Cross-Sectional Anatomy Lesson I

Human Development Fertilization of the ovum occurs in the fallopian tubes After fertilization, the united sperm and ovum (zygote) travel into the uterus Major cellular events occur in the uterus, one of the events being the formation of a blastocyst which is implanted in the uterine wall (Endometrial Layer) Implantation occurs approximately seven (7) days after ovulation or around the twenty-first (day 21) of the menstrual cycle. Implantation occurs as the endometrial tissue grows and surrounds the blastocyst (takes about 7 days)

Human Development HCG secreted by the blastocyst keeps it from being aborted HCG- Human Chorionic Gonadotropin secretion is only produced in pregnant women Pregnancy tests are based on the presence of this hormone in blood or urine

Formation of Primary Germ Layers During implantation, changes occur in the inner cell mass of the blastocyst, resulting in the formation of the primary germ layers Amnionic cavity develops in the inner cell mass Two layered flattened embryonic disk develops Ectoderm, endoderm, and mesoderm develop ALL TISSUES AND ORGANS DEVELOP FROM THESE THREE PRIMARY GERM LAYERS Embryonic stage of development begins, lasting 6 weeks (from beginning of week three thru week eight)

Development of the Human Brain CNS (brain and spinal cord) develops from a thickened area of the embryonic ectoderm called the neural plate 18 – 20 days after conception Neural plate is altered to form a neural tube and neural crest Neural tube forms the brain and spinal cord Neural crest forms most structures within the peripheral nervous system

Neural Tube Temporarily open at both caudal and cranial ends (portions) Cranial portion closes approximately 24 days after conception Caudal portion closed about 2 days later Differentiation and growth of the neural tube is greatest at the cranial end

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Neural Tube Development Four weeks after conception – cranial end differentiates into three primary vesicles; Prosencephalon (forebrain) Mesencephalon (midbrain) Rhombencephalon (hindbrain)

Further Differentiation of the Neural Tube Divisions During the next week: Prosencephalon (forebrain) = telencephalon and diencephalon Rhomencephalon (hindbrain) = metencephalon and myelencephalon 3 now equal 5

Ectoderm Epidermis of the skin Hair, nails, skin glands Lens of the eye Enamel of the teeth All nervous tissue Adrenal medulla Sense organ receptor cells Linings of the oral and nasal cavities, vagina, and anal canal

Endoderm Epithelial lining of digestive tract Epithelium of the liver and pancreas Epithelium of urinary bladder & urethra Epithelium of the respiratory tract Thyroid, parathyroid, and thymus glands

Mesoderm Dermis of the skin Skeletal, smooth, cardiac muscle Connective tissue including cartilage and bone Epithelium of serous membranes Epithelium of joint cavities Epithelium of blood vessels Kidneys and ureters Adrenal cortex Epithelium of gonads and reproductive ducts

Fetal Images

Fetal Images

The Brain Weighs about 3 lbs.(2% of body weight) Larger in the male than in the female Composed of organized regions of gray and white matter Gray matter = nerve cell bodies and unmyelinated fibers (regions of synapse –electrical communication between neurons) Gray matter grouping = Basal Ganglia White matter = nerve fibers covered in myelin (fatty substance)

Basal Ganglia

Regions of the Brain Cerebrum Diencephalon Brainstem Cerebellum

Brain Regions