Mature vertebrate erythrocytes always:
Lack a nucleus
Have a bi-concave shape
Use haemoglobin to carry oxygen
All of the above

Correct option: The statement is not true for “mature vertebrate erythrocytes” as a group; it is only fully correct for mammals. Among the given MCQ choices, the intention in most exam contexts is that “Use haemoglobin to carry oxygen” is always true for vertebrate erythrocytes, whereas “lack a nucleus” and “have a biconcave shape” are not universal to all vertebrates. So “All of the above” is not correct for vertebrates in general.

Introduction

Mature vertebrate erythrocytes are central to oxygen transport, but their structure is not identical across all vertebrate groups. In mammals they are classically described as anucleate, biconcave discs packed with haemoglobin, but fishes, amphibians, reptiles and birds retain nucleated red cells with different shapes while still transporting oxygen efficiently. Understanding these differences is crucial for solving conceptual MCQs on erythrocyte structure and function and for writing accurate, SEO-optimized explanations in physiology and zoology.

Analyzing the question

The question:
“Mature vertebrate erythrocytes always:

1. Lack a nucleus

2. Have a bi-concave shape

3. Use haemoglobin to carry oxygen

4. All of the above”

To answer correctly, it is necessary to recall how red blood cells differ between mammals and non-mammalian vertebrates.

Option 1: “Lack a nucleus”

• In mammals, mature circulating erythrocytes are anucleate; their nucleus is expelled during late erythropoiesis to increase internal space for haemoglobin and improve deformability in narrow capillaries.

• In non-mammalian vertebrates (fishes, amphibians, reptiles and birds), mature erythrocytes typically retain a nucleus; these red blood cells are nucleated yet functionally active in gas transport.

Therefore, “mature vertebrate erythrocytes always lack a nucleus” is false for vertebrates as a group, because it holds only for mammals and not for other vertebrate classes.

Option 2: “Have a bi-concave shape”

• Human and most mammalian erythrocytes are classic biconcave discs, which increases surface area-to-volume ratio for gas exchange and facilitates flexibility.

• In other vertebrates, red blood cells are usually oval or ellipsoidal and nucleated, not typical biconcave discs; even within mammals, there are minor shape variations among species.

Thus, “have a bi-concave shape” is not a universal property of all vertebrate erythrocytes, and the word “always” makes this statement incorrect for vertebrates in general.

Option 3: “Use haemoglobin to carry oxygen”

• Across vertebrates, the standard respiratory pigment in erythrocytes is haemoglobin, an iron-containing protein that binds oxygen reversibly and transports it from respiratory surfaces to tissues.

• While some invertebrates use other pigments (e.g., haemocyanin), vertebrate red blood cells characteristically depend on haemoglobin for oxygen transport, regardless of whether the cells are nucleated or biconcave.

Hence, “use haemoglobin to carry oxygen” is the only statement that is true for all vertebrate erythrocytes in the context of this question.

Option 4: “All of the above”

For “All of the above” to be correct, all three previous statements must be true for mature vertebrate erythrocytes:

• Lack a nucleus → true only for mammals, false for birds, reptiles, amphibians and fishes.

• Have a bi-concave shape → true for typical mammalian RBCs, not universal for all vertebrates.

• Use haemoglobin to carry oxygen → true for vertebrate erythrocytes.

Since the first two statements fail for non-mammalian vertebrates, “All of the above” is not correct. In many exam keys, the intended correct concept is that only the haemoglobin-based oxygen transport is universal, while absence of nucleus and biconcave shape are mammalian specializations.

Final exam-oriented takeaway

• Universal vertebrate feature:

  • Mature erythrocytes use haemoglobin for oxygen transport.

• Mammalian special features (not universal to all vertebrates):

  • Mature erythrocytes lack a nucleus.

  • They typically have a biconcave disc shape.

So, when the question is strictly about “mature vertebrate erythrocytes always”, the only globally correct statement is about haemoglobin-based oxygen transport, and therefore “All of the above” cannot be accepted as correct for vertebrates as a whole.