9. Cell Differentiation
By the end of the first week, cell differentiation occurs to
create two tissue types:
Ectoderm – forms outer skin, sensory organs, entire brain and
nervous system
Endoderm – forms the internal organs (gut, lungs, liver, glands
By the end of the second week, the third cell type
differentiation occurs
Mesoderm – forms the bones, muscles, connective tissues,
circulatory system, and smooth muscles of the digestive
system
10. Spinal Cord and Nervous System Formation
Neurogenesis starts in the third week after fertilization, creating
neurons at a rate of over 250,000 per minute
The synapses for each neuron are created – up to 15,000
synapses per neuron – at a rate of 1.8 million synapses per
second between 8 weeks gestation and 2 years after birth
By 9 weeks the spinal cord is formed and starting to function
11. Brain
Yolk sac
Eye
Placenta
Heart
Umbilical Liver
cord
Arm bud
Leg bud
Embryo at 5 weeks from conception
Close your eyes, and imagine a 1-year-old child – what does she look like, what is she doing?
Is this what you imagined: a toddler, able to sit up, walking, babbling
Or this: infant lying on its back I hope your vision will be such that you will recognize the experience of life from the baby’s point of view. At one year of life, an infant is only 3 months post-birth (assuming a full-term gestation) but has also experienced a rich and full life before being born. Babies indeed do not just come alive at birth for the first time, but have been alive and filled with senses and experience from the moment of conception, and perhaps even before…
Fertilization occurs inside the fallopian tube normally Cell divisions occur within the fallopian tube as the fertilized egg makes its way toward the uterus Around day 8 the fertilized egg, now called a blastocyst (or the blackberry-like’ clump of cells) will drop into the uterus and implant on the uterine wall
Ovum and sperm Recent research shows that the egg is ‘selective’ in the sperm it allows to fertilize it, allowing a certain sperm to penetrate it when others may have tried before it Once the sperm penetrates the cell membrane, a protective barrier forms preventing any other sperm from penetrating. Fertilization has occurred. Each cell (ovum and sperm) carry 23 chromosomes which will form the genetic makeup of the new person. The sperm carries either an X or a Y chromosome which will determine if the person will be a male or a female.
Blastocyst The developing fetus itself is the area marked as "ICM" (inner cell mass) The blastocoel cavity in the center is marked as "C" The trophectoderm cells that will form the placenta surround the cavity - one is marked with a "T" The momentous decision about whether to become body or not is made entirely on the basis of chance – whatever position a given cell happens to be in. This is one of the earliest instances of how the environment profoundly affects development. Any one cell from a blastocyst is capable of forming an entire baby if removed and left to divide on its own.
Embryo at 5 weeks. The embryo produces hormones which stop the mother’s menstrual cycle. This is just the first of many instances where the baby will create chemical messengers (hormones) to instruct the mother’s body what to do. Embryo is the size of a raisin. By day twenty-one, the embryo's tiny heart has begun beating. The neural tube enlarges into three parts, soon to become a very complex brain. Beginning of eyes and ears The spine and spinal cord grows faster than the rest of the body at this stage and give the appearance of a tail. This disappears as the child continues to grow. Rudimentary blood runs through main vessels
6 week old embryo in sac
Embryo at 6 weeks All essential internal and external structures are present – organogenesis Hands and feet have digits but are still webbed Nipples and hair follicles form
Embryo in sac at 8 weeks Placenta begins to function Heart beat may be heard with a Doppler Rotation of intestines
Close up of hands at 10 weeks Discuss LMP vs. conception dates
Fetus at 12 weeks Face with eyes, nose, ears, moth, lips, tongue Can suck thumb, smile, frown, swallow, make breathing movements, urinate 3 inches long, weighs a little over 1 ounce
Hand at 12 weeks Has fingerprints, soft nails on fingers and toes
Fetus at 3+ months Muscular activity begins Reflex responses Gender clearly distinguishable Eyelids fuse and will not reopen until about 20 weeks Tooth buds appear
Ear at 3+ months
Fetus at 15 weeks
Fetus at 15 weeks Notice eyelid, ear, umbilical cord Taste buds are mature 4” long
Foot at 16 weeks Notice blood vessels Red blood cells are produced in the liver
Sucking thumb at 18 weeks Neurogenesis is complete – all the 100 billion neurons in the brain have been created – for the most part, all the neurons that will ever be created for this person
Girl fetus at 18 weeks Rapid body growth – 9” long Mother may feel fetal movements and hiccups Heartbeat heard with fetoscope Vernix caseosa covers entire body
Ear at 5 months Has been hearing mother’s heartbeat, blood swooshing, digestive noises, and voice since about 18 weeks. Will develop voice recognition in the next trimester.
Face at 5 months – notice lanugo
Fetus at 24 weeks – notice lanugo and vernix 15” Has hand and startle reflex (illustrated in this picture) Alveoli (air sacs) forming in lungs Skin wrinkled and red Covered with fine, soft hair – lanugo Eyelashes and eyebrows form Eyelids begin to part and eyes open Brown fat formed Weights 1-1 ½ pounds Could survive if born at this time
Fetus at 32 weeks – notice blood vessels within umbilical cord, chorionic villi Neural circuits are as developed as they will be at birth. Cerebral cortex is equally developed, meaning babies as early as 28 weeks can think, feel and remember Memory tracks begin being laid down in the third trimester Fetus can smell and taste – swallows up to a liter of amniotic fluid a day which can smell or taste strongly of curry, cumin, garlic, onion,… serves as a flavor bridge to breast milk which also carries food flavors. Fetus is sensitive to heat, cold, pressure and pain
Fetus at 36 weeks – note scrotum, penis, umbilical cord, placenta Skin smooth as subcutaneous fat becomes thicker Rapid brain growth continues Rhythmic breathing movements Bones are fully developed, but soft and pliable 18” long 5 pounds
Weeks 37-40 Finishing touches Fetus gains about ½ pound a week Lanugo disappears from most of body Fingernails extend beyond ends of fingertips Head hair is coarse and thicker Testes descend in males 19-21” long 6-9 pounds Can smell amniotic fluid on its hands which “match” the smell of mother’s breast, helping it find its source of food The brain will triple its size and triple its birth weight in the first year
