Human Development

Differentiation vs Development

Differentiation is cell specialization; development is overall growth from fertilization to birth.

Early differentiation (first days)

Results from rapid cell division and gene activation that begins cell specialization.

Prenatal development

Development from fertilization to birth.

Germinal period

0–2 weeks; fertilization → zygote → cleavage → morula → blastocyst → implantation.

Embryonic period

Weeks 3–8; major organs begin forming; 3 germ layers develop.

Fetal period

Week 9 to birth; growth, organ maturation, and body refinement.

Cleavage

Rapid mitotic cell division without increase in size.

Blastulation

Formation of blastocyst (hollow ball of cells with fluid cavity).

Implantation

Blastocyst attaches and embeds into uterine lining (~6–7 days after fertilization).

Placentation

Formation of placenta for exchange of nutrients, gases, and wastes.

Blastocyst

Hollow structure with inner cell mass and trophoblast.

Inner cell mass

Becomes the embryo.

Trophoblast

Outer layer that forms placenta and supports implantation.

Morula

Solid ball of cells formed after cleavage.

Gastrulation

Process forming three germ layers: ectoderm, mesoderm, endoderm.

Ectoderm

Forms skin, nervous system, sensory organs.

Mesoderm

Forms muscles, bones, blood, reproductive system.

Endoderm

Forms digestive tract lining, respiratory system, liver.

Neural tissue origin

Ectoderm.

Reproductive system origin

Mesoderm.

Ovulation

Release of mature egg from ovary into fallopian tube.

FSH role

Stimulates ovarian follicle development.

Fertilization

Fusion of sperm and egg to form a zygote (usually in fallopian tube).

Sperm migration

Sperm travel vagina → cervix → uterus → fallopian tube.

Hyaluronidase

Enzyme released by sperm to penetrate egg layers.

Zygote

Single cell formed after fertilization with full chromosome set.

Cleavage (definition)

Rapid cell division of zygote without growth in size.

Blastulation (definition)

Formation of blastula from dividing cells.

Implantation (definition)

Embedding of blastocyst into uterine wall.

Placentation (definition)

Formation of placenta from chorion and uterine tissue.

Germ layers formation time

During gastrulation.

Gastrula

Stage where embryo has three germ layers.

Amnion

Fluid-filled sac protecting embryo.

Chorion

Outer membrane contributing to placenta.

Yolk sac

Early nutrient supply and blood cell formation.

Allantois

Contributes to waste removal and umbilical blood vessels.

Placenta

Organ for exchange of nutrients, gases, wastes, and hormones.

hCG

Maintains corpus luteum and progesterone production.

Progesterone

Maintains uterine lining and prevents contractions.

Estrogen

Supports uterine growth and fetal development.

hPL

Alters maternal metabolism to support fetus.

Relaxin

Relaxes pelvic ligaments for childbirth.

Hormone preventing contractions

Progesterone.

Labor stages

Dilation stage, expulsion stage, placental stage.

Dilation stage

Cervix opens and contractions increase.

Expulsion stage

Baby is delivered.

Placental stage

Placenta is expelled after birth.

Postnatal development

Development after birth.

First trimester (0–12 weeks)

Organ formation; heart, brain, limbs develop; placenta forms.

Second trimester (13–27 weeks)

Rapid growth; fetal movement begins; organ maturation.

Third trimester (28–birth)

Weight gain, lung maturation, and preparation for birth.

Fastest fetal growth period

Second trimester.

When fetus looks human

End of embryonic period (~week 8–9).