What tissue type forms most of the dermis?

The skin is composed of three main layers: the epidermis, dermis, and hypodermis (subcutaneous layer). The epidermis is the outermost layer and is responsible for forming a protective barrier against environmental damage. It undergoes continuous renewal through a process called keratinization, where cells move from the basal layer to the surface and are eventually shed. Disorders affecting desquamation, such as ichthyosis, primarily involve abnormalities in this outer epithelial layer, leading to accumulation of dead keratinized cells and scaly skin.

A. Epidermis: ichthyosis affects the epidermis, specifically the stratum corneum, which is the outermost portion of the epidermis. Normally, keratinocytes undergo a regulated process of differentiation and desquamation, where dead cells are shed from the skin surface. In ichthyosis, this process is disrupted, leading to excessive accumulation of keratinized cells and a thick, scaly appearance. The epidermis is responsible for barrier function and continuous renewal, making it the primary site of pathology.

B. Dermis: The dermis is the deeper layer of the skin located beneath the epidermis and is composed of connective tissue containing collagen, elastin, blood vessels, nerves, and hair follicles. It provides structural support, elasticity, and nourishment to the epidermis. While it plays an important supportive role, it is not involved in keratinization or surface cell shedding.

C. Hypodermis: The hypodermis, also known as the subcutaneous layer, is the deepest layer of the skin and is primarily composed of adipose tissue and loose connective tissue. It functions in insulation, energy storage, and cushioning of underlying structures. It does not participate in epidermal cell turnover or keratinization. As a result, it is not involved in the pathological process seen in ichthyosis.

D. Subcutaneous layer: The subcutaneous layer is another term for the hypodermis and shares the same structure and functions. It lies beneath the dermis and consists mainly of fat and connective tissue. Its role is primarily supportive and metabolic rather than epithelial renewal. Since ichthyosis is a disorder of epidermal desquamation, the subcutaneous layer is not involved in this condition.

The marked structure is the carpals, a group of eight small bones that form the wrist (carpus). These bones are arranged in two rows and create the connection between the forearm and the hand. The carpals provide flexibility and stability to the wrist joint, allowing a wide range of movements including flexion, extension, abduction, and adduction. They also play a key role in absorbing and distributing forces transmitted from the hand to the forearm during gripping and weight-bearing activities.

A. Metacarpals: The metacarpals are five long bones located in the palm of the hand, positioned between the carpals and the phalanges. They form the framework of the hand and support finger movements and grip strength. Compared to the carpals, they are longer and more tubular, forming the “palm bones” rather than the wrist structure.

B. Radius: The radius is the lateral forearm bone on the thumb side that extends from the elbow to the wrist. It articulates distally with the carpals to form part of the wrist joint but is not part of the wrist bones themselves. Unlike the carpals, it is a long bone of the forearm rather than a cluster of small bones.

C. Carpals: The carpals are eight small, irregular bones forming the wrist joint between the forearm and the hand. They are arranged in proximal and distal rows and provide both stability and flexibility to wrist movements. Their function includes shock absorption during hand impact and facilitating smooth movement between the forearm and hand. Since the circled structure is a compact cluster at the wrist, it corresponds to the carpals.

D. Phalanges: The phalanges are the bones of the fingers and thumb, arranged in three segments per finger (except the thumb, which has two). They allow fine motor movements such as grasping and writing. Compared to the carpals, they are distal and elongated structures forming the digits rather than the wrist joint itself.