TBL 1 Objectives

Question 1

Summarize gastrulation and formation of the trilaminar germ disc

Answer 1

at onset of the 3rd week, epiblast cells proliferate and some pass through the primitive streak to form mesodermal cells in between the hypoblast and epiblast

Question 2

Describe the vitelline vessels and fate of the yolk sac

Answer 2

Vitelline artery: carries blood from the embryo to capillaries in the yolk sac, where nutritive substances in the lumen of the sac enter capillaries

Vitelline vein: drain blood from yolk sac back to embryo

fate of yolk sac: by 10th week, amniotic cavity enlarges, compressing yolk sac against connecting stalk

Question 3

Describe the fate of the connecting stalk

Answer 3

the connecting stalk contains umbilical vessels. These susstain continued growth of the embryo as progressive obliteration of the yolk sac occurs.

Question 4

Cite representative derivatives of the ectoderm and endoderm

Answer 4

ectoderm: forms spinal cord and epidermis
endoderm: initially forms lining of the yolk sac and later forms epithelia, such as those lining the derivatives of the cut tube

Question 5

Compare the paraxial, intermediate, and lateral plate mesoderm

Answer 5

-mesenchymal cells are stem cells derived from mesoderm
mesenchymal cells create bilateral longitudinal columns composed initially of paraxial mesoderm and intermediate mesoderm

• intermediate cells: urinary system and gonads
• paraxial mesoderm: forms block-like somites adjacent to the neural tube. Somites form the vertebral column

lateral plate mesoderm: mesoderm remains thin and splits into parietal and visceral layers:
-parietal layer: dermis of skin, bone and muscles of the body wall, and extremities
visceral layer: connective tissue and smooth muscle of the gut tube derivatives

Question 6

Describe the blastocyst and distinguish the trophoblast and embryoblast

Answer 6

After entering the lumen of the uterus, fluid penetrates between cells of the zygotes to form the blastocyst with an inner cell mass and an outer cell mass. The inner cell mass becomes the embryoblast with pluripotent cells that can differentiate into all cells of the embryo but not the placenta. The outer cell mass becomes the trophoblast that penetrates the uterine epithelium to initiate blastocyst implantation by the end of the 1st week.

Question 7

Compare the two-cell and eight-cell zygotes

Answer 7

Up to the 8 cell stage, the 8 embryonic stem cells are totipotent, meaning they can differentiate into all cell types in the embryo and placenta. Each cell is known as a blastomere.

Question 8

Describe the connecting stalk, yolk sac, and amniotic cavity

Answer 8

The embryoblast differentiates into the epiblast and the hypoblast. The epiblast surrounds the amniotic cavity and the hypoblast lines the yolk sac.
The amniotic cavity is filled by amniotic fluid derived from maternal blood to provide a protective cushion.
The yolk sac contains nutritive substances, which are essential prior to development of the umbilical circulation. The connecting stalk aka primitive umbilical cord maintains continuity of the blastocyst with the trophoblast.

Question 9

Define the bilaminar germ disc

Answer 9

At the end of the 2nd week, the epiblast and hypoblast form the bilaminar (2 layered) germ disc.
A linear depression in the epiblast creates the primitive streak, which is located in the caudal region of the developing embryo.

Question 10

Why do cells of the inner cell mass have clinical potential?

Answer 10

Embryonic stem cells are derived from the inner cell mass of the embryo. They are pluripotent, thus they have the potential for curing a variety of diseases.

Question 11

Duration and clinical relevance of embryonic period

Answer 11

3-8 weeks during this period embryo is specially susceptible to teratogens

Question 12

Duration of fetal period

Answer 12

9-38 weeks

Question 13

How is the primitive streak related to the formation of teratomas and why do the tumors contain a mixture of tissue types?

Answer 13

sometimes primitive streak remains in the sacrococcygeal region then pluripotent cells infiltrate and produce tumors that contain multiple tissues.

Question 14

Epidermis

Answer 14

Most outer layer of skin, composed of contiguous cells in multiple layers, covered by keratin

Question 15

Basement membranes

Answer 15

It is located in the boundary between epithelium and underlying connective tissue. It supports and cushions the epithelia. It is a semipermeable/selective filtration barrier and controls epithelial cell differentiation in growth and tissue repair.
Hemi-desmosomes link the basal surface of the stratified epithelium (epidermis) to the basement membrane which counteracts disruptive forces at the dermo-epidermal junction (boundary of the dermis and epidermis).
It is essential for recovery after damage of skin.

Question 16

• Compare white fat and brown fat

Answer 16

Both originate from primitive mesenchymal cells
White Fat
1. Located in the superficial fascia beneath dermis
2. Uniocular (appearance under microscope from fat droplet)
3. Fat storage

Brown Fat

1. Located in thighs, back, neck
2. Multiocular (multiple smaller droplets)
3. Nonshivering thermogenesis

Question 17

Define the microcirculation and interpret its thermoregulatory function

Answer 17

Arterioles, capillaries, venules comprise microcirculation. Deep dermal plexus supplies dermis, superficial plexus make of the papillary loops of dermal papillae that supply superficial dermis and avascular epidermis. Blood diverted from superficial to deep plexus to avoid heat loss in response to cold temperature.

