Human Development and Aging (McGraw Hill Anatomy & Physiology)

Introduction to Prenatal and Developmental Biology

Embryology is defined as the formal study of prenatal development. Expanding upon this, developmental biology is a broader field that examines the progressive changes in form and function an individual undergoes from the stage of a fertilized egg throughout the entire lifespan, concluding at old age.

The Course of Pregnancy

Pregnancy is traditionally divided into three-month intervals known as trimesters:

• First Trimester: Extends from the moment of fertilization through the end of week $12$.
• Second Trimester: Spans from week $13$ through week $24$.
• Third Trimester: Spans from week $25$ until the event of birth.

Fertilization and the Prevention of Polyspermy

Defining the Embryo

There is academic variation regarding the definition of an "embryo":

• Some authorities consider the individual an embryo starting from the fertilized egg or the basic two-cell stage.
• Other authorities assert that the embryonic stage begins at $16$ days post-fertilization, once the individual consists of the three primary germ layers: ectoderm, mesoderm, and endoderm.
• Embryogenesis refers to the specific events leading up to the formation of these germ layers.
• The pre-embryonic stage encompasses the first $16$ days of development following fertilization.

The Process of Fertilization

Fertilization involves the fusion of the sperm and egg membranes. Upon fusion, the sperm nucleus, midpiece, and the majority of the tail sink into the egg cytoplasm.

• Mitochondrial Inheritance: Sperm mitochondria are not passed to the offspring. They are tagged with the protein ubiquitin, which marks them for subsequent destruction by the egg.

Prevention of Polyspermy

Polyspermy is the penetration of an egg by multiple sperm, which would result in a non-viable polyploid zygote. The slow block to polyspermy involves several steps:

1. Penetration of the cumulus oophorus: The sperm passes through the outer layer of granulosa cells.
2. Acrosome reaction: The release of enzymes (such as hyaluronidase) to penetrate the zona pellucida.
3. Sperm-egg membrane fusion.
4. Release of sperm nucleus into the egg cytoplasm.
5. Cortical reaction: Cortical granules release their contents into the extracellular space.
6. Rejection of additional sperm: A fertilization membrane forms to prevent further sperm entry.

The Pre-embryonic Stage and Early Development

Timeline of Early Development

1. Fertilization: ($0$ hours) The zygote is formed. The egg pronucleus and sperm pronucleus are present.
2. Cleavage: The zygote undergoes mitotic divisions to produce smaller cells called blastomeres.
3. 2-celled stage: Occurs at approximately $30$ hours.
4. 4-celled and 8-celled stages: Continued division.
5. Morula: A solid ball of cells formed by approximately $72$ hours.
6. Blastocyst: A hollow sphere that undergoes implantation into the uterine wall at approximately day $6$.

Embryogenesis

This process involves the arrangement of blastomeres into the three primary germ layers within the embryoblast:

• Amniotic cavity: Forms above the embryonic disc.
• Embryonic disc: The structure that will differentiate into the germ layers.

The Embryonic and Fetal Stages

Organogenesis

The embryonic stage officially begins at day $16$ when the three primary germ layers are established. During this stage, organogenesis occurs, where germ layers differentiate into specific organs and organ systems. Significant early structures include:

• Neural tube: Forerunner of the central nervous system.
• Somites: Segmented blocks of tissue that give rise to muscle and bone.

Developmental Milestones and Size

• 4 weeks: Length is approximately $0.3\,cm$. Features include the heart bulge, pharyngeal arches, arm and leg buds, somites, and a tail.
• 7 weeks: Length is approximately $1.0\,cm$. Features include the eye, ear, and digital rays on the limb buds.
• 8 weeks: Length is approximately $2.0\,cm$. The liver bulge and umbilical cord are prominent.
• 12 weeks: Length is approximately $5.0\,cm$. The fetus is characterized by its umbilical cord and amniotic sac.
• 20 weeks: The fetus is well-developed; organs like the placenta and membranes like the chorion are clearly functional.

