Development II

what happens after cleavage

• normal cell cycle continues

• cell division slow

• morphogenesis

what is morphogenesis

• cellular and tissue-based process by which animal body takes shape

• occurs in gastrulation and organogenesis

what is gastrulation

• process by which hollow blastula becomes layered embryo (gastrula)

what is gastrulation in sea urchins

• involves cell migration and invagination (infolding)

• forms archenteron (early gut tube)

• blastopore becomes anus

• second opening becomes mouth

what is gastrulation in birds

• are vertebrae so different than urchins

• germ layer fates

• occurs on active cap

• have two layers

• cell migrates

• use hypoblast cells

what is the active cap (birds)

• cannot occur in enchinoderms

• meroblastic cleavage

what are the two layers in birds

• epiblast: upper layer (where embryo is derived from)

• hypoblast: lower layer

how do cells migrate in birds

• involution: epiblast cells ingrate towards midline

• ectoderm: some stay on top layer

• mesoderm: some move inwards towards yolk then laterally outwards

• endoderm: some move downwards and displace hypoblast

what is a primitive streak

• midline where cells migrate inwards

• blastopore

what are hypoblast cells

• contribute to membranes around yolk

• connection between yolk and embryo

• do not become part of embryo itself

what is gastrulation in humans

• the end of cleavage

• implantation then gastrulation begins

what happens at end of cleavage for humans

• blastocyst has formed which is the mammalian version of a blastula

what are the parts of a blastocyst

• trophoplast: outer single layer of cells

• inner cell mass: cluster of cells, becomes embryo and source of germ layers

what is implantation like in humans

• blastocyst arrives in uterus about six days after fertilization

• trophoblast contacts uterine lining

  • erodes endometrium

• embryo penetrates

• thickens and extends projections into endometrium

what is gastrulation like in humans

• takes about one week during the second or third week of pregnancy

• inner cell mass forms two layers: epiblast and hypoblast

• same process as in birds but both layers contribute to endoderm

• gastrula also contains extra embryonic membranes for fluid and blood vessels

what is organogenesis

• development of forming organs

• nervous system first

what is the process of neurulation

• cells of dorsal mesoderm form notochord

• induces dorsal ectoderm

• thickens and formed neural plate

what is induction

• process by which cells stimulate or influence differentiation of neighboring cells

what is a neural plate

• embryonic region that folds into neural tubes

• develops into central nervous system

  • anterior becomes brain

  • remainder becomes spinal cord

what are somites

• blocks of mesoderm later to the notochord

• form segmentally repeated structures

the implanted embryo secretes what

• human chorionic gonadotrophin

• hCG

• acts like LH and maintains progesterone and estrogen form corpus luteum

what is the placenta

• trophoblast mixes its endometrium

• organ of exchange of nutrients and oxygen between parent and embryo

• connects by umbilical cord

what are the types of twins

• monozygotic: inner cell mass splits during first month (identical)

• dizygotic: two eggs fertilized by two sperm develop at the same time (fraternal)

what is birth

• occurs at 9 months

• begins with labor

• uterus contracts to push fetus and placenta out of body

• regulated by prostaglandins, estradiol and oxytocin

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