01. Early Human Embryonic Development Flashcards
1
Q
Why study human embryology?
A
- For a better understanding of adult anatomy
- To correlate basic science (e.g. anatomy and physiology) with clinical science (e.g. obstetrics, pediatrics, and surgery)
- Allows the physician to accurately advise patients on issues, such as:
-> Reproduction
-> Contraception
-> Birth defects
-> Prenatal development
-> In vitro fertilisation
-> Stem cells
-> Genome editing
-> Cloning
2
Q
Subdivisions of human pregnancy according to Prospective Parents and Physicians:
A
Three trimesters / 3 month period:
- 1 to 3 months
- 4 to 6 months
- 7 to 9 months
Starting from the date of the onset of the last menstrual period and ending at birth.
3
Q
Subdivisions of human pregnancy according to Embryologists
A
Three different periods:
-> The period of the egg (from fertilisation to the end of the third week)
-> The period of the embryo (from the beginning of the fourth week to the end of the eighth week)
-> The period of the fetus (from the beginning of the third month to birth)
4
Q
Phases of human embryogenesis
A
- Gametogenesis
- Fertilization
- Cleavage
- Gastrulation
- Formation of the tube - within - a - tube body plan
- Organogenesis
5
Q
What is Gametogenesis?
A
The formation of the gametes
6
Q
What is Fetilization?
A
The joining of the gametes to form the zygote
7
Q
What is cleavage?
A
A series of rapid cell divisions that result first in the formation of the morula, and then in the formation of the blastocyst.
8
Q
What is gastrulation?
A
The arrangement of cells in the embryonic region of the implanted blastocyst into three primary germ layers (ectoderm,mesoderm, endoderm) to form the embryonic disc.
9
Q
What is the formration of the tube - within - a - tube body plan?
A
Conversion, through body folding, of the embryonic disc into a C - shaped embryonic body consisting of an outer ectoderm tube (the future skin) and an inner endodermal tube (the gut tube), within the mesoderm interposed between the two tubes.
10
Q
What is Organogenesis?
A
The formation of organ rudiments and organ systems.
11
Q
Type of Gametes:
A
- Spermatozoon / Spermatozoa (male)
- Oocyte / Ocytes (females)
12
Q
Describe the tructure of the spermatozoon
A
- Head: Nucleus, Acrosome
- Neck: Junction between head and tail
- Tail: Middle piece, Principal piece, End piece
13
Q
How many chromosomes do the gametes contain?
A
23 chromosomes. Haploid cells
14
Q
What does the Acrosome of the spermatozoon contain?
A
Enzymes important for penetration of the oocyte.
15
Q
What does the tail facilitate?
A
Sperm motility
16
Q
what does the middle piece of the tail contain?
A
Mitochondria that produce energy
17
Q
Describe the structure of the oocyte
A
- Secondary oocyte: Nucleus, Cytoplasm
- Zona pellucida: Amorphous material
- Corona radiata: Layer of follicular cells
18
Q
What are primordial germ cells (PGCs)?
A
Undifferentiated sex cells
e.g. Spermatogonia, Oogonia
19
Q
Are Primordial germ cells haploid or diploid?
A
Diploid (contain 46,XY or 49,XX)
20
Q
Main porpose of gametogenesis
A
- Meiosis to reduce the number of chromosomes
- Cytodifferentiation to complete maturation
21
Q
What is the purpose of Meiosis I?
A
To separate the homologous chromosomes
22
Q
What are the stages of Interphase I?
A
G1: Cell growth
S: DNA synthesis
G2: Preparation for division
23
Q
Describe Prophase I
A
The homologous chromosomes are aligned in pairs (synapsis)
- Relocation of segments of maternal and paternal chromosomes by crossing over of chrmose segments => Genetic Recombination
- X, Y chromosomes are not homologous, but they synapse in regions of homology ( they act as homologous, but they are not homologous as they do not have the same shape and genes)
24
Q
Describe Metaphase I
A
The pairs of homologous chromosomes align along the equatorof the cell
25
Describe Anaphase I
1. The homologous chromosomes are pulled towards the opposite poles
2. Each pole contains a full set of chomosomes
3. Final step of karyokinesis
26
Describe Telophase I
The chromosomes of each pair end up in opposite spolse
27
Describe Cytokinesis I
The single cell pinches in the middle
- the cytoplasm divides
- two daughter cells are formed
28
What is the purpose of Meiosis II
To separate the sister chromatids
29
Describe Prophase II
1. The membrane around the nucleus breaks
2. The centrioles duplicate
3. The meotic spindle forms again
30
Describe Metaphase II
The chromosomes (pair of sister chromatids) align along the equator of the cell.
