Development Of Placenta And Extraembryonic Membranes

Front 1

zygote formation

Back 1

zygote undergoes cleavage (division of cells) to form a sphere of cells within the zona pellucida = morula, cleavage is cell division but without growth and once the zona pellucida breaks down fluid goes into the morula creating hte blastocyst

Front 2

blastocele

Back 2

cavity (blastocele) forms within the morula, now called the blastocyst, cells are segregating on the inside
1. trophoblast - outer surrounding layer of cells, contributes to the formation of the placenta
2. inner mass cells - cells clustered at one pole of the blastocyst, will become future embryo

Front 3

implantation

Back 3

makes contact with the unterine wall (9-10 days after fertilization)
1. blastocyst contacts uterus endometrium
2. trophoblast cells organize into 2 layers(differentiating into 2 layers)
3. cytotrophoblast (cellular trophoblast) inner layer of cells around the blastocele and seperates the inner cell mass from the syncticial trophoblast

Front 4

syncticial trophoblast

Back 4

outer cell layer of fused cells which invades the endometrium, erodes uterine glands and blood vessels, secretes HCG, estrogen, progesterone

Front 5

10 days after fertilization

Back 5

blastocyst embeds completely in the endometrium, as the syncticial trophoblast erodes away endometrial blood vessels, maternal blood accumulate in spaces within the synctitial trophoblasts = lacunae (pool of blood), finger-like projections of trophoblast project into the lacunae = primary (stem) villi (inner core of cytotrophoblast and outter core of synctial trophoblasts)

Front 6

decidualization of endometrial stroma (lamina propria)

Back 6

the embedding blastocyst stimulates the CT cells to enlarge and secrete proteins, endometrium is now called decidua (specialized endometrium of pregnancy)
1. decidua basalis - most important portion of endometrium b/w developing embryo and myometrium, contributes to the formation of placenta
2. decidua capsularis - thin layer of endometrium b/w blastocyst (embryo) and uterine cavity
3. decidua parietalis - remainder of endometrium lining of the uterine cavity

Front 7

formation of extraembryonic membranes and cavities

Back 7

1. inner cell mass develops into 3 germ layers (ectoderm, mesoderm, endoderm)
2. outpocketing of ectoderm creates ectoderm-lined cavity above the embryo = amnionic cavity, lined with extra embryonic ectoderm (cells that arise from the epiblast) it is the space above the developing embryo
3. outpocketing of endoderm creates an endoderm-lined pouch below the endoderm = yolk sac (connects to developing embryo midgut), the extraembryonic endoderm
4. small outpocketing of endoderm creates a endoderm-lined pouch caudal to the yolk sac = allantois

Front 8

mesoderm

Back 8

proliferates and streams out from the middle layer of embryo and creates an extensive continous layer
1. it forms the outer layer of the amnion, the yolk sac, the allantois, by streaming out
2. it lines the inner surface of the blastocsyt, the trophoblast and adjacent mesoderm is now referred to as the chorion
3. from streaming out it forms a thick stock, a layer outside the amnion (lined with ectoderm covered with endoderm), yolk sac (lined with endoderm and covered w/ mesoderm), and allantois

Front 9

chorion

Back 9

it is adjacent to the decidua basalis undergoes the greatest development contributing to the formation of the placenta
1. mesoderm penetrates into primary villi, now called chroionic villi or secondary villi (outer synctial trophoblast, middle cytotrophoblast and mesoderm core)
2. fetal blood vessels develop within chorionic villi (at this stage chorionic villi are sometimes called tertiary villi), blood vessels develop from the mesoderm

Front 10

placenta constituents

Back 10

1. decidua basalis (maternal component)
2. chorion (fetal component - all fetal membranes)
functions: exchange of nutrients, wastes, gases, hormone production

Front 11

development of the umbilical cord (attaches the fetus to the placenta)

Back 11

the mesoderm surrounding the allantois is continuous with chorion and as the embryo develops, elongates to form the body stalk
the stalk of the yolk sac fuses with the body stalk to form the umbilical cord
fetal blood vessels develop with in the umbilical cord (2 umbilical arteries and 1 umbilical vein)

Front 12

fetal circulation

Back 12

uterine blood vessels to and from the maternal blood in lacunae to and from diffusion across chorionic villi to and from across exchange of gases and wastes occurs in chorionic capillaries to and from umbilical vein (oxygenated blood) to the fetus to the umbilical arteries (deoxygenated blood)

Front 13

history of placenta

Back 13

chronionic villi - will have an outer flat layer of synctinial trophoblast, the cells are fused so you will see individual cells, internal to the synctian layer is a layer of polygonal cytotrophoblast cells, diffuse mesynchemal cells fill the core of the villus, depending on the stage of the villus, there may be several cross sections of chorionic blood vessels in the core, the lacunae filled with the maternal blood appear as spaces surrounding the chorionic villi

Front 14

stages of villi development

Back 14

1. primary (stem) villi: synctyial trophoblast with core of cytotrophoblasts
2. chronionic villi:
secondary villi - same as primary but with core of loose CT (mesoderm)
tertiary villi - CT becomes vascularized
as the pregnancy progresses , the cytotrophoblast layer of the villi regresses
placental insufficiency/ intrauterine growth resctriction