153.The answer is e.(Ross, p 254. Junqueira, p 238. Kumar, p 620.)The
bone marrow begins to function in the second month and becomes the pre-
dominant hematopoietic site during months 5 to 9 of gestation, The first
site of blood cell development (hematopoiesis) is extraembryonic, in the
yolk sac (answer b),which produces hematocytoblasts and primitive ery-
throblasts from the third week through the second month of gestation.
Hepatic erythropoiesis (answer a)begins during the sixth week, reaches
its maximum in the third month, and then ceases about the seventh month.
Whereas, the spleen (answer c)is involved specifically in the production
of red blood cells (erythropoiesis) from months 2 to 5 of gestation with
some activity continuing postnatally. The spleen continues to produce
monocytes and lymphocytes throughout life. From the second month of
gestation, the lymph nodes produce lymphocytes, and the thymus
(answer d)is responsible for the education of T cells. Those T lympho-
cytes are seeded to T-dependent areas, such as the deep cortex of the lymph
node and periarteriolar lymphoid sheath (PALS) of the spleen.
154.The answer is a.(Kumar, p 620. Ross, p 254. Junqueira, p 238.)The
iliac crest is the best location for bone marrow sampling. The sternum is
not as safe a place for bone marrow aspiration and biopsy because of pos-
sible damage to thoracic structures (answer b).The patient in the vignette
suffers from chronic lymphocytic leukemia CLL more common in adults
over 60 years old. Hematopoiesis occurs in the flat bones (answer c)and
other bones in the adult human. Although most bones in the body are
involved in hematopoiesis during growth, the marrow of the sternum, ribs,
vertebrae, iliac crest, skull, and proximal femora are the primary sites of
blood cell development by the time that skeletal maturity is achieved. It
also occurs in the long bones (answers d, and e)during development, but
many of those areas become dominated by yellow marrow that contains
many fat cells (adipose tissue). The inactive yellow marrow can be reacti-
vated on exposure to the proper stimulus (e.g., severe blood loss).
155.The answer is a.(Young, pp 139, 203. Junqueira, pp 168, 212–213,
266–267. Ross, pp 423–424. Kierszenbaum, pp 327–330.) The capillary
endothelia in the brain and thymus are continuous, as is the basal lamina.
The blood-thymus barrier provides the appropriate microenvironment for
education of T cells without exposure to self. The capillary is further sur-
rounded by perivascular connective tissue and epithelial cells and their
262 Anatomy, Histology, and Cell Biology