Human Development: Fertilization to Embryo

• Human development refers to the continuous body changes from fertilization to death.
• This development has two stages: prenatal (before birth) and postnatal (after birth).
• A single cell develops into an embryo, then a fetus, before birth.

Fertilization

• Fertilization involves the fusion of an egg and sperm cell.
• It begins with sperm entering the vagina post-ejaculation.
• Sperm cells move through the uterus into the uterine tube by flagellum.
• Enzymes released from the sperm head help with penetration.
• Once the zona pellucida is penetrated, changes occur to prevent more sperm from attaching.
• The united egg and sperm form a zygote.
• Within 36 hours, the zygote starts dividing while traveling through the fallopian tube.
• The original cell divides into two cells.
• From the two-cell stage to about nine weeks after fertilization, the growing structure is an embryo.
• A blastocyst is an early-stage in a fertilized egg forming approximately 5-6 days post-fertilization; ready to implant into the uterine wall.

Three functions of fertilization.

• Restoring the diploid number (2n) of chromosomes as sperm contributes its haploid (n) set to the haploid set in the ovum
• Determining the offspring's sex in mammals and many other animals.
• Stimulating reactions in the egg, allowing for development to occur.

Development Before Birth

• Prenatal or anternal development begins with fertilization and continues to fetal development until birth.
• The embryo is formed early in the development stage at week eight.
• Fetal development involves the process by which a fertilized egg (zygote) grows and develops into a fully formed baby inside the mother's womb and occurs in three main stages.

Germinal Stage (0–2 weeks)

Embryonic Stage (3–8 weeks)

Fetal Stage (9 weeks – birth)

• Amniotic sacs protect the developing baby inside the uterus.
• The placenta, formed from tissue, is a station between the mother and embryo.
• The embryo connects to the placenta via the umbilical cord.
• The umbilical cord's blood vessels transport food and oxygen, with waste moving between the embryo and placenta.

Stages of Fetal Development

• First Stage: From zygote formation to the end of the 6th week; the head starts to differentiate, eyes appear distinctly, and the heart develops.
• Second Stage: From the 7th to the 12th week; the developing embryo shows human features, organs develop completely, and the reproductive organs begin to develop.
• Third Stage: Begins at week 13 and goes to the end of the 22nd week.
• Fourth Stage: From beginning of the 23rd week up until delivery; all body systems complete development, leading to the ability to move hands and feet.
• The body inside the uterus develops and grows after 38 weeks.
• During labor, the baby is still connected to the placenta through the umbilical cord, clamped and then cut.
• Contractions happen in the uterus, and the mother experiences labor pains.
• A piece of the umbilical cord remains attached to the newborn, dries, and falls off, leaving the navel as a lifelong scar.

Development after Birth

• The postpartum or postnatal time frame starts right after childbirth and lasts about six weeks while a bodies hormones/ uterus goes back a non pregnant.
• Shortly after birth, the babys begins breathin.
• The mother and the baby are advised to stay at around three to four days in the hospital following a caesarean section delivery.
• Humans go through various stages of development
• Childhood begins around 18 months and continues until the age of 13.
• Adolescence begins after the thirteenth year and continues through the teenage years.
• Adulthood begins at about 20 years of age.

Human development involves prenatal and postnatal stages. Fertilization unites egg and sperm, forming a zygote. The zygote divides into two cells, eventually becoming an embryo around nine weeks after fertilization.