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Human Development: From Gamete to Adult

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Overview

• Prenatal (before birth): Zygote → Embryo → Fetus
• Postnatal (after birth): Neonate → Infant → Child → Adolescent → Adult

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PHASE 1 - GAMETOGENESIS (Formation of Sex Cells)

Spermatogenesis (in males)

• Occurs in the seminiferous tubules of the testes, starting at puberty
• Spermatogonia (diploid, 2n) undergo mitosis to produce primary spermatocytes
• Meiosis I produces secondary spermatocytes (haploid, n)
• Meiosis II produces spermatids
• Spermiogenesis transforms spermatids into mature spermatozoa
• Each spermatogonium yields 4 functional sperm

Oogenesis (in females)

• Begins before birth; oogonia multiply in the fetal ovary
• Primary oocytes arrest in prophase I at birth; resume at puberty
• Each month, one primary oocyte completes meiosis I → secondary oocyte + first polar body
• Meiosis II completes only after fertilization
• Each oogonium yields 1 functional egg + polar bodies

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PHASE 2 - FERTILIZATION (Day 0)

• Occurs in the ampulla of the uterine tube, approximately 14 days after the last menstrual period
• One sperm penetrates the secondary oocyte's zona pellucida
• The oocyte completes meiosis II → mature ovum
• The two pronuclei (male + female) fuse → zygote (diploid, 2n, 46 chromosomes)
• The zygote is the first cell of a new human being

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PHASE 3 - CLEAVAGE & TRANSPORT (Days 1-4)

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PHASE 4 - BLASTOCYST & IMPLANTATION (Days 5-10)

• Blastocyst has two cell populations:
  • Inner cell mass (embryoblast) - will become the embryo
  • Outer cell layer (trophoblast) - will become the placenta and fetal membranes
• Hatching: blastocyst escapes the zona pellucida
• Implantation (Days 6-10): trophoblast invades the endometrium of the uterine wall
  • Syncytiotrophoblast (invasive outer layer) penetrates the decidua
  • Cytotrophoblast (inner proliferating layer) supports growth
  • Human chorionic gonadotropin (hCG) is secreted - basis of pregnancy tests

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PHASE 5 - EMBRYONIC PERIOD (Weeks 3-8)

Week 3 - Gastrulation

• Primitive streak forms on the epiblast
• Three primary germ layers established via gastrulation:
  • Ectoderm - skin, nervous system
  • Mesoderm - muscles, bones, heart, kidneys, blood
  • Endoderm - gut, lungs, liver, pancreas
• Notochord forms → induces neural plate formation
• Neurulation begins: neural plate → neural folds → neural groove

Week 4 - Folding & Organogenesis Begins

• Embryo folds (head, tail, and lateral folds) - converts flat disc to cylindrical shape
• Heart begins to beat (day 22-23) - this is the first organ to function
• Neural tube closes (rostral neuropore closes day 25; caudal neuropore day 28)
• Pharyngeal arches form (jaw, neck structures)
• Upper limb buds appear (day 26-27)
• Primitive circulatory system forms
• Somites (blocks of paraxial mesoderm) form rapidly - 25-30 pairs by end of week 4

Week 5

• Rapid brain growth; large head
• Lens pits and optic cups form (eye development)
• Nasal pits forming
• Lower limb buds appear
• Crown-rump length (CRL): ~5-8 mm

Week 6

• Hand plates develop; digital rays visible
• Upper lip and nose formed
• Oral and nasal cavities become confluent
• Retinal pigment visible
• CRL: ~9-13 mm

Week 7

• Eyelids begin forming
• Fingers evident; toes developing
• Intestines herniate into the umbilical cord (physiological herniation)
• Wrist and fingers still fused
• CRL: ~13 mm

Week 8 - End of Embryonic Period

• All major structures established
• Fingers distinct but webbed; toes present
• External genitalia begin to differentiate (still indifferent)
• Face has distinctly human features
• CRL: ~30 mm

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PHASE 6 - FETAL PERIOD (Weeks 9-38/Birth)

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POSTNATAL STAGES

Neonatal Period (0-4 weeks / first month)

• Key events: adaptation to extrauterine life
• Critical cardiovascular changes: foramen ovale closes, ductus arteriosus closes
• Respiratory system must start functioning independently
• Thermoregulation begins
• Weight may drop initially (physiological), then regained by ~2 weeks
• By 1 month: reflexes (rooting, sucking, Moro, grasp) present

Infancy (1 month - 1 year)

• Fastest period of postnatal growth
• Length increases by ~50%; weight triples by age 1
• 6-8 deciduous teeth erupt by age 1
• Motor development: head control → rolling → sitting → standing → first steps
• Neurological: smiling (6 weeks), tracking, reaching, pincer grasp develop
• Language: cooing → babbling → first words (~12 months)

Early Childhood (1-6 years)

• Growth continues but slows compared to infancy
• All 20 primary (deciduous) teeth emerge by age 2-3
• Fine and gross motor skills mature
• Language rapidly expands
• Cognitive: egocentric thinking, symbolic play (Piaget's preoperational stage)
• Social: attachment, parallel then cooperative play

Late Childhood (6-12 years)

• Slow, steady growth; ~6 cm/year
• Primary teeth replaced by permanent teeth
• Active ossification; skeletal growth
• Logical thinking develops (Piaget's concrete operational)
• Fine motor skills refined (writing, drawing)
• Peer relationships become central

Puberty / Adolescence (females ~8-16 yrs; males ~9-18 yrs)

• Prepubertal growth spurt precedes puberty
• Triggered by hypothalamic-pituitary-gonadal axis activation
• Females: breast development (thelarche), pubic hair (adrenarche), menarche (first menstruation), ovulation
• Males: testicular enlargement, pubic hair, voice deepening, spermarche (first ejaculation), facial hair
• Bone density increases; epiphyseal growth plates eventually fuse
• Psychological: identity formation (Erikson's identity vs. role confusion)

Adulthood (18-21+ years)

• Full physical growth and maturity attained between ages 18-21
• Ossification and growth virtually complete by 21-25 years
• Brain development (prefrontal cortex maturation) continues into the mid-20s
• Peak physical performance in early adulthood
• Reproductive capacity established and maintained through middle adulthood
• Aging processes begin gradually (decreased bone density, reduced cellular repair, organ changes)

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Summary Table: Development at a Glance

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