The integumentary, skeletal and muscular systems

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Chapter 4 The integumentary, skeletal and muscular systems

The integumentary system

The hairs and glands of the skin

Hairs develop during the fetal period as proliferations of the stratum germinativum of the epidermis growing into the underlying dermis. The tip of the hair bud becomes a hair bulb and is soon invaginated by the mesenchymal hair papilla in which the vessels and nerve endings develop (Fig. 4.2A, B). The epidermal cells in the centre of the hair bud become keratinized to form the hair shaft, and the surrounding mesenchymal cells differentiate into the dermal root sheath (Fig. 4.2C). Small bundles of smooth muscle fibres called arrector pili muscles develop in the mesenchyme and are attached to the dermal sheath (Fig. 4.2C). Most sebaceous glands develop as buds from the side of the epithelial root sheath growing into the dermis; these glands produce an oily secretion that lubricates the hair and skin. The sweat glands develop as epidermal buds into the underlying dermis which become coiled to form the secretory part of the glands (Fig. 4.2C).

The musculoskeletal system

The mesenchyme gives rise to the musculoskeletal system. Most of the mesenchyme is derived from the mesodermal cells of the somites and the somatopleuric layer of lateral plate mesoderm (see Chapter 1). The mesenchyme in the head region comes from the neural crest cells. Regardless of their sources, a common feature of mesenchymal cells is their ability to migrate and differentiate into many different cell types, e.g. myocytes, fibroblasts, chondroblasts or osteoblasts. This differentiation often requires interaction with either epithelial cells or the components of the surrounding extracellular matrix.

The skeletal system

The origin of mesenchymal cells forming the skeletal tissues varies in different regions of the body. Mesenchymal cells forming the axial skeleton arise from the mesodermal somites, whereas the bones of the appendicular skeleton are derived from the somatopleuric mesenchyme of the lateral plate mesoderm. After reaching their destination the mesenchymal cells condense and form models of bones. The subsequent differentiation of mesenchymal cells into chondroblasts or osteoblasts is genetically controlled. Various molecular processes, therefore, play a significant role in determining whether mesenchymal cells undergo a membranous ossification or transform into cartilage models, which later become ossified by endochondral ossification.

Development of the axial skeleton

The axial skeleton is composed of the skull, vertebral column, sternum and ribs. This part of the skeleton is derived from the paraxial mesoderm, which is soon organized into the somites. The first somites appear on day 20 in the cranial region, and by 30 days approximately 37 pairs are formed. Somites appear as rounded elevations under the surface ectoderm on the dorsal aspect of the embryo from the base of the skull to the tail region. Each somite subdivides into two parts: the sclerotome and the dermomyotome (Fig. 4.4). The cells of the sclerotome give rise to the vertebrae and ribs, and those of the dermomyotome form muscle and the dermis of the skin.