Human Growth and Development

Human Growth and Development

High School

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Learning Objectives

• Distinguish between the prenatal and postnatal periods of development.

• Describe the process of fertilization and the initial stages of embryonic development.

• Explain the key hormonal changes that occur during pregnancy to support the developing offspring.

• Outline the major events of the embryonic and fetal stages, including the formation of primary germ layers.

• Identify the main support structures for the embryo and their functions.

Key Vocabulary

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Fertilization

The fusion of sperm and egg nuclei to form a diploid zygote, initiating pregnancy.

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Zygote

A single diploid cell that results from the successful fusion of an egg and a sperm cell.

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Embryo

A developing offspring from fertilization to the end of the eighth week of gestation.

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Fetus

The term for a developing offspring from the end of the eighth week until birth.

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Placenta

A vital organ that develops in the uterus to nourish the fetus and remove waste products.

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Fundamentals of Growth and Development

Cell Division

• ​One parent cell divides to form two identical daughter cells through a process called mitosis.

• ​​This method is responsible for increasing the total number of cells within an organism.

• ​It is also essential for repairing tissues and replacing old or damaged cells.

Cell Enlargement

• ​This process involves each individual cell increasing in its own physical size after division.

• ​​Cells expand by absorbing water and synthesizing molecules like proteins, leading to increased volume.

• ​This contributes to the permanent increase in an organism's overall size and weight.

Multiple Choice

Which of the following statements best describes the primary mechanisms of growth?

• The process of changing from one life phase to another, like from prenatal to postnatal.

• The division of a parent cell into two, followed by a decrease in cell size.

• A permanent increase in size and weight through cell division and enlargement.

• A temporary increase in weight due to fluid retention controlled by hormones.

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From Fertilization to Implantation

• A sperm fertilizes an egg in the uterine tube, creating a diploid zygote.

• The zygote undergoes rapid mitosis, called cleavage, to form a morula.

• The morula develops into a blastocyst and travels to the uterus.

• The blastocyst implants in the uterus and is now called an embryo.

Multiple Choice

What is the correct sequence of events from fertilization to the establishment of pregnancy?

• Zygote formation, cleavage, blastocyst development, implantation.

• Zygote formation, blastocyst development, implantation, cleavage.

• Cleavage, implantation, zygote formation, blastocyst development.

• Implantation, zygote formation, cleavage, blastocyst development.

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Hormonal Control of Pregnancy

• The blastocyst secretes human chorionic gonadotropin (hCG) to maintain the uterine lining.

• ​High hCG levels detect pregnancy; the placenta later produces its own hormones.

• The placenta makes placental lactogen for breast development, plus estrogen and progesterone.

• Aldosterone increases fluid retention, and parathyroid hormone boosts calcium for fetal bones.

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Multiple Choice

What is the primary role of human chorionic gonadotropin (hCG) in early pregnancy?

• To stimulate breast development for lactation.

• To increase fluid retention in the mother's body.

• To maintain the uterine lining and sustain the pregnancy.

• To regulate calcium levels for fetal bone growth.

The Embryonic Stage (Weeks 2-8)

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Ectoderm: Outer Layer

• This is the outermost germ layer of the developing embryo.

• It gives rise to the nervous system and skin epidermis.

• This layer also forms hair, nails, and tooth enamel.

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Mesoderm: Middle Layer

• This middle germ layer is found between the other two layers.

• It is responsible for forming all muscle and bone tissues.

• It also creates blood, bone marrow, and reproductive organs.

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Endoderm: Inner Layer

• This is the innermost germ layer of the developing embryo.

• It produces linings of the digestive and respiratory tracts.

• The endoderm also forms the liver, pancreas, and thyroid.

Multiple Choice

Which primary germ layer is responsible for forming the nervous system and skin?

• Ectoderm

• Mesoderm

• Endoderm

• Embryonic Disc

Development of Support Structures

• The chorion forms the placenta for gas and nutrient exchange.

• The amnion surrounds the embryo with cushioning amniotic fluid.

• The umbilical cord connects the embryo to the placenta.

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Solved Example 1
A human zygote has 46 chromosomes. If an embryo undergoes 3 rounds of cleavage (mitosis), and all resulting cells are normal, how many total chromosomes would be found across all the cells produced?

