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Human development

Depends on temporal and spatial cues in dynamic niches

Conceptus

Developing embryo or fetus

gestation

Time of development within the uterus

Embryo

From day 1 to 8th week of gestation

Fetus

From the 8th week until birth

Zona pellicuda

Glycoprotein shell surrounding the embryo until it hatches out in the late blastula stage. It must hatch out of this to implant

Zygote

Oocyte

Ampulla of fallopian tube

Where does fertilization occur

Two cell embryo

Occurs approximately 30 hours after fertilization. Each cell is a blastomere

Four cell embryo

48 hours gestation

Morula

Solid ball of cells. About 16 blastomeres and occurs around day 3 after fertilization

Totipotenet

Each has the potential to develop into a complete individual, identical twins, or clones

Blastula

Hollow ball of cells, begins around day 4

inner cell mass

Will become the embryo proper

Trophoblasts

Outer wall of the embryo and will contribute to the placenta. Makes hCH to keep uterus ready for pregnancy

Blasstocoel

Fluid filled space

Nidation

Implantation. Occurs around day 6-9 after fertilization at blastula stage

Bilaminar blastula

Day 7, the inner cell mass differentiates into two layers

Epiblast

Will continue to produce the amnion, the embryo proper, and eventually the ectoderm, mesoderm, and endoderm of embryo proper

Hypoblast

Critical in establishing various axis of embryo, anterior vs posterior and medial vs lateral. Necessary for normal development. Forms primary yolk sack

Late blastula

Around day 8

Cytotrophoblasts

Surround the entire embryo and give rise to synctiotrophoblasts

Synctiotrophoblasts

Multinucleated. They are aggressively invasive cells that digest into the uterus to establish placenta

Lacunae

Spaces that form within the synctiotrophoblasts and are eventually filled with maternal blood which will bath the chorionic villi

Amnion

Surrounding the embryo/fetus. Filled with amniotic fluid to protect embryo/fetus. There is about 1000 ml of amniotic fluid that is removed and replaced each day

Chorion

Will surround the amnion, epiblast, hypoblast, and yolk sac. Lines the inside surface of the cytotrophoblasts

Day 12

Some lacunae fuse and maternal blood seeps in. Embryonic blood vessels will enter lacunae. Embryonic blood vessels will receive nutrients and oxygen from uterine arteries. Carbon and dioxide and other wastes will return to be picked up

Secondary yolk sack

Pushes down from the hypoblast and pushes the primary yolk sac down where it regresses

Trilaminar embryonic disc

Referred to as gastrula in lower animals.q

Day 15-16

Event is marked by the development of the three primary germ layers. Ectoderm, mesoderm, endoderm from the epiblast.

Embryogenesis

Development of embryo from the zygote to the trilaminar embryo

Gastrulation

Development of the 3 primary germ layers in the trilaminar embryonic disc

Ectoderm

Gives rise to nervous system, epidermis, pituitary gland, and adrenal medulla

mesoderm

Gives rise to connective tissue, muscle, bone, blood, dermis, adrenal cortex, mesenchyme

Endoderm

Gives rise viscera, 4 weeks, heart begins to pump blood

Organogenesis

development of organs and their systems

28 day

About 5 mm long

Somites

Mesodermal masses that lie on either side of the notocord, giving the embryo a segmented appearance. Can also be used to date embryo progress. Develops into some parts of skeleton, skin, and muscles

Notochord

A median cylindrical structure eventually replaced by the vertebral column

Sclerotome cells

Produce vertebra and rib at each level

Dermatome cells

Form dermis of the skin on the dorsal part of the body

Myotome cells

Form skeleton muscles of the neck, trunk, and limbs

Placentation

The development of the placenta

Chorion

The fetal contribution to the placenta

Decidua

The maternal contribution to the placenta. Participates in the exchange of nutrients, waste, oxygen, carbon dioxide. Aid in protecting the fetus from maternal immune attack

Placenta functions

Provides oxygen and nutrients, removes carbon dioxide and other waste products such as uric acid and creatinine. Immune functions. Produces hCG, progesterone, and estrogen

Immune functions

Allows passage of maternal igG’s to fetus

Human chorionic gonadotropins

Keeps corpus luteum functioning so it can continue to release progesterone to keep the uterus prepared for continued pregnancy

Progesterone

Once the corpus luteum regresses, the placenta takes over this production

Estrogen

Helps for embryonic development but also helps with maternal physiology necessary to maintain a healthy pregnancy

Neurulation

Neural groove, soon to become the neural tube forms in the dorsum of the embryo

Day 25

Closure of the anterior portion (cerebral cortex)

Day 27

closure of the posterior portion (spinal cord)

Neural tube defects

Due to failure of the tube to close as seen with folic acid deficiency. Indicated by increased alpha fetal protein

Anencephaly

Anterior neural tube closure defect

Spina bifida

Posterior neural tube closure defect

Amniocentesis

Sampling the amniotic fluid w/ a needle to examine the fetal chromosomes found in sloughed skin cells. Performed at 15-16 weeks for potential genetic diseases

Chorion

develops Finger like projections called villi

Chorionic villus sampling

Biopsy of villus cells to examine fetal chromosomes for potential genetic diseases. Test usually done during 10-12 weeks

Allantois

Will become part of the umbilical cord and part urinary bladder. Critical in the proper development of Extraembryonic membranes such as chorion, amnion, and yolk sack. Becomes the urachis

8 weeks

no longer called an embryo, not referred to as a fetus until birth. 22 mm long

Parturition

The process of birth

Cortisol

What does the fetus release to start labor?

Hypothalamus

Where is oxytocin made

Posterior pituitary gland

Where is oxytocin released

Dilation stage

Cervix will dilate to 10 cm

Effacement

As the fetal head pushes down into the cervix, the cervix thins and softens

Expulsion stage

From dilation to delivery

Crowning

Fetal head begins to push through the vaginal orifice

Placental stage

Usually within 30 minutes of delivery. The placenta separates from the uterine wall and is delivered with the fetal membranes

Ductus venosum

Hepatic bypasses liver

Foramen ovale

Opening in interaterial septum. Bypasses pulmonary circulation

Ductus arteriosus

Bypasses pulmonary circulation