In the third week of development the embryonic trilaminar disc is formed, giving the embryo three germ layers: ectoderm, mesoderm and endoderm (see Figure 14.4). From these germ layers almost all of the structures of the embryo will develop.
The ectoderm will form the external surface of the embryo: the epidermis of the skin (Figure 15.1). The dermis is formed from the mesoderm layer. Melanocytes, the cells that give the skin its pigment, are derived from neural crest cells. These cells are themselves ectodermal and are involved in the development of a range of structures (see Chapters 17 and 18).
The nervous system is also formed from ectoderm (Figure 15.1), as we see when we study neurulation (see Chapter 17). This probably reflects the evolutionary internalisation of sensory apparatus. Simpler, early animals had external sensory apparatus that allowed the animal to sense nutrients, chemicals, light, and so on. This apparatus developed from ectoderm, the external layer. In humans much of this sensory apparatus remains external to some extent (the retina, touch, temperature and pain senses in the skin), but the nervous system that has evolved is now located internally.
The mesoderm is a major contributor to the embryo and its cells are used to build the bones, cartilage and connective tissues of the skeleton, striated skeletal muscle, smooth muscle, most of the cardiovascular system and lymphatic system, the reproductive system, kidneys, the suprarenal cortex, ureters, the linings of body cavities such as the peritoneum, the dermis of the skin and the spleen the cardiovascular and immune systems rmed in the bone marrow are also derived from mesoderm.
So the remaind e formed from endoderm. What is left? Epithelia derived from endoderm line internal passages exposed to external substances, including the gastrointestinal tract, the lungs and respiratory tracts (Figure 15.3). Glands that open into the gastrointestinal tract and the glandular cells of organs associated with the gastrointestinal tract, such as the pancreas and liver, are also derived from the endoderm (Figure 15.3).
The epithelia of the urethra and bladder come from endoderm cells, as do the tonsils, the thymus, the thyroid gland and the parathy- roid glands (Figure 15.3). You can find out how these latter structures form in the pharyngeal arch chapters (see Chapters 40–43).
Endoderm and ectoderm meet at the openings of the mouth and anus. Thus, the oral cavity and part of the pharynx have an epithelium derived from ectoderm, and the remainder of the pharynx has a lining derived from endoderm. The same thing occurs at the anus, and this has important anatomical ramifications for the development of the vasculature there, with respect to portosystemic venous anastomoses for example.
The germ layers should not be confused with germ cells. Germ cells migrate from the yolk sac through the gut tube and dorsal mesentery into the dorsal mesenchyme of the embryo (see Chapter 38). Here they differentiate to form gametes; either oocytes or spermatocytes.
A gamete is a reproductive cell with a haploid (half) set of chromosomes that will combine with another gamete during fertilisation to produce a new cell with a full, diploid complement of chromosomes (see Chapter 10). That cell (zygote) will become the embryo and its supporting structures.
A gamete, then, is an example of a cell that does not develop from embryonic ectoderm, endoderm or mesoderm.
With the meeting of the ectoderm and endoderm near the anal opening of the gut tube, the rectum develops with links partly to the ectoderm and partly to the endoderm. The superior part of the rectum (endoderm, gut tube) drains blood back to the liver via the superior rectal vein and subsequently the inferior mesenteric vein and the portal vein. The inferior part of the rectum (ectoderm, not gut tube) drains blood via the inferior and middle rectal veins to pelvic veins, iliac veins, the inferior vena cava and thus into the systemic circulation. Venous anastomoses (links) exist between the superior, middle and inferior rectal veins. An impedance to the flow of blood through the liver from the portal vein to the inferior vena cava will cause the blood to find an alternate route, one example of which are the rectal portosystemic venous anastomoses. The veins here stretch and enlarge, causing haemorrhoids.
Germ cell tumours are growths that develop from germ cells.
They may occur within or outside the gonads, possibly from aberrant or normal migration, and can be congenital.
Germ cell tumours of different types exist, including teratomas. A teratoma may form structures of any of the three germ layers, uding thyroid, liver or lung tissues, or occasionally hair, teeth or bone.