Question 18

Summarize the acute inflammatory response to skin lacerations

Answer 18

Mast cells of dermis, associated with microcirculation, release histamine that opens capillary tight junctions.

2. Plasma proteins and monocytes leak into the interstitial fluid, tissue swells.
3. Monocytes become macrophages and engulf pathogens using psuedopods. Phagocytic vacuole fuses with cell membrane antigen is presents to CD4+ T helper cells. Langarhans cells also help in immune response.

Question 19

• Compare the locations and functions of sensory nerve terminals in the skin

Answer 19

1. Free nerve endings: pain, thermal stimuli (basal layer of epidermis)
2. Merkel cell: respond to punctate pressure and bending of hairs (basal layer of epidermis)
3. Meissner Corpuscles: touch (dermal papillae)
4. Pacinian Corpuscles: deep pressure and vibratory stimuli (subcutaneous, a.k.a. superfiscial fascia)

Question 20

Explain skin remodeling in response to laceration.

Answer 20

Lacerations and skin remodeling: wound healing includes an inflammatory response, nearby epithelial cells regenerate, change shape, migrate across the wound. Cells secrete growth factors and activators for angiogenesis. The basement membrane is critical for healing, without it healing is slow.

Question 21

Compare locations and functions of melanocytes and Langerhans cells.

Answer 21

Both in epidermis.

Melanocytes- dispersed between basal cells (deeper)
Langerhans cells- dispersed between keratinocytes (more superficial)

Melanocytes produce melanin pigment in melanosomes, to be deposited in the cytoplasm of keratinocytes.

Langerhans cells are immune cells, and function to engulf invading pathogens and assist in activating immune responses for their destruction.

Other random facts from TBL:
Skin and hair color is determined by the rate of melanosome production, not the melanocyte number. Melanocytes are the only epidermal cells derived from neural crest cells.

Question 22

How does tanning of the skin occur and why isn’t it a permanent change?

Answer 22

Melanin is produced in melanosomes. When cells receive UV rays, the melanosomes rearrange themselves: they cluster near cell centers and rapidly re-distribute along microtubules to the end of dendritic processes of melanocytes. Keratinocytes then phagocytose these dendritic tips, releasing melanosomes into the keratinocyte cytoplasm. There, they are often packaged into secondary lysosomes.

Tanning is a result of increased eumelanin content in melanosomes in response to UV exposure. This is the cell’s protective measure against damaging effects of this UV radiation on DNA. *note- I could not find out why it is only a temporary effect but will ask Dr Grace and update.

Question 23

Describe hair follicles and hair shafts.

Answer 23

Hair follicles
The hair follicle is formed when proliferating basal cells form hair buds that penetrate into the dermis. In the dermis, the invagination of a hair bud’s terminal end creates the hair papillae, from which where the hair shaft sprouts.

Hair shafts
The hair shaft is formed from basal keratinocytes of the hair papillae (bottom of the hair follicle). This hair shaft moves progressively upward in the follicle.

Note that hair follicles and shafts do not exist in thick, highly keratinized skin.

Question 24

Define roles of sebaceous glands and arrector pili muscles.

Answer 24

Sebaceous glands and arrector pili muscles are both associated with hair follicles. Sebaceous glands produce sebum and arrector pili muscles help its secretion.

Sebaceous glands arise from cells of the hair bud. These glands secrete sebum, comprised of lipid-rich decomposed glandular cells.

The arrector pili muscles attach to the follicular wall near sebaceous glands. They are smooth muscles that involuntarily contract to compress glands in order to expedite sebum secretion into the hair follicle.

Question 25

Distinguish locations and functions of eccrine and odiferous sweat glands.

Answer 25

Eccrine and odiferous sweat glands both secrete sweat.

Eccrine sweat glands are located all around the body except for in the glans penis, clitoris, and labia minora. These glands reside in the dermis and superficial fascia. They are tubular and coiled.

Odiferous (apocrine) sweat glands are located in the axillae, scrotum, prepuce, labia minora, nipples, and perianal region. They are large and branched, and their sweat secretions has an acrid or musky odor in response to bacterial decomposition.

Eccrine sweat glands help maintain body temperature via evaporative heat loss. Clear cells secrete water and electrolytes, and dark cells elaborate macromolecular substances in sweat.

Odiferous sweat glands function in response to sex hormones. They are innervated by adrenergic sympathetic nerve fibers and start to function at puberty.

Question 26

Distinguish the superficial dermis and deep dermis.

Answer 26

Superficial dermis: loose connective tissue w Type I collagen fibers lending tensile strength, many small blood vessels, elastic fibers enabling recoil after stretching, lymphocytes and plasma cells for immune reaction, macrophages and mast cells.

Deep dermis: Between superficial dermis and superficial fascia. Type I collagen fibers more densely packed here. Fewer cells and less ISF than the looser connective tissue of the superficial dermis.