Vertebrate Skeletal System Structure and Development

Skeletal System Fundamentals

During vertebrate embryonic development, the notochord develops, which is the precursor of the adult skeleton. (The lamprey retains its notochord in adulthood, while in other groups it is replaced by the spine).

Spine Composition

The spine includes articulated parts and is enlarged in its anterior section, forming the skull. The skeleton may be composed of cartilage (e.g., sharks and rays) or bone (e.g., other vertebrates). Bone is harder and more resistant.

Human Skeletal Development

The human skeleton in embryonic stages consists of cartilage that is later replaced by bone. Ossification begins in the sixth week of embryonic life and continues until adulthood is reached.

The human skeleton is divided into two main parts:

1. Axial Skeleton

This includes the head and trunk. The head is formed by the bones that protect the brain and the facial structure.

Bones of the Skull (Cranium)

• Frontal
• Parietal
• Temporal
• Occipital
• Ethmoid
• Sphenoid bone

Facial Bones

These bones allow for chewing and define the shape of the face. They are divided into:

• Upper Jaw (Maxilla): Includes the zygomatic (cheek), nasal, lacrimal, turbinate bones of the nose, vomer, palatine, and the upper jaw itself.
• Lower Jaw (Mandible): The lower jaw bone.

2. Trunk (Vertebral Column and Rib Cage)

The trunk is divided into:

• Spine (Vertebral Column): Protects the spinal cord and serves to anchor other bones. It is composed of 33 or 34 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 4 or 5 fused coccygeal vertebrae.
• Rib Cage: Includes 12 pairs of ribs (10 joined to the sternum and 2 floating ribs) and the sternum (the bone to which the first 10 pairs of ribs attach).

3. Appendicular Skeleton

This is formed by the bones of the upper and lower extremities.

1. Upper Extremities

• Pectoral Girdle: Formed by the clavicle and scapula. It allows for the insertion and union of the upper limbs with the axial skeleton.
• Arm: Humerus, ulna, and radius.
• Hand: 8 short bones called carpals, 5 metacarpals, and phalanges (3 per finger, except the thumb, which has 2).

2. Lower Extremities

• Pelvic Girdle (Waist): Formed by the coxal bones (ilium, pubis, and ischium), which fuse with the sacrum of the spine. It allows for the insertion and union of the lower extremities with the axial skeleton.
• Leg: Femur, patella, tibia, and fibula.
• Foot: Tarsus (7 bones), metatarsus (5 bones), and phalanges.