31
Describe Anaphase II
The sister chromatids are pulled towards the opposite poles of the cell
32
Describe Telophase II
1. The sister chromatids are now on opposite poles
2. Final step of karyokinesis
33
Describe Cytokinesis II
1. The cytoplasm divides
2. Each single cell forms two daughter cells
34
Number of daughter cells from Meiosis I vs Meiosis II
Meiosis I: two haploid cells
Meiosis II: four haploid cells
35
What are Spermatogonia?
1. These are the earliest germ cells in themale testes
2. They are haploid and located in the basal compartment of the seminiferous tubules.
3. They divide by mitosis o produce more spermatogonia or differentiate into primary spermatocytes.
Spermatogonia → Mitotic division → Primary spermatocytes
36
What are Spermatocytes?
1. These are the intermediate stage between spermatogonia and spermatozoa.
2. Primary spermatocytes (2n) undergo meiosis I to form secondary spermatocytes (haploid, n).
3. Secondary spermatocytes undergo meiosis II to produce spermatids (haploid, n).
Primary spermatocytes → Meiosis I → Secondary spermatocytes → Meiosis II → Spermatids
37
What are Spermatozoa?
1. These are the fully developed, motile sperm cells.
2. They result from the differentiation of spermatids through a process called spermiogenesis.
3. They have a head (containing the nucleus and acrosome), a midpiece (rich in mitochondria), and a tail for motility.
Spermatids → Spermiogenesis → Spermatozoa (mature sperm cells)
38
What are primordial germ cells (PGCs) called in males?
Spermatogonia
39
What happens to spermatogonia at puberty?
They proliferate by mitosis and enlarge to form primary spermatocytes.
40
What is spermatogenesis?
The process of sperm cell development, including mitosis, meiosis, and spermiogenesis.
41
What happens in the first meiotic division of spermatogenesis?
Primary spermatocytes (diploid, 2n) → Secondary spermatocytes (haploid, n)
- Size is halved
- Half contain 23,X and half contain 23,Y (determines the embryo’s sex)
42
What happens in the second meiotic division of spermatogenesis?
Secondary spermatocytes (haploid, n) → Spermatids (haploid, n)
Size is halved again
43
What is spermiogenesis?
The maturation of spermatids into spermatozoa (final functional sperm).
44
What key changes occur during spermiogenesis?
The nucleus condenses
The acrosome forms (important for egg penetration)
45
Where are spermatozoa stored and matured?
In the epididymis (epididymal maturation).
46
What are the key features of epididymal maturation?
Fertilizing ability
Forward motility
47
What are the primordial germ cells (PGCs) in females called?
Oogonia
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49
What happens to oogonia during fetal life?
They proliferate by mitosis and enlarge to form primary oocytes.
50
At birth, what stage are all primary oocytes in?
They have completed Prophase I and remain arrested until puberty.
51
What happens prior to each ovulation?
A primary oocyte (diploid, 2n) completes Meiosis I and forms a secondary oocyte (haploid, n).
52
What is the result of Cytokinesis I in oogenesis?
Unequal distribution of cytoplasm, forming:
A secondary oocyte (haploid, 23,X)
A first polar body
53
At ovulation, what phase is the secondary oocyte arrested in?
Metaphase II of Meiosis II
54
When does Meiosis II resume in the secondary oocyte?
Upon fertilization
55
What is formed when Meiosis II is completed?
The second polar body
56
How big is the secondary oocyte?
Large enough to be visible to the unaided eye.
57
How many primary oocytes are present in a neonate’s ovaries?
Up to 2 million
58
What happens to most of the primary oocytes during childhood?
They regress (degenerate).
59
How many primary oocytes remain at puberty?
No more than 40,000.
60
How many oocytes mature into secondary oocytes and are ovulated during a woman’s reproductive life?
Approximately 400.
61
What is the normal chromosomal composition of gametes?
23,X or 23,Y
62
What causes abnormal gametogenesis?
Non-disjunction of chromosomes during meiosis.
63
What happens in Meiosis I non-disjunction?
Homologous chromosomes fail to separate.
64
What happens in Meiosis II non-disjunction?
