11.0, 12.0 EARLY DEVELOPMENTAL ANATOMY
Upon completion of this lecture, students will be able to:
1. Describe embryo development and stages; when uterine attachment occurs, where
embryonic stem cells reside.
2. Understand the blastocyst layers and its differentiation
3. Describe gastrulation and the germinal layers
4. Describe critical periods of development for the dog
5. Identify the origin of all basic organs from embryonic germ layers.
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EARLY DEVELOPMENTAL ANATOMY
I.
II.
III.
IV.
Introduction
A. Early embryonic
1. Fertilization and egg activation
2. Cleavage and formation of blastocyst
3. Gastrulation and formation of germ layers
4. Regional development
B. Late embryonic
1. Ectoderm forms neural crest, eye, cutaneous (skin)
2. Meosoderm form early heart
3. Limb development, germ line
C. Post embryonic
Fertilization
(to be covered in reproduction)
Formation of the Blastocyst
A. Zygote
1. Formed by the fusion of sperm and egg
2. Divides into 2 blastomeres
3. Continuous cleavage of the blastomeres
B. Morula
1. Day 10-12 post fertilization in dog
2. Solid group of cells, 16-64
3. Surrounded by zona pellucida from original egg
C. Blastocyst
1. Forms fluid filled cavity; about day14 in dog
2. Polarized blastodisc
a. Surrounded by single layer of cells – trophoblast
b. (inner cell mass) forms the embryo; source of ES cells
c. differentiates into 2 cell types
i. epiblast cells give rise to 3 germ layers
ii. hypoblast cells are extra embryonic tissues of the placenta
3. Ultrasound detectable about day 18 (> 1mm diameter)
4. Trophoblast cells start to invade uterine swellings about days 21-22 in dog
Chick versus primate and other mammal embryo
A. We study the chick embryo because development of all the germ layers is much faster
than in the mammal. Within 24 hours, the 3 germ layers are well formed and within 48
hours you can see the beginning eye, heart, liver, trachea and gut. Whereas, such
development would take about 18-20 days in the fetal pig. In the human, development is
about the same as in the pig.
B. Gastrulation
1. Formation of the 3 germ layers
2. Primitive streak forms
a. First sign of gastrulation
b. A furrow in midline of disk
c. Future caudal end
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d. Epiblast cells invaginate—will form endoderm and mesoderm
3. Trilaminar embryo, also called germinal disk
a. Ectoderm, endoderm, mesoderm
4. Notochord
a. Cells from mesoderm form a flexible rod
b. Defines the axis; follows the neural tube
c. Vertebral column forms around notochord
d. Induces ectoderm to form neural plate
5. Avian
a. Embryonic disc (blastodisc) forms on the surface of the egg yolk
V.
EMBRYONIC FORMATION OF ORGANS
A. Ectoderm
1.
2.
3.
4.
5.
Outer layer
Forms skin, hair, hoof, nails, teeth, lens and cornea of eye
Forms neural crest; ganglia of nervous system
Forms neural tube; then brain, spinal cord, retina, posterior pituitary
Forms adrenal medulla
From: Wikipedia
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B. Mesoderm
1. Paraxial
a. Bi-lateral to the neural tube
b. Gives rise to somites,
somitomeres and branchial
arches
c. Somites
i. Form vertebral
column and skeletal
muscle
d. Branchial arches
i. Form facial muscle
and cartilage
2. Lateral
a. Splits into 2 layers; outer
next to ectoderm =
somatopleure; inner layer
adheres to the endoderm =
splanchnopleure
b. Forms circulatory system
and gut wall
3. Intermediate
a. Forms kidneys and gonads
From: Wikipedia
C. Endoderm
1. Forms entire
gastrointestinal tract
(except for parts of
mouth and pharynx)
2. Forms lining of digestive
glands
3. Epithelium of auditory
tube
4. Trachea, lungs
5. Urinary bladder
6. Follicles of thyroid and
thymus
From: Wikipedia
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D. Embryo folding
1. Cephalic or neural folds
a. Head and subcephalic pocket
2. Lateral
3. Caudal
E. Branchial apparatus
1. Groove
a. External acoutic meatus
2. Pharyngeal pouches
a. 5 Endodermal lined pockets of the pharynx
b. Form the inner ear, middle ear, auditory tube, guttural pouch (horse),
palatine tonsils, facial nerve, parathyroid glands, cells of thymus,
ultimobranchial body
3. Branchial arches
a. Lateral to pharynx
b. Contain artery, cartilage, muscle, nerve
c. in the adult form: artery; bones of ear,
mandible, hyoid, larynx; muscles of
head and neck; nerves
d. Somitomeres are cranial mesoderm
that form muscles of face, jaw and
throat
F. Somites
1. Masses from the paraxial mesoderm
2. Bilateral along the neural tube
3. Form dermis (beneath skin)
4. Form skeletal muscle (myotome)
5. Form vertebrae (sclerotome)
From: Wikipedia
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