学病理首先明确组织学知识
这是大家都很明白的
但是到底如何下手？
谁都难说
有幸搞到本The third edition of Histology for Pathologists
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Introduction
The skin accounts for about 15% of the total body weight and is the largest organ of the body. It is composed of three layers: (a) epidermis, (b) dermis and (c) the subcutaneous adipose tissue. Each component has its unique and complex structure and function ，with variation according to age, gender, race, and anatomic location. Functions of the skin are extremely diverse. It serves as a mechanical barrier against external physical, chemical, and biological noxious substances and as an immunologic organ. It participates in body temperature and electrolyte regulation. It is an important organ of sensuality and psychological well-being. In addition, it is a vehicle that expresses not only primary diseases of the skin, but also diseases of the internal organs. An understanding of the skin's normal histology is essential to the understanding of pathologic conditions.
Embryology
Epidermis
Embryologically, the ectoderm gives rise to epidermis and its appendages. The mesoderm provides the mesenchymal elements of the dermis and subcutaneous fat .
At first, the embryo is covered by a single layer of ectodermal cells. By the sixth to eighth week of development, it differentiates into two layers, the basal layer and an overlying second layer called periderm. The surface of the periderm is covered by microvilli and is in contact with the amniotic fluid. The mitotic activity of the basal layer predominates over that of the periderm, and soon the basal layer becomes the germinative layer. Additional rows of cells develop from this proliferating layer, forming a multilayer of cells between ectoderm and periderm . By the twenty-third week, keratinization has taken place in the upper stratum, and the cells of the periderm have already been shed . Interestingly, it has been demonstrated that many of the cell junction proteins are expressed in the early two-layered embryonic epidermis and as early as the eighth week of estimated gestational age. By the end of the first trimester, the dermal epidermal junction with its component is ultrastructually similar to that of mature skin . Thus, the characteristic neonatal epidermis is well developed by the fourth month.
The majority of cells in the epidermis are keratinocytes (90 to 95%). The rest of the epidermal cells are nonkeratinocytes (5 to 10%), and they include melanocytes, Langerhans cells, and Merkel cells. The nonkeratinocytes are seen in the epidermis of 8- to10-week-old embryos. The precursor cells of melanocytes migrate from the neural crest to the dermis and then to the epidermis, where they differentiate into melanocytes during the first three months of development. During this migration, melanocytes can reside in other organs and tissue. Ultrastructually, recognizable melanosomes in melanocytes may be seen in the fetal epidermis at 8 to10 weeks of gestational age .
Langerhans cells are derived from the CD34+ hematopoietic precursor cell of the bone marrow. The characteristic cytoplasmic marker, the Birbeck granule, is seen ultrastructurally in 10-week-old embryos . The expression of a more characteristic immunohistochemical marker, CD1a is completed by 12 to 13 weeks of estimated gestational age .
Merkel cells can also be seen in the epidermis of 8- to 10-week-old embryos. The origin of Merkel cells is debatable. Some have suggested a neural crest derivation , whereas others suggest epidermal origin through a process of differentiation from neighboring keratinocytes . Merkel cells in the epidermis are initially numerous and later diminish with increasing gestational age, which suggests their role in growth and development .