Embryonic Membranes and Accessory Organs

The development of the embryo is supported by accessory organs and four specialized membranes:

• Placenta: Provides nutrients and waste removal.
• Umbilical cord: Contains two umbilical arteries and one umbilical vein.
• Amnion: A transparent sac that encloses the embryo/fetus and contains amniotic fluid.
• Yolk sac: Provides early blood cell and germ cell precursors.
• Allantois: Forms the foundation of the umbilical cord and urinary bladder.
• Chorion: The outermost membrane; features chorionic villi that interface with maternal blood in the uterus.

Fetal and Neonatal Circulatory Systems

Fetal Circulation (Bypassing Lungs and Liver)

In the fetus, blood circulation is adapted to rely on the placenta for oxygenation rather than the lungs:

1. Foramen ovale: A hole in the interatrial septum that allows blood to flow directly from the right atrium to the left atrium, bypassing the lungs.
2. Ductus arteriosus: A shunt that directs blood from the pulmonary trunk into the aorta, bypassing the lungs.
3. Umbilical arteries: Two vessels that carry oxygen-poor, waste-laden blood to the placenta.
4. Placenta: Disposes of $\text{CO}_2$ and wastes while reoxygenating the blood.
5. Umbilical vein: Returns oxygenated blood from the placenta to the fetus.
6. Ductus venosus: A shunt that allows placental blood to bypass the liver and flow directly into the inferior vena cava (IVC).

Neonatal Circulation (Changes at Birth)

Upon the first breath and the clamping of the umbilical cord, the circulatory system shifts:

• Foramen ovale: Closes and becomes the fossa ovalis.
• Ductus arteriosus: Constricts and becomes the ligamentum arteriosum.
• Umbilical arteries: Degenerate and become the median umbilical ligaments.
• Umbilical vein: Constricts and becomes the round ligament of the liver.
• Ductus venosus: Degenerates and becomes the ligamentum venosum of the liver.
• Post-birth status: Blood returning to the heart is exclusively oxygen-poor systemic blood.

The Neonate and Prematurity

Stages of Early Infancy

• Transitional period: The first $6$ to $8$ hours following birth. This is when the neonate must adapt to self-sustaining life and external environments.
• Neonatal period: The first $4$ weeks ($28$ days) of life. An infant in this stage is known as a neonate.

Premature Infants

• Definition: Infants born before week $37$ of gestation.
• Respiratory Challenges: Infants born before $7$ months ($28$ weeks) often lack pulmonary surfactant. This leads to infant respiratory distress syndrome (IRDS), making the first breaths extremely difficult as the lungs struggle to remain inflated.

Aging, Senescence, and Life Expectancy

Definitions

• Aging: Refers to all changes occurring in the body over time, encompassing growth, development, and the degenerative changes associated with later life.
• Senescence: Specifically defines the degeneration that occurs in organ systems following the age of peak functional efficiency.

Dynamics of Senescence

• Major causes of death after age $55$ are typically related to the senescence of organ systems, including heart disease, cancer, stroke, diabetes, COPD, and infectious lung diseases.
• The senescence of one system often triggers the decline of others.
• Different organ systems do not degenerate at the same rate.

Slowing Senescence through Exercise

Good nutrition and exercise are the primary methods for slowing senescence:

• Resistance exercise: Maintains muscle mass and reduces the incidence of bone fractures.
• Endurance exercise: Reduces body fat and increases both cardiac output and oxygen uptake.
• Quality of Life: Exercise helps maintain strength, joint mobility, and endurance, while reducing the severity of hypertension, osteoporosis, obesity, and diabetes mellitus.

Death terminology

• Life expectancy: The average length of life observed in a given population.
• Life span: The maximum age theoretically attainable by humans.
• Clinical death: The cessation of breathing and blood flow, though the heart may still beat arrhythmically or ineffectively for a short time.

Assisted Reproductive Technology (ART)

Reproductive technologies help address infertility but introduce various legal and ethical dilemmas:

• Surrogate mother: A woman who carries a child for another woman who lacks a functional uterus. The surrogate may or may not also provide the egg.
• Gamete intrafallopian transfer (GIFT): A procedure where eggs and sperm are introduced directly into the fallopian tube proximal to an obstruction.
• Embryo adoption: A process where a man's sperm is used to artificially inseminate a woman, and the resulting embryo is then transferred back to the uterus of the expected mother.