Human Life Cycle and Development

• Human life cycle begins with fertilization
• Followed by series of growth and development that allow formation of fully functional individual
• Male sperm cell unites with female egg cell during fertilization
• Gametes serve to combine paternal and maternal genetic material in offspring
• Fertilized egg (zygote) develops into an embryo through cell division and differentiation

Human Development

• Embryo develops into a fetus inside the womb
• Fetus obtains oxygen and nourishment through placenta
• Gestation period: ninth month of pregnancy
• Newborns (0-1 year old) are infants
• Infants continue developing functionality of organs like eyes and brain

Stages of Development

• Childhood: 1-12 years; locomotor and cognitive skills develop
• Adolescence: puberty prepares individual for adulthood (13-19 years)
• Puberty involves changes in body parts and functions and development of secondary sex traits
• Adulthood ensures transmission of genes to next generation

Adulthood Stages

• Young adults: 20–35 years old
• Middle-aged adults: 36–54 years old
• Elderly: 55-65 years old

Human Reproduction

• Exclusively sexual; involves production/fusion of sex cells (gametes)
• Fertilization ensures zygote formation
• Complete genetic instructions needed to grow into adult

Male Reproductive Structures

• Located in pelvic region
• Produce and convey male gametes
• Testes: primary reproductive glands, contained in scrotum (maintains temperature for sperm production)
• Sperm exits through vas deferens during ejaculation, ascends into abdomen, loops into urinary bladder, goes down into urethra
• Seminal vesicle: contributes 60% to semen, alkaline solution nourishes/protects sperm
• Prostate gland: 30% of semen, neutralizes pH of semen/vaginal fluids
• Bulbourethral gland: lubricating fluid for intercourse
• Penis: male organ for erection and semen expulsion.

Female Reproductive Structures

• Female reproductive system also in pelvic region
• Ovaries: produce eggs
• Fallopian tubes (oviducts): catch eggs
• Fimbriae: finger-like appendages
• Uterus (womb): chamber with vascularized endometrium to house/nourish embryo; muscular layer expels baby during labor
• Vagina: receives penis during intercourse, Elastic channel (birth canal)
• Cervix: prevents entry of foreign materials to the uterus
• Clitoris: homologous to penis; functions for sexual stimulation

Gametogenesis

• Testes and ovaries (gonads) produce gametes and nourish sex cells
• Gametogenesis: diploid chromosome set of precursor cells reduced to haploid

Fertilization Process

• Male releases sperm (spermatozoa) during copulation/intercourse
• Sperm travels to fallopian tube to meet egg cell
• Fertilization: fusion of sperm and egg cells forms diploid zygote
• Sperm uses motility to reach egg cell
• Fertilization: sperm penetrates between granulosa cells, then passes through zona pellucida

Egg Cell Layers

• Granulosa cells protect/nourish egg during development
• Acrosomal enzymes degrade zona pellucida (jelly coat layer)
• Membranes of sperm and egg fuse, paternal genetic material combines

Polyspermy Prevention

• Fast-block mechanism: Electrical charge changes within egg cell membrane
• Slow-block mechanism: Cortical granule reaction facilitates fertilization membrane formation

Human Development: Pre-Implantation

• Zygote travels to uterus, implanted, develops into human
• Preimplantation/germinal stage: fertilized egg travels to uterus, undergoes a series of mitotic cell divisions
• This is called germinal embryonic development

Cell division

• Two-cell, four-cell, eight-cell stages occur
• Morula stage: 16-cell phase; arrives in uterus three days post-fertilization
• Blastocyst: ball of cells developed by the continued division of the Zygote
• Trophoblast: outer shell; develops into chorionic sac and fetal portion of placenta
• Trophoblast and the inner mass develops into the chorionic sac and the fetal function of the placenta

Cellular Differentiation

• Trophoblast cells undergo differentiation forming connections between developing embryo and uterine wall during eighth/ninth day (embryonic implantation)
• Connects embryo to the womb, supplies blood and nutrients until birth

Extraembryonic Membranes and Placental Development:

• During the second week, Embryonic implanted cells form extraembryonic membranes
• These consist of the: amnion, yolk sac, allantois, and chorion
• Membranes support and protect the growing embryo

Extraembryonic Membrane Facts

• Amnion: contains fluid to surround the embryo; found in amniotes
• Yolk sac: provides initial nourishment in early pregnancy
• Allantois: collects liquid waste, facilitates gas exchange; forms umbilical cord along with yolk sac
• Chorion: allows villi to exchange nutrients, oxygen, and waste with mother

Third Week of Development (Gastrulation Stage)

• The embryo becomes a three layered disc of cells through the process of gastrulation
• The embryo becomes an oval-shaped disc
• The three germ layers (formed after gastrulation) developing into specific tissues and organs in the embryo.
• At four to twelve weeks During the prenatal period the placenta develops allowing the feeding of the developing embryo
• The Placenta derived from the tissues of the embryo and maternal tissues connected to developing through the umbilical cord
• Networks facilitate the transfer of material between the baby and the mother

Embryonic Development

• Embryo becomes susceptible to teratogens at embryonic stage, which induces developmental defects
• Organogenesis: organ formation starts in third week after fertilization; heart develops
• Limbs rotate, fingers separate, sex determined, and face becomes human

Fetal Development

• At fourth week, eyes, arms, legs become prominent, embryo size increases
• External genitalia form during eighth/ninth weeks predicts Phenotypic sex through ultrasound
• Further active organ formation occurs from ninth to thirty-eighth weeks of pregnancy and formation of different organs