Sister chromatids fail to separate.
65
What types of aneuploid gametes can result from non-disjunction?
-> n-1 (e.g. 22,O)
-> n+1 (e.g. 24,XY / 24,XX / 24,X+21)
66
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What are the two major complications of aneuploidy?
Monosomies & Trisomies
68
What usually happens to monosomic embryos?
They are usually fatal, leading to miscarriage
69
What is Trisomy 21, and what condition does it cause?
Down syndrome (47,XY,+21)
70
What hormone does the developing oocyte produce?
Estrogen
71
What triggers the LH surge?
High estrogen levels
72
What does the LH surge induce?
Ovulation
73
What is ovulation?
The rupture of the secondary follicle and release of the secondary oocyte into the fallopian tube.
74
What happens to the secondary oocyte after ovulation?
It is now called an ovum (egg)
75
What part of the fallopian tube helps capture the oocyte?
The fimbriated end of the uterine tube.
76
What is the function of the fimbriae in ovulation?
They perform a sweeping movement to bring the oocyte into the fallopian tube.
77
How does the secondary oocyte enter the fallopian tube?
It is swept into the funnel-shaped infundibulum of the uterine tube.
78
Where does the oocyte pass after the infundibulum?
It enters the ampulla of the uterine tube.
79
How does the oocyte move through the fallopian tube?
Through peristalsis, which is the alternate contraction and relaxation of uterine tube muscles.
80
Where does fertilization occur?
In the ampulla of the uterine tube
81
What role do muscular contractions of the uterus and uterine tube play?
They facilitate the movement of sperm toward the egg.
82
What do the cumulus cells of the ovulated egg release?
Chemoattractants that guide the sperm to swim towards the egg.
83
What is the first step after sperm meets the egg?
The sperm penetrates the corona radiata and binds to the zona pellucida, triggering the acrosomal reaction.
84
What marks the formation of the zygote?
The fusion of the sperm and egg nuclei.
85
What is cleavage in embryonic development?
The mitotic division of the zygote to form a morula, which is a solid ball of cells.
86
What is the blastocyst?
A stage in development after cleavage where the morula forms a hollow structure consisting of the trophoblast (outer layer) and inner cell mass.
87
When does implantation occur?
Around days 6-7, when the blastocyst embeds into the endometrial lining of the uterus.
88
Where do the sperm and egg move towards during fertilization?
The ampulla of the uterine tube.
89
What happens to the sperm's plasma membrane over the acrosome?
It becomes destabilized, allowing the sperm to pass through the corona cells and undergo the acrosomal reaction.
90
What occurs after the sperm binds to the zona pellucida?
The acrosomal reaction occurs, releasing hyaluronidase to break down cumulus cells and acrosin to penetrate the zona pellucida.
91
What prevents polyspermy after sperm penetration?
The zona pellucida becomes impermeable to other sperm.
92
What happens after the sperm and egg plasma membranes fuse?
The head and tail of the sperm enter the cytoplasm of the oocyte, while the sperm's plasma membrane and mitochondria remain behind.
93
What happens to the egg after fertilization in terms of meiosis?
Meiosis II resumes, leading to the extrusion of the second polar body and the formation of the female pronucleus.
94
How is the egg metabolically activated after fertilization?
It is mediated by an influx of Ca2+ ions.
95
What happens to the sperm's nucleus after fertilization?
The sperm’s nucleus enlarges to form the male pronucleus.
96
What happens to the sperm's tail after fertilization?
The tail degenerates.
97
What happens to the chromosomes after fertilization?
Maternal and paternal chromosomes condense and arrange for mitotic cell division.
98
What is the result of the combination of chromosomes from both pronuclei?
The formation of a zygote with 46 chromosomes.
99
What is the first event after fertilization in terms of cell division?
The first cleavage division of the zygote occurs.
100
When do the mitotic divisions of the zygote begin?
30 hours after fertilization
101
What are the individual cells of the dividing zygote called?
Blastomeres
102
103
What happens to the size of blastomeres with each mitotic division?
They decrease in size.
104
What structure still surrounds the dividing zygote during cleavage?
The zona pellucida.
105
What happens after the 8-cell stage?
The blastomeres change shape and undergo compaction, forming a morula.
106
What is the morula?
A solid ball of 12-32 blastomeres that enters the uterus and has improved cell-to-cell interactions.
107
What happens when the morula enters the uterus?
