Basic Embryology
EmbryologyDefinition: the study of the origin and development of an organismPrenatal period: before birth38 weeks from conception to birth (average) “fetal” ageGynecologic timing has been from LMP therefore refers to 40 weeks “gestational” age Date of conception has been difficult to timeLMP is on average two weeks before ovulation
Traditional (artificial) division:
“Embryonic” period: first 8 weeksAll major organs formed“Fetal” period: remaining 30 weeksOrgans grow larger and become more complexFertilization to Implantation
Conception (biology) or fertilisation, the fusion of gametes to produce a new organism of the same species (Wikipedia)Ovulation: egg released into the peritoneal cavity Travels down fallopian tube in which fertilization occurs At conception in fallopian tube, maternal and paternal genetic material join to form a new human life (zygote) Cell division occurs with travel down the tube and into the uterus
Week 1 post conception
Zygote divides repeatedly moving down tube toward uterus (cleavage) The daughter cells are called blastomeres Morula: the solid cluster of 12-16 blastomeres at about 72 hours Day 4: late 60 cell morula enters uterus, taking up fluid becoming blastocyst_____inner cell mass
______trophoblastBlastocyst stage
Two distinct types of cells Inner cell mass: forms the embryo Trophoblast: layer of cells surrounding the cavity which helps form the placenta Floats for about 3 days Implantation on about day 6 post conception Trophoblast erodes uterine wall Takes 1 week to complete If inner cell mass of a single blastocyst divides: monozygotic (identical) twinsWeek 2
Inner cell mass divides into epiblast and hypoblast 2 fluid filled sacs Amniotic sac from epiblast Yolk sac from hypoblast Bilaminar embryonic disc: area of contact (gives rise to the whole body)
Week 3
Bilaminar to trilaminar discThree primary “germ” layers: all body tissues develop from theseEctodermEndodermMesodermFormation of the 3 “germ” layers
Primitive streak (groove) on dorsal surface of epiblast Grastrulation: invagination of epiblast cells Days 14-15: they replace hypoblast becoming endoderm Day 16: mesoderm (a new third layer) formed in between Epiblast cells remaining on surface: ectodermThe three “germ” tissues
“Germ” as in germinate, not germsEarly specialization of cellsAre precursorsEctoderm and endoderm are epithelial tissue (form sheets of tissue)Mesoderm is a mesenchyme tissueMesenchyme cells are star shaped and do not attach to one another, therefore migrate freelyNotochord
Days 16-18 Primitive node epiblast cells invaginate and migrate anteriorly with some endoderm cells Rod defining the body axis is formed Future site of the vertebral column
Neurulation
Notochord signals overlying ectoderm Formation begins of spinal cord and brain (neurulation) Neural plate to neural groove to neural tube: pinched off into bodyClosure of neural tube: begins at end of week 3; complete by end of week 4 (folic acid important for this step) Extends cranially (eventually brain) and caudally (spinal cord) Neural crest, lateral ectodermal cells, pulled along and form sensory nerve cells and other structures
Mesoderm begins to differentiateLateral to notochord, week 3Extends cranially and caudally (from head to tail or crown to rump)Division of mesoderm into three regionsSomites: 40 pairs of body segments (repeating units, like building blocks) by end week 4Intermediate mesoderm: just lateral to somitesLateral plate: splits to form coelom (“cavity”)
Divisions of the mesodermal lateral plate
Somatic mesoderm: apposed to the ectoderm Splanchnic mesoderm: apposed to the endoderm Coelom in between will become the serous cavities of the ventral body cavity: Peritoneal Pericardial Pleural
Folding begins at week 4
(main difference between the 3 week embryo and the adult body is that the embryo is still a flat disc)24 day embryro; protrudes into amniotic cavity
Day 23, beginning to foldLateral folds will join ventrally