Embryogenesis and Development

cleavage

rapid mitotic cell division that the zygote undergoes as it moves to the uterus for implantation

indeterminate cleavage

results in cells that can still develop into complete organisms

determinate cleavage

results in cells that are already fated to differentiate into a particular cell type

morula

solid ball of cells that is the embryo after several rounds of cleavage

blastula

hollow ball of cells with a fluid-filled inner cavity known as the blastocoel

blastocyst

mammalian blastula that consists of the inner cell mass and trophoblasts

inner cell mass

gives rise to the organism, protrudes into the blastocoel

trophoblast cells

surround the blastocoel and give rise to the chorion (later the placenta)

chorion

extraembryonic membrane that eventually develops into the placenta; given rise to by trophoblast cells

gastrulation

generation of three distinct cell layers

primary germ layers

ectoderm, mesoderm, endoderm

ectoderm

gives rise to the integument (including epidermis, hair, nails, epithelia of the nose, mouth, and lower anal canal), lens of the eye, nervous system, and inner ear

mesoderm

gives rise to the musculoskeletal, circulatory, and most of the excretory systems, gonads, muscular/connective tissue layers of the digestive and respiratory systems, and the adrenal cortex

endoderm

gives rise to the epithelial linings of the digestive/respiratory tracts, pancreas, thyroid, bladder, distal urinary tracts, and parts of the liver

embryonic origin of the adrenal glands

adrenal cortex is derived from the mesoderm, but the adrenal medulla is derived from the ectoderm

neurulation

development of the nervous system, begins after the three germ layers are formed

stages of development

zygote → embryo → morula → blastula → gastrula

autocrine signaling

signal acts on the same cell that secreted the signal in the first place

paracrine signaling

signals act on cells in the local area

juxtracrine signaling

signal directly stimulates receptors of an adjacent cell, but does not involve diffusion

endocrine signaling

secreted hormones travel through the bloodstream to a distant target tissue

umbilical arteries

carry deoxygenated blood away from the fetus and toward the placenta

umbilical veins

carry oxygenated blood away from the placenta and toward the fetus

fetal shunts

foramen ovale, ductus arteriosus, ductus venosus

foramen ovale

one-way valve that connects the right atrium to the left atrium, bypasses the lungs

ductus arteriosus

shunts leftover blood from the pulmonary artery to the aorta, bypasses the lungs

ductus venosus

shunts blood returning from the placenta via the umbilical vein directly into the inferior vena cava, bypasses the liver

first trimester

major organs develop

second trimester

fetus starts moving, facial features develop, fingers and toes elongate

third trimester

brain development occurs, growth rate slows, less fetal movement

first phase of birth

cervix thins, amniotic sac ruptures (“water breaking”)

second phase of birth

uterus contractions begin, baby is born

third phase of birth

placenta and umbilical cord are expelled; commonly known as afterbirth