Gastrulation

Gastrulation

• Germ layer formation

• Stage of embryological development during which the single‐layered blastula is converted into a trilaminar structure

Ectoderm

Gives rise to:

• Epidermis (skin and its derivatives such as hair)

• Nervous system (brain, spinal cord, peripheral nerves)

• Sensory organs (eyes, ears, etc.)

Mesoderm (middle layer)

Forms:

• Muscles (skeletal, smooth, cardiac)

• Skeletal connective tissues (bones, cartilage)

• Urogenital system (kidneys, gonads, reproductive ducts)

• Cardiovascular system (heart, blood vessels, blood cells)

Endoderm

Develops into:

• Primitive gut or archenteron

• Lining of the gastrointestinal tract

• Respiratory tract

• Organs such as the liver, pancreas, thyroid, and bladder

Primitive streak

• A linear structure that forms on the surface of the epiblast during early gastrulation.

• It marks the beginning of germ layer formation and defines the body’s longitudinal axis (cranial–caudal direction).

• Primitive groove

• Primitive ridges

• Primitive node (Hensen’s node)

• Primitive pit

Primitive streak is composed of the following key structural regions:

Primitive groove

• A narrow depression that forms along the midline of the primitive streak.

• Site of cell ingression for mesoderm and endoderm formation.

Primitive ridges

• The elevated lateral margins or raised edges on either side of the primitive groove.

• Arises due to cell accumulation and thickening of epiblast cells on either side of the groove.

Primitive node (Hensen’s node)

• A rounded, elevated structure at the cranial end of the primitive streak

• Functions as a signaling center or organizer

• Analogous to the Spemann organizer in amphibians

Primitive pit

• A depression within the primitive node

• Continuation of the primitive groove within the node region

• Cells migrate through this pit to form the notochord.

Notochord Formation

• The embryonic disc grows more at the cephalic end than the caudal end because of continuous migration of cells from primitive streak and primitive node in the cephalic direction.

• This causes the rounded embryonic disc to become elongated with broad cephalic and narrow caudal end.

Mesoderm-forming

__ cells migrate cranially into the future head region where they become the notochord.

Notochord

• Rod-shaped, midline mesodermal structure that forms during gastrulation.

• Acts as a temporary axial skeleton of the embryo

• Induces overlying ectoderm to form the neural plate (which later becomes the neural tube)

Nucleus pulposus

In adults, notochord persists as the __ of intervertebral discs.

1. Dizygotic (fraternal) twins

2. Monozygotic (identical) twins

Types of Twins

Dizygotic (fraternal) twins

• Originate from two separate oocytes, each fertilized by a different sperm

• Results from ovulation of two follicles during a single reproductive cycle

• Genetically distinct

Monozygotic (identical) twins

• Develop from a single fertilized oocyte which later splits into two embryos

• Genetically identical

Two‐blastomere stage

The __ is the earliest point in embryological development at which monozygotic twins can arise, each blastomere giving rise to a separate individual with its own foetal membranes.

Duplication of the inner cell mass

__ results in twins with separate amnions but with a shared yolk sac and chorion.

Embryonic disc stage with the formation of two primitive streaks

The third stage in development at which monozygotic twins may arise is at the __, each giving rise to separate individuals. Such twins would share a common amnion, yolk sac and chorion

Conjoined twins

Anomalous incomplete separation of two primitive streaks results in __.

Superfecundation

• Fertilization of two or more ova released during a single estrus cycle, by sperm from different mating events.

• In polytocous animals (species that routinely give birth to more than one offspring per gestation), offspring are referred to as littermates.

• Example: Dogs: A female in heat mates with two different males → some puppies are sired by male A, others by male B.