Lecture 20: Embryology and Development
I. Overview
A. preembryonic period (preembryo)
B. embryonic period (embryo)
C. fetal period (fetus)
conception -> Week 2
Week 3 -> Week 8
Week 9 -> birth
II. Preembryonic Period
A. Fertilization
1.
2.
3.
4.
sperm fertilizes egg in the uterine tube
sperm membrane must be capacitated in order to fuse
acrosomal reaction - allows sperm the penetrate egg
nuclei of sperm and egg join to form diploid zygote
B. Cleavage and Implantation
1. cleavage - rapid mitotic divisions of the zygote
a. zygote -> blastomeres (2 cells)
-> morula
(16+ cells)
-> blastocyst (100+ cells)
2. implantation in endometrium of uterus
36 hours
72 hours
5 days
6 -> 14 days
a. human chorionic gonadotropin - overrides menses
i. used for pregnancy testing (urine/blood)
3. formation of the embryonic membranes
10 -> 21 days
a. amnion - water sac surrounding embryo/fetus
i. protects and stores waste products
b. yolk sac - source of germ cells for growth
c. chorion - outermost bag (part of placenta)
d. allantois - base for the umbilical cord
III. Embryonic Development
A. Gastrulation - the formation of the 3 Primary Germ Layers
1. embryonic disc -> 3 layered gastrula
a. ectoderm - on the amniotic surface of embryo
i. nervous system, skin epidermis
b. mesoderm - middle of the "sandwich"
i. muscle, bone, blood, connective tissues
ii. mesenchyme - cells free to migrate
c. endoderm - on the yolk sac surface of embryo
i. epithelia - digest, resp, urogen, glands
B. Ectoderm Specialization
1. neurulation -> brain & spinal cord development
a. neural plate -> n. groove -> n. fold -> n. tube
b. anterior neural tube - develops into brain
c. posterior neural tube - develops into spinal c.
i. notochord - early spinal tube
d. neural crest cells - develop into cranial,
spinal, autonomic nerves and ganglia
C. Mesoderm Specialization
1. somite - paired blocks of mesoderm along "notochord"
a. sclerotome - develop into vertebrae
b. myotome - develop into muscles and bones
c. dermatome - develop into dermis of skin
D. Endoderm Specialization
1. embryo folds inward to form a tube
2. endoderm layer lines this "early gut tube"
3. endoderm forms: lining of GI and Respiratory tracts;
thyroid, parathyroid, thymus, liver, pancreas
E. Fetal Circulation - Adult Vestiges
FETUS
1.
2.
3.
4.
umbilical vein ->
ductus venosus ->
ductus arteriosus ->
foramen ovale ->
ADULT
ligamentum teres (liver)
ligamentum venosum (liver)
ligamentum arteriosum (heart)
fossa ovalis (heart)