Human Development

What is differentiation?

The process by which unspecialized cells become specialized in structure and function.

What is development?

The series of changes an organism undergoes from fertilization to adulthood.

What is the relationship between differentiation and development?

Differentiation allows specialized cells to form, making growth and development of tissues and organs possible.

Differentiation during the first few days of development results from what?

Selective activation and inactivation of genes.

What is prenatal development?

The period from fertilization until birth.

What are the three prenatal periods?

Germinal period, Embryonic period, Fetal period.

When does the germinal period occur?

Fertilization through the end of week 2.

Major events of the germinal period

Fertilization, cleavage, morula formation, blastocyst formation, implantation begins.

What is formed at fertilization?

A zygote.

What is cleavage?

Rapid mitotic cell division without growth in cell size.

What does cleavage produce?

Blastomeres.

What does the zygote become after cleavage?

A morula.

What is a morula?

A solid ball of blastomeres.

What does the morula become?

A blastocyst.

What is a blastocyst?

A hollow ball of cells with an inner cell mass and trophoblast.

When does implantation begin?

Around day 6–7 after fertilization.

When does the embryonic period occur?

Weeks 3–8.

Major events of the embryonic period Implantation, gastrulation, organ formation, germ layer formation, placenta development.

What develops during the embryonic period?

Major organs and body systems.

When are the three germ layers formed?

During gastrulation.

When does the fetal period occur?

Week 9 until birth.

Major events of the fetal period

Rapid growth, organ maturation, weight gain, recognizable external features.

What is the main characteristic of the fetal period?

Growth and maturation of organs already formed.

What is embryological development?

The process of forming tissues, organs, and body structures during the embryonic period.

What is fetal development?

The growth and maturation of organs from week 9 until birth.

What is ovulation?

Release of a mature secondary oocyte from the ovary into the uterine tube.

What hormone triggers ovulation?

LH surge.

Where does the ovulated egg enter?

The uterine (fallopian) tube.

If fertilization does not occur, what happens to the egg?

It degenerates.

If fertilization occurs, what happens to meiosis II?

It is completed.

What is fertilization?

Fusion of sperm and egg to form a zygote.

Where does fertilization usually occur?

Upper one-third (ampulla) of the uterine tube.

Describe sperm migration.

Sperm travel from the vagina through the cervix, uterus, and into the uterine tube.

Approximately how many sperm reach the site of fertilization?

About a dozen.

What is the corona radiata?

The outer protective layer surrounding the egg.

What is the zona pellucida?

The protective glycoprotein layer surrounding the egg.

What enzyme helps sperm penetrate the corona radiata and zona pellucida?

Hyaluronidase.

Function of hyaluronidase

Breaks down hyaluronic acid to allow sperm penetration.

What happens after sperm penetrates the egg?

The sperm and egg membranes fuse.

What forms after fusion of sperm and egg?

A zygote.

What chromosomes does the zygote contain?

Half from the mother and half from the father.

What happens after fertilization?

The zygote undergoes cleavage.

What happens as the embryo travels to the uterus?

It continues dividing into many cells.

What happens after the embryo reaches the uterus?

It implants into the endometrium.

What is implantation?

Attachment of the blastocyst to the uterine lining.

Why is implantation important?

It allows nourishment and support for embryonic development.

What are the three trimesters of pregnancy?

First, Second, and Third trimesters.

When is the first trimester?

Weeks 1–12.

Major events of the first trimester

Organ formation, placenta development, heartbeat begins, embryo becomes fetus.

When is the second trimester?

Weeks 13–27.

Major events of the second trimester

Rapid fetal growth, fetal movement, organs continue developing.

When can fetal movement usually be felt?

During the second trimester.

By the end of the second trimester, what can the fetus do?

Survive outside the womb with intensive medical care.

When is the third trimester?

Weeks 28–birth.

Major events of the third trimester

Rapid weight gain, organ maturation, lungs mature, fetus moves into birth position.

What position does the fetus usually assume before birth?

Head-down (vertex) position.

What begins near the end of the third trimester?

Labor contractions.

During which trimester do organs and organ systems complete most of their development?

First trimester.

During which trimester does the fetus look distinctly human?

First trimester.

During which trimester does rapid fetal growth occur?

Second trimester.

During which trimester do the lungs finish maturing?

Third trimester.

Define cleavage

Rapid mitotic cell divisions of the zygote without increasing its overall size.

What is the purpose of cleavage?

To increase the number of cells (blastomeres) while maintaining the same overall embryo size.

What are blastomeres?

The smaller cells produced during cleavage.

Does cleavage increase embryo size?

No, it only increases the number of cells.

Define blastulation

The process in which the morula develops into a blastocyst.

What is formed during blastulation?

A blastocyst.

What is a blastocoel?

The fluid-filled cavity inside the blastocyst.

What are the main parts of a blastocyst?

Trophoblast, inner cell mass (embryoblast), and blastocoel.

What is the function of the blastocoel?

Provides space for cell organization during development.

Define implantation

The attachment and embedding of the blastocyst into the uterine lining (endometrium).

When does implantation occur?

About 6–7 days after fertilization.

Why is implantation important?

It establishes pregnancy and allows the embryo to receive nutrients and oxygen.

Which part of the blastocyst attaches to the uterus?

The trophoblast.

Define placentation

The formation and development of the placenta.

What is the placenta?

A temporary organ connecting the mother and fetus for exchange of nutrients, gases, wastes, and hormones.

What forms the placenta?

The trophoblast and maternal uterine tissue.

What are chorionic villi?

Finger-like projections that increase surface area for exchange between maternal and fetal blood.

Functions of the placenta

Exchange nutrients, oxygen, wastes; produce hormones; protect the fetus.

What is gastrulation?

The process that forms the three primary germ layers.

When does gastrulation occur?

During the embryonic period after blastulation.

Why is gastrulation important?

It establishes the tissues that form all organs of the body.

What are the three primary germ layers?

Ectoderm, mesoderm, and endoderm.

Which germ layer is the outermost?

Ectoderm.

Which germ layer is the middle layer?

Mesoderm.

Which germ layer is the innermost?

Endoderm.

What does the ectoderm develop into?

Nervous system, skin (epidermis), hair, nails, sensory organs.

What body system develops from the ectoderm?

Nervous system.

What organs develop from the ectoderm?

Brain, spinal cord, epidermis, eyes, ears.

What does the mesoderm develop into?

Muscles, bones, connective tissue, blood, heart, kidneys, reproductive organs.

Which body systems develop from the mesoderm?

Muscular, skeletal, cardiovascular, urinary, reproductive systems.

Which germ layer forms the reproductive system?

Mesoderm.

Which germ layer forms blood vessels?

Mesoderm.

Which germ layer forms the heart?

Mesoderm.

What does the endoderm develop into?

Digestive tract lining, respiratory tract lining, liver, pancreas, thyroid, bladder lining.

Which body systems develop from the endoderm?

Digestive and respiratory systems.

Which germ layer forms the liver?

Endoderm.

Which germ layer forms the pancreas?

Endoderm.

Which germ layer forms the lungs?

Endoderm.

What is a blastocyst?

A hollow ball of cells consisting of a trophoblast, inner cell mass, and blastocoel.