Uterine fluid passes through the zona pellucida, forming the blastocystic cavity (blastocoele).
108
What are the two main parts of the blastocyst?
Trophoblast (outer layer) – primordium of the placenta.
Embryoblast (inner cell mass) – primordium of the embryo proper.
109
What is hatching in embryonic development?
The degeneration and disappearance of the zona pellucida, allowing the embryo to grow in size.
110
Why is hatching important?
The hatched blastocyst can now implant into the receptive endometrium.
111
When does the blastocyst attach to the endometrial epithelium?
6 days post-fertilization.
112
What does the trophoblast rapidly proliferate and differentiate into?
Cytotrophoblast (inner layer; mononucleated cells, mitotically active).
Syncytiotrophoblast (outer layer; multinucleated, formed by cell fusion).
113
How is the syncytiotrophoblast formed?
Cells from the cytotrophoblast undergo mitosis, migrate into the syncytiotrophoblast, and fuse, losing their cell membranes.
114
What is the function of the syncytiotrophoblast?
It invades the endometrial connective tissue, reaching the uterine capillaries and glands.
115
What happens at the end of the first week of development?
The blastocyst is superficially implanted in the endometrium.
116
What enables the blastocyst to burrow into the endometrium?
The syncytiotrophoblast produces proteolytic enzymes that erode maternal tissues.
117
What new cell layer appears on the embryoblast during implantation?
The hypoblast, a layer of cuboidal cells facing the blastocoele.
118
When is implantation completed?
During the second week of development.
119
What happens to endometrial cells in the center of the implantation site?
They undergo apoptosis, facilitating implantation.
120
What hormone does the syncytiotrophoblast produce, and where does it enter?
Human chorionic gonadotropin (hCG), which enters the maternal blood through lacunae in the syncytiotrophoblast.
121
What is the role of hCG?
Maintains spiral artery development in the myometrium.
Supports the formation of the syncytiotrophoblast.
122
How does a pregnancy test detect pregnancy?
By detecting hCG levels in maternal blood or urine
123
What does the embryoblast differentiate into?
The bilaminar embryonic disc, consisting of:
Epiblast: Thick layer of high columnar epithelial cells.
Hypoblast: Thin layer of small cuboidal cells.
124
What are the key extraembryonic structures that develop during the second week?
Amniotic cavity and amnion
Umbilical vesicle (yolk sac)
Chorionic cavity
Connecting stalk
125
What is the primordium of the amniotic cavity?
A small cavity that appears between the epiblast and cytotrophoblast.
126
What forms the amnion?
Amniogenic (amnioblast) cells migrate from the epiblast to form a thin membrane enclosing the amniotic cavity.
127
What is the role of the epiblast in relation to the amniotic cavity?
It forms the floor of the amniotic cavity and is continuous peripherally with the amnion.
128
What forms the exocoelomic membrane (Heuser’s membrane)?
Cells migrating from the hypoblast.
129
What is the function of the exocoelomic membrane?
It covers the inner surface of the cytotrophoblast, forming the exocoelomic cavity (primitive yolk sac/primary umbilical vesicle).
130
Where does the bilaminar embryonic disc lie?
Between the amniotic cavity and the primary umbilical vesicle.
131
What forms at the site of initial embryo attachment and invasion?
A closing plug.
132
What new cell population appears between the cytotrophoblast and primitive yolk sac?
The extraembryonic mesoderm
133
What are the two layers of the extraembryonic mesoderm?
Extraembryonic somatic mesoderm – lines the trophoblast and covers the amnion.
Extraembryonic splanchnic mesoderm – surrounds the umbilical vesicle.
134
What happens to small cavities that appear in the extraembryonic mesoderm?
They fuse to form the chorionic cavity (extraembryonic coelom).
135
What does the trophoblast become lined by after the formation of the chorionic cavity?
Extraembryonic somatic mesoderm, making it the chorion.
136
What happens as the chorionic cavity enlarges?
A portion of the exocoelomic cavity is pinched off, forming the secondary yolk sac.
137
What is the possible function of the umbilical vesicle?
It may play a role in selective nutrient transfer to the embryonic disc.
138
How is the bilaminar embryonic disc attached to the trophoblast?
By the connecting stalk, which later becomes the primordium of the umbilical cord.
139
What does the chorionic cavity surround?
The amnion and umbilical vesicle, except where they are attached to the chorion by the connecting stalk.