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Step 1: Analyze and Sketch the Problem

• Goal: Calculate the total number of chromosomes in all cells after 3 rounds of cleavage.

• Knowns: The starting cell (zygote) has 46 chromosomes, and it undergoes 3 rounds of cleavage (mitosis).

• Unknown: The total number of chromosomes found across all the cells produced.

• Formula: Total chromosomes = (Total number of cells) × (Number of chromosomes per cell). The number of cells is calculated by 2ⁿ.

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Solved Example 1
A human zygote has 46 chromosomes. If an embryo undergoes 3 rounds of cleavage (mitosis), and all resulting cells are normal, how many total chromosomes would be found across all the cells produced?

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Step 2: Solve for the Unknown

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Solved Example 1

A human zygote has 46 chromosomes. If an embryo undergoes 3 rounds of cleavage (mitosis), and all resulting cells are normal, how many total chromosomes would be found across all the cells produced?

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Step 3: Evaluate the Answer

Multiple Choice

Which structure is the first site of blood cell formation and also gives rise to immune system stem cells?

• Amnion

• Placenta

• Yolk sac

• Chorion

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The Fetal Stage: Week 8 to Birth

• This stage begins at week 8 and continues until birth.

• It involves rapid growth and maturation of the structures formed earlier.

• Body proportions change as the head's growth slows and limbs lengthen.

• Existing organs are refined, preparing the fetus for life outside the uterus.

Multiple Choice

What is the main characteristic of the fetal stage of development?

• Implantation of the blastocyst into the uterine wall.

• Rapid growth and maturation of existing structures.

• Formation of the three primary germ layers.

• The initial formation of all major internal organs.

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Common Misconceptions about Pregnancy

Misconception	Correction
Mother's and baby's blood mix in the placenta.	Blood supplies are separate; nutrients and gases are exchanged through a membrane.
A fetus is just a tiny baby and doesn’t change much.	Development is complex, with distinct stages for organ formation and maturation.
Pregnancy hormones only affect the mother's reproductive organs.	Hormones affect the mother’s entire body, like fluid balance and calcium levels.

Multiple Choice

What is the primary difference in the developmental process between the embryonic and fetal stages?

• The embryonic stage lasts from week 8 to birth, while the fetal stage is the first 8 weeks.

• The embryonic stage is about forming organs, while the fetal stage is about growing and maturing them.

• The embryonic stage is when the heart starts beating, while in the fetal stage the placenta develops.

• The embryonic stage involves rapid growth, while the fetal stage involves the formation of germ layers.

Multiple Choice

Why is the secretion of human chorionic gonadotropin (hCG) critical for the establishment of a successful pregnancy?

• It prevents the breakdown of the corpus luteum, thus maintaining the uterine lining for the embryo.

• It directly provides all the nutrients the embryo needs for the first eight weeks of development.

• It triggers the development of the fetal nervous system from the ectoderm layer.

• It stimulates the mother's body to retain extra fluid and calcium for the growing fetus.

Multiple Choice

A prenatal scan reveals that the mesoderm layer of an embryo is not developing correctly. What would be the most likely outcome of this issue?

• The placenta and umbilical cord would fail to form.

• Defects in the development of muscle, bone, and the circulatory system.

• Improper formation of the brain, spinal cord, and skin.

• Failure of the digestive and respiratory tracts to develop linings.

Multiple Choice

If a pregnant woman has a condition that significantly reduces the function of her placenta after 12 weeks, what would be a primary concern?

• The initial signal for pregnancy (hCG) would not be produced.

• Inadequate hormone production to maintain pregnancy and insufficient nutrient/gas exchange for the fetus.

• The embryo would fail to implant properly in the uterine wall.

• The primary germ layers would not differentiate correctly.

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Summary

• Development includes prenatal and postnatal periods, with growth as a key part.

• A fertilized zygote develops and implants in the uterus.

• Hormones like hCG are crucial for maintaining a healthy pregnancy.

• The embryonic and fetal stages involve organ formation and then rapid growth.

Poll

On a scale of 1-4, how confident are you with the concepts of human growth and development covered today?

• 1

• 2

• 3

• 4