Germ layers | Embryology | Anjani Mishra

Germ layers and its derivatives

Hey Guys, In this post you are going to know about ectoderm, mesoderm and endoderm & their derivatives which is a part of Embryology/Developmental Anatomy.

Now we are going to run the series of Embryology that is going on.

 At first, you must know about the ectoderm and its origin.

So the question is, how the word ectoderm is originated/derived ?

Let's start with the origin of ectoderm first.

Ectoderm

The word ectoderm is derived from the Greek word ectos means "outside" and derma means "skin" means the outer layer of skin, i.e. epidermis.

As epidermis arises/develops from ectoderm and covers dermis as well as the whole body, similarly the ectoderm appears as epidermis and covers other germ layers.

Do you know how many layers are there in the skin?

 Primarily, there are mainly two layers, i.e. epidermis(outer) and dermis(inner).

Microscopically, epidermis consists of five layers, and dermis consists of two layers.

In an aquatic invertebrate animals like cnidarians and ctenophores, epidermis is occasionally called ectoderm.

The next question must be in your mind that when the word ectoderm was recorded?

Any guesses?

Well, It was recorded in 1859-1865 and came in use for the first time.

I think now you are almost clear that how and when the word ectoderm was originated and came in use for the first time.

Just wait for a while, you will be more clear about the nomenclature of ectoderm.

Definition: Ectoderm is one of the outermost part of the three primary germ layers, that appears in early embryonic development, arises from the epiblast cell layer.

What is epiblast cell layer ?

A layer of high columnar cells adjacent to the amniotic cavity is known as the epiblast cell layer. Click Here For More Detailed Information.

After knowing the history and definition of ectoderm, we must proceed further as we are dealing with the ectoderm, which is developing component of Developmental Anatomy.

I think at this moment, several questions arises in your mind that what next?

Just guess....

I just want to make you understand that what happens after the formation of ectoderm.

Let's see the next events that takes place after the formation of ectoderm is Differentiation and its derivatives. 

Differentiation of ectoderm

Primarily, the ectoderm differentiates into two zones, such as

A. Surface ectoderm (external parts)

B. Neuro ectoderm (internal parts)

    The Neuroectoderm further divides into two zones, namely;

       a. Neural tube, &

       b. Neural crest

A. Surface ectoderm-- forms the outer protecting covering of the embryo and gives rise to the various tissues, cells and organs.

Derivatives of surface ectoderm

Outer epithelium of the body, epidermis

• Nasal+Olfactory epithelium
• Oral epithelium
• Enamel of teeth
• Anal epithelium
• Hair, Nail
• Sweat gland, sabeceous gland
• Mammary gland
• Lens of eye
• Auditory vesicle

Inner epithelium of the body

• Adenohypophysis
• Utricle, semilunar ducts and vestibular ganglion of the cranial nerve(vii)
• Saccule, cochlear duct, organ of corti, spinal ganglion   of   cranial nerve(viii)

B. Neuro ectoderm-- mainly gives rise to the central nervous system and peripheral nervous system.

Differentiation of neuro ectoderm

 a. Neural tube-- Neural plate is the premordium of the CNS. the neural plate dorsally close up forming a neural tube.

Derivatives of neural tube

• Brain, spinal cord, neurohypophysis, pineal gland
• Retina, optic nerve, optic vesicle
• Motor spinal nerve root
• Astrocytes, oligodendrocytes, ependymal cells, choroid plexus 

 b. Neural crest-- Cells between superficial ectoderm and neural tube is the neural crest.

Derivatives of neural crest

• Spinal dorsal root ganglia
• Cranial sensory ganglia
• Arachnoid, piamater
• Schwann cells, microglial cells      

Mesoderm

The mesoderm is the middle layer of one of the three primary germ layer(ectoderm, mesoderm and endoderm) formed by the process of gastrulation.

Differentiation of mesoderm

Primarily, the mesoderm differentiates peripherally into somatic mesoderm and splanchnic mesoderm. The cavity in between these two layer is the coelom. The somatic mesoderm together with trophectoderm forming primary or extraembryonic somatopleurae. The splanchnic mesoderm together with endoderm forming secondary or intraembryonic  splanchnopleurae.

In this way, the mesoderm differentiate into extraembryonic and intraembryonic segments. Further intraembryonic segment differentiate into three zones, such as;

A. Paraxial mesoderm

B. Intermediate cell mass

C. Lateral plate mesoderm

A. Paraxial mesoderm-- The paraxial mesoderm, which lines the sides of the notochord compose most of the body skeleton and its muscles and the connective tissue layers of the skin.

                                        The paraxial mesoderm becomes subdivided into a number of cubical blocks of tissue masses and are called somites or metemeres. All the somites do not appear at a time. They start appearing at the end of first month of gestation. The somites are formed in cranio-caudal direction and their time of appearance is variable depending upon the species. For example; in case of chick embryo, the somites begin to start on 22 hours of incubation. Eight pairs of somites appear by 27 hours of incubation. Subsequent number of somites appear with the progress of incubation period.

                                        Each cubical somite divides into a ventro-medial part called sclerotome and a dorso-lateral part dermomyotome.The sclerotome moves ventro-medially around the notochord and forms primitive vertebra. The dermomyotome again divides into a dermal plate and a muscular plate. The dermal plate forms the dermis and subcutaneous tissue and the muscular plate(myotome) gives rise to skeletal muscle.

Derivatives of paraxial mesoderm

Skeletal muscles of the trunk, head, neck, extra-ocular muscles, intrinsic muscles of the tongue

• Vertebral column, ribs, occipital bone
• Dermis
• duramater

B. Intermediate cell mass-- The intermediate mesoderm, which lines further away from the center of the embryo marked by the notochord, which later become the kidney and most of the genitalia.

Derivatives of intermediate mesoderm

• Uro-genital system

C. Lateral plate mesoderm-- The lateral mesoderm, which lines further away peripherally from the notochord, which will turn in part to the extra embryonic mesoderm and in part to the somatic and splanchnic mesoderm and mainly gives rise to the cardio vascular system, pericardium, pleura, and peritoneum.

Derivatives of lateral plate mesoderm

• Cardio-vascular system and lymphatic system
• Blood cells, kupffer cells
• Serous membrane of the body cavities
• Mucous membrane of the gasto-intestinal tract
• Adrenal gland
• Laryngeal cartilage
• Bones of the limb, sternum
• Skeletal muscles of the limb

Endoderm

Differentiation of endoderm

As the cells at the center of the flat germ disc proliferate rapidly than those of periphery, the embryonic area develops a head fold, a tail fold and two lateral folds at the end of first month. Therefore a cylindrical shape of the embryo is formed.

During the folding, growth of the yolk sac slows down and the amniotic cavity enlarges. It surrounds the whole embryo so that the latter floats in the amniotic fluid.

Derivatives of endoderm

• Digestive tube, liver, pancreas            
• Epithelial lining of gasto-intestinal tract
• Epithelial lining and glands of trachea, bronchi, lungs
• Epithelial lining of urinary bladder
• Epithelial lining of vagina and female urethra
• Epithelial lining of auditory tube and middle air cavity
• Allantois and yolk sac

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