140
When do primary chorionic villi appear?
At the end of the second week.
141
What forms the primary chorionic villi?
Cytotrophoblastic cells proliferate and extend into the syncytiotrophoblast.
142
What is the significance of primary chorionic villi?
They are the primordia of the chorionic villi of the placenta.
143
What structures appear in the syncytiotrophoblast during the second week of development?
Lacunae (small holes).
144
How do lacunar networks form?
Lacunae fuse to form interconnected lacunar networks.
145
What happens to capillaries in the endometrium?
They dilate to form maternal sinusoids.
146
What is the role of proteolytic enzymes from the syncytiotrophoblast?
Rupture maternal sinusoids.
Erode the lining of endometrial glands.
147
What enters the lacunar networks after maternal sinusoids rupture?
Maternal blood from maternal sinusoids.
Uterine gland secretions.
148
What does this exchange establish?
The primordial uteroplacental circulation.
149
How does embryotroph (fluid in the lacunae) reach the embryonic disc?
By diffusion via the syncytiotrophoblast.
150
Where does oxygenated blood in the lacunae come from?
The spiral arteries in the endometrium.
151
How is deoxygenated blood removed from the lacunae?
Through endometrial veins.
152
What is abnormal (extrauterine) implantation?
The blastocyst implants outside the uterus.
153
Where do most ectopic pregnancies occur?
95% - 98% occur in the uterine tubes, mainly in the ampulla and isthmus.
154
How common is ectopic pregnancy in North America?
1 in 200 pregnancies.
155
What causes ectopic pregnancies?
Delay or prevention of the cleaving zygote’s transport to the uterus.
156
What is a specific cause of transport failure?
Blockage of the uterine tube, even though sperm could still pass through.
157
When do symptoms of an ectopic pregnancy appear?
Between the 4th and 12th weeks of pregnancy.
158
What are the symptoms of ectopic pregnancy?
Usual signs of pregnancy.
Abdominal pain (due to distention of the uterine tube).
Abnormal bleeding.
159
What are the main complications of ectopic pregnancy?
Rupture of the uterine tube.
Hemorrhage.
Embryo death.
160
What is the primary treatment for ectopic pregnancy?
Laparoscopy to remove the fertilized egg.
161
What is gastrulation?
Gastrulation is the process that establishes the three germ layers and marks the beginning of morphogenesis (development of body structures).
162
What are the three germ layers formed during gastrulation?
Ectoderm (outer layer)
Mesoderm (middle layer)
Endoderm (inner layer)
163
What major structure develops during gastrulation?
The notochord, which plays a key role in nervous system development.
164
When does the primitive streak appear?
At the beginning of the third week on the dorsal side of the embryonic disc.
165
Where do the cells of the primitive streak come from?
They proliferate and migrate from the epiblast toward the median plane of the embryonic disc.
166
What does the primitive streak establish in the embryo?
It defines:
Craniocaudal axis (head-to-tail orientation)
Dorsal and ventral surfaces
Right nad left sides
167
How does the primitive streak elongate?
By adding cells to its caudal end.
168
What structure forms at the cranial end of the primitive streak?
The primitive node.
169
What is the primitive groove and where does it lead?
A narrow groove that runs along the primitive streak, ending in a small depression in the primitive node, called the primitive pit.
170
What does the primitive streak help to differentiate during the third week?
The primitive streak is key for the differentiation of the three germ layers and the development of the notochord.
171
What is the source of all three germ layers?
The epiblast is the source of all three germ layers: ectoderm, mesoderm, and endoderm.
172
How does the differentiation of the germ layers occur?
Epiblast cells migrate through the primitive groove.
Some displace the hypoblast to become the endoderm.
Others lie between the epiblast and endoderm to form the mesoderm.
The remaining epiblast cells form the ectoderm.
173
What is the mesoderm formed from and what is its structure?
The mesoderm is formed from epiblast cells that lie between the endoderm and epiblast. It forms a loose network called mesenchyme.
174
How long does the primitive streak actively form mesoderm?
The primitive streak actively forms mesoderm until the early part of the fourth week. After this, its production slows down.
175
What happens to the primitive streak after the fourth week?
The primitive streak diminishes in size and becomes an insignificant structure.
176
What does the cranial-most part of the primitive node form?
The notochord (n), a crucial structure for body development.
177
What does the more posterior part of the node and the cranial-most part of the primitive streak form?
