Developmental Fate: A Comprehensive Overview of Ectoderm, Mesoderm, and Endoderm Derivatives

 

Developmental Fate: A Comprehensive Overview of Ectoderm, Mesoderm, and Endoderm Derivatives

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Name of different germ layers:

1. Ectoderm 2. Mesoderm and 3. Endoderm

Ectoderm

Parts of ectoderm: surface ectoderm, neuro-ectoderm, neural crest, ectodermal placode

Derivatives of Surface ectoderm: epidermis of skin

Derivatives of Neuro-ectoderm: it has two parts: neural tube and neural crest

Derivatives of neural tube: Brain and  Spinal cord

Derivatives of neural crest:

1.       Connective tissue and bones of face and skull

2.      C cells of the thyroid gland

3.      Cono-truncal septum in the heart

4.      Cranial nerve ganglia

5.      Schwann cells 

6.      Spinal ganglia,

7.      Sympathetic ganglia

8.    Parasympathetic ganglia

9.      Meninges, 

10.  Melanocytes

Derivatives of Ectodermal placode:Lens placode: give rise to lens.

Otic placode :it form otic vesicles.

It is divided into two parts

i) Ventral component – give rise saccule and cochlear duct

ii) Dorsal component forms the utricle, semicircular canals and endolymphatic duct

Nasal placode: nose and nasal cavity

Mesoderm:they are two types

Intra-embryonic mesoderm and extra-embryonic mesoderm

Parts of intra-embryonic mesoderm:

Paraxial mesoderm, intermediate mesoderm and lateral plate mesoderm, septum transversum

Formation of intra embryonic mesoderm

At first the intra embryonic mesodermal cells are distributed as a loose sheet on each side of the notochord in between the ectoderm and endoderm . then intra embryonic mesoderm subdivided into three parts,

Paraxial mesoderm: on each side of the notochord and beneath the developing neural plate.

Intermediate mesoderm:  middle part of mesoderm.

Lateral plate mesoderm: it is continuous with extra embryonic primary mesoderm.

1.      Paraxial mesoderm:

It is a thick plate of mesoderm located on each side of the midline. They organized into segment and form somitomeres (1-7) in head region which contribute mesoderm to the pharyngeal arches and somites(42-44 pairs )in occipital and caudal region

Each somite has three parts

i)                   Myotome- give rise to epimeric and hypomeric muscle tissue

ii)                 Sclerotome – give rise bone and cartilage of vertebral column

iii)              Dermatome – subcutaneous tissue of body 

2. Intermediate mesoderm:  developed genitourinary system

Urinary system: kidney, ureter,

Genital system: In case female para-mesonephric duct developed from intermediate mesoderm –

Derivatives from para-mesonephric duct: uterus, uterine tube and ovary, upper part of vagina

In case male mesonephric duct developed from intermediate mesoderm

Derivatives from mesonephric duct – testes, vas deferens)

3. Lateral plate mesoderm: it has two parts –

i) somatopleuric layer:  develops  parietal layer of serous sac like parietal layer of pleura, peritoneum, pericardium and muscles of body wall 

ii) Splanchnopleuric layer:  develops viceral layer of serous sac like visceral layer of pleura, peritoneum, pericardium, smooth muscle and connective tissue of GIT.

Septum transversum

The septum transversum is a sheet of mesoderm that appears on day 22 rostral to the developing heart.

Derivatives of septum transversum

The cranial part of the septum transversum gives rise to the central tendon of the diaphragm and The fibrous pericardium

The caudal part of the septum transversum is form stroma of liver, the ventral mesentery of the foregut,  the lesser omentum, the visceral peritoneum of the liver and the falciform ligament

Derivatives of endoderm: 

Lining epithelium of GIT, lining epithelium of respiratory tract, lining epi of urinary bladder.

Parenchyma of thyroid gland, parathyroid, liver and pancreas

 

Fig : formation of neural tube, intra embryonic mesoderm, primitive gut tube

Foldings of the embryo

Up till now the trilaminar germ disc is flattened and pear shaped .

Cranio-caudal folding  appear due to rapid growth of the central nervous system

Lateral folding appear due to rapid growth of somites . due to this folding primitive gut tube is formed

During these process of embryonic folds the growth of yolk sac is retarded and the amniotic cavity enlarges and cover whole fetus

Stages of development.

Other Developments in the Embryo

Several other major developments that occur during the embryonic stage are summarized chronologically below, starting with the fifth week after fertilization

.

Week Five

By week five after 

Fertilization, the embryo measures about 4 mm (0.16 in.) in length and has begun to curve into a C shape. During this week, the following developments take place:

• Grooves called pharyngeal arches form. These will develop into the face and neck.
• The inner ears begin to form.
• Arm buds are visible.
• The liver, pancreas, spleen, and gallbladder start to form.

Week Six

By week six after fertilization, the embryo measures about 8 mm (0.31 in.) in length. During the sixth week, some of the developments that occur include:

• The eyes and nose start to develop.
• Leg buds form and the hands form as flat paddles at the ends of the arms.
• The precursors of the kidneys begin to form.
• The stomach starts to develop.

Week Seven

By week seven, the embryo measures about 13 mm (0.51 in.) in length. During this week, some of the developments that take place include:

• The lungs begin to form.
• The arms and legs have lengthened, and the hands and feet have started to develop digits.
• The lymphatic system starts to develop.
• The primary prenatal development of the sex organs begins.

Week Eight

By week eight — which is the final week of the embryonic stage — the embryo measures about 20 mm (0.79 in.) in length. During this week, some of the developments that occur include:

• Nipples and hair follicles begin to develop.
• External ears start to form.
• The face takes on a human appearance.
• Fetal heartbeat and limb movements can be detected by ultrasound.
• All essential organs have at least started to form.