,to\,55$.

  • Ovaries cease responding to gonadotropic hormones and stop secreting estrogen and progesterone.

  • Symptoms include "hot flashes," headaches, dizziness, insomnia, and depression.

  • Life Expectancy: Females generally live longer than males, partially because estrogen offers protection against cardiovascular disorders.

Questions & Discussion

• Is mitosis the process by which a new gamete is formed?: According to the text, mitosis is the process by which a new gamete is formed.

• Which endocrine gland controls the formation and release of an egg?: The formation and release of an egg is ultimately controlled by an endocrine gland.

• Which phase of the ovarian cycle involves increased release of progesterone?: The phase due to the developing corpus luteum involves increased progesterone.

• Which hormone promotes follicle development in the first phase of the ovarian cycle?: A specific hormone promotes this development.

• What happens during days 1-5 of the uterine cycle?: Levels of estrogen and progesterone are low, causing the endometrium to disintegrate and menstruation to occur.

• When is estrogen highest during the uterine cycle?: During the phase when the egg is still developing in the ovary.

• Which day does ovulation usually occur?: Ovulation occurs on a specific day of the uterine cycle (typically day $14$).

• What birth control method is 100% effective?: One specific method is identified as $100\%$ effective.

• Do birth control pills protect against STDs?: Birth control pills are evaluated for whether they protect against sexually transmitted diseases.

• What is the male procedure involving the vasa deferentia?: The procedure consists of cutting and sealing the vasa deferentia.

• Are HIV, genital warts, and herpes bacterial STDs?: According to the text, HIV, genital warts, genital herpes, and hepatitis are all examples of bacterial sexually transmitted diseases.