Paraxial mesoderm (pm), which will give rise to somitomeres and somites.
178
What does the next portion of the primitive streak form?
Intermediate mesoderm (im), which will contribute to the urogenital system.
179
What does the more caudal part of the primitive streak form?
Lateral plate mesoderm (lpm), which will form the body wall.
180
What does the most caudal part of the primitive streak form?
Extraembryonic mesoderm (eem), which contributes to the formation of the chorion.
181
182
How is the notochord formed?
Mesenchymal cells migrate cranially from the primitive node and pit, forming the notochordal process.
183
How does the notochordal process grow?
The notochordal process grows cranially until it reaches the prechordal plate, which gives rise to the oropharyngeal membrane, the future site of the oral cavity.
184
What is located caudal to the primitive streak, and what will it become?
The cloacal membrane, which will become the future site of the anus.
185
What are the key functions of the notochord?
Defines the axis of the embryo.
Serves as the basis for the development of the axial skeleton.
Indicates the future site of the vertebral bodies.
Functions as the primary inductor in the early embryo, guiding surrounding tissue development.
186
What is neurulation?
Neurulation is the formation of the neural plate and neural folds, which close to form the neural tube, completed by the end of the fourth week of development.
187
How does the developing notochord influence neurulation?
The developing notochord induces the embryonic ectoderm to thicken and form an elongated neural plate of thickened neuroepithelial cells.
188
What does the ectoderm of the neural plate give rise to?
The ectoderm of the neural plate (neuroectoderm) gives rise to:
Central nervous system (CNS): Brain, spinal cord
Other structures: Retina
189
How does the neural plate form the neural groove?
On day 18, the neural plate invaginates along its central axis, forming a median longitudinal neural groove with neural folds on each side. The neural folds are prominent at the cranial end and are the first signs of brain development.
190
What happens at the end of the third week of development regarding the neural folds?
The neural folds move together and fuse, forming the neural tube, which becomes the primordium of the brain vesicles and spinal cord.
191
How does the neural tube separate from the surface ectoderm?
The neural tube separates from the surface ectoderm, and the free edges of the ectoderm fuse, becoming continuous over the neural tube and the back of the embryo. The surface ectoderm then differentiates into the epidermis of the skin.
192
Where are the neural crest cells located, and what is their role?
The neural crest is located between the neural tube and the overlying surface ectoderm. Neural crest cells migrate on each side of the neural tube and within the mesenchyme, differentiating into various cell types, including spinal ganglia and ganglia of cranial nerves.
193
What are neural tube defects (NTD)?
Neural tube defects (NTD) occur when the neural tube does not close properly, leading to openings in the brain, skull, or spine. This can affect:
Brain: e.g., anencephaly
Skull: e.g., craniorachischisis
Spine: e.g., spina bifida
194
What are the causes of neural tube defects (NTD)?
The causes of NTD include:
Genetics
Folic acid deficiency
195
How can neural tube defects (NTD) be prevented?
Folic acid supplementation before and during pregnancy can help prevent NTDs.
196
Can neural tube defects (NTD) be treated?
NTDs often result in permanent damage with limited treatment options.
197
What does "cranial" refer to in anatomical positioning?
Cranial refers to towards the head.
198
What does "caudal" refer to in anatomical positioning?
Caudal refers to towards the tail.
199
What does "ventral" refer to in anatomical positioning?
Ventral refers to towards the front of the body.
200
What does "dorsal" refer to in anatomical positioning?
Dorsal refers to towards the back of the body.
201
What does "superior" refer to in anatomical positioning?
Superior refers to towards the top of the head.
202
What does "inferior" refer to in anatomical positioning?
Inferior refers to towards the feet.
203
What does "anterior" refer to in anatomical positioning?
Anterior refers to the front of the body.
204
What does "posterior" refer to in anatomical positioning?
Posterior refers to behind the body.
205
What does "distal" mean in anatomical terms?
Distal refers to being away from or the farthest from the trunk or the point of origin of a body part.
206
What does "proximal" mean in anatomical terms?
Proximal refers to being closer to or towards the trunk or the point of origin of a body part.
207
What does "median" refer to in anatomical terms?
Median refers to the midline of the body.
208
What does "medial" refer to in anatomical terms?
Medial refers to being towards the median or midline of the body.
209
What does "lateral" refer to in anatomical terms?
Lateral refers to being away from the median or